Heater apparatus and methods

Abstract

Apparatus for drying and / or styling hair comprises: a heater 6a comprising a hair contacting surface and a plurality of independently controllable heater electrodes that define a plurality of heating zones 642; and a controller configured to control the apparatus to determine the resistance of each heater electrode, and independently control the supply of power to each of the heater electrodes, 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. Alternatively, heating zone temperature control may be based upon signals from temperature sensors for measuring respective heating zone temperatures.

Description

Field of the Invention The present invention relates to heating apparatus and methods. The heaters can be used for drying and / or styling hair. 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 drying or styling devices comprising one or more low thermal mass heaters. Background to the Invention Heated 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). The present invention aims to address or at least partially ameliorate one or more of the above problems. Summary In one 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. The controller may be 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. The controller may be configured to determine, for each heater electrode, the target duty cycle for the drive signal applied to the heater electrode based on a difference between the resistance of the heater electrode and a target resistance for the heater electrode; wherein the controller is configured to determine a scaling factor based on the maximum current limit of the apparatus; wherein the controller is configured to scale the target duty cycles by multiplying or dividing the target duty cycles by the scaling factor; and wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between the average duty cycle of a drive signal applied to the heater electrode and the scaled target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in the time window. The controller may be configured to determine, for each heater electrode, the target duty cycle for the drive signal applied to the heater electrode based on a difference between the temperature of the corresponding heating zone and a target temperature for the corresponding heating zone; wherein the controller is configured to determine a scaling factor based on the maximum current limit of the apparatus; wherein the controller is configured to scale the target duty cycles, to reduce the target duty cycles, by multiplying or dividing the target duty cycles by the scaling factor; and wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between the average duty cycle of a drive signal applied to the heater electrode and the scaled target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in the time window. The controller may be configured to determine the scaling factor based on the current that would be drawn by the heater if all of the heater electrodes were supplied with power using the corresponding target duty cycles. The controller may be configured to determine the scaling factor based on the maximum current that would be drawn by an individual heater electrode if all of the heater electrodes were supplied with power. The controller may be configured to determine a dynamic current limit based on the sum of: the current that would be drawn by the heater if all of the heater electrodes were supplied with power using the corresponding target duty cycles, and the maximum current that would be drawn by an individual heater electrode if all of the heater electrodes were supplied with power; wherein the controller is configured to control the supply of power to each of the heater electrodes so that the total current drawn by the heater electrodes does not exceed the dynamic current limit. The controller may be configured to modulate the dynamic current limit to smooth a change in the current drawn by the heater in the time domain. The controller may be configured to randomise the order of the sequential determination of whether power is to be supplied to each heater electrode in the time window. A first of the heater electrodes may have a higher priority for being supplied with power than a second of the heater electrodes; wherein the controller is configured to determine whether the first heater electrode is to be supplied with power in the time window before determining whether the second heater electrode is to be supplied with power in the time window. The controller may be configured to determine a current that would be drawn by the heater if the next heater electrode in the sequence were supplied with power in the time window; wherein the controller is configured to determine to not supply the next heater electrode in the sequence with power in the time window in a case where the controller determines that the current that would be drawn by the heater would exceed a maximum instantaneous current limit and / or a maximum average current limit of the apparatus. The controller may be configured to determine a power that would be drawn by the heater if the next heater electrode in the sequence were supplied with power in the time window; wherein the controller is configured to determine to not supply the next heater electrode in the sequence with power in the time window in a case where the controller determines that the power that would be drawn by the heater would exceed a maximum instantaneous power limit and / or a maximum average power limit of the apparatus. The controller may be configured to determine to not supply a heater electrode with power in the time window if the average duty cycle of the drive signal applied to the heater electrode is greater than the target duty cycle for the drive signal. The controller may be configured to determine, sequentially for each of the heater electrodes and based on the difference between the average duty cycle of the drive signal applied to the heater electrode and the target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a series of discrete time windows; wherein the controller is configured to determine the average duty cycle of the drive signals applied to the heater electrodes in each of the time windows. The controller may be configured to control the supply of power to each of the heater electrodes such that a maximum average power limit of the apparatus is not exceeded. The controller may be configured to control the supply of power to each of the heater electrodes such that a maximum instantaneous power limit of the apparatus is not exceeded. The heating zones may be substantially thermally isolated from each other. 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 / cm2 and less than 15 W / cm2, preferably greater than 2 W / cm2 and 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. 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. Brief Description of the Drawings Embodiments 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 1b 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 11b 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 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; and Figures 29 to 34b show graphs of the current drawn from the power source during operation of the device. Detailed Description of Preferred Embodiments The 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 Overview Figure 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 1b, 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 the device. 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 15 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. In 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 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 1b). 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. 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 coiled spring. 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. Heating Zones Independently 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 the longitudinal 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 hair-contacting surface. However, the heater 6 is not limited to having a flat hair-contacting surface and could alternatively have a tubular form (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 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 Diagram Figure 7 is a simplified block diagram of control circuitry 15 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 users 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)) 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 63 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 63 may include, for example, an operating system and a heater control module suitable for implementing one or more of the methods described below. The temperature measurement circuitry 25 may comprise temperature sensors such as thermistors or may use circuitry that senses the resistance of heater electrodes that are 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. 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 zones A 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 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 zones Z1, 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 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 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 11a 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 11b 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 11b 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 11a and 11b, it will be appreciated that table 11b 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 zone can 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 Configuration Figures 12a and 12b 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 30pm to 1000pm in the case of Safe Extra Low Voltage (SELV) operation (less than 42.4 Volts) and 0.8mm to 2.0mm 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 non-stick 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 if the layer 62 does not itself have such non-stick properties. 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. 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 circuitry 23 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). 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 layer 62 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. 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 Circuitry Control 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 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 sense amplifier, 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) 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 Load Methods of monitoring and controlling the temperature of the heating zones 64 will now be described in more detail. Heating Zone Temperature Monitoring and Control 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. 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 an indication 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 Determination The measurements 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. 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 = (Vcurrent / 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 heater electrode 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, where Vt is the voltage over the heater electrode that was determined based on the attenuated power supply potential measured in step S1802, and lt 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 / It) - Rsense, 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, where Telectrode is the temperature of the heater electrode, Reiectrode is the resistance of the heater electrode 64 obtained in step S1805, Rcai is 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 64. 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 resistors and operational amplifiers are connected to a respective different ADC input of the same microcontroller 29), measurements for heater electrodes 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 Control The 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. 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, 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 Teiectrode 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, 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 = in + pt, 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, 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 ■ ka), where kp, ki and ka 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 Cycle The 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. 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 <0.5 In step S2002 the fractional duty cycle, rt, is calculated by adding 0.5 to the clipped PID value: rt = Ut + 0.5 As 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 rt 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 rt 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 Determination of Firing Sequence The determination of the firing sequence for the heater electrodes 64 will now be described in more detail. 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] = -1 In 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] = W F[n][1] = A-W A=F[n][1] S[n][1] = 0 In 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 Sequence The firing sequence can then be executed as follows. The firing sequence duration is set as 14 / 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][1 ] = 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 to the 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] = t, or F[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][1 ] = 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 t) 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 Control Further details of controlling the temperature of the independently controllable heating zones will now be described. Device closure The 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, T0Pen, and one for when the heater arms are closed, Tdosed. 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 Tdosed. Response to Thermal Load As 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 Pthresh[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 P thresh-sty Ie, 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 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-mtermediate. 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 be detected 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 Zones When each of the arms 4a, 4b of the device 1 is provided with a respective heater 6a, 6b as illustrated in Figure 1a, 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 Ts^e. 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). Hair Speed Across Heater Surface As 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 contacting surface, 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 Conductivity As 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 Methods Particularly 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 target temperature). 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, Rcai, 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, Reiectrode, using the equation: Teiectrode — (Reiectrode Real) / (k X Rcal) "*■ 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 firing window 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 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 in the firing window exceeds the requested duty cycle, and will be powered if the average duty cycle 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 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 |z), where x is the total current, Di is the requested duty cycle for zone i, and I, is the current that would be drawn if zone i is supplied with power. In step S272 the maximum current that can be drawn from the power source is determined. 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, I 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 I,: I LIMIT — X + lmax By 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 +1 When the method proceeds to the next time slot the value of the time slot counter is increased by 1: n = n +1 For any time slot n, the actual fractional duty cycle az of a heating zone can be calculated by dividing the integrator value by the time slot counter value: az = lz / n At the beginning of a new firing window, the value of lz for 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: Sz.n — clz.n-1 * k + Pz * (1 ~ k), where Pz has a value of 1 if the heating zone z is powered, Pz has 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, Ilimit, 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. In step S283, the actual duty cycle az of 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 heating zones 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 Draw It 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 the dynamic 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. 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. 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. Modifications and alternatives Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. The invention has been described above by way of implementation in a hair styling device for straightening hair (‘hair straighteners’) which employ flat hair styling heaters 6. However, it could alternatively be implemented in any form of hair styling or hair drying device, such as (but not limited to) crimpers, curlers or heated brushes. The heaters 6 may define a heating surface that is flat, curved, ridged or in the shape of a barrel. The hair styling device may have two arms like the device illustrated in Figure 1 or it may be a single armed device. The heaters described above may also be used in hair dryers or in combination devices that use conductive heating and air to dry and style the user’s hair (such as those described in the applicant’s earlier PCT application WO 2021 / 019239). In embodiments where air is used, the heaters 6 may be perforated so that air passes through the heater and is warmed by the heater as the air passes through. Whilst a target styling temperature of 200°C has been described above, the target styling temperature need not necessarily be 200°C. The target temperature for styling hair can range from, for example, 30°C to 230°C. This advantageously allows for a variety of styling options, including “wet to style” where the hair styler is applied to wet hair. In this case, the water content of the hair can be measured, for example by using a detector which compares the amount of radiant energy in two absorption bands in the spectrum of light emitted by an infra-red source and reflected by the hair. Based on this measurement, the target temperature can be adjusted accordingly to stop damage to the hair or ‘sizzle’ occurring. In the above embodiments, Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switches were used to control powering and sensing of the heater electrodes. As those skilled in the art will appreciate, other switches could be used instead. For example, Field Effect Transistors (FETs) could be used, such as Gallium Nitride FETs or bipolar junction transistors (BJTs). Whilst in the examples described above the heater electrodes 64 were used for temperature sensing, in alternative embodiments, separate temperature sensors may be provided for sensing the temperature of each heating zone defined by the heater electrodes 64. For example, referring to Figure 12a, a separate layer of temperature sensors may be provided under dielectric layer 66 or on top of the layer 62. In the above embodiments, a DC power source was used to provide electrical power for heating the heater electrodes 64. This DC power source will typically be a battery, although DC supplies that derive their power from a mains power AC signal may be used. In embodiments where separate temperature sensors are provided, then AC mains power may be used to heat the heater electrodes. Thicker dielectric layers may be provided in this case between the heater electrodes 64 and the hair contacting surface of the hair styler 1. The device 10 may be partially or entirely formed of a unitary structure, e.g. by 3D printing. In the above-described examples the device 10 may comprise a single heater 6, or may alternatively comprise two or more heaters 6 (e.g. one provided for each heater arm 4a, 4b). Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps. The expressions “to dry hair”, “drying hair” or “decrease a moisture level of hair” and 5 the like, as used in the present disclosure, can refer both to the removal of “unbound” water that exists on the outside of hair when wet, or the removal of “bound” water, which exists inside individual hairs, and which can be interacted with when heat styling hair. The “bound” water need not necessarily be removed when drying hair, although removal of some bound water may occur during a drying or styling process. 10 It should also be noted that the term “wet” as used in the present disclosure should be interpreted broadly, to encompass not only hair wetted by water, but also hair wetted by liquids other than water. For example, hair may be wetted by a solvent-based colourant, which the invention may be used to dry and / or style. 15 Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

Claims

1. 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; anda 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; andindependently 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.

2. The apparatus according to claim 1, 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.

3. The apparatus according to claim 2,wherein the controller is configured to determine, for each heater electrode, the target duty cycle for the drive signal applied to the heater electrode based on adifference between the resistance of the heater electrode and a target resistance for the heater electrode;wherein the controller is configured to determine a scaling factor based on the maximum current limit of the apparatus;wherein the controller is configured to scale the target duty cycles by multiplying or dividing the target duty cycles by the scaling factor; andwherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between the average duty cycle of a drive signal applied to the heater electrode and the scaled target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in the time window.

4. The apparatus according to claim 2,wherein the controller is configured to determine, for each heater electrode, the target duty cycle for the drive signal applied to the heater electrode based on a difference between the temperature of the corresponding heating zone and a target temperature for the corresponding heating zone; andwherein the controller is configured to determine a scaling factor based on the maximum current limit of the apparatus;wherein the controller is configured to scale the target duty cycles, to reduce the target duty cycles, by multiplying or dividing the target duty cycles by the scaling factor; andwherein the controller is configured to determine, sequentially for each of the heater electrodes and based on a difference between the average duty cycle of a drive signal applied to the heater electrode and the scaled target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in the time window.

5. The apparatus according to claim 3 or 4, wherein the controller is configured to determine the scaling factor based on the current that would bedrawn by the heater if all of the heater electrodes were supplied with power using the corresponding target duty cycles.

6. The apparatus according to claim 5, wherein the controller is configured to determine the scaling factor based on the maximum current that would be drawn by an individual heater electrode if all of the heater electrodes were supplied with power.

7. The apparatus according to any preceding claim, wherein the controller is configured to determine a dynamic current limit based on the sum of:the current that would be drawn by the heater if all of the heater electrodes were supplied with power using the corresponding target duty cycles, andthe maximum current that would be drawn by an individual heater electrode if all of the heater electrodes were supplied with power; and wherein the controller is configured to control the supply of power to each of the heater electrodes so that the total current drawn by the heater electrodes does not exceed the dynamic current limit.

8. The apparatus according to claim 7, wherein the controller is configured to modulate the dynamic current limit to smooth a change in the current drawn by the heater in the time domain.

9. The apparatus according to any preceding claim, wherein the controller is configured to randomise the order of the sequential determination of whether power is to be supplied to each heater electrode in the time window.

10. The apparatus according to any one of claims 1 to 8, wherein a first of the heater electrodes has a higher priority for being supplied with power than a second of the heater electrodes; andwherein the controller is configured to determine whether the first heater electrode is to be supplied with power in the time window before determining whether the second heater electrode is to be supplied with power in the time window.

11. The apparatus according to any preceding claim,wherein the controller is configured to determine a current that would be drawn by the heater if the next heater electrode in the sequence were supplied with power in the time window; andwherein the controller is configured to determine to not supply the next heater electrode in the sequence with power in the time window in a case where the controller determines that the current that would be drawn by the heater would exceed a maximum instantaneous current limit and / or a maximum average current limit of the apparatus.

12. The apparatus according to any preceding claim,wherein the controller is configured to determine a power that would be drawn by the heater if the next heater electrode in the sequence were supplied with power in the time window; andwherein the controller is configured to determine to not supply the next heater electrode in the sequence with power in the time window in a case where the controller determines that the power that would be drawn by the heater would exceed a maximum instantaneous power limit and / or a maximum average power limit of the apparatus.

13. The apparatus according to any preceding claim, wherein the controller is configured to determine to not supply a heater electrode with power in the time window if the average duty cycle of the drive signal applied to the heater electrode is greater than the target duty cycle for the drive signal.

14. The apparatus according to any preceding claim, wherein the controller is configured to determine, sequentially for each of the heater electrodes and based on the difference between the average duty cycle of the drive signal applied to the heater electrode and the target duty cycle for the drive signal applied to the heater electrode, whether power is to be supplied to the heater electrode in a series of discrete time windows; andwherein the controller is configured to determine the average duty cycle of the drive signals applied to the heater electrodes in each of the time windows.

15. The apparatus according to any preceding claim, wherein the controller is configured to control the supply of power to each of the heater electrodes such that a maximum average power limit of the apparatus is not exceeded.

16. The apparatus according to any preceding claim, wherein the controller is configured to control the supply of power to each of the heater electrodes such that a maximum instantaneous power limit of the apparatus is not exceeded.

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

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

19. 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 / cm2 and less than 15 W / cm2, preferably greater than 2 W / cm2 and more preferably greater than 8 W / cm2.

20. 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 than50°C / s and less than 500 °C / s, preferably greater than 185°C / s, more preferably greater than 400°C / s.

21. 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.

22. 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; anda 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.

23. 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, whereineach of the heater electrodes has a resistance that varies with the temperature of the heater electrode; anda 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; andindependently 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; andwherein 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.

24. 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; andindependently controlling the supply of power to each of the heater electrodes, based on the determined resistance of the corresponding heaterelectrode, to control the temperature of the corresponding heating zone towards a respective target temperature; anddetermining, 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.

25. 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; anddetermining, 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. 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 ofindependently 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 5 voltage supplied to the heater electrode and a respective current passing through the heater electrode; andindependently 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 10 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 15 time window; anddetermining the target duty cycle for the drive signal applied to each of the heater electrodes based on a maximum current limit of the apparatus.

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