Heater apparatus for a hair styling appliance and methods of control

Abstract

A hair styling appliance comprises a heater (6a, fig.3) with a plurality of heating zones (Z1-Z7, fig.3) on the hair contacting surface and a plurality of touch sensors proximal to each heating zone c

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 hair drying or styling appliances / devices (e.g., a hair curler, or the like) comprising one or more low thermal mass heaters, and means of controlling those low thermal mass heaters in response to detecting user skin contact with one or more heated surfaces of the styling appliance / device (e.g., finger / hand contact and / or scalp contact). Background to the Invention Heated hair styling tools or hair drying tools use heat to increase the temperature of hair to a desired styling or drying temperature. For example, a hair 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 or drying. Hair may be styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry. Existing hair styling appliances typically use relatively thick heating plates or heating tubes that provide a certain amount of thermal mass to the hair styling appliance. These heating plates or tubes are heated by a heater that is mounted on an inner surface of the heating plate / tube. As a result of the thermal mass, the heating plates / tubes take time to heat up and, once heated, they can take quite a long time to cool down. This thermal mass makes it quite difficult to control the heating of the hair. Furthermore, overheating or underheating of the hair can also occur with such hair styling appliances that use relatively thick heating plates or heating tubes. 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 quicker. Additionally, low thermal mass heaters are more responsive and are easier to control. Nevertheless, it will be appreciated that whilst rapid heat up and cool down of low thermal masses provides many benefits when implemented in hair styling appliances as alluded to above, the provision of such low thermal mass heaters does raise safety concerns. In particular, given the rapid heat up of such low thermal mass heaters, there is a risk of an end user experiencing skin contact burning if their skin comes into contact with a heat surface of the hair appliance unknowingly as it is heating up. This may especially be the case where a user pre-loads their hair into the hair styler and then activates heating. There is therefore a need for mechanisms to address the risk of skin contact burning in hair stylers that use low thermal mass heaters. The present invention aims to address or at least partially solve the issue of skin contact burning that arises from a user touching a heat surface of a hair styler, or the like. Summary of Invention In one aspect the invention provides a hair styling appliance comprising a multilayer heater comprising a plurality of functional layers that are bonded together. The multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating. At least one of the layers of the multilayer heater is a heater electrode layer comprising a plurality of independently powerable heater electrodes that define a corresponding plurality of heating zones on the hair contacting surface of the multilayer heater, each heater electrode being formed of a conductive material that generates heat when a current is passed through it. The hair styling appliance further comprises a plurality of touch sensors, wherein at least one touch sensor is provided proximal to each heating zone and is configured to detect user skin contact with the heating zone. Furthermore, the hair styling appliance comprises control circuitry comprising at least one processor configured to perform a control action in respect of at least one heating zone of the plurality of heating zones in response to a touch sensor detecting user skin contact with the at least one heating zone. In an alternative aspect, the hair styling appliance comprises a plurality of touch sensors, wherein at least one touch sensor is provided proximal to each heating zone and is configured to output signals indicating user skin contact with the heating zone, and control circuitry comprising at least one processor configured to process the signals from the plurality of touch sensors to detect user skin contact with any of the heating zones and, in response to detecting user skin contact with at least one heating zone, configured to perform a control action in respect of the at least one heating zone. The control action in respect of at least one heating zone may comprise control of a temperature of the at least one heating zone. The control action in respect of at least one heating zone may, for example, comprise controlling a power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone. Controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone may comprise controlling the power provided to the independently powerable heater electrode to reduce a temperature of the at least one heating zone to a temperature below e.g., 60 degrees Celsius. Alternatively (or additionally), the power provided to the independently powerable heater electrode may be controlled to reduce a temperature of the at least one heating zone to a temperature equal to (or below) a ‘safe touch’ temperature, wherein the ‘safe touch’ temperature is a temperature deemed low enough to prevent burning and discomfort to the skin / scalp of an end user of the hair styler during use. It will be appreciated that the temperature deemed low enough to prevent burning and discomfort to the skin / scalp of an end user of the hair styler during use may be defined in terms of both the temperature of the at least one heating zone, and the duration of skin contact. For example, a safe touch temperature may be any temperature T for which it is deemed safe for a user’s skin / scalp to touch a heater of the hair styler for s seconds. Controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone may comprise reducing the power provided to the independently powerable heater electrode to a power level corresponding to a heating zone temperature of below e.g., 60 degrees Celsius. Controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone may comprise controlling the power provided to the independently powerable heater electrode by preventing power being provided to the independently powerable heater electrode. Controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone may comprise sending a control signal to drive circuitry of the control circuitry to control the power provided to the independently powerable heater electrode. Each touch sensor of the hair styling appliance may form part of a corresponding electrical circuit with a capacitance. Each touch sensor may comprise a respective transceiver electrode that is provided with an excitation signal, and upon user skin contact with the transceiver electrode, the capacitance of the corresponding electrical circuit may increase, indicating that user skin contact has occurred in the at least one heating zone. The increase of the capacitance of the corresponding electrical circuit may cause an electrical signal output by the transceiver electrode to have a lower frequency and / or a lower voltage than the excitation signal provided to the transceiver electrode, indicating that user skin contact has occurred in the at least one heating zone. The hair styling appliance may further comprise at least one synchronous detector connected to the transceiver electrode of the at least one touch sensor. That at least one synchronous detector configured to receive the excitation signal provided to the at least one touch sensor, receive the electrical signal output from the at least one touch sensor, and processes both the received excitation signal and the electrical signal output to generate an electrical signal for output from the at least one synchronous detector to the processor of the control circuitry, wherein an electrical signal for output from the at least one synchronous detector with a non-zero amplitude indicates that user skin contact has occurred in the at least one heating zone. In another aspect each touch sensor of the hair styling appliance may form part of a corresponding electrical circuit with a capacitance and comprises a transmitter electrode and a receiver electrode, the transmitter electrode being provided with an excitation signal and, upon user skin contact with the touch sensor, the capacitance of the corresponding electrical circuit may decrease, indicating that user skin contact has occurred in the at least one heating zone. The decrease of the capacitance of the corresponding electrical circuit may cause an electrical signal output by the receiver electrode to have a higher frequency and / or a higher voltage than the excitation signal provided to the transmitter electrode, indicating that user skin contact has occurred in the at least one heating zone. In this aspect of the invention the hair styling appliance may also comprise at least one synchronous detector connected to the receiver electrode of the at least one touch sensor. The at least one synchronous detector may be configured to receive the excitation signal provided to the at least one sensor, receive the electrical signal output from the at least one touch sensor, and process both the received excitation signal and the electrical signal output to generate an electrical signal for output from the at least one synchronous detector to the processor of the control circuitry, wherein an electrical signal for output from the at least one synchronous detector with a non-zero amplitude indicates that user skin contact has occurred in the at least one heating zone. In an aspect of the invention, each touch sensor of the hair styling appliance comprises an electrical circuit with a capacitor, each capacitor comprises at least one transceiver electrode, or respective transmitter and receiver electrodes. In an aspect of the invention, the transmitter electrode of each touch sensor of the hair styling appliance may be connected to a multiplexer configured to provide a single excitation signal to each touch sensing capacitor of the plurality of touch sensing capacitors. In this case, the receiver electrode of each touch sensing capacitor may be commoned. In another aspect of the invention, the receiver electrode of each touch sensor may be connected to a multiplexer configured to receive an electrical signal output from each touch sensor of the plurality of touch sensors for provision to a single synchronous detector. In this case, the transmitter electrode of each touch sensor is commoned. In another aspect of the invention, the transmitter electrode and the receiver electrode of each touch sensor may be connected to a respective first and second multiplexer. In this case, the first multiplexer may be configured to provide a single excitation signal to each touch sensor of the plurality of touch sensors, and the second multiplexer may be configured to receive an electrical signal output from each touch sensor of the plurality of touch sensors for provision to a single synchronous detector. In another aspect of the invention, the transmitter electrode of each touch sensor is formed from a heater track of a respective heater electrode. The transmitter electrode of each touch sensor may be formed from a heater track of one of the corresponding independently powerable heater electrodes. In another aspect of the invention there is provided a method performed by the hair styling appliance in use for styling and / or drying hair. The method comprises sending, by a signal generator, an excitation signal to a plurality of touch sensors, wherein one or more touch sensors of the plurality of touch sensors are provided proximal to each heating zone of the hair styling appliance and are configured to detect user skin contact with the heating zones of the hair styling appliance; detecting user skin contact with at least one of the heating zone of the hair styling appliance; receiving, in response to detecting user skin contact with at least one of the heating zone of the hair styling appliance, a response signal indicating that user skin contact with the at least one of the heating zone has occurred; and performing, by control circuitry of the hair styling appliance comprising at least one processor, a control action in respect of the at least one heating zone of the plurality of heating zones in response to a touch sensor detecting user skin contact with the at least one heating zone. Performing the control action in respect of the at least one heating zone may comprise controlling a power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone. Controlling the power provided to the independently powerable heater electrode may comprise: reducing a temperature of the at least one heating zone to a temperature below 60 degrees Celsius; and / or reducing the power provided to the independent powerable heater electrode to a power level corresponding to a heating zone temperature of below 60 degrees Celsius; or controlling the power provided to the independently powerable heater electrode by preventing power being provided to the independently powerable heater electrode. Brief Description of the Drawings Figure 1a illustrates a handheld (portable) hair styler; Figure 1b illustrates the handheld (portable) hair styler of Figure 1a in use; Figure 2a illustrates an exploded view of a low thermal mass heater implemented in the hair styling device of Figure 1a; Figure 2b illustrates a perspective see-through assembled view of the low thermal mass heater shown in Figure 2a; Figure 3a illustrates a set of heating zones Z1 to Z7 of the hair styling device 1; Figure 3b schematically illustrates the heating zones on the heating surface of the heater shown in Figure 3a; Figure 3c schematically illustrates an alternative arrangement of heating zones; Figure 3d schematically illustrates a further alternative arrangement of heating zones that are of different sizes and shapes; Figure 3e 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 3f 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 4 is a simplified block diagram of control circuitry of the hair styling device of Figure 1a; Figure 5 illustrates example circuitry for a skin sense / touch sense electrode that may be implemented in the hair styling device of Figure 1a; Figure 6 illustrates another example circuitry for a skin sense / touch sense electrode that may be implemented in the hair styling device of Figure 1a; Figure 7 illustrates example circuitry for a synchronous detector that may be used in the example circuitry of Figures 5 and 6; Figure 8a illustrates an example input signal that may be input to the synchronous detector circuitry of Figure 7 in response to a skin touch detection by a sensor; Figure 8b illustrates an example of the input signal of Figure 8a once it has been passed through a low-pass filter; Figure 8c illustrates an example of an output signal output by the synchronous detector of Figure 7; Figure 9 illustrates example circuitry for a quadrature synchronous detector that may be used in the example circuitry of Figures 5 and 6; Figure 10a and 10b illustrate real-world scenarios in which the hair styling device of Figure 1a may be used; Figure 11 illustrates an example of a 1-to-8 multiplexer arrangement for the implementation of a multizone touch sensor in the hair styling device of Figure 1a; and Figure 12 illustrates an example of an adaptation of a heater in the hair styling device of Figure 1a to use said heater as a touch sensor. Overview Figure 1a illustrates a handheld (portable) hair styler 1. The hair styler 1 includes a first movable arm 4a and a second movable arm 4b, which are coupled at proximal ends thereof to a shoulder 2. The first arm 4a bears a first heater 6a at its distal end, and the second arm 4b bears a second heater 6b at its distal end. The first and second heaters 6a, 6b oppose one another and are brought together as the first and second arms 4a, 4b are moved from an open configuration to a closed configuration. As shown in Figure 1 b, during use, a tress of hair 40 is sandwiched between the two arms 4 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 6a, 6b. Therefore, as the user pulls the hair styler 1 along the tress of hair 40, the tress of hair 40 is heated by conductive heating to a suitable temperature to facilitate styling. The heaters 6a, 6b are low thermal mass heaters and are therefore able to heat up and cool down rapidly. Whereas the heaters of a typical conventional hair styler may take, for example, around 30 seconds to reach an operating temperature of about 200°C from ambient temperature, a low thermal mass heater operating at maximum power may take less than one second to reach the same operating temperature. It will be appreciated that the exact operating temperature and heat-up time depend on the particular requirements of the device, and on the maximum power output of the device. By way of example, a low thermal mass heater may heat up at a rate of approximately 410 °Cs’1, and may cool from an operating temperature for styling to a touchable temperature in a timescale of the order of a few seconds. A user interface 11 is provided to allow the user to turn the device on or off and could also be used to enable the user to set user-definable parameters of the device 1 or to output information to the user. For example, a desired operating temperature for the heaters 6a, 6b could be input via the user interface 11. The user interface 11 may have a dial, button 14, or touch display for allowing the user to input information into the device 1 and the user interface 11 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. The user interface 11 may also comprise an indicator light 15 to indicate whether the device is on. A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 1 and carries the control circuitry for controlling the operation of the device 1 and for controlling the interaction with the user via the user interface 11. In this example, electrical power is provided to the device 1 by means of a power supply located at an end of the device, via a power supply cord 3. The power supply may be an AC mains power supply. However, in an alternative embodiment the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g., from the mains ora 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 heater 106 the hair absorbs a significant amount of the heat energy. The heaters 6 must quickly supply the lost heat energy back to the heating surface otherwise the temperature of the heating surface will drop and potentially impact on the quality of the thermal styling. If the temperature of the heaters 106 fall below that required to raise the hair temperature above the glass transition temperature of the hair, the hair will not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo significant damage. As such, the device 1 must be able to control the temperature so that the heating surface of the heaters 6 remains within a particular temperature range. Furthermore, it must maintain the temperature range both when hair is frequently and quickly loaded and unloaded onto the heating surface, and when hair is held on the heating surface for a prolonged period of time. 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 to detect closure and opening of the arms 4a, 4b. The present invention is not limited to the type of device illustrated in Figure 1a. For example, a device that additionally transfers heat to hair using heated air could be used. When heated air is used to heat the user’s hair, the device may comprise a heater having an electrically powered heating coil (or any other suitable type of heating element), operable to heat air drawn in by a fan assembly. It will be appreciated that the apparatus and methods described below are applicable to any suitable device (e.g., hair styling or hair drying device) comprising one or more low thermal mass heaters. It will also be appreciated that while Figure 1a depicts a ‘hair-straightener’ type hair styling device, other types / designs of hair styling devices are possible and that the following description regarding skin contact detection may apply equally to such other hair styling devices. By way of example only, the hair styling device may alternatively be a hair curler / curling type device that is used to curl hair. In this scenario, the hair curler may have (but need not necessarily have) a generally tubular heater 6 (or set of heaters 6). Exemplary Heater Configuration Figures 2a and 2b 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 2a, the heaters 6a, 6b include an upper dielectric (electrically insulating) layer 62, an electrode layer 63 that has a plurality of separate heater electrodes 64, and a lower dielectric layer 66 which electrically insulates the heater electrodes 64 from other components mounted behind the heater 6a, 6b. The three layers 62, 63 and 66 are bonded together (mechanically or chemically) and define a heater 6 that is very thin (the three layers have an overall thickness of between 30pm to 1000pm in the case of low voltage operation (less than about 40 Volts) and 0.8mm to 2.0mm in the case of AC operation) and with very low thermal mass. The upper dielectric layer 62 provides the hair contacting surface of the heater 6, although a nonstick coating may be applied to the upper surface of the layer 62 to facilitate the passage of the user’s hair 40 over the heating surface (although the dielectric layer 62 itself may have non-stick properties and so a separate non-stick coating may not be required). 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 2a and 2b 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 2b) 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 2b) of the heater 6. The arrangement of heating zones illustrated in Figure 3c, 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 3c 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. Heating Zones Independently controllable heating zones 642 of the heaters 6a, 6b will now be described with reference to Figure 3. Figure 3a shows a set of heating zones 642 (Z1 to Z7) of the hair styling device 1. Whilst the zones 642 are illustrated only for the first heater 6a, it will be appreciated that 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 3b schematically illustrates the heating zones 642 on the heating surface of the heater shown in Figures 1 and 3a. Whilst in the example shown in Figure 3b 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 3c 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 3b and 3c are all the same size, this need not necessarily be the case. Different sized heating zones 646 may be provided, as illustrated in Figure 3d, 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 3e), for example for use in a hair curler device. In a further alternative the heater could have a curved form as illustrated in Figure 3f, for example for use in a heated hairbrush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device. Device Control Circuitry Figure 4 is a simplified block diagram of control circuitry 15 (e.g., a microcontroller unit) that controls the operation of the hair styler device 1 shown in Figure 1a. As shown, the control circuitry 15 comprises a power supply 21 that, in this embodiment, derives power from a battery power source (e.g., a battery having a voltage between 3 V and 42 V, although any other suitable voltage battery could alternatively be used). A mains power supply input may be provided to charge the battery via an AC to DC converter (not shown), which may be external or internal to the device 1. Alternatively, the power supply 21 may derive power from an AC mains supply input. In this example, power is provided to the heaters 6 for heating the user’s hair. The power supplied to the heaters 6 is controlled by a controller 28 having a microprocessor 29. The power supplied to the heaters 6 is controlled by drive circuitry 23 (which may include one or more power semiconductor switching devices (triacs)) which controls the application of an AC mains voltage, or a DC voltage derived from the AC mains or from a battery to the heaters 6 in accordance with instructions from the microprocessor 29. The microprocessor 29 is coupled to a memory 30 (which is typically a non-volatile memory) that stores processor control code for implementing one or more control methods that control the heating of the heaters 6 in accordance with a desired operating temperature of the heaters 6 (for each of the independently controllable heating zones) and sensed temperatures of the heaters obtained from temperature measurement circuitry 25. The memory 30 may store, for example, one or more operating profiles or parameters (e.g., a target temperature for each of the heating zones, a maximum current limit, or a maximum power limit). Software stored in the memory 30 may include, for example, an operating system and a heater control module suitable for implementing one or more of the methods described below. The temperature measurement circuitry 25 may comprise temperature sensors such as thermistors or may use circuitry that senses the resistance of heater electrodes that are used to heat the heaters 6, which resistance depends on the temperature of the heater electrode. The temperature measurement circuitry 25 may comprise a 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 4 also shows a user interface 11 that is coupled to the microprocessor 29, for example to provide one or more user controls and / or output indications such as a visual indication or an audible alert. The output(s) may be used to indicate to the user, for example, if they have inserted too much hair between the heaters 6, if they are moving the device 1 too quickly along the hair tress 40, or simply if the device is on or off. In this example, the control circuitry 15 also comprises communications circuitry 27 to allow the device to communicate with a remote sensor, a remote server, or a remote application (e.g., on a mobile telephone). The communications circuitry 27 may use, for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the remote device. The communications circuitry 27 could be used to receive an input parameter from a remote device, for example a desired operating temperature for the heaters 6 for styling hair. As those skilled in the art will appreciate, the device 1 does not necessarily need to have all of the blocks illustrated in Figure 4. For example, if the device 10 is a hair straightener, then there is not necessarily a need for the communications circuitry 27. Skin Contact Detection Self-Capacitance Figure 5 illustrates an example of a circuit implemented in hardware in the hair styler 1 of Figure 1a to detect skin contact with the hair styler 1. It may be understood that whilst skin contact is depicted relative to a hand, this is for illustrative purposes only and skin contact may more generally be understood to comprise any form of skin contact. As shown in Figure 5, by way of example only, skin contact of a user 402 with the hair styler 1 is detected by capacitive coupling. For example, an array of capacitors 404 (404-1, 404-2, etc.) - i.e., an array of touch sensors - is provided proximal to the surface of the hair styler 1 at regions corresponding to locations of the heating zones Z1-Z7 642 to detect whether a user’s skin comes into contact with the surface of those regions (hereafter referred to as ‘heat surfaces’) e.g., to detected whether the skin of a user 402 comes into contact with an electrode 406 of at least one capacitor 404-1 of the array of capacitors 404. For simplicity, only one capacitor (‘C4’) 404-1 of the array is depicted in Figure 4. By way of example only, that capacitor 404-1 (hereafter referred to as ‘a sensing capacitor’ 404-1) may be a sensing capacitor located proximal to a heat surface of the hair styler 1. As shown in Figure 5, the sensing capacitor 404-1 comprises a transmitter / receiver (e.g., a single transceiver) electrode 406. The electrode 406 of the sensing capacitor 404-1 is connected to ground 410 via a capacitor 408 (‘C3’) and is provided with an excitation signal 412 that first passes via a pre-defined impedance 414 (‘Z1’). The excitation signal 412 is also provided to a synchronous detector 420. The transceiver electrode 406 is also connected to an amplifier 418 which is in turn connected to the synchronous detector 420. During operation, an excitation signal 412 is provided to the transceiver electrode 406 of each sensing capacitor of the array of sensing capacitors 404 for sensing a touch of a user 402. Each sensing capacitor of the array is provided proximal to a corresponding heat surface of the hair styler 1 corresponding to where the heating zones Z1-Z7 642 are located. When the user’s skin comes into contact with, or is at close proximity to a heat surface of the hair styler 1, that skin contact results in the user forming a capacitor 403 (‘C2’) with body capacitance, parallel to the sensing capacitor 404-1 provided proximal to the heat surface. The formed capacitor 403 (‘C2’) arises because once the user 402 touches the heat surface with their skin, they become a human capacitor between the surface of the hair styler 1 and Earth 410. By the user 402 becoming a capacitor of the electrical circuit that is parallel to the sensing capacitor 404-1, the apparent capacitance of the overall circuit increases, thereby altering the electrical signal output by the transceiver electrode 406. For example, by increasing the overall capacitance of the circuit of Figure 4, the electrical signal output by the transceiver electrode 406 may have a lower frequency and / or a lower voltage than that of the excitation signal 412 provided to the transceiver electrode 406. The electrical signal output by the transceiver electrode 406 is subsequently provided to the synchronous detector 420 to detect whether a user’s skin is touching a heat surface of the hair styler 1. The synchronous detector 420, for example, may receive both the electrical signal output from the transceiver electrode 406 and the initial excitation signal 412 as a reference signal to determine changes between the initial excitation signal 412 and the electrical signal output from the transceiver electrode 406. A signal is then output from the synchronous detector 420 that indicates that the user 402 is touching a heat surface of the hair styler 1 with their skin; for example, the signal output from the synchronous detector 420 may be analysed / processed by a processor of the hair styler 1 to determine that the user 402 is touching a heat surface proximal to the touched sensing capacitor that is being touched with their skin. Operation of the synchronous detector 420 is disclosed in further detail below with reference to Figure 7. Prior to the output electrical signal being provided to the synchronous detector 420, the signal may first be passed through an amplifier 418 to amplify the signal from the transceiver electrode 406. Although, Figure 5 illustrates hardware for detecting skin contact with the hair styler 1 via capacitive coupling it will be appreciated nevertheless that the hardware could be reconfigured for detecting skin contact with the hair styler 1 via resistive coupling. By way of example only, in such hardware a circuit with a pressure sensor may be provided that detects skin contact based on pressure being applied to the pressure sensor. Such pressure sensors typically comprise two resistive coated layers with a gap or spacing layer therebetween. The top resistive coated layer is semi-flexible (e.g., a flexible plastic) and may be deformed via the application of pressure. The gap or spacing layer typically consists of air or inert gas and / or spacers. During a touch event by a user, the top resistive coated layer is deformed and / or bent to make contact with the lower resistive coated layer (e.g., glass), thereby closing an electrical circuit. During operation, a small voltage is applied across one of the two resistive coated layers. Upon completion of the circuit between the resistive coated layers a change in resistance and voltage across the circuit can be detected which is interpreted as a touch. Mutual Capacitance Figure 6 illustrates another example of a circuit implemented in hardware in the hair styler 1 of Figure 1a to detect skin contact with the hair styler 1. As shown in Figure 6, by way of example only, skin contact of a user 402 with the hair styler 1 is detected by capacitive coupling. For example, an array of capacitors 404 (404-1, 404-2, etc.) - i.e., an array of touch sensors - is provided proximal to the surface at regions corresponding to locations of the heating zones Z1-Z7 642 of the hair styler 1 to detect whether a user’s skin comes into contact with the surface of those regions (hereafter referred to as a ‘heat surfaces’) e.g., to detected whether the skin of a user comes into contact with an electrode 406 of at least one capacitor 404-1 of the array of capacitors 404. For simplicity, only one capacitor (‘C4’) 404-1 of the array is depicted in Figure 5. By way of example only, that capacitor 404-1 (hereafter referred to as ‘a sensing capacitor’ 404-1) may be a sensing capacitor located proximal to a heat surface of the hair styler 1. As shown in Figure 6, the sensing capacitor 404-1 proximal to a heat surface (e.g., a heat surface corresponding to the location of heating zone Z1, or the like) of the hair styler 1 comprises a separate transmitter electrode 406a and receiver electrode 406b. The transmitter electrode 406a of the sensing capacitor 404-1 is connected to ground 410 via a capacitor 408 (‘C3’) and is provided with (connected to) an excitation signal 412. The excitation signal 412 is also provided to (connected to) a synchronous detector 420. The receiver electrode 406b of the sensing capacitor 404a is connected to an amplifier 418 which is in turn connected to the synchronous detector 420. During operation, an excitation signal 412 is provided to the transmitter electrode 406a of each sensing capacitor of the array of sensing capacitors 404 for sensing a touch of a user. Each sensing capacitor of the array is provided proximal to a corresponding heat surface of the hair styler 1 corresponding to where the heating zones Z1-Z7 642 are located. During use, when the user’s skin comes into contact with a heat surface of the hair styler 1 (e.g., heat surface corresponding to the location of heating zone Z1, or the like), that skin contact results in a change in the charge stored on the sensing capacitor 404-1. Specifically, when the user’s skin comes into contact with the heat surface, charge is transferred from the sensing capacitor 404-1. By removing charge from the sensing capacitor 404-1, the overall apparent capacitance of the circuit of Figure 5 is decreased, and thus the electrical signal output by the receiver electrode 406b may have a higher frequency and / or a higher voltage than that of the excitation signal 412 provided to the transmitter electrode 406a. Nevertheless, it will be appreciated that the circuit of Figure 6 may be suitably configured such that when the user’s skin comes into contact with a heat surface of the hair styler 1, the electrical signal output by the receiver electrode 406b has a lower frequency and / or a lower voltage than that of the excitation signal 412 provided to the transmitter electrode 406a. It will be appreciated that whether an increase or a decrease in frequency and / or voltage occurs in response to a user’s skin coming into contact with a heat surface of the hair styler 1 is dependent on the layout of the electrodes and the user’s capacitive coupling to Earth 410. The electrical signal output by the receiver electrode 406b is subsequently provided to the synchronous detector 420 to detect whether a user’s skin is touching a heat surface of the hair styler 1. The synchronous detector 420, for example, may receive both the electrical signal output from the receiver electrode 406b and the initial excitation signal 412 as a reference signal to determine changes between the initial excitation signal 412 and the electrical signal output from the receiver electrode 406b. A new signal is then output from the synchronous detector 420 that indicates that the user 402 is touching the heat surface A with their skin; for example, the new signal output from the synchronous detector 420 may be analysed / processed by a processor of the hair styler 1 to determine that the user 402 is touching the heat surface A with their skin. Beneficially, by allowing the detection of skin contact of heat surfaces of the hair styler 1 as described above with reference to Figures 4 and 5, reliable detection of skin contact of hot surfaces of the hair styler 1 can be achieved even if only a square millimetre of flesh comes into contact with the hot surface. Operation of the synchronous detector 420 is disclosed in further detail below with reference to Figure 7. Prior to the electrical signal output by the transceiver electrode 406 being provided to the synchronous detector 420, the signal may first be passed through an amplifier 418 to amplify the signal from the transceiver electrode 406. Although Figure 6 illustrates hardware for detecting skin contact with the hair styler 1 via capacitive coupling it will be appreciated nevertheless that the hardware could be reconfigured for detecting skin contact with the hair styler 1 via resistive coupling. By way of example only, in such hardware a circuit with a pressure sensor would be provided that detects skin contact based on pressure being applied to the pressure sensor. Such pressure sensors typically comprise two resistive coated layers with a gap or space layer between them. The top resistive coated layer is semi-flexible (e.g., a flexible plastic) and may be deformed via the application of pressure. The gap or space layer typically consists of air or inert gas and spacers. During a touch event by a user, the top resistive coated layer is deformed and / or bent to make contact with the lower resistive coated layer (e.g., glass), thereby closing an electrical circuit. During operation, a small voltage is applied across one of the two resistive coated layers. Upon completion of the circuit between the resistive coated layers a change in resistance and voltage across the circuit can be detected which is interpreted as a touch. Response to touch detection Irrespective of whether the self-capacitance detection scheme of Figure 5 or the mutual capacitance scheme of Figure 6 is implemented in the hair styler 1, in response to determining that the user 402 is touching a heat surface with their skin, the processor of the hair styler 1 may be configured to trigger and / or send an appropriate command message to control a function / action of (e.g., a temperature of, and / or a power provided to) at least the heating zone corresponding to the location of the heat surface (e.g., heating zone Z1 when the heat surface being touched is proximal to heating zone Z1) It will be appreciated that controlling a function / action of (e.g., a temperature of, and / or a power provided to) a heating zone corresponding to the location of the heat surface may comprise controlling a power provided to a heat electrode or the like (e.g., heater 6) of the heating zone that is configured to heat the heating zone. For example, in response to determining that the user 402 is touching a heat surface with their skin, the processor of the hair styler 1 may be configured to send an appropriate message and / or signal to drive circuitry 23 to control a power provided to a heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched. By way of example only, control of the power provided to the heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched may include, controlling a power to the heater 6 (i.e., an electrode of a heater responsible for heating a heating zone of the hair styling appliance) to reduce a temperature of the at least one heating zone to a temperature below a ‘safe touch’ temperature; that is to say, any temperature for which it is safe for a user’s skin to touch the heater for a defined period of time. For example, a safe touch temperature may be a temperature below 60 degrees Celsius. Nevertheless, it will be appreciated that a safe touch temperature may be defined as any temperature T for which it is safe for a user’s skin to touch the heater for a period of s seconds. In another example, control of the power provided to the heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched may include, reducing the power provided to the independently powerable heater electrode to a power level corresponding to a heating zone temperature of below a touch safe temperature as defined above. In yet another example, control of the power provided to the heater 6 of the heating zone corresponding to the location of the heat surface of the hair styler 1 being touched may include, controlling the power provided to the independently powerable heater electrode by preventing power being provided to the independently powerable heater electrode. For example, in response to detecting that a user’s skin is touching a heating zone heated by a heater electrode of the heater 6 that defines the heating zone, power to that heater electrode may be switched off until skin contact is removed, thereby preventing the heater electrode heating up (further). Synchronous detector Figure 7 illustrates an example synchronous detector 420 implemented in hardware in the hair styler 1 of Figure 1a. As shown in Figure 7, the synchronous detector 420 comprises numerous resistors, capacitors, amplifiers, op-amp amplifiers, low-pass filters, and switches. During operation, the output signal from the amplifier 418 (see Figures 5 &6) is combined with (e.g., multiplied by) the initial excitation signal 412 to produce an input signal 602 (‘Vin’) for the synchronous detector 420. Figure 8a depicts an example an input signal 602 (‘Vin’) for the synchronous detector 420. As show in Figure 8a, the input signal 602 comprises two sine waves, a first sine wave 702 with a first frequency, and a second sine wave 704 with a second frequency that is greater than the first frequency. The second wave 704 with the higher frequency is often referred to as a ‘carrier wave’ that ‘carries’ the first sine wave 702. The first sine wave 702 for example, may correspond to the output signal from the amplifier 418 (see Figures 4 &5), while the second sine wave may correspond to the original excitation signal 412. Retuning to Figure 7, at the synchronous detector 420, the input signal 602 is passed through an AC coupling circuit (e.g., a low-pass filter and / or AC op-amp integrator circuit) 603 comprising: an op-amp 604, a first resistor 606-1, and the first capacitor 608-1 in parallel with one another, and a second resistor 606-1, and a second capacitor 608-1 in serious with the first resistor 606-1. The AC coupling circuit is designed to remove any signal from input signal 602 above a specific (predefined) frequency threshold, wherein the specific frequency threshold is defined by the properties (e.g., resistance, capacitance etc.,) of the components of the AC coupling circuit. For example, the low pass filter 603 may be configured such that the wave of Figure 7a is filtered to remove all high frequency components (e.g., the second sine wave 704). Having been filtered by the low pass filter 603, the filtered output signal is passed down Paths A and B of the circuit. Path A comprises a changeover switch 612, a low-pass filter / integrator 616, and a DC amplifier circuit 618 comprising a DC amplifier 620, and a third resistor 606-3 grounded through resistor 606-4. A third capacitor 608-3 is also provided after the low pass filter 603 and which connects to ground. Path B comprises a synchronous inverter (‘X1 invertor’) 610, the changeover switch 612, the low-pass filter / integrator 616 and the circuit 618 comprising a DC amplifier 620, and a third resistor 606-3 grounded through resistor 606-4. The synchronous inverter 610 inverts the signal Vin by multiplying it by -1, thus peaks of the signal Vin become troughs, and the troughs of signal Vin become peaks. The changeover switch 612 is controlled by a clock 614 that is synchronous with the Vin signal i.e., the clock 614 switches the changeover switch between Path A and Path B at the same frequency as the Vin signal. As the changeover switch switches between Path A and Path B at the same frequency as the Vin signal, only positive voltage features of the Vn signal are passed on to the low-pass filter / integrator 616. Figure 8b illustrates an example resultant signal passed to the low filter / integrator 616. The resultant signal is passed through the low-pass filter / integrator 616 to filter out high-frequency noise, and to convert the signal into a steady, DC output proportional to the amplitude of the resultant signal passed to the low-fiIter / integrator 616. It will be appreciated that the steady, DC output may be a digitized signal. The steady, DC output is then subsequently passed through DC amplifier circuit 618 to amplify the DC signal for output at 706 (‘Vout’). Figure 8c illustrates an example output signal (‘Vout’) 706 from the synchronous detector 420. That signal is a DC signal whose existence indicates the occurrence of skin contact with a heat surface of the hair styler 1. In response to the DC signal, the processor 28 of the hair styler 1 may process the signal and determine that a user 402 is touching a heat surface of the hair styler 1 with their skin. In response to determining that a heat surface of the hair styler 1 is being touched by a user 402, the processor may be further configured to trigger and / or send an appropriate command message to control a function of (e.g., a temperature of, and / or a power provided to) at least one heating zone 642 of the hair styler 1 corresponding with the touched heat surface. Quadrature Synchronous Detector It will be appreciated that the synchronous detector described above, and implemented in the self-capacitance and mutual capacitance topologies described with reference to Figures 5 and 6, is only capable of detecting skin contact with heat surfaces of the hair styler 1 by detecting differences between an excitation signal provided to an electrode of a capacitor proximal to the heat surface, and the response signal from the capacitor upon a user touching the heat surface. The coupling that occurs between the capacitor and the user when they touch the heat surface, however, as well as causing changes in the amplitude (e.g., voltage) and / or frequency of the response signal, may also cause the response signal to be phase shifted with respect to the original excitation signal. It is therefore beneficial to also be able to detect changes in phase between the response signal and the initial excitation signal. Figure 9 illustrates an example quadrature synchronous detector 800 that may be implemented in hardware in the hair styler of Figure 1a. It will be appreciated that the example quadrature synchronous detector 800 may be implemented in the hardware in place of the synchronous detector 420. As shown in Figure 9, the quadrature synchronous detector 800 comprises numerous components including two synchronous detectors 420-1, 420-2. During operation, the output signal from the amplifier 418 (see Figures 6 &7) is combined with (e.g., multiplied by) the initial excitation signal 412 to produce an input signal 602 (‘Vin’) for the quadrature synchronous detector 800. Figure 8a depicts an example input signal 602 (‘Vn’) for the quadrature synchronous detector 800. At the quadrature synchronous detector 800, the input signal 602 is passed down Paths ‘A’ and ‘B’ of the circuit. Path ‘A’ comprises a first synchronous detector 420-1 (as described above with reference to Figure 7) and path ‘B’ comprises a second synchronous detector 420-2 (described above with reference to Figure 6). Both the first synchronous detector 420-1 and the second synchronous detector 420-2 are clocked using clock 802 such that they are phase shifted from one another by 90° (tt / 2 rads) such that an output signal from the first synchronous detector 420-1 is an output signal with only positive voltage features of the Vin signal as described above with reference to Figure 7, and an output signal from the second synchronous detector 420-2 is an output signal with only positive voltage features of the Vin signal as described above with reference to Figure 7, but which is phase shifted by 90° (tt / 2 rads). Those two output signals thus constitute the real (‘VOut(reai)’) and imaginary (‘Vout(imaginary)’) amplitude components of the detected signal from the first and second synchronous detectors 420-1,420-2, respectively. Those output signals are subsequently passed onto a processor 28 of the hair styler 1 to combine both the real (‘VOut(reai)’) and imaginary (‘Vout(imaginary)’) amplitude components of the detected signal to produce a single magnitude value in software. Real-world Scenarios Above two possible topologies (self-capacitance &mutual capacitance) are described that may be used to detect the presence of a user’s skin on heat surfaces of the hair styler 1. In either topology, a synchronous detector or a quadrature synchronous detector may be implemented. Figures 10a and 10b illustrate two example real-world scenarios of use of the hair styler 1 described above. As is shown in Figures 10a and 10b, the hair styler 1 may be used by a user on their own (Figure 10a), alternatively, the hair styler 1 may be used by a stylist who uses the hair styler 1 to style a customer’s hair (Figure 10b). In the scenario where a user uses the hair styler 1 on themselves, skin contact of the user may be detected in response to capacitive coupling, where the user becomes a capacitor connected to ground. In the scenario where a stylist uses the hair styler 1 to style a customer’s hair, both capacitive and resistive coupling may arise. The coupling to Earth between a stylist and a client thus can be complex and arbitrary. It will be appreciated therefore that it cannot be assumed that a client and stylist are not directly in contact with each other, or not in direct contact with a metallic connection to Earth, or that a home user is sitting in complete isolation when implementing the two example topologies described above. Multizone Touch Detection In all the examples described above, for the purposes of simplicity only, skin contact detection is described with reference to a single capacitive touch sensor proximal to a single heat surface of the hair styler 1. For example, in Figures 5 and 6 only one touch sensor capacitor 404-1 is shown, which is described as proximal to a heat surface of the hair styler 1. However, it will be appreciated that the hair styler 1 comprises multiple heat surfaces (corresponding to the multiple heating zones Z1-Z7), and that depending on the desired level of granularity of detection desired, there may be one touch sensor capacitor per heating zone, or multiple touch sensor capacitors per heating zone. For example, each heating zone may itself be divided up into smaller sub-zones, with a touch sensor capacitor in each sub-zone. Irrespective of whether there is one touch sensor capacitor per zone, or multiple however, there may be a need to adapt the topologies of Figures 5 and 6 to allow a single excitation signal to be sent to different touch sensor capacitors, rather than having to implement multiple excitation signal generators to simplify the overall detection system. One such way of using a single excitation signal for the plurality of touch sensor capacitors is to implement a multiplexer. Figure 11 illustrates an example 1-to-8 multiplexer arrangement 900 for providing the same excitation signal to eight capacitor electrodes that may be implemented in the topologies of Figures 5 and 6. As shown in Figure 11, an excitation signal 412 may be generated by a single signal generator which is input to a 1 -to-8 multiplexer 902. Upon entry to the 1 -to-8 multiplexer 902 the signal may be provided to any one of the 8 capacitor Tx electrodes (X(1)...X(8)) of a touch sense capacitor 404 (e.g., a Tx electrode X(1) of touch sense capacitor 404-1) that is proximal to a heat surface of the hair styler 1, depending on the inputs provided to the Tx address line. For example, where the Tx address line allows the 3 inputs SO, S1, and S2, the mapping of X to a specific one of the 8 capacitor Tx electrodes (X(1)...X(8)) using inputs SO, S1, and S2 may be implemented as shown in Table I below. Input Output (i.e., Capacitor that receives excitation signal) SO S1 S2 X1 X2 X3 X4 X5 X6 X7 X8 0 0 0 X 0 0 0 0 0 0 0 0 0 1 0 X 0 0 0 0 0 0 0 1 0 0 0 X 0 0 0 0 0 0 1 1 0 0 0 X 0 0 0 0 1 0 0 0 0 0 0 X 0 0 0 1 0 1 0 0 0 0 0 X 0 0 1 1 0 0 0 0 0 0 0 X 0 1 1 1 0 0 0 0 0 0 0 X Table I: Mapping between 1-to-8 multiplexer Tx address line inputs and multiplexer output. Similarly, the Rx electrodes of the 8 touch sense capacitors 402 may be connected to an 8-to-1 multiplexer 904 that allows response signals from the 8 different touch sense 5 capacitors to be fed to the synchronous detector 420 (or the quadrature synchronous detector 800) depending on the inputs provided to the Rx address line. For example, where the Rx address line allows 3 inputs SO, S1, and S2, the signal output from the 8-to-1 multiplexer may be controlled using inputs SO, S1, and S2 as shown in Table II below. 10 Beneficially, by using a separate multiplexer for Tx and Rx electrodes of the touch sense capacitor 402 individual selection of both Tx and Rx channels is provided, which in turn means extra information can be deduced. For example, non-corresponding electrode pairs could be activated to provide extra information about the shape and proximity of the operator’s hand. 15 Furthermore, the Rx and Tx electrodes of the touch sense capacitors may also be formed into interlocking patterns of electrodes on the heat surfaces of the hair styler 1 to ensure a touch from even a small area of skin would cause a skin contact detection event to occur. Alternatively, to simplify the circuitry, the Tx or the Rx electrode may be ‘commoned’ thereby allowing one of the multiplexers to be omitted from the circuitry. However, in such an arrangement isolation between the multiple channels would not be as clean, as stray capacitance between the Tx electrodes and the Rx electrodes may arise. 5 It will be appreciated that the 1-to-8 multiplexer described above is by way of example only and that other multiplexer configurations (e.g., 1-to-4) are possible. Input SO S1 0 0 0 0 0 1 0 1 1 0 1 0 1 1 1 1 Output S2 Y 0 Y1 1 Y2 0 Y3 1 Y4 0 Y5 1 Y6 0 Y7 1 Y8 Table II: Mapping between 8-to-1 multiplexer Rx address line inputs and multiplexer output. Capacitor Touch Sensor-Heater 10 In the above-described examples, the hair styler 1 has an array of capacitors 404 (404-1, 404-2, etc.) that are provided proximal to the surface at regions corresponding to locations of the heating zones Z1-Z7 642 of the hair styler 1 to detect whether a user’s skin comes into contact with the surface of those regions (i.e., heat surfaces). Those capacitors 404 are separate and distinct components of the hair styler 1 from the 15 heaters 6. However, it will nevertheless be appreciated that it may be beneficial to make use of conductive layers of the heaters 6 to detect whether a user’s skin comes into contact with the heat surfaces. As shown in Figure 12, rather than providing an array of capacitors 404, a heater track 1102 of each heater electrode (i.e., an independently controllable / powerable heater electrode) may be adapted to so as to form a capacitive transmitter electrode of a sensing capacitor 1100. For example, a heater track 1102 of each heater electrode may be adapted through the inclusion of an appropriate touch sensor layer 1104 nearby (e.g., on top of) the heater track and which acts as a transmitter electrode. That transmitter electrode 1102, may by way of example only, be similar to the transmitter electrode 406a of the sensing capacitor 404-1 shown in the ‘Mutual Capacitance’ arrangement of Figure 6. In this scenario the power supply to the heater track 1102 of each heater electrode should be able to be isolated from the heater track 1102 by suitable switches. It will be appreciated that this may be necessary given that the voltage provided to the heater electrode to heat the heater may be significantly higher than the voltages associated with an excitation signal provided to the transmitter electrode of the sensing capacitor 1100. To allow isolation of the heater electrode from the higher voltage power supply 1108 for heating the electrode during the periods when an excitation signal may be provided to the transmitter electrode, a first switch 1106a may be provided on the low voltage side of the power supply 1108 (e.g., heater PSU). Additionally, a second switch 1106b may also be provided on the low voltage side of the power supply 1108 (e.g., heater PSU). Alternatively, the transmitter electrode 1102, may by way of example only, act as a shield layer beneath an appropriate touch sensor layer 1104. In this scenario, the arrangement may correspond with the ‘Self Capacitance’ arrangement of Figure 5. In this scenario, the power supply to the heater track 1102 of each heater electrode should also be able to be isolated from the heater track 1102 by suitable switches as described above. As shown in Figure 12, it will be appreciated that the appropriate touch sensor layer 1104 may be connected to an excitation signal generator 412 for providing an excitation signal to the touch sensor layer similar to the scenarios described above with reference to Figures 5 and 6. The output signal in response to a user touching the touch sensor layer 1104 may then be processed in a similar manner to that described above with reference to Figures 5 and 6 in order to detect whether a user is touching the touch sensor layer 1104, and to control the power provided to the heater track 1102 of each heater electrode accordingly. 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 hair styler 1 may be partially or entirely formed of a unitary structure, e.g., by 3D printing. In the above-described examples the hair styler 1 may comprise a single heat source or may alternatively comprise two or more heat sources. More generally, the hair styler 1 may comprise any suitable means for transferring heat to the hair of the user, such as any suitable conductive heater, thick film printed heater, steam heater, or radiative heater. In some of the above-described examples, the array of capacitors 404 provided proximal to the surface of sections, S1-S7, of the hair styler 1 may comprise capacitive electrodes formed of dedicated conductive layer(s) in the heater assembly itself. For example, the capacitive electrodes may comprise dedicated conductive layer(s) of the controllable heating zones 642. Alternatively, where heat spreaders are provided between the heaters and the controllable heating zones 642 the capacitive electrodes may comprise dedicated conductive layer(s) in the heat spreaders. Alternatively, the capacitive electrodes may comprise dedicated conductive layer(s) of the heaters themselves. In some of the above-described examples, the array of capacitors 404 provided proximal to the surface of sections, S1-S7, of the hair styler 1 may be configured such that a single large sensor area across the surface of the sections S1-S7 is provided. Beneficially, this would be simpler to manufacture than providing multiple smaller sensor areas. However, such a configuration could increase background capacitance and electrical interference. This in turn would mean that some detections would be difficult. For example, if a user were to touch 1mm2 of the heat surface it would be difficult to detect (e.g. attempting to detect 2pF of extra capacitance against a background of 1nF). In some of the above-described examples, the array of capacitors 404 provided proximal to the surface of sections, S1-S7, of the hair styler 1 may be configured to provide multiple, ‘pixellated’ sensors, which are able to detect small areas of skin contact. For example, by providing multiple, ‘pixellated’ capacitive touch sensors a much higher proportional change in capacitance can be detected when skin is touching, thus improving sensitivity of the system. In some of the above-described examples, the array of capacitors 404 provided proximal to the surface of sections, S1-S7, of the hair styler 1 and the heaters of the hair styler 1 may be configured to allow more possibilities in functionality e.g. localised disabling of heating due to touch rather than the whole device being disabled. In some of the above-described examples, the array of capacitors 404 provided proximal to the surface of sections, S1-S7, of the hair styler 1 may be configured to provide a self-referencing system e.g. a detection threshold is derived from the median capacitance detected in any sections, S1-S7 on the assumption that all sections, S1-S7 are unlikely to be being touched at the same time. In addition, constraints (e.g., absolute limits) may be implemented, which may be compared to the detected median capacitance to ensure proper functioning of the capacitance touch sensors. Furthermore, where the array of capacitors 404 is configured to provide a self-referencing system, they may also be configured such that patterns of touch may be detected (e.g., the shape of the thing touching the capacitive touch sensors). It will be appreciated that all of the data / signals obtained from the array of capacitors 404 in the above-described examples, may be interpreted in the context of inertial measurement unit information obtained from accelerometers, gyroscopes, and magnetometers implemented in the hair styler 1. In some of the above-described examples, the multiple zones Z1-Z7 and their corresponding sensor or sensors may be multiplexed to allow a single excitation signal &synchronous detector to be used in conjunction with multiple capacitive touch sensors. Alternatively, a complete capacitive sensing circuit may be provided per capacitive touch sensor (e.g., an excitation signal &synchronous detector per sensor). In some of the above-described examples, the skin detection may require techniques such as active shielding to allow signals to be routed through a PCB without stray coupling until the touch sensor capacitor electrode is reached. In some of the above-described examples, a virtual-earth current sensor may be implemented for any capacitive touch sensor receiver electrodes in the mutual capacitance configuration to simplify the routing of received signals. In some of the above-described examples, performance of the system may be maximised if circuitry is located right next to the capacitive touch sensors electrodes. In some of the above-described examples, the excitation signal used may simply be a single-frequency sinusoid that is lower than the minimum frequency used in electromagnetic compatibility (EMC) susceptibility testing. Alternatively, to glean more information, the excitation signal used may comprise multiple frequencies. By using multiple frequencies improved sensitivity can be achieved, and phase &litude information can be obtained to improve discrimination between skin and hair. The use of multiple frequencies may also result in better rejection of interference. Excitation signals with multiple frequency components could be achieved efficiently through time division multiplexing across different frequencies, or the like. In some of the above-described examples, the hair styler 1 may comprise moisture sensors for sensing the moisture of the hair. Alternatively, for example, the hair styler 1 could be configured for heating the hair using a predetermined set of operating parameters based on a target moisture level. For example, a table of target moisture levels and corresponding operating temperatures needed to achieve that target moisture level could be determined in advance and stored in the memory. In this configuration, the hair styler 1 need only identify the operating temperature (and any other relevant operating parameters) to be used from the table, and it is not necessary to sense the moisture of the hair. However, providing a moisture sensor for sensing the moisture of the hair is nevertheless beneficial for verifying that the hair has reached the intended moisture level, and enables more precise control of the moisture level. In any of the above-described embodiments, the device may be provided with a motion sensor (for example, a motion sensor arranged in the main body of the hair styler 1). The motion sensor may be used to sense whether the device is currently in use by a user (by sensing movement of the device), or whether the device is laying idle (for example, by sensing that the device has not been moved for a predetermined amount of time). The motion sensor may comprise a gyroscope, accelerometer, a switch on a docking station onto which the device is placed, and / or may comprise any other suitable type of sensor. When the data from the motion sensor indicates that the device has not been moved for a predetermined amount of time (for example, 1 minute), the device may be put into an idle mode in which the temperature of the head portion 16 is reduced (for example, by switching off the heaters, or reducing the power output of the heaters). In the idle mode, the temperature of the heaters may be reduced to an idle mode temperature that is above ambient temperature, but below the normal operating temperature for drying and / or styling hair. For example, the operating temperature may be 120 °C, and the idle temperature may be 90 °C. When the data from the motion sensor indicates that the user has picked up the device, the heater 30 is then controlled to return the temperature to the operating temperature for drying and / or styling hair. Advantageously, the use of the idle mode increases the efficiency of the device and may also reduce the damage due to thermal stress that may be caused to a surface when the device is placed on that surface for a prolonged period of time. When the device is in the idle mode and the data from the motion sensor indicates that the user has picked up the device, the heater (or heaters) may be operated in a ‘boost mode’ to return to the operating temperature. More generally the heater(s) may be temporarily operated at a higher power output to increase the temperature from the idle temperature to the operating temperature for drying and / or styling hair. 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. Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

Claims

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

2. The hair styling appliance of claim 1, wherein the control action in respect of at least one heating zone comprises controlling a temperature of the at least one heating zone.

3. The hair styling appliance of claim 2, wherein the control action in respect of at least one heating zone comprises controlling a power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone.

4. The hair styling appliance of claim 3, wherein controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone comprises:controlling the power provided to the independently powerable heater electrode to reduce a temperature of the at least one heating zone to a temperature below 60 degrees Celsius.

5. The hair styling appliance of claim 3, wherein controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone comprises:reducing the power provided to the independently powerable heater electrode to a power level corresponding to a heating zone temperature of below 60 degrees Celsius.

6. The hair styling appliance of claim 3, wherein controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone comprises:controlling the power provided to the independently powerable heater electrode by preventing power being provided to the independently powerable heater electrode.

7. The hair styling appliance of any one of claims 3 to 6, wherein controlling the power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone comprises:sending a control signal to drive circuitry of the control circuitry to control the power provided to the independently powerable heater electrode.

8. The hair styling appliance of any preceding claim, wherein each touch sensorforms part of a corresponding electrical circuit with a capacitance and comprises a respective transceiver electrode that is provided with an excitation signal and,wherein upon user skin contact with the transceiver electrode, the capacitance of the corresponding electrical circuit increases, indicating that user skin contact has occurred in the at least one heating zone.

9. The hair styling appliance of claim 8, wherein the increase of the capacitance of the corresponding electrical circuit causes an electrical signal output by the transceiver electrode to have a lower frequency and / or a lower voltage than the excitation signal provided to the transceiver electrode, indicating that user skin contact has occurred in the at least one heating zone.

10. The hair styling appliance of claim 9, wherein the hair styling appliance further comprises at least one synchronous detector connected to the transceiver electrode ofthe at least one touch sensing capacitor, the at least one synchronous detector configured to:receive the excitation signal provided to the at least one touch sensorreceive the electrical signal output from the at least one touch sensor; andprocess both the received excitation signal and the electrical signal output to generate an electrical signal for output from the at least one synchronous detector to the processor of the control circuitry, wherein an electrical signal for output from the at least one synchronous detector with a non-zero amplitude indicates that user skin contact has occurred in the at least one heating zone.

11. The hair styling appliance of any of claims 1 to 7, wherein each touch sensor forms part of a corresponding electrical circuit with a capacitance and comprises a transmitter electrode and a receiver electrode, the transmitter electrode being provided with an excitation signal and,wherein upon user skin contact with the receiver electrode, the capacitance of the corresponding electrical circuit decreases, indicating that user skin contact has occurred in the at least one heating zone.

12. The hair styling appliance of claim 11, wherein the decrease of the capacitance of the corresponding electrical circuit causes an electrical signal output by the receiver electrode to have a higher frequency and / or a higher voltage than the excitation signal provided to the transmitter electrode, indicating that user skin contact has occurred in the at least one heating zone.

13. The hair styling appliance of claim 12, wherein the hair styling appliance further comprises at least one synchronous detector connected to the receiver electrode of the at least one touch sensing capacitor, the at least one synchronous detector configured to:receive the excitation signal provided to the at least one touch sensor;receive the electrical signal output from the at least one touch sensor; andprocess both the received excitation signal and the electrical signal output to generate an electrical signal for output from the at least one synchronous detector to the processor of the control circuitry, wherein an electrical signal for output from the at least one synchronous detector with a non-zero amplitude indicates that user skin contact has occurred in the at least one heating zone.

14. The hair styling appliance of any one of claims 11 to 13, wherein the transmitter electrode of each touch sensor is connected to a multiplexer configured to provide a single excitation signal to each touch sensor of the plurality of touch sensors, and wherein the receiver electrode of each touch sensor is commoned.

15. The hair styling appliance of any one of claims 11 to 13, wherein the receiver electrode of each touch sensing capacitor is connected to a multiplexer configured to receive an electrical signal output from each touch sensor of the plurality of touch sensor for provision to a single synchronous detector, andwherein the transmitter electrode of each touch sensor is commoned.

16. The hair styling appliance of any one of claims 11 to 13, wherein the transmitter electrode and the receiver electrode of each touch sensor are connected to a respective first and second multiplexer, the first multiplexer configured to provide a single excitation signal to each touch sensor of the plurality of touch sensor, and the second multiplexer configured to receive an electrical signal output from each touch sensor of the plurality of touch sensor for provision to a single synchronous detector.

17. The hair styling appliance of claim any one of claims 11 to 16, wherein the transmitter electrode of each touch sensor is formed from a heater track of a respective heater electrode.

18. The hair styling appliance of claim any one of claims 11 to 16, wherein the transmitter electrode of each touch sensor is formed from a heater track of one of the corresponding independently powerable heater electrodes.

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

20. The method of claim 19, wherein performing the control action in respect of at least one heating zone comprises controlling a power provided to the independently powerable heater electrode that defines the corresponding at least one heating zone.

21. The method of claim 20, wherein controlling the power provided to the independently powerable heater electrode comprises:reducing a temperature of the at least one heating zone to a temperature below 60 degrees Celsius; and / orreducing the power provided to the independently powerable heater electrode to a power level corresponding to a heating zone temperature of below 60 degrees Celsius; orcontrolling the power provided to the independently powerable heater electrode by preventing power being provided to the independently powerable heater electrode.

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