Heater apparatus and methods

Abstract

The present disclosure relates to apparatus and methods for drying or styling hair. In some examples the apparatus comprises: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: change a temperature or power output of a first heating zone of the plurality of independently controllable heating zones; and determine, based on a corresponding change in temperature of a second heating zone of the plurality of independently controllable heating zones, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a speed at which hair is moving across the hair contacting surface.

Description

HEATER APPARATUS AND METHODSField of the InventionThe present invention relates to heating apparatus and methods, and to heater calibration apparatus and heater calibration methods. The heaters can be used for drying and / or styling hair (e.g. using a hair curler). Such drying and / or styling of the hair may be performed by a user in respect of their own hair, for example, or by a hair stylist. The invention has particular, but not exclusive, relevance to a drying or styling devices comprising one or more low thermal mass heaters. The invention also has relevance to the calibration of hair styling and hair drying devices, and to means of controlling low thermal mass heaters in response to detecting user skin contact with one or more heated surfaces of the styling appliance / device (e.g., finger / hand contact and / or scalp contact).Background to the InventionHeated hair styling tools or hair drying tools use heat to increase the temperature of hair to a desired styling or drying temperature. For example, a hair straightener having a heated plate applies heat directly via conduction to heat the hair, which may be either wet or dry, to achieve the desired temperature for styling. The hair may be heated to a temperature that is particularly suitable for styling hair (for example, to or beyond a glass transition phase temperature). At lower temperatures, the user may have to make many passes with the hair straightener over the hair to achieve a desired styling effect, whereas at higher temperatures, there is a risk of causing permanent damage to the hair.Similarly, a heated brush or hair dryer can also be used to style hair by heating the hair to a temperature suitable for styling or drying. Hair may be styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry.Existing hair styling appliances typically use relatively thick heating plates or heating tubes that provide a certain amount of thermal mass to the hair styling appliance. These heating plates or tubes are heated by a heater that is mounted on an inner surface of the heating plate / tube. As a result of the thermal mass, the heating plates / tubes take time to heat up and, once heated, they can take quite a long time to cool down. This thermal mass makes it quite difficult to control the heating of the hair and over heating or under heating of the hair can result. There has been a general desire to move towards hair styling appliances that use heaters that have a lower thermal mass and can therefore heat up and cool down much more quickly. Low thermal mass heaters are more responsive and are easier to control. However, there is a need for improved control of the power delivered to such low thermal mass heaters. For example, there is a need for improved apparatus and control methods for providing more energy efficient devices having reduced energy consumption, that take advantage of the high responsiveness of low thermal mass heaters. Improved energy efficiency increases runtime of battery powered devices, but is also beneficial even for corded mains-powered devices (e.g. to enable a smaller power supply to be used, and for environmental reasons).Whilst rapid heat up and cool down of low thermal masses provides many benefits when implemented in hair styling appliances as alluded to above, the provision of such low thermal mass heaters does raise safety concerns. In particular, given the rapid heat up of such low thermal mass heaters, there is a risk of an end user experiencing skin contact burning if their skin comes into contact with a heat surfaceof the hair appliance unknowingly as it is heating up. This may especially be the case where a user pre-loads their hair into the hair styler and then activates heating. There is therefore a need for mechanisms to address the risk of skin contact burning in hair stylers that use low thermal mass heaters.There is also a need for improved apparatus and control methods for preventing possible overheating of low thermal mass heaters, as when such heaters overheat there is a risk of damage not only to the heater itself, but also to other parts and components of the hair styling or hair drying device.Hair styling and hair drying devices typically pass current through one or more heater electrodes to generate heat for subsequent transfer of the heat to the hair of a user (for example via conductive heating, convective heating or radiative heating). The voltage supplied to the heater is controlled in order to modulate the heat output from the heater electrodes. The heat output will depend on both the applied voltage and the resistance of the heater electrodes. However, whilst the heater electrodes are manufactured to have a particular resistance, the actual resistance of the heater electrodes will have some variation that will depend on the manufacturing tolerances. Since the heat output depends on the heater electrode resistance, the heat output at a particular applied voltage will therefore have some variation between devices. It is therefore beneficial to test and calibrate the heat output from the heaters after manufacture, to enable the heat output to be more accurate and consistent between devices. Testing the devices after manufacture also enables devices for which the heater electrode resistance falls outside of an acceptable tolerance to be identified. However, there is a need for improved methods for calibration and testing of hair styling and hair drying devices. For example, there is a need for faster and more accurate calibration methods to enable large numbers of devices to be efficiently calibrated. There is also a need for improved methods for detecting devices that have faults, for example an irregularity in the response of a heater to a supply of power.The present invention aims to address or at least partially ameliorate one or more of the above problems.Summary of the InventionIn one aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: change a temperature or power output of a first heating zone of the plurality of independently controllable heating zones; and determine, based on a corresponding change in temperature of a second heating zone of the plurality of independently controllable heating zones, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a speed at which hair is moving across the hair contacting surface.Changing the temperature or power output of the first heating zone may comprise increasing the temperature or power output of the first heating zone at a first time; and determining the speed at which the hair is moving across the hair contacting surface comprises: detecting a corresponding decrease in the power needed to maintain the second heating zone at the target temperature at a second time; and determining the speed at which hair is moving across the hair contacting surface based on a difference between the second time and the first time.Changing the temperature or power output of the first heating zone may comprise decreasing the temperature or power output of the first heating zone at a first time; and determining the speed at which the hair is moving across the hair contacting surface may comprise: detecting a corresponding increase in the power needed to maintain the second heating zone at the target temperature at a second time; and determining the speed at which hair is moving across the hair contacting surface based on a difference between the second time and the first time.The apparatus may comprise a temperature sensor for measuring the temperature of the second heating zone.The first and second heating zones may be arranged across the width of the heater.The first and second heating zones may be adjacent.The first and second heating zones may be substantially thermally isolated from each other.The controller may be configured to control the apparatus to: determine the resistance of a heater electrode of the second heating zone based on a voltage supplied to the heater electrode and the current passing through the heater electrode; and determine the temperature of the second heating zone based on the determined resistance.The controller may be configured to control the apparatus to supply power to a heater electrode of the heater to determine the resistance of the heater electrode even in a case where it is determined that the corresponding heating zone is not engaged with hair.The apparatus may further comprise one or more optical sensors for detecting whether hair is engaged with the hair-contacting surface.The apparatus may further comprise a pair of opposing heating zones, wherein heat can flow between the opposing heating zones when the apparatus is in use, and wherein the controller is configured to control the apparatus to: determine whether hair is engaged with at least one of the pair of opposing heating zones based on a sum or average of a power needed to maintain each of the pair of opposing heating zones at a target temperature.The heater may be a low thermal mass heater.At least one of the heating zones may be operable to have a power density that is greater than 0.8 W / cm2and less than 15 W / cm2preferably greater than 2 W / cm2and more preferably greater than 8 W / cm2.At least one of the heating zones may have an outer surface heat up rate capability of greater than 50°C / s and less than 500 °C / s, preferably greater than 185°C / s, more preferably greater than 400°C / s.The apparatus may be a hair straightener, a hair dryer, a hot paddle brush, a hot round brush, a heater roller, or a hair curler.The controller may be configured to: control the apparatus to maintain a first heating zone of the plurality of independently controllable heating zones at a first targettemperature for drying or styling hair even if it is determined that hair is not in contact with the first heating zone; and control the apparatus to maintain a second heating zone of the plurality of independently controllable heating zones at a second temperature that is lower than a target temperature for the second heating zone for drying or styling hair if it is determined that hair is not in contact with the second heating zone.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: determine, based on a temperature of a heating zone of the plurality of independently controllable heating zones or based on the power needed to maintain the heating zone at a target temperature, whether hair is engaged with the heating zone; control power supplied to a heater electrode of the heating zone to control the temperature of the heating zone towards a first target temperature in a case where it is determined that the hair is engaged with the heating zone, and to control the temperature of the heating zone towards a second target temperature, lower than the first target temperature, in a case where it is determined that the hair is not engaged with the heating zone; and intermittently supply power to the heater electrode to determine the resistance of the heater electrode even in the case where it is determined that the heating zone is not engaged with the hair, and determine the temperature of the heating zone based on the resistance.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: when hair is in contact with a first heating zone and a second heating zone of the plurality of independently controllable heating zones for transfer of heat from the first heating zone to the second heating zone via thermal conduction along the hair, provide power to a heater electrode of the first heating zone to heat the hair in the first heating zone; and determine, based on a corresponding change in the temperature of the second heating zone, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a thermal conductivity of the hair.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: provide power to each of the plurality of independently controllable heater electrodes based on the power demand at the respective heating zone; wherein the power provided to each heater electrode is interleaved with the power provided to the other heater electrodes.The controller may be configured to determine a duty cycle for each of the heating zones, and wherein the interleaved power supplied to each heating zone is based on the determined duty cycle.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, and the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: changing a temperature or power output of a first heating zone of the plurality of independently controllable heating zones; and determining, based on a corresponding change in temperature of a second heating zone of the plurality of independently controllable heating zones, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a speed at which hair is moving across the hair contacting surface.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: determining, based on a temperature of a heating zone of the plurality of independently controllable heating zones or based on the power needed to maintain the heating zone at a target temperature, whether hair is engaged with the heating zone; controlling power supplied to a heater electrode of the heating zone to control the temperature of the heating zone towards a first target temperature in a case where it is determined that the hair is engaged with the heating zone, and controlling the temperature of the heating zone towards a second target temperature, lower than the first target temperature, in a case where it is determined that the hair is not engaged with the heating zone; and intermittently supplying power to the heater electrode to determine the resistance of the heater electrode even in the case where it is determined that the heating zone is not engaged with the hair, and determining the temperature of the heating zone based on the resistance.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: when hair is in contact with a first heating zone and a second heating zone of the plurality of independently controllable heating zones for transfer of heat from the first heating zone to the second heating zone via thermal conduction along the hair, providing power to a heater electrode of the first heating zone to heat the hair in the first heating zone; and determining, based on a corresponding change in the temperature of the second heating zone, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a thermal conductivity of the hair.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: providing power to each of the plurality of independently controllable heater electrodes based on the power demand at the respective heating zone; wherein the power provided to each heater electrode is interleaved with the power provided to the other heater electrodes.In another aspect the invention provides apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; and a controller configured to control the apparatus to: determine the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently control the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, to control the temperature of the corresponding heating zone towards a respective target temperature; wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.In another aspect the invention provides apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; a plurality temperature sensors, each temperature sensor being configured for measuring the temperature of a respective heating zone; and a controller configured to control the apparatus to: independently control the supply of power to each of the heater electrodes, based on the temperature of the corresponding heating zone, to control the temperature of the corresponding heating zone towards a respective target temperature; wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.In another aspect the invention provides apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; and a controller configured to control the apparatus to: determine the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently control the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, to control the temperature of the corresponding heating zone towards a respective target temperature; wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between a current duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window; and wherein the controller is configured to determine the target duty cycle for the drive signal applied to each of the heater electrodes based on a maximum current limit of the apparatus.In another aspect the invention provides a method performed by apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; wherein the method comprises: determining the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently controlling the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, to control the temperature of the corresponding heating zone towards a respective target temperature; and determining, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.In another aspect the invention provides a method performed by apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a plurality temperature sensors, each temperature sensor being configured for measuring the temperature of a respective heating zone; wherein the method comprises: independently controlling the supply of power to each of the heater electrodes, based on the temperature of the corresponding heating zone, to control the temperature of the corresponding heating zone towards a respective target temperature; and determining, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.In another aspect the invention provides a method performed by apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; wherein the method comprises: determining the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently controlling the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, to control the temperature of the corresponding heating zone towards a respective target temperature; determining, sequentially for each of the heater electrodes and based on a difference between an actual duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window; and determining the target duty cycle for the drive signal applied to each of the heater electrodes based on a maximum current limit of the apparatus.In another aspect the invention provides apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; a memory storing separate calibration information for each of the heating zones that indicates a respective relationship between at least one of: the resistance of the corresponding heater electrode and the temperature of the corresponding heating zone, the voltage supplied to the corresponding heater electrode and the temperature of the corresponding heating zone, or a measured temperature of the corresponding heating zone and an actual temperature of the corresponding heating zone; and a controller configured to control the apparatus to: independently control the supply of power to each of the heater electrodes, based on the respective calibration information stored in the memory, to control the temperature of each of the heating zones towards a respective target temperature.In another aspect the invention provides calibration apparatus for calibrating the apparatus for drying and / or styling hair according to the above aspect, the calibration apparatus comprising: a plurality of temperature sensing zones, wherein each of the plurality of temperature sensing zones is configured for thermal contact with a respective one of the independently controllable heating zones of the hair contacting surface, and wherein each of the temperature sensing zones has a resistance that varies with the temperature of the temperature sensing zone; and communication circuitry configured for communication with the apparatus for drying and / or styling hair; wherein the calibration apparatus is configured for: sensing the temperature of each of the heating zones based on the resistance of the respective temperature sensing zone; obtaining calibration information for each of the heating zones; and transmitting the calibration information to the apparatus for drying and / or styling hair using the communication circuitry.In another aspect the invention provides a calibration system comprising: at least one calibration apparatus according to the above aspect for calibrating the apparatus for drying and / or styling hair; and a calibration controller configured for communication with the calibration apparatus; wherein the calibration controller is configured for: receiving, from the calibration apparatus, measurement results corresponding to the sensed temperatures for each of the independently controllable heating zones; and transmitting, to the apparatus for drying and / or styling hair, the calibration information for each of the heating zones.In another aspect the invention provides a hair styling apparatus comprising: a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating; a plurality of microcontroller units, MCUs, in communication with one another, each one of the MCUs being configured to control a subset of the plurality of heaters, wherein at least one MCU of the plurality of MCUs comprises a processor configured to: detect malfunctions of the hair styling apparatus; and control a function of the hair styling apparatus in response to the processor detecting malfunctions of the hair styling apparatus.In another aspect the invention provides a hair styling apparatus comprising: communication means for communicating with at least one external computing device for controlling the hair styling apparatus; a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating; a plurality of microcontroller units, MCUs, in communication with one another, eachone of the MCUs being configured to control a subset of the plurality of heaters, wherein at least one MCU of the plurality comprises a processor configured to: detect malfunctions of the hair styling apparatus; and control a function of the hair styling apparatus in response to the processor detecting malfunctions of the hair styling apparatus.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller; wherein the plurality of independently controllable heating zones comprise one or more first heating zones and one or more second heating zones; and wherein the controller is configured to perform control to cause the apparatus to, when hair passes from the one or more first heating zones to the one or more second heating zones before leaving the one or more second heating zones: determine a first power or first duty cycle needed to maintain the one or more first heating zones at a first target temperature for the one or more first heating zones; determine a second power or second duty cycle needed to maintain the one or more second heating zones at a second target temperature for the one or more second heating zones; estimate, based on the first power and the second power, or based on the first duty cycle and the second duty cycle, a temperature of the hair as the hair leaves the one or more second heating zones; and control the power supplied to the independently controllable heater electrodes based on the estimated temperature of the hair.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, and a controller; wherein the plurality of independently controllable heating zones comprise one or more first heating zones and one or more second heating zones; and wherein the method comprises, when hair passes from the one or more first heating zones to the one or more second heating zones before leaving the one or more second heating zones: determining a first power or first duty cycle needed to maintain the one or more first heating zones at a first target temperature for the one or more first heating zones; determining a second power or second duty cycle needed to maintain the one or more second heating zones at a second target temperature for the one or more second heating zones; estimate, based on the first power and the second power, or based on the first duty cycle and the second duty cycle, a temperature of the hair as the hair leaves the one or more second heating zones; and control the power supplied to the independently controllable heater electrodes based on the determined temperature of the hair.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; an accelerometer for sensing an acceleration of the apparatus; and a controller configured to: determine a speed that the apparatus is moving across the hair using the acceleration sensed by the accelerometer; and determine, based on the speed and based on a target hair temperature for the hair, a target temperature for theheating zones or an amount of power to be supplied to heat each of the heating zones to control the temperature of the hair towards the target hair temperature.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to: determine whether hair is in contact with each of the heating zones; determine that a first heating zone is to be heated to a corresponding target temperature for drying or styling the hair when it is determined that hair is in contact with the first heating zone; determine that one or more additional heating zones are also to be heated to respective target temperatures for drying or styling the hair even when contact between the one or more additional heating zones and the hair is not detected; and control a supply of power to the heater electrodes to control the temperature of the first heating zone and the temperature of the one or more additional heating zones towards the corresponding target temperatures for drying or styling the hair.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising at least one heater electrode; and a controller configured to: control a supply of power to the at least one heater electrode to maintain the temperature of the hair contacting surface at a target temperature for drying or styling the hair; determine that a temperature of hair that is in contact with the hair contacting surface has exceeded a first threshold temperature; after it is determined that the temperature of the hair has exceeded the first threshold temperature, determine that the temperature of the hair contacting surface is to be allowed to cool towards an ambient temperature; and determine, as the temperature of the hair contacting surface is allowed to cool towards the ambient temperature, that the temperature of hair that is in contact with the hair contacting surface has fallen below a second threshold temperature.In another aspect the invention provides apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to: determine that skin of a user is in contact with a first set of one or more of the heating zones; in response to determining that the that the skin of the user is in contact with the first set of one or more of the heating zones, disable a supply of power to the first set of one or more heating zones; and control the supply of power to at least one other heating zone that is not determined to be in contact with the skin of the user to control the temperature of the heating zone towards a target temperature for drying or styling the hair, whilst the supply of power to the first set of one or more heating zones is disabled.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, an accelerometer for sensing an acceleration of the apparatus, and a controller; wherein the method comprises the controller: determining, a speed that theapparatus is moving across the hair using the acceleration sensed by the accelerometer; and determining, based on the speed and based on a target hair temperature for the hair, a target temperature for the heating zones or an amount of power to be supplied to heat the heating zones to control the temperature of the hair towards the target hair temperature.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, and a controller; wherein the method comprises the controller: determining whether hair is in contact with each of the heating zones; determining that a first heating zone is to be heated to a corresponding target temperature for drying or styling the hair when it is determined that hair is in contact with the first heating zone; determining that one or more additional heating zones are also to be heated to respective target temperatures for drying or styling the hair even when contact between the one or more additional heating zones and the hair is not detected; and controlling the temperature of the first heating zone and the temperature of the one or more additional heating zones towards the corresponding target temperatures for drying or styling the hair.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising at least one heater electrode, and a controller; wherein the method comprises the controller: controlling a supply of power to the at least one heater electrode to maintain the temperature of the hair contacting surface at a target temperature for drying or styling the hair; determining that a temperature of hair that is in contact with the hair contacting surface has exceeded a first threshold temperature; when it is determined that the temperature of the hair has exceeded the first threshold temperature, determining that the temperature of the hair contacting surface is to be allowed to cool towards an ambient temperature; and determining, as the temperature of the hair contacting surface is allowed to cool towards the ambient temperature, that the temperature of hair that is in contact with the hair contacting surface has fallen below a second threshold temperature.In another aspect the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, and a controller; wherein the method comprises the controller: determining that skin of a user is in contact with a first set of one or more of the heating zones; in response to determining that the that the skin of the user is in contact with the first set of one or more of the heating zones, disabling a supply of power to the first set of one or more heating zones; and controlling the supply of power to at least one other heating zone that is not determined to be in contact with the skin of the user to control the temperature of the heating zone towards a target temperature for drying or styling the hair, whilst the supply of power to the first set of one or more heating zones is disabled.In another aspect the invention provides a hair styling appliance comprising: a multilayer heater comprising a plurality of functional layers that are bonded together,wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein at least one of the layers of the multilayer heater is a heater electrode layer comprising a plurality of independently powerable heater electrodes that define a corresponding plurality of heating zones on the hair contacting surface of the multilayer heater, each heater electrode being formed of a conductive material that generates heat when a current is passed through it; a plurality of touch sensors, wherein at least one touch sensor is provided proximal to each heating zone and is configured to detect user skin contact with the heating zone; and control circuitry comprising at least one processor configured to perform a control action in respect of at least one heating zone of the plurality of heating zones in response to a touch sensor detecting user skin contact with the at least one heating zone.In another aspect the invention provides a method performed by the hair styling appliance in use for styling and / or drying hair, the method comprising: sending, by a signal generator, an excitation signal to a plurality of touch sensors, wherein one or more touch sensors of the plurality of touch sensors are provided proximal to each heating zone of the hair styling appliance and are configured to detect user skin contact with the heating zones of the hair styling appliance; detecting user skin contact with at least one of the heating zone of the hair styling appliance; receiving, in response to detecting user skin contact with at least one of the heating zone of the hair styling appliance, a response signal indicating that user skin contact with the at least one of the heating zone has occurred; and performing, by control circuitry of the hair styling appliance comprising at least one processor, a control action in respect of the at least one heating zone of the plurality of heating zones in response to a touch sensor detecting user skin contact with the at least one heating zone.Brief Description of the DrawingsEmbodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:Figure 1a shows an overview of an exemplary hair styling device;Figure 1 b shows a hair styling device in use;Figure 2 shows a set of heating zones of the hair styling device;Figure 3 schematically illustrates the heating zones on the heating surface of the heater shown in Figure 2;Figure 4 schematically illustrates an alternative arrangement of heating zones;Figure 5 schematically illustrates a further alternative arrangement of heating zones that are of different sizes and shapes;Figure 6a illustrates the way in which the heating zones may be formed on a tubular substrate for use in a curling tong or the like;Figure 6b illustrates the way in which the heating zones may be arranged on a curved substrate which may be used on a heated brush or the like;Figure 7 shows a simplified block diagram illustrating the main electronic components of the hair styling device shown in Figure 1 ;Figure 8 illustrates a method of controlling heating zones of the hair styling device;Figure 9 shows a simplified schematic illustration of heating zones and tresses of hair on two of the heating zones;Figure 10 shows a simplified schematic illustration of heating zones and a tress of hair;Figure 11 a shows a table of target temperatures for a heating zone;Figure 11 b shows a table of target temperatures assigned for each heating zone;Figure 12a illustrates an exploded view of a low thermal mass heater;Figure 12b illustrates a perspective see-through assembled view of the low thermal mass heater shown in Figure 12a;Figure 12c illustrates a perspective opaque assembled view of the low thermal mass heater shown in Figure 12a;Figure 13 shows a simplified schematic illustration of a configuration of layers of a low thermal mass heater;Figure 14 shows a simplified schematic illustration of an alternative configuration of layers of a low thermal mass heater;Figure 15 shows a simplified schematic illustration of a further alternative configuration of layers of a low thermal mass heater;Figure 16 shows schematic circuit diagram of the hair styling device illustrating heaters, a controller, and control electronics;Figure 17 shows a flow diagram of a method of monitoring the temperature of the heating zones, and controlling the power output of each heating zone;Figure 18 illustrates a method of measuring the resistance of the heater electrodes, and determining the corresponding temperature of the respective heating zones;Figure 19 illustrates a method of determining a PID controller output;Figure 20 illustrates a method of determining a duty cycle for a heater electrode;Figure 21 illustrates a method of determining to change a target temperature for a heating zone to a styling temperature from an intermediate temperature;Figure 22 shows an example in which two opposing arms of the hair styling device are provided with heating zones;Figures 23a and 23b schematically illustrate a tress of hair being moved across the hair-contacting surface of the heater in the transverse direction;Figure 24 shows a graph illustrating the temperature of a heating zone, and the power required to maintain a target temperature at another heating zone, that can be used to determine a speed at which hair is moved across the hair contacting surface;Figure 25 shows a flow diagram illustrating a method of controlling the power output to the heating zones;Figure 26 shows a flow diagram illustrating a first part of a method of controlling the power output to the heating zones;Figure 27 shows a flow diagram illustrating a second part of a method of controlling the power output to the heating zones;Figure 28 shows a third part of the method of controlling the power output to the heating zones;Figures 29 to 34b show graphs of the current drawn from the power source during operation of the device;Figure 35 shows a simplified schematic illustration of a heater having a number of heating zones, and a temperature sensor having a number of sensing zones;Figure 36 shows a further simplified schematic illustration of a of heater having a number of heating zones, and a temperature sensor having a number of sensing zones;Figure 37 shows a simplified schematic illustration of a pair of heaters and a corresponding pair of temperature sensors;Figure 38 shows a simplified schematic illustration of a further example of a pair of heaters and a corresponding pair of temperature sensors;Figure 39 shows a schematic block diagram of apparatus for calibrating devices;Figure 40 shows a further schematic block diagram of apparatus for calibrating devices;Figure 41 shows a schematic block diagram of apparatus for calibrating devices;Figure 42 shows a flow diagram of a method of calibrating a heater;Figure 43 shows a table of exemplary calibration measurements;Figure 44 shows a graph of hair temperature along a tress of hair, and temperature of a corresponding heating zone;Figure 45 shows an example of a hair distribution across heating zones;Figure 46 shows a further example of a hair distribution across heating zones;Figure 47 shows a further example of a hair distribution across heating zones; andFigure 48 shows a plot of hair temperature, heater temperature and power supplied to the heater against time;Figure 49 shows a graph of hair temperature as hair passes over two adjacent heating zones;Figure 50 shows an example of how heating zones may be indexed;Figure 51 shows a simplified schematic illustration in which two heating zones are sub-divided into smaller elements;Figure 52 shows a simplified schematic illustration of various parameters that may be measured by the device and used to control the power supplied to the heating zones;Figure 53 shows a flow diagram illustrating a method of controlling the power supplied to the heating zones;Figure 54 shows a simulated temperature distribution for a tress of hair;Figure 55 shows a further simulated temperature distribution for a tress of hairFigure 56 shows a further simulated temperature distribution for a tress of hair;Figure 57 shows a further simulated temperature distribution for a tress of hair;Figure 58 shows a simplified schematic diagram of a neural network for estimating an exit temperature of the hair;Figure 59 shows a simplified schematic diagram of a further example of a neural network for estimating an exit temperature of the hair;Figure 60 shows a further simulated temperature distribution for a tress of hair;Figure 61 shows a further simulated temperature distribution for a tress of hair;Figure 62 illustrates example circuitry for a skin sense I touch sense electrode that may be implemented in the device of Figure 1a;Figure 63 illustrates another example circuitry for a skin sense I touch sense electrode that may be implemented in the device of Figure 1 a;Figure 64 illustrates example circuitry for a synchronous detector that may be used in the example circuitry of Figures 62 and 63;Figure 65a illustrates an example input signal that may be input to the synchronous detector circuitry of Figure 64 in response to a skin touch detection by a sensor;Figure 65b illustrates an example of the input signal of Figure 65a once it has been passed through a low-pass filter;Figure 65c illustrates an example of an output signal output by the synchronous detector of Figure 64;Figure 66 illustrates example circuitry for a quadrature synchronous detector that may be used in the example circuitry of Figures 62 and 63;Figures 67a and 67b illustrate real-world scenarios in which the hair styling device of Figure 1 a may be used;Figure 68 illustrates an example of a 1-to-8 multiplexer arrangement for the implementation of a multizone touch sensor in the hair styling device of Figure 1a;Figure 69 illustrates an example of an adaptation of a heater in the hair styling device of Figure 1 a to use said heater as a touch sensor;Figure 70 illustrates a weak link which may be used in a thermal safety circuit;Figures 71a and 71 b illustrate possible arrangements of heating zones that may be implemented in the hair styling device of Figure 1a;Figures 72 to 76 illustrate further possible arrangements of heating zones that may be implemented in the hair styling device of Figure 1a;Figure 77 is a temperature-time plot of a heating response of the low thermal mass heater;Figure 78 is a schematic circuit diagram that illustrates the operation of a thermal safety circuit that is used to isolate a heating circuit of the hair styler;Figure 79 illustrates one form of a thermal safety circuit;Figure 80 illustrates an alternative form of a thermal safety circuit;Figure 81 illustrates a further example of a thermal safety circuit;Figure 82 illustrates a further example of a thermal safety circuit;Figure 83 is a simplified block diagram of the heater control sub-circuitry of the hair styling device of Figure 1a;Figure 84 shows a schematic view of the microcontroller units, software, and other hardware implemented in the hair styling device of Figure 1a;Figure 85 illustrates a tabular example summary of the malfunctions that may occur with the hair styling device of Figure 1a;Figure 86 illustrates the hair styling device of Figure 1 in wireless communication with an external computing deviceFigure 87 is a partially exploded cross-sectional and perspective view of another heater assembly;Figure 88a is an exploded view from above of a flexible heater, an adhesive layer and a heater support;Figure 88b is an exploded view from below of the flexible heater, an adhesive layer and a heater support shown in Figure 88a;Figure 89 illustrates a main heating electrode layer forming part of the heater shown in Figure 88;Figure 90 illustrates a heat spreading a fuse layer forming part of the heater shown in Figure 88;Figure 91 illustrates in more detail fuse circuitry shown in Figure 90;Figure 92a is a cross-sectional view of the heater assembly shown in Figure 88 illustrating a fuse when intact;Figure 92b is a cross-sectional view of the heater assembly shown in Figure 88 illustrating when the fuse has melted due to overheating; andFigure 93 is a simplified schematic diagram of drive and control circuitry that can be used to control the heating of the heater shown in Figure 88.Detailed Description of Preferred EmbodimentsThe present embodiments represent the best ways known to the applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved.Device OverviewFigure 1a illustrates a handheld (portable) hair styler 1. The hair styler 1 includes a first movable arm 4a and a second movable arm 4b, which are coupled at proximal ends thereof to a shoulder 2. The first arm 4a bears a first heater 6a at its distal end, and the second arm 4b bears a second heater 6b at its distal end. The first and second heaters 6a, 6b oppose one another and are brought together as the first and second arms 4a, 4b are moved from an open configuration to a closed configuration. As shown in Figure 1 b, during use, a tress of hair 40 is sandwiched between the two arms 4 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 6a, 6b. Therefore, as the user pulls the hair styler 1 along the tress of hair 40, the tress of hair 40 is heated by conductive heating to a suitable temperature to facilitate styling.The heaters 6a, 6b are low thermal mass heaters and are therefore able to heat up and cool down rapidly. Whereas the heaters of a typical conventional hair styler may take, for example, around 30 seconds to reach an operating temperature of about 200°C from ambient temperature, a low thermal mass heater operating at maximum power may take less than one second to reach the same operating temperature. It will be appreciated that the exact operating temperature and heat-up time depend on the particular requirements of the device, and on the maximum power output of thedevice. By way of example, a low thermal mass heater may heat up at a rate of approximately 410 °Cs-1, and may cool from an operating temperature for styling to a touchable temperature in a timescale of the order of a few seconds.A user interface 11 is provided to allow the user to turn the device on or off, and could also be used to enable the user to set user-definable parameters of the device 1 or to output information to the user. For example, a desired operating temperature for the heaters 6a, 6b could be input via the user interface 11. The user interface 11 may have a dial, button 14, or touch display for allowing the user to input information into the device 1 and the user interface 11 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. The user interface 11 may also comprise an indicator light 17 to indicate whether the device is on.A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 1 and carries the control circuitry for controlling the operation of the device 1 and for controlling the interaction with the user via the user interface 11. In this example, electrical power is provided to the device 1 by means of a power supply located at an end of the device, via a power supply cord 3. The power supply may be an AC mains power supply. However, in an alternative embodiment the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g. from the mains or a DC supply via a charging lead), thereby enabling the device 1 to be a cordless product. Where the device 101 is a cordless product, the power supply may be incorporated within the device 101In use, the device 1 is turned on, energising the heaters 6 to cause them to heat up. The user then opens the first and second arms 4a, 4b (or they may be biased open initially) and, normally starting from the roots of the hair (i.e. near the scalp), a length or tress of hair 40 (which may be clumped) is introduced between the arms 4a, 4b, transversely across the heaters 6a, 6b. The user then closes the arms 4a, 4b so that the length of hair 40 is held between the first and second arms 4a, 4b and then the user pulls the hair through the closed arms (as illustrated in Figure 1 b). The outer (hair contacting) surface of the heaters 6 is flat in this embodiment and so the hair styler 1 can be used to straighten the user’s hair (but alternatively the device 1 could comprise a cylindrical heater 6, or another shape of heater 6 having a curved surface). The hair styling device 1 shown in Figure 1 can also be used to curl the hair by turning the device 1 through approximately 180 degrees or more after clamping the hair between the arms 4a, 4b and before moving the device 1 along the tress of hair 40.Hair has a relatively high thermal mass, and when in contact with the heating surface of the heaters 6 the hair absorbs a significant amount of heat energy. The heaters 6 must quickly supply the lost heat energy back to the heating surface otherwise the temperature of the heating surface will drop and potentially impact on the quality of the thermal styling. If the temperature of the heaters 6 falls below that required to raise the hair temperature above the glass transition temperature of the hair, the hair may not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo damage. Therefore, the device 1 is configured to control the temperature of the heaters 6 so that the hair-contacting surfaces remain within a particular temperature range when in use for styling the hair.A hinge may be provided at the elbow 2. The hinge may comprise a spring for biasing the first and second arms 4a, 4b into the open configuration, such that the user is required to apply pressure to the arms 4a, 4b to close them together (overcoming the effect of the spring). For example, the hinge may include a leaf spring or a coiledspring. The spring itself can be used to couple the first and second arms 4a, 4b together, thereby avoiding the need to provide a separate mechanical hinge and simplifying the overall construction of the device. Alternatively, the first and second arms 4a, 4b may be formed in a unitary manner (e.g. from a plastics material) with a “U” shaped middle part at the elbow 2, the “U” shaped middle part being able to resiliently flex to allow opening and closing of the first and second arms 4a, 4b.Opening and closing of the arms 4a, 4b may be detected using a switch. For example, a microswitch or Hall effect sensor and passive magnet could be used detect closure and opening of the arms 4a, 4b.The present invention is not limited to the type of device illustrated in Figure 1a. For example, a device that additionally transfers heat to hair using heated air could be used. When heated air is used to heat the user’s hair, the device may comprise a heater having an electrically-powered heating coil (or any other suitable type of heating element), operable to heat air drawn in by a fan assembly. It will be appreciated that the apparatus and methods described below are applicable to any suitable device (e.g. hair styling or hair drying device) comprising one or more low thermal mass heaters. It will also be appreciated that while Figure 1a depicts a ‘hairstraightener’ type hair styling device, other types / designs of hair styling devices are possible and that the following description regarding skin contact detection may apply equally to such other hair styling devices. By way of example only, the hair styling device may alternatively be a hair curler / curling type device that is used to curl hair. In this scenario, the hair curler may have (but need not necessarily have) a generally tubular heater 6 (or set of heaters 6).Heating ZonesIndependently controllable heating zones 642 of the heaters 6a, 6b will now be described with reference to Figures 2 to 6b.Figure 2 shows a set of heating zones 642, Z1 to Z7, of the hair styling device 1. Whilst the zones 642 are illustrated for the first heater 6a, a corresponding set of zones is also provided for the second heater 6b. The power output to each heating zone 642, and therefore the temperature of each heating zone, is independently controllable. Advantageously, this enables the temperature distribution along the longitudinal length of the heater 6 to be controllable. Moreover, the use of independently controllable heating zones 642 also enables better mitigation against reductions in temperature of the styling surface due to transfer of heat to the user’s hair 40. For example, when the user places a tress of wet or damp hair on heating zones Z3 and Z4, the power output to heating zones Z3 and Z4 can be independently increased in order to maintain the hair-contacting surface at the styling temperature. As will be described in more detail later, the use of independently controllable heating zones 642 also enables some of the zones 642 to be operated at a temperature below the styling temperature even when the device is in use (e.g. when only some of the heating zones 642 are in contact with the user’s hair), increasing the energy efficiency of the device 1. Each heating zone can be independently controlled towards a respective target temperature. Control is performed to reduce the difference between the actual temperature of the heating zone and the target temperature. The heating zones can be switched off when the device is not in use to style hair, which can be detected, for example, by sensing that the heater arms are in the open position.Figure 3 schematically illustrates the heating zones 642 on the heating surface of the heater shown in Figures 1 and 2. Whilst in the example shown in Figure 3 the temperature of the hair contacting surface is independently controllable along thelongitudinal direction of the heater 6 by virtue of the arrangement of the heating zones 642, the independent temperature control is not limited to being along the longitudinal direction. For example, Figure 4 illustrates an alternative arrangement of independently controllable heating zones 644 in which the temperature of the hair contacting surface is also controllable along the transverse direction, by virtue of two heating zones (e.g. zones Z1 and Z2) being provided across the width of the heater.Whilst the heating zones 642, 644 illustrated in Figures 3 and 4 are all the same size, this need not necessarily be the case. Different sized heating zones 646 may be provided, as illustrated in Figure 5, which shows a heater 6 having seven different sized heating zones (labelled Z1 to Z7). The way in which the heater electrodes 64 would be arranged to define these different sized zones would be understood by the skilled reader and will not be described in detail here.The heating zones described above form part of a heater 6 having a flat haircontacting surface. However, the heater 6 is not limited to having a flat hair-contacting surface and could alternatively have a tubular form, with heating zones labelled Z1 to Z4, (as illustrated in Figure 6a), for example for use in a hair curler device. In a further alternative the heater could have a curved form, with heating zones labelled Z1 to Z6, as illustrated in Figure 6b, for example for use in a heated hair brush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device.Illustrative Block DiagramFigure 7 is a simplified block diagram of control circuitry 15 (e.g. a microcontroller unit) that controls the operation of the hair styler device 1 shown in Figure 1 . As shown, the control circuitry 15 comprises a power supply 21 that, in this embodiment, derives power from a battery power source (not shown) (e.g. a battery having a voltage between 3 V and 42 V, although any other suitable voltage battery could alternatively be used). A mains power supply input may be provided to charge the battery via an AC to DC converter (not shown), which may be external or internal to the device 1. Alternatively, the power supply 21 may derive power from an AC mains supply input.In this example, power is provided to the heaters 6 for heating the user’s hair. The power supplied to the heaters 6 is controlled by a controller 28 having a microprocessor 29. The power supplied to the heaters 6 is controlled by drive circuitry 23 (which may include one or more power semiconductor switching devices (triacs) or Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switches) which controls the application of an AC mains voltage, or a DC voltage derived from the AC mains or from a battery, to the heaters 6 in accordance with instructions from the microprocessor 29. The microprocessor 29 is coupled to a memory 30 (which is typically a non-volatile memory) that stores processor control code for implementing one or more control methods that control the heating of the heaters 6 in accordance with a desired operating temperature of the heaters 6 (for each of the independently controllable heating zones) and sensed temperatures of the heaters obtained from temperature measurement circuitry 25. The memory 30 may store, for example, one or more operating profiles or parameters (e.g. a target temperature for each of the heating zones, a maximum current limit, or a maximum power limit). Software stored in the memory 30 may include, for example, an operating system and a heater control module suitable for implementing one or more of the methods described below. For example, the software stored in the memory 30 may include a calibration and testing module for use when the device 1 is being calibrated and tested.The temperature measurement circuitry 25 may comprise temperature sensors such as thermistors or may use circuitry that senses the resistance of heater electrodes thatare used to heat the heaters 6, which resistance depends on the temperature of the heater electrode. The temperature measurement circuitry 25 may comprise a noncontact type of temperature sensor (for example, an infrared sensor) for sensing the temperature of the user’s hair, and / or for sensing the temperature of a part of the device 1 (for example the temperature of the hair-contacting surface of the heater 6). The temperature sensor(s) could be provided inside a body portion of the device 1 , or could be provided on an exterior surface of the device 1. A temperature measurement of a part of the device 1 can be measured directly or indirectly. For example, a heat pipe could be used to transfer heat via conduction from the part whose temperature is to be measured to an internal sensor.In the example illustrated in Fig. 7, heaters 6 and drive circuitry 23 may be collectively referred to as the heater control sub-circuitry of the control circuitry 15. The heater control sub-circuitry of the control circuitry 15, may include the heating circuit and safety circuitry shown in Figures 78 to 82, described in more detail later.Figure 7 also shows a user interface 11 that is coupled to the microprocessor 29, for example to provide one or more user controls and / or output indications such as a visual indication or an audible alert. The output(s) may be used to indicate to the user, for example, if they have inserted too much hair between the heaters 6, if they are moving the device 1 too quickly along the hair tress 40, or simply if the device is on or off.In this example, the control circuitry 15 also comprises communications circuitry 27 to allow the device to communicate with a remote sensor, a remote server, or a remote application (e.g. on a mobile telephone). The communications circuitry 27 may use, for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the remote device. The communications circuitry 27 could be used to receive an input parameter from a remote device, for example a desired operating temperature for the heaters 6 for styling hair.As those skilled in the art will appreciate, the device 1 does not necessarily need to have all of the blocks illustrated in Figure 7. For example, if the device 10 is a hair straightener, then there is not necessarily a need for the communications circuitry 27.Control of Heating zonesA method of controlling the independently controllable heating zones of the heaters 6 will now be described with reference to Figure 8.Figure 8 shows a flow diagram of a method of controlling power output to the heating zones of the heaters 6. In step S801 , the heating zones that are in use for styling the user’s hair 40 are determined. In the present example, a heating zone is determined to be in use for styling the user’s hair 40 when the user’s hair 40 is engaged with (in contact with) the hair-contacting surface of the heating zone. Figure 9 shows an example in which a tress of hair 40 is in contact with heating zone Z2, and a further tress of hair 40 is in contact with heating zone Z4. Therefore, zones Z2 and Z4 are determined to be in use for styling the user’s hair 40. Since the user’s hair 40 is not in contact with heating zones Z1 , Z3, Z5 or Z6, these heating zones are determined not to be in use for styling the user’s hair 40, and can therefore be (but need not necessarily be) operated at a reduced temperature (which could be the ambient temperature, in which case the supply of power to these zones can be switched off). Figure 10 shows a further example in which a tress of hair 40 is in contact with heating zones Z2 and Z3, and therefore zones Z2 and Z3 are determined to be in use for styling the user’s hair 40. Since the user’s hair 40 is not in contact with heating zonesZ1 , Z4, Z5 or Z6, these heating zones are determined not to be in use for styling the user’s hair 40 and as previously described can be operated at a reduced temperature.Sensing the presence of hair 40 in a heating zone can be achieved by measuring the change in thermal load at the heating zone. As will be described in more detail later, the presence of the user’s hair can be determined based on a change in the power required to maintain the heating zone at a particular temperature, or based on a change in temperature of the heating zone (which could be an increase in temperature as hair is removed from the zone, or a decrease in temperature as hair comes into contact with the zone). Alternatively, the presence of the user’s hair in a heating zone could be measured directly using a sensor. For example, an optical sensor could be used to sense the presence of hair in a particular heating zone. Light pipes could be used to sense the brightness of light through openings in the casework of the device 1 , arranged so that the brightness of the detected light will change depending on whether a tress of hair 40 is present on the surface of the heater 6 in a particular heating zone.In step S802, a target temperature for each of the heating zones is determined. For heating zones that are determined, in step S801 , to be in use for styling the user’s hair, the target temperature is set to a styling temperature. For the heating zones that are not determined to be in use for styling hair, the target temperature is set lower than the styling temperature in order to reduce the power needed to operate the device 1. The target temperature for use in styling the hair of the user may be, for example, 200°C. For zones determined not to be in use for styling the hair 40 of the user, the lower target temperature may be, for example, 100°C. Alternatively, a target temperature may not be defined (or ‘set’) for the zones determined not to be in use for styling the hair of the user, and the zones may be allowed to cool towards the ambient temperature. If the arms 4a, 4b of the device 1 are determined to be in the open configuration (e.g. using a switch or sensor as described above) then step S801 comprises determining that none of the heating zones are in use for styling.A table indicating the target styling temperature and one or more intermediate target temperatures may be stored in the memory 30 of the device 1. Figure 11 a shows an example of a table indicating that the styling temperature, Tstyie, is 200°C and an intermediate target temperature, Tint, is 120°C. Figure 11 b shows a further table that could be stored in the memory 30 of the device 1 , indicating the target temperature assigned to each heating zone in step S802. Therefore, step S802 may comprise updating the target temperature stored in the table for each of the heating zones. The table shown in Figure 11 b corresponds to the situation illustrated in Figure 9, in which the user’s hair is in contact with heating zones Z2 and Z4. Therefore, the table indicates that the target temperature for zones Z2 and Z4 is Tstyie, whereas the target temperature for the other heating zones is Tint (which is lower than Tstyie, and therefore results in less power being required to maintain those heating zones at the target temperature). Whilst two separate tables are illustrated in Figures 11 a and 11 b, it will be appreciated that table 11 b could simply indicate the target temperature in degrees directly, rather than by reference to an index of table 11a.Advantageously, since the heaters 6 of the device 1 are low thermal mass heaters, each of the heating zones can be controlled to rapidly return to the styling temperature (e.g. from ambient temperature, or the intermediate temperature Tint) in response to a change in the distribution of the user’s hair amongst the heating zones. The power output to a heating zone that is not in use for styling hair can be reduced in order to increase the energy efficiency of the device 1 , whilst ensuring that the heating zonecan be rapidly returned to the styling temperature when hair 40 comes into contact with that heating zone.Whilst in the present example each of the heating zones is controlled towards a lower target temperature (lower than the styling temperature) if the heating zone is determined not to be in use for styling hair, the method could alternatively be applied to a subset of the heating zones. For example, referring to Figure 2, the method of reducing the power output to the heating zones when the zones are not in contact with the user’s hair could be applied to zones Z1 , Z2, Z5, Z6 and Z7, whereas zones Z3 and Z4 could be operated at the styling temperature even when the presence of hair is not detected in zones Z3 and Z4. This alternative operation of zones Z3 and Z4 could be performed for heating zones that are in a region of the heater 6 that is more likely to be in regular contact with the user’s hair (e.g. zones in a central region of the heater 6), reducing the risk that hair is present in those regions but not detected.In step S803, the temperature of each heating zone is controlled towards the corresponding target temperature by controlling the power output to each heating zone. Methods of controlling the power output to each heating zone based on the difference between the target temperature and the actual temperature of the heating zone will be described in more detail later. The method then returns to step S801 , in which the heating zones in contact with the user’s hair are determined.Exemplary Heater ConfigurationFigures 12a to 12c show an exemplary embodiment of the low thermal mass heaters 6a, 6b of the device of Figure 1. In this example, the heaters 6a, 6b comprise a stack of thin layers. Referring firstly to Figure 12a, the heaters 6a, 6b include an upper dielectric (electrically insulating) layer 62, an electrode layer 63 that has a plurality of separate heater electrodes 64, and a lower dielectric layer 66 which electrically insulates the heater electrodes 64 from other components mounted behind the heater 6a, 6b. The three layers 62, 63 and 66 are bonded together (mechanically or chemically) and define a heater 6 that is very thin (the three layers have an overall thickness of between 30 pm to 1000 pm (preferably between 75 pm to 300 pm, or between 30 pm to 40 pm) in the case of Safe Extra Low Voltage (SELV) operation (less than about 40 Volts, e.g. less than 42.4 Volts) and 0.8mm to 2.0mm (e.g. 0.8mm to 1.0mm) in the case of AC operation) and with very low thermal mass. The upper dielectric layer 62 provides the hair contacting surface of the heater 6, although a nonstick coating may be applied to the upper surface of the layer 62 to facilitate the passage of the user’s hair 40 over the heating surface (e.g. if the layer 62 does not itself have such non-stick properties). The hair contacting surface of the heater is a single smooth surface over which the hair can pass. The bonded layers 62, 63 and 66 define a flexible heater 6 and rigidity of the heater 6 is provided in the illustrated embodiment by mounting the heater layers 62, 63 and 66 onto a rigid support 68 which forms a base. If a flexible heater is desired, then there is no need for the rigid support 68. Thus, in this embodiment, there is no heater plate or tube that is heated by the heaters 6, and instead the heaters 6 directly heat the user’s hair 40. This provides a hair styler 1 having a very low thermal mass which can therefore heat up and cool down rapidly. The total thickness across the multiple bonded layers may be between 30 microns and 2 mm, for example.In the illustrated embodiment, there are ten heater electrodes 64 that each snake across and back across the width of the heater 6, folding twice such that they each cross the width three times. The ends of each of the heater electrodes 64 are electrically connected through the lower dielectric layer 66 to electrical connections within the rigid support 68, which connect to an electrical connector 70. Drive circuitry23 (not shown) that is mounted within one of the arms 4 connects to the heater electrodes 64 via the electrical connector 70 and applies electrical power to the individual heater electrodes 64 to control the heat generated by each heater electrode 64. The electrical connector 70 extends from a surface of the rigid support 68 facing away from the surface layer 62 (shown in Figures 12a and 12b as extending directly away from the upper layer 62, but it could also be provided as extending in a perpendicular direction).It may be generally understood that this is a non-limiting arrangement and other serpentine layouts that snake across and back across the width of the heater 6, folding once or more, may be envisioned. The ends of each of the heater electrodes 64 are electrically connected through the base substrate 66 to electrical connections within the rigid support 68, which connect to an electrical connector 70. Drive circuitry (not shown) that is mounted within one of the arms 4 connects to the heater electrodes 64 via the electrical connector 70 and applies current to the individual heater electrodes 64 to control the heat generated by each heater electrode 64. The electrical connector 70 extends from a surface of the rigid support 68 facing away from the surface layer 62 (shown in Figures 2a to 2c as extending directly away from the upper layer 62, but it could also be provided as extending in a perpendicular direction).Each of the heater electrodes 64 thus creates an individual heating zone on the hair contacting surface of the heater 6, which spans the width (which we shall refer to as the x-direction, as illustrated in Figure 12b) of the heater 6 and the heater electrodes 64 are arranged sequentially one after the other along the length (the y-direction as illustrated in Figure 12b) of the heater 6. The arrangement of heating zones illustrated in Figure 4, in which heating zones 644 are arranged along both the x- and y- directions, can be provided by arranging two sets of heater electrodes 64 side by side in the width (x-) direction. The heaters 6 may be separated in this way into any number of heating zones and may comprise any number of heating zones along the x- and y-directions. For example, whilst Figure 4 shows two zones along the transverse direction, a greater number of zones in the transverse direction could also be provided. The heating zones 644 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce hot / cold spots when using very low thermal mass heaters 6.As described above, the temperature of each heating zone is independently controllable. Each heating zone can be set to a target temperature. The target temperature of each heating zone may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone which is fed back to the microprocessor 29 to allow the microprocessor 29 to control the delivery of power to the heater electrode 64 of the corresponding heating zone 642. Alternatively, if the heater electrodes 64 are formed of a material having a Positive Temperature Coefficient (PTC) or a Negative Temperature Coefficient (NTC) (such that its resistance varies with its temperature), then the temperature of each heating zone can be determined by sensing the resistance of the corresponding heater electrode 64. The microprocessor 29 controls the heating in order to reduce the difference between the actual temperature of the heating zone and the target temperature for that heating zone.Figure 13 shows a simplified schematic illustration of a configuration of layers of a low thermal mass heater 6. In this example, the heater 6 comprises the layer of electrically conductive heater tracks 63 (e.g. formed from copper or stainless steel, or any other suitable electrically conductive material), and the electrically insulating dielectric layer62 that forms the hair-contacting surface for engaging with the hair 40 of the user. However, as illustrated in Figure 14, an additional layer 141 may be provided in between the dielectric layer 62 and the hair 40, to form the hair contacting surface. For example, a friction-reducing layer 141 , or any other suitable type of coating, may be provided between the dielectric layer and the hair 40. The coating may be a non-stick coating to facilitate the passage of the user’s hair over the heating surface. The coating may be between, for example, 1 pm and 50 pm thick, and preferably between 1 pm and 3 pm thick. The coating may be applied as a spray coating (e.g. Cerasol) or as a wash or deposition layer, for example physical vapor deposition, chemical vapor deposition, or any other suitable method of applying such a coating. The coating may comprise multiple layers including, for example, a primer layer (of about 6pm), a base coat layer (of about 25pm) and a top coat layer (of about 10pm). The coating may also have combined functionalities with the other layers (e.g. more than just offering low friction) such as dielectric strength I electrical separation provided the material is sufficiently electrically insulative.Whilst in the examples illustrated in Figures 12a to 14 the heater electrodes 64 are provided in a layer 63 that is separate from the dielectric layer, this need not necessarily be the case. For example, Figure 15 shows an alternative in which the heater electrodes 64 are arranged within the dielectric layer 62, but are nevertheless electrically insulated from the hair 40 of the user.Exemplary Control CircuitryControl circuitry for monitoring the temperature of the heating zones, and controlling the power output to the heating zones, will now be described with reference to Figure 16.Figure 16 shows a schematic view of the way in which the heater electrodes 64 may be connected together and to the drive circuitry 23 and the power supply 21 , which in this embodiment is a battery 21. As shown in Figure 16, each heater electrode 64 (for example 64-1 to 64-3) is connected at one end to a power supply 21 and at the other end to a respective switch (in this case a metal-oxide-semiconductor field-effect transistor, MOSFET, switch) 95-1 to 95-3. The switches 95 are controlled by the microprocessor 29. When a heater electrode 64 is to provide heat, the corresponding switch 95 is closed thereby connecting the heater electrode 64 to ground through the resistor R. As a result, current flows from the power supply 21 to ground causing the heater electrode 64 to heat up. The microprocessor 29 can control the position of each switch 95 independently thereby allowing each heater electrode 64 to be powered independently.When the temperature of a selected heating zone is to be determined, the switch 95 of the corresponding heater electrode 64 is closed and all other switches 95 are opened. In this way, the selected heater electrode 64 is provided in series with the resistor R. Since the heater electrodes 64 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, the temperature of the heating zone can be determined based on the resistance of the heater electrode 64. The resistance of the heater electrode 64 at room temperature may be, for example, between 0.5 Q and 40 Q, although it will be appreciated that the heater electrodes 64 may have any other suitable resistance at room temperature. The heater electrodes 64 may be formed from, for example, copper or stainless steel, although any other suitable electrically conductive material could alternatively be used. As described in more detail below, the resistance (and therefore temperature) of the heater electrode 64 can be determined based on measurements of the attenuated power supply potential, VSUppiy, and the voltage at the output of the current senseamplifier, Vcurrent. If the determined temperature is above the desired temperature for that heating zone, then the microprocessor 29 can reduce the power applied to that heater electrode 64; or if the heating zone is at a lower temperature than that desired, then the microprocessor 29 can increase the power applied to the corresponding heater electrode 64. Any suitable ON / OFF control or PWM (pulse width modulation) or Pulse Density Modulation (PDM) control can be used to vary the power applied to the different heater electrodes 64. The microprocessor 29 can select each heater electrode 64 in turn in order to determine the temperature of each heater electrode 64 / heating zone 642.The microprocessor 29 may be a pre-programmed microcontroller unit with in-built timing references, digital outputs and analogue-to-digital converter (ADC) inputs (e.g. from the operational amplifier 97). The microprocessor 29 may also receive an input signal from a microswitch, or Hall effect sensor and passive magnet, (not shown in the figure) to detect closure / opening of the arms 4a, 4b of the device. The voltage source supply, VSS, and voltage common collector, VCC, of the microprocessor are also illustrated in Figure 16. A DC-to-DC converter arranged between the microprocessor and the heater electrodes 64, and a Vsupply monitor input for monitoring the voltage provided by the power supply, are also illustrated.Whilst all of the heater electrodes could be controlled by a single microprocessor 29 via the corresponding set of switches 95, this need not necessarily be the case. Alternatively, a plurality of microprocessors could be provided, each for controlling the operation of a respective set of heater electrodes 64. Advantageously, the use of a plurality of microprocessors 29 (and a corresponding plurality of resistors, R, and operational amplifiers 97) enables the resistance (and therefore temperature) of a plurality of heater electrodes 64 to be measured independently and simultaneously.Temperature Measurement and Thermal LoadMethods of monitoring and controlling the temperature of the heating zones 64 will now be described in more detail.Heating Zone Temperature Monitoring and ControlFigure 17 shows a flow diagram of a method of monitoring the temperature of the heating zones, and controlling the power output of each heating zone.In step S1701 the resistance of the heater electrode 64 corresponding to each heating zone is measured by operating the switches 95 and the operational amplifier 97 as described above. In step S1702 the measured resistances are converted into a corresponding set of temperatures of the heating zones. For example, the device may store a lookup table or formula that maps the measured resistance to the temperature of the heating zone.In step S1703, the differences between each of the temperatures determined in step S1702 and the target temperature of the respective heating zone are determined. Alternatively, rather than storing a target temperature for each heating zone, the device 1 could store a target resistance for each heating zone (which nevertheless corresponds to a target temperature). In this case, step S1702 could be omitted, and the method instead comprises determining the difference between the measured resistance and the target resistance for each heating zone. Based on the difference (or ‘error’) between the measured temperature and the target temperature for a heating zone (or based on the difference between the measured resistance and the target resistance), a proportional-integral-derivative (PID) controller output is determined for each heating zone. The PID controller output values can be used as anindication of the thermal load at each heating zone, and used as an input for determining the appropriate target temperature, for example in step S802 of Figure 8 (e.g. based on whether hair is present in a zone or not).In step S1704, the duty cycle to be applied for each heater electrode 64 (the duty cycle of the corresponding drive signal) is determined. For each heating zone, the PID controller output is mapped to a duty cycle for the corresponding electrode 64 (which can be applied by opening and closing the corresponding switch 95).In step S1705, a firing sequence is determined based on the duty cycle determined for each heater electrode. In other words, the timing of the opening and closing of each switch is determined in order to implement the determined duty cycles. In step S1705 the firing sequence is executed, thereby controlling the temperature of each heating zone towards the respective target temperature. The method then returns to step S1701 in which the resistance of each heater electrode 64 is measured.Resistance Measurement for Temperature DeterminationMeasurements of the resistance of the heater electrodes 64, and the determination of the corresponding temperatures of the heating zones performed in step S1702 of Figure 17, will now be described in more detail with reference to the flow diagram shown in Figure 18. However, it will be appreciated that the resistance measurements and the determination of the temperatures of the heating zones could be performed in any other suitable manner.In step S1801 power is supplied to one of the heater electrodes 64 by operating the corresponding switch 95.In step S1802 the attenuated power supply potential, VSUppiy as shown in Figure 16, is measured using an ADC input of the microprocessor 29. It will be appreciated that the attenuation of the power supply potential measured by the microprocessor will depend on the resistances of the corresponding resistors 161 , 162 illustrated in Figure 16. Since the resistance of these resistors 161 , 162 is known, the attenuation of the power supply potential can be compensated for to obtain the voltage applied to the heater electrodes 64.In step S1803 the current passing through the current sense resistor at time t, It, is measured. This is achieved by measuring the voltage at the output of the current sense amplifier (VCUrrent, as illustrated in Figure 16). Since the resistance of the current sense resistor R is known, the current passing through the current sense resistor (and therefore the current passing through the heater electrode 64 that is connected in series with the current sense resistor) can be calculated using the equation It = (V current I R). It will be appreciated that the measurement of VCUrrent is performed taking into account the gain of the operational amplifier 97.Steps S1802 and S1803 may be repeated a number of times, accumulating a summed voltage and summed current for the heater electrode 64. Mean values of the current through the heater electrode and the power supply potential can then be obtained by dividing the summed values by the number of times steps S1802 and S1803 are repeated. This averaging helps to mitigate against the effects of random noise on the VSUppiy and VCUrrent signals, and also helps to mitigate against ADC rounding errors at the microprocessor 29.In step S1804 the supply of power to the heater electrode 64 is disabled, by operating the corresponding switch 95. It will be appreciated that the power supply to the heaterelectrode 64 could be disconnected at any suitable point following the measurements of the power supply potential and current, but before the measurements for the next heater electrode 64 (using the same current sense resistor and operational amplifier 97) are performed.In step S1805 the resistance of the heater electrode 64 is determined. The resistance of the heater electrode 64 is given by:Relectrode=Vt / It, (Equation 1 ) where Vt is the voltage over the heater electrode that was determined based on the attenuated power supply potential measured in step S1802, and It is the current passing through the current sense resistor measured in step S1803. In a case where the resistance of the heater electrode 64 is sufficiently large compared to the resistance of the current sense resistor, the resistance of the current sense resistor may be ignored when calculating the resistance of the heater electrode 64. Alternatively, the resistance of the current sense resistor may be taken into account using the formula:Relectrode=(Vt I It) - Rsense, (Equation 2) where Rsense is the resistance of the current sense resistor. The determined resistance of the heater electrode 64, Relectrode, may be stored in the memory of the device in association with an indication of the time at which the measurement was performed (e.g. in association with a time-based index that increments following the execution of each complete firing sequence, described in more detail later). The time between each measurement of the resistance of the heater electrode 64 may be, for example, 40 ms.In step S1806 the temperature of the heater electrode 64 is determined based on the resistance of the heater electrode, Relectrode, obtained in step S1805. The device 1 may store any suitable equation or table that maps the determined resistance value to the temperature of the heater electrode 64. For example, the relationship between Relectrode and the temperature of the heater electrode 64 could be determined experimentally in advance, and a corresponding mapping could be provided in a table stored in the memory of the device 1. The temperature of the heater electrode 64 could also be determined based on a stored calibration resistance for that heater electrode 64 for a particular reference temperature, and based on a known coefficient of resistance of the heater electrode material. As described above, in this case, the temperature of the heater electrode 64 may be determined using the formula:Telectrode=(Relectrode-Rcal) / (k*Rcal)+Teal, (Equation 3) where Telectrode is the temperature of the heater electrode, Relectrode is the resistance of the heater electrode 64 obtained in step S1805, Rcaiis the stored calibration resistance when the heater electrode 64 is at a corresponding calibration temperature Tcai, and k is the coefficient of resistance (that corresponds to the change in the resistance of the heater electrode material with temperature).Steps S1801 to S1806 are then repeated for the next heater electrode 64 for which the temperature is to be determined. As described above, the measurements of the current passing through the heater electrodes 64 that are connected to the same current sense resistor are performed separately. However, when a plurality of microcontrollers are provided (or when each of a plurality of current sense resistorsand operational amplifiers are connected to a respective different ADC input of the same microcontroller 29), measurements for heater electrodes 64 connected to different current sense resistors may be performed simultaneously.Since the measurement of the current performed in step S1803 requires power to be supplied to the heater electrode 64, some heat will be generated in the corresponding heating zone whilst the measurement is being performed. This additional heat output may be taken into account when determining the duty cycle to be used for the heater electrode in step S1704 of Figure 17. Beneficially, therefore, the temperature of the corresponding heating zone can be more accurately controlled towards the target temperature.PIP ControlThe determination of the PID controller output in step S1703 of Figure 17 will now be described in more detail with reference to the flow diagram shown in Figure 19. However, it will be appreciated that the PID control could be performed in any other suitable manner.In step S1901 the proportional error value for a heating zone is calculated by subtracting the temperature of the heating zone determined in step S1702 of Figure 17 from the target temperature for that heating zone (e.g. determined in step S802 of Figure 8):Pt=Ttarget - Telectrode, (Equation 4) where pt is the proportional error value obtained for time t (or an index t corresponding to a particular measurement cycle), Ttarget is the target temperature for the heating zone and Telectrode is the measured temperature of the corresponding heater. Alternatively, the proportional error value could be calculated by subtracting the measured resistance of the heater electrode 64 from a target resistance using the formula:Pt=Rtarget - Relectrode, (Equation 5) where Rtarget is the target resistance for the heater electrode 64 (e.g. based on experimental measurements of the correspondence between the temperature of the heating zone and the resistance of the heater electrode) and Relectrode is the actual resistance of the heater electrode (e.g. determined in step S1805 of Figure 18).In step S1902 an integral value is accumulated for the heating zone, by adding the proportional value pt obtained in step S1901 to the previous integral value: it = it-i + pt, (Equation 6) where it is the new integral value and it-i is the previous integral value.In step S1903 the proportional value pt is subtracted from the previous proportional value for the heating zone to obtain a derivative value: dt = pt - pt-i , (Equation 7) where dt is the derivative value and pt-i is the previous proportional value (based on the previous temperature or resistance measurement for the same heater electrode).In step S1904 the PID value, Ut, is determined using the formula:Ut = (pt ■ kp) + (it ■ ki) + (dt ■ kd), (Equation 8) where kp, k; and kd are predefined constants. Steps S1901 to S1904 are then repeated for the other heater electrodes 64 to obtain the corresponding PID values. Each of the PID values Ut can then be used in step S1704 of Figure 17 to determine the duty cycles to be used for the respective heater electrodes.Duty CycleThe determination of the duty cycles for the heater electrodes 64 performed in step S1704 of Figure 17 will now be described in more detail with reference to Figure 20, which shows a method of mapping the PID values Ut to corresponding duty cycles. However, the determination of the duty cycles could alternatively be performed in any other suitable manner.In step S2001 the PID value Ut is clipped to a predefined range. In this example, the PID value is clipped to be between -0.5 and 0.5:-0.5 < Ut < 0.5In step S2002 the fractional duty cycle, rt, is calculated by adding 0.5 to the clipped PID value: rt = Ut + 0.5As described above, the fractional duty cycle may be corrected to account for power supplied to the heater electrode 64 in order to measure the resistance of the heater electrode 64 (e.g. by decreasing the value of n based on the measurement time and the power output to the heater electrode 64). Steps 2001 and 2002 are then repeated for each heater electrode 64, to obtain a value of n for each heater electrode 64.The power demand Pt, or thermal load, of the heating zone (which can be used to determine the presence of hair 40 in contact with the heating zone, since hair coming into contact with the heater zone will increase the power required to maintain the heater zone at the target temperature) can also be estimated at this stage using the formula, based on the assumption that the power supply voltage from the power source is constant:Pt = Vt ■ It ■ rt (Equation 11 )Determination of Firing SequenceOne method for determining the firing sequence for the heater electrodes 64 based on an interleaved Pulse Width Modulation (PWM) will now be described in more detail. However, the firing sequence could alternatively be determined in any other suitable matter. An alternative method based on Pulse Density Modulation (PDM) is described later.A pulse width modulation (PWM) pattern is determined for each heating zone that delivers the determined duty cycle. However, beneficially, the number of heating zones that are powered simultaneously is limited to reduce the peak instantaneous current demand on the power supply 21. This is achieved by preparing a firing pattern and interleaving the PWM pulses to reduce the number of heating zones that are powered simultaneously.The start times and finish times for the firing sequence for each heating zone are stored in the memory 30 of the device 1 . Start times for each of the N heating zones may be stored as an array: S[1 ... N][2]. Similarly, finish times may similarly be stored as an array: F[1 ... N][2].An accumulator index, A, is initially set to zero. Then, for the first heating zone ‘n’ of the N heating zones a first set of value assignments is performed:S[n][0] = A S[n][1] = -1 F[n][0] = -1 F[n][1] = -1In other words, the first start time for heating zone n is set to the value of A (which is initially zero), the second start time for heating zone n is set to -1 , and the first and second finish times for heater zone n are also set to -1. (The value of -1 is an indication that the value has not been populated yet and can be ignored in the execution of the firing sequence).The duty cycle value for heating zone n, rt[n] (which can have a value between 0.0 and 1.0 in this example), (determined in step S1704 of Figure 17) is multiplied by the duration of the subsequent firing window which is set as a constant W (e.g. in milliseconds). The value of rt[n]*W is then added to the previous value of the accumulator index A. If the value of the of the accumulator index A is less than W, then F[n][0] is set to be equal to A. Alternatively, if the value of the accumulator index A is greater than or equal to W, then the following values are set:F[n][0] = WF[n][1] = A-W A = F[n][1] S[n][1] = 0In other words, the first finish time for heater n is set to W, and the second finish time for heater n is set to A-W. A is then set to the value of F[n][1], and the second start time for heater n is set to be equal to zero. At this stage of the method, the values of S[n][0], S[n][1], F[n][0] and F[n][1] have been determined for heating zone ‘n’. The value of n is then incremented by 1 (i.e. n = n+1), corresponding to the next heating zone, and the method returns to the assignments.This process is repeated (including the updating of the accumulator index A and the conditional steps depending on whether A is less than W, or greater or equal to W) until n=N (in other words until the values of S[n][0], S[n][1], F[n][0] and F[n][1] have been determined for each of the N heating zones. Advantageously, therefore, the method results in the power delivered to each heating zones being interleaved, reducing the peak power demand on the power source 21 .Execution of Firing SequenceThe firing sequence can then be executed as follows. The firing sequence duration is set as HZ milliseconds, a time index t is initially set to be equal to zero, and the heating zone index n is set to 1.Then, if S[n][0] = t, or if S[n]

[0001] = t, then the power is supplied to the heating zone by controlling the corresponding switch 95-n. In other words, if the value of t is equal tothe first start time or the second start time for heating zone n, then the corresponding heater electrode 64 is switched on.If F[n][0]= orF[n][1] = t, then the power supply to the heater electrode 64 is switched off by controlling the corresponding switch 95-n.The value of n is then incremented by one (i.e. n = n +1), and the determinations of whether S[n][0] = t, S[n]

[0001] = t, F[n][0] = t, or F[n][1] = t are performed for the new value of n. This process is repeated until n > N. The value of t is then incremented by one (i.e. t = t+1). This increase in the value of t may be performed, for example, following a 1 ms pause.Following the value of t being incremented by one, the value of n is set to 1 , and the method is repeated (but with the new value of f) until t = W. In other words, the firing sequence is performed for W milliseconds.Beneficially, the methods for controlling the temperature of the heating zones described above with reference to Figures 17 to 20 enable the temperatures to be more precisely controlled, and adjusted between different target temperatures more rapidly than conventional heaters and algorithms, on a timescale of milliseconds as opposed to seconds. The thermal load for a heating zone can also be measured on a timescale of milliseconds, compared to a conventional heater whose response time is typically of the order of several seconds. This enables the heating zones to be maintained at the appropriate target temperatures more accurately.Temperature ControlFurther details of controlling the temperature of the independently controllable heating zones will now be described.Device closureThe target temperatures for the heating zones can be changed in response to whether the arms 4a, 4b of the device are detected in the open or closed position using a sensor. The sensor is monitored periodically by the microprocessor 29, monitoring for a change of state. Two target temperature values can be pre-defined: one for when the heater arms are open, Topen, and one for when the heater arms are closed, Tciosed . Topen may be defined to be so low that no power (or no additional power - some power may be supplied in order to measure the resistance of the heater electrodes as described above) is supplied to the heater electrodes 64 when the sensor reading indicates that the styler is in the open configuration (e.g. by setting all of the duty cycles to zero). Alternatively, Topen may not be defined, and the microprocessor may be configured to determine that no power (or no additional power) is to be supplied to the heater electrodes 64. When closure of the arms 4a, 4b is detected, the target temperature for each heating zone is set to the respective value of Tciosed .Response to Thermal LoadAs described above, the target temperature for a heating zone may be set to an intermediate temperature Tint between the styling temperature Tstyie and the ambient temperature.A method of determining when to change a target temperature for a heating zone from Tint to Tstyie will now be described with reference to Figure 21. The method illustrated in Figure 21 is performed when the arms 4a, 4b of the device 1 are in the closed position (if the arms 4a, 4b of the device 1 are in the open position, then the heating zones can be allowed to cool to ambient temperature).In step S2101 the target temperature for each of the heating zones is set to the intermediate temperature Tint (cooler than the styling temperature Tstyie, but hotter than the ambient temperature), and each of the heating zones is controlled towards Tint using the PID control, duty cycles and firing sequences described above. A threshold power value Pthresh[n] is defined for each heating zone, n, of the N heating zones. The value of Pthresh[n] is initialised to a value of Pthresh-intermediate for each of the heating zones.In step S2102 a determination of whether the power required to maintain each heating zone at the target temperature Tint exceeds the threshold power value set for that heating zone is made.If the power required to maintain a heating zone at the target temperature Tint exceeds the threshold value Pthresh[n], then in step S2103 the target temperature for that heating zone is updated to the styling temperature Tstyie, since the relatively large amount of power required to maintain the heating zone at the target temperature indicates that hair 40 is in contact with the heating zone. The threshold power value thresh[n] for that heating zone is also updated from Pthresh-intermediate to a value of Pthresh- style ■If the power required to maintain a heating zone at the target temperature Tint does not exceed the threshold value Pthresh[n], then the target temperature is maintained at Tint.The method then returns to step S2102, and the comparison of the power required to maintain each heating zone at target temperature with the corresponding threshold power values is performed. However, some of the heating zones may now have a target temperature of Tstyie, and a corresponding value of Pthresh[n] of Pthresh-styie, depending on the updates to the target temperatures performed in step S2103.If the target temperature for a heating zone is set to Tstyie, and the power required to maintain the heating zone at Tstyie falls below the Pthresh[n], then the target temperature for that heating zone is set to Tstyie. This is because when the power required to maintain the heating zone at the target temperature falls below the threshold value, it indicates that the hair is no longer in contact with the heating zone. The threshold power value Pthresh[n] for the heating zone is also set to Pthresh-intermediate.It will be appreciated, therefore, that in the method of Figure 21 the threshold temperature assigned to each heating zone can be changed from Tint to Tstyie, and from Tstyie to Tint, based on a comparison of the power required to maintain the heating zone at the target temperature with a threshold power value. In other words, the target temperature for a heating zone is determined based on the thermal load at the heating zone. Beneficially, therefore, the presence of hair 40 in a heating zone can be determined, and the heating zone can be controlled to increase in temperature to the styling temperature Tstyie, in order to style the hair. The removal of the hair 40 from the heating zone can also be detected, and the target temperature lowered to Tint in order to reduce the energy usage of the device 1 .When the target temperature for a heating zone has changed from Tstyie to Tint, and the heating zone is cooling from Tstyie to Tint, there is no power demand at the heating zone (since the zone is cooling). However, since there is no power demand to maintain the zone at the target temperature, the power demand of the heating zone will be lower than the threshold value Pthresh-intermediate even when hair 40 is placed onto the heating zone, meaning that the presence of hair on the heating zone may not bedetected by monitoring the power demand during the cooling period. This will result in a delay in returning the heating zone to the styling temperature Tstyie, decreasing the styling performance. However, the presence of hair 40 can nevertheless be detected during the cooling period by monitoring the rate of cooling, since the heating zone cools faster when hair is in contact with the heating zone. For example, the method of determining the temperature of the heater electrode 64 illustrated in Figure 18 can be performed to monitor the rate of cooling of the heating zone. When the rate of cooling exceeds a threshold rate of cooling (which may also be referred to as a threshold temperature gradient), a determination that hair is in contact with the heating zone can be made, and the target temperature for the heating zone is set to Tstyie. Beneficially, therefore, the temperature of the heating zone can be rapidly returned to the styling temperature Tstyie to style the hair 40. The threshold rate of cooling may be set to the rate of cooling of the unloaded heating zone (or slightly below that unloaded cooling rate, to avoid false determinations that hair 40 is present in the heating zone).Opposing Heating ZonesWhen each of the arms 4a, 4b of the device 1 is provided with a respective heater 6a, 6b as illustrated in Figure 1 a, heat can be transferred between opposing heating zones when the arms 4a, 4b are in the closed position. Figure 22 shows a simplified schematic illustration of such opposing heating zones. As shown in the figure, in this example heat transfer will occur between zones 1 and 4, between zones 2 and 5, and between zones 3 and 6. The inventors have realised that the methods for controlling the temperature of the heating zones can be further improved by taking into account this heat transfer. For example, a situation may occur in which the target temperature of Zone 1 of Figure 22 is set to Tint or ambient, but the target temperature of Zone 4 is set to Tstyie. In this situation, heat will be transferred from Zone 4 to Zone 1 , and the power reguired to maintain Zone 4 at Tstyie will be relatively high. This may lead to a false determination that hair is present in Zone 4 (since the power to maintain the zone at the threshold temperature will be higher than Pthresh[n]), and therefore the target temperature for Zone 4 will be maintained at Tstyie. This effect can be mitigated against by determining a target temperature for a pair of opposing heating zones based on the sum of the powers (or the average of the powers) reguired to maintain each heating zone of the opposing pair at a target temperature. In other words, each pair of opposing heating zones can be considered to be a single logical heating zone, and control performed for the single logical heating zone rather than individually for the two separate opposing heating zones. Control of the target temperatures and threshold power values can then be performed as described above with reference to Figure 21 , but by assigning target temperatures and threshold power values to the pairs of opposing heating zones rather than to each heating zone of the opposing pair separately (and by comparing on the total power supplied to the pair of opposing heating zones, or the average power supplied to the pair of opposing heating zones, to the threshold power values). Whilst in the example of Fig. 22 the two heaters (6a, 6b) have the same number of heating zones, this need not necessarily be the case. Each of the heaters may have any suitable number of heating zones (and any suitable shape for each heating zone), and the heating zones of one heater 6a need not necessarily be aligned (e.g. perfectly overlapping) with the heating zones of the other heater 6b.Hair Speed Across Heater SurfaceAs illustrated schematically in Figure 23a, the device 1 may be provided with a plurality of heating zones across the transverse width of the heater 6 (in the x-direction illustrated in Figure 23a), as well as a plurality of heating zones along the longitudinal length of the heater 6 (in the y-direction illustrated in Figure 23a). When the hair 40 moves across a plurality of heating zones as it moves across the hair contactingsurface, the speed at which the hair 40 is moving across the hair contacting surface (i.e. the speed at which the device is moved along a tress of hair 40) can be determined by detecting changes in temperature or thermal load of the heating zones. The determined speed of the hair 40 across the surface of the heater 40 can then be used to generate feedback to output to the user via the user interface 11. For example, if the user is moving the device 1 too quickly along the hair 40, then insufficient heat may be transferred to the hair 40 resulting in poor styling performance, and an indication that the user should move the device more slowly along the hair could be output. Alternatively, for example, the temperature of the heating zones could be controlled based on the determined speed of the hair, to increase the temperature of the heating zones when the hair is moving more quickly across the surface of the heater 6.As described above with reference to Figure 18, the temperature of each heating zone can be measured. Methods of monitoring the power required to maintain a heating zone at a target temperature have also been described, with reference to Figure 21. The inventors have realised that when the hair 40 moves from a first heating zone to a second heating zone, by modifying the temperature of the first heating zone and monitoring the subsequent change in temperature or thermal load at the second heating zone the speed of the hair across the surface of the heater 6 can be determined. As illustrated schematically in Figures 23a and 23b, in this example a tress of hair 40 is moving in the x-direction, across the transverse width of the heater 6, from zone Z4 to zone Z3. It will be appreciated that the hair 40 could also move in the opposite direction, from zone Z4 to Z3 (e.g. if the device is being held in a left hand rather than a right hand of the user), in which case the operation of zones Z3 and Z4 described below is swapped. The direction in which the hair 40 is being moved across the heating zones can be determined by monitoring the temperature or thermal load of the heating zones, since the thermal load at the first zone the hair passes over will be greater than the thermal load at the second zone the hair passes over (since the hair received in the first heating zone is cooler than the hair that passes into the second heating zone).Figure 24 shows a graph of the temperature of zone Z4 and the power required to maintain zone Z3 at a target temperature (corresponding to the thermal load at zone Z3). As shown in Figure 24, in this example the temperature of zone Z4 is controlled to increase at a time T1 , by temporarily increasing the power output of zone Z4. This increase in temperature causes additional heat to be transferred to the hair 40, increasing the temperature of a section of the tress 40. The hotter section of hair 40 then passes into zone Z3 as the device is moved along the tress of hair 40. As the hotter section of hair 40 passes into zone Z3, the power required to maintain zone Z3 at a target temperature decreases, since less heat is transferred from heating zone Z3 to the hotter section of hair 40. Therefore, at time T2, a dip in the power needed to maintain zone Z3 at the target temperature (e.g. Tstyie) is observed. The time difference between time T1 and T2, shown as AT in Figure 24, can then be used to determine the speed at which the hair is moving across the surface of the heater 6. Since the dimensions of the heater are known, and the time taken for the hair to traverse zones Z3 and Z4 is known, the speed can be calculated (or the speed of the hair corresponding to different values of AT could be measured experimentally in advance, and stored in the memory of the device as a lookup table or formula). It will be appreciated that the heating zones used to determine the speed of the hair need not necessarily be adjacent, for example when three or more heating zones are provided across the transverse width of the heater 6, or if the heating zones are spaced apart.Whilst in the example shown in Figure 24 the dip in power required to maintain a heating zone at a target temperature is used to determine the hair speed, alternatively the temperature of zone Z4 could be temporarily decreased, and the subsequent increase in power required to maintain zone Z3 at the target temperature could be used to determine the hair speed. In a further alternative, rather than monitoring the power required to maintain zone Z3 at the target temperature, the power output to zone Z3 could be maintained at a constant level (or disabled), and the change in temperature of zone Z3 as the hotter or cooler section of hair passes over zone Z3 could be detected. Moreover, whilst in the example shown in Figure 24 the change in temperature is a ‘pulse’ of increased temperature, this need not necessarily be the case and any other suitable change in the temperature could be used. For example, a step increase in the temperature could be used. In other words, the change in the temperature of the heating zone need not be a transient pulse.Hair Thermal ConductivityAs illustrated in Figure 23b the hair of the user may span two or more heating zones of the heater. Advantageously, the heating zones can be used to measure the thermal conductivity of the hair, which is indicative of the health of the hair (damaged hair is less thermally conductive than healthier hair). The amount of heat transferred between heating zones via thermal conduction along the hair can also advantageously be used to determine amount of hair in contact with the heating zones (since an increase in the amount of hair results in more heat being transferred). In this example, a tress of hair is placed onto the hair-contacting surface and a first heating zone (e.g. zone Z4 in Figure 23b) is used to heat the hair whilst there is no (or little) relative movement between the heater and the hair along the length of the tress of hair (in other words, the device is held substantially stationary at a particular point along a tress of hair). Heat will then pass from the first heating zone to a second heating zone via thermal conduction along the hair. The temperature of the second heating zone (e.g. zone Z3 in Figure 23b) is then measured (e.g. using a resistance measurement as described above, or using a temperature sensor) to determine the thermal conductivity of the hair. Alternatively, the change in power required to maintain the second heating zone at a target temperature could be measured to determine the thermal conductivity of the hair. The first and second heating zones may be adjacent heating zones (but need not necessarily be adjacent, since even when a further heating zone is arranged in between the first and second heating zones some heat will be transferred from the first heating zone to the second heating zone via conduction along the tress of hair). The first and second heating zones may be substantially thermally isolated from each other. In other words, the amount of heat that passes directly from the first heating zone to the second heating zone may be relatively small. The device may be configured to output an indication to the user of the health of the hair, based on the measurement of the thermal conductivity of the hair. Whilst the heating zones used for the measurement of the thermal conductivity could be adjacent zones (e.g. zones Z3 and Z4 of Figure 23b), the heating zones could also be opposing zones (e.g. zone 1 and zone 4 of Figure 22).Further Heater Control MethodsParticularly advantageous methods of controlling the power output to the heaters 6 will now be described in more detail, with reference to figures 25 to 33. Advantageously, the methods reduce the instantaneous peak current demand on the power supply, improve the stability of the temperature regulation, and reduce the dead time of the heaters.As described above, the temperature of each of the heating zones can be controlled towards a respective setpoint (in other words, controlled towards a respective targettemperature). Depending on the mode of operation, those setpoints may be constant or may vary between, for example, 100°C to 230°C. The temperature of a heating zone can be controlled towards a new setpoint by modulating the power supplied to the respective heater(s) 6. In the present example, the power output to heat the heating zones is controlled using pulse density modulation (PDM).Figure 25 shows a flow diagram illustrating a method of controlling the power output to the heating zones. In step S251 the system is initialised. In step S252 the temperatures of the heating zones are determined. Methods of determining the temperature of the heating zones, by outputting a transient supply of power to the corresponding heater electrode(s) and measuring the attenuated power supply potential and the current passing through a current sense resistor, have been described above with reference to figures 16 to 18. The measured voltage and current can be used to determine the resistance of the heater electrode(s), and so the temperature of the heating zone can be inferred using the temperature coefficient of resistance of the heater electrode(s). If a calibration resistance, Real, of a heating zone has been previously measured at a reference temperature, Tcai, then the temperature of the heating zone, Teiectrode, can be determined from the measured resistance, eiectrode, using the equation:Teiectrode=(Reiectrode—Rea |) / (k X R cal) + Teal, where k is the temperature coefficient of resistance of the heater electrode. However, it will be appreciated that any other suitable method could be used to determine the temperatures of the heating zones in step S252. For example, the heating zone temperatures could be measured using temperature sensors such as thermocouples, negative temperature coefficient (NTC) transducers, or positive temperature coefficient (PTC) transducers.The temperature of each heating zone may be measured periodically. For example, the temperature of each heating zone may be measured at 50 ms intervals. However, it will be appreciated that the temperature of each heating zone may be measured using any other suitable time interval, and the time between temperature measurements for a heating zone may be variable. Moreover, if temperature sensors are provided for measuring the temperature of the heating zones, then the temperature of the heating zones could alternatively be measured continuously (since in that case the temperature measurements can be performed independently of the supply of power to the heater electrodes).As described above, the power supplied to the heating zones may be supplied using a DC power source such as a battery 21. However, the power source is not limited to being a battery 21 , and an AC supply of power could alternatively be used.In step S253 configuration for the firing sequence is performed, based on the determined temperatures of the heating zones and the target temperatures for the heating zones. The configuration for the firing sequence may also be based on an instantaneous current limit of the power source, a maximum average power limit of the power source, or a maximum instantaneous power limit of the power source. In step S254 power is output to the heating zones during the firing window. Following step S254 the method returns to step S252. Steps S252 and S253 may be collectively referred to as the measurements and setup phase, which may have a time duration of between 1 ms and 3 ms for example, although any other suitable time duration could alternatively be used. However, reducing the time duration of steps S252 and S253 beneficially reduces the dead time of the heaters, enabling the duration the firingwindow of step S254 to be increased. During the firing window of step S254 power is output to control the temperature of the heating zones towards the target temperatures). A method of determining the temperature of the heating zones, performing configuration for a firing sequence for the heating zones, and executing the firing sequence will now be described in more detail with reference to figures 26 to 33.Figure 26 shows a flow diagram illustrating a first part of a method of controlling the power output to the heating zones. Step S261 corresponds to step S251 of Figure 25, in which the system is initialised. Steps S262 to S264 may be referred to as the ‘measurement phase’. Step S262 corresponds to step S252 of Figure 25, and so will not be described again here.In step S263, a proportional-integral-differential (PID) control determination is made for each heating zone. For each heating zone, the difference between the target temperature for the heating zone and the temperature measured in step S262 is determined. This temperature difference is used to generate a PID value that is proportional to the power to be output to the heating zone to control the heating zone towards the target temperature. As described above with reference to Figure 20, the PID values may be generated such they fall in a range between -0.5 and 0.5, which can be converted into a fractional duty cycle for the heating zone by adding 0.5 to the PID value.In step S264, a duty cycle is determined for each of the heating zones based on the corresponding PID values determined in step S263. The duty cycle corresponds to the fraction of time that the heating zone is to be powered during the firing window. For example, referring to Figure 16, the determined duty cycle for each heating zone corresponds to the duty cycle of the respective switch 95, e.g., a MOSFET gate signal (and the duty cycle of the corresponding drive signal). Following step S264 the method proceeds to the setup phase, which will now be described with reference to Figure 27.Figure 27 shows a flow diagram illustrating a second part of the method of controlling the power output to the heating zones. Steps S271 to S274 may be referred to as the setup phase, in which configuration for the firing sequence to be performed in the firing window is performed. If all of the heating zones were to be powered simultaneously then a maximum instantaneous current limit of the power source (or a maximum power output limit, which could be an average power limit or an instantaneous power limit) could be exceeded. Therefore, in the setup phase of Figure 27 configuration is performed to enable a firing sequence in which power is output to the heating zones without exceeding the maximum instantaneous current limit. Whilst a method of determining a firing sequence comprising interleaving the power delivered to the heating zones has been described above with reference to step S1705 of Figure 17, in the setup phase of Figure 27 a modified method is used which beneficially reduces the dead time of the heating zones, reduces the peak power and peak current demands on the power source, and reduces the peak computational load needed to calculate the firing sequence (by spreading out the computational load in the time domain). The power output using the firing sequence determined in the method of Figure 27 is particularly smooth, advantageously enabling the duration of the subsequent firing window to be extended. This is because when the power delivery is smoother, the PID control of the power output for the heating zones is more stable, and so less time needs to be allocated for temperature measurements. This reduces the dead time and increases the time available for the firing window. Moreover, whilst in the method of Figure 17 the computational demand of determining the firing sequence occurs before the execution of the firing sequence in step S1706,in the present example the firing sequence need not be pre-calculated, enabling the computational demand on the controller(s) to be distributed throughout the firing window, beneficially reducing the peak computational demand (although this need not necessarily be the case, and in the method of Figures 26 to 28 the firing sequence could nevertheless be pre-calculated before the firing window).The method of generating the firing sequence for the heating zones may be referred to as Pulse Density Modulation (PDM), which is analogous to the operation of a delta sigma digital-to-analogue converter (DAC). As will be described later with reference to Figure 28, the average duty cycle that has been used for a heating zone is integrated during the firing window, and the error between the average duty cycle and the target duty cycle is determined and used to control the power output to the heating zone. The average duty cycle is determined periodically in a series of time slots (e.g. at 1 ms intervals resulting in 50 time slots within a 50 ms firing window, although any other suitable time interval and duration for the firing window could alternatively be used). Beneficially, since the average duty cycle is determined periodically during the firing window, control of the power output is also performed periodically in a series of time slots during the firing window, leading to smoother power delivery since the power output for a heating zone is constant during a time slot.Whilst the power to be output for each heating zone is determined periodically during the firing window, the total current and power demands on the power source also need to be considered. Advantageously, in the present example the total current being drawn from the power source by the heating zones is monitored during (e.g. throughout) the firing window, and the predicted current that would be drawn from the power source if the next heating zone in the sequence were powered in the next time slot is determined. If the predicted current would exceed a threshold current limit, then it is determined not to power that heating zone in the time slot (and the powering on of the heating zone is ‘held over’ to the next time slot). However, other heating zones in the sequence may still be powered in the time slot if they have a lower current demand and would not result in the current limit being exceeded. This results in a ‘self-meshing’ behaviour of the power delivery to the heating zones that avoids the current limit from being exceeded but enables efficient use of the time available during the firing window. If no heating zones need to be held over until the next time slot, then the PDM for each heating zone follows an on-off sequence to achieve the requested duty cycle. In each time slot the heating zone will not be powered if the average duty cycle thus far in the firing window exceeds the requested duty cycle, and will be powered if the average duty cycle thus far in the firing window is less than the requested duty cycle. If power cannot be output to a heating zone because it would result in the instantaneous current limit being exceeded, then power is not output to the heating zone in that time slot but the on-off ratio for the requested duty cycle can nevertheless be maintained by enabling the power output in subsequent time slots later in the firing window.If power was output to the heating zones to satisfy all of the requested duty cycles determined in step S264 of Figure 26, then it could result in the maximum current limit of the power source being exceed. This can result in relatively few heating zones being supplied with power during the firing window to avoid exceeding the current limit. Therefore, to mitigate against this effect and to provide more evenly distributed power output for the heating zones, in the setup phase of Figure 27 the requested duty cycles are scaled based on the current limit of the power source. In step S271 , the total current, x, that would be drawn from the power source if all of the zones were powered using the requested duty cycles is calculated by summing the value of the product of the requested duty cycle and the predicted current for each heating zone:x = Z(D / X b), where x is the total current, D; is the requested duty cycle for zone i, and I, is the current that would be drawn if zone / is supplied with power.In step S272 the maximum current drawn by any single zone that can be drawn from the power source in the upcoming firing window is determined based on the measured zone resistances and PSU voltage. The maximum current could be stored in a look-up table in the memory of the device, for example.In step S273, a dynamic current limit value, LIMIT, is determined by adding the value of the total current, x, to the maximum current of the heating zones, lmax, where lmax is the largest value of lz:LIMIT = X + lmaxBy setting the dynamic current limit to the value of x + lmax, during the firing window the powering of heating zones can be held over until the next time slot whilst increasing the likelihood that there will be time available for activation of that heating zone later in the firing window. This beneficially results in more efficient use of the time slots in the firing window and reduces the peak current demand on the current source. If the calculated value of LIMIT exceeds the actual current limit of the power source, then the value of LIMIT is set to the actual current limit of the power source to avoid exceeding the current limit.In step S274, if the value of the total current, x, is greater than the dynamic current limit of the power source, LIMIT, then a scaling factor equal to LIMIT divided by the value of x is calculated. The requested duty cycle of each heating zone is then multiplied by the scaling factor. This has the effect of ‘compressing’ the firing sequence by reducing the requested duty cycles, to enable a more even power distribution amongst the heating zones during the firing window (rather than the firing window being dominated by relatively few heating zones having large requested duty cycles and preventing power being supplied to other heating zones as it would breach the current limit of the power source). Following step S274 the method proceeds to the firing window phase illustrated in Figure 28.Figure 28 shows a third part of the method of controlling the power output to the heating zones. Steps S281 to S283 are performed during the firing window, during which power is output to the heating zones based on the scaled requested duty cycles to control the temperature of the heating zones towards the target temperatures.As described above, the firing window is divided into a number of time slots. The duration of the time slot may be, for example, 1 ms, although any other suitable duration of the time slot could alternatively be used. An integrator value for each heating zone, lz, is initialised to zero at the beginning of the first firing window. A time slot counter, n, is also initialised to zero. The actual duty cycle, az, (or ‘current’ duty cycle) of a heating zone can be calculated at time slot n using the integrator value. In a particular time slot, for each heating zone z a value of 1 is added to the integrator value if the heating zone z is powered: lz,n=lz,n-1+1When the method proceeds to the next time slot the value of the time slot counter is increased by 1 :n = n +1For any time slot n, the actual fractional duty cycle azof a heating zone can be calculated by dividing the integrator value by the time slot counter value: az= lz / nAt the beginning of a new firing window, the value of lzfor each heating zone can be normalized and used as the initial value of the next firing window (but the value of n is reset to zero).Alternatively, rather than using an integrator for each heating zone to calculate the actual duty cycle, the duty cycle a in each time slot can be estimated from a low pass filtered (LPF) pulse train using the equation:3z,n=3z,n-1xk + P zx(1 ~ k), where Pzhas a value of 1 if the heating zone z is powered, Pzhas a value of 0 if the heating zone is not powered, and k is a constant filter coefficient between 0 and 1 . This low pass filter provides an approximation of the average duty cycle, and provides an alternative to the integration method described above.In step S281 , for each heating zone that is currently powered, it is determined not to supply power to the heating zone if the actual duty cycle for that heating zone is greater than the scaled requested duty cycle determined in step S274.In step S282, for each heating zone that is not currently powered, it is determined to supply power to the heating zone if the actual duty cycle for that heating zone is less than the scaled requested duty cycle (which may also be referred to as the scaled target duty cycle) determined in step S274 and if powering the heating zone would not cause the dynamic current limit value, LIMIT, to be exceeded. Advantageously, the determination of whether to power each heating zone is performed in a random order, to avoid prioritising particular heating zones (which could result in the last heating zone of the sequence rarely being supplied power due to the current limit, when the scaled requested duty cycle of the other heating zones is large). Alternatively, the determination could be made first for one or more prioritised heating zones, for example heating zones provided generally towards the middle of the hair contacting surface. Alternatively, prioritisation can be based on each zone’s shortfall from the target temperature i.e. preferentially powering the coldest zones first. Another scheme might be based on the geometric position of the zone on the hair contacting surface, optionally prioritising the zones in the middle. Several criteria can be combined in a single scheme.In step S283, the actual duty cycle azof each heating zone z is calculated as described above. Steps S281 to S283 are repeated for each time slot until the end of the firing window has been reached. After the end of the firing window, the method returns to step S252 of the measurement phase.Figures 29 to 33 show graphs of the current drawn from the power source during various modes of operation of the device. Figure 29 shows an exemplary graph of the current draw from the power source when the power demand at the heating zones is relatively low. Small ‘blips’ of power used to measure the temperature of the heatingzones in step S262 are shown. Figure 30 shows a similar graph but for a higher power demand at the heating zones.Figure 31 shows an exemplary graph of the current drawn from the power source when the dynamic current limit determined in step S273 is not in use (e.g. when the actual current limit of the power source is used instead, to determine whether a heating zone can be powered), and Figure 32 shows an example of the current drawn from the power source when the dynamic current limit is used. As shown in Figure 32, the use of the dynamic current limit beneficially results in a smoother draw of current from the power source.Figure 33 shows an exemplary graph of the current drawn from the power source during a 50 ms firing window 332 during a period of high power demand at the heating zones. Advantageously, by virtue of the use of the dynamic current limit (in this example, approximately 10 A), the current draw from the current source is smooth. The measurement phase 330 arranged before the start of the firing window 332 can also be seen.For a given power source voltage, the resistance of the heating element(s) determines whether a heating zone can provide full power at less than 100% duty cycle. A low zone resistance results in the power pulses being more sparsely distributed in the time domain during the firing window. For a low zone resistance, the instantaneous currents are relatively high, and so fewer zones can be activated before exceeding the current limit of the power source. This can limit the achievable power during the firing window. In contrast, the use of relatively high resistances results in higher pulse density within the firing window for the same power output. The use of relatively high resistances results in lower instantaneous currents, enabling more zones to be activated simultaneously before reaching the power supply current limit. Nevertheless, it will be appreciated that any suitable resistances can be used for heater electrodes.Modulating Changes in Current DrawIt can be challenging for the power supply 21 to handle sudden changes in power demand and current draw. For example, when the heater electrodes are supplied with power there may be a transition from no load (or minimal load) to full load, or vice versa. In the present example, the current drawn from the power supply 21 by the heater 6 is modulated to smooth (or ‘shape’, or ‘modulate’) large transitions in the current draw in the time domain. Beneficially, this enables the use of a smaller, lighter, and lower-cost power supply 21 than could otherwise be used.Figs. 34a and 34b show two examples of the current drawn from the power source 21 by the heater electrodes. In figure 34a, an example is illustrated in which there is a relatively abrupt transition from a current draw of 10 A to a current draw of around 0 A, and then a further abrupt transition from a current draw of around 0 A back to a current draw of 10 A. As the power supplied to each heater electrode is switched on and off, these near-instantaneous transitions in the current draw can occur many times per second (for example, 15 times per second), which is challenging for the power source 21.The graph in Fig. 34b shows a particularly advantageous example in which the dynamic current limit, LIMIT, is modulated to smooth out the changes in the current draw in the time domain. The dynamic current limit may be modulated in this manner when it is determined that an abrupt transition in the current draw will occur (for example, based on the current that will be drawn by the heater electrodes that are to be supplied with power in the next firing window). As described above, varying thedynamic current limit affects whether the supply of power to a heater electrode will be held-over until the next time slot. Therefore, by controlling the dynamic current limit, the total current draw by the heater can be controlled to add intermediate ‘steps’ to smooth out the transition. This may be limited in ‘granularity’ by the number, and resistance of, zones in the system i.e. a system containing only two, similar zones could only provide a single, approximate 50%, step. In the example illustrated in Fig. 34b, an intermediate value of LIMIT of approximately 50% of the maximum current limit is used to smooth out the change in the current draw. However, it will be appreciated that any other suitable value could be used. For example, changes in the current draw of 80% of the maximum current limit could be used provided there are sufficient discrete, smaller loads available in the system as described above. Moreover, the number of intermediate steps, and the magnitude of the discrete changes in the current draw, need not necessarily be the same for transitions from high current draw to low current draw, as for transitions from low current draw to high current draw.Calibration Apparatus and MethodsImproved methods and apparatus for calibrating the heater(s) 6 of the device 1 will now be described. As described above, the heater 6 may be provided with a number of heating zones 642, and the device 1 comprises temperature measurement circuitry 25 for monitoring the temperatures of the heating zones 642. The power supplied to the heater electrodes can then be controlled in order to control the temperature of each heating zone 642 towards a respective target temperature. However, the actual heat output at the heating zones (and the sensed temperature) at a particular supplied voltage will depend on various manufacturing tolerances. For example, the actual heat output and sensed temperature at the heating zones will depend on:• Heater track resistance tolerances• Analogue component tolerances• Analogue-to-Digital (ADC) inaccuracies• Power source voltage errors and fluctuations under load• Wiring and printed circuit board (PCB) track impedances• MOSFET drain-source resistancesIt is desirable, therefore, to calibrate the actual temperature of each heating zone as a function of the supplied voltage.Temperature SensorsFigure 35 shows a simplified schematic illustration of a heater 6 and a temperature sensor 230. As described above with reference to Figures 2 to 6b, the heater 6 is provided with a set of heating zones 642. The temperature sensor 230 is provided with a corresponding set of temperature sensing zones 232, and may also be referred to as a ‘temperature sensing array’ 230. As will be described in more detail later, during the calibration method the heater 6 is supplied with power, resulting in heat output at the heating zones 642. The temperature of each heating zone is measured using a respective temperature sensing zone 232.In this example each of the temperature sensing zones 232 is aligned with a respective heating zone 642 of the heater 6 during the calibration measurements. For example, temperature sensing zone S1 is generally aligned with (overlaps with) heating zone Z1 during the calibration measurements. It will be appreciated that the temperature sensing zones 232 need not necessarily be completely aligned with the heating zones 642. For example, the temperature sensor 230 may be larger than the heater 6 (temperature sensing zones 232 may be larger than the correspondingheating zones 642) and extend past the edges of the heater 6 as illustrated in Figure 41. Alternatively, the temperature sensing zones 232 could be smaller than the corresponding heating zones 642, and the temperature sensing zones 232 could be separated from one another by thermally insulating material (the temperature sensing zones 232 need not necessarily be adjacent).Figure 36 illustrates an alternative arrangement of temperature sensing zones 232 in which there are two temperature sensing zones 232 provided for each heating zone 642. In this example, temperature sensing zone S1a overlaps half of heating zone Z1 , and temperature sensing zone S1 b overlaps with the other half of heating zone Z1. This configuration can be used to provide a redundant temperature sensing zone 232 for each heating zone 642, to mitigate against failure of a temperature sensing zone 232 (or, for example, to verify an abnormal temperature measurement from one of the temperature sensing zones 232). It will be appreciated that any other suitable arrangement and size of temperature sensing zones 232 could alternatively be used.Figure 37 illustrates an example in which the device 1 is provided with a pair of heaters 6a, 6b. For example, the device 1 may be of the type illustrated in Figure 1a, in which a heater 6 is provided in each of the two arms 4 of the device 1. A corresponding pair of temperature sensors 230a, 230b are provided for temperature calibration measurements. Whilst in the example illustrated in Figure 37 the two temperature sensors 230a, 230b are provided separately, this need not necessarily be the case. Figure 38 shows an example in which the two temperature sensors 230a, 230b are provided together as part of a single temperature sensing device 262 for sensing the temperatures of the heating zones 642 of both heaters 6a, 6b. In the example of Figure 38 the two temperature sensors 230a, 230b are separated by a layer of thermally insulating material 260, to prevent heat transfer between the temperature sensors 230a, 230b. The layer of thermally insulating material 260 may comprise, for example, a high temperature glass-filled polymer. The heaters 6a, 6b are moved into thermal contact with the temperature sensing device 262 before performing the calibration measurements, for thermal conduction from the heating zones 642 to the temperature sensing zones 232. For example, when the device 1 is of the type illustrated in Figure 1a, the arms 4 of the device 1 can be closed to bring the heaters 6 into contact with the temperature sensors 230a, 230b (by sandwiching the temperature sensing device 262 between the heaters 6). Alternatively, for example when separate temperature sensors 230a, 230b are provided as illustrated in Figure 37, each temperature sensor 230 could be clipped or otherwise fastened onto the respective heater 6, to engage the temperature sensor 230 with the heater and to align the temperature sensing zones 232 with the corresponding heating zones 642.Whilst the temperature sensors 230 illustrated in Figures 35 to 38 are generally rectangular, and the temperature sensing zones 232 are also rectangular, this need not necessarily be the case. The temperature sensors 230 and the temperature sensing zones 232 could have any other suitable shape. For example, when the heater 6b is a curved heater as illustrated in Figure 6b, the temperature sensor 230 could also be curved to ensure good thermal contact with the heater 6b. The temperature sensing zones 232 need not necessarily have a uniform size or shape, for example when the corresponding heating zones 646 do not have a uniform size or shape as illustrated in Figure 5.The temperature sensors 232 may be formed from any suitable material and may have any suitable form. For example, each temperature sensing zone 232 may comprise a silicone heatsink coupled to a thermocouple. As described above, a heater 6 can be provided with power in order to determine the temperature of a heating zoneelectrode. The heater electrodes 64 can be formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, and the temperature can be determined based on the measured resistance of the heater electrode 64. Therefore, alternatively, the temperature sensor 230 illustrated in Figures 35 to 38 could simply be an additional heater 6 but operated in a temperature measurement mode (or in the example illustrated in Figure 38, a pair of heaters 6 separated by the layer of thermally insulating material 260 to form the temperature measurement device 262). The temperature sensor 230 could be provided with an arrangement of layers as described above with reference to Figures 13 to 15, and temperature measurement circuity as described above with reference to Figures 7 and 16.In the present example, the temperature of each of the temperature sensing zones 232 is determined by measuring a resistance of each temperature sensing zone 232 (which varies with temperature) as described above. However, the temperature sensor 230 need not necessarily be arranged for thermal conduction from the heater 6 to the temperature sensor 230. For example, a temperature sensor 230 could be arranged at a particular distance from the heater 6 for measuring the radiative heat output of the heater 6 (for example, using an array of infrared sensors). Moreover, even when the temperature sensor 230 is arranged for thermal conduction from the heater 6 to the temperature sensor 230, the temperature sensor 230 and the heater 6 need not necessarily be in direct physical contact. For example, a removable thermally conductive pad could be placed between each heating zone 642 and the respective temperature sensing zone 232 to reduce the risk of damage to the temperature sensor 230 or to the heater 6, or the temperature sensor 230 could be provided with a protective coating.The temperature sensor 230 can be calibrated by heating the temperature sensor 230 (e.g. in an oven) to two or more reference temperatures. The resistances (or voltages) for each temperature sensing zone 232 can then be measured at each reference temperature, enabling the measured temperature to be calibrated as a function of the measured resistance (e.g. using a linear fit to the calibration data, or any other suitable type of calibration fit). Advantageously, this calibration method takes into account both the sensing zone resistances and variations in the measurement electronics as a whole, including manufacturing tolerances in any amplifiers, ADCs, or the like. The method also advantageously does not require the resistances of the temperature sensing zones 232 to have previously been measured. However, the resistances of the temperature sensing zones 232 could nevertheless be measured in order to detect defects in the manufacturing of the temperature sensor 230.Calibration SystemA system 2700 for calibrating the devices 1 will now be described with reference to Figures 39 to 41 .Figure 39 shows a schematic block diagram of the system 2700 for calibrating the devices 1. The calibration system 2700 comprises a number of calibration stations 274 (or ‘bays’). Each calibration station 274 is provided with a temperature sensor 230 and is configured for receiving a device 1 for calibration. Calibration circuitry 280 of the calibration stations 274 will be described in more detail later. The calibration station 274 may comprise a device holder for receiving the device 1 , configured such that each of the heating zones 642 of the heater(s) 6 of the device 1 is brought into alignment with the respective temperature sensing zones 232 of the temperature sensor 230 when the device 1 is positioned in the device holder, for subsequenttemperature calibration measurements. The calibration station 274 may comprise a flat surface (e.g. formed of plastic) with the temperature sensor 230 rigidly mounted perpendicular to the surface.A power source 270 is provided for supplying the devices 1 with power, for example to power the heaters 6 of the devices. The power source 270 could be, for example, a supply of mains power. The calibration system 2700 may be provided with any suitable number of calibration stations 274 (e.g. 10, 50, or 100 calibration stations).Each calibration station 274 is provided with communication circuity 292 for communicating with a calibration controller 272. The calibration station 274 may communicate with the calibration controller 272 to receive calibration measurement instructions, and to transmit measurement results to the calibration controller 272. The calibration controller 272 performs overall control of the calibration method. The calibration controller 272 is configured to determine calibration information for each device 1 , based on the corresponding calibration measurement results, to be stored on the device 1 , and to transmit the calibration information to the device 1 via the calibration station 274. Alternatively, the calibration information could be transmitted directly from the calibration controller 272 to the device 1 , for example via a wireless communication link (e.g. Wifi or Bluetooth). In a further alternative, the calibration information could be determined at each of the calibration stations 274 rather than at the calibration controller 272. The calibration information, and methods of determining the calibration information, will be described in more detail later.The calibration controller 272 may also communicate with the power source 270 to control the supply of power from the power source 270 to each of the devices 1 . Alternatively, the supply of power to the devices 1 may be controlled by a controller 284 provided at the calibration station 274 (which may be under the overall control of the calibration controller 272). In the example shown in Figure 39 the calibration controller 272 has a separate communication connection to each of the calibration bays 274. However, this need not necessarily be the case, and Figure 40 shows an alternative arrangement in which the communication connections are ‘daisy chained’ along the calibration stations to the calibration controller 272. Any suitable communication circuity 292 could be used to connect the calibration stations 274 to the calibration controller, for example an ethernet connection or a wireless communication link. The supply of power from the power source 270 could also be daisy chained along the calibration stations.Figure 41 shows a schematic block diagram of the calibration station 274 including the calibration circuitry 280. A device 1 to be calibrated is situated in the calibration station 274, and is arranged so that the temperature of each of the heating zones 642 of the heater 6 of the device 1 can be measured using the respective temperature sensing zone 232 of the temperature sensor 230 (e.g., when the device is of the type illustrated in Figure 1a, by clamping the temperature sensor 230 between the heater plates. During the calibration measurements, the device 1 is supplied power from the power source 270, for example between 25 V and 40 V of DC power. Alternatively, if the device 1 is a corded device, the device 1 could simply be plugged into a mains power source.The calibration circuitry 280 comprises a calibration station controller 284 that operates in accordance with software stored in the memory 285. The calibration station controller 284 controls the communication circuitry 292 to communicate with the overall calibration controller 272 (e.g. via any suitable wired or wireless connection). The calibration circuitry 280 also comprises a sensor interface 286 forcommunicating with the temperature sensor 230 in order to obtain the temperature measurements. The sensor interface 286 may also be configured to communicate with a sensor for measuring the power drawn by the device 1 from the power source 270 (or to communicate with the power source 270 directly to obtain the power draw information). The power draw information could be used to detect that the device 1 has a fault, for example if the device 1 is drawing an abnormally large amount of power, or is not drawing power at all.The connection from the power source 270 to the device 1 for supply of power to the device may be provided using a magnetic power jack. However, any other suitable connection could alternatively be used. A power cable of the magnetic power jack may be sacrificial, for example designed to be replaced every 1000 calibrations, with an industrial grade power connector provided for connection to the device 1 . If the power is provided to the device 1 using a calibrated power source (e.g. calibrated laboratory power supply), then advantageously the voltage sense scaling of the device 1 can be accurately calibrated.The calibration circuitry also comprises a device interface 282 for communicating with the device 1 (for example, to upload the final calibration information to the device 1). The device interface 282 may comprise any suitable wired or wireless connection (e.g. Wifi or a short-range wireless communication link such as Bluetooth). The provision of a wireless device interface 282 is particularly advantageous since it removes the need to connect a physical wire from the calibration circuitry 280 to the device 1 for communication, improving the efficiency of the calibration method. The calibration circuitry 280 also includes a status indicator 290 for indicating a status of the calibration station 274 or a status of the device 1. The status indicator 290 may comprise a coloured light for indicating the status. For example, the status indicator light may illuminate green to indicate that the device 1 has been successfully calibrated, or could illuminate red to indicate that the calibration has been unsuccessful (or that the device 1 or calibration station 274 has a fault). The light could also illuminate blue when power is first supplied to the device 1 , and illuminate orange whilst the calibration measurements are in progress.When the device interface 282 comprises a wireless communication link, the communication link may initially be configured to use a relatively low transmission power to minimize radio interference with the other calibration stations 274 and devices 1. If communication with the device 1 is unsuccessful, the transmission power could then be increased to attempt to achieve communication. Radio frequency (RF) shielding could be provided around each calibration station 274 to reduce the risk of interference.Calibration MethodFigure 42 shows a flow diagram of a method of calibrating the device 1 using the calibration station 274. The calibration method enables efficient and accurate calibration of large numbers of devices 1 . Beneficially, the assembled device 1 can be calibrated, rather than calibrating the individual components of the device 1 separately (for example, by separately measuring the resistance of the heater tracks, the resistance of the current sense resistor, and so on). This is more efficient than performing separate calibration procedures on the un-assembled components, and advantageously takes into account all of the contributing errors using a single calibration. Moreover, the calibration method does not require the device 1 or component to be placed within a controlled environmental chamber, and the calibration can be performed particularly fast (e.g. a few seconds). Furthermore, themethod also enables the detection of hardware faults and anomalies for example, by detecting that the calibration parameters are outside of an acceptable range.In step S301 , pre-calibration configuration is performed. The calibration station 274 receives the device 1 , bringing the heater 6 into thermal contact with the temperature sensor 230 for thermal conduction from each of the heating zones 642 to the respective temperature sensing zone 232. The device 1 is connected to the power source 270 for supply of power to the heater 6, and establishes communication via the device interface 282. The device interface 282 can be used to communicate with the device 1 to register the device 1 when it is received by the calibration station 274. The device 1 may be configured to enter a calibration mode when the device 1 is first powered on, but could alternatively be controlled to enter the calibration mode by the controller 284 via the device interface 282.In step S302, calibration measurements are performed. The calibration of the device 1 can be performed in an automatic manner, with calibration status indicated by the status indicator 290. The device 1 outputs power to the heater(s) 6, to heat the heating zones 642 to a target temperature as measured using the temperature sensing circuitry of the device 1. The device sequentially increases the target temperature under the control of the calibration controller 272. Two or more target temperatures are used for the calibration, for example a first target temperature of 100 °C and a second target temperature of 240 °C, but it will be appreciated that the actual calibration temperatures used will depend on the temperatures that the device 1 is expected to operate at during normal use. The actual temperatures of the heating zones 642 are measured using the temperature sensor 230, and the measurements are transmitted to the calibration controller 272. Based on the differences between the target temperatures and the temperatures measured using the temperature sensor 230, the calibration controller generates calibration information to be stored at the device, to calibrate the device 1. The generation of the calibration information will be described in more detail later. The calibration measurements could also be performed using a plurality of different supply voltages, for example if the design of the device 1 means this is a potential source of errors. If the calibration controller 272 detects an unexpected or anomalous relationship between the target temperatures and the temperatures measured by the temperature sensor 230, then the device 1 may be rejected as faulty or requiring further investigation.Self-measurements of voltage and current within the device 1 can also be performed in order to identify possible hardware faults, and can also be transmitted to the calibration controller. Large temperature errors or anomalous power readings at this stage can be used as a trigger to abort of the calibration, and the device 1 is rejected with diagnostic information stored. The calibration controller 272 monitors and controls calibration process and records the calibration results automatically (each device 1 could have a unique, pre-programmed software-readable serial number that can be stored in association with the calibration results).In step S303, the calibration information generated by the calibration controller 272 is uploaded to the device 1. Temperature, power or voltage corrections are determined and stored in the device 1 , so that the device 1 can accurately control the heating zones 642 towards target temperatures using the calibration information. The calibration information is provided for each heating zone 642. The calibration information includes the equivalent of a gain and offset per heating zone (if a linear calibration is used). A calibration report for the device 1 may also be stored at the calibration controller 272. The device 1 is then controlled by the calibration controller 272 to exit the calibration mode and to shut down, and will enter a standard, calibratedoperation mode when the device 1 is next powered on. The device 1 is then removed from the calibration station 274.Temperature Calibration ExampleFigure 43 shows a table of exemplary calibration measurements, obtained as part of the method of Figure 42, that can be used to generate the calibration information to be stored and used by the device 1. As shown in the table, in this example target temperatures of 100°C and 240°C are used.In this example, when the device controlled the temperature of the heater to 100 °C, the actual temperature of the heating zones 642 was measured by the temperature sensor 230 to be 103 °C. Therefore, the uncalibrated device 1 has a temperature error of 3°C at a target temperature of 100°C. Similarly, when the device controlled the temperature of the heater to 240 °C, the actual temperature of the heating zones 642 was measured by the temperature sensor 230 to be 232 °C. Therefore, the uncalibrated device 1 has a temperature error of 8°C at a target temperature of 100°C. Therefore, calibration information (e.g. gain and offset) is to be generated to correct the uncalibrated temperature of 100°C sensed by the device to 103°C, and to correct the uncalibrated temperature of 240°C sensed by the device to 232°C.If the device 1 measures a zone temperature of TSENSERAW° C, then the device 1 can correct its internal temperature measurement using the following equation: TCALO CTWAL) __ - + TCALOACT[IALWhere TSENSECAL is the calibrated temperature of a heating zone 642 sensed by the device 1 , TSENSERAW the uncalibrated temperature of the heating zone 642 sensed by the device 1 , TCALORAW is a first uncalibrated calibration temperature sensed by the device 1 , TCALOACTUAL is the first calibration temperature as measured by the temperature sensor 230, TCAL1RAW is a second uncalibrated calibration temperature sensed by the device 1 , and TCALIACTUAL is the second calibration temperature as measured by the temperature sensor 230.In this example, the calibration information is generated so that if the device 1 senses the uncorrected temperature to be 180.0 °C, the calibration information can be used to determine the actual temperature of:(180 - 100) X (232 - 103)TSENSECAL= - - . .. . - - + 103 = 176.71(240 - 100)Therefore, given a target temperature of 180 °C, the device 1 will set the heater 6 temperature higher than it would otherwise have done to achieve an actual temperature of 180 °C.By way of example, during a calibration procedure the heater 6 may be controlled from a target temperature of 125 °C to a target temperature of 195 °C, in 10 °C increments. For each target temperature, the actual temperature obtained for each heating zone is measured using the temperature sensor 230. These measurements enable a relationship between the target temperature and the actual temperature to be determined. This relationship may be, but need not necessarily be, a linear relationship. For example, a slope parameter ‘m’ and an offset parameter ‘C’ may bedetermined for a linear relationship between the target temperature and the actual temperature.For a given temperature of a heating zone sensed by the device 1 , the device 1 may determine a corrected temperature for the heating zone based on the calibration information comprising the slope parameter and the offset parameter. The corrected temperature, TCorrected, for a heating zone is given by:"["Corrected="["Measured (^."["Measured) Q where TMeasuredis the temperature of a heating zone as measured by the device 1 . The value of TCorrecledfor each heating zone can then be controlled towards the respective target temperature.Alternatively, rather than storing a gain and an offset for each temperature zone, the calibration information stored on the device may comprise an indication (e.g., a table) of a temperature offset per heating zone at a number of calibration temperatures. Linear interpolation could then be used to determine the value of the temperature offset, C, for any measured temperature, using the table.Hair Speed and AccelerometerWhen a user passes the device 1 through the hair 40, due to the heat transfer to the hair 40 the temperature of the hair engaging surface can drop. This problem is particularly acute for low thermal mass heaters, since a relatively small amount of heat transfer results in a significant change in temperature of the hair engaging surface. The drop in temperature can result in ineffective drying or styling towards the end of a pass through the hair. As described above, the temperature of the heating zones can be controlled based on the determined speed of the hair over the heating zones, to increase the temperature of the heating zones when the hair is moving more quickly across the surface of the heater 6. However, whilst in some examples above the speed that the hair passes over the heating zones is determined based on a change in the power needed to maintain heating zones at a particular temperature, in the present example the speed that the device 1 is being passed through the hair 40 is determined based on data from an accelerometer of the device 1 . Advantageously, the power supplied to each of the heating zones can be rapidly controlled based on the speed of the device 1 , to achieve more even heating of the hair 40.In this example, the heating zones are used to estimate the temperature of the hair 40, and the accelerometer is used to determine the speed that the device 1 is passing over the hair 40. The heating zones can be used to estimate the temperature of the hair 40 based on changes in the power needed to maintain a heating zone at a particular target temperature. As the hair becomes hotter, the power needed to maintain a heating zone at a particular temperature decreases, since the heat transfer to the hair 40 decreases. The relationship between the temperature of the hair and the power needed to maintain a heating zone at a target temperature could be calibrated in advance (e.g. in a laboratory) and stored as calibration information on the device 1.The speed of the device 1 is determined using measurements from an accelerometer of the device 1. Methods of determining a speed of a device based on accelerometer measurements are well known and need not be described in detail here. As the device 1 is passed through the hair 40, the determined speed of the device 1 is used to control the amount of power supplied to each of the heating zones. When the device 1 is moving quickly through the hair 40, rapid heat transfer away from the heating zones will occur due to the heating zones engaging with a larger amount of relatively coolhair. Therefore, when the accelerometer indicates that the device 1 is being moved relatively quickly through the hair 40, a determination is made to supply a relatively large amount of power to one or more of the heating zones (e.g. all of the heating zones, or only the heating zones that are determined to be in contact with the hair 40). Conversely, when the device 1 is being passed relatively slowly through the hair 40, less heat transfer from the heating zones will occur. Therefore, when the accelerometer indicates that the device 1 is being moved relatively slowly through the hair 40, a determination is made to supply a smaller amount of power to each heating zone. Advantageously, therefore, the measurements from the accelerometer can be used to modify the power supplied to the heaters 6, to help maintain the temperature of the hair engaging surface at the styling / drying temperature. This enables more responsive control of the hair temperature, to achieve more even heating of the hair. A relationship between the speed of the device 1 and a power to supply to a heating zone could be calibrated in advance (e.g. in a laboratory) and stored on the device as calibration information. This calibration information may comprise, for example, a factor to multiply a default power level by, for a particular target temperature and speed of the device 1 .Figure 44 shows a graph of hair temperature along a tress of hair 40, and temperature profiles of a corresponding heating zone. The temperature of the hair 40 along the length of the tress, illustrated by the solid line in Figure 44, may be referred to as the temperature profile of the hair 40. Firstly, the user positions the hair 40 on the heating zone (e.g. by clamping the hair between the heater plates of a straightener). The heating zone is then heated to the target temperature. An indication may be provided to the user that the user is to begin moving the device 1 along the tress of hair, to pass the tress of hair 40 over the heating zone. As the device 1 is passed along a tress of hair 40 (e.g. for the first time during the styling process), the temperature of the hair 40 is measured based on the power needed to maintain the heating zone at the target temperature. The temperature of the hair 40 along the length of the tress is indicated by the solid line 241 in Figure 44. As illustrated in the figure, in this example a significant amount of heat has been transferred to the hair 40 at the beginning of the tress, but the other end of the tress is relatively cool. Based on the measurement of the hair temperature along the tress, and based on the speed the device 1 was moved along the tress (as measured using the accelerometer), the device 1 is configured to determine a temperature profile for the heating zone for a subsequent pass along the tress. The determination of the temperature profile for the heating zone may be performed only during the first pass along the tress of hair 40, but could also be performed during subsequent passes along the hair 40. Two temperature profiles for the heating zone are illustrated in Figure 44 by the dashed line 242 and the dot-dash line 243. The temperature profile for a heating zone indicates the target temperature for the heating zone at each position along the tress of hair. The temperature profile indicated by the dashed line 242 corresponds to lower temperatures, and is mapped to a case where the user moved the device 1 relatively slowly (as measured using the accelerometer) along the tress. The temperature profile indicated by the dot-dash line corresponds to higher temperatures, and is mapped to a case where the user moved the device 1 relatively quickly (as measured using the accelerometer) along the tress. In other words, the device is configured to control the temperature of the heating zones based on the speed of the device 1 measured using the accelerometer, and based on the measured temperature of the hair 40. Faster movement of the device 1 results in the heating zones being heater to higher temperatures, to ensure that the hair 40 is consistently heated over the entire length of the tress. The mapping between the heating zone temperature profiles, measured hair temperature and measured device speed may be calibrated in advance (e.g. in a laboratory) and stored on the device 1 .Hair Detection and Adjacent ZonesAs described above with reference to Figure 21 , the power needed to maintain a heating zone at a target temperature can be used to determine whether the heating zone is in contact with hair 40. For heating zones that are determined not to be in contact with the hair 40, the target temperature of those heating zones can be set to the ambient temperature (in which case no power will be provided to the corresponding heater electrode(s)), or an intermediate temperature Tint between the ambient temperature and the styling temperature Tstyie. However, as illustrated in Figure 45, a situation can occur in which one or more stray hairs are present in a neighbouring heating zone. In the example illustrated in figure 45, a significant amount of hair 40 is present in zones Z2 and Z3, and the heat transfer to the hair 40 from those zones is sufficient for a determination by the device 1 that there is hair present in zones Z2 and Z3 (based on the increase in power needed to maintain zones Z2 and Z3 at their respective target temperatures, or based on a rate of cooling of the heating zones). However, stray hairs are present in the neighbouring zone Z4, and the relatively low heat transfer to those stray hairs makes it difficult to detect the presence of the hair 40 in zone Z4. There is a problem, therefore, that the stray hairs in zone Z4 are undetected by the device 1 , and the device 1 determines to set the target temperature for zone Z4 to the ambient temperature (or the intermediate temperature nt). This will result in the stray hairs in zone Z4 not being heated to the desired temperature for sty ling / dry ing the hair 40.In the present example, to mitigate against this problem, when hair 40 is determined to be present in a heating zone, the device 1 determines to also set the target temperature of one or more neighbouring (e.g. adjacent) heating zones to the target styling temperature Tstyie. Advantageously, therefore, the risk of stray hairs not being heated to the desired temperature is reduced. In the example of Figure 45, the device 1 determines that hair is present in zones Z2 and Z3. The device 1 therefore determines to set the target temperature for zones Z2 and Z3 to Tstyie. However, the device 1 also determines to set the target temperature of the neighbouring heating zones Z1 and Z4 to Tstyie- Whilst there is no hair present in zone Z1 and so the power supplied to the corresponding heater electrodes will be wasted, advantageously the stray hairs in zone Z4 will be heated to the desired temperature, resulting in more effective styling of the hair 40.Figure 46 illustrates a further example in which the presence of hair 40 is detected in only one of the heating zones. In this example the hair 40 is detected in zone Z3. The device 1 therefore determines to set the target temperature for zone Z3 to Tstyie. However, the device also determines to set the target temperature of the neighbouring heating zones Z2 and Z4 to Tstyie, beneficially resulting in the undetected stray hairs in zones Z2 and Z4 being heated to the desired temperature. It will be appreciated that the neighbouring zones that are additionally heated to the target temperature for styling need not necessarily be limited to the heating zones that are immediately adjacent to a zone in which the presence of hair 40 has been detected. Alternatively, for example, the device 1 may be configured to set the target temperature to Tstyie for the four heating zones that are closest to the heating zone in which the presence of hair 40 has been detected. In the example illustrated in Figure 46, based on the detection of hair 40 in zone Z3, the device 1 may be configured to set the target temperatures for zones Z1 , Z2, Z4 and Z5 to Tstyie. Zones Z1 and Z2 form a contiguous set of heating zones, and at least one of the heating zones (Z2) is adjacent to a heating zone (Z3) in which the presence of hair has been detected. Similarly, zones Z4 and Z5 form a contiguous set of heating zones, and at least one of theheating zones (Z4) is adjacent to a heating zone (Z3) in which the presence of hair has been detected.It will be appreciated that the method of setting the target temperature to Tstyie for zones that neighbour a zone in which hair 40 has been detected is not limited to a simple row of heating zones as illustrated in Figures 45 and 46. The method is applicable to any other suitable arrangement of heating zones, for example any of the arrangements of heating zones illustrated in Figures 3 to 6b. Figure 47 illustrates an example in which the method is applied to a grid of heating zones. In this example, the device 1 determines that hair 40 is in contact with heating zones Z3 to Z6, and therefore determines to set the target temperature for zone Z3 to Z6 to Tstyie. However, the device 1 additionally determines to set the target temperature to Tstyie for the neighbouring zones Z1 , Z2, Z7 and Z8. Beneficially, therefore, the stray hairs present in zones Z7 and Z8 are heated to the desired temperature.The target temperature Tstyie need not necessarily be the same for each of the heating zones. For example, in the example illustrated in Figure 47, the device may determine to set the target temperature for zones Z1 to Z5 to a first target temperature for styling, T1style, may determine to set the target temperature for zone Z6 to a second target temperature for styling, T2styie, and may determine to set the target temperature for zones Z7 and Z8 to a third target temperature for styling, T3styie.Temperature Inference and Power CutoffWhilst in some of the examples described above the device 1 has been described with reference to a hair straightener, this need not necessarily be the case. Alternatively, for example, the device 1 may be a hair curler. The hair curler may have (but need not necessarily have) a generally tubular set of heating zones, as illustrated for example in Figure 6a. Particularly advantageous apparatus and methods for providing a ‘one- shot’ curler will now be described.By virtue of the rapid heat-up and cool-down times of the low thermal mass heater, a ‘one-shot’ curling iron can be provided. In use, the user wraps a tress of hair 40 around the device 1 whilst the heating zones are at ambient (or near ambient) temperature. A ‘shot’ (transient pulse) of heat is then applied to the hair 40 to set the curl, by supplying power to the heater electrodes. Due to the low thermal mass of the heaters 6, the heating zones (and the hair 40) then cool rapidly back towards the ambient temperature, and the curled hair 40 can then be removed from the device. In order to achieve more efficient styling, it is advantageous to minimise the time period during which the heater electrodes are supplied with power (the time period during which the target temperature for the heating zones is set to Tstyie, also referred to as the ‘heating period’). It is also beneficial to minimise the time period between the time at which the supply of power to the heater electrodes is stopped and the time at which the user removes the curled hair 40 from the device 1 (the ‘cooling period’), in order to achieve more rapid styling. Improved apparatus and methods for reducing these time periods will now be described. Advantageously, rather than using preconfigured time periods for the heating period and the cooling period, the improved apparatus and methods enable the device 1 to determine the time at which hair has reached the target temperature, and to determine the time at which the hair 40 is to be removed from the device 1. This enables more efficient styling and improved styling performance. The improved apparatus and methods enable inference of the hair temperature in the heating and cooling phases, enabling the heating period and the cooling period to be minimised (or reduced).The heating zones of the device 1 in this example may be (but need not necessarily be) of the cylindrical form illustrated in Figure 6a. However, it will be appreciated that any other suitable arrangement of heating zones could alternatively be used. As described above, a corresponding target styling temperature, Tstyie, may be defined for each of the heating zones, and the target styling temperature need not necessarily be the same for each of the heating zones. In use, the user wraps the hair 40 around the device 1 when the heating zones are at the ambient temperature (or near the ambient temperature). The heating of the hair 40 then begins, for example in response to the user pressing a button on the device 1. The target temperature for the heating zones is set to Tstyie, and the supply of power to the heater electrodes is controlled to control the temperature of each heating zone towards the respective target temperature. The heating zones remain at the temperature Tstyie for a period of time, before the supply of power to the corresponding heater electrodes is stopped. Advantageously, the device 1 is configured to power a heating zone until it is determined that the hair 40 in contact with that heating zone has reached the desired target temperature. As described above, the temperature of the hair 40 can be determined based on the power needed to maintain a heating zone at the target temperature. After the supply of power to the heater electrodes has stopped, the heating zones rapidly cool towards the ambient temperature. Advantageously, the device 1 in this example is configured to provide an indication to the user of when the user is to remove the hair 40 from the device 1 . The indication that the hair 40 is to be removed from the device 1 may comprise, for example, an optical signal (e.g. a coloured light), an audible signal (e.g. a beep), or a haptic signal (e.g. a vibration generated by a motor of the device 1). It will be appreciated, however, that any other suitable indication could be provided to the user to indicate that the hair 40 can be removed from the device 1 .Since the temperature of the hair will not exceed the target temperature Tstyie of the heating zones, there is no risk of overheating the hair. Therefore, a preconfigured time period could be used for the heating period. Since there is no risk of hair damage, this time period could be set arbitrarily long, to ensure that all types of hair 40 will reach the desired temperature. However, the inventors have realised that by configuring the device 1 to dynamically determine when the heating of the hair 40 is complete (i.e. when to set the target temperature of the heating zones back to the ambient temperature), the heating period can be reduced in duration in many scenarios (for example, when the user has a hair type that reaches the target temperature more rapidly), resulting in a more energy efficient device and more rapid styling.Figure 48 shows a plot of hair temperature 281 , heater temperature 287 and power 283 supplied to the heater against time, illustrating a method of heating the hair 40 to the target temperature and determining when the hair 40 is to be removed from the device 1. The power supplied to the heater electrode is indicated by the solid line 283, the temperature of the hair 40 is indicated by the dot-dash line 281 , and the temperature of the heating zone is indicated by the dashed line 287.The method begins at time to, at which both the hair 40 and the heating zones are at ambient temperature. The hair 40 is arranged in thermal contact with the heating zone. The supply of power to the heater electrode then begins, for example in response to the user pressing a button on the device 1. As illustrated in Figure 48 by the solid line 283, the supply of power to the heater electrode increases rapidly, and the temperature of the corresponding heating zone indicated by the dashed line 287 also increases rapidly due to the low thermal mass of the heater. The temperature 281 of the hair 40 also begins to rise towards the target temperature, as heat is transferred from the heating zone to the hair 40.In order to determine when the heating of the hair 40 is complete, the device 1 is configured to determine the temperature of the hair based on the power needed to maintain the heating zones at their respective target temperatures. This enables the heating of the hair 40 to be stopped shortly after the inferred hair temperature 40 reaches the target temperature. After the temperature of the heating zone has reached the target temperature Tstyie, the power supplied to the heater electrode begins to decrease. The decrease in power to maintain the heating zone at the temperature Tstyie is due to the increasing temperature of the hair 40, which results in less heat transfer from the heating zone to the hair 40. The power needed to maintain the heating zone at the target temperature is approximately proportional to the temperature difference between the heating zone and the hair 40. The temperature of the hair 40 rises asymptotically towards the target temperature Tstyie. The power needed to maintain the heating zone at the temperature Tstyie continues to decrease until a residual power lever, Presiduai, is reached. The value of Presiduai is non-zero due to, for example, heat transfer from the heating zone to the air around the device 1 , and heat transfer to the casework of the device 1. As illustrated in Figure 48, when the power supplied to the heater electrode reaches the residual power level Presiduai, the temperature of the hair 40 has reached the target temperature Tstyie. Advantageously, therefore, the device 1 can determine that the hair 40 has reached the target temperature based on the power supplied to the heater electrode. When the device comprises a plurality of heating zones and heater electrodes, it will be appreciated that the power measurement and the residual power Presiduai may correspond to the average power supplied to the powered heater electrodes to heat the heating zones, or to the summed power supplied to the powered heater electrodes, or any other suitable parameters calculated based on the power supplied to the heater electrodes.At time ti the power supplied to the heater electrode has reached the residual power level Presiduai, and therefore the device 1 determines that the temperature of the hair has reached the target temperature Tstyie. A determination that the hair has reached the desired temperature could alternatively, or additionally, be made based on the shape (e.g. gradient) of the power draw curve 283, which beneficially mitigates against the problem that the value of Presiduai can depend on the ambient temperature (the value of Presiduai will be larger when the ambient temperature is lower, due to increased heat transfer to the ambient air). Optionally, the device 1 may be configured to maintain the target temperature for the heating zone at Tstyie for a further period of time, illustrated as the time between ti and t2 in the figure. This optional time delay helps to ensure that all of the hair 40 has been heated to the target temperature.At time t2(or at time ti if the optional delay is not used) the target temperature for the heating zone is set to the ambient temperature, and the supply of power to the corresponding heater electrode is stopped. Therefore, both the heating zone and the hair 40 begin to cool towards the ambient temperature. Since the hair 40 and the heating zone are in thermal contact, the hair 40 and the heating zone cool at a similar (or related) rate. Therefore, the temperature of the heating zone can be used as a proxy measurement for the temperature of the hair 40. As described above, for example with reference to step S1702 of Figure 17, the temperature of the heating zone during this period can be measured by providing transient pulses of power to the heater electrode, and measuring the resistance of the heater electrode. At time ts the temperature of the heating zone reaches the threshold temperature Tremove, and the device 1 determines that the hair 40 is to be removed from the device 1 . When the device 1 comprises a plurality of heating zones, the temperature that is compared to the threshold temperature Tremove may be the average temperature of the heating zones (or any other suitable parameter for comparison to a threshold value could be calculated based on the temperatures of the heating zones).As described above, the device 1 may be configured to provide any suitable indication to the user that the hair 40 is to be removed from the device 1 . Advantageously, by virtue of the device 1 determining when the hair has reached the target temperature Tstyie and when the heating zone has reached the removal temperature Tremove, the heating period and the cooling period can be reduced compared to when preconfigured time periods are used. This results in more efficient and effective styling of the hair 40.The indication that the hair 40 is to be removed from the device 1 may comprise, for example, an audible countdown to the time at which the hair is to be removed from the device 1. The countdown could be determined using the temperature of the heating zone, and / or the gradient of the temperature curve 287 of the heating zone, to predict when the heating zone will reach the threshold temperature Tremove.Whilst in the example of Figure 48 the indication to the user that the hair 40 is to be removed from the device 1 is generated when the temperature of the heating zone reaches the threshold temperature Tremove, a preconfigured timeout timer could additionally be used. The use of a backup timeout timer ensures that the indication will be provided to the user after a maximum cooling time has elapsed, irrespective of the measured temperature of the heating zone. A corresponding maximum heating time may also be configured for the heating period.Whilst in the example of Figure 48 both the duration of the heating period and the duration of the cooling period are dynamically determined by the device 1 , alternatively only one of those periods may be dynamically determined by the device, and a preconfigured time period could be used for the other of the periods. For example, the duration of the heating period may be determined dynamically by the device 1 based on the power needed to maintain the heating zone at the temperature Tstyie, and a preconfigured time period may be used for the cooling period.When the device 1 comprises a plurality of heating zones, the method may be applied to each heating zone individually rather than using an average power level or average temperature. In this case, the indication to the user that the hair is to be removed from the device 1 may be output once each of the heating zones has cooled to the threshold temperature Tremove. Advantageously, when the method is applied to each applied to each heating zone individually, heating zones in contact with a relatively small amount of hair will have a shorter heating period, reducing the overall power usage compared to the case where the average power (or summed power) of the heating zones is used for a collective determination of when to terminate the heating period for all of the heating zones. The shorter heating periods for the heating zones in contact with a relatively small amount of hair also enables the cooling of those zones to begin earlier, beneficially resulting in a cooler device 1 overall.Optionally, the initial target temperature for each heating zone at time to may be the intermediate target temperature Tint (between the ambient temperature and the styling temperature Tstyie). If the subsequent power draw of a heating zone is less than a predetermined threshold value, then it is determined that no hair is in contact with the heating zone, and the target temperature for that heating zone is set back to the ambient temperature. The method of heating zones that neighbour those in which hair has been detected, described above with reference to Figures 45 to 47, may optionally be used to ensure that stray hairs are heated to the temperature Tstyie.The device 1 beneficially makes use of the rapid thermal response of the low thermal mass heaters. The rapid cooling of the heaters makes it practical to allow the curl to remain undisturbed on the device for a period of time following the heating before removing the hair from the device 1 , increasing the effectiveness of the styling. In contrast, with a conventional heater having a high thermal mass, the user wraps the hair 40 around an already-heated barrel and removes the hair whilst the barrel is at high temperature, which risks the user suffering a burn. Removing the hair from the device whilst the hair is still hot also risks disturbing the curl (or other styling effect), decreasing the styling performance. In a conventional curling wand, the user must slide the curl off the wand while it is still hot, which risks spoiling the style. In contrast, the heating zones of the device 1 of the present disclosure will cool rapidly (and therefore the hair on the heating zones will also cool rapidly), setting the style before the hair is removed from the device.Whilst the present example has been described with reference to a curler, the device 1 need not necessarily be a curler. The methods described above with reference to Figure 48 could be applied to any suitable device 1 that heats the hair 40 to a target temperature before allowing it to cool.Target Hair TemperatureIn many conventional hair styling devices, the target temperature of the hair-engaging surface is set to a particular static temperature, and the power supplied to the heater(s) is controlled to try to maintain the hair-engaging surface at that target temperature. In such devices, the temperature of the hair 40 is unknown, and the hairengaging surface is simply maintained at the desired temperature for the hair. However, the inventors have realised that the rapid thermal response of the low thermal mass heaters, and the use of a plurality of heating zones, enables the temperature of the hair 40 as it exits the device 1 to be estimated and controlled towards a target temperature for the hair 40. Advantageously, therefore, in this example by dynamically adjusting the temperature of the heating zones, the temperature of the hair 40 as it exits the device 1 (as device is moved across the hair) can be controlled towards a target temperature. The apparatus and methods enable reliable and efficient control of the hair temperature, even for a range of hair types and a range of speeds at which the device 1 is moved across the hair 40.The present example will be described with reference to an N x 2 array of heating zones, such as the arrangement of heating zones illustrated in Fig. 4. However, it will be appreciated that any other suitable arrangement of heating zones could be used (e.g. an N x 3 array, or N x 4 array of heating zones). It will also be appreciated that the value of N may be any suitable number. For example, as illustrated in Fig. 4, a 6 x 2 array of heating zones could be used.When an N x 2 array of heating zones is used, in normal operation of the device 1 a section of hair 40 will pass over two of the heating zones in sequence. Fig. 49 shows a graph of hair temperature as the hair 40 passes over the two adjacent zones (Zone 0 and Zone 1), and Fig. 50 shows an example of how each of the heating zones may be assigned an identifying index. As illustrated in Fig. 50, the hair engaging surface of the device 1 may be divided into N heating zones along the longitudinal direction (10 heating zones along the x-direction for each arm of the device 1 in the example illustrated Fig. 50) and two transverse zones (zone 0 and zone 1) in the y-direction at each position along the hair engaging surface. Whilst a 10 x 2 arrangement of heating zones and two arms are illustrated in Fig. 50, this need not necessarily be the case (e.g. a 30 x 4 arrangement of heating zones and a single arm could be used). The device 1 need not necessarily comprise arms as illustrated in Fig 50, for example ifthe device 1 is a hair curler and the heating zones are provided in a cylindrical configuration similar to that illustrated in Fig. 6a.In the example illustrated in Fig. 49, the two heating zones are maintained at the same temperature, Thtr, but this need not necessarily be the case. As illustrated in Fig. 49, the hair that comes into contact with the first heating zone (Zone 0) is heated from an initial temperature TEntry (e.g. ambient temperature) to an intermediate temperature To. The intermediate temperature To is less than the heater temperature Thtr by a value of Ao. The hair 40 then passes over the second heating zone (Zone 1) and is heated to the intermediate temperature Ti, before reaching the exit temperature TExit. The intermediate temperature Ti is less than the heater temperature Thtr by a value of Ai.Due to the first heating zone receiving hair 40 at a lower initial temperature (TEntry) than the second heating zone, the power needed to maintain the first heating zone at the temperature Thtr will be greater than the power needed to maintain the second heating zone at the temperature Thtr (assuming that the section of hair 40 moves at approximately the same speed over both heating zones, which is likely to be the case).The ratio of powers drawn by the two heating zones to maintain the zones at the target temperature Thtr is proportional to the difference in temperature between each heating zone and the temperature of the hair 40 as it passes over the heating zone (Ao for the first heating zone and Ai for the second heating zone). The ratio of the powers drawn by the two heating zones therefore provides an indication of the relative average temperature of the hair engaged with the heating zones. When the heating zones are moved across a tress of hair 40, there is a longitudinal temperature gradient along the hair 40 engaged with each heating zone (as illustrated in Fig. 49), but it can be assumed that the average hair temperature occurs at the same longitudinal position on each zone.The temperature of the hair 40 as it exits the second heating zone is given by:TExit = Thtr - (Thtr - TEntry) * (Ai / A0)2(Equation 12)If Pois the average power drawn by the first heating zone as the hair 40 passes across the first heating zone, and Pi is the average power drawn by the second heating zone as the hair passes across the second heating zone, then the ratio P1 / P0 represents the proportional change in power drawn in the distance between the centres of the heating zones. However, each section of the hair 40 traverses twice that distance in total, which results in the squared term (A1 / A0)2. The ratio of the powers Po and Pi is equal to the ratio of the temperature differences Ai and Ao:Pi / Po = Ai / Ao (Equation 13)Therefore, the estimated temperature of the hair 40 as it exits the second heating zone, TExitEst, is given by:TExitEst = Thtr - (Thtr - KyEntry) * (Pi / Po)2(Equation 14) where KyEntry is a constant temperature assumed for the hair entry temperature TEntry, for example 25°C. Therefore, it can be seen that TExitEst can be obtained by measuring the temperature of the heating zones Thtr (e.g. using the method described above with reference to Fig. 18) and the power output to the two heating zones, Po and Pi. Advantageously, therefore, the target temperature of the heating zones can becontrolled in response to the power drawn by the heating zones, to guide the value of TExitEst towards a target temperature for the hair 40 (e.g. a particularly effective drying or styling temperature for the hair 40). In other words, by controlling the target temperature of the heating zones based on the power drawn by the heating zones, the device 1 is operable to maintain the temperature of the hair 40 as it exits the final heating zone at a target hair temperature.Figure 51 shows a simplified schematic illustration in which the two heating zones are sub-divided into smaller elements, useful for understanding why the (P1 / P0) term is squared in equation 14. The relationship Pi / Po = Ai / Ao would apply for each pair of corresponding sub-elements (a to ri) between the heating zones, if this could be measured. The distance between the first sub-element, a, of Zone 0 (the sub-element at which the hair 40 first engages with the first heating zone) and the last sub-element, n, of Zone 1 (the sub-element at which the hair 40 exits the second heating zone), is doubled compared to the distance between the same sub-element of both heating zones. In this case, the power ratio is chained twice, i.e. squared. Therefore, the power ratio P1 / P0 is squared when determining TExitEst using equation 14.Equation 14 used to calculate the value of TExitEst is based on approximations that the temperatures of the heating zones are constant as the hair 40 passes over the heating zones, and that the speed of the hair 40 over the heating zones is also constant. Dynamic variations in the speed of the device 1 and / or the thermal contact between the hair and the heating zones can introduce variations in the temperature gradient of the hair 40 along the zones, resulting in errors in the inferred hair temperature. Similarly, dynamic variations in the temperatures of the heating zones can also cause variations in the temperature gradient of the hair 40. Errors in the estimate of TExitEst could result in unpredictable modulation of the target temperatures for the heating zones, which can cause additional uncertainty in the power draw measurements and further errors in the estimation of TExitEst. To mitigate against these effects, the target temperatures for the heating zones may optionally only be adjusted when the determined value of TExitEst is sufficiently far from the target hair temperature (i.e. the magnitude of the difference between TEXitEst and the target hair temperature is greater than a threshold value). This results in more stable control of the temperature of the heating zones and hair 40. Optionally, the temperature of a heating zone may be determined and adjusted only when the absolute power demand for the heating zone is above a fixed threshold value (otherwise, the temperature of the heating zone is allowed to drift downwards until the power demand rises above the threshold value).Whilst the method described above with reference to Fig. 18 could be used to measure the temperature of the heating zones, this need not necessarily be the case. Alternatively, for example, one or more dedicated temperature sensors could be used to measure the temperatures of the heating zones (e.g. one or more contact sensors, or one or more far infrared sensors). However, using the heating zones themselves to determine the temperature of the hair is particularly advantageous, since a dedicated temperature sensor may only sense the temperature of the outer layer of the tress of hair 40, whereas the measurements using the heating zones are indicative of the core temperature of the hair 40. The use of dedicated temperature sensors also requires mor space on or around the styling surface, and so using the heating zones to infer the hair 40 temperature enables a particularly compact configuration. Moreover, if an optical temperature sensor was used, such sensors can become occluded with dirt and require calibration (which may drift over the lifetime of the device 1).In a further alternative, rather than using all of the transverse heating zones at one position (along the y-direction illustrated in Fig. 50) to heat the hair, one of the heatingzones could be operated as a dedicated temperature sensor. In the example illustrated in Fig. 49, Zone 1 could be operated as a dedicated heater maintained at the target hair temperature, and Zone 0 could be operated as a dedicated temperature sensor for sensing the hair entry temperature TEntry. Alternatively, Zone 0 could be operated as a dedicated heater maintained at the target hair temperature, and Zone 1 could be operated as a dedicated temperature sensor for sensing the hair exit temperature.It will be appreciated that in the example illustrated in Fig. 49, the direction of movement of the device 1 may be reversed, such that Zone 1 is the first heating zone to engage with the hair rather than Zone 0. The direction of movement of the device 1 may be inferred by determining which heating zone has the largest amount of power needed to maintain that zone at the target temperature. In the example illustrated in Fig. 49, more power will be needed to maintain zone 0 at its target temperature than to maintain zone 1 at its target temperature. Therefore, the device 1 is operable to determine that the hair 40 is moving from zone 0 to zone 1 .Whilst in equation 14 the power ratio P1 / P0 is used to estimate the temperature of the hair as it leaves the hair contacting surface, additional information could also be used to help improve the accuracy of the estimation. For example, the device 1 may comprise an accelerometer for determining an acceleration of deceleration (and therefore velocity) of the device 1 , and the acceleration / deceleration may be used to help estimate the temperature of the hair (or may be used to determine that the speed of the device 1 over the hair 40 was more variable than usual, and so the estimate of the temperature of the hair 40 may be unreliable). The acceleration, deceleration and / or velocity may be used to help correct under-heating at the start of a tress, or to help prevent overheating when the styler reaches the end of the tress. For example, the user may decrease the velocity of the device 1 as it is approaching the end of the tress, and the device 1 may therefore determine that less power is to be supplied to the heaters 1 to avoid overheating the hair 40 (since a given section of the tress will be in contact with the heating zones for a longer period of time when the device 1 is being moved more slowly across the hair 40).In practice, measurements of the power supplied for each heating zone (P0 and P1 in this example) will be subject to measurement uncertainties and jitter. Sources of uncertainty in the power measurements include: analogue to digital converter (ADC) quantisation (due to the finite resolution of the voltage and current measurements); analogue noise in the signals; pulse width modulation (PWM) quantisation (due to the finite timing resolution of zone power PWM generation); maxing out of the PWM under heavy loads; electrical noise from the power supply; instantaneous voltage drops and / or ringing in the power cable; thermal cross-coupling between zones; and transient loading effects (e.g. inconsistent thermal contact between the heating zones and the hair 40). Therefore, smoothing may be applied to the power measurements, in order to improve the estimate of the hair temperature (and to reduce large fluctuations in the estimate of the hair temperature which could cause instability in the control of the amount of power supplied for each of the heating zones).To achieve such smoothing, when the device 1 comprises two opposing heaters (e.g. as illustrated in Fig. 2) the power determined to maintain a zone at the target temperature and the power determined to maintain the opposing zone at the target temperature may be summed. For example, referring to Fig. 50, the power needed to maintain zone (0,0,1) at its target temperature and the power needed to maintain zone (1 ,0,1) at its target temperature may be summed. Similarly, the power needed tomaintain zone (0,1 ,1) at its target temperature and the power needed to maintain zone (1 ,1 ,1) at its target temperature may be summed, and so on.Alternatively, or additionally (and including devices for which there is no opposing heater), the mean power for zone 0 and zone 1 which span the transverse direction across the heater may be determined for a set of the heating zones. For example, referring to Fig. 50, the power determined to be needed to maintain zones (0,1 ,1), (0,2,1) and (0,3,1) at the target temperature of the heating zones may be calculated by determining the average power needed to maintain zones (0,1 ,1), (0,2,1) and (0,3,1) at the target temperature. Similarly, the power determined to be needed to maintain zones (0,1 ,0), (0,2,0) and (0,3,0) at the target temperature of the heating zones may be calculated by determining the average power needed to maintain zones (0,1 ,0), (0,2,0) and (0,3,0) at the target temperature. The subset of heating zones used to determine the average powers may be, but need not necessarily be, the subset of heating zones that are determined to be loaded with hair 40.Alternatively, or additionally, the power measurements for each heating zone may be subject to low-pass filtering. A nonlinear filter could be used, where the cutoff frequency for the nonlinear filter is higher for large amplitude input transitions, and thus helps to speed up response to large changes, but smoothes steady state signals.The determined temperature of the hair 40, TExit, as it leaves the hair contacting surface can be compared to a target hair temperature (e.g. a predetermined target hair temperature) which may be, for example, 155°C. However, it will be appreciated that any other suitable target temperature for the hair 40 may be used, and that the target hair temperature may depend on the particular styling method in use. As described above with reference to Fig. 17, resistance measurements can be used to determine the temperature of the heating zones, and the temperature of each heating zone can be controlled towards a target temperature for the heating zone by determining corresponding duty cycles and a firing sequence. In this example, if the temperature TExit is lower than the target temperature for the hair, then the target temperature for the heating zones is increased. Therefore, more power will be supplied to the heating zones, increasing the temperature of the heating zones, and increasing the temperature of the hair 40 towards the target temperature for the hair. Advantageously, by controlling the supply of power to the heating zones based on the estimated hair temperature, the target temperature that is set for a heating zone can be higher than the target temperature for the hair, which helps to more rapidly control the temperature of the hair towards the target temperature of the hair 40 without risk of damage to the hair 40. In contrast, if the target temperature for a heater of a conventional device was set to a temperature that is higher than the desired temperature for the hair 40, there would be a significant risk of overheating and damaging the hair, since the temperature of the hair 40 is unknown.If none of the heating zones are determined to be loaded with hair (e.g. based on the power needed to maintain a heating zone at a target temperature being lower than a predetermined threshold value), the temperature of the heating zones may be allowed to drift down to a predetermined idle temperature. The idle temperature may be equal to the target temperature for the hair 40 (which is a temperature that can be maintained on stationary hair without appreciable damage). Alternatively, the idle temperature may be higher than the target temperature for the hair, to ensure a more rapid heating of the hair when the user begins to pass the device 1 over the next tress.Fig. 52 shows a simplified schematic diagram, similar to Fig. 16, illustrating various parameters that may be measured by the device 1 and used to control the power supplied to the heating zones. Fig. 52 schematically illustrates various methods that are performed by the device 1 , and it will be appreciated that these methods (e.g. the PID control) may be performed under the overall control of a controller of the device 1 . In this example, two heating zones (zone 0 and zone 1) across the transverse width of the heater are illustrated. As illustrated in Fig. 52, the controller 28 is configured to output a target temperature for the heating zones, which is determined based on a determined temperature for the hair and a target temperature for the hair. The controller 28 may also be provided with the maximum target temperature that can be requested for one or more of the heating zones. The controller 28 may also be provided with other sensor data, such as data from an accelerometer of the device 1 (which, as described above, may be used to control the supply of power for the heating zones). As described above with reference to Fig. 17, a duty cycle can be determined for each of the heating zones, and in this example the determined duty cycles are provided to the controller 28. The target temperature for the heating zones is used as an input for the PID control for each of the heating zones. In this example the target temperature for Zone 0 and the target temperature for Zone 1 is the same temperature, but this need not necessarily be the case. As described above with reference to Fig. 17, the output of the PID determination is used in the determination of the duty cycle for the heating zones. The determined duty cycle is determined based on the difference between the target temperature for the heating zone and a measured temperature for the heating zone (which, as described above, can be determined by suppling a transient pulse of power to the corresponding heater electrode(s)). The duty cycle may also determined based on a maximum average power that can be supplied for the heating zone and / or a maximum instantaneous power that can be supplied for the heating zone. Fig. 52 also schematically illustrates the determination of the actual power supplied to each heating zone based on the determined duty cycle and the current passing through the corresponding heater electrode.Fig. 53 shows a flow diagram illustrating an exemplary method of controlling the temperature of the heating zones, to control the temperature of the hair 40 towards a target hair temperature. In Fig 53, N is the number of heating zones along the length of the heater. Kpmin is the minimum duty cycle threshold, for determining whether a position is loaded with hair, n is the number of loaded positions detected. pO and p1 are the average duty cycles for loaded zone 0 and loaded zone 1 , respectively. p0_av and p1_av are the smoothed values of pO and p1. ratiopoWer is the duty cycle ratio (and therefore the power ratio) between zones 1 and 0. Thtr_req is the requested temperature for the heating zones. Kris a constant used to modify the value of ratiopoWer and is proportional toTerror. Koffset is a temperature assumed for the hair as it comes into contact with the heater (the hair entry temperature). ThairEst is the determined temperature of the hair as it exits the device 1. Kratiomin and Kratiomax are constant thresholds that are used to help determine the value of ratiopoWer. Kpis a constant that is multiplied by Terror as a minimum threshold for p0_av_ and p1_av. Ki-target is the target hair exit temperature. Terror is the difference between ThairEst and T-rtar et. Khyst is a hysteresis threshold for Terror, below which the value of Thtr_req is not changed. KramPis a temperature increment by which the value of Thtr_req is changed.In this example the device 1 comprises two opposing heaters (although this need not necessarily be the case), and in step S280 the duty cycles are summed for each pair of opposing zones. In the example illustrated in Fig. 50, the duty cycles for zones (0,0,1) and (1 ,0, 1) are summed, the duty cycles for zones (0, 1 ,1) and (1 , 1 ,1) are summed, and so on.In step S281 , a comparison of the summed duty cycles calculated in step S280 with the value of Kpmin is performed in order to determine which of the heating zones are loaded with hair. In other words, the determined duty cycle for a heating zone is compared to a threshold duty cycle to determine whether the heating zone is loaded with hair. This corresponds to comparing the power supplied to the heating zone to maintain the heating zone at a target temperature with a threshold power lever, to determine whether the heating zone is loaded with hair. In this example the determination of step S282 is performed only for each ‘zone 0’ (i.e. at a particular position in the transverse direction across the heater), but this need not necessarily be the case.In step S282, the power supplied for each ‘zone 0’ is summed, and the power supplied for each ‘zone T is summed, for all of the heating zones that are determined to be loaded with hair. For example, referring to Fig. 50, it may be determined that the zones that are loaded with hair are zones: (0,2,0), (0,3,0), (0,4,0), (0,2,1), (0,3,1), (0,4,1), (1 ,2,0), (1 ,3,0), (1 ,4,0), (1 ,2,1), (1 ,3,1) and (1 ,4,1). In this case, the power supplied to zones (0,2,0), (0,3,0) and (0,4,0) may be summed, and the power supplied to zones (0,2,1), (0,3,1) and (0,4,1) may be summed. Similarly, the power supplied to zones (1 ,2,0), (1 ,3,0) and (1 ,4,0) may be summed, and the power supplied to zones (1 ,2,1), (1 ,3,1) and (1 ,4,1) may be summed.In step S283 the sums for pO and p1 calculated in step S282 is normalised by dividing the sum by the number of heating zones used to calculate that sum.In step S284 the normalised sums are smoothed using a low pass filter, to determine p0_av and p1_av.In step S285, the power ratio between zone 0 and zone 1 is calculated using the equation: ratiopower = p1_av I p0_av (Equation 15)In step 286 the direction that the styler is passing over the hair is determined. In other words, it is determined whether it is zone 0 or zone 1 that first contacts a particular section of hair. If the value of ratiopoWer is less than one, then it is determined that zone 0 contacts the hair before zone 1 . In contrast, if the value of ratiopoWer is greater than 1 , then it is determined that zone 1 contacts the hair before zone 0, and in this case the value of ratioPower is inverted (the reciprocal of the original value of ratiopoWer is determined and used as the value of ratiopoWer in future calculations).In step S287, the temperature of the hair 40 as it leaves the hair contacting surface is estimated using the equation:T hairEst=T htr_req (T htr_req Koffset) * ratiopower * ratiopower (Equation 16)Alternatively, as will be described in more detail later, a neural network could be used to estimate the value of ThairEst based on the values of p0_av, p1_av, Thtr_req.In step S288 a determination is made of whether ratiopOwer > Kratiomin, and ratiopower < Kratiomax. If the result of this determination is ‘NO’, then the method proceeds to step S289a, and the temperature of the heating zones is allowed to drift downwards (in this example, by the value of Kramp in each iteration of the control loop). If the result of the determination of step S288 is ‘YES’, then the method proceeds to step S289b,in which the estimated temperature of the hair is compared to the target temperature of the hair to determine the value of Terror:Terror=Knarget ■ ThairEst (Equation 17)In step S290b, a new target temperature for the heating zones is determined based on the value of Terror. In this example, the current target temperature for the heating zones is increased by the value of (Terror * K). In other words, K is a constant that is used to control the magnitude of the change in the target temperature for the heating zones for a given difference between the estimated hair temperature and the target hair temperature.It will be appreciated that the values of the constants in the method illustrated in Fig. 53 may have any suitable values, and will depend on the particular heater configuration used. Optionally, simulated annealing may be used to determine values for one or more of those constants. In simulated annealing, the variables are initially set randomly (or semi-randomly within an acceptable range), and then each variable is incremented in random increments and tested (e.g. by measuring the temperature of the hair as it exits the device 1 using a thermal imaging camera). If the performance of the device 1 is improved using that set of parameters, then the incremented variables become the new base value for the next random iteration. Various further strategies that will be well known to the skilled person can also be used to avoid the iterative updating of the values of the constants from becoming stuck in local minima (e.g. by progressively increasing or reducing the size of the random increments).Simulated Hair HeatingThe results of the control method illustrated by the flow diagram of Fig. 53 can be visualised using a software simulation of the device 1 and virtual tress of hair 40. In the simulation, the simulated device 1 is passed over the tress of hair, and the temperature of the hair 40 at various positions is simulated. The heaters and the tress of hair can be simulated as thermal masses separated by a predetermined thermal resistance. The simulations can be performed using various different (and variable) speeds for the device 1 passing over the hair 40, enabling the results to be visualised for a variety of different user behaviours and heater control methods.Fig. 54 schematically illustrates the temperature distribution of the hair 40 after the device 1 has been passed over a section of hair for the first time (a ‘first pass’ over the hair 40). As illustrated in Fig. 54, the hair 40 has been divided into a grid of 4x16 virtual areas, and the temperature of the hair is simulated for each virtual area. The heater first comes into contact with the tress at the virtual areas at the top of the figure (in other words, the virtual areas at the top of Fig. 54 corresponds to the start of the tress). Each value in Fig. 54 indicates the temperature of that virtual area in °C. Fig. 30 shows the results of a first pass of the device 1 over the hair when the heaters are maintained at a constant temperature of 185 °C. As illustrated in Fig. 54, this results in some overheating at the start of the tress, to approximately 180 °C. Fig. 55 schematically illustrates the temperature distribution of the hair 40 after the device 1 has been passed over a section of hair for the second time (a ‘second pass’ over the hair 40), and the heaters continue to be maintained at a constant temperature of of 185 °C. As illustrated in Fig. 55, significant overheating of the hair 40 has occurred throughout the tress. Fig. 55 illustrates why the temperature of the heaters of conventional devices cannot be set significantly higher than the target hair temperature, since without controlling the temperature of the heaters based on the estimated hair temperature there is a risk of overheating and damaging the hair if thetemperature of the heater is allowed to be higher than the target temperature for the hair.Fig. 56 schematically illustrates the temperature distribution of the hair 40 using the method illustrated in Fig. 53 to control the target temperature for the heating zones based on the difference between the target exit temperature for the hair and the estimated exit temperature of the hair, and using the method illustrated in Fig. 17 to determine the temperature of the heating zones and determine the firing sequences. In this example the temperature of the heating zones varied dynamically between 155°C and 220°C, and the target temperature for the hair is 155°C. Advantageously, by virtue of the control of the target temperature for the heating zones being based on the estimated exit temperature of the hair, overheating is greatly reduced compared to the example of Fig. 55, despite the temperature of the heating zones exceeding the target temperature for the hair (which enables more rapid heating of the hair to the target hair temperature). Fig. 57 illustrates a second pass of the device 1 over the hair using the same control method as for Fig. 56. As shown in the figure, near-uniform heating of the hair 40 to temperatures very close to the target hair temperature has been achieved.As described above, advantageously the target temperature for the heating zones can be controlled based on the estimated temperature of the hair as it leaves the hair contacting surface. The change in the target temperature for a heating zone may be proportional to the difference between the target temperature for the hair and the estimated temperature for the hair. However, if the changes in the target temperature for a heating zone in each control loop are too large, then the PWM drive to the heater may become saturated, resulting in instability in the control of power to the heater(s). In other words, if the target temperature for one or more heating zones is suddenly changed by a large amount, then it may not be possible to supply sufficient power for all of those heating zones in one control iteration, to control the heating zones towards the new target temperature, without exceeding instantaneous power limits or average power limits of the heater(s). To mitigate against this, a maximum allowed change in the target temperature for a heating zone may be defined. A time period may also be configured during which, following the increase in the target temperature for a heating zone, the target temperature cannot be increased again until the period has expired.Neural NetworkAs described above, the temperature of the hair can be estimated based on the ratio of the power needed to maintain zone 0 at the target temperature for zone 0, and the power needed to maintain zone 1 at the target temperature for zone 1 (e.g. using Equation 14 or Equation 16). Alternatively, a neural network can be used to estimate the temperature of the hair as it leaves the device 1. In this example, the use of Equation 14 or 16 is replaced by the use of a network of nodes and static weights. Fig. 58 shows a simplified schematic illustration of a neural network that could be used to estimate the exit temperature of the hair 40. As illustrated in the figure, the network 200 comprises an input layer 3100, one or more hidden layers 320, and an output layer 350. Parameters that can be measured, determined or estimated by the device 1 out used as the inputs in the input layer 3100, and the output layer 350 outputs the estimated exit temperature for the hair 40. In this example the input layer comprises three nodes, and there are two hidden layers each comprising four nodes. However, it will be appreciated that the number of nodes and layers is not limited, and any suitable number of nodes and layers could alternatively be used. The variables input into the input layer 3100 for the case of two heating zones arranged across the transverse width of the heater (zone 0 and zone 1) may be, for example, the average power supplied to the first heating zone (zone 0) in a time period, the average powersupplied to the second heating zone (zone 1) in the time period, and the target temperatures for those heating zones (which may be the same target temperature, but need not necessarily be the same target temperature).It will be appreciated that the neural network can be trained, during a training phase, to output the estimated hair temperature based on the input into the input layer 3100. Methods of training neural networks of the type illustrated in Fig. 58 will be well known to the skilled person. During the training phase, the actual exit temperature of the hair 40 could be measured using any suitable sensor (e.g. an infrared camera), and the difference between the actual exit temperature and the exit temperature estimated using the neural network can be backpropagated into the network, to adjust the weights of the network.Fig. 59 shows a further example of a neural network that could be used to generate an estimate of the hair exit temperature. In this example, the inputs into the neural network are the average power supplied to the first heating zone (zone 0) in a time period, the average power supplied to the second heating zone (zone 1) in the time period, and the target temperatures for those heating zones (which may be the same target temperature, but need not necessarily be the same target temperature). However, different from the example illustrated in Fig. 58, in this example the values of those parameters at four different times are provided as inputs. In other words, in the example of Fig. 59, the current values of the parameters are used, but additional the values of the parameters at three previous times are also used as inputs. Advantageously, this enables the neural network to output the estimated exit temperature for the hair based on changes over time of the state of the heating zones. The inputs into the neural network may be referred to as a ‘multivariate time series’. In other words, in additional to the latest 3-value vector (it will be appreciated that more than three values could alternatively be used), a sliding timeline of previous vectors are also provided as input. Whilst in the example illustrated in Fig. 59 three previous sets of values are used, this need no necessarily be the case. Alternatively one previous set of values, two previous sets of values, or four or more previous sets of values may be used as inputs into the input layer of the neural network. For example, when 5 sets of previous values are used, the neural network may have 18 input nodes, one hidden layer of 7 nodes (which may have a sigmoid characteristic), and one output node. In this case the input nodes form a 6-stage, multivariate, sequential series of 3-element vectors. Whilst in this example each 3-element vector comprises the average power supplied to the first heating zone (zone 0) in a time period, the average power supplied to the second heating zone (zone 1), alternatively the duty cycle for the first zone during the time period and the duty cycle for the second zone during the time period could be used. The duty cycles may be the average duty cycle for all of the loaded zones at that position (i.e. the average duty cycle for each loaded ‘zone O’, and the average duty cycle for each loaded ‘zone T). Similarly, the average powers may be the average power supplied to each loaded ‘zone 0’ and the average power supplied to each loaded ‘zone T in the time period. Alternatively, rather than determining which heating zones are loaded and therefore to be used to generate the inputs for the neural network, alternatively the input to the neural network may comprise the power or duty cycle for every heating zone, and the neural network will therefore effectively have information regarding which zones are loaded. In this case, the neural network may be configured to output an additional indication, via the output layer, or which heating zones are loaded with hair 40 (it will be appreciated that the neural network could be trained to do this in the training phase, and which zones are actually loaded with hair could be monitored using a camera for the training phase). Similarly, the neural network may be configured to output an indication of the tress width.Fig. 60 shows a further example of a simulated temperature distribution along a tress of hair when control of the heaters using the method illustrated in Fig. 53 is performed, and Fig. 61 shows the temperature distribution when the neural network is used to estimate the exit temperature of the hair (rather than Equation 16). In the example of Figs. 60 and 61 , a movement of the device 1 over the hair 40 is simulated in which initially the heaters are stationary and engaged with the hair for a few seconds. The device is then moved over the hair at a speed of 40 mm / s. The device is then stopped part-way along the tress for a few seconds, before being moved along to the end of the tress. This corresponds to a relatively extreme situation of erratic movement of the device along the hair 40 by the user. Nevertheless, even under these challenging conditions, as illustrated in Figs. 60 and 61 both control methods achieve relatively uniform heating of the hair to around the target temperature of 155°C, without significant overheating of the hair 40.It is noted that, in additional to verifying the performance of the device 1 , the simulated temperature distributions and simulated movements of the device 1 over the hair 40 can be used to generate additional training data for the neural network. For example, training data can be generated using 300 ‘pulls’ (or any other suitable number of pulls) of the device 1 along the hair 40, with random variations in the heater to hair thermal resistance, target temperature, and hair entry temperature.Skin Contact DetectionWhen using the device 1 (e.g. during the method described above with reference to Figure 48), there is a chance that the user will make skin contact with a heating zone whilst the heating zone is heated to a high temperature, damaging the epidermal tissue of the user. Improved apparatus and methods for mitigating against this problem will now be described.In this example the device 1 is configured for detecting which of the heating zones are in contact with skin. A determination of whether a heating zone is in contact with skin can be achieved by passing a relatively small current through the corresponding heater electrode, or by using any other suitable capacitive characteristic of the heating zone. Since hair is an electrical insulator, it can be distinguished from skin in contact with the heating zones. In this example, heating zones that are determined to be in contact with skin are prevented from being heated. Advantageously, therefore, the device 1 can still be used to heat the hair 40, even when some of the heating zones are in contact with skin. For example, when the device is a curler, some of the heating zones may be in contact with the scalp of the user. The invention enables the user to position the curler near the scalp without fear of damage to the skin, since any heating zone in contact with the scalp will not be heated. The user can therefore more effectively style the hair closer to the roots, achieving better lift. The improved device 1 also enables tighter curls of hair 40 to be formed, since a user can safely hold the hair tension around the curler with their hand, without risk of skin damage.Even when the device 1 is not a curler, the disabling of the heating zones that are in contact with skin enables the user to hold the device 1 in new ways when styling or drying hair 40, which results in easier and more effective styling / drying. The device 1 may be any other suitable device 1 for drying and / or styling hair. The disabling of the heating zones in contact with skin also acts as a safety feature when the device is left unattended.Self-CapacitanceFigure 62 illustrates an example of a circuit implemented in hardware in the hair styler 1 of Figure 1 a to detect skin contact with the hair styler 1. It may be understood that whilst skin contact is depicted relative to a hand, this is for illustrative purposes only and skin contact may more generally be understood to comprise any form of skin contact.As shown in Figure 62 by way of example only, skin contact of a user 402 with the hair styler 1 is detected by capacitive coupling. For example, an array of capacitors 404 (404-1 , 404-2, etc.) - i.e., an array of touch sensors - is provided proximal to the surface of the hair styler 1 at regions corresponding to locations of the heating zones Z1-Z7 642 to detect whether a user’s skin comes into contact with the surface of those regions (hereafter referred to as ‘heat surfaces’) e.g., to detected whether the skin of a user 402 comes into contact with an electrode 406 of at least one capacitor 404-1 of the array of capacitors 404. For simplicity, only one capacitor (‘C4’) 404-1 of the array is depicted in Figure 62. By way of example only, that capacitor 404-1 (hereafter referred to as ‘a sensing capacitor’ 404-1) may be a sensing capacitor located proximal to a heat surface of the hair styler 1 .As shown in Figure 62, the sensing capacitor 404-1 comprises a transmitter / receiver (e.g., a single transceiver) electrode 406. The electrode 406 of the sensing capacitor 404-1 is connected to ground 410 via a capacitor 408 (‘C3’) and is provided with an excitation signal 412 that first passes via a pre-defined impedance 414 (‘Z1’). The excitation signal 412 is also provided to a synchronous detector 420. The transceiver electrode 406 is also connected to an amplifier 418 which is in turn connected to the synchronous detector 420.During operation, an excitation signal 412 is provided to the transceiver electrode 406 of each sensing capacitor of the array of sensing capacitors 404 for sensing a touch of a user 402. Each sensing capacitor of the array is provided proximal to a corresponding heat surface of the hair styler 1 corresponding to where the heating zones Z1-Z7 642 are located.When the user’s skin comes into contact with, or is at close proximity to a heat surface of the hair styler 1 , that skin contact results in the user forming a capacitor 403 (‘C2’) with body capacitance, parallel to the sensing capacitor 404-1 provided proximal to the heat surface. The formed capacitor 403 (‘C2’) arises because once the user 402 touches the heat surface with their skin, they become a human capacitor between the surface of the hair styler 1 and Earth 410. By the user 402 becoming a capacitor of the electrical circuit that is parallel to the sensing capacitor 404-1 , the apparent capacitance of the overall circuit increases, thereby altering the electrical signal output by the transceiver electrode 406. For example, by increasing the overall capacitance of the circuit of Figure 62, the electrical signal output by the transceiver electrode 406 may have a lower frequency and / or a lower voltage than that of the excitation signal 412 provided to the transceiver electrode 406.The electrical signal output by the transceiver electrode 406 is subsequently provided to the synchronous detector 420 to detect whether a user’s skin is touching a heat surface of the hair styler 1. The synchronous detector 420, for example, may receive both the electrical signal output from the transceiver electrode 406 and the initial excitation signal 412 as a reference signal to determine changes between the initial excitation signal 412 and the electrical signal output from the transceiver electrode 406. A signal is then output from the synchronous detector 420 that indicates that the user 402 is touching a heat surface of the hair styler 1 with their skin; for example, the signal output from the synchronous detector 420 may be analysed / processed by aprocessor of the hair styler 1 to determine that the user 402 is touching a heat surface proximal to the touched sensing capacitor that is being touched with their skin.Operation of the synchronous detector 420 is disclosed in further detail below with reference to Figure 64.Prior to the output electrical signal being provided to the synchronous detector 420, the signal may first be passed through an amplifier 418 to amplify the signal from the transceiver electrode 406.Although Figure 62 illustrates hardware for detecting skin contact with the hair styler 1 via capacitive coupling it will be appreciated nevertheless that the hardware could be reconfigured for detecting skin contact with the hair styler 1 via resistive coupling. By way of example only, in such hardware a circuit with a pressure sensor may be provided that detects skin contact based on pressure being applied to the pressure sensor.Such pressure sensors typically comprise two resistive coated layers with a gap or spacing layer therebetween. The top resistive coated layer is semi-flexible (e.g., a flexible plastic) and may be deformed via the application of pressure. The gap or spacing layer typically consists of air or inert gas and / or spacers. During a touch event by a user, the top resistive coated layer is deformed and / or bent to make contact with the lower resistive coated layer (e.g., glass), thereby closing an electrical circuit. During operation, a small voltage is applied across one of the two resistive coated layers. Upon completion of the circuit between the resistive coated layers a change in resistance and voltage across the circuit can be detected which is interpreted as a touch.Mutual CapacitanceFigure 63 illustrates another example of a circuit implemented in hardware in the hair styler 1 of Figure 1a to detect skin contact with the hair styler 1 .As shown in Figure 63, by way of example only, skin contact of a user 402 with the hair styler 1 is detected by capacitive coupling. For example, an array of capacitors 404 (404-1 , 404-2, etc.) - i.e., an array of touch sensors - is provided proximal to the surface at regions corresponding to locations of the heating zones Z1-Z7 642 of the hair styler 1 to detect whether a user’s skin comes into contact with the surface of those regions (hereafter referred to as a ‘heat surfaces’) e.g., to detected whether the skin of a user comes into contact with an electrode 406 of at least one capacitor 404-1 of the array of capacitors 404. For simplicity, only one capacitor (‘C4’) 404-1 of the array is depicted in Figure 63. By way of example only, that capacitor 404-1 (hereafter referred to as ‘a sensing capacitor’ 404-1) may be a sensing capacitor located proximal to a heat surface of the hair styler 1 .As shown in Figure 63, the sensing capacitor 404-1 proximal to a heat surface (e.g., a heat surface corresponding to the location of heating zone Z1 , or the like) of the hair styler 1 comprises a separate transmitter electrode 406a and receiver electrode 406b. The transmitter electrode 406a of the sensing capacitor 404-1 is connected to ground 410 via a capacitor 408 (‘C3’) and is provided with (connected to) an excitation signal 412. The excitation signal 412 is also provided to (connected to) a synchronous detector 420. The receiver electrode 406b of the sensing capacitor 404a is connected to an amplifier 418 which is in turn connected to the synchronous detector 420.During operation, an excitation signal 412 is provided to the transmitter electrode 406a of each sensing capacitor of the array of sensing capacitors 404 for sensing a touch of a user. Each sensing capacitor of the array is provided proximal to a corresponding heat surface of the hair styler 1 corresponding to where the heating zones Z1-Z7 642 are located.During use, when the user’s skin comes into contact with a heat surface of the hair styler 1 (e.g., heat surface corresponding to the location of heating zone Z1 , or the like), that skin contact results in a change in the charge stored on the sensing capacitor 404-1. Specifically, when the user’s skin comes into contact with the heat surface, charge is transferred from the sensing capacitor 404-1. By removing charge from the sensing capacitor 404-1 , the overall apparent capacitance of the circuit of Figure 63 is decreased, and thus the electrical signal output by the receiver electrode 406b may have a higher frequency and / or a higher voltage than that of the excitation signal 412 provided to the transmitter electrode 406a.Nevertheless, it will be appreciated that the circuit of Figure 63 may be suitably configured such that when the user’s skin comes into contact with a heat surface of the hair styler 1 , the electrical signal output by the receiver electrode 406b has a lower frequency and / or a lower voltage than that of the excitation signal 412 provided to the transmitter electrode 406a. It will be appreciated that whether an increase or a decrease in frequency and / or voltage occurs in response to a user’s skin coming into contact with a heat surface of the hair styler 1 is dependent on the layout of the electrodes and the user’s capacitive coupling to Earth 410.The electrical signal output by the receiver electrode 406b is subsequently provided to the synchronous detector 420 to detect whether a user’s skin is touching a heat surface of the hair styler 1. The synchronous detector 420, for example, may receive both the electrical signal output from the receiver electrode 406b and the initial excitation signal 412 as a reference signal to determine changes between the initial excitation signal 412 and the electrical signal output from the receiver electrode 406b. A new signal is then output from the synchronous detector 420 that indicates that the user 402 is touching the heat surface A with their skin; for example, the new signal output from the synchronous detector 420 may be analysed / processed by a processor of the hair styler 1 to determine that the user 402 is touching the heat surface A with their skin.Beneficially, by allowing the detection of skin contact of heat surfaces of the hair styler 1 as described above with reference to Figures 62 and 63, reliable detection of skin contact of hot surfaces of the hair styler 1 can be achieved even if only a square millimetre of flesh comes into contact with the hot surface.Operation of the synchronous detector 420 is disclosed in further detail below with reference to Figure 64.Prior to the electrical signal output by the transceiver electrode 406 being provided to the synchronous detector 420, the signal may first be passed through an amplifier 418 to amplify the signal from the transceiver electrode 406.Although Figure 63 illustrates hardware for detecting skin contact with the hair styler 1 via capacitive coupling it will be appreciated nevertheless that the hardware could be reconfigured for detecting skin contact with the hair styler 1 via resistive coupling.By way of example only, in such hardware a circuit with a pressure sensor would be provided that detects skin contact based on pressure being applied to the pressuresensor. Such pressure sensors typically comprise two resistive coated layers with a gap or space layer between them. The top resistive coated layer is semi-flexible (e.g., a flexible plastic) and may be deformed via the application of pressure. The gap or space layer typically consists of air or inert gas and spacers. During a touch event by a user, the top resistive coated layer is deformed and / or bent to make contact with the lower resistive coated layer (e.g., glass), thereby closing an electrical circuit. During operation, a small voltage is applied across one of the two resistive coated layers. Upon completion of the circuit between the resistive coated layers a change in resistance and voltage across the circuit can be detected which is interpreted as a touch.Response to touch detectionIrrespective of whether the self-capacitance detection scheme of Figure 62 or the mutual capacitance scheme of Figure 63 is implemented in the hair styler 1 , in response to determining that the user 402 is touching a heat surface with their skin, the processor of the hair styler 1 may be configured to trigger and / or send an appropriate command message to control a function / action of (e.g., a temperature of, and / or a power provided to) at least the heating zone corresponding to the location of the heat surface (e.g., heating zone Z1 when the heat surface being touched is proximal to heating zone Z1). It will be appreciated that controlling a function / action of (e.g., a temperature of, and / or a power provided to) a heating zone corresponding to the location of the heat surface may comprise controlling a power provided to a heat electrode or the like (e.g., heater 6) of the heating zone that is configured to heat the heating zone.For example, in response to determining that the user 402 is touching a heat surface with their skin, the processor of the hair styler 1 may be configured to send an appropriate message and / or signal to drive circuitry 23 to control a power provided to a heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched.By way of example only, control of the power provided to the heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched may include, controlling a power to the heater 6 (i.e. , an electrode of a heater responsible for heating a heating zone of the hair styling appliance) to reduce a temperature of the at least one heating zone to a temperature below a ‘safe touch’ temperature; that is to say, any temperature for which it is safe for a user’s skin to touch the heater for a defined period of time. For example, a safe touch temperature may be a temperature below 60 degrees Celsius. Nevertheless, it will be appreciated that a safe touch temperature may be defined as any temperature T for which it is safe for a user’s skin to touch the heater for a period of s seconds.In another example, control of the power provided to the heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched may include, reducing the power provided to the independently powerable heater electrode to a power level corresponding to a heating zone temperature of below a touch safe temperature as defined above.In yet another example, control of the power provided to the heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched may include, controlling the power provided to the independently powerable heater electrode by preventing power being provided to the independently powerable heater electrode. For example, in response to detecting that a user’s skin is touching a heating zone heated by a heater electrode of the heater 6 that defines the heatingzone, power to that heater electrode may be switched off until skin contact is removed, thereby preventing the heater electrode heating up (further).Synchronous detectorFigure 64 illustrates an example synchronous detector 420 implemented in hardware in the hair styler 1 of Figure 1a.As shown in Figure 64, the synchronous detector 420 comprises numerous resistors, capacitors, amplifiers, op-amp amplifiers, low-pass filters, and switches. During operation, the output signal from the amplifier 418 (see Figures 5 & 6) is combined with (e.g., multiplied by) the initial excitation signal 412 to produce an input signal 602 (‘Vin’) for the synchronous detector 420.Figure 65a depicts an example an input signal 602 (‘ jn’) for the synchronous detector 420. As show in Figure 65a, the input signal 602 comprises two sine waves, a first sine wave 702 with a first frequency, and a second sine wave 704 with a second frequency that is greater than the first frequency. The second wave 704 with the higher frequency is often referred to as a ‘carrier wave’ that ‘carries’ the first sine wave 702. The first sine wave 702 for example, may correspond to the output signal from the amplifier 418 (see Figures 62 & 63), while the second sine wave may correspond to the original excitation signal 412.Retuning to Figure 64, at the synchronous detector 420, the input signal 602 is passed through an AC coupling circuit (e.g., a low-pass filter and / or AC op-amp integrator circuit) 603 comprising: an op-amp 604, a first resistor 606-1 , and the first capacitor 608-1 in parallel with one another, and a second resistor 606-1 , and a second capacitor 608-1 in serious with the first resistor 606-1 .The AC coupling circuit is designed to remove any signal from input signal 602 above a specific (predefined) frequency threshold, wherein the specific frequency threshold is defined by the properties (e.g., resistance, capacitance etc.,) of the components of the AC coupling circuit. For example, the low pass filter 603 may be configured such that the wave of Figure 65a is filtered to remove all high frequency components (e.g., the second sine wave 704).Having been filtered by the low pass filter 603, the filtered output signal is passed down Paths A and B of the circuit. Path A comprises a changeover switch 612, a low- pass filter / integrator 616, and a DC amplifier circuit 618 comprising a DC amplifier 620, and a third resistor 606-3 grounded through resistor 606-4. A third capacitor 608-3 is also provided after the low pass filter 603 and which connects to ground. Path B comprises a synchronous inverter (‘X1 invertor’) 610, the changeover switch 612, the low-pass filter / integrator 616 and the circuit 618 comprising a DC amplifier 620, and a third resistor 606-3 grounded through resistor 606-4. The synchronous inverter 610 inverts the signal Vjnby multiplying it by -1 , thus peaks of the signal Vjnbecome troughs, and the troughs of signal Vjnbecome peaks.The changeover switch 612 is controlled by a clock 614 that is synchronous with the Vin signal i.e., the clock 614 switches the changeover switch between Path A and Path B at the same frequency as the Vnsignal. As the changeover switch switches between Path A and Path B at the same frequency as the V;nsignal, only positive voltage features of the V;nsignal are passed on to the low-pass filter / integrator 616.Figure 65b illustrates an example resultant signal passed to the low filter / integrator 616. The resultant signal is passed through the low-pass filter / integrator 616 to filterout high-frequency noise, and to convert the signal into a steady, DC output proportional to the amplitude of the resultant signal passed to the low-filter / integrator 616. It will be appreciated that the steady, DC output may be a digitized signal. The steady, DC output is then subsequently passed through DC amplifier circuit 618 to amplify the DC signal for output at 706 (‘Vout’).Figure 65c illustrates an example output signal (‘Vout’) 706 from the synchronous detector 420. That signal is a DC signal whose existence indicates the occurrence of skin contact with a heat surface of the hair styler 1 .In response to the DC signal, the processor 28 of the hair styler 1 may process the signal and determine that a user 402 is touching a heat surface of the hair styler 1 with their skin. In response to determining that a heat surface of the hair styler 1 is being touched by a user 402, the processor may be further configured to trigger and / or send an appropriate command message to control a function of (e.g., a temperature of, and / or a power provided to) at least one heating zone 642 of the hair styler 1 corresponding with the touched heat surface.Quadrature Synchronous DetectorIt will be appreciated that the synchronous detector described above, and implemented in the self-capacitance and mutual capacitance topologies described with reference to Figures 62 and 63, is only capable of detecting skin contact with heat surfaces of the hair styler 1 by detecting differences between an excitation signal provided to an electrode of a capacitor proximal to the heat surface, and the response signal from the capacitor upon a user touching the heat surface.The coupling that occurs between the capacitor and the user when they touch the heat surface, however, as well as causing changes in the amplitude (e.g., voltage) and / or frequency of the response signal, may also cause the response signal to be phase shifted with respect to the original excitation signal. It is therefore beneficial to also be able to detect changes in phase between the response signal and the initial excitation signal.Figure 66 illustrates an example quadrature synchronous detector 800 that may be implemented in hardware in the hair styler of Figure 1a. It will be appreciated that the example quadrature synchronous detector 800 may be implemented in the hardware in place of the synchronous detector 420.As shown in Figure 66, the quadrature synchronous detector 800 comprises numerous components including two synchronous detectors 420-1 , 420-2. During operation, the output signal from the amplifier 418 (see Figures 6 & 7) is combined with (e.g., multiplied by) the initial excitation signal 412 to produce an input signal 602 (‘Vn’) for the quadrature synchronous detector 800. Figure 8a depicts an example input signal 602 (‘Vin’) for the quadrature synchronous detector 800.At the quadrature synchronous detector 800, the input signal 602 is passed down Paths ‘A’ and ‘B’ of the circuit. Path ‘A’ comprises a first synchronous detector 420-1 (as described above with reference to Figure 64) and path ‘B’ comprises a second synchronous detector 420-2 (described above with reference to Figure 63). Both the first synchronous detector 420-1 and the second synchronous detector 420-2 are clocked using clock 802 such that they are phase shifted from one another by 90° (TT / 2 rads) such that an output signal from the first synchronous detector 420-1 is an output signal with only positive voltage features of the Vnsignal as described above with reference to Figure 64, and an output signal from the second synchronous detector 420-2 is an output signal with only positive voltage features of the Vnsignal asdescribed above with reference to Figure 64, but which is phase shifted by 90° (TT / 2 rads). Those two output signals thus constitute the real (‘Vout(reai)’) and imaginary ('Vout(imaginary)’) amplitude components of the detected signal from the first and second synchronous detectors 420-1 , 420-2, respectively.Those output signals are subsequently passed onto a processor 28 of the hair styler 1 to combine both the real (‘Vout(reai)’) and imaginary (‘Vout(imaginary)’) amplitude components of the detected signal to produce a single magnitude value in software.Real-world ScenariosAbove two possible topologies (self-capacitance & mutual capacitance) are described that may be used to detect the presence of a user’s skin on heat surfaces of the hair styler 1. In either topology, a synchronous detector or a quadrature synchronous detector may be implemented.Figures 67a and 67b illustrate two example real-world scenarios of use of the hair styler 1 described above. As is shown in Figures 67a and 67b, the hair styler 1 may be used by a user on their own (Figure 67a), alternatively, the hair styler 1 may be used by a stylist who uses the hair styler 1 to style a customer’s hair (Figure 67b). In the scenario where a user uses the hair styler 1 on themselves, skin contact of the user may be detected in response to capacitive coupling, where the user becomes a capacitor connected to ground. In the scenario where a stylist uses the hair styler 1 to style a customer’s hair, both capacitive and resistive coupling may arise. The coupling to Earth between a stylist and a client thus can be complex and arbitrary. It will be appreciated therefore that it cannot be assumed that a client and stylist are not directly in contact with each other, or not in direct contact with a metallic connection to Earth, or that a home user is sitting in complete isolation when implementing the two example topologies described above.Multizone Touch DetectionIn all the examples described above, for the purposes of simplicity only, skin contact detection is described with reference to a single capacitive touch sensor proximal to a single heat surface of the hair styler 1. For example, in Figures 62 and 63 only one touch sensor capacitor 404-1 is shown, which is described as proximal to a heat surface of the hair styler 1. However, it will be appreciated that the hair styler 1 comprises multiple heat surfaces (corresponding to the multiple heating zones Z1-Z7), and that depending on the desired level of granularity of detection desired, there may be one touch sensor capacitor per heating zone, or multiple touch sensor capacitors per heating zone. For example, each heating zone may itself be divided up into smaller sub-zones, with a touch sensor capacitor in each sub-zone.Irrespective of whether there is one touch sensor capacitor per zone, or multiple however, there may be a need to adapt the topologies of Figures 62 and 63 to allow a single excitation signal to be sent to different touch sensor capacitors, rather than having to implement multiple excitation signal generators to simplify the overall detection system. One such way of using a single excitation signal for the plurality of touch sensor capacitors is to implement a multiplexer.Figure 68 illustrates an example 1-to-8 multiplexer arrangement 900 for providing the same excitation signal to eight capacitor electrodes that may be implemented in the topologies of Figures 62 and 63.As shown in Figure 68, an excitation signal 412 may be generated by a single signal generator which is input to a 1-to-8 multiplexer 902. Upon entry to the 1-to-8 multiplexer 902 the signal may be provided to any one of the 8 capacitor Tx electrodes(X(1)...X(8)) of a touch sense capacitor 404 (e.g., a Tx electrode X(1) of touch sense capacitor 404-1) that is proximal to a heat surface of the hair styler 1 , depending on the inputs provided to the Tx address line. For example, where the Tx address line allows the 3 inputs SO, S1 , and S2, the mapping of X to a specific one of the 8 capacitor Tx electrodes (X(1)...X(8)) using inputs SO, S1 , and S2 may be implemented as shown in Table I below.Table I: Mapping between 1-to-8 multiplexer Tx address line inputs and multiplexer output.Similarly, the Rx electrodes of the 8 touch sense capacitors 402 may be connected to an 8-to-1 multiplexer 904 that allows response signals from the 8 different touch sense capacitors to be fed to the synchronous detector 420 (or the quadrature synchronous detector 800) depending on the inputs provided to the Rx address line. For example, where the Rx address line allows 3 inputs SO, S1 , and S2, the signal output from the 8-to-1 multiplexer may be controlled using inputs SO, S1 , and S2 as shown in Table II below.Beneficially, by using a separate multiplexer for Tx and Rx electrodes of the touch sense capacitor 402 individual selection of both Tx and Rx channels is provided, which in turn means extra information can be deduced. For example, noncorresponding electrode pairs could be activated to provide extra information about the shape and proximity of the operator’s hand.Furthermore, the Rx and Tx electrodes of the touch sense capacitors may also be formed into interlocking patterns of electrodes on the heat surfaces of the hair styler 1 to ensure a touch from even a small area of skin would cause a skin contact detection event to occur.Alternatively, to simplify the circuitry, the Tx or the Rx electrode may be ‘commoned’ thereby allowing one of the multiplexers to be omitted from the circuitry. However, in such an arrangement isolation between the multiple channels would not be as clean, as stray capacitance between the Tx electrodes and the Rx electrodes may arise.It will be appreciated that the 1-to-8 multiplexer described above is by way of example only and that other multiplexer configurations (e.g., 1-to-4) are possible.Table II: Mapping between 8-to-1 multiplexer Rx address line inputs and multiplexer output.Capacitor Touch Sensor-HeaterIn the above-described examples, the hair styler 1 has an array of capacitors 404 (404-1 , 404-2, etc.) that are provided proximal to the surface at regions corresponding to locations of the heating zones Z1-Z7 642 of the hair styler 1 to detect whether a user’s skin comes into contact with the surface of those regions ( / .e., heat surfaces). Those capacitors 404 are separate and distinct components of the hair styler 1 from the heaters 6. However, it will nevertheless be appreciated that it may be beneficial to make use of conductive layers of the heaters 6 to detect whether a user’s skin comes into contact with the heat surfaces.As shown in Figure 69, rather than providing an array of capacitors 404, a heater track 1102 of each heater electrode ( / .e., an independently controllable / powerable heater electrode) may be adapted to so as to form a capacitive transmitter electrode of a sensing capacitor 1100. For example, a heater track 1102 of each heater electrode may be adapted through the inclusion of an appropriate touch sensor layer 1104 nearby (e.g., on top of) the heater track and which acts as a transmitter electrode.That transmitter electrode 1102, may by way of example only, be similar to the transmitter electrode 406a of the sensing capacitor 404-1 shown in the ‘Mutual Capacitance’ arrangement of Figure 63.In this scenario the power supply to the heater track 1102 of each heater electrode should be able to be isolated from the heater track 1102 by suitable switches. It will be appreciated that this may be necessary given that the voltage provided to the heater electrode to heat the heater may be significantly higher than the voltages associated with an excitation signal provided to the transmitter electrode of the sensing capacitor 1100. To allow isolation of the heater electrode from the higher voltage power supply 1108 for heating the electrode during the periods when anexcitation signal may be provided to the transmitter electrode, a first switch 1106a may be provided on the low voltage side of the power supply 1108 (e.g., heater PSU). Additionally, a second switch 1106b may also be provided on the low voltage side of the power supply 1108 (e.g., heater PSU).Alternatively, the transmitter electrode 1102, may by way of example only, act as a shield layer beneath an appropriate touch sensor layer 1104. In this scenario, the arrangement may correspond with the ‘Self Capacitance’ arrangement of Figure 62. In this scenario, the power supply to the heater track 1102 of each heater electrode should also be able to be isolated from the heater track 1102 by suitable switches as described above.As shown in Figure 69, it will be appreciated that the appropriate touch sensor layer 1104 may be connected to an excitation signal generator 412 for providing an excitation signal to the touch sensor layer similar to the scenarios described above with reference to Figures 62 and 63.The output signal in response to a user touching the touch sensor layer 1104 may then be processed in a similar manner to that described above with reference to Figures 62 and 63 in order to detect whether a user is touching the touch sensor layer 1104, and to control the power provided to the heater track 1102 of each heater electrode accordingly.Further Examples of Arrangements of Heating ZonesFigures 71a and 71 b show schematic views of different arrangements of heating zones 642 applicable to any of the above-described examples, where appropriate. Figure 71a shows an arrangement corresponding to that of Figures 12a and 12b, in which the heating zones 642-1 to 642-10 are arranged along the y-direction only. Figure 3c shows an alternative arrangement, in which heating zones 642-1 to 642-16 are arranged in both the x- and y-directions. Such an arrangement of heating zones 642 can be provided by arranging two sets of heater electrodes 64 like those shown in Figure 12a side by side in the width (x-) direction.The heaters 6 may be separated in this way into any number of heating zones 642, and may comprise any number of heating zones along the x- and y-directions. In particular, whilst Figure 71 b shows two zones along the x-direction, a greater number of zones in the y-direction could also be provided, with the heater electrodes and their electrical connections being designed in a suitable manner to fit such an arrangement. The heating zones 642 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce hot / cold spots when using very low thermal mass heaters 6 such as those shown in Figures 12a to 12c. It may be generally understood that the arrangement of the heater electrodes 64 may comprise any suitable layout to enable the desired arrangement of heating zones 642Heating Zone SizingOne issue with low thermal mass heaters 6 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 40 onto the heaters 6, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 6 to ensure that all regions on the hair contacting surface can be retained within and / or recovered back to the desired operating temperature limits. The low thermal mass heaters 6 described above are relatively thin and the dielectric layers are formed of materials with relativelylow thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 64 running across the whole length and whole width of the heater 6, then when more power is supplied to the heater 6 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, or cause the overheated regions to burn relatively small bundles / strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and / or by increasing the thicknesses of the layers that constitute the heater and / or by dividing the heater 6 into multiple separately powered and controlled heating zones 642 across its length or its length and width. Increasing the thickness of the layers increases the thermal mass of the heater 6 which is undesired and there are limited materials that have the required dielectric strength and high thermal diffusivity (and which are available for use in mass produced consumer products). Therefore, the inventors have divided the heaters 6 up into plural heating zones. These heating zones can be equally and / or unequally sized and can be arranged regularly and / or irregularly across the width and length of the heater.However, overheating can still occur within a single heating zone. For example, if half of the heating zone is loaded with hair (which is assumed to be the realistic worst case scenario during operation) and the other half is not loaded with hair, then the half that is loaded with hair will cause the temperature of that part of the heating zone to drop which will cause more power to be applied to that heating zone in its entirety. That applied power will bring the average temperature of the heating zone back up to the desired operating temperature, but the unloaded part of the heating zone will be above the average temperature of the heating zone. This temperature increase may be sufficient to cause the unloaded part to overheat. At the same time the loaded part of the heating zone will be below the average temperature causing a reduction in heat transfer and reduced styling performance. This situation is illustrated in Figure 72, which shows a tress of hair 40 overlying heating zones Z2, Z3 and Z4, with heating zone Z3 being fully loaded with hair and heating zones Z2 and Z4 being only partially loaded with hair. This problem can be reduced by making the heating zones very small - but that is costly due to all the connections needed to connect each heater electrode for each heating zone back to the drive circuitry 23 as well as the number of control switches in the drive circuitry 23 needed to control the powering of each heater electrode 64. The inventors have found that for a given permitted maximum temperature within the heater, a maximum size of the heating zones can be defined which depends on the maximum power density to hair that can be extracted from the heating zone and the material characteristics and thicknesses of the layers forming the heating zone.Specifically, if it is assumed that only one half of a heating zone 642 is loaded with hair, upon loading with hair, the maximum temperature that occurs in the unloaded half of a heating zone can be defined with the equation below:where,Tmax= maximum temperature (°C) on the surface of the heater which would occur in the unloaded half (worst case) of an individual heating zone;Trar= target operational temperature or average temperature (°C) of individual heating zones;9 = power density (Wm-2) required to heat hair passing over the surface to the desired temperature for styling;W = width of a heating zone measured perpendicular to the motion of hair over the surface; t = total thickness of the layers that constitute the heating zone; and= the thickness averaged thermal conductivity of the layers that constitute the heating zone.If it is assumed that the thickness averaged thermal conductivity of the constituent layers of a heating zone 642 and the total thickness of the layers that form the heating zone 642 are known and fixed (for any given device), then the above equation can be used to determine the required zone width (W) and hence a number of divisions along the length of the heater that will prevent overheating of the unloaded halves, when their other halves are loaded with hair, and more power is supplied to maintain and / or recover the hair contacting surface temperatures back to the desired operating limits. Consequently, for a given surface area that must be covered with the considered heater technology, the equation above can be used to determine the number of heating zones that should be positioned along the length of the given surface area, so that each heating zone 642 can be operated without exceeding the maximum operating temperature of the heater materials and without causing the temperature of the unloaded part of a heating zone 642 to exceed the maximum temperature (ATmax) that could cause burning of relatively small bundles / strands of hair that come in contact with such overheated regions of the heating zone.Specifically, the required divisions along the length can be determined from:Where,L = length of the heater plate (perpendicular to the direction that hair typically travels across the surface);= number of zonal divisions along the length of the heater plate;Tmax= maximum permitted temperature (°C) on the surface of the heater (which would occur in the unloaded half (worst case) of an individual heating zone) needed to avoid damage to hair or the heater;TTat= target operational temperature or average temperature (°C) of individual heating zones; fe = the thickness averaged thermal conductivity (Wm’1oC’1) of the layers that constitute the heating zone; t = total thickness of the layers that constitute the heating zone; and£? = power density (Wrrr2) required to heat hair passing over the surface to the desired temperature for styling.For a hair styling device, the inventors have found the following suitable ranges for these parameters:• Power density required for styling (q ) is greater than 40,000 W / m2and less than 100,000 W / m2.• The average thermal conductivity of the layers forming the heating zone k (averaged through the depth of the various layers) is between 80 and 200 W / m.K.• The maximum permitted temperature of a heating zone to manage (ideally avoid) hair damage is less than 250°C, more preferably less than 220°C and most preferably less than 200°C.• The total thickness of the layers (t) which make up the heater is less than 300pm but no less than 75pm due to manufacturing limitations.• The target operational temperature of the heater (Trar) is between 150°C and 230°C.Operating within these ranges, the inventors have found that the required number of heating zones per unit length (cm) along the length of the heater is between 0.6 and 2.5 per cm which is equivalent to a zone width (in the lengthwise direction of the heater) of between 0.4 cm and 1 .7 cm.Of course, this is for the case of there not being multiple zones in the width direction of the heater as well (e.g. this is for the single row case shown in Figure 71a). If multiple rows of heating zones 642 are provided along the length of the heater (such as is shown in Figure 71 b), then each row of heating zones 642 should meet the limits defined above if the above described overheating problem is to be avoided.Further Alternative Heater ArrangementAn alternative flexible heater 6’ is illustrated in Figure 73, which shows on the lefthand side an exploded cross-sectional view of the heater 6’ and substrate 68’ and on the right-hand side a perspective view of the heater 6’ and substrate 68’. As shown in Figure 73, the heater 6’ has curved edges 72-1 and 72-2 that are shaped to match the shape of an upper surface 74 of the rigid support substrate 68’ so that the flexible heater 6’ can be bonded securely using an adhesive or diffusion bonding of the underlying materials to the upper surface of the rigid substrate 68’. The curved edges of the heater 6’ can be formed, for example, using a heat forming process. Figure 73 also illustrates that one or more surface mounted electronic components 76 may be attached to an underside of the heater 6’. These components may be, for example thermistors for sensing the temperature of the heating zones 642 of the heater 6’. Figure 73 also shows a control printed circuit board (PCB) 78 that carries the drive and control electronics 15 illustrated in Figure 7 that controls the heating of the different heating zones 642 of the heater 6’.As before, the heater 6’ is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 150 pm. The different layers forming part of the heater 6’ are shown in exploded cross-sectional and perspective views in Figure 74. A description of each layer is given below.Low Friction Coating 81 (Optional)This is an optional layer and can be added to create a smooth, low friction surface to enhance the user experience by making the heater 6’ feel less grippy against the hair. This layer would be as thin as possible (for example, between 1 and 150 pm) to reduce the thermal resistance from the heater 6’ to the hair, whilst still being sufficiently durable and scratch resistant.This layer would typically be applied last, possibly as a spray coating (e.g. Cerasol), after the rest of the heater 6’ has been produced and assembled around the rigidifying substrate 68’. This is needed because the coating is prone to cracking when flexed,and once applied the coating will reduce the natural flexibility of the heater, and so it should be applied once the heater 6’ has been formed into its final shape.Alternatively, this coating may itself comprise multiple layers including, for example, a primer layer (of about 6pm), a base coat layer (of about 25pm) and a topcoat layer (of about 10pm).Heat Spreading Laver 82 (optional)This is also an optional layer and, when provided, helps to spread the heat within each heating zone 642 to ensure that the temperature of individual heating zones 642 is able to maintain an acceptable degree of homogeneity during typical use. As discussed above, if a heating zone 642 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 642 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot region to the cold region. This problem is exacerbated by the anisotropic thermal characteristics of the serpentine like heater electrodes 64, and by the fact the control electronics would typically work to maintain an “average” temperature within the heating zone 64 based on the overall resistance of the heater electrode that forms the heating zone 642 - from the perspective of the control electronics, the heating zone 642 would be at the “correct” temperature despite having hot and cold regions.Each heating zone 642 would have its own heat spreader 91 , which is thermally separated (there is a high thermal impedance / low thermal conductivity) from the heat spreaders for adjacent zones. This is desirable to prevent heating zones 642 from heating neighbouring heating zones 642 which might otherwise increase power consumption, reduce warm up time, and complicate algorithms based on zonal power consumption by adding crosstalk. Figure 75 illustrates an example form of the heat spreader layer 82. As shown, in this example there are 20 heat spreaders 91-1 to 91- 20, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 91 is separated from its neighbouring heat spreaders 91 and in effect forms an island of thermally conductive material over the corresponding heating zone that substantially does not touch neighbouring heat spreaders to reduce heat spreading from one heating zone to an adjacent heating zone. This also helps signal to noise for sensing and the independent control of the different heating zones. Alternatively, those islands may be electrically interconnected but thermally decoupled by minimising the area of contact between neighbouring islands (as opposed to ensuring they do not touch each other at all). For example, it may be desirable to provide an electrical connection between neighbouring heat spreaders - for example to ground the heat spreading layer. In this case the individual heat spreaders may have some conducting material connecting them with at least some of their neighbouring heat spreaders. Even though an electrical connection is provided between adjacent heat spreading elements, as long as the connection is relatively small (for example less than 1 / 1 Oth of the length / width of the heat spreader), there will still be, in effect, a thermal break or decoupling between neighbouring heat spreaders.The heat spreaders 91 may be separated from each other by a solid material having a thermal conductivity lower than 35 W / mK or they may be separated by air. The heat spreaders 91 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 91 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 91 , it is difficult for heat from one heating zone 642 to pass into neighbouring heating zones 642. The solid material that is provided in the gap between adjacent heatspreaders 91 may be provided by a coating or a wash that is applied to the heat spreading layer 82 after the etching process has formed the gaps between adjacent heat spreaders 91 and may be the coating layer 81 described above. Alternatively other suitable methods may be used to form substantially or fully physically separated individual heat spreaders 91 and any suitable method may be used to provide solid material in the gaps between individual heat spreaders 91 , such as masking and vapour deposition, etc.This layer 82 can provide mechanical integrity to the overall heater 6’, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes 64, which in turn could lead to short circuits or loss of functionality.Polyimide Separator layer 83The polyimide separator layer 83 provides electrical isolation between the hair contacting surface of the heater 6’ (which may be the upper surface of this layer 83 if the optional layers 81 and 82 are not provided) and the main heater electrode layer. This layer 83 would have as low thermal resistance as possible whilst still achieving the dielectric requirements of the layer. As the name suggests, this layer is formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, the in-plane thermal diffusivity or thermal conductivity of this layer (in a plane perpendicular to its thickness) is quite low (less than 35 W / mK). This helps to prevent heat spreading from one heating zone 642 to an adjacent heating zone 642.Main Heater Electrode & Sensing Laver 84This layer 84 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries).This layer 84 comprises a number of independently controllable heater electrodes 64 each defining a corresponding heating zone 642. Figure 76 illustrates in more detail the form that this layer 84 takes in this example heater 6’. As shown, in this example, there are twenty independently controllable heater electrodes 64-1 to 64-20 that each defines a corresponding heating zone 642. Each heater electrode 64 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance and peak power requirements for the relevant power source.Each heater electrode 64 is formed into a serpentine pattern using, for example, chemical etching as a manufacturing process. In more detail, a solid layer of conductive material is provided and then etched to form the different heater electrodes 64. The straight lines shown in Figure 3k are the etched parts of the layer 84 and the white parts of the figure show the serpentine conductor paths that form the heater electrodes 64. In this illustrated example, adjacent heater electrodes 64 share a common positive terminal (although in other embodiments they may share a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 78 and the heater 6’. This common positive terminal is connected to the different heater electrodes at suitable vias 65-1 to 65-5, which connect through to connection circuitry below (not shown) that connects to the drive and control board 78. The other end of each heater electrode connects through a respective switch (not shown) to the drive and control board 78 to allow independent control of current flow through each heater electrode 64. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each heater electrode 64 may be physically separate from all other heater electrodes64 in which case, each end of each heater electrode 64 would be connected separately back to the drive and control board 78.Alternatively, other processes such as printing, thick film printing, physical vapour deposition and the like could be used to form the heater electrodes 64.As schematically illustrated in Figure 76, the end of each heater electrode 64 that is connected to the switch is provided at the side of the heater and the direction of the serpentine tracks changes in this edge portion (which corresponds to the portion of the heater which is curved over the upper surface 74 of the rigid support substrate 68’). The inventors have found that this arrangement helps heat generated in the heater electrodes 64 in these edge portions to pass up to the top surface of the heater which is more likely to come into contact with the user’s hair. However, if the device is twisted in use such that the user’s hair comes into contact with the curved edge portion, then the hair will still be heated as this curved edge portion is heated.The conductive material used in the layer 84 is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 64 depends upon its temperature - and so the temperature of the heating zone 642 can be determined by measuring a parameter that varies with the resistance of the corresponding heater electrode 64.Polyimide Separator (Optional) 85When an auxiliary heater electrode layer is provided, this layer is required to provide the required electrical separation between that auxiliary heater electrode layer and the main heater electrode layer 84 described above. This polyimide layer 85 would have a low thermal resistance in the thickness direction whilst still achieving the dielectric requirements. Due to this layer being relatively thin, it will have a low thermal conductivity in the plane perpendicular to its thickness of less than about 35 W / mK. Other dielectric materials could be used instead of polyimide.Auxiliary Heater Electrode Layer (Optional) 86Some embodiments of the heater 6’ may benefit from the presence of an additional heating element layer 86. This additional layer 86 could be used to dissipate power (create heat) from a secondary power source that operates at a different voltage to the main power source 21 , for example the main power source could be a power supply and the power source for the auxiliary heater electrode layer 86 could be one or more batteries. In other embodiments the primary source could be one or more batteries and the auxiliary one or more supercapacitors. Alternatively, the conductors on this auxiliary layer 86 could be used for temperature sensing, in which case, the heater electrodes 64 in the main layer 84 may only be used for heating.The heater electrodes on the auxiliary layer 86 will typically have the same form as the heater electrodes 64 used in the main heater electrode layer 84 - so that they will define the same heating zones 642 as the heating zones 642 defined by the heater electrodes 64 on the main heater electrode layer 84. The path taken by the heater electrodes on the auxiliary layer 86 do not need to follow the same path as the corresponding heater electrodes 64 formed on the main heater electrode layer 84. For example, whilst the main part of each heater electrode 64 on the main heater electrode layer 84 (ignoring the edge part of each heater electrode 64) serpentines in the longitudinal direction of th...

Claims

CLAIMS1 . Apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: change a temperature or power output of a first heating zone of the plurality of independently controllable heating zones; and determine, based on a corresponding change in temperature of a second heating zone of the plurality of independently controllable heating zones, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a speed at which hair is moving across the hair contacting surface.

2. The apparatus according to claim 1 , wherein changing the temperature or power output of the first heating zone comprises increasing the temperature or power output of the first heating zone at a first time; and determining the speed at which the hair is moving across the hair contacting surface comprises: detecting a corresponding decrease in the power needed to maintain the second heating zone at the target temperature at a second time; and determining the speed at which hair is moving across the hair contacting surface based on a difference between the second time and the first time.

3. The apparatus according to claim 1 , wherein changing the temperature or power output of the first heating zone comprises decreasing the temperature or power output of the first heating zone at a first time; and determining the speed at which the hair is moving across the hair contacting surface comprises: detecting a corresponding increase in the power needed to maintain the second heating zone at the target temperature at a second time; and determining the speed at which hair is moving across the hair contacting surface based on a difference between the second time and the first time.

4. The apparatus according to claim 1 , wherein the apparatus comprises a temperature sensor for measuring the temperature of the second heating zone.

5. The apparatus according to any preceding claim, wherein the first and second heating zones are arranged across the width of the heater.

6. The apparatus according to any preceding claim, wherein the first and second heating zones are adjacent.

7. The apparatus according to any preceding claim, wherein the first and second heating zones are substantially thermally isolated from each other.

8. The apparatus according to any preceding claim, wherein the controller is configured to control the apparatus to:determine the resistance of a heater electrode of the second heating zone based on a voltage supplied to the heater electrode and the current passing through the heater electrode; and determine the temperature of the second heating zone based on the determined resistance.

9. The apparatus according to any preceding claim, wherein the controller is configured to control the apparatus to supply power to a heater electrode of the heater to determine the resistance of the heater electrode even in a case where it is determined that the corresponding heating zone is not engaged with hair.

10. The apparatus according to any preceding claim, further comprising one or more optical sensors for detecting whether hair is engaged with the hair-contacting surface.

11. The apparatus according to any preceding claim, wherein the apparatus further comprises a pair of opposing heating zones, wherein heat can flow between the opposing heating zones when the apparatus is in use, and wherein the controller is configured to control the apparatus to: determine whether hair is engaged with at least one of the pair of opposing heating zones based on a sum or average of a power needed to maintain each of the pair of opposing heating zones at a target temperature.

12. The apparatus according to any preceding claim, wherein the heater is a low thermal mass heater.

13. The apparatus according to any preceding claim, wherein at least one of the heating zones is operable to have a power density that is greater than 0.8 W / cm2and less than 15 W / cm2(or greater than 0.8 W / cm2and less than 30 W / cm2), preferably greater than 2 W / cm2and more preferably greater than 8 W / cm2.

14. The apparatus according to any preceding claim, wherein at least one of the heating zones has an outer surface heat up rate capability of greater than 50°C / s and less than 500 °C / s, preferably greater than 185°C / s, more preferably greater than 400°C / s.

15. The apparatus according to any preceding claim, wherein the apparatus is a hair straightener, a hair dryer, a hot paddle brush, a hot round brush, a heater roller, or a hair curler.

16. The apparatus according to any preceding claim, wherein the controller is configured to: control the apparatus to maintain a first heating zone of the plurality of independently controllable heating zones at a first target temperature for drying or styling hair even if it is determined that hair is not in contact with the first heating zone; and control the apparatus to maintain a second heating zone of the plurality of independently controllable heating zones at a second temperature that is lower than a target temperature for the second heating zone for drying or styling hair if it is determined that hair is not in contact with the second heating zone.

17. Apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: determine, based on a temperature of a heating zone of the plurality of independently controllable heating zones or based on the power needed tomaintain the heating zone at a target temperature, whether hair is engaged with the heating zone; control power supplied to a heater electrode of the heating zone to control the temperature of the heating zone towards a first target temperature in a case where it is determined that the hair is engaged with the heating zone, and to control the temperature of the heating zone towards a second target temperature, lower than the first target temperature, in a case where it is determined that the hair is not engaged with the heating zone; and intermittently supply power to the heater electrode to determine the resistance of the heater electrode even in the case where it is determined that the heating zone is not engaged with the hair, and determine the temperature of the heating zone based on the resistance.

18. Apparatus for drying or styling hair, the apparatus comprising: a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: when hair is in contact with a first heating zone and a second heating zone of the plurality of independently controllable heating zones for transfer of heat from the first heating zone to the second heating zone via thermal conduction along the hair, provide power to a heater electrode of the first heating zone to heat the hair in the first heating zone; and determine, based on a corresponding change in the temperature of the second heating zone, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a thermal conductivity of the hair.

19. Apparatus for drying or styling hair, the apparatus comprising: a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: provide power to each of the plurality of independently controllable heater electrodes based on the power demand at the respective heating zone; wherein the power provided to each heater electrode is interleaved with the power provided to the other heater electrodes.

20. The apparatus according to claim 19, wherein the controller is configured to determine a duty cycle for each of the heating zones, and wherein the interleaved power supplied to each heating zone is based on the determined duty cycle.

21. A method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, and the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: changing a temperature or power output of a first heating zone of the plurality of independently controllable heating zones; and determining, based on a corresponding change in temperature of a second heating zone of the plurality of independently controllable heating zones, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a speed at which hair is moving across the hair contacting surface.

22. A method performed by apparatus for drying or styling hair, the apparatus comprising a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: determining, based on a temperature of a heating zone of the plurality of independently controllable heating zones or based on the power needed to maintain the heating zone at a target temperature, whether hair is engaged with the heating zone; controlling power supplied to a heater electrode of the heating zone to control the temperature of the heating zone towards a first target temperature in a case where it is determined that the hair is engaged with the heating zone, and controlling the temperature of the heating zone towards a second target temperature, lower than the first target temperature, in a case where it is determined that the hair is not engaged with the heating zone; and intermittently supplying power to the heater electrode to determine the resistance of the heater electrode even in the case where it is determined that the heating zone is not engaged with the hair, and determining the temperature of the heating zone based on the resistance.

23. A method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: when hair is in contact with a first heating zone and a second heating zone of the plurality of independently controllable heating zones for transfer of heat from the first heating zone to the second heating zone via thermal conduction along the hair, providing power to a heater electrode of the first heating zone to heat the hair in the first heating zone; and determining, based on a corresponding change in the temperature of the second heating zone, or based on a corresponding change in the power needed to maintain the second heating zone at a target temperature, a thermal conductivity of the hair.

24. A method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: providing power to each of the plurality of independently controllable heater electrodes based on the power demand at the respective heating zone; wherein the power provided to each heater electrode is interleaved with the power provided to the other heater electrodes.

25. Apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; and a controller configured to control the apparatus to: determine the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently control the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, tocontrol the temperature of the corresponding heating zone towards a respective target temperature; wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.

26. Apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; a plurality temperature sensors, each temperature sensor being configured for measuring the temperature of a respective heating zone; and a controller configured to control the apparatus to: independently control the supply of power to each of the heater electrodes, based on the temperature of the corresponding heating zone, to control the temperature of the corresponding heating zone towards a respective target temperature; wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.

27. Apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; and a controller configured to control the apparatus to: determine the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently control the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, to control the temperature of the corresponding heating zone towards a respective target temperature; wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between a current duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window; and wherein the controller is configured to determine the target duty cycle for the drive signal applied to each of the heater electrodes based on a maximum current limit of the apparatus.

28. A method performed by apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; wherein the method comprises:determining the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently controlling the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, to control the temperature of the corresponding heating zone towards a respective target temperature; and determining, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.

29. A method performed by apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a plurality temperature sensors, each temperature sensor being configured for measuring the temperature of a respective heating zone; wherein the method comprises: independently controlling the supply of power to each of the heater electrodes, based on the temperature of the corresponding heating zone, to control the temperature of the corresponding heating zone towards a respective target temperature; and determining, sequentially for each of the heater electrodes and based on a difference between an average duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window.

30. A method performed by apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; wherein the method comprises: determining the resistance of each heater electrode based on a respective voltage supplied to the heater electrode and a respective current passing through the heater electrode; and independently controlling the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heater electrode, to control the temperature of the corresponding heating zone towards a respective target temperature; determining, sequentially for each of the heater electrodes and based on a difference between an actual duty cycle of a drive signal applied to the heater electrode and a target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a particular time window; and determining the target duty cycle for the drive signal applied to each of the heater electrodes based on a maximum current limit of the apparatus.31 . Apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode;a memory storing separate calibration information for each of the heating zones that indicates a respective relationship between at least one of: the resistance of the corresponding heater electrode and the temperature of the corresponding heating zone, the voltage supplied to the corresponding heater electrode and the temperature of the corresponding heating zone, or a measured temperature of the corresponding heating zone and an actual temperature of the corresponding heating zone; and a controller configured to control the apparatus to: independently control the supply of power to each of the heater electrodes, based on the respective calibration information stored in the memory, to control the temperature of each of the heating zones towards a respective target temperature.

32. Calibration apparatus for calibrating the apparatus for drying and / or styling hair according to claims 31 , the calibration apparatus comprising: a plurality of temperature sensing zones, wherein each of the plurality of temperature sensing zones is configured for thermal contact with a respective one of the independently controllable heating zones of the hair contacting surface, and wherein each of the temperature sensing zones has a resistance that varies with the temperature of the temperature sensing zone; and communication circuitry configured for communication with the apparatus for drying and / or styling hair; wherein the calibration apparatus is configured for: sensing the temperature of each of the heating zones based on the resistance of the respective temperature sensing zone; obtaining calibration information for each of the heating zones; and transmitting the calibration information to the apparatus for drying and / or styling hair using the communication circuitry.

33. A calibration system comprising: at least one calibration apparatus according to claim 32 for calibrating the apparatus for drying and / or styling hair; and a calibration controller configured for communication with the calibration apparatus; wherein the calibration controller is configured for: receiving, from the calibration apparatus, measurement results corresponding to the sensed temperatures for each of the independently controllable heating zones; and transmitting, to the apparatus for drying and / or styling hair, the calibration information for each of the heating zones.

34. A hair styling apparatus comprising: a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating; a plurality of microcontroller units, MCUs, in communication with one another, each one of the MCUs being configured to control a subset of the plurality of heaters, wherein at least one MCU of the plurality of MCUs comprises a processor configured to: detect malfunctions of the hair styling apparatus; and control a function of the hair styling apparatus in response to the processor detecting malfunctions of the hair styling apparatus.

35. A hair styling apparatus comprising: communication means for communicating with at least one external computing device for controlling the hair styling apparatus;a plurality of heaters for heating hair in contact with hair contacting surfaces of the plurality of heaters by conductive heating; a plurality of microcontroller units, MCUs, in communication with one another, each one of the MCUs being configured to control a subset of the plurality of heaters, wherein at least one MCU of the plurality comprises a processor configured to: detect malfunctions of the hair styling apparatus; and control a function of the hair styling apparatus in response to the processor detecting malfunctions of the hair styling apparatus.

36. Apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller; wherein the plurality of independently controllable heating zones comprise one or more first heating zones and one or more second heating zones; and wherein the controller is configured to perform control to cause the apparatus to, when hair passes from the one or more first heating zones to the one or more second heating zones before leaving the one or more second heating zones: determine a first power or first duty cycle needed to maintain the one or more first heating zones at a first target temperature for the one or more first heating zones; determine a second power or second duty cycle needed to maintain the one or more second heating zones at a second target temperature for the one or more second heating zones; estimate, based on the first power and the second power, or based on the first duty cycle and the second duty cycle, a temperature of the hair as the hair leaves the one or more second heating zones; and control the power supplied to the independently controllable heater electrodes based on the estimated temperature of the hair.

37. A method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, and a controller; wherein the plurality of independently controllable heating zones comprise one or more first heating zones and one or more second heating zones; and wherein the method comprises, when hair passes from the one or more first heating zones to the one or more second heating zones before leaving the one or more second heating zones: determining a first power or first duty cycle needed to maintain the one or more first heating zones at a first target temperature for the one or more first heating zones; determining a second power or second duty cycle needed to maintain the one or more second heating zones at a second target temperature for the one or more second heating zones; estimate, based on the first power and the second power, or based on the first duty cycle and the second duty cycle, a temperature of the hair as the hair leaves the one or more second heating zones; and control the power supplied to the independently controllable heater electrodes based on the determined temperature of the hair.

38. Apparatus for drying or styling hair, the apparatus comprising:a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; an accelerometer for sensing an acceleration of the apparatus; and a controller configured to: determine a speed that the apparatus is moving across the hair using the acceleration sensed by the accelerometer; and determine, based on the speed and based on a target hair temperature for the hair, a target temperature for the heating zones or an amount of power to be supplied to heat each of the heating zones to control the temperature of the hair towards the target hair temperature.

39. Apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to: determine whether hair is in contact with each of the heating zones; determine that a first heating zone is to be heated to a corresponding target temperature for drying or styling the hair when it is determined that hair is in contact with the first heating zone; determine that one or more additional heating zones are also to be heated to respective target temperatures for drying or styling the hair even when contact between the one or more additional heating zones and the hair is not detected; and control a supply of power to the heater electrodes to control the temperature of the first heating zone and the temperature of the one or more additional heating zones towards the corresponding target temperatures for drying or styling the hair.

40. Apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising at least one heater electrode; and a controller configured to: control a supply of power to the at least one heater electrode to maintain the temperature of the hair contacting surface at a target temperature for drying or styling the hair; determine that a temperature of hair that is in contact with the hair contacting surface has exceeded a first threshold temperature; after it is determined that the temperature of the hair has exceeded the first threshold temperature, determine that the temperature of the hair contacting surface is to be allowed to cool towards an ambient temperature; and determine, as the temperature of the hair contacting surface is allowed to cool towards the ambient temperature, that the temperature of hair that is in contact with the hair contacting surface has fallen below a second threshold temperature.41 . Apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to: determine that skin of a user is in contact with a first set of one or more of the heating zones;in response to determining that the that the skin of the user is in contact with the first set of one or more of the heating zones, disable a supply of power to the first set of one or more heating zones; and control the supply of power to at least one other heating zone that is not determined to be in contact with the skin of the user to control the temperature of the heating zone towards a target temperature for drying or styling the hair, whilst the supply of power to the first set of one or more heating zones is disabled.

42. A method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, an accelerometer for sensing an acceleration of the apparatus, and a controller; wherein the method comprises the controller: determining, a speed that the apparatus is moving across the hair using the acceleration sensed by the accelerometer; and determining, based on the speed and based on a target hair temperature for the hair, a target temperature for the heating zones or an amount of power to be supplied to heat the heating zones to control the temperature of the hair towards the target hair temperature.

43. A method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, and a controller; wherein the method comprises the controller: determining whether hair is in contact with each of the heating zones; determining that a first heating zone is to be heated to a corresponding target temperature for drying or styling the hair when it is determined that hair is in contact with the first heating zone; determining that one or more additional heating zones are also to be heated to respective target temperatures for drying or styling the hair even when contact between the one or more additional heating zones and the hair is not detected; and controlling the temperature of the first heating zone and the temperature of the one or more additional heating zones towards the corresponding target temperatures for drying or styling the hair.

44. A method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising at least one heater electrode, and a controller; wherein the method comprises the controller: controlling a supply of power to the at least one heater electrode to maintain the temperature of the hair contacting surface at a target temperature for drying or styling the hair; determining that a temperature of hair that is in contact with the hair contacting surface has exceeded a first threshold temperature; when it is determined that the temperature of the hair has exceeded the first threshold temperature, determining that the temperature of the hair contacting surface is to be allowed to cool towards an ambient temperature; and determining, as the temperature of the hair contacting surface is allowed to cool towards the ambient temperature, that the temperature of hair that is incontact with the hair contacting surface has fallen below a second threshold temperature.

45. A method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, and a controller; wherein the method comprises the controller: determining that skin of a user is in contact with a first set of one or more of the heating zones; in response to determining that the that the skin of the user is in contact with the first set of one or more of the heating zones, disabling a supply of power to the first set of one or more heating zones; and controlling the supply of power to at least one other heating zone that is not determined to be in contact with the skin of the user to control the temperature of the heating zone towards a target temperature for drying or styling the hair, whilst the supply of power to the first set of one or more heating zones is disabled.

46. A hair styling appliance comprising: a multilayer heater comprising a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein at least one of the layers of the multilayer heater is a heater electrode layer comprising a plurality of independently powerable heater electrodes that define a corresponding plurality of heating zones on the hair contacting surface of the multilayer heater, each heater electrode being formed of a conductive material that generates heat when a current is passed through it; a plurality of touch sensors, wherein at least one touch sensor is provided proximal to each heating zone and is configured to detect user skin contact with the heating zone; and control circuitry comprising at least one processor configured to perform a control action in respect of at least one heating zone of the plurality of heating zones in response to a touch sensor detecting user skin contact with the at least one heating zone.

47. A method performed by the hair styling appliance in use for styling and / or drying hair, the method comprising: sending, by a signal generator, an excitation signal to a plurality of touch sensors, wherein one or more touch sensors of the plurality of touch sensors are provided proximal to each heating zone of the hair styling appliance and are configured to detect user skin contact with the heating zones of the hair styling appliance; detecting user skin contact with at least one of the heating zone of the hair styling appliance; receiving, in response to detecting user skin contact with at least one of the heating zone of the hair styling appliance, a response signal indicating that user skin contact with the at least one of the heating zone has occurred; and performing, by control circuitry of the hair styling appliance comprising at least one processor, a control action in respect of the at least one heating zone of the plurality of heating zones in response to a touch sensor detecting user skin contact with the at least one heating zone.

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