Hair drying and / or styling appliance and method

Abstract

A hair dryer or styler 1 comprising first and second moveable arms 4a and 4b, each with first and second heaters 6a and 6b. The arms being mutually opposed and adapted for movement between an open con

Description

Field of the Invention The present invention relates to a hair drying and / or styling appliance, to parts for use in such appliances and to methods of making and using such appliances. Such styling of the hair may be performed by a user in respect of their own hair, for example, or by a hair stylist. The invention has particular, but not exclusive, relevance to a hair drying and / or styling appliance comprising a sensor for detecting whether the appliance is in an open or a closed configuration. 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 straightener having a heated plate applies heat directly via conduction to heat the hair, which may be either wet or dry, to achieve the desired temperature for styling. The hair may be heated to a temperature that is particularly suitable for styling hair (for example, to or beyond a hair 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. 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 a long time to cool down. This thermal mass makes it difficult to control the heating of the hair and over heating or under heating of the hair can result. There has been recent development by the applicant and other companies in developing hair styling appliances that use heaters having a lower thermal mass that can therefore heat up and cool down much more quickly. Such low thermal mass heaters are therefore more responsive and are easier to dynamically vary the temperature with time. United Kingdom Patent No. 2,477,834 discloses a styling appliance comprising a heater having a plurality of heating zones which are independently operable. A control system includes sensing means and predicts the intended use of the appliance. The heating zones are then operated accordingly. It is desirable for further improvements to hair styling appliances to be made. Summary of the Invention Aspects of the invention provide a hair drying and / or styling appliance comprising any or all of the following features: a first arm; a second arm; the first and second arms being mutually opposed and adapted for movement between an open configuration in which a tress of hair can be positioned between the first and second arms and a closed configuration in which the tress of hair is sandwiched between the first arm and the second arm; a sensor configured to sense relative movement of the first arm and the second arm between said open configuration and said closed configuration; and a controller configured to: determine whether the appliance is in the open configuration or the closed configuration based on the sensed relative movement; determine a pull duration that the appliance is pulled along the tress of hair based on a duration that the appliance is determined to be in the closed configuration, and perform a control action based on the determined pull duration. One or both arms may include a heater for heating the hair when sandwiched between the first and second arms. In this case the information based on whether the appliance is in an open configuration or a closed configuration can be used to influence how the heaters are controlled. Calculating the pull duration and using it to perform a control action of the appliance provides additional data on which a control algorithm can operate, for example by classifying the current operating mode. In particular, this allows the appliance to be controlled in a targeted manner with respect to the hair that is being styled (for example, the length of the hair) and the behaviour of the user (for example, the speed at which a user pulls a tress of hair through the appliance). The control action may comprise displaying information to the user. The control action may comprise storing information in a memory of the appliance. This provides options for the appliance to learn behaviour about how the styler is used and to learn the characteristics of the hair being styled. The appliance may be configured to determine a velocity of the appliance as it is moved along the tress of hair. The appliance may be configured to determine a tress length based on the determined velocity and the determined pull duration. The control action may be based on the determined tress length. This advantageously combines information gathered from the appliance to calculate additional behaviours of how the appliance is used and the characteristics of the hair being styled. The controller may be configured to determine whether the determined pull duration is within a predefined range before performing the control action. This allows the appliance to ensure that the pull duration is neither too long nor too short before using this information to influence the behaviour of the styler or infer characteristics of the hair being styled. The control action may comprise controlling the power supplied to the heaters. This has the advantage of controlling the temperature of the heaters dynamically based on the pull duration, which may be determined by the length of a tress and how quickly a user moves the appliance along the tress. Controlling the power may include initially increasing the temperature of the heaters from a standby temperature to a start temperature, being higher than the standby temperature, when the controller determines that the appliance is in the closed configuration. Controlling the power may include maintaining the temperature of the heaters at the start temperature for a predetermined time. This advantageously delays heating up the roots of the hair (i.e. the hair near the scalp) further while the styler is presumed to be momentarily stationary on the root at the moment the appliance is closed. Controlling the power may further include increasing the temperature of the heaters during the pull from the start temperature to an end temperature, being higher than the start temperature. This provides a higher temperature towards the tip where the hair may be older and more heat resistant. Controlling the power may comprise increasing the power supplied to the heaters to gradually increase the temperature of the heaters during the pull at a rate based on the determined pull duration. The rate may be proportional to the determined pull duration. The rate may be linearly proportional to the determined pull duration. This can provide an advantageous mode of operation in which lower temperatures are applied toward the root and the heaters gradually heat up toward the tip of a tress. The controller may be further configured to determine an open duration during which the appliance has remained in the open configuration and is configured to reduce the temperature of the heaters to a standby temperature if the open duration is greater than a first threshold. This provides leniency in controlling the appliance such that, for example, if the user were to mistakenly open the appliance during a tress pull, it would not reduce the temperature of the heaters straight away. The controller may be further configured to switch off the heaters if the open duration is greater than a second threshold, being greater than the first threshold. This has the advantage of allowing the controller to infer that the appliance is no longer in use and switching off the heaters accordingly. The sensor may comprise at least one sensor selected from the group consisting of: a microswitch; a magnetic field sensor; an optical sensor; a capacitive sensor; and a strain gauge transducer. Brief Description of the Drawings Embodiments will now be described, by way of example only, and with reference to the drawings in which: Figure 1a shows an overview of an exemplary hair styling device; Figure 1b shows a hair styling device in use; Figure 2 is a block diagram illustrating the main electronic components of the hair styling device shown in Figure 1; Figure 3a is an exploded view of a heater forming part of the hair styling device shown in Figure 1; Figure 3b is an assembled partially transparent view of the heater shown in Figure 3a; Figure 4a schematically illustrates the heating zones on the heating surface of the heater shown in Figure 3; Figure 4b 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; Figure 7 illustrates a tress of hair that partly overlaps with zones Z2 and Z4 of a heater; Figure 8 illustrates a cross-sectional view of a further example of a low thermal mass heater that has curved edges and a supporting substrate onto which the heater is attached with an adhesive or via a diffusion bonding process (e.g. by melting them together); Figure 9 is a partially exploded cross-sectional and perspective view of the different layers that form the heater shown in Figure 8; Figure 10 is a plan view illustrating the form of a heat spreading layer forming part of the heater illustrated in Figure 8; Figure 11 illustrates a main heating element layer forming part of the heater shown in Figure 8; and Figure 12 is a simplified block diagram illustrating the way in which the heater electrodes of the heater shown in Figure 8 are used to heat the heater and to sense the temperature of the heating zones; Figure 13a shows an overview of an exemplary hair styling device; Figure 13b shows a hair styling device in use; Figure 14 is a block diagram illustrating the main electronic components of the hair styling device shown in Figure 13; Figure 15 illustrates a control sequence for the hair styling device; Figure 16 is a graph illustrating a control sequence based on the method of Figure 15. Detailed Description of the Drawings Overview of Hair Styling Device 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. A user interface 11 is provided to allow the user to set user defined parameters and for the device to output information to the user. For example, a desired operating temperature may be set via the user interface 11. The user interface 11 may have a dial, button or touch display for allowing the user to input information to 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. In this embodiment, the user interface 11 also comprises a control button or switch 14 to enable the user to turn the device 1 on or off; and an indicator light 15 to show whether the power 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. 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 6 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 6 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. Control Circuitry Figure 2 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)) 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 and sensed temperatures of the heaters obtained from temperature measurement circuitry 25. The temperature measurement circuitry 25 may be 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. Figure 2 also shows that the user interface 11 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 or if they are moving the device 1 too quickly along the hair tress 40. Finally, the control circuitry includes 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, WiFi and / or 3GPP communication protocols to communicate with the remote device. Heaters The heaters 6a, 6b may be low thermal mass heaters and can therefore heat up and cool down quickly. Figures 3a and 3b show an exemplary embodiment of such heaters 6a, 6b, which comprise a stack of thin layers. Referring in particular to Figure 3a, 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 either through an adhesive layer (pressure set or thermoset) or through diffusion bonding of the contacting materials (e.g. melting them together) and define a heater 6 that is very thin (the three layers have an overall thickness of between 30pm to 1000pm in the case of Safe Extra Low Voltage (SELV) operation (less than 42.4 Volts) and 0.8mm to 2.0mm in the case of AC operation) and with very low thermal mass. The upper surface of the 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 over the heating surface if the layer 62 does not itself have such non-stick properties. The bonded layers 62, 63 and 66 define a flexible heater 6 and rigidity of the heater is provided in the illustrated embodiment by mounting the heater layers 62, 63 and 66 into a rigid support 68 which forms a base. These layers may be mounted onto the rigid support after the layers themselves have been bonded together or they may be bonded one at a time (or multiple at a time) onto the rigid support 68. If a flexible heater is desired, then there is no need for the rigid support 68 or if a support is used, this may be a non-rigid support. 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. This provides a hair styler 1 having a very low thermal mass which can therefore heat up and cool down much more quickly than prior art stylers. 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 3a and 3b 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 642 on the hair contacting surface of the heater 6, which spans the width (which we shall refer to as the x-direction) of the heater 6 and the heater electrodes 64 are arranged sequentially one after the other along the length (the y-direction) of the heater 6. Figures 4a and 4b show schematic views of different arrangements of such heating zones 642. Figure 4a shows an arrangement corresponding to that of Figures 3a and 3b, in which the heating zones 642-1 to 642-10 are arranged along the y-direction only. Figure 4b shows an alternative arrangement, in which heating zones 642-1 to 642-16 are arranged in both the x- and y-directions. Such an arrangement of heating zones 642 can be provided by arranging two sets of heater electrodes 64 like those shown in Figure 3a side by side in the width (x-) direction. The heaters 6 may be separated in this way into any number of heating zones 642 and may comprise any number of heating zones along the x- and y-directions. In particular, whilst Figure 4b shows two zones along the x-direction, a greater number of zones in the x-direction could also be provided. The heating zones 642 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce hot / cold spots when using very low thermal mass heaters 6 such as those shown in Figure 3. The heating zones illustrated in Figure 4 are all the same size. Of course, different sized heating zones 642 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 642 described above form part of a heater having a flat hair contacting surface. The heater is not limited to flat hair contacting surfaces and can be configured for use in a tubular form (as illustrated in Figure 6a) for example for use in a hair curler device or in 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. The temperature of each heating zone 642 is independently controllable. Each heating zone 642 can be set to a target temperature. The target temperature of each heating zone 642 may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone 642 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 642 can be determined by determining the resistance of the corresponding heater electrode 64. The microprocessor 28 controls the heating in order to reduce the difference between the actual temperature of the heating zone 642 and the target temperature for that heating zone 642. Heating Zone Sizing One issue with low thermal mass heaters 6 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 40 onto the heaters 6, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 6 to ensure that all regions on the hair contacting surface can be retained within and / or recovered back to the desired operating temperature limits. The low thermal mass heaters 6 described above are relatively thin and the dielectric layers are formed of materials with relatively low thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 64 running across the whole length and whole width of the heater 6, then when more power is supplied to the heater 6 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, orcause the overheated regions to burn relatively small bundles / strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and / or by increasing the thicknesses of the layers that constitute the heater and / or by dividing the heater 6 into multiple separately powered and controlled heating zones 642 across its length and width. Increasing the thickness of the layers increases the thermal mass of the heater 6 which is undesired and there are limited materials that have the required dielectric strength and high thermal diffusivity (and which are available for use in mass produced consumer products). Therefore, the inventors have divided the heaters 6 up into plural heating zones. These heating zones can be equally and / or unequally sized and can be arranged regularly and / or irregularly across the width and / or length of the heater. However, overheating can still occur within a single heating zone. For example, if half of the heating zone is loaded with hair and the other half is not loaded with hair, then the half that is loaded with hair will cause the temperature of that part of the heating zone to drop which will cause more power to be applied to that heating zone. That applied power will bring the loaded part of the heating zone back up to the desired operating temperature, but the unloaded part of the heating zone will overheat. This situation is illustrated in Figure 7, which shows a tress of hair 40 overlying heating zones Z2, Z3 and Z4, with heating zone Z3 being fully loaded with hair whilst heating zones Z2 and Z4 being only partially loaded with hair. This problem can be reduced by making the heating zones very small - but that is costly due to all the connections needed to connect each heater electrode 64 for each heating zone back to the drive circuitry 23 as well as the control switches of the drive circuitry 23 needed to control the powering of each heater electrode 64. The inventors have found that for a given permitted maximum temperature difference between the loaded and unloaded halves of a heating zone, a maximum size of the heating zones can be defined which depends on the maximum power that can be applied to the heating zone and the material characteristics of the layers forming the heating zone. Specifically, if it is assumed that only one half of a heating zone 642 is loaded with hair, upon loading with hair, the maximum temperature difference that occurs between the hair loaded and unloaded halves of a heating zone 642 can be defined with the equation below: ^Tmax=K{^TloadAP,A,a,t}LTload.^P.A / a.t where, ^Tmax = maximum permitted temperature difference between the hair loaded and unloaded halves of a heating zone toad = initial temperature difference that is created between the hair loaded and unloaded halves of a heating zone upon loading with hair △P = increase in power supplied to the heating zone A = surface area of the heating zone a = combined thermal diffusivity of the layers that constitute the heating zone t = total thickness of the layers that constitute the heating zone K = a constant whose value is also dependent on the values of &Tioad, &P, A, a, and t If it is assumed that the combined thermal diffusivity of a heating zone 642 and the total thickness of the layers that form the heating zone 642 are known and fixed (for any given device), then the above equation can be used to determine the area (A) and hence a length and width of each heating zone that will prevent overheating of the unloaded halves, when their other halves are loaded with hair, and more power is supplied to retain and / or recover the hair contacting surface temperatures back to the desired operating limits. Consequently, for a given surface area that must be covered with the considered heater technology, the equation above can be used to determine the number of heating zones that must be positioned across the length and width of the given surface area, so that each heating zone 642 can be operated without exceeding the maximum operating temperature of heater materials and without causing the differential temperature between a loaded part and an unloaded part of a heating zone 642 from exceeding a maximum differential temperature (LTmax) that could cause burning of relatively small bundles / strands of hair that come in contact with the overheated regions of the heating zone. Specifically, the maximum area can be determined from: Amax — Mk * AT max *( (K * t) / Pdmax ) Where, hTmax = maximum permitted temperature difference between the hair loaded and unloaded halves of a heating zone (in Kelvin) to avoid damage to the hair k = the combined lateral thermal conductivity of the layers that constitute the heating zone (in W / m.K) t = total thickness of the layers that constitute the heating zone (in mm) Pdmax = the maximum power density of the heating zone (in W / cm2) Mk = a constant For a hair styling device, the inventors have found the following suitable ranges for these parameters: - Maximum power density (Pdmax) is greater than 0.8 W / cm2 and less than 100 W / cm2 and preferably greater than 2 W / cm2 and most preferably greater than 8 W / cm2 and less than 15 W / cm2. - The combined thermal conductivity of the layers forming the heating zone (k) (in a plane perpendicular to the thickness of the heating zone) is between 15 and 100 W / m.K. In comparison the combined thermal conductivity of the multilayer heater across the entire heater and measured from end to end along a straight line passing along the longest dimension of the heater 6) is between 0.1 and 15 W / m.K. - The maximum permitted temperature difference between the hair loaded and unloaded halves of a heating zone to avoid hair damage is preferably less than 20 Kelvin, more preferably less than 10 Kelvin and most preferably less than 5 Kelvin. Operating within these ranges, the inventors have found that the maximum area of each heating zone is 5 cm2. The inventors have also found that a heater zone area of between 1 cm2 and 2 cm2 provides a good compromise between not having too many heating zones whilst avoiding the risks of burning the user’s hair. Alternative Heater Arrangement An alternative flexible heater 6’ is illustrated in Figure 8, which shows on the left hand side an exploded cross-sectional view of the heater 6’ and substrate 68’ and on the right hand side a perspective view of the heater 6’ and substrate 68’. As shown in Figure 8, the heater 6’ has curved edges 72-1 and 72-2 that are shaped to match the shape of an upper surface 74 of the rigid support substrate 68’ so that the flexible heater 6’ can be bonded securely using an adhesive or diffusion bonding of the underlying materials to the upper surface of the rigid substrate 68’. The curved edges of the heater 6’ can be formed, for example, using a heat forming process. Figure 8 also illustrates that one or more surface mounted electronic components 76 may be attached to an underside of the heater 6’. These components may be, for example thermistors for sensing the temperature of the heating zones 642 of the heater 6’. Figure 8 also shows a control printed circuit board (PCB) 78 that carries the drive and control electronics 15 illustrated in Figure 3 that controls the heating of the different heating zones 642 of the heater 6’. As before, the heater 6’ is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 100 pm. The different layers forming part of the heater 6’ are shown in exploded cross-sectional and perspective views in Figure 9. A description of each layer is given below. Low Friction Coating 81 (Optional) This is an optional layer and can be added to create a smooth, low friction surface to enhance the user experience by making the heater 6’ feel less grippy against the hair. This layer would be as thin as possible (for example, between 1 and 3 pm) to reduce the thermal resistance from the heater 6’ to the hair, whilst still being sufficiently durable and scratch resistant. This layer would typically be applied last, possibly as a spray coating (e.g. Cerasol), after the rest of the heater 6’ has been produced and assembled around the rigidifying substrate 68’. This is needed because the coating is inherently rigid, and once applied the coating will reduce the natural flexibility of the heater, and so it should be applied once the heater 6’ has been formed into its final shape. Heat Spreading Layer 82 (optional) This is also an optional layer and, when provided, helps to spread the heat within each heating zone 642 to ensure that the temperature of individual heating zones 642 is able to maintain an acceptable degree of homogeneity during typical use. As discussed above, if a heating zone 642 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 642 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot regions to the cold. This problem is exacerbated by the anisotropic thermal characteristics of the serpentine like heater electrodes 64, and by the fact the control electronics 15 would typically work to maintain an “average” temperature within the heating zone 64 based on the overall resistance of the heater electrode that forms the heating zone 642 - from the perspective of the control electronics 15, the heating zone 642 would be at the “correct” temperature despite having hot and cold regions. Each heating zone 642 would have its own heat spreader, which is thermally separated (there is a high thermal impedance / low thermal conductivity) from the heat spreaders for adjacent zones. This is desirable to prevent heating zones 642 from heating neighbouring heating zones 642 which might otherwise increase power consumption and reduce warm up time. Figure 10 illustrates an example form of the heat spreader layer 82. As shown, in this example there are 20 heat spreaders 91-1 to 91-20, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 91 is separated from its neighbouring heat spreaders 91 and in effect forms an island of thermally conductive material over the corresponding heating zone. The heat spreaders 91 may be separated from each other by a solid material having a thermal conductivity lower than 35 W / mK or they may be separated by air. The heat spreaders 91 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 91 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 91, it is difficult for heat from one heating zone 642 to pass into neighbouring heating zones 642. The solid material that is provided in the gap between adjacent heat spreaders 91 may be provided by a coating or a wash that is applied to the heat spreading layer 82 after the etching process has formed the gaps between adjacent heat spreaders 91 and may be the coating layer 81 described above. This layer 82 can provide mechanical integrity to the overall heater 6’, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes 64, which in turn could lead to short circuits or loss of functionality. Polyimide Separator layer 83 The polyimide separator layer 83 provides electrical isolation between the hair contacting surface of the heater 6’ (which may be the upper surface of this layer 83 if the optional layers 81 and 82 are not provided) and the main heater electrode layer. This layer 83 would have as low thermal resistance as possible whilst still achieving the dielectric requirements of the layer. As the name suggests, this layer is formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, the in-plane thermal diffusivity or thermal conductivity of this layer (in a plane perpendicular to its thickness) is quite low (less than 35 W / mK). This helps to prevent heat spreading from one heating zone 642 to an adjacent heating zone 642. Main Heater Electrode &Sensing Layer 84 This layer 84 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries). This layer 84 comprises a number of independently controllable heater electrodes 64 each defining a corresponding heating zone 642. Figure 11 illustrates in more detail the form that this layer 84 takes in this example heater 6’. As shown, in this example, there are twenty independently controllable heater electrodes 64-1 to 64-20 that each defines a corresponding heating zone 642. Each heater electrode 64 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance and peak power requirements for the relevant power source. Each heater electrode 64 is formed into a serpentine pattern using chemical etching as a manufacturing process. In more detail, a solid layer of conductive material is provided and then etched to form the different heater electrodes 64. The straight lines shown in Figure 11 are the etched parts of the layer 84 and the white parts of the figure show the serpentine conductor paths that form the heater electrodes 64. In this illustrated example, adjacent heater electrodes 64 share a common positive terminal (although in other embodiments they may share a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 78 and the heater 6’. This common positive terminal is connected to the different heater electrodes at the central vias 65-1 to 65-5, which connect through to connection circuitry below (not shown) that connects to the drive and control board 78. The other end of each heater electrode connects through a respective switch (not shown) to the drive and control board 78 to allow independent control of current flow through each heater electrode 64. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each heater electrode 64 may be physically separate from all other heater electrodes 64 in which case, each end of each heater electrode 64 would be connected separately back to the drive and control board 78. As schematically illustrated in Figure 11, the end of each heater electrode 64 that is connected to the switch is provided at the side of the heater and the direction of the serpentine tracks changes in this edge portion (which corresponds to the portion of the heater which is curved over the upper surface 74 of the rigid support substrate 68’). The inventors have found that this arrangement helps heat generated in the heater electrodes 64 in these edge portions to pass up to the top surface of the heater which is more likely to come into contact with the user’s hair. However, if the device is twisted in use such that the user’s hair comes into contact with the curved edge portion, then the hair will still be heated as this curved edge portion is heated. The conductive material used in the layer 84 is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 64 depends upon its temperature - and so the temperature of the heating zone 642 can be determined by measuring a parameter that varies with the resistance of the corresponding heater electrode 64. Figure 12 is 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 12, each heater electrode 64 is connected at one end to the power supply 21 and at the other end to a respective switch (in this case a MOSFET switch) 95-1 to 95-20. 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 642 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, by measuring the voltage dropped across the resistor R (using the operational amplifier 97), the microprocessor 29 can determine the resistance of the selected heater electrode 64 and hence can determine the temperature of the corresponding heating zone 642. If the determined temperature is above the desired temperature for that heating zone 642, then the microprocessor 29 can reduce the power applied to that heater electrode 64; or if the heating zone 642 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. Polyimide Separator (Optional) 85 When an auxiliary heater electrode layer is provided, this layer is required to provide the required electrical separation between that auxiliary heater electrode layer and the main heater electrode layer 84 described above. This polyimide layer 85 would have a low thermal resistance in the thickness direction whilst still achieving the dielectric requirements. Due to this layer being relatively thin, it will have a low thermal conductivity in the plane perpendicular to its thickness of less than about 35 W / mK. Other dielectric materials could be used instead of polyimide. Auxiliary Heater Electrode Layer (Optional) 86 Some embodiments of the heater 6’ may benefit from the presence of an additional heating element layer 86. This additional layer 86 could be used to dissipate power (create heat) from a secondary power source that operates at a different voltage to the main power source 21, for example the main power source could be a power supply and the power source for the auxiliary heater electrode layer 86 could be one or more batteries. In other embodiments the primary source could be one or more batteries and the auxiliary one or more supercapacitors. Alternatively still, the conductors on this auxiliary layer 86 could be used for temperature sensing, in which case, the heater electrodes 64 in the main layer 84 may only be used for heating. The heater electrodes on the auxiliary layer 86 will typically have the same form as the heater electrodes 64 used in the main heater electrode layer 84 - so that they will define the same heating zones 642 as the heating zones 642 defined by the heater electrodes 64 on the main heater electrode layer 84. The path taken by the heater electrodes on the auxiliary layer 86 do not need to follow the same path as the corresponding heater electrodes 64 formed on the main heater electrode layer 84. For example, whilst the main part of each heater electrode 64 on the main heater electrode layer 84 (ignoring the edge part of each heater electrode 64) serpentines in the longitudinal direction of the heater 6’ in Figure 11, the corresponding heater electrodes of the auxiliary heater electrode layer 86 could be arranged to serpentine in the width direction of the heater 6’. Such an arrangement may help to spread the heat flow within the heating zone 642 particularly if the heating zone 642 is only partially loaded with hair. Polyimide backing 87 This layer encapsulates the bottom heating layer (either the main or the auxiliary heating layer) so as not to allow it’s accidental exposure and to prevent moisture ingress. This backing layer 87 electrically separates the bottom heating layer from any surface mounted components that are mounted in the surface mounting layer 88 (discussed below) on the bottom of the heater 6’. If desired, this dielectric layer 87 can be made thicker than the upper dielectric layers to provide enhanced structural integrity of the flexible part of the heater system. As with the other dielectric layers, this backing layer 87 does not need to be a polyimide layer and other dielectric materials could be used. Rear Side Surface Mount Components (Optional) 88 This layer is used to mount components on to the rear of the flexible heater 6. These components may be temperature sensors (e.g. thermistors) or other components involved in providing fusing functionality for the heater (e.g. solder links). This layer would be produced using standard chemical etching methods from the PCB manufacturing process. Additional surface mount components would be added later. High Temperature Adhesive 89 The function of this layer is to enabling bonding of the flexible heater 6’ to the rigid substrate 68 (shown in Figure 8) that forms the final shape of the overall heater. Various types of adhesive could be used such as a pressure activated adhesive (PAA) or a heat activated adhesive (HAA). It could also be a thermoplastic film which sets after heat and pressure have been applied in a forming tool. Detecting pull duration Disclosed herein is a hair styling appliance, such as a set of hair straighteners, having a pair of opposing jaws or arms, connected by a hinge, and a heating surface on each jaw. The heating surfaces face each other so that, when the jaws are closed, hair can be sandwiched between the heating surfaces for straightening or otherwise styling the hair. Hair can be styled by systematically running tresses of hair between the heating surfaces. A single tress pull can be done by pulling the hair styling appliance along the tress of hair from the root of the tress (i.e., near the scalp) to the tip of the tress (i.e., at the free end of the tress). The heaters may be low thermal mass heaters such as those described above that can heat up and cool down quickly. When using such a hair drying and / or styling appliance, it is advantageous for the heaters to switch off or to reduce their temperature between successive tress pulls. In some modes of styling, it may also be considered advantageous for the heaters to have a relatively lower temperature at the start of the tress pull, i.e. near the roots where the hair is younger (and more susceptible to heat damage), and to increase the temperature of the heaters at a later stage in the tress pull, i.e. towards the tip of the tress where the hair is older and less sensitive to heat. The hair styling appliance disclosed herein senses whether the jaws are open or closed using a hinge switch. Based on whether the jaws are open or closed, a controller of the appliance can perform a control action to control the heaters according to the advantageous behaviours mentioned above. For example, the controller can switch off or reduce the temperature of the heaters when the jaws are opened. Also, the controller can time the duration of a tress pull, for example by determining the time between the jaws closing and subsequently opening. This information can be used to perform an action, such as displaying information on a display or storing information in a memory or controlling the temperature of the heaters by altering the power supplied to the heaters. The controller can also infer the position of the appliance along a tress of hair based on how long the jaws have been closed as a fraction of the determined duration of a tress pull. In this way, the controller can control the temperature of the heaters based on the position of the appliance along the tress, so as to increase the temperature of the heaters for example at a rate appropriate to the length of the hair and the speed of the tress pull. Referring now to the figures, Figure 13a illustrates a hand held (portable) hair styler 13-1. The hair styler 13-1 includes a first arm 13-4a and a second arm 13-4b, which are coupled at proximal ends thereof by a hinge, such as a shoulder 13-2. The first arm 13-4a bears a first heater 13-6a at its distal end, and the second arm 13-4b bears a second heater 13-6b at its distal end. The first and second heaters 13-6a, 13-6b oppose one another and are brought together as the first and second arms 13-4a, 13-4b are moved from an open configuration to a closed configuration. As shown in Figure 13b, during use, a tress of hair 13-40 is sandwiched between the two arms 13-4 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 13-6a, 13-6b. Therefore, as the user pulls the hair styler 13-1 along the tress of hair 13-40, the tress of hair 13-40 is heated by conductive heating to a suitable temperature to facilitate styling. A user interface 13-11 is provided to allow the user to set user defined parameters for the device and to output information to the user. For example, a desired operating temperature may be set via the user interface 13-11. The user interface 13-11 may have a dial, button or touch display for allowing the user to input information to the device 13-1 and the user interface 13-11 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. In this embodiment, the user interface 13-11 also comprises a control button or switch 13-14 to enable the user to turn the device 13-1 on or off; and an indicator light 13-15 to show whether the power is on. A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 13-1 and carries the control circuitry for controlling the operation of the device 13-1 and for controlling the interaction with the user via the user interface 13-11. In this example, electrical power is provided to the device 13-1 by means of a power supply located at an end of the device, via a power supply cord 13-3. 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 13-1 to be a cordless product. In use, the device 13-1 is turned on, energising the heaters 13-6 to cause them to heat up. The user then opens the first and second arms 13-4a, 13-4b and, normally starting from the roots of the hair (i.e. near the scalp), a length or tress of hair 13-40 (which may be clumped) is introduced between the arms 13-4a, 13-4b, transversely across the heaters 13-6a, 13-6b. The user then closes the arms 13-4a, 13-4b so that the length of hair 13-40 is held between the first and second arms 13-4a, 13-4b and then the user pulls the hair through the closed arms (as illustrated in Figure 13b). The outer (hair contacting) surface of the heaters 13-6 is flat in this embodiment and so the hair styler 13-1 can be used to straighten the user’s hair. The hair styling device 13-1 shown in Figure 13 can also be used to curl the hair by turning the device 13-1 through approximately 180 degrees or more after clamping the hair between the arms 13-4a, 13-4b and before moving the device 13-1 along the tress of hair 13-40. The device 13-1 also comprises a sensor (not shown). The sensor may be comprised in sensor circuitry 14-50 (see Figure 14). The sensor is configured to detect relative movement between the first arm 13-4a and the second arm 13-4b. The sensor may therefore be considered a hinge switch or hinge sensor. The sensor may comprise a switch configured to be actuated upon opening and / or closing of the device 13-1. In one example, the sensor may comprise an electromechanical contact, such as a microswitch, which may be open when the appliance is in the open configuration and closed when the appliance is in the closed configuration, or vice versa. The sensor may comprise a first sensing component and a second sensing component, which can interact with one another to detect relative movement therebetween. For example, the first sensing component may be provided on the first arm 13-4a and the second sensing component may be provided on the second arm 13-4b. The sensor may be connected to the controller 14-28 (see Figure 14) in order to output a detection signal to the controller 14-28. In one such arrangement, the sensor comprises a magnetic switch arrangement. The magnetic switch arrangement may comprise a magnet and a magnetic field sensor. As such, the magnet may be the first sensing component and the magnetic field sensor may be the second sensing component. The magnet may be provided on the first arm 13-4a and the magnetic field sensor may be provided on the second arm 13-4b. The magnetic field sensor may comprise a semiconductor hall effect sensor. The sensor may be configured to output a signal based on the strength of the magnetic field detected by the magnetic field sensor. The magnetic field sensor may also be a reed switch. In such an arrangement, the sensor may be configured to output a signal based on whether the reed switch is closed by virtue of its interaction with the magnet. In another arrangement, the sensor comprises an optical sensor arrangement. The optical sensor arrangement may comprise a retro-reflective sensor comprising an emitter / receiver and a retroreflector. In this arrangement, the emitter / receiver may be the first sensing component and the retroreflector may be the second sensing component. In another example, the optical sensor comprises an interruptive sensor comprising an emitter and a receiver. In this arrangement, the emitter may be the first sensing component and the receiver may be the second sensing component. The sensor may be configured to output a signal based on the intensity of the light received by the receiver, or based on whether or not the optical sensor is interrupted. In some arrangements, the sensor comprises a capacitive sensor arrangement. The capacitive sensor arrangement may comprise a capacitive sensor and a conductive object. In this arrangement, the capacitive sensor may be the first sensing component and the conductive object may be the second sensing component. The sensor may be configured to output a signal based on the strength of the electric field detected by the capacitive sensor. In a different example, the sensor comprises a strain gauge arrangement. The sensor can comprise a flexible member with a strain gauge transducer configured to measure the strain of the flexible member. The flexible member may be positioned in the appliance so as to measure a strain proportional to the separation between the arms 13-4a, 13-4b. In this respect, the flexible member may be provided between the first arm 13-4a and the second arm 13-4b and may be incorporated into the shoulder 13-2. The sensor may be configured to output a signal based on the strain detected by the strain gauge transducer. In view of the foregoing, the sensor can be configured to output a signal to the sensor circuitry 14-50 representative of the separation between the arms 13-4a, 13-4b, in order to detect whether the appliance is in the open configuration or the closed configuration. In some examples, the sensor can be configured to measure a degree of opening of the arms 13-4a, 13-4b, such as an angle between the arms 13-4a, 13-4b, and to output a signal based on the degree of opening. Figure 14 is a simplified block diagram of control circuitry 14-16 that controls the operation of the hair styler device 13-1 shown in Figure 13. Labels in the figures that indicate similar features to other figures have been allocated the same reference numerals prefixed with the figure number. The control circuitry 14-16 comprises a power supply 14-21 that, in this embodiment, derives power from a battery power source. 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 13-1. Alternatively, the power supply 14-21 may derive power from an AC mains supply input. In this example, power is provided to the heaters 14-6 for heating the user’s hair. The power supplied to the heaters 14-6 is controlled by a controller 14-28 having a microprocessor 14-29. The power supplied to the heaters 14-6 is controlled by drive circuitry 14-23 (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 14-6 in accordance with instructions from the microprocessor 14-29. The microprocessor 14-29 is coupled to a memory 14-30 (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 14-6 in accordance with a desired operating temperature of the heaters 14-6 and sensed temperatures of the heaters obtained from temperature measurement circuitry 14-25. The temperature measurement circuitry 14-25 may be temperature sensors such as thermistors or they may use circuitry that senses the resistance of heater electrodes that are used to heat the heaters 14-6, which resistance depends on the temperature of the heater electrode. Figure 14 also shows that the user interface 14-11 is coupled to the microprocessor 14-29, 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 14-6 or if they are moving the device 13-1 too quickly along the hair tress 13-40. The control circuitry 14-16 includes communications circuitry 14-27 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 14-27 may use, for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the remote device. The control circuitry 14-16 further comprises hinge sensor circuitry 14-50. The hinge sensor circuitry 14-50 is configured to transmit signals from the sensor to microprocessor 14-29 in the controller 14-28. The controller 14-28 is configured to interpret the signals to determine whether the appliance is in the open configuration or the closed configuration. For example, in the case of a microswitch, the controller 14-28 may determine based on the microswitch being closed that the appliance is in the closed configuration, and may determine that the appliance is in the open configuration based on the absence of a signal if the microswitch is open. In the case of a magnetic switch arrangement, the controller 14-28 may determine that the appliance is in the closed configuration when the magnetic field sensor detects a magnetic field above a threshold strength, and may determine that the appliance is in the open configuration when the magnetic field sensor detects a magnetic field below a threshold strength. In the case of an optical sensor arrangement, the controller 14-28 may determine that the appliance is in the closed configuration based on an intensity of light received by the receiver being above a threshold, and vice versa for the open configuration. In the case of a capacitive sensor arrangement, the controller 14-28 may determine that the appliance is in the closed configuration based on the detected strength of the electric field being above a threshold, and vice versa for the open configuration. In the case of a strain gauge arrangement, the controller 14-28 may determine that the appliance is in the closed configuration based on the detected strain being below a threshold, and vice versa for the open configuration. The control circuitry 14-16 may further comprises accelerometer circuitry 14-51. The accelerometer circuitry 14-51 is configured to detect an acceleration of the appliance and to transmit acceleration information to the microprocessor 14-29 in the controller 14-28. Figure 15 illustrates a control sequence 15-S that the controller 14-28 can be configured to perform. The controller 14-28 is configured to detect 15-S1 a first closure based on determining that the appliance is in the closed configuration. The appliance is configured to detect 15-S2 a first opening based on subsequently determining that the appliance is in the open configuration. The controller 14-28 is configured to measure 15-S3 a pull duration between determining that the appliance is in the closed configuration and determining that the appliance is in the open configuration. In other words, once it is detected via the hinge sensor circuitry 14-50 that the appliance is in the closed configuration, the controller 14-28 is configured to measure the time taken until the appliance is detected to be in the open configuration again. The determined pull duration represents how long it takes a user to perform one tress pull, i.e. pulling the appliance from the root of the hair tress to the tip of the hair tress. The appliance is configured to perform 15-S4 a control action based on the determined pull duration. The control action can comprise displaying information to the user via the user interface 13-11 and / or storing information in the memory 14-30. For example, the controller can be configured to store the tress pull duration in the memory for association with a user profile, in order to build data on the styling behaviour of a particular user. The controller can also be configured to determine a velocity of the appliance at a given time. In one example, the velocity can be determined based on the acceleration detected by the accelerometer 14-51 and on determining that the appliance is in the closed configuration. In particular, it may be inferred that, at the moment when the closed configuration is detected, the appliance is stationary with respect to the user’s hair. Therefore, the controller can combine the acceleration from the accelerometer with the time elapsed since the appliance was closed in order to calculate the velocity of the appliance with respect to the user’s hair at a given time. The variation of the determined velocity over the course of a tress pull (i.e. between the appliance closing and the appliance opening) can be used by the controller 14-28 to calculate the tress length, for example by integrating the velocity with respect to time. In this way, the controller can be configured to determine a tress length based on the determined velocity and the determined pull duration. The determined velocity and / or tress length can be used in addition to the pull duration to perform the control action. For example, the control action can comprise storing or displaying such information as described above, or controlling the power supplied to the heaters based on this information. Figure 16 is a graph of temperature against time according to an example of how the controller 14-28 can control the temperature of the heaters based on the pull duration. Once the appliance is powered on, the controller 14-28 may be configured to supply power to the heaters to achieve a standby temperature, Tstby, which may be 100°C for example. Initially, when the first closure is detected at time 16-t1, the controller 14-28 is configured to increase the temperature of the heaters from Tstby to Tstart- Tstart may be a temperature suitable for heating hair at the roots of a tress, in that the temperature is high enough to achieve efficient styling (i.e., in a minimal number of passes) but sufficiently cool to prevent damage to the relatively young hair at the root. At this stage, the controller 14-28 may be configured to maintain the heaters at Tstart until the first opening is detected at time 16-t2. In other examples, the controller 14-28 may be configured to increase the temperature at a predefined rate to a predefined maximum temperature. Once the first opening is detected, the controller 14-28 is configured to reduce the temperature of the heaters to Tstby. In some examples, the reduction to Tstby may be immediate upon detecting that the appliance is open. In other examples, the controller 14-28 is configured to determine an open duration during which the appliance has remained in the open configuration, and reduce the temperature of the heaters, for example to the standby temperature, if the open duration is greater than a first threshold. Following the first closure and the first opening, the controller 14-28 is configured to measure the pull duration. This can be calculated using the time elapsed between the appliance closing and the appliance subsequently opening, i.e. the difference between time 16-t2 and time 16-t1 in Figure 16. In the example shown, at time 16-t3, a closure is detected such that, as before, the controller 14-28 is configured to increase the temperature of the heaters from Tstby to Tstart. Then, the controller 14-28 is configured to increase the temperature of the heaters from Tstart to Tend, which represents a temperature higher than Tstart for efficient styling of the hair that is further down the tress and hence older and less prone to heat damage. If the power supplied to the heaters is such that the temperature of the heaters increases too slowly, then the tress pull will be finished before Tend is reached and the styling may be inefficient, for example requiring many passes. Conversely, if the power supplied to the heaters is such that the temperature is increased too quickly, then hair closer to the root may be more liable to heat damage. Using the pull duration, the controller 14-28 can be configured to calculate the position of the appliance along the tress at a given point in time during a tress pull. For example, assuming each tress of hair has about the same length and assuming an approximately constant velocity of the appliance once a tress pull begins, the proportional position of the appliance along the tress at a given time can be approximated by dividing the elapsed time since the appliance was closed by the pull duration of the first pull. In some modes, the controller 14-28 can be configured to increase the temperature of the heaters linearly, from Tstart to Tend, in proportion to the proportional position of the appliance along the tress. For example, when the controller 14-28 infers, based on the elapsed time into a tress pull and the pull duration, that the appliance is 50% along the length of the tress, then the controller 14-28 may set the temperature of the heaters to be halfway between Tstart and Tend. It will be appreciated that other heating modes, which may be non linear, may be employed, for example to increase the temperature slowly for the majority of the length of the tress and then increase the temperature more quickly towards the end of the tress, or vice versa. In the illustrated example, when the controller 14-28 determines that the appliance is in the open configuration at time 16-t4, the controller 14-28 reduces the temperature of the heaters from Tend to Tstby. This temperature pattern can be repeated for subsequent tress pulls, as indicated by the dashed lines. The controller 14-28 can be configured to measure how long the appliance remains in the open configuration and, if this duration is determined to be greater than a second threshold, then the controller 14-28 is configured to switch off the heaters. The second threshold is greater than the first threshold. In this way, upon detecting that the appliance is open and remains in the open configuration, the controller 14-28 is configured to wait for a first time period before reducing the temperature to Tstby, and to wait for a second time period before switching off the heaters. In the arrangement shown, the duration of time between time 16-t6 and time 16-t5 is greater than the second threshold such that the heaters are switched off. In some arrangements, after determining that the appliance is in the closed configuration, the controller 14-28 may be configured to set the temperature to Tstart and maintain it for a predetermined delay, before increasing the temperature to Tend. In other words, the controller 14-28 is configured to account for the appliance being momentarily stationary when it is first closed, and to wait until the appliance is inferred to be moving along the tress before increasing the temperature of the heaters. While the control sequence has been described as using the first tress pull to control the temperature of the heaters for subsequent tress pulls, the controller 14-28 may be configured to measure the pull duration of multiple tress pulls and control the power based on the determined pull duration of multiple preceding tress pulls, for example by calculating an average pull duration. The controller 14-28 may also be configured to determine whether the determined pull duration is within a predefined range and to control the power supplied to the heaters based on the determined pull duration and also based on whether the determined pull duration falls within the predefined range. In other words, before using the determined pull duration to determine how the heaters are controlled, the controller 14-28 can check whether the determined pull duration is too long or too short. 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 aspects embodied therein. It will therefore be understood that the disclosure 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 disclosure has been described 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, this includes devices with a hinge to move just one arm relative to a fixed arm or any device with one stationary component such as a curling wand. 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, which is hereby incorporated by reference in its entirety). 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. In this respect, some embodiments may be incorporated into a wet-to-style product, which may incorporate heaters (such as heaters 13-6a, 13-6b) for straightening hair, as well as means to dry hair, for example by delivering heated airflow to the hair. Such means may comprise a fan assembly to generate airflow, which may be driven by a motor, and an air heater (which may be separate to the heaters 13-6a, 13-6b) to heat the airflow. The inventors have recognised that high thrust airflow can be important in enhancing heat transfer efficiency through the entirety of the hair section, which helps to minimise the size, power consumption and performance requirements of a hair styling and drying product. However, while high thrust airflow enhances drying efficiency, it can cause hair to fly around when the tress is inserted into such a product, leading to poor hair alignment, shine and smoothness due to the creation of flyaway hairs and frizz. Therefore, it may be advantageous to control the fan speed, in particular by reducing the fan speed while loading a tress of hair into the product (to reduce the thrust force acting on the hair, thereby reducing or preventing the creation of flyaway hairs and frizz) and to increase the fan speed once a tress of hair has been loaded (to benefit from maximum heat transfer efficiency). In addition or instead of controlling the temperature of the heaters, any of the sensing methods discussed above may be used to control the fan speed. For example, in the case of the sensor configured to sense relative movement of the first arm and the second arm between the open and closed configurations, the controller may be configured to control the fan speed based on an output of the sensor. In other words, by determining whether the appliance is in the open configuration or the closed configuration based on the sensed relative movement, the controller can control the fan speed accordingly. The controller can therefore be configured to reduce the fan speed (e.g., to a first predefined speed) when the appliance is determined to be in the open configuration and to increase the fan speed (e.g., to a second predefined speed, being higher than the first predefined speed) when the appliance is determined to be in the closed configuration. The reduction of the fan speed upon opening the appliance may be selected to generate a 20-50% air flow rate reduction with respect to the intended flow rate for drying. Since the output air temperature is a function of the flow rate and the air heater input power, it may be advantageous to control the air heater power to prevent or reduce spikes in air temperature at the outlet. Therefore, the controller may be configured to control the air heater power based on an output of the sensor. In other words, by determining whether the appliance is in the open configuration or the closed configuration based on the sensed relative movement, the controller can control the air heater power accordingly. The controller can therefore be configured to reduce the air heater power (e.g., to a first predefined power) when the appliance is determined to be in the open configuration and to increase the air heater power (e.g., to a second predefined power, being higher than the first predefined power) when the appliance is determined to be in the closed configuration. In some examples, after determining that the appliance is in the open configuration, the controller is configured to reduce the air heater power before reducing the fan speed. Similarly, after determining that the appliance is in the closed configuration, the controller may be configured to increase the fan speed before increasing the air heater power. The above examples may be performed by implementing a delay between controlling the fan speed and the air heater power. 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). 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 one or more batteries, although DC supplies that derive their power from a mains power AC signal may be used. Thicker or more dielectric layers are typically used between the heater electrodes 64 and the hair contacting surface of the hair styler when AC power is used to heat the heaters. In the above-described examples the hair styling device 10 may comprise a single heater 6, or may alternatively comprise two or more heaters 6. 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. 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 drying and / or styling appliance comprising:a first arm;a second arm;heaters comprising a first heater on the first arm and a second heater on the second arm, the first and second arms being mutually opposed and adapted for movement between an open configuration in which a tress of hair can be positioned between the first and second arms and a closed configuration in which the tress of hair is sandwiched between the first heater and the second heater;a sensor configured to sense relative movement of the first arm and the second arm between said open configuration and said closed configuration; anda controller configured to:determine whether the appliance is in the open configuration or the closed configuration based on the sensed relative movement;determine a pull duration that the appliance is pulled along the tress of hair based on a duration that the appliance is determined to be in the closed configuration, andperform a control action based on the determined pull duration.

2. The appliance of claim 1, wherein the control action comprises displaying information to the user and / or storing information in a memory of the appliance.

3. The appliance of claim 1 or claim 2, wherein the appliance is configured to determine a velocity of the appliance as it is moved along the tress of hair, and to determine a tress length based on the determined velocity and the determined pull duration, wherein the control action is based on the determined tress length.

4. The appliance of any preceding claim, wherein the controller is configured to determine whether the determined pull duration is within a predefined range before performing the control action.

5. The appliance of any preceding claim, wherein the control action comprises controlling the power supplied to the heaters.

6. The appliance of claim 5, wherein controlling the power includes initially increasing the temperature of the heaters from a standby temperature to a start temperature, being higher than the standby temperature, when the controller determines that the appliance is in the closed configuration.

7. The appliance of claim 6, wherein controlling the power includes maintaining the temperature of the heaters at the start temperature for a predetermined time.

8. The appliance of claim 6 or claim 7, wherein controlling the power further includes increasing the temperature of the heaters during the pull from the start temperature to an end temperature, being higher than the start temperature.

9. The appliance of any of claims 5 to 8, wherein controlling the power comprises increasing the power supplied to the heaters to gradually increase the temperature of the heaters during the pull at a rate based on the determined pull duration.

10. The appliance of claim 9, wherein the rate is proportional to the determined pull duration.

11. The appliance of claim 10, wherein the rate is linearly proportional to the determined pull duration.

12. The appliance of any preceding claim, wherein the controller is further configured to determine an open duration during which the appliance has remained in the open configuration and is configured to reduce the temperature of the heaters to a standby temperature if the open duration is greater than a first threshold.

13. The appliance of claim 12, wherein the controller is further configured to switch off the heaters if the open duration is greater than a second threshold, being greater than the first threshold.

14. The appliance of any preceding claim, wherein the sensor comprises at least one sensor selected from the group consisting of: a microswitch; a magnetic field sensor; an optical sensor; a capacitive sensor; and a strain gauge transducer.

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