Heater apparatus and method

Abstract

A hair drying or styling apparatus 1 comprising a heater 6 with a plurality of independently controllable heating zones (642, fig.6b) and a controller (28, fig.2) that: determines a loaded hair tress size based on a drop of the temperature of the heating zones or an increase in the power output needed to maintain them at the target temperature, and an amount of hair engaged; determines whether the hair tress size is greater than a threshold value; and, if so, outputs a feedback message, such as sound, light and / or haptic feedback to a user interface 11, indicating that too much hair has been loaded and controls the temperature or the power output of the heating zones which are engaged with the hair to avoid burning the hair. The controller may also determine whether the hair tress size is smaller than a second threshold value. The feedback message may be output to a mobile phone or smart watch. The controller may receive hair type data and / or output from a pressure sensor to refine the hair tress size determination. A method performed by the apparatus is also disclosed.

Description

Field of the Invention The present invention relates to heating apparatus and methods. The heaters can be used for styling hair. 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 styling device comprising one or more low thermal mass heaters. Background to the Invention Heated hair styling tools use heat to increase the temperature of hair to a desired styling temperature. For example, a hair 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. Similarly, a heated brush or hair dryer can also be used to style hair by heating air which in turn heats the hair to a temperature suitable for styling. The hair is typically styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry. Existing hair styling appliances typically use heaters that provide a certain amount of thermal mass to the hair styling appliance. The amount of thermal energy provided to the hair styling appliance correspond to one mode of operation that is particularly suitable for a desired amount of hair. However, there is a need for improvements to such existing hair styling appliances, as users generally do not know the amount of hair they should be loading into the hair styling appliance. If the user loads too much hair, the hair will not be heated adequately, resulting in a frustrating styling experience. However, if the user does not load enough hair into the styler, will have their hair damaged when styling. This issue might be exacerbated depending on the type of hair. The present invention aims to address or at least partially ameliorate one or more of the above problems. Summary of the Invention Aspects and embodiments of the invention are set out in the appended claims. These and other aspects of the invention, and aspects and embodiments which are useful in understanding the invention set out in the appended claims, are also described in the disclosure herein. Any feature in one aspect of the disclosure may be applied to other aspects of the disclosure, in any appropriate combination. In particular, method aspects may be applied to device and computer program aspects, and vice versa. Furthermore, features implemented in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly. In one aspect, the invention provides apparatus fordryingor styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair of a user that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; and a controller configured to control the apparatus to: determine, based on a drop of a temperature of one or more heating zones of the plurality of independently controllable heating zones or based on an increase in a power output needed to maintain the one or more heating zones at a target temperature, whether hair is engaged with the one or more heating zones and, if it is, to determine an amount of hair which is engaged with the one or more heating zones; determine, based on the determined amount of hair engaged with the one or more heating zones across all of the one or more heating zones, a hair tress size which has been loaded by the user into the apparatus; determine whether the determined hair tress size is greater than a threshold value and, if it is, to cause the apparatus to: output a feedback message indicating that too much hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the hair styling device accordingly, and / or control the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt. The threshold value may be a first threshold value. The controller may further be configured to determine whether the determined hair tress size is smaller than a second threshold value, smaller than the first threshold value and, if it is, to cause the apparatus to: output a feedback message indicating that too little hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the apparatus accordingly, and / or control the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt. The apparatus may comprise: a user interface for outputting a feedback message to the user. The controller may be configured to cause the user interface to output the feedback message indicating whether too much hair has been loaded into the apparatus. The user interface may comprise at least one of a sound generator, lighting means and / or a haptic feedback generator. The lighting means may comprise one or more lamps arranged along a length the apparatus, sideways of the heater, the controller being configured to control the apparatus to light up one or more lights of the lighting means on a length corresponding to the extent of the one or more heating zones which are engaged with the hair, in a length of the heater. The controller may be configured to control the apparatus to cause the lighting means to produce a first colour, such as green, when it is determined that the hair tress size corresponds to a desired amount of hair, and a second colour different from the first colour, such as red, otherwise, thus prompting the user to modify the amount of hair loaded in the apparatus. The controller may be configured to control the apparatus to: cause the sound generator to produce a first sound, when it is determined that the hair tress size corresponds to a desired amount of hair, and to produce a second sound different from the first sound, otherwise, thus prompting the user to increase or decrease the amount of hair loaded in the apparatus, and / or cause the haptic feedback generator to produce a first haptic feedback, when it is determined that the hair tress size corresponds to a desired amount of hair, and to produce a second haptic feedback differentfrom the first haptic feedback, otherwise thus prompting the user to increase or decrease the amount of hair loaded in the apparatus. The apparatus may further comprise communication circuitry such that the apparatus is configured to output the feedback message to an external processing device and wherein the output feedback message is output on a user interface associated with the user. The external processing device may be part of a mobile phone or smart watch and may be configured to run an application, remote from the apparatus. If it is determined that the determined hair tress size is greater than the threshold value and thus is bigger than a desired amount of hair, the controller may be configured to cap the increase in the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, so as to enable a better drying or styling of the hair which is loaded in the apparatus while avoiding burning of the hair. If it is determined that the determined hair tress size is smaller than the second threshold value and thus is smaller that a desired amount of hair, the controller may be configured to turn off the heater or to reduce the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to prevent the hair being burnt. The one or more heating zones may be adjacent to each other along a length and / or across a width of the heater. The one or more heating zones may be substantially thermally isolated from each other. The controller may be configured to control the apparatus to: determine the resistance of a heater electrode of the one or more heating zones based on a voltage supplied to the heater electrode and the current passing through the heater electrode; and determine the temperature of the one or more heating zones based on the determined resistance. The controller may be configured to control the apparatus to supply power to a heater electrode of the heater to determine the resistance of the heater electrode even in a case where it is determined that the corresponding heating zone is not engaged with hair The apparatus may further comprise a pair of opposing heating zones, heat being allowed to flow between the opposing heating zones when the apparatus is in use. The controller may be configured to control the apparatus to: determine whether hair is engaged with at least one of the pair of opposing heating zones based on a sum or average of a power needed to maintain each of the pair of opposing heating zones at a target temperature. The controller may be configured to control power supplied to a heater electrode of the one or more heating zones to control the temperature of the one or more heating zones towards a first target temperature in a case where it is determined that the hair is engaged with the one or more heating zones, and to control the temperature of the one or more heating zones towards a second target temperature, lower than the first target temperature, in a case where it is determined that the hair is not engaged with the one or more heating zones; and intermittently supply power to the heater electrode to determine the resistance of the heater electrode even in the case where it is determined that the one or more heating zones is not engaged with the hair, and determine the temperature of the one or more heating zones based on the resistance. The apparatus may further comprise a pressure sensor configured to sense and output data indicative of a pressure between a first movable arm and a second movable arm of the apparatus, to enable the controller to verify the determination of the hair tress size, as larger hair tresses require more pressure to stay in place in the apparatus. The controller may be configured to receive data indicative of hair type of the user, to enable the controller to refine the determination of the hair tress size, as thick curly hair has larger hair tress sizes than fine straight hair. The apparatus may be a hair straightener, a hair dryer, a hot paddle brush, a hot round brush, a heater roller, or a hair curler. In another aspect, the invention provides a method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, and the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises: decreasing a temperature or increasing a power output of one or more heating zones of the plurality of independently controllable heating zones; determining, based on the drop of a temperature of one or more heating zones of the plurality of independently controllable heating zones or based on the increase in a power output needed to maintain the one or more heating zones at a target temperature, whether hair is engaged with the one or more heating zones and, if it is, to determine an amount of hair which is engaged with the one or more heating zones; determining whether the determined hair tress size is greater than a threshold value and, if it is, the method further comprising: outputting a feedback message indicating that too much hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the hair styling device accordingly, and / or controlling the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt. The threshold value may be a first threshold value. The method may further comprise determining whether the determined hair tress size is smaller than a second threshold value, smaller than the first threshold value and, if it is, the method may further comprise: outputting a feedback message indicating that too little hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the apparatus accordingly, and / or controlling the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt. The apparatus may comprise a user interface for outputting a feedback message to the user, the user interface comprising at least one of a sound generator, lighting means and / or a haptic feedback generator. The method may furthercomprise: producing a first feedback, when it is determined that the hair tress size corresponds to a desired amount of hair, and producing a second feedback different from the first feedback, otherwise, thus prompting the user to modify the amount of hair loaded in the apparatus. If it is determined that the determined hair tress size is greater than the threshold value and thus is bigger than a desired amount of hair, the method may further comprise: capping the increase in the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, so as to enable a better drying or styling of the hair which is loaded in the apparatus while avoiding burning of the hair. If it is determined that the determined hair tress size is smaller than the second threshold value and thus is smaller that a desired amount of hair, the method may further comprise: turning off the heater or reducing the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to prevent the hair being burnt. In another aspect, the invention provides a corresponding computer program product comprising computer implementable instructions. Brief Description of the Drawings Embodiments of the invention will now be described, byway 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 shows a flow chart illustrating an example method for operating an appliance according to the disclosure; Figures 9A and 9B show an interface of the appliance comprising lighting means, the lighting means comprising one or more lamps arranged along a length the appliance, sideways of a heater; Figure 10 shows a flow chart illustrating an example method for operating an appliance according to the disclosure. Overview of Hair Styling Device Figure 1a illustrates a hand-held (portable) hair styler 1 (or hair styling appliance or apparatus in the present disclosure). 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 input information about them to the device and / or for the device to output information to the user. 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 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. Furthermore, different hair types require different amount of heat energy for hair styling (because of different thickness, quality, condition, thermal mass of hair). Typically, fine straight hair requires less heat than thick curly hair for styling. Depending on the type of hair, the glass transition temperature may be a temperature in the range of approximately 160°C-200°C. 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 fora 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. 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. I n 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, ora 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 microprocessor 29 allows for complex control of the heaters 6. For example, the controller 28 may be configured to adjust the power delivered to heaters by using an on / off triac based upon the output of the temperature measurement circuitry 25. The memory 30 may store a number of transfer functions such as: simple on-off control means or bang-bang control means; proportional-integral-derivative (PID) control means; a set of deterministic rules; data for an artificial intelligence algorithm; fuzzy logic; feed back control means; feed forward control means. The controller 28 comprises means to measure the input voltage or alternatively to detect the speed at which the heaters 6 heat up, so as to detect the type of input voltage. A high input voltage would lead to a faster heat up of the heaters 6 and hence a control loop can react appropriately. The input voltage and / or speed of heat up can also be used to detect a failure. The controller 28 may comprise means to detect the use of the hair styling appliance and control the power supply to the heaters accordingly. This feature helps to reduce power consumption and improve safety. For example, the controller 28 may comprise means to reduce the temperature of the heaters when they are not active and then rapidly heat them up when they are about to be used. The controller 28 may allow a heater to power down to a standby temperature if a user momentarily places the hair styling appliance on a table. The controller 28 may then power up the heater to an operating temperature when the hair styling appliance is picked up to be used. If the controller 28 detects that the hair styling appliance has not been used fora longer period of time, then the control means may shut down the hair styling appliance. This enables the hair styling appliance to meet the mandatory requirement of the safety standard that the appliance must turn off after 30 minutes whether it is being used or not. Detection of use may be achieved by detecting the opening and closing of the first movable arm and a second movable arm of the hair styling appliance, or through the use of one or more motion detection devices to detect the motion of the hair styling appliance or the used of a capacitive touch system. The one or more motion detection devices are shown in Figure 2 and referred to with numerical reference 31. The one or more motion detection devices 31 may comprise any one of an accelerometer, a gyrometer, a magnetometer, an inclination sensor. The hair styler 1 includes a pressure sensor 32 to measure a pressure between the first movable arm 4a and the second movable arm 4b, e.g., when styling the hair. 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, input and / or output indications such as a visual indication or an audible alert. Finally, the control circuitry includes communications circuitry 27 to allow the device to communicate with a remote sensor, a remote server, ora remote application (e.g., on a mobile telephone or a smart watch). The communications circuitry 27 may use, for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the remote device. As already stated, the user interface 11 comprises feedback means for providing feedback to the user of the styler. The operation of the feedback means will be described in greater detail below. The feedback means may comprise a sound generator configured to provide a polyphonic sound. The means may provide a particular sound brand or jingle when switching on and / or off. The means may provide a sound to indicate particular events, such as reaching a desired operating temperature and / or sleep mode. Additionally or alternatively, the feedback means may comprise lighting means. The lighting means may provide a pleasing aesthetic appearance as well as indicate temperature or other events. The lighting means may comprise an electroluminescent backlight as it enables wide angle, wide area viewing. Alternatively or additionally, the lighting means may comprise an LED lighting with a suitable light-pipe and / or optical diffuser. Additionally or alternatively, the feedback means may comprise a haptic feedback generator for outputting information to the user. Heaters The heaters 6a, 6b are 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 electrodelayer 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 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 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. 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. 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 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) ora 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. When hair is present on the hair contacting surface, the temperature of the hair contacting surface drops more rapidly than when hair is not present. Therefore, when the hair styling appliance is unpowered, presence of hair can be detected by measuring a rate of change of temperature gradient of the hair contacting surface. Alternatively or additionally, and as discussed in more detail below, hair presence can be detected by zonal power loading. 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. The controller may be configured to determine the resistance of a heater electrode of the one or more heating zones based on a voltage supplied to the heater electrode and the current passing through the heater electrode; and determine the temperature of the one or more heating zones based on the determined resistance. The controller may be configured to control the supply of power to a heater electrode of the heater to determine the resistance of the heater electrode even in a case where it is determined that the corresponding heating zone is not engaged with hair. When there is a pair of opposing heating zones, heat can flow between the opposing heating zones when the apparatus is in use, and the controller may be configured to determine whether hair is engaged with at least one of the pair of opposing heating zones based on a sum or average of a power needed to maintain each of the pair of opposing heating zones at a target temperature. In some examples, the controller may be configured to control power supplied to a heater electrode of the one or more heating zones to control the temperature of the one or more heating zones towards a first target temperature in a case where it is determined that the hair is engaged with the one or more heating zones, and to control the temperature of the one or more heating zones towards a second target temperature, lower than the first target temperature, in a case where it is determined that the hair is not engaged with the one or more heating zones; and intermittently supply power to the heater electrode to determine the resistance of the heater electrode even in the case where it is determined that the one or more heating zones is not engaged with the hair, and determine the temperature of the one or more heating zones based on the resistance. Heating Zone Sizing 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, and using the already described temperature measurement circuitry 25, 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. Figure 7 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. Operation based on different input streams of data Tress Load The microprocessor 28 can detect when the heaters have detected the presence of hair, by using a measure of a rate of change of temperature gradient of the hair contacting surface and / or a measure of the zonal power loading. In other words, based on the temperature loss of the heaters and on how many heating zones are activated for heating, the microprocessor 28 can calculate how much hair was loaded into it. The Figure 7 extent of the heating zones gives the microcontroller 28 an indication of the size of the tress of hair 40 introduced between the arms of the styling appliance. In the present disclosure, the size of the tress of hair can refer to a volume of hair, a number of hair fibres, a surface area of hair, or any combinations of the previous, such as a hair density. The size of the tress of hair 40 gives an indication of the amount of hair introduced between the arms of the styling appliance. Additionally or alternatively, the styler may comprise sensing means to determine the amount of hair introduced in the styling apparatus. The sensing means may include capacitive sensing means to detect the amount of hair between the heaters. Figure 8 shows a flow chart illustrating an example method 100 for operating an appliance according to the disclosure. In the method 100 of Figure 8, the microcontroller 28 determines, at S1, based on a drop of a temperature of one or more heating zones of the plurality of independently controllable heating zones or based on an increase in a power output needed to maintain the one or more heating zones at a target temperature, whether hair is engaged with the one or more heating zones. If it is determined that hair is present, the method 100 moves to S2 where the microcontroller 28 determines an amount of hair which is engaged with the one or more heating zones. The determined amount of hair in contact with each heating zone is based on the amplitude of the drop of temperature or the amplitude of the increase in power output. The method 100 moves to S3, where the microcontroller 28 determines, based on the determined amount of hair engaged with the one or more heating zones across all of the one or more heating zones, a hair tress size which has been loaded by the user into the apparatus. The method 100 moves to S4, where the microcontroller 28 determines whether the determined hair tress size is greater than a threshold value. If it is determined that the determined hair tress size is greater than a threshold value, this means that too much hair has been loaded in the styler, and the method moves to S5. In S5, the microcontroller 28 causes the appliance to output a feedback message indicating that too much hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the hair styling device accordingly. Alternatively or additionally, in S5, the microcontroller 28 causes the styler to control the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt. Therefore, in an example, when it is determined that the determined hair tress size is greater than the threshold value and thus is bigger than a desired amount of hair, the microcontroller 28 causes the appliance to cap the increase in the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, so as to enable a better drying or styling of the hair which is loaded in the apparatus while avoiding burning of the hair. Alternatively or additionally, as shown in Figure 9A, the interface of the appliance comprises lighting means and the lighting means comprise one or more lamps 111 arranged along a length the apparatus, sideways of the heater 6a. In the example of Figure 9A, when it is determined that the determined hair tress size is greater than the threshold value and thus is bigger than a desired amount of hair, the controller controls the appliance to light up one or more lights 111 of the lighting means on a length corresponding to the extent of the one or more heating zones which are engaged with the hair, in a length of the heater, thus prompting the user to modify the amount of hair loaded in the styler. As shown in Figure 9A, the lighting means are configured to produce, in such a case, a first colour, such as red, to prompt the user to modify the amount of hair loaded in the appliance. Alternatively or additionally, as shown in Figure 9B, when it is determined that the hair tress size corresponds to a desired amount of hair, the lighting means are configured to produce, a second colour, different from the first colour, such as green, reassuring the user that the amount of hair corresponds to the desired amount of hair. Other examples of feedback messages are envisaged. When the interface comprises a sound generator, the controller is configured to control the apparatus to cause the sound generator to produce a first sound, when it is determined that the hair tress size corresponds to a desired amount of hair, and to produce a second sound different from the first sound, otherwise, thus prompting the user to decrease the amount of hair loaded in the apparatus. Alternatively or additionally, when the interface comprises a haptic feedback generator, the controller is configured to control the apparatus to cause the haptic feedback generator to produce a first haptic feedback, when it is determined that the hair tress size corresponds to a desired amount of hair, and to produce a second haptic feedback different from the first haptic feedback, otherwise thus prompting the user to decrease the amount of hair loaded in the apparatus. Alternatively or additionally, when the user is connected to an application running on e.g., their mobile phone, the communication circuitry of the appliance may output the feedback message to the user interface associated with the user on the mobile phone. In some examples the haptic feedback may be produced by a smart watch without needing to install a haptic feedback generator on the styler. In an optional step, in S4 the microcontroller 28 may set the threshold value discussed above to a first threshold value, and determine whether the determined hair tress size is smaller than a second threshold value, smaller than the first threshold value. If it is determined in S4 that the determined hair tress size is smaller than the second threshold value, the controller is configured to cause the apparatus to, in S5: output a feedback message indicating that too little hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the apparatus accordingly, and / or control the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt. Similarly to what has been already described, in such a case the feedback message is different from the feedback produced when it is determined that the hair tress size corresponds to a desired amount of hair. Additionally, in such a case the feedback message may be different or similar to the feedback produced when it is determined that too much hair has been loaded in the appliance, thus prompting the user to modify, here increase, the amount of hair loaded in the appliance. In some examples, controlling at S5 the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, comprises turning off the heater or reducing the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to prevent the hair being burnt. Grip Pressure The hair styler 1 includes a pressure sensor 32 to enable the microcontroller 28 to measure a pressure between the first movable arm 4a and the second movable arm 4b, e.g., when styling the hair. The determination of the hair tress size can be furtherverified by the controller 28, using the grip pressure sensor data, as larger hair tresses require more pressure to stay in place. Hair Type Knowledge of the hair style of the user may be used to determine the size of the hair tress more accurately, as thick curly hair has larger hair tress sizes than fine straight hair. The data indicative of hair type of the user may be provided directly by the user, possibly by utilising responses to a hair quiz on their mobile phone, or may be provided as the result of a determination by the appliance (e.g., using a set of deterministic rules or an artificial intelligence algorithm). Using the determined hair tress size, the settings applied to the appliance correspond to the determined hair tress size, such that the settings are tailored to the amount of hair of the user loaded in the appliance. The user has thus a better styling experience, a better styling result and the hair is less likely to get damaged when styling. Figure 10 shows an example method 200 of operating the appliance. At step SO the appliance is turned on. At S01, it is determined whether the appliance is linked to a remote application running e.g., on a mobile telephone, via the communications circuitry 27 (for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the mobile telephone). If the appliance is linked to the application, then at S02, the settings from the application are applied to the appliance (this may include the indication of the hair type of the user). If the appliance is not linked to an application or in any case after S02, the method 200 moves to the method 100 described above. The method 100 is run until it is determined at S10 that the appliance should be turned off, and in that case the appliance is turned off and the method 200 stops. 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 furtheralternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. The invention has been described above by way of implementation in a hair styling device for straightening hair (‘hair straighteners’) which employ flat hair styling heaters 6. However, it could alternatively be implemented in any form of hair styling device, such as (but not limited to) crimpers, curlers or heated brushes. The heaters 6 may define a heating surface that is flat, curved, ridged or in the shape of a barrel. The hair styling device may have two arms like the device illustrated in Figure 1 or it may be a single armed device. The heaters described above may also be used in hair dryers or in combination devices that use conductive heating and air to dry and style the user’s hair (such as those described in the applicant’s earlier PCT application WO 2021 / 019239). In embodiments where air is used, the heaters 6 may be perforated so that air passes through the heater and is warmed by the heater as the air passes through. 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. Apparatus for drying or styling hair, the apparatus comprising:a heater comprising a hair contacting surface for heating hair of a user that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface; anda controller configured to control the apparatus to:determine, based on a drop of a temperature of one or more heating zones of the plurality of independently controllable heating zones or based on an increase in a power output needed to maintain the one or more heating zones at a target temperature, whether hair is engaged with the one or more heating zones and, if it is, to determine an amount of hair which is engaged with the one or more heating zones;determine, based on the determined amount of hair engaged with the one or more heating zones across all of the one or more heating zones, a hair tress size which has been loaded by the user into the apparatus;determine whether the determined hair tress size is greater than a threshold value and, if it is, to cause the apparatus to:output a feedback message indicating that too much hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the hair styling device accordingly, and / orcontrol the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt.

2. The apparatus of claim 1, wherein the threshold value is a first threshold value, and wherein the controlleris further configured to determine whether the determined hair tress size is smaller than a second threshold value, smaller than the first threshold value and, if it is, to cause the apparatus to:output a feedback message indicating that too little hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the apparatus accordingly, and / orcontrol the temperature orthe power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt.

3. The apparatus of claim 1 or 2, comprising:a user interface for outputting a feedback message to the user;the controller being configured to cause the user interface to output the feedback message indicating whether too much hair has been loaded into the apparatus.

4. The apparatus of claim 3, wherein the user interface comprises at least one of a sound generator, lighting means and / or a haptic feedback generator.

5. The apparatus of claim 4, wherein the lighting means comprise one or more lamps arranged along a length the apparatus, sideways of the heater, the controller being configured to control the apparatus to light up one or more lights of the lighting means on a length corresponding to the extent of the one or more heating zones which are engaged with the hair, in a length of the heater.

6. The apparatus of claim 5, wherein the controller is configured to control the apparatus to cause the lighting means to produce a first colour, such as green, when it is determined that the hair tress size corresponds to a desired amount of hair, and a second colour different from the first colour, such as red, otherwise, thus prompting the user to modify the amount of hair loaded in the apparatus.

7. The apparatus of any of claims 4 to 6, wherein the controller is configured to control the apparatus to:cause the sound generator toproduce a first sound, when it is determined that the hair tress size corresponds to a desired amount of hair, and toproduce a second sound different from the first sound, otherwise, thus prompting the user to increase or decrease the amount of hair loaded in the apparatus, and / orcause the haptic feedback generator toproduce a first haptic feedback, when it is determined that the hair tress size corresponds to a desired amount of hair, and toproduce a second haptic feedback different from the first haptic feedback, otherwise thus prompting the userto increase or decrease the amount of hair loaded in the apparatus.

8. The apparatus according to any of claims 1 to 7, further comprising communication circuitry such that the apparatus is configured to output the feedback message to an external processing device and wherein the output feedback message is output on a user interface associated with the user.

9. The apparatus according to claim 8, wherein the external processing device is part of a mobile phone or a smart watch and is configured to run an application, remote from the apparatus.

10. The apparatus according to any of claims 1 to 9, wherein, if it is determined that the determined hair tress size is greater than the threshold value and thus is bigger than a desired amount of hair, the controller is configured to cap the increase in the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, so as to enable a better drying or styling of the hair which is loaded in the apparatus while avoiding burning of the hair.

11. The apparatus according to any of claims 2 to 10, wherein, if it is determined that the determined hair tress size is smaller than the second threshold value and thus is smaller that a desired amount of hair, the controller is configured to turn off the heater or to reduce the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to prevent the hair being burnt.

12. The apparatus according to any preceding claim, wherein the one or more heating zones are adjacent to each other along a length and / or across a width of the heater, and / or wherein the one or more heating zones are substantially thermally isolated from each other.

13. The apparatus according to any preceding claim, wherein the controller is configured to control the apparatus to:determine the resistance of a heater electrode of the one or more heating zones based on a voltage supplied to the heater electrode and the current passing through the heater electrode; anddetermine the temperature of the one or more heating zones based on the determined resistance, and / orwherein the controller is configured to control the apparatus to supply power to a heater electrode of the heaterto determine the resistance of the heater electrode even in a case where it is determined that the corresponding heating zone is not engaged with hair14. The apparatus according to any preceding claim, wherein the apparatus further comprises a pair of opposing heating zones, wherein heat can flow between the opposing heating zones when the apparatus is in use, and wherein the controller is configured to control the apparatus to:determine whether hair is engaged with at least one of the pair of opposing heating zones based on a sum or average of a power needed to maintain each of the pair of opposing heating zones at a target temperature.

15. The apparatus according to any preceding claim, wherein the controller is configured tocontrol power supplied to a heater electrode of the one or more heating zones to control the temperature of the one or more heating zones towards a first target temperature in a case where it is determined that the hair is engaged with the one or more heating zones, and to control the temperature of the one or more heating zones towards a second target temperature, lower than the first target temperature, in a case where it is determined that the hair is not engaged with the one or more heating zones; andintermittently supply power to the heater electrode to determine the resistance of the heater electrode even in the case where it is determined that the one or more heating zones is not engaged with the hair, and determine the temperature of the one or more heating zones based on the resistance.

16. The apparatus according to any preceding claim, further comprising a pressure sensor configured to sense and output data indicative of a pressure between a first movable arm and a second movable arm of the apparatus, to enable the controller to verify the determination of the hair tress size, as larger hair tresses require more pressure to stay in place in the apparatus.

17. The apparatus according to any preceding claim, wherein the controller is configured to receive data indicative of hair type of the user, to enable the controller to refine the determination of the hair tress size, as thick curly hair has larger hair tress sizes than fine straight hair.

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

19. A method performed by apparatus for drying or styling hair, the apparatus comprising a heater having a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, and the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein the method comprises:decreasing a temperature or increasing a power output of one or more heating zones of the plurality of independently controllable heating zones;determining, based on the drop of a temperature of one or more heating zones of the plurality of independently controllable heating zones or based on the increase in a power output needed to maintain the one or more heating zones at a target temperature, whether hair is engaged with the one or more heating zones and, if it is, to determine an amount of hair which is engaged with the one or more heating zones;determining whether the determined hair tress size is greater than a threshold value and, if it is, the method further comprising:outputting a feedback message indicating that too much hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the hair styling device accordingly, and / orcontrolling the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt.

20. The method according to claim 19, wherein the threshold value is a first threshold value, and wherein the method further comprises determining whether the determined hair tress size is smaller than a second threshold value, smaller than the first threshold value and, if it is, the method further comprising:outputting afeedback message indicating that too little hair has been loaded into the apparatus, so as to prompt the user to modify the amount of hair loaded into the apparatus accordingly, and / orcontrolling the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to avoid the hair loaded into the apparatus to be burnt.

21. The method according to claim 19 or 20, wherein the apparatus comprises a user interface for outputting a feedback message to the user, the user interface comprising at least one of a sound generator, lighting means and / or a haptic feedback generator, the method further comprising:producing a first feedback, when it is determined that the hair tress size corresponds to a desired amount of hair, andproducing a second feedback different from the first feedback, otherwise, thus prompting the user to modify the amount of hair loaded in the apparatus.

22. The method according to any of claims 19 to 21, wherein, if it is determined that the determined hair tress size is greater than the threshold value and thus is bigger than a desired amount of hair, the method further comprising:capping the increase in the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, so as to enable a better drying or styling of the hair which is loaded in the apparatus while avoiding burning of the hair.

23. The method according to any of claims 20 to 22, wherein, if it is determined that the determined hair tress size is smaller than the second threshold value and thus is smaller that a desired amount of hair, the method further comprising:turning off the heater or reducing the temperature or the power output of the one or more heating zones of the plurality of controllable heating zones which are engaged with the hair, to prevent the hair being burnt.

24. A computer program product comprising computer implementable instructions for causing a programmable device to carry out the method of any of claims 19 to 23.31

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