Heater apparatus and methods

Abstract

Apparatus (1, fig.1a) for drying or styling hair comprises a heater with a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones

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 use heat to increase the temperature of hair to a desired styling temperature. For example, a hair straighter 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. The hair is typically 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. Such low thermal mass heaters are therefore more responsive and are easier to control. However, there is a need for further improvements heater hair styling and / or drying tools. For example, there is a need for more energy efficient and responsive devices having reduced energy consumption and improved styling / drying performance. 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 or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller; wherein the plurality of independently controllable heating zones comprise one or more first heating zones and one or more second heating zones; and wherein the controller is configured to perform control to cause the apparatus to, when hair passes from the one or more first heating zones to the one or more second heating zones before leaving the one or more second heating zones: determine a first power or first duty cycle needed to maintain the one or more first heating zones at a first target temperature for the one or more first heating zones; determine a second power or second duty cycle needed to maintain the one or more second heating zones at a second target temperature for the one or more second heating zones; estimate, based on the first power and the second power, or based on the first duty cycle and the second duty cycle, a temperature of the hair as the hair leaves the one or more second heating zones; and control the power supplied to the independently controllable heater electrodes based on the estimated temperature of the hair. The apparatus may be configured to: determine a third target temperature for the one or more first heating zones or a power to be supplied to the one or more first heating zones based on the estimated temperature of the hair; and determine a fourth target temperature for the one or more second heating zones or a power to be supplied to the one or more second heating zones based on the estimated temperature of the hair. The apparatus may be configured to determine the third target temperature and the fourth target temperature based on a difference between a target temperature for the hair and the estimated temperature of the hair. The estimation of the temperature of the hair may be based on a square of a ratio of the first power and the second power. The first target temperature may be the same as the second target temperature. The apparatus may be configured to determine that the hair is passing from the one or more first heating zones to the one or more second heating zones based on the first power being greater than the second power. The apparatus may be configured to estimate the temperature of the hair as the hair leaves the one or more second heating zones using a neural network. The neural network may comprise: an input layer; one or more hidden layers; and an output layer; wherein inputs for the input layer comprise: the first power or first duty cycle needed to maintain the one or more first heating zones at the first target temperature, and the second power or second duty cycle needed to maintain the one or more second heating zones at the second target temperature; and wherein an output of the output layer comprises the estimated temperature of the hair as the hair leaves the one or more second heating zones. The inputs for the input layer may comprise at least one of the first target temperature for the one or more first heating zones or the second target temperature for the one or more second heating zones. The inputs for the input layer may comprise: the first power or first duty cycle needed to maintain the one or more first heating zones at the first target temperature in a first time period; the second power or second duty cycle needed to maintain the one or more second heating zones at the second target temperature in the first time period; the first power or first duty cycle needed to maintain the one or more first heating zones at the first target temperature in a second time period, different from the first time period; and the second power or second duty cycle needed to maintain the one or more second heating zones at the second target temperature in the second time period. The apparatus may comprise a plurality of the first heating zones and a plurality of the second heating zones; wherein the first power or first duty cycle is a first average power or first average duty cycle to maintain the first heating zones at the first target temperature; and wherein the second power or second duty cycle is a second average power or second average duty cycle to maintain the second heating zones at the second target temperature. The apparatus may be configured to: determine which of the first heating zones are loaded with hair; determine the first average power or first average duty cycle using only the first heating zones that are determined to be loaded with hair; determine which of the second heating zones are loaded with hair; and determine the second average power or second average duty cycle using only the second heating zones that are determined to be loaded with hair. The apparatus may comprise a plurality of the first heating zones and a plurality of the second heating zones; wherein the one or more first heating zones are arranged along a longitudinal length of the heater at a first position along a transverse width of the heater; and wherein the one or more second heating zones are arranged along the longitudinal length of the heater at a second position along a transverse width of the heater, different from the first position. The one or more first heating zones and the one or more second heating zones may be arranged across a width of the heater. The heater may be a low thermal mass heater. The heater 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. The heater 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 a method performed by apparatus for drying or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, and a controller; wherein the plurality of independently controllable heating zones comprise one or more first heating zones and one or more second heating zones; and wherein the method comprises, when hair passes from the one or more first heating zones to the one or more second heating zones before leaving the one or more second heating zones: determining a first power or first duty cycle needed to maintain the one or more first heating zones at a first target temperature for the one or more first heating zones; determining a second power or second duty cycle needed to maintain the one or more second heating zones at a second target temperature for the one or more second heating zones; estimate, based on the first power and the second power, or based on the first duty cycle and the second duty cycle, a temperature of the hair as the hair leaves the one or more second heating zones; and control the power supplied to the independently controllable heater electrodes based on the determined temperature of the hair. 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 1 b shows a hair styling device in use; Figure 2 shows a set of heating zones of the hair styling device; Figure 3 schematically illustrates the heating zones on the heating surface of the heater shown in Figure 2; Figure 4 schematically illustrates an alternative arrangement of heating zones; Figure 5 schematically illustrates a further alternative arrangement of heating zones that are of different sizes and shapes; Figure 6a illustrates the way in which the heating zones may be formed on a tubular substrate for use in a curling tong or the like; Figure 6b illustrates the way in which the heating zones may be arranged on a curved substrate which may be used on a heated brush or the like; Figure 7 shows a simplified block diagram illustrating the main electronic components of the hairstyling 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 on the heating zones; Figure 11a 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 graph of hair temperature as hair passes over two adjacent heating zones; Figure 26 shows an example of how heating zones may be indexed; Figure 27 shows a simplified schematic illustration in which two heating zones are sub-divided into smaller elements; Figure 28 shows a simplified schematic illustration of various parameters that may be measured by the device and used to control the power supplied to the heating zones; Figure 29 shows a flow diagram illustrating a method of controlling the power supplied to the heating zones; Figure 30 shows a simulated temperature distribution for a tress of hair; Figure 31 shows a further simulated temperature distribution for a tress of hair Figure 32 shows a further simulated temperature distribution for a tress of hair; Figure 33 shows a further simulated temperature distribution for a tress of hair; Figure 34 shows a simplified schematic diagram of a neural network for estimating an exit temperature of the hair; Figure 35 shows a simplified schematic diagram of a further example of a neural network for estimating an exit temperature of the hair; Figure 36 shows a further simulated temperature distribution for a tress of hair; and Figure 37 shows a further simulated temperature distribution for a tress of hair. 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 hand held (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 17 to indicate whether the device is on. A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 1 and carries the control circuitry for controlling the operation of the device 1 and for controlling the interaction with the user via the user interface 11. In this example, electrical power is provided to the device 1 by means of a power supply located at an end of the device, via a power supply cord 3. The power supply may be an AC mains power supply. However, in an alternative embodiment the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g. from the mains or a DC supply via a charging lead), thereby enabling the device 1 to be a cordless product. 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 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. 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. 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). 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), or Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switches) which controls the application of an AC mains voltage, or a DC voltage derived from the AC mains or from a battery, to the heaters 6 in accordance with instructions from the microprocessor 29. The microprocessor 29 is coupled to a memory 30 (which is typically a non-volatile memory) that stores processor control code for implementing one or more control methods that control the heating of the heaters 6 in accordance with a desired operating temperature of the heaters 6 (for each of the independently controllable heating zones) and sensed temperatures of the heaters obtained from temperature measurement circuitry 25. The memory 30 may store, for example, one or more operating profiles or parameters (e.g. a target temperature for each of the heating zones). Software stored in the memory 30 may include, for example, an operating system and a heater control module suitable for implementing one or more of the methods described below. For example, the software stored in the memory 30 may include a calibration and testing module for use when the device 1 is being calibrated and tested. The temperature measurement circuitry 25 may comprise temperature sensors such as thermistors or they 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 non-contact 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 10 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 (but need not necessarily be) operated at a reduced temperature (which could be the ambient temperature, in which case the supply of power to these zones can be switched off). Figure 10 shows a further example in which a tress of hair 40 is in contact with heating zones Z2 and Z3, and therefore zones Z2 and Z3 are determined to be in use for styling the user’s hair 40. Since the user’s hair 40 is not in contact with heating zones Z1, Z4, Z5 or Z6, these heating zones are determined not to be in use for styling the user’s hair 40. Sensing the presence of hair 40 in a heating zone can be achieved by measuring the change in thermal load at the heating zone. As will be described in more detail later, the presence of the user’s hair can be determined based on a change in the power required to maintain the heating zone at a particular temperature, or based on a change in temperature of the heating zone (which could be an increase in temperature as hair is removed from the zone, or a decrease in temperature as hair comes into contact with the zone). Alternatively, the presence of the user’s hair in a heating zone could be measured directly using a sensor. For example, an optical sensor could be used to sense the presence of hair in a particular heating zone. Light pipes could be used to sense the brightness of light through openings in the casework of the device 1, arranged so that the brightness of the detected light will change depending on whether a tress of hair 40 is present on the surface of the heater 6 in a particular heating zone. In step S802, a target temperature for each of the heating zones is determined. For heating zones that are determined, in step S801, to be in use for styling the user’s hair, the target temperature is set to a styling temperature. For the heating zones that are not determined to be in use for styling hair, the target temperature is set lower than the styling temperature in order to reduce the power needed to operate the device 1. The target temperature for use in styling the hair of the user may be, for example, 200°C. For zones determined not to be in use for styling the hair 40 of the user, the lower target temperature may be, for example, 100°C. Alternatively, a target temperature may not be defined (or ‘set’) for the zones determined not to be in use for styling the hair of the user, and the zones may be allowed to cool towards the ambient temperature. If the arms 4a, 4b of the device 1 are determined to be in the open configuration (e.g. using a switch or sensor as described above) then step S801 comprises determining that none of the heating zones are in use for styling. A table indicating the target styling temperature and one or more intermediate target temperatures may be stored in the memory 30 of the device 1. Figure 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 0.3mm to 0.4mm in the case of low voltage operation (less than about 40 Volts) and 0.8mm to 1.0mm in the case of AC operation) and with very low thermal mass. The upper dielectric layer 62 provides the hair contacting surface of the heater 6, although a 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. The bonded layers 62, 63 and 66 define a flexible heater 6 and rigidity of the heater 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, 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. The coating may be a non-stick coating to facilitate the passage of the user’s hair over the heating surface. The coating may be between, for example, 1 pm and 50 pm thick, and preferably between 1 pm and 3 pm thick. The coating may be applied as a spray coating (e.g. Cerasol) or as a wash or deposition layer, for example physical vapor deposition, chemical vapor deposition, or any other suitable method of applying such a coating. The coating may comprise multiple layers including, for example, a primer layer (of about 6pm), a base coat layer (of about 25pm) and a top coat layer (of about 10pm). The coating may also have combined functionalities with the other layers (e.g. more than just offering low friction) such as dielectric strength I electrical separation provided the material is sufficiently electrically insulative. Whilst in the examples illustrated in Figures 12a to 14 the heater electrodes 64 are provided in a layer 63 that is separate from the dielectric layer, this need not necessarily be the case. For example, Figure 15 shows an alternative in which the heater electrodes 64 are arranged within the dielectric layer 62, but are nevertheless electrically insulated from the hair 40 of the user. Exemplary Control 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. As shown in Figure 16, each heater electrode 64 (for example 64-1 to 64-3) is connected at one end to a power supply 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. 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 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 Exemplary 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. However, it will be appreciated that the resistance measurements and the determination of the temperatures of the heating zones could be performed in any other suitable manner. In step S1801 power is supplied to one of the heater electrodes 64 by operating the corresponding switch 95. In step S1802 the attenuated power supply potential, VSUppiy as shown in Figure 16, is measured using an ADC input of the microprocessor 29. It will be appreciated that the attenuation of the power supply potential measured by the microprocessor will depend on the value 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: Reiectrode — Vt / It, (Equation 1) where Vt is the voltage over the heater electrode that was determined based on the attenuated power supply potential measured in step S1802, and 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: Reiectrode = (Vt I It) ■ Rsense, (Equation 2) where Rsense is the resistance of the current sense resistor. The determined resistance of the heater electrode 64, Reiectrode, 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, Reiectrode, 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 Reiectrode and the temperature of the heater electrode 64 could be determined experimentally in advance, a corresponding mapping 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. In this case, the temperature of the heater electrode 64 may be determined using the formula: Telectrode — (Relectrode-Rcai) / (k*Rcai) + Tcai, (Equation 3) 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. 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 64 connected to different current sense resistors may be performed simultaneously. Since the measurement of the current performed in step S1803 requires power to be supplied to the heater electrode 64, some heat will be generated in the corresponding heating zone whilst the measurement is being performed. This additional heat output may be taken into account when determining the duty cycle to be used for the heater electrode in step S1704 of Figure 17. Beneficially, therefore, the temperature of the corresponding heating zone can be more accurately controlled towards the target temperature. PIP Control An exemplary determination of the PID controller output in step S1703 of Figure 17 will now be described in more detail with reference to the flow diagram shown in Figure 19. However, it will be appreciated that the PID control could be performed in any other suitable manner. In step S1901 the proportional error value for a heating zone is calculated by subtracting the temperature of the heating zone determined in step S1702 of Figure 17 from the target temperature for that heating zone (e.g. determined in step S802 of Figure 8): Pt = Ttarget - Telectrode, (Equation 4) where pt is the proportional error value obtained for time t (or an index t corresponding to a particular measurement cycle), Ttarget is the target temperature for the heating zone and Telectrode is the measured temperature of the corresponding heater. Alternatively, the proportional error value could be calculated by subtracting the measured resistance of the heater electrode 64 from a target resistance using the formula: Pt = Rtarget - Relectrode, (Equation 5) where Rtarget is the target resistance for the heater electrode 64 (e.g. based on experimental measurements of the correspondence between the temperature of the heating zone and the resistance of the heater electrode) and Relectrode is the actual resistance of the heater electrode (e.g. determined in step S1805 of Figure 18). In step S1902 an integral value is accumulated for the heating zone, but adding the proportional value pt obtained in step S1901 the previous integral value: it = it-i + pt, (Equation 6) where it is the new integral value and it-i is the previous integral value. In step S1903 the proportional value pt is subtracted from the previous proportional value for the heating zone to obtain a derivative value: dt = pt - pt-i, (Equation 7) where dt is the derivative value and pt-i is the previous proportional value (based on the previous temperature or resistance measurement for the same heater electrode). In step S1904 the PID value, Ut, is determined using the formula: Ut = (pt ■ kp) + (it ■ ki) + (dt ■ kd), (Equation 8) where kp, ki and kd are predefined constants. Steps S1901 to S1904 are then repeated for the other heater electrodes 64 to obtain the corresponding PID values. Each of the PID values Ut can then be used in step S1704 of Figure 17 to determine the duty cycles to be used for the respective heater electrodes. Duty Cycle An exemplary determination of the duty cycles for the heater electrodes 64 performed in step S1704 of Figure 17 will now be described in more detail with reference to Figure 20, which shows a method of mapping the PID values Ut to corresponding duty cycles. However, the determination of the duty cycles could alternatively be performed in any other suitable manner. In step S2001 the PID value Ut is clipped to a predefined range. In this example, the PID value is clipped to be between -0.5 and 0.5: -0.5 <Ut <0.5 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 (Equation 11) Determination of Firing Sequence An exemplary determination of the firing sequence for the heater electrodes 64 will now be described in more detail. However, the firing sequence could alternatively be determined in any other suitable matter. 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 21 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. 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 Pthresh-styie, depending on the updates to the target temperatures performed in step S2103. If the target temperature for a heating zone is set to Tstyie, and the power required to maintain the heating zone at Tstyie falls below the Pthresh[n], then the target temperature for that heating zone is set to Tstyie. This is because when the power required to maintain the heating zone at the target temperature falls below the threshold value, it indicates that the hair is no longer in contact with the heating zone. The threshold power value Pthresh[n] for the heating zone is also set to Pthresh-intermediate. It will be appreciated, therefore, that in the method of Figure 21 the threshold temperature assigned to each heating zone can be changed from Tint to Tstyie, and from Tstyie to Tint, based on a comparison of the power required to maintain the heating zone at the target temperature with a threshold power value. In other words, the target temperature for a heating zone is determined based on the thermal load at the heating zone. Beneficially, therefore, the presence of hair 40 in a heating zone can be determined, and the heating zone can be controlled to increase in temperature to the styling temperature Tstyie, in order to style the hair. The removal of the hair 40 from the heating zone can also be detected, and the target temperature lowered to Tint in order to reduce the energy usage of the device 1. When the target temperature for a heating zone has changed from Tstyie to Tint, and the heating zone is cooling from Tstyie to Tint, there is no power demand at the heating zone (since the zone is cooling). However, since there is no power demand to maintain the zone at the target temperature, the power demand of the heating zone will be lower than the threshold value Pthresh-intermediate even when hair 40 is placed onto the heating zone, meaning that the presence of hair on the heating zone may not 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 inventors have realised that 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 Tstyie. In this situation, heat will be transferred from Zone 4 to Zone 1, and the power reguired to maintain Zone 4 at Tstyie will be relatively high. This may lead to a false determination that hair is present in Zone 4 (since the power to maintain the zone at the threshold temperature will be higher than Pthresh[n]), and therefore the target temperature for Zone 4 will be maintained at Tstyie- The inventors have realised that this effect can be mitigated against by determining a target temperature for a pair of opposing heating zones based on the sum of the powers (or the average of the powers) reguired to maintain each heating zone of the opposing pair at a target temperature. In other words, each pair of opposing heating zones can be considered to be a single logical heating zone, and control performed for the single logical heating zone rather than individually for the two separate opposing heating zones. Control of the target temperatures and threshold power values can then be performed as described above with reference to Figure 21, but by assigning target temperatures and threshold power values to the pairs of opposing heating zones rather than to each heating zone of the opposing pair separately (and by comparing on the total power supplied to the pair of opposing heating zones, or the average power supplied to the pair of opposing heating zones, to the threshold power values). Whilst in the example of Fig. 22 the two heaters (6a, 6b) have the same number of heating zones, this need not necessarily be the case. Each of the heaters may have any suitable number of heating zones (and any suitable shape for each heating zone), and the heating zones of one heater 6a need not necessarily be aligned (e.g. perfectly overlapping) with the heating zones of the other heater 6b. Hair Speed Across Heater 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). The inventors have realised that 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. Target Hair Temperature In many conventional hair styling devices, the target temperature of the hair-engaging surface is set to a particular static temperature, and the power supplied to the heater(s) is controlled to try to maintain the hair-engaging surface at that target temperature. In such devices, the temperature of the hair 40 is unknown, and the hair-engaging surface is simply maintained at the desired temperature for the hair. However, the inventors have realised that the rapid thermal response of the low thermal mass heaters, and the use of a plurality of heating zones, enables the temperature of the hair 40 as it exits the device 1 to be estimated and controlled towards a target temperature for the hair 40. Advantageously, therefore, in this example by dynamically adjusting the temperature of the heating zones, the temperature of the hair 40 as it exits the device 1 (as device is moved across the hair) can be controlled towards a target temperature. The apparatus and methods enable reliable and efficient control of the hair temperature, even for a range of hair types and a range of speeds at which the device 1 is moved across the hair 40. The present example will be described with reference to an N x 2 array of heating zones, such as the arrangement of heating zones illustrated in Fig. 4. However, it will be appreciated that any other suitable arrangement of heating zones could be used (e.g. an N x 3 array, or N x 4 array of heating zones). It will also be appreciated that the value of N may be any suitable number. For example, as illustrated in Fig. 4, a 6x2 array of heating zones could be used. When an N x 2 array of heating zones is used, in normal operation of the device 1 a section of hair 40 will pass over two of the heating zones in sequence. Fig. 25 shows a graph of hair temperature as the hair 40 passes over the two adjacent zones (Zone 0 and Zone 1), and Fig. 26 shows an example of how each of the heating zones may be assigned an identifying index. As illustrated in Fig. 26, the hair engaging surface of the device 1 may be divided into N heating zones along the longitudinal direction (10 heating zones along the x-direction for each arm of the device 1 in the example illustrated Fig. 26) and two transverse zones (zone 0 and zone 1) in the y-direction at each position along the hair engaging surface. Whilst a 10 x 2 arrangement of heating zones and two arms are illustrated in Fig. 26, this need not necessarily be the case (e.g. a 30 x 4 arrangement of heating zones and a single arm could be used). The device 1 need not necessarily comprise arms as illustrated in Fig 26, for example if the device 1 is a hair curler and the heating zones are provided in a cylindrical configuration similar to that illustrated in Fig. 6a. In the example illustrated in Fig. 25, the two heating zones are maintained at the same temperature, Thtr, but this need not necessarily be the case. As illustrated in Fig. 25, the hair that comes into contact with the first heating zone (Zone 0) is heated from an initial temperature TEntry (e.g. ambient temperature) to an intermediate temperature To. The intermediate temperature To is less than the heater temperature Thtr by a value of Ao. The hair 40 then passes over the second heating zone (Zone 1) and is heated to the intermediate temperature Ti, before reaching the exit temperature TExit. The intermediate temperature Ti is less than the heater temperature Thtr by a value of Ai. Due to the first heating zone receiving hair 40 at a lower initial temperature (TEntry) than the second heating zone, the power needed to maintain the first heating zone at the temperature Thtr will be greater than the power needed to maintain the second heating zone at the temperature Thtr (assuming that the section of hair 40 moves at approximately the same speed over both heating zones, which is likely to be the case). The ratio of powers drawn by the two heating zones to maintain the zones at the target temperature Thtr is proportional to the difference in temperature between each heating zone and the temperature of the hair 40 as it passes over the heating zone (Ao for the first heating zone and Ai for the second heating zone). The ratio of the powers drawn by the two heating zones therefore provides an indication of the relative average temperature of the hair engaged with the heating zones. When the heating zones are moved across a tress of hair 40, there is a longitudinal temperature gradient along the hair 40 engaged with each heating zone (as illustrated in Fig. 25), but it can be assumed that the average hair temperature occurs at the same longitudinal position on each zone. The temperature of the hair 40 as it exits the second heating zone is given by: TExit = Thtr - (Thtr - TEntry) * (Ai / A0)2 (Equation 12) If Po is the average power drawn by the first heating zone as the hair 40 passes across the first heating zone, and Pi is the average power drawn by the second heating zone as the hair passes across the second heating zone, then the ratio P1 / P0 represents the proportional change in power drawn in the distance between the centres of the heating zones. However, each section of the hair 40 traverses twice that distance in total, which results in the squared term (A1 / A0)2. The ratio of the powers Po and Pi is equal to the ratio of the temperature differences Ai and Ao: Pi / Po — Ai / Ao (Equation 13) Therefore, the estimated temperature of the hair 40 as it exits the second heating zone, TExitEst, is given by: TExitEst - Thtr- (Thtr- KyEntry) * (Pl / Po)2 (Equation 14) where KTEntry is a constant temperature assumed for the hair entry temperature TEntry, for example 25°C. Therefore, it can be seen that TExitEst can be obtained by measuring the temperature of the heating zones Thtr(e.g. using the method described above with reference to Fig. 18) and the power output to the two heating zones, Po and Pi. Advantageously, therefore, the target temperature of the heating zones can be controlled in response to the power drawn by the heating zones, to guide the value of TExitEst towards a target temperature for the hair 40 (e.g. a particularly effective drying or styling temperature for the hair 40). In other words, by controlling the target temperature of the heating zones based on the power drawn by the heating zones, the device 1 is operable to maintain the temperature of the hair 40 as it exits the final heating zone at a target hair temperature. Figure 27 shows a simplified schematic illustration in which the two heating zones are sub-divided into smaller elements, useful for understanding why the (P1 / P0) term is squared in equation 14. The relationship Pi / Po = Ai / Ao would apply for each pair of corresponding sub-elements (a to n) between the heating zones, if this could be measured. The distance between the first sub-element, a, of Zone 0 (the sub-element at which the hair 40 first engages with the first heating zone) and the last sub-element, n, of Zone 1 (the sub-element at which the hair 40 exits the second heating zone), is doubled compared to the distance between the same sub-element of both heating zones. In this case, the power ratio is chained twice, i.e. squared. Therefore, the power ratio P1 / P0 is squared when determining TExitEst using equation 14. Equation 14 used to calculate the value of TExitEst is based on approximations that the temperatures of the heating zones are constant as the hair 40 passes over the heating zones, and that the speed of the hair 40 over the heating zones is also constant. Dynamic variations in the speed of the device 1 and / or the thermal contact between the hair and the heating zones can introduce variations in the temperature gradient of the hair 40 along the zones, resulting in errors in the inferred hair temperature. Similarly, dynamic variations in the temperatures of the heating zones can also cause variations in the temperature gradient of the hair 40. Errors in the estimate of TExitEst could result in unpredictable modulation of the target temperatures for the heating zones, which can cause additional uncertainty in the power draw measurements and further errors in the estimation of TExitEst. To mitigate against these effects, the target temperatures for the heating zones may optionally only be adjusted when the determined value of TExitEst is sufficiently far from the target hair temperature (i.e. the magnitude of the difference between TExitEst and the target hair temperature is greater than a threshold value). This results in more stable control of the temperature of the heating zones and hair 40. Optionally, the temperature of a heating zone may be determined and adjusted only when the absolute power demand for the heating zone is above a fixed threshold value (otherwise, the temperature of the heating zone is allowed to drift downwards until the power demand rises above the threshold value). Whilst the method described above with reference to Fig. 18 could be used to measure the temperature of the heating zones, this need not necessarily be the case. Alternatively, for example, one or more dedicated temperature sensors could be used to measure the temperatures of the heating zones (e.g. one or more contact sensors, or one or more far infrared sensors). However, using the heating zones themselves to determine the temperature of the hair is particularly advantageous, since a dedicated temperature sensor may only sense the temperature of the outer layer of the tress of hair 40, whereas the measurements using the heating zones are indicative of the core temperature of the hair 40. The use of dedicated temperature sensors also requires mor space on or around the styling surface, and so using the heating zones to infer the hair 40 temperature enables a particularly compact configuration. Moreover, if an optical temperature sensor was used, such sensors can become occluded with dirt and require calibration (which may drift over the lifetime of the device 1). In a further alternative, rather than using all of the transverse heating zones at one position (along the y-direction illustrated in Fig. 26) to heat the hair, one of the heating zones could be operated as a dedicated temperature sensor. In the example illustrated in Fig. 25, Zone 1 could be operated as a dedicated heater maintained at the target hair temperature, and Zone 0 could be operated as a dedicated temperature sensor for sensing the hair entry temperature TEntry. Alternatively, Zone 0 could be operated as a dedicated heater maintained at the target hair temperature, and Zone 1 could be operated as a dedicated temperature sensor for sensing the hair exit temperature. It will be appreciated that in the example illustrated in Fig. 25, the direction of movement of the device 1 may be reversed, such that Zone 1 is the first heating zone to engage with the hair rather than Zone 0. The direction of movement of the device 1 may be inferred by determining which heating zone has the largest amount of power needed to maintain that zone at the target temperature. In the example illustrated in Fig. 25, more power will be needed to maintain zone 0 at its target temperature than to maintain zone 1 at its target temperature. Therefore, the device 1 is operable to determine that the hair 40 is moving from zone 0 to zone 1. Whilst in equation 14 the power ratio P1 / P0 is used to estimate the temperature of the hair as it leaves the hair contacting surface, additional information could also be used to help improve the accuracy of the estimation. For example, the device 1 may comprise an accelerometer for determining an acceleration of deceleration (and therefore velocity) of the device 1, and the acceleration / deceleration may be used to help estimate the temperature of the hair (or may be used to determine that the speed of the device 1 over the hair 40 was more variable than usual, and so the estimate of the temperature of the hair 40 may be unreliable). The acceleration, deceleration and / or velocity may be used to help correct under-heating at the start of a tress, or to help prevent overheating when the styler reaches the end of the tress. For example, the user may decrease the velocity of the device 1 as it is approaching the end of the tress, and the device 1 may therefore determine that less power is to be supplied to the heaters 1 to avoid overheating the hair 40 (since a given section of the tress will be in contact with the heating zones for a longer period of time when the device 1 is being moved more slowly across the hair 40). In practice, measurements of the power supplied for each heating zone (P0 and P1 in this example) will be subject to measurement uncertainties and jitter. Sources of uncertainty in the power measurements include: analogue to digital converter (ADC) quantisation (due to the finite resolution of the voltage and current measurements); analogue noise in the signals; pulse width modulation (PWM) quantisation (due to the finite timing resolution of zone power PWM generation); maxing out of the PWM under heavy loads; electrical noise from the power supply; instantaneous voltage drops and / or ringing in the power cable; thermal cross-coupling between zones; and transient loading effects (e.g. inconsistent thermal contact between the heating zones and the hair 40). Therefore, smoothing may be applied to the power measurements, in order to improve the estimate of the hair temperature (and to reduce large fluctuations in the estimate of the hair temperature which could cause instability in the control of the amount of power supplied for each of the heating zones). To achieve such smoothing, when the device 1 comprises two opposing heaters (e.g. as illustrated in Fig. 2) the power determined to maintain a zone at the target temperature and the power determined to maintain the opposing zone at the target temperature may be summed. For example, referring to Fig. 26, the power needed to maintain zone (0,0,1) at its target temperature and the power needed to maintain zone (1,0,1) at its target temperature may be summed. Similarly, the power needed to maintain zone (0,1,1) at its target temperature and the power needed to maintain zone (1,1,1) at its target temperature may be summed, and so on. Alternatively, or additionally (and including devices for which there is no opposing heater), the mean power for zone 0 and zone 1 which span the transverse direction across the heater may be determined for a set of the heating zones. For example, referring to Fig. 26, the power determined to be needed to maintain zones (0,1,1), (0,2,1) and (0,3,1) at the target temperature of the heating zones may be calculated by determining the average power needed to maintain zones (0,1,1), (0,2,1) and (0,3,1) at the target temperature. Similarly, the power determined to be needed to maintain zones (0,1,0), (0,2,0) and (0,3,0) at the target temperature of the heating zones may be calculated by determining the average power needed to maintain zones (0,1,0), (0,2,0) and (0,3,0) at the target temperature. The subset of heating zones used to determine the average powers may be, but need not necessarily be, the subset of heating zones that are determined to be loaded with hair 40. Alternatively, or additionally, the power measurements for each heating zone may be subject to low-pass filtering. A nonlinear filter could be used, where the cutoff frequency for the nonlinear filter is higher for large amplitude input transitions, and thus helps to speed up response to large changes, but smoothes steady state signals. The determined temperature of the hair 40, TExit, as it leaves the hair contacting surface can be compared to a target hair temperature (e.g. a predetermined target hair temperature) which may be, for example, 155°C. However, it will be appreciated that any other suitable target temperature for the hair 40 may be used, and that the target hair temperature may depend on the particular styling method in use. As described above with reference to Fig. 17, resistance measurements can be used to determine the temperature of the heating zones, and the temperature of each heating zone can be controlled towards a target temperature for the heating zone by determining corresponding duty cycles and a firing sequence. In this example, if the temperature TExit is lower than the target temperature for the hair, then the target temperature for the heating zones is increased. Therefore, more power will be supplied to the heating zones, increasing the temperature of the heating zones, and increasing the temperature of the hair 40 towards the target temperature for the hair. Advantageously, by controlling the supply of power to the heating zones based on the estimated hair temperature, the target temperature that is set for a heating zone can be higher than the target temperature for the hair, which helps to more rapidly control the temperature of the hair towards the target temperature of the hair 40 without risk of damage to the hair 40. In contrast, if the target temperature for a heater of a conventional device was set to a temperature that is higher than the desired temperature for the hair 40, there would be a significant risk of overheating and damaging the hair, since the temperature of the hair 40 is unknown. If none of the heating zones are determined to be loaded with hair (e.g. based on the power needed to maintain a heating zone at a target temperature being lower than a predetermined threshold value), the temperature of the heating zones may be allowed to drift down to a predetermined idle temperature. The idle temperature may be equal to the target temperature for the hair 40 (which is a temperature that can be maintained on stationary hair without appreciable damage). Alternatively, the idle temperature may be higher than the target temperature for the hair, to ensure a more rapid heating of the hair when the user begins to pass the device 1 over the next tress. Fig. 28 shows a simplified schematic diagram, similar to Fig. 16, illustrating various parameters that may be measured by the device 1 and used to control the power supplied to the heating zones. Fig. 28 schematically illustrates various methods that are performed by the device 1, and it will be appreciated that these methods (e.g. the PID control) may be performed under the overall control of a controller of the device 1. In this example, two heating zones (zone 0 and zone 1) across the transverse width of the heater are illustrated. As illustrated in Fig. 28, the controller 29 is configured to output a target temperature for the heating zones, which is determined based on a determined temperature for the hair and a target temperature for the hair. The controller 29 may also be provided with the maximum target temperature that can be requested for one or more of the heating zones. The controller 29 may also be provided with other sensor data, such as data from an accelerometer of the device 1 (which, as described above, may be used to control the supply of power for the heating zones). As described above with reference to Fig. 17, a duty cycle can be determined for each of the heating zones, and in this example the determined duty cycles are provided to the controller 29. The target temperature for the heating zones is used as an input for the PID control for each of the heating zones. In this example the target temperature for Zone 0 and the target temperature for Zone 1 is the same temperature, but this need not necessarily be the case. As described above with reference to Fig. 17, the output of the PID determination is used in the determination of the duty cycle for the heating zones. The determined duty cycle is determined based on the difference between the target temperature for the heating zone and a measured temperature for the heating zone (which, as described above, can be determined by suppling a transient pulse of power to the corresponding heater electrode(s)). The duty cycle may also determined based on a maximum average power that can be supplied for the heating zone and / or a maximum instantaneous power that can be supplied for the heating zone. Fig. 28 also schematically illustrates the determination of the actual power supplied to each heating zone based on the determined duty cycle and the current passing through the corresponding heater electrode. Fig. 29 shows a flow diagram illustrating an exemplary method of controlling the temperature of the heating zones, to control the temperature of the hair 40 towards a target hair temperature. In Fig 29, N is the number of heating zones along the length of the heater. Kpmin is the minimum duty cycle threshold, for determining whether a position is loaded with hair, n is the number of loaded positions detected. pO and p1 are the average duty cycles for loaded zone 0 and loaded zone 1, respectively. p0_av and p1_av are the smoothed values of pO and p1. ratiopoWer is the duty cycle ratio (and therefore the power ratio) between zones 1 and 0. Thtr_req is the requested temperature for the heating zones. Kr is a constant used to modify the value of ratiopoWer and is proportional to Terror. Koffset is a temperature assumed for the hair as it comes into contact with the heater (the hair entry temperature). ThairEst is the determined temperature of the hair as it exits the device 1. Kratiomin and Kratiomax are constant thresholds that are used to help determine the value of ratiopoWer. Kp is a constant that is multiplied by Terror as a minimum threshold for p0_av_ and p1_av. KTtarget is the target hair exit temperature. Terror is the difference between ThairEst and TTtarget. Khyst is a hysteresis threshold for Terror, below which the value of Thtr_req is not changed. Kramp is a temperature increment by which the value of Thtr_req is changed. In this example the device 1 comprises two opposing heaters (although this need not necessarily be the case), and in step S280 the duty cycles are summed for each pair of opposing zones. In the example illustrated in Fig. 26, the duty cycles for zones (0,0,1) and (1,0,1) are summed, the duty cycles for zones (0,1,1) and (1,1,1) are summed, and so on. In step S281, a comparison of the summed duty cycles calculated in step S280 with the value of Kpmin is performed in order to determine which of the heating zones are loaded with hair. In other words, the determined duty cycle for a heating zone is compared to a threshold duty cycle to determine whether the heating zone is loaded with hair. This corresponds to comparing the power supplied to the heating zone to maintain the heating zone at a target temperature with a threshold power lever, to determine whether the heating zone is loaded with hair. In this example the determination of step S282 is performed only for each ‘zone 0’ (i.e. at a particular position in the transverse direction across the heater), but this need not necessarily be the case. In step S282, the power supplied for each ‘zone 0’ is summed, and the power supplied for each ‘zone T is summed, for all of the heating zones that are determined to be loaded with hair. For example, referring to Fig. 26, it may be determined that the zones that are loaded with hair are zones: (0,2,0), (0,3,0), (0,4,0), (0,2,1), (0,3,1), (0,4,1), (1,2,0), (1,3,0), (1,4,0), (1,2,1), (1,3,1) and (1,4,1). In this case, the power supplied to zones (0,2,0), (0,3,0) and (0,4,0) may be summed, and the power supplied to zones (0,2,1), (0,3,1) and (0,4,1) may be summed. Similarly, the power supplied to zones (1,2,0), (1,3,0) and (1,4,0) may be summed, and the power supplied to zones (1,2,1), (1,3,1) and (1,4,1) may be summed. In step S283 the sums for pO and p1 calculated in step S282 is normalised by dividing the sum by the number of heating zones used to calculate that sum. In step S284 the normalised sums are smoothed using a low pass filter, to determine p0_av and p1_av. In step S285, the power ratio between zone 0 and zone 1 is calculated using the equation: ratiopower = p1_av I p0_av (Equation 15) In step 286 the direction that the styler is passing over the hair is determined. In other words, it is determined whether it is zone 0 or zone 1 that first contacts a particular section of hair. If the value of ratiopoWer is less than one, then it is determined that zone 0 contacts the hair before zone 1. In contrast, if the value of ratiopoWer is greater than 1, then it is determined that zone 1 contacts the hair before zone 0, and in this case the value of ratioPower is inverted (the reciprocal of the original value of ratiopoWer is determined and used as the value of ratiopoWer in future calculations). In step S287, the temperature of the hair 40 as it leaves the hair contacting surface is estimated using the equation: T hairEst — T htr_req (Thtr_req Koffset) ratiOpower ratiOpower (Equation 16) Alternatively, as will be described in more detail later, a neural network could be used to estimate the value of ThairEst based on the values of p0_av, p1_av, Thtr_req. In step S288 a determination is made of whether ratiopoWer >Kratiomin, and ratiopower <Kratiomax. If the result of this determination is ‘NO’, then the method proceeds to step S289a, and the temperature of the heating zones is allowed to drift downwards (in this example, by the value of KramP in each iteration of the control loop). If the result of the determination of step S288 is ‘YES’, then the method proceeds to step S289b, in which the estimated temperature of the hair is compared to the target temperature of the hair to determine the value of Terror: Terror — Kytarget _ ThairEst (Equation 17) In step S290b, a new target temperature for the heating zones is determined based on the value of Terror. In this example, the current target temperature for the heating zones is increased by the value of (Terror * K). In other words, K is a constant that is used to control the magnitude of the change in the target temperature for the heating zones for a given difference between the estimated hair temperature and the target hair temperature. It will be appreciated that the values of the constants in the method illustrated in Fig. 29 may have any suitable values, and will depend on the particular heater configuration used. Optionally, simulated annealing may be used to determine values for one or more of those constants. In simulated annealing, the variables are initially set randomly (or semi-randomly within an acceptable range), and then each variable is incremented in random increments and tested (e.g. by measuring the temperature of the hair as it exits the device 1 using a thermal imaging camera). If the performance of the device 1 is improved using that set of parameters, then the incremented variables become the new base value for the next random iteration. Various further strategies that will be well known to the skilled person can also be used to avoid the iterative updating of the values of the constants from becoming stuck in local minima (e.g. by progressively increasing or reducing the size of the random increments). Simulated Hair Heating The results of the control method illustrated by the flow diagram of Fig. 29 can be visualised using a software simulation of the device 1 and virtual tress of hair 40. In the simulation, the simulated device 1 is passed over the tress of hair, and the temperature of the hair 40 at various positions is simulated. The heaters and the tress of hair can be simulated as thermal masses separated by a predetermined thermal resistance. The simulations can be performed using various different (and variable) speeds for the device 1 passing over the hair 40, enabling the results to be visualised for a variety of different user behaviours and heater control methods. Fig. 30 schematically illustrates the temperature distribution of the hair 40 after the device 1 has been passed over a section of hair for the first time (a ‘first pass’ over the hair 40). As illustrated in Fig. 30, the hair 40 has been divided into a grid of 4x16 virtual areas, and the temperature of the hair is simulated for each virtual area. The heater first comes into contact with the tress at the virtual areas at the top of the figure (in other words, the virtual areas at the top of Fig. 30 corresponds to the start of the tress). Each value in Fig. 30 indicates the temperature of that virtual area in °C. Fig. 30 shows the results of a first pass of the device 1 over the hair when the heaters are maintained at a constant temperature of 185 °C. As illustrated in Fig. 30, this results in some overheating at the start of the tress, to approximately 180 °C. Fig. 31 schematically illustrates the temperature distribution of the hair 40 after the device 1 has been passed over a section of hair for the second time (a ‘second pass’ over the hair 40), and the heaters continue to be maintained at a constant temperature of of 185 °C. As illustrated in Fig. 31, significant overheating of the hair 40 has occurred throughout the tress. Fig. 31 illustrates why the temperature of the heaters of conventional devices cannot be set significantly higher than the target hair temperature, since without controlling the temperature of the heaters based on the estimated hair temperature there is a risk of overheating and damaging the hair if the temperature of the heater is allowed to be higher than the target temperature for the hair. Fig. 32 schematically illustrates the temperature distribution of the hair 40 using the method illustrated in Fig. 29 to control the target temperature for the heating zones based on the difference between the target exit temperature for the hair and the estimated exit temperature of the hair, and using the method illustrated in Fig. 17 to determine the temperature of the heating zones and determine the firing sequences. In this example the temperature of the heating zones varied dynamically between 155°C and 220°C, and the target temperature for the hair is 155°C. Advantageously, by virtue of the control of the target temperature for the heating zones being based on the estimated exit temperature of the hair, overheating is greatly reduced compared to the example of Fig. 31, despite the temperature of the heating zones exceeding the target temperature for the hair (which enables more rapid heating of the hair to the target hair temperature). Fig. 33 illustrates a second pass of the device 1 over the hair using the same control method as for Fig. 32. As shown in the figure, near-uniform heating of the hair 40 to temperatures very close to the target hair temperature has been achieved. As described above, advantageously the target temperature for the heating zones can be controlled based on the estimated temperature of the hair as it leaves the hair contacting surface. The change in the target temperature for a heating zone may be proportional to the difference between the target temperature for the hair and the estimated temperature for the hair. However, if the changes in the target temperature for a heating zone in each control loop are too large, then the PWM drive to the heater may become saturated, resulting in instability in the control of power to the heater(s). In other words, if the target temperature for one or more heating zones is suddenly changed by a large amount, then it may not be possible to supply sufficient power for all of those heating zones in one control iteration, to control the heating zones towards the new target temperature, without exceeding instantaneous power limits or average power limits of the heater(s). To mitigate against this, a maximum allowed change in the target temperature for a heating zone may be defined. A time period may also be configured during which, following the increase in the target temperature for a heating zone, the target temperature cannot be increased again until the period has expired. Neural Network As described above, the temperature of the hair can be estimated based on the ratio of the power needed to maintain zone 0 at the target temperature for zone 0, and the power needed to maintain zone 1 at the target temperature for zone 1 (e.g. using Equation 14 or Equation 16). Alternatively, a neural network can be used to estimate the temperature of the hair as it leaves the device 1. In this example, the use of Equation 14 or 16 is replaced by the use of a network of nodes and static weights. Fig. 34 shows a simplified schematic illustration of a neural network that could be used to estimate the exit temperature of the hair 40. As illustrated in the figure, the network 200 comprises an input layer 310, one or more hidden layers 320, and an output layer. Parameters that can be measured, determined or estimated by the device 1 out used as the inputs in the input layer 310, and the output layer 330 outputs the estimated exit temperature for the hair 40. In this example the input layer comprises three nodes, and there are two hidden layers each comprising four nodes. However, it will be appreciated that the number of nodes and layers is not limited, and any suitable number of nodes and layers could alternatively be used. The variables input into the input layer 310 for the case of two heating zones arranged across the transverse width of the heater (zone 0 and zone 1) may be, for example, the average power supplied to the first heating zone (zone 0) in a time period, the average power supplied to the second heating zone (zone 1) in the time period, and the target temperatures for those heating zones (which may be the same target temperature, but need not necessarily be the same target temperature). It will be appreciated that the neural network can be trained, during a training phase, to output the estimated hair temperature based on the input into the input layer 310. Methods of training neural networks of the type illustrated in Fig. 34 will be well known to the skilled person. During the training phase, the actual exit temperature of the hair 40 could be measured using any suitable sensor (e.g. an infrared camera), and the difference between the actual exit temperature and the exit temperature estimated using the neural network can be backpropagated into the network, to adjust the weights of the network. Fig. 35 shows a further example of a neural network that could be used to generate an estimate of the hair exit temperature. In this example, the inputs into the neural network are the average power supplied to the first heating zone (zone 0) in a time period, the average power supplied to the second heating zone (zone 1) in the time period, and the target temperatures for those heating zones (which may be the same target temperature, but need not necessarily be the same target temperature). However, different from the example illustrated in Fig. 34, in this example the values of those parameters at four different times are provided as inputs. In other words, in the example of Fig. 35, the current values of the parameters are used, but additional the values of the parameters at three previous times are also used as inputs. Advantageously, this enables the neural network to output the estimated exit temperature for the hair based on changes over time of the state of the heating zones. The inputs into the neural network may be referred to as a ‘multivariate time series’. In other words, in additional to the latest 3-value vector (it will be appreciated that more than three values could alternatively be used), a sliding timeline of previous vectors are also provided as input. Whilst in the example illustrated in Fig. 35 three previous sets of values are used, this need no necessarily be the case. Alternatively one previous set of values, two previous sets of values, or four or more previous sets of values may be used as inputs into the input layer of the neural network. For example, when 5 sets of previous values are used, the neural network may have 18 input nodes, one hidden layer of 7 nodes (which may have a sigmoid characteristic), and one output node. In this case the input nodes form a 6-stage, multivariate, sequential series of 3-element vectors. Whilst in this example each 3-element vector comprises the average power supplied to the first heating zone (zone 0) in a time period, the average power supplied to the second heating zone (zone 1), alternatively the duty cycle for the first zone during the time period and the duty cycle for the second zone during the time period could be used. The duty cycles may be the average duty cycle for all of the loaded zones at that position (i.e. the average duty cycle for each loaded ‘zone O’, and the average duty cycle for each loaded ‘zone T). Similarly, the average powers may be the average power supplied to each loaded ‘zone 0’ and the average power supplied to each loaded ‘zone T in the time period. Alternatively, rather than determining which heating zones are loaded and therefore to be used to generate the inputs for the neural network, alternatively the input to the neural network may comprise the power or duty cycle for every heating zone, and the neural network will therefore effectively have information regarding which zones are loaded. In this case, the neural network may be configured to output an additional indication, via the output layer, or which heating zones are loaded with hair 40 (it will be appreciated that the neural network could be trained to do this in the training phase, and which zones are actually loaded with hair could be monitored using a camera for the training phase). Similarly, the neural network may be configured to output an indication of the tress width. Fig. 36 shows a further example of a simulated temperature distribution along a tress of hair when control of the heaters using the method illustrated in Fig. 29 is performed, and Fig. 37 shows the temperature distribution when the neural network is used to estimate the exit temperature of the hair (rather than Equation 16). In the example of Figs. 36 and 37, a movement of the device 1 over the hair 40 is simulated in which initially the heaters are stationary and engaged with the hair for a few seconds. The device is then moved over the hair at a speed of 40 mm / s. The device is then stopped part-way along the tress for a few seconds, before being moved along to the end of the tress. This corresponds to a relatively extreme situation of erratic movement of the device along the hair 40 by the user. Nevertheless, even under these challenging conditions, as illustrated in Figs. 36 and 37 both control methods achieve relatively uniform heating of the hair to around the target temperature of 155°C, without significant overheating of the hair 40. It is noted that, in additional to verifying the performance of the device 1, the simulated temperature distributions and simulated movements of the device 1 over the hair 40 can be used to generate additional training data for the neural network. For example, training data can be generated using 300 ‘pulls’ (or any other suitable number of pulls) of the device 1 along the hair 40, with random variations in the heater to hair thermal resistance, target temperature, and hair entry temperature. 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 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 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. 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. 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 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; anda controller;wherein the plurality of independently controllable heating zones comprise one or more first heating zones and one or more second heating zones; andwherein the controller is configured to perform control to cause the apparatus to, when hair passes from the one or more first heating zones to the one or more second heating zones before leaving the one or more second heating zones:determine a first power or first duty cycle needed to maintain the one or more first heating zones at a first target temperature for the one or more first heating zones;determine a second power or second duty cycle needed to maintain the one or more second heating zones at a second target temperature for the one or more second heating zones;estimate, based on the first power and the second power, or based on the first duty cycle and the second duty cycle, a temperature of the hair as the hair leaves the one or more second heating zones; andcontrol the power supplied to the independently controllable heater electrodes based on the estimated temperature of the hair.

2. The apparatus according to claim 1, wherein the apparatus is configured to:determine a third target temperature for the one or more first heating zones or a power to be supplied to the one or more first heating zones based on the estimated temperature of the hair; anddetermine a fourth target temperature for the one or more second heating zones or a power to be supplied to the one or more second heating zones based on the estimated temperature of the hair.

3. The apparatus according to claim 2, wherein the apparatus is configured to determine the third target temperature and the fourth target temperature based on a difference between a target temperature for the hair and the estimated temperature of the hair.

4. The apparatus according to any preceding claim, wherein the estimation of the temperature of the hair is based on a square of a ratio of the first power and the second power.

5. The apparatus according to any preceding claim, wherein the first target temperature is the same as the second target temperature.

6. The apparatus according to any preceding claim, wherein the apparatus is configured to determine that the hair is passing from the one or more first heating zones to the one or more second heating zones based on the first power being greater than the second power.

7. The apparatus according to any preceding claim, wherein the apparatus is configured to estimate the temperature of the hair as the hair leaves the one or more second heating zones using a neural network.

8. The apparatus according to claim 7, wherein the neural network comprises: an input layer;one or more hidden layers; andan output layer;wherein inputs for the input layer comprise: the first power or first duty cycle needed to maintain the one or more first heating zones at the first target temperature, and the second power or second duty cycle needed to maintain the one or more second heating zones at the second target temperature; andwherein an output of the output layer comprises the estimated temperature of the hair as the hair leaves the one or more second heating zones.

9. The apparatus according to claim 8, wherein the inputs for the input layer comprise at least one of the first target temperature for the one or more first heating zones or the second target temperature for the one or more second heating zones.

10. The apparatus according to claim 8 or 9, wherein the inputs for the input layer comprise:the first power or first duty cycle needed to maintain the one or more first heating zones at the first target temperature in a first time period;the second power or second duty cycle needed to maintain the one or more second heating zones at the second target temperature in the first time period;the first power or first duty cycle needed to maintain the one or more first heating zones at the first target temperature in a second time period, different from the first time period; andthe second power or second duty cycle needed to maintain the one or more second heating zones at the second target temperature in the second time period.

11. The apparatus according to any preceding claim, wherein the apparatus comprises a plurality of the first heating zones and a plurality of the second heating zones;wherein the first power or first duty cycle is a first average power or first average duty cycle to maintain the first heating zones at the first target temperature; andwherein the second power or second duty cycle is a second average power or second average duty cycle to maintain the second heating zones at the second target temperature.

12. The apparatus according to claim 11, wherein the apparatus is configured to: determine which of the first heating zones are loaded with hair;determine the first average power or first average duty cycle using only the first heating zones that are determined to be loaded with hair;determine which of the second heating zones are loaded with hair; anddetermine the second average power or second average duty cycle using only the second heating zones that are determined to be loaded with hair.

13. The apparatus according to any preceding claim,wherein the apparatus comprises a plurality of the first heating zones and a plurality of the second heating zones;wherein the one or more first heating zones are arranged along a longitudinal length of the heater at a first position along a transverse width of the heater; andwherein the one or more second heating zones are arranged along the longitudinal length of the heater at a second position along a transverse width of the heater, different from the first position.

14. The apparatus according to any preceding claim, wherein the one or more first heating zones and the one or more second heating zones are arranged across a width of the heater.

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

16. The apparatus according to any preceding claim, wherein the heater 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.

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

18. 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.

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

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