Hair styling apparatus

Abstract

A hair styling apparatus 102 comprising a hair contact surface 105 and a heater 107 for heating the hair contact surface. A controller 120 is adapted to monitor (210, figure 2) an electrical character

Description

BACKGROUND Various haircare devices such as hairbrushes and hair stylers can be used in combination with the application of heat, in order to style and / or dry hair. SUMMARY According to a first aspect, there is provided a hair styling apparatus comprising: a hair contact surface for contacting a user’s hair; a heater for heating the hair contact surface so as to heat the user’s hair when the hair is in contact with the hair contact surface; and a controller adapted to: monitor an electrical characteristic of the heater; and determine an amount of hair in contact with the hair contact surface based on a profile of the electrical characteristic of the heater. At its most general, the invention provides a means of determining how much hair is present at a surface based on the behaviour of a heater for heating said surface. The invention provides a means of determining, or inferring, the amount of hair in contact with a hair contact surface of a hair styling apparatus based on the electrical characteristic of a heater for heating the hair contact surface. In other words, there is provided a means of using the electrical characteristics of the heater of a hair styling apparatus to determine how much hair has been provided to, or at, the hair styling apparatus for styling. Tresses of hair comprising different amounts of hair will exhibit different thermal characteristics, for example in terms of the rate and amount of heat absorption by the hair. The amount of hair may refer to: a length of the tress of hair; a thickness of the tress of hair, the thickness being the dimension substantially perpendicular to the hair contact surface; and / or a width of the tress of hair, the width being the dimension substantially parallel to the hair contact surface. The electrical characteristics of a heater attempting to reach a given temperature will therefore vary depending on the amount of hair being heated. In particular, the profile of the electrical characteristic, i.e., the electrical characteristic over time and the rate of change of the electrical characteristic, will vary depending on the amount of hair being heated, i.e., the amount of hair in contact with the hair contact surface. The heater heats the hair contacting surface in order to heat the hair in contact with the hair contacting surface. For example, where the electrical characteristic is a power draw of the heater and for a heater at a starting temperature, which may be ambient temperature, the initial power draw will be relatively high and will decay over time as the temperature of the heater rises, as the resistance of the heater increases and the current flow through the heater decreases assuming a constant voltage is being applied. In the case where a large amount of hair is provided on the hair contact surface, the hair may have a large capacity for absorbing thermal energy from the hair contact surface, and so from the heater, meaning that the heater will lose a greater amount of heat to the hair and take longer to reach the target temperature. Accordingly, in the example above, the decay of the power draw over time may be slower. In contrast, where a small amount of hair is provided on the hair contact surface, the hair may have a smaller capacity for absorbing thermal energy from the hair contact surface, and so from the heater, meaning that the heater will lose a smaller amount of heat to the hair and reach the target temperature quicker. Accordingly, in the example above, the decay of the power draw over time may be quicker. The profile of the electrical characteristic of the heater may include one or more of: an amplitude of the electrical characteristic; a maximum amplitude of the electrical characteristic; a rate of change of the electrical characteristic over time; and a total amount of power provided to the heater over time, i.e., an energy provided to the heater. The controller may be a controller provided locally on the hair styling apparatus. Alternatively, the controller may comprise multiple distributed controllers with at least one distributed controller being provided remotely to the hair styling apparatus. Determining the amount of hair in contact with the hair contact surface based on the electrical characteristic of the heater may facilitate the apparatus to alter an operation of the apparatus, or to alter the user, in order to reduce style variation, provide feedback to the user (for example, to style for longer or shorter times, or provide a time to style), avoid hair damage and the like. The hair styling apparatus may be: a hair curling apparatus comprising a barrel extending along a longitudinal axis of the hair curling apparatus, wherein the hair contact surface is provided on an outer surface of the barrel; or a hair straightening apparatus comprising a pair of plates having a cavity therebetween for receiving the hair, wherein the hair contact surface is provided on one or both of the plates. In an example, determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater may comprise: determining a rate of change of the electrical characteristic of the heater; and determining an amount of hair in contact with the hair contact surface based on the rate of change of the electrical characteristic of the heater. In this way, the rate of change in the electrical characteristic of the heater, or the speed of decay of the electrical characteristic of the heater, may be used to determine, infer, recognise or categorise the amount of hair in contact with the hair contact surface. In the example where the electrical characteristic is the power draw of the heater, on a graph of power draw of the heater over time, the rate of change of power draw of the heater may be represented by the gradient of the plot. Accordingly, the amount of hair in contact with the hair contact surface may be determined from the gradient of the power draw plot over time. The gradient may be determined between two points in time defined by, for example, a monitoring period, for example a rate monitoring period. In an example, the rate of change of the electrical characteristic of the heater may be indicative of the thickness of the tress of hair in contact with the hair contact surface. In other words, the distance that the tress of hair extends in a direction perpendicular to the hair contact surface may be determined, or inferred, from the rate of change of the electrical characteristic of the heater. For example, the rate of change of the electrical characteristic of the heater may be relatively high when a relatively thin tress of hair is in contact with the hair contact surface. In contrast, the rate of change of the electrical characteristic of the heater may be relatively low when a relatively thick tress of hair is in contact with the hair contact surface. In an example, determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater may comprise: determining an amount of energy delivered to the heater; and determining an amount of hair in contact with the hair contact surface based on the amount of energy delivered to the heater. In this way, the energy delivered to the heater, and correspondingly the thermal energy delivered to the hair, may be used to determine, infer, recognise or categorise the amount of hair in contact with the hair contact surface. In the example where the electrical characteristic is the power draw of the heater, on a graph of power draw of the heater over time, the energy delivered to the heater may be presented by the area under the plot. Accordingly, the amount of hair in contact with the hair contact surface may be determined from the area under the power draw plot over time, i.e., the integral of the power draw plot over time. The energy delivered to the heater may be determined between two points in time defined by, for example, a monitoring period, for example an area monitoring period. In an embodiment, the rate monitoring period and the area monitoring period may be different. In an embodiment, the rate monitoring period and the area monitoring period may be the same. In an example, the energy delivered to the heater may be indicative of the cross-sectional area (e.g. approximated by the width of the tress and the thickness of the tress) of the tress of hair in contact with the hair contact surface. In other words, the cross-sectional area (e.g. combined width and thickness) of the tress of hair in contact with the hair contact surface may be determined, or inferred, from the rate of change of the electrical characteristic of the heater. For example, the amount of energy delivered to the heater may be relatively high when a tress of hair with a relatively large cross-sectional area is in contact with the hair contact surface. In contrast, the amount of energy delivered to the heater may be relatively low when a tress of hair with a relatively small cross sectional area is in contact with the hair contact surface. The amount of energy delivered to two tresses with the same cross sectional area may be the same. For example, a thick, narrow tress of hair may receive the same amount of energy as a thin, wide tress, if the cross-sectional area of the tresses is the same. However, a rate of change of electrical characteristic of the heater may be different between the two example tresses. In an example, determining an amount of energy delivered to the heater based on the profile of the electrical characteristic of the heater may comprise: monitoring the electrical characteristic of the heater over a monitoring period; and calculating the amount of energy delivered to the heater based on the electrical characteristic of the heater over the monitoring period. The monitoring period may be a period of any suitable length. For example, the average time required for hair to be in contact with a hair contact surface in order to impart a style to the hair may be 10 seconds. Accordingly, the monitoring period may be less than 10 seconds, for example less than or equal to 5 seconds, for example one second. The monitoring period may be the rate monitoring period and / or the area monitoring period. In an example, determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater may comprise: determining a maximum amplitude of the electrical characteristic of the heater; and determining an amount of hair in contact with the hair contact surface based on the maximum amplitude of the electrical characteristic of the heater. In this way, the amplitude of the electrical characteristic to the heater, and correspondingly the thermal energy delivered to the hair, may be used to determine, infer, recognise or categorise the amount of hair in contact with the hair contact surface. In the example where the electrical characteristic is the power draw of the heater, on a graph of power draw of the heater over time, the energy delivered to the heater may be presented by the area under the plot. Accordingly, the amount of hair in contact with the hair contact surface may be determined from the area under the power draw plot over time, i.e., the integral of the power draw plot over time. The energy delivered to the heater may be determined between two points in time defined by, for example, a monitoring period. In an example, the maximum amplitude of the electrical characteristic of the heater may be indicative of the width of the tress of hair in contact with the hair contact surface. In other words, the distance that the tress of hair extends in a direction parallel to the hair contact surface, or the contact area of the tress of hair with the hair contact surface, may be determined, or inferred, from the maximum amplitude of the electrical characteristic of the heater. For example, the maximum amplitude of the electrical characteristic of the heater may be relatively high when a relatively wide tress of hair is in contact with the hair contact surface. In contrast, the maximum amplitude of the electrical characteristic of the heater may be relatively low when a relatively narrow tress of hair is in contact with the hair contact surface. In an example, the hair styling apparatus may comprise a plurality of heaters, the plurality of heaters including the heater. Each heater of the plurality of heaters may be associated with a respective styling zone of the hair contact surface. The controller may be adapted to: monitor an electrical characteristic of each of the plurality of heaters; and determine an amount of hair in contact with each styling zone of the hair contact surface based on a profile of the electrical characteristic of the respective heater of the plurality of heaters. The amount of hair present in a tress of hair may not necessarily be consistent along the length of the tress, or across the width of the tress. For example, across the width of a tress certain portions of the tress may be thicker or thinner than other portions of the tress. Accordingly, the electrical characteristic of the heater may vary across the length of the heater and, by providing a plurality of heaters distributed across, or along, the hair styling apparatus (for example, beneath the hair contact surface), the variation in the electrical characteristic resulting from the variation in the tress of hair may be more accurately monitored. In this way, the accuracy of the determination of the amount of hair in contact with the hair contact surface may be improved. In an example, the hair styling apparatus may comprise a temperature sensor for detecting a temperature of the heater. The controller may be adapted to: monitor a temperature of the heater detected by the temperature sensor; and determine an amount of hair in contact with the hair contact surface based on the temperature of the heater and the profile of the electrical characteristic of the heater. As outlined above, the electrical characteristic of the heater may be indicative of the amount of hair in contact with the hair contact surface of the hair styling apparatus. However, the electrical characteristic of the heater and the profile of the electrical characteristic of the heater will change depending on the temperature of the heater. Therefore, by taking into account the temperature of the heater as well as the electrical characteristic of the heater, the accuracy of the determination of the amount of hair in contact with the hair contact surface may be improved. For example, a heater with a lower starting temperature may exhibit a higher initial electrical characteristic, which can affect aspects of the profile of the electrical characteristic of the heater, for example by increasing the maximum amplitude of the electrical characteristic of the heater and / or causing a greater change in the electrical characteristic of the heater to occur. In a further example, a heater with a higher starting temperature may exhibit a lower initial electrical characteristic, which can affect aspects of the profile of the electrical characteristic of the heater, for example by decreasing the maximum amplitude of the electrical characteristic of the heater and / or causing a smaller change in the electrical characteristic of the heater to occur. In an example, determining the amount of hair in contact with the hair contact surface based on the temperature of the heater and the profile of the electrical characteristic of the heater comprises: determining an initial temperature of the heater; monitoring the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to a target temperature; and determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to the target temperature. The initial temperature of the heater may be an ambient temperature, for example where the hair styling apparatus has not been used for a sufficient time such that the heater has cooled to the ambient, or environmental, temperature. The initial temperature of the heater may be higher than an ambient temperature, for example where the hair styling apparatus has been used recently and sufficient time has not elapsed for the heater has to cool to the ambient, or environmental, temperature. When hair, and in particular damp hair, is brought into contact with the hair contact surface, the hair will absorb heat from the hair contact surface and the temperature of the hair contact surface, and correspondingly the temperature of the heater, will approach the temperature of the hair, i.e., cool. Accordingly, the initial temperature of the heater may be determined to be the temperature of the heater after the temperature has equalised following the contact of the hair with the hair contact surface. For example, if the temperature of the heater is high and then the damp hair is brought into contact with the hair contact surface, the temperature of the heater will dip, forming a local minimum in the temperature profile of the heater, before the temperature of the heater begins to rise once more. Accordingly, determining the initial temperature of the heater may comprise identifying a local minimum temperature of the heater as the initial temperature of the heater. In an example, determining the initial temperature of the heater may comprise identifying a local minimum temperature of the heater as the initial temperature of the heater. In an example, the hair styling apparatus may comprise a plurality of temperature sensors, the plurality of temperature sensor including the temperature sensor. Each temperature sensor of the plurality of temperature sensors may be associated with a respective heater of the plurality heaters. The controller may be adapted to: monitor a temperature of each of the plurality of the heaters detected by a respective temperature sensor of the plurality of temperature sensors; and determine an amount of hair in contact with each hair styling zone of the hair contact surface based on the temperature of the respective heater of the plurality of heaters and the profile of the electrical characteristic of the respective heater of the plurality of heaters. The amount of hair present in a tress of hair is not necessarily consistent along the length of the tress, or across the width of the tress. For example, across the width of a tress certain portions of the tress may be thicker or thinner than other portions of the tress. Accordingly, the effects of the hair on the temperature of the heater and correspondingly on the electrical characteristic of the heater may vary across the length of the heater and, by providing a plurality of heaters and temperature sensors distributed across, or along, the hair styling apparatus (for example, beneath the hair contact surface), the variation in the temperature and electrical characteristic resulting from the variation in the tress of hair may be more accurately monitored. In this way, the accuracy of the determination of the amount of hair in contact with the hair contact surface may be improved. The hair styling apparatus further may comprise a sensor arrangement adapted to generate one or more signals, each signal being indicative of a presence of hair at the hair contact surface. The controller may be adapted to: determine a proportion of the hair contact surface in contact with the hair and a proportion of the hair contact surface exposed to air based on the one or more signals; determine a thermal energy loss based on the proportion of the hair contact surface exposed to air; and determine an amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater and the thermal energy loss. In addition to the amount of hair in contact with the hair contact surface, the proportion of the hair contact surface covered by the hair, and the proportion of the hair contact surface exposed to the air, will also affect the electrical characteristic and the electrical characteristic profile of the heater. Therefore, by accounting for the proportion of the hair contact surface exposed to air and the thermal energy lost to the air, the accuracy of the determination of the amount of hair in contact with the hair contact surface may be improved. The thermal characteristics of the hair contact surface may be known, meaning that the thermal energy loss can be calculated by the controller. In combination with a thermal sensor as described above, the accuracy of the calculated thermal energy loss may be improved. In some examples, the sensor arrangement may comprise a non-contact sensor. The noncontact sensor may comprise a self-capacitance sensor and / or a mutual capacitance sensor. A capacitive sensor (sometimes known as a capacitance sensor) may comprise at last one electrode (e.g., a plate or wire) of conductive material positioned with or upon a nonconductive medium (e.g., dielectric such as: air, plastic, etc.). The electrode is configured to be capable of being electrically charged, and the time taken to fully charge the electrode in response to a pre-set applied voltage, may be returned as the sensor response value. Two types of capacitive sensors exist, and each type exploits a different respective capacitance mechanism. A first type of capacitive sensor is known as a self-capacitive sensor (sometimes known as a self-capacitance sensor), and a second type of capacitive sensor is known as a mutual-capacitive sensor (sometimes known as a mutual-capacitance sensor). A self-capacitive sensor measures a change in capacitance with respect to earth ground. Here, the electrode forms one electrode of a notional capacitor, with the other electrode being provided either by earth ground or by the user’s quantity of hair when in proximity to (or in contact with) the electrode. Proximity to (or contact with) the electrode causes the electrode capacitance to increase, as the quantity of hair “adds” capacitance to that of the system. The greater the quantity of water (i.e., moisture content) in the hair in question then the greater is the capacitance “added” to that of the system. Self-capacitive measurement may employ a single electrode and may measure a capacitance, and / or a change in capacitance with respect to ground caused by the user’s quantity of hair when in proximity to (or contact with) the electrode. A self-capacitive sensor’s electrode, in use, projects salient electric field lines, some of which change direction so as to terminate on charges present within the user’s quantity of hair (as opposed to elsewhere beyond the sensor) when that hair is brought into proximity with (or contact with) the electrode. The hair may result in a higher capacitance as compared to the baseline measured value. A mutual-capacitive sensor employs two electrodes that together represent a two-electrode capacitor. The sensor measures a change in capacitance of the capacitor. The user’s quantity of hair when in proximity to (or contact with) the two electrodes, changes the electric field between the two electrodes and reduces the capacitive coupling between the electrodes. Electrodes of a mutual-capacitive sensor project salient electric field lines from one of the two electrodes to the other electrode. Once more, some electric field lines may change direction so as to terminate not on an electrode of the sensor but, instead, on charges present within the user’s quantity of hair when that hair is brought into proximity with (or contact with) either or both electrodes. Put in other words, the user’s quantity of hair when in proximity to (or contact with) the electrode(s), in effect “steals” electric field lines from the two-electrode capacitor system. This may result in a lower capacitance as compared to a baseline measured value of the capacitance of the system. In some examples, the sensor arrangement may comprise a plurality of sensors. Each sensor of the plurality of sensors may be associated with a respective heater of the plurality of heaters. Each sensor of the plurality of sensors may be adapted to generate a signal indicative of a presence of hair at a respective hair styling zone of the hair contact surface. Once again, the provision of multiple sensors or monitoring locations on the hair contact surface improves the accuracy of the determination of the amount of hair in contact with the hair contact surface. In some examples, determining the amount of hair in contact with the hair contact surface comprises classifying the amount of hair in contact with the hair contact surface as one of a plurality of categories using a machine learning algorithm, wherein the machine learning algorithm takes the electrical characteristic profile of the heater as an input. In some examples, the electrical characteristic comprises one or more of: a power draw of the heater; a current draw of the heater; a voltage of the heater; and a resistance of the heater. The power draw of a heater may be defined as the product of the current supplied to the heater and the voltage applied to the heater. For a constant voltage, a change in the power draw of the heater may represent a change in the current supplied to the heater. Similarly, for a constant current, a change in the power draw of the heater may represent a change in the voltage applied to the heater. According to a second aspect, there is provided a computer-implemented method for determining an amount of hair in contact with a hair contact surface of a hair styling apparatus, the hair styling apparatus comprising a heater for heating the hair contact surface, the method comprising: monitoring an electrical characteristic of the heater; and determining an amount of hair in contact with the hair contact surface based on a profile of the electrical characteristic of the heater. In some examples, the method may further comprise: determining a rate of change of electrical characteristic of the heater; and determining an amount of hair in contact with the hair contact surface based on the rate of change of electrical characteristic of the heater. In some examples, the method may further comprise: determining an amount of energy delivered to the heater; and determining an amount of hair in contact with the hair contact surface based on the amount of energy delivered to the heater. In some examples, the method may further comprise: monitoring the electrical characteristic of the heater over a monitoring period; and calculating the amount of energy delivered to the heater based on the electrical characteristic of the heater over the monitoring period. In some examples, the method may further comprise: determining a maximum amplitude of the electrical characteristic of the heater; and determining an amount of hair in contact with the hair contact surface based on the maximum amplitude of the electrical characteristic of the heater. In some examples, the hair styling apparatus may further comprise a temperature sensor for detecting a temperature of the heater, and wherein the method may further comprise: monitoring a temperature of the heater detected by the temperature sensor; and determining an amount of hair in contact with the hair contact surface based on the temperature of the heater and the profile of the electrical characteristic of the heater. In some examples, the method may further comprise: determining an initial temperature of the heater; monitoring the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to a target temperature; and determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to the target temperature. In some examples, the hair styling apparatus may further comprise a sensor arrangement adapted to generate one or more signals, each signal being indicative of a presence of hair at the hair contact surface. The method may further comprise: determining a proportion of the hair contact surface in contact with the hair and a proportion of the hair contact surface exposed to air; determining a thermal energy loss based on the proportion of the hair contact surface exposed to air based on the one or more signals; and determining an amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater and the thermal energy loss. In some examples, determining the amount of hair in contact with the hair contact surface may comprise classifying the amount of hair in contact with the hair contact surface as one of a plurality of categories using a machine learning algorithm, wherein the machine learning algorithm takes the electrical characteristic profile of the heater as an input. The categories may, for example, include one or more of: high, medium or low length tress; high, medium or low width tress; and high, medium or low thickness tress. The number of categories may be increased for greater classification granularity and accuracy, or may be decreased for greater classification speed. The apparatus may output a control signal in response to the classification for generating an indication to be provided to the user of how long the hair should be kept in contact with the hair contact surface to achieve the desired style. The indication may be provided on the hair styling apparatus, for example by way of visual, audible or haptic means. Alternatively, or in addition, the signal may be used to generate an indication on a device which is remote from the hair styling apparatus but in communication (e.g. wireless communication) with the hair styling apparatus. The remote device may be a smartphone or smartwatch. According to a third aspect, there is provided a computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the methods described above. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic representation of a hair styling apparatus. Figure 2 shows a method for controlling the hair styling apparatus of Figure 1. Figures 3A to 3C show schematic representations of different amounts of hair in contact with a hair contact surface of a hair styling apparatus and corresponding graphs of power draw of the heater over time. Figures 4A to 4C show schematic representations of different hair windings around a hair contact surface of a hair styling apparatus and corresponding graphs of sensor coverage against time. DETAILED DESCRIPTION Figure 1 shows a schematic representation of a hair styling apparatus 102. The haircare appliance 102 comprises a body 104 having an axis A around which hair is wrapped in use. In the example shown in Figure 1, the body 104 comprises a hair contact surface 105 for contacting a user’s hair and a heater 107 for heating the hair contact surface 105 so as to heat the user’s hair when the hair is in contact with the hair contact surface 105. The haircare appliance 102 is for curling hair. The haircare appliance 102 comprises a main part 106 and the body 104 is provided as an attachment 104 that is attachable and / or detachable from the main part 106. The main part 106 provides a handle for the haircare appliance 102. The body 104 is generally in the form of a cylinder. The body 104 is elongate along the axis A. When hair is wrapped around the body 104, around the axis A, the heat provided to the hair contact surface 105 by the heater 107 may act to set the hair into a curl. Accordingly, a user may use the haircare appliance 102 to style hair. The haircare appliance 102 comprises a controller 120. As described in more detail below, the controller 120 is configured to monitor an electrical characteristic of the heater 105 and determine an amount of hair in contact with the hair contact surface 105 based on a profile of the electrical characteristic of the heater 107. In the example shown in Figure 1, the haircare appliance 102 comprises an input interface 126 configured to receive a user input. For example, the input interface 126 may allow a user to input a particular number of curls or style that they intend to curl the hair in. In examples, the input interface 126 may comprise one or more buttons, and / or a touch screen. Other input interfaces are possible. The haircare appliance 102 comprises a feedback device 124 configured to provide feedback to a user of the haircare appliance 102 in use. In this example, the feedback device 124 comprises a display configured to display the feedback to a user. For example, the display may comprise a LED screen or the like. In examples, the feedback device 124 may alternatively, or additionally comprise a speaker (not shown) configured to generate sounds to convey the feedback to the user, and / or a haptic device (not shown) configured to generate haptic feedback for the user. Using the feedback device 124, the haircare appliance 102 may provide feedback to the user during use of the haircare appliance 102, so as to guide the user’s operation of the haircare appliance 102. In some examples, the input interface 126 and the feedback device 124 may be provided by the same device, such as a touch screen display. Figure 2 shows a method 200 for controlling the hair styling apparatus 102 of Figure 1. In the example shown in Figure 2, the method 200 comprises two fundamental steps, which are monitoring 210 the electrical characteristic of the heater 107 and determining 220 the amount of hair in contact with the hair contact surface 105 based on the monitored profile of the electrical characteristic of the heater 105. The step 210 of monitoring the electrical characteristic of the heater 107 may comprise a number of optional sub-steps. Each of the example sub-steps described with reference to Figure 2 are illustrated in detail further below with reference to Figure 3. In the example shown in Figure 2, the step 210 of monitoring the electrical characteristic of the heater 107 comprises determining 230 a rate of change of the electrical characteristic of the heater 107. The step of determining 220 the amount of hair in contact with the hair contact surface 105 may therefore be based on the rate of change of the electrical characteristic of the heater 107. In the example shown in Figure 2, the step 210 of monitoring the electrical characteristic of the heater 107 further comprises determining 240 the maximum amplitude of the electrical characteristic of the heater 107. The step of determining the amount of hair in contact with the hair contact surface 105 may therefore also be based on the maximum amplitude of the electrical characteristic of the heater 107. In the example shown in Figure 2, the step 210 of monitoring the electrical characteristic of the heater 107 further comprises determining 250 the amount of energy delivered to the heater 107. The step of determining the amount of hair in contact with the hair contact surface 105 may therefore also be based on the amount of energy delivered to the heater 107. Determining 250 the amount of energy delivered to the heater 107 may comprise the steps of monitoring 260 the electrical characteristic of the heater over a monitoring period and calculating 270 the amount of energy delivered to the heater based on the electrical characteristic of the heater over the monitoring period. Figures 3A to 3C show a schematic representation of different amounts of hair in contact with a hair contact surface 105 of a hair styling apparatus 102, the hair styling surface being heated by a heater 107. In addition, Figures 3A to 3C each show a corresponding graph of power draw, in Watts, of the heater 107 over time after the hair is brought into contact with the hair contact surface 105. In the examples shown in Figures 3A to 3C, the electrical characteristic of the heater 107 being monitored and used to determine the amount of hair in contact with the hair contact surface 105 is the power draw of the heater. The electrical characteristics of the heater 107 attempting to reach a given temperature will vary depending on the amount of hair being heated. In particular, the profile of the power draw over time and the rate of change of the power draw will vary depending on the amount of hair being heated, i.e., the amount of hair in contact with the hair contact surface 105. In each of the examples shown in Figures 3A to 3C, the initial power draw of the heater 107 is relatively high and decays over time as the temperature of the heater 107 rises causing the resistance of the heater 107 to increase and the current flow through the heater 107 decrease for a constant voltage. In general terms, when a large amount of hair is provided on the hair contact surface 105, the hair has a large capacity for absorbing thermal energy from the hair contact surface 105, and so from the heater 107. Therefore, the heater 107 will lose a greater amount of heat to the hair initially and take longer to reach the target temperature. Further, when a small amount of hair is provided on the hair contact surface 105, the hair has a smaller capacity for absorbing thermal energy from the hair contact surface 105, and so from the heater 107. Therefore, the heater 107 will lose a smaller amount of heat to the hair and reach the target temperature quicker. In the example shown in Figure 3 A, the tress of hair 310 in contact with the hair contact surface 105 is narrow in the direction parallel to the hair contact surface 105 and thick in the direction perpendicular to the hair contact surface 105. The tress of hair 310 shown in Figure 3 A may be referred to as a narrow, thick tress of hair. Figure 3 A shows a graph 311 of power draw over time, showing a plot 312 illustrating the power draw of the heater 107 after the tress of hair 310 has been brought into contact with the hair contact surface 105. One or more parameters of the plot 312 may form part of the profile of the electrical characteristic for determining the amount of hair in contact with the hair contact surface 105. The gradient of the plot 312 represents the rate of change of the power draw of the heater 107. The amount of hair in contact with the hair contact surface 105 may be determined from the gradient of the plot 312. The gradient may be determined over a rate monitoring period between two points in time, such as between t=0 and a first point in time 313. In particular, the rate of change of the power draw of the heater 107, and so the gradient of the plot 312, is indicative of the thickness of the tress of hair 310 (i.e., the distance that the tress of hair 310 extends in a direction perpendicular to the hair contact surface 105) in contact with the hair contact surface 105. As illustrated in the example shown in Figure 3 A, the rate of change of the power draw of the heater 107, i.e., the gradient of the plot 312, is relatively low when a thick tress of hair 310 is in contact with the hair contact surface 105. Accordingly, in the example shown in Figure 3 A, the controller 120 may infer that a thick tress of hair 310 is in contact with the hair contact surface 105 as the rate of the rate of change of the power draw of the heater 107, i.e., the gradient of the plot 312, is relatively low. The maximum amplitude 314 of the power draw of the heater 107 may also be used to infer the amount of hair in contact with the hair contact surface 105 in addition, or as an alternative, to the rate of change of the power draw of the heater 107. In particular, the maximum amplitude 314 of the power draw of the heater 107 is indicative of the width of the tress of hair 310 (i.e., the distance that the tress of hair 310 extends in a direction parallel to the hair contact surface 105, or the contact area of the tress of hair 310 with the hair contact surface 105) in contact with the hair contact surface 105. As illustrated in the example shown in Figure 3 A, the maximum amplitude 314 of the power draw of the heater 107 is relatively low when a relatively narrow tress of hair 310 is in contact with the hair contact surface 105. Accordingly, in the example shown in Figure 3 A, the controller 120 may infer that a thick tress of hair 310 is in contact with the hair contact surface 105 as the rate of the rate of change of the power draw of the heater 107, i.e., the gradient of the plot 312, is relatively low. The area 315 under the plot 312 represents the energy delivered to the heater 107. The amount of hair in contact with the hair contact surface 105 may be determined from the area 315 under the plot 312, i.e., the integral of the power draw plot over time. The energy delivered to the heater, i.e., the area 315, may be determined over an area monitoring period, such as the first point in time 313 and a second point in time 316. In some examples, the area monitoring period and the rate monitoring period are the same period of time. In particular, the energy delivered to the heater 107, i.e., the area 315, is indicative of the cross-sectional area (e.g., approximated by the width of the tress of hair 310 and the thickness of the tress of hair 310) of the tress of hair 310 in contact with the hair contact surface 105. As illustrated in the example shown in Figure 3A, the amount of energy delivered to the heater 107, i.e., the area 315, is relatively low when a tress of hair 310 with a relatively small cross sectional area is in contact with the hair contact surface 105. Accordingly, in the example shown in Figure 3 A, the controller 120 may infer that a tress of hair 310 with a relatively low cross sectional area, such as a narrow, thick tress of hair 310, is in contact with the hair contact surface 105 as the amount of energy delivered to the heater 107, i.e., the area 315, is relatively low. In the example shown in Figure 3B, the tress of hair 320 in contact with the hair contact surface 105 is wide in the direction parallel to the hair contact surface 105 and thick in the direction perpendicular to the hair contact surface 105. The tress of hair 320 shown in Figure 3B may be referred to as a wide, thick tress of hair. Figure 3B also shows a graph 321 of power draw over time, showing a plot 322 illustrating the power draw of the heater 107 after the tress of hair 320 has been brought into contact with the hair contact surface 105. As described above, the gradient of the plot 322 is indicative of the thickness of the tress of hair 320 in contact with the hair contact surface 105. As illustrated in the example shown in Figure 3B, the gradient of the plot 322 is relatively low due to the thickness of the tress of hair 320 in contact with the hair contact surface 105. Comparing the gradient of the plot 322 in Figure 3B and the gradient of the plot 312 in Figure 3A, it can be seen that the gradients of the two plots are similar. This similarity in gradient is due to the similar thicknesses of the tress of hair 310 in Figure 3 A and the tress of hair 320 in Figure 3B. Accordingly, in order to differentiate between the thick, narrow tress of hair 310 and the thick, wide tress of hair 320, an additional parameter of the plots 312, 322 may be investigated. In the example shown in Figure 3B, the maximum amplitude 324 of the power draw of the heater 107 is relatively high when a relatively wide tress of hair is in contact with the hair contact surface 105. In addition, the amount of energy delivered to the heater 107, i.e., the area 325, is relatively high when a tress of hair with a relatively large cross-sectional area is in contact with the hair contact surface 105. Comparing the maximum amplitude 324 of the power draw of the heater 107 in Figure 3B to the maximum amplitude 314 of the power draw of the heater 107 in Figure 3 A, it can be seen that the maximum amplitude 324 of the power draw of the heater 107 in Figure 3B is higher. Accordingly, the controller 120 may infer that the tress of hair 320 in contact with the hair contact surface 105 in Figure 3B is wider than the tress of hair 310 in contact with the hair contact surface 105 in Figure 3 A. Comparing the area 325 in Figure 3B to the area 315 in Figure 3 A, it can be seen that the area 325 in Figure 3B is larger. Accordingly, the controller 120 may infer that the tress of hair 320 in contact with the hair contact surface 105 in Figure 3B is has a larger cross sectional area than the tress of hair 310 in contact with the hair contact surface 105 in Figure 3 A. As both tresses of hair 310, 320 are known to be relatively thick, the controller may interpret the larger cross-sectional area of the tress of hair 320 in Figure 3B to mean that the tress of hair 320 in contact with the hair contact surface 105 in Figure 3B is wider than the tress of hair 310 in contact with the hair contact surface 105 in Figure 3 A. In the example shown in Figure 3C, the tress of hair 330 in contact with the hair contact surface 105 is wide in the direction parallel to the hair contact surface 105 and thin in the direction perpendicular to the hair contact surface 105. The tress of hair 330 shown in Figure 3C may be referred to as a wide, thin tress of hair. Figure 3C also shows a graph 331 of power draw over time, showing a plot 332 illustrating the power draw of the heater 107 after the tress of hair 330 has been brought into contact with the hair contact surface 105. As described above, the area 335 under the plot 332 represents the energy delivered to the heater 107 and the amount of hair in contact with the hair contact surface 105 may be determined from the area 335 under the plot 332, i.e., the integral of the power draw plot over time. In particular, the energy delivered to the heater 107, i.e., the area 335, is indicative of the cross-sectional area (e.g., approximated by the width of the tress of hair 330 and the thickness of the tress of hair 330) of the tress of hair 330 in contact with the hair contact surface 105. Comparing the area 335 in Figure 3C and the area 325 in Figure 3B, it can be seen that the amount of energy delivered to the tresses of hair 310, 330 is similar over the same time periods. The amount of energy delivered to two tresses with the same cross sectional area will be the same, or similar. For example, a thick, narrow tress of hair 310 as shown in Figure 3A may receive the same amount of energy as a thin, wide tress of hair 330 as shown in Figure 3C, if the cross-sectional areas of the tresses 310, 330 are the same. Accordingly, in order to differentiate between the thick, narrow tress of hair 310 and the thin, wide tress of hair 330, an additional parameter of the plots 312, 322 may be investigated. Comparing the maximum amplitude 334 of the power draw of the heater 107 in Figure 3C to the maximum amplitudes 314, 324 of the power draw of the heater 107 in Figures 3 A and 3B, it can be seen that the maximum amplitude 334 of the power draw of the heater 107 in Figure 3C is higher than the maximum amplitude 314 of the power draw of the heater 107 in Figure 3 A and similar to the maximum amplitude 324 of the power draw of the heater 107 in Figure 3B. Accordingly, the controller 120 may infer that the tress of hair 330 in contact with the hair contact surface 105 in Figure 3C is wider than the tress of hair 310 in contact with the hair contact surface 105 in Figure 3 A and is a similar width to the tress of hair 320 in contact with the hair contact surface 105 in Figure 3B. In order to have a cross-sectional area similar to the thick, narrow tress of hair 310, a tress of hair 330 with a similar width to the thick, wide tress of hair 320 would have to be relatively thin. Accordingly, the controller 120 may infer that the tress of hair 330 in contact with the hair contact surface 105 in Figure 3C is thinner and wider than the tress of hair 310 in contact with the hair contact surface 105 in Figure 3A and thinner than the tress of hair 320 in contact with the hair contact surface 105 in Figure 3B. Comparing the gradient of the plot 332 in Figure 3C to the gradients of the plots 312, 322 in Figures 3A and 3B, it can be seen that the gradient of the plot 332 in Figure 3C is steeper than the gradients of the plots 312, 322 in both Figures 3 A and 3B. Accordingly, the controller 120 may infer that the tress of hair 330 in contact with the hair contact surface 105 in Figure 3C is thinner than the tresses of hair 310, 320 in contact with the hair contact surface 105 in Figures 3 A and 3B. Figures 4A to 4C show a schematic representation of different hair windings around a hair contact surface 405 of a hair styling apparatus, such as hair styling apparatus 102 shown in Figure 1. In the examples shown in Figures 4A to 4C, the hair contact surface 405 is heated by a plurality of heaters 407a, 407b, 407c and 407d. Each heater of the plurality of heaters 407a, 407b, 407c and 407d is associated with a respective styling zone of the hair contact surface 405, which aligns with the respective heater. At least one electrical characteristic, such as the power draw, of each heater is monitored as described above to determine an amount of hair in contact with each styling zone of the hair contact surface 405 based on the profile of the monitored electrical characteristic of the respective heater of the plurality of heaters 407a, 407b, 407c and 407d. In addition, in the examples shown in Figures 4A to 4C the hair styling apparatus further comprises a plurality of temperature sensors 408a, 408b, 408c and 408d each being associated with a respective heater of the plurality heaters 407a, 407b, 407c and 407d. The temperature of each of the plurality of the heaters 407a, 407b, 407c and 407d is detected by a respective temperature sensor of the plurality of temperature sensors 408a, 408b, 408c and 408d and monitored by the controller 120. The detected temperatures of the plurality of the heaters 407a, 407b, 407c and 407d are taken into account with the profiles of the monitored electrical characteristics of the plurality of the heaters 407a, 407b, 407c and 407d when determining the amount of hair in contact with each styling zone of the hair contact surface 405. As outlined above, the electrical characteristic, e.g., the power draw, of the heaters 407a, 407b, 407c and 407d may be indicative of the amount of hair in contact with the hair contact surface 405 of the hair styling apparatus. However, the electrical characteristics of the heaters 407a, 407b, 407c and 407d and the profile of the electrical characteristics will change depending on the temperature of the heaters 407a, 407b, 407c and 407d. Therefore, by taking into account the temperature of the heaters 407a, 407b, 407c and 407d as well as the electrical characteristics, the accuracy of the determination of the amount of hair in contact with the hair contact surface may be improved. For example, heaters 407a, 407b, 407c and 407d with a lower starting temperature may exhibit a higher initial power draw, which can affect aspects of the profile of the power draw of the heaters 407a, 407b, 407c and 407d, for example by increasing the maximum amplitude (314, 324, 334 in Figures 3A to 3C) of the power draw of the heaters 407a, 407b, 407c and 407d and / or causing a greater change in the power draw (i.e., the gradients of plots 312, 322, 332 in Figures 3 A to 3C) of the heaters 407a, 407b, 407c and 407d when compared with heaters 407a, 407b, 407c and 407d with a higher starting temperature for the same amount of hair in contact with the hair contact surface. In addition, in the examples shown in Figures 4A to 4C the hair styling apparatus further comprises a plurality of non-contact sensors 409a, 409b, 409c and 409d adapted to generate a signal indicative of the presence of hair at the hair contact surface 405, and in particular generate a signal indicative of the presence of hair at a respective hair styling zone, corresponding to the plurality of heaters 407a, 407b, 407c and 407d, of the hair contact surface 405. Figures 4A to 4C each show a graph of sensor coverage, as a percentage, against time illustrating the action of wrapping hair about the hair contact surface 405 of the hair styling apparatus. In the example shown in Figure 4A, three wraps of hair 410a, 410b and 410c are provided about the hair styling surface 405 of the hair styling apparatus. Figure 4A shows a corresponding graph 411 of sensor coverage, as a percentage, against time for the plurality of non-contact sensors 409a, 409b, 409c and 409d. It has been assumed that the hair has been wrapped around the hair styling surface 405 at a constant speed to form the three wraps of hair 410a, 410b and 410c. In the example shown in Figure 4A, the first wrap of hair 410a covers the majority of the first non-contact sensor 409a. The first non-contact sensor 409a generates a first signal 412 in response to the contact of the first wrap of hair 410a with the hair contact surface 405. As the proportion of the hair contact surface 405, in the first hair styling zone, in contact with the first wrap of hair 410a is relatively large, the first signal is relatively high. The second wrap of hair 410b covers a portion of the second non-contact sensor 409b and the third non-contact sensor 400c. The second non-contact sensor 409b generates a second signal 413 and the third non-contact sensor 409c generates a third signal 414 in response to the contact of the second wrap of hair 410b with the hair contact surface 405. As the proportion of the hair contact surface 405, in each of the second and third hair styling zones, in contact with the second wrap of hair 410b is relatively small, the second and third signals are relatively low. The third wrap of hair 410c covers the majority of the fourth non-contact sensor 409d. The fourth non-contact sensor 409d generates a fourth signal 415 in response to the contact of the third wrap of hair 410d with the hair contact surface 405. As the proportion of the hair contact surface 405, in the fourth hair styling zone, in contact with the third wrap of hair 410c is relatively large, the fourth signal is relatively high. Based on the signals 412, 413, 414 and 415 generated by the plurality of non-contact sensors 409a, 409b, 409c and 409d, and in particular the amplitude of the signals, the controller may determine a proportion of the hair contact surface 405 in contact with the hair and a proportion of the hair contact surface exposed to air. The thermal energy loss of the hair contact surface 405 to air per unit surface area is known based on the material characteristics of the hair contact surface. Therefore, the total thermal energy loss of the system to air can be determined based on the proportion of the hair contact surface 405 exposed to air, as indicated by the signals from the plurality of non-contact sensors 409a, 409b, 409c and 409d. The total thermal energy delivered to the hair can therefore be calculated based on the power draw of the plurality of heaters 407a, 407b, 407c and 407d over time and the total thermal energy loss of the system to air. The total thermal energy delivered to the hair may then be used in conjunction with the electrical characteristics of the plurality of heaters 407a, 407b, 407c and 407d in order to determine the amount of hair in contact with the hair contact surface 405. In addition, the dimensions of the hair contact surface 405 are known to the controller 120. Therefore, the length of the tress of hair may be calculated based on the number of times the tress of hair has been wrapped about the hair contact surface 405, which is indicated by the number of times one or more of the plurality of non-contact sensors 409a, 409b, 409c and 409d is triggered. In the example shown in Figure 4B, three wraps of hair 420a, 420b and 420c are provided about the hair styling surface 405 of the hair styling apparatus. Figure 4B shows a corresponding graph 421 of sensor coverage, as a percentage, against time for the plurality of non-contact sensors 409a, 409b, 409c and 409d. It has been assumed that the hair has been wrapped around the hair styling surface 405 at a constant speed to form the three wraps of hair 420a, 420b and 420c. In the example shown in Figure 4B, the each of the first wrap of hair 420a, the second wrap of hair 420b and the third wrap of hair 420c cover the majority of the first noncontact sensor 409a, second non-contact sensor 409b and the third non-contact sensor 409c, respectively. The first non-contact sensor 409a generates a first signal 422 in response to the contact of the first wrap of hair 420a with the hair contact surface 405. The second non-contact sensor 409b generates a second signal 423 in response to the contact of the first wrap of hair 420b with the hair contact surface 405. The third non-contact sensor 409c generates a third signal 424 in response to the contact of the third wrap of hair 420c with the hair contact surface 405. The fourth non-contact sensor 409d As the proportion of the hair contact surface 405, in each hair styling zone, in contact with the first wrap of hair 420a, the second wrap of hair 420b and the third wrap of hair 420c is equal, the first signal 422, the second signal 423 and the third signal 424 are equal in amplitude. Compared to the example shown in Figure 4A, the controller may infer from the signals 422, 423 and 424 in Figure 4B that the hair in Figure 4B is wrapped more tightly around the hair styling surface 405, for example to produce a tighter curl to the tress of hair, that the hair in Figure 4A. In the example shown in Figure 4C, three wraps of hair 430a, 430b and 430c are provided about the hair styling surface 405 of the hair styling apparatus. Figure 4C shows a corresponding graph 431 of sensor coverage, as a percentage, against time for the plurality of non-contact sensors 409a, 409b, 409c and 409d. It has been assumed that the hair has been wrapped around the hair styling surface 405 at a constant speed to form the three wraps of hair 430a, 430b and 430c. In the example shown in Figure 4C, the first wrap of hair 430a covers all of the first noncontact sensor 409a and a small proportion of the second non-contact sensor 409b. The first non-contact sensor 409a generates a first signal 432 in response to the contact of the first wrap of hair 430a with the hair contact surface 405 in the first hair styling zone associated with the first heater 407a. The second non-contact sensor 409b generates a second signal 433, which has a lower amplitude than the first signal, in response to the contact of the first wrap of hair 420b with the hair contact surface 405 in the second hair styling zone associated with the second heater 407b. The second wrap of hair 430b covers a further portion of the second non-contact sensor 409b and the third non-contact sensor 409c. The amplitude of the second signal 433 increases as more of the second non-contact sensor 409b is covered in hair and the third non-contact sensor 410c generates a third signal 434 in response to the contact of the second wrap of hair 410b with the hair contact surface 405. The third wrap of hair 430c covers a further portion of the third non-contact sensor 409c and the fourth non-contact sensor 409d. The amplitude of the third signal 434 increases as more of the third non-contact sensor 409c is covered in hair and the fourth non-contact sensor 410d generates a fourth signal 435 in response to the contact of the third wrap of hair 410c with the hair contact surface 405. 5 Compared to the examples shown in Figures 4A and 4B, the controller may infer from the signals 432, 433 434 and 435 in Figure 4C that the tress of hair in Figure 4C is wider than the tresses of hair in Figures 4A and 4B due to the increase in the proportion of the hair contact surface 405 in contact with hair.

Claims

1. A hair styling apparatus comprising:a hair contact surface for contacting a user’s hair;a heater for heating the hair contact surface so as to heat the user’s hair when the hair is in contact with the hair contact surface; anda controller adapted to:monitor an electrical characteristic of the heater; anddetermine an amount of hair in contact with the hair contact surface based on a profile of the electrical characteristic of the heater.

2. The hair styling apparatus as claimed in claim 1, wherein determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater comprises:determining a rate of change of the electrical characteristic of the heater; and determining an amount of hair in contact with the hair contact surface based on the rate of change of the electrical characteristic of the heater.

3. The hair styling apparatus as claimed in any of claims 1 to 2, wherein determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater comprises:determining an amount of energy delivered to the heater; anddetermining an amount of hair in contact with the hair contact surface based on the amount of energy delivered to the heater.

4. The hair styling apparatus as claimed in claim 3, wherein determining an amount of energy delivered to the heater based on the profile of the electrical characteristic of the heater comprises:monitoring the electrical characteristic of the heater over a monitoring period; and calculating the amount of energy delivered to the heater based on the electrical characteristic of the heater over the monitoring period.

5. The hair styling apparatus as claimed in any preceding claim, wherein determining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater comprises:determining a maximum amplitude of the electrical characteristic of the heater; and determining an amount of hair in contact with the hair contact surface based on the maximum amplitude of the electrical characteristic of the heater.

6. The hair styling apparatus as claimed in any preceding claim, wherein the hair styling apparatus comprises a plurality of heaters, the plurality of heaters including the heater, each heater of the plurality of heaters being associated with a respective styling zone of the hair contact surface, and wherein the controller is adapted to:monitor an electrical characteristic of each of the plurality of heaters; and determine an amount of hair in contact with each styling zone of the hair contact surface based on a profile of the electrical characteristic of the respective heater of the plurality of heaters.

7. The hair styling apparatus as claimed in any preceding claim, wherein the hair styling apparatus further comprises a temperature sensor for detecting a temperature of the heater, and wherein the controller is further adapted to:monitor a temperature of the heater detected by the temperature sensor; and determine an amount of hair in contact with the hair contact surface based on the temperature of the heater and the profile of the electrical characteristic of the heater.

8. The hair styling apparatus as claimed in claim 7, wherein determining the amount of hair in contact with the hair contact surface based on the temperature of the heater and the profile of the electrical characteristic of the heater comprises:determining an initial temperature of the heater;monitoring the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to a target temperature; anddetermining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to the target temperature.

9. The hair styling apparatus as claimed in claim 8, wherein determining the initial temperature of the heater comprises identifying a local minimum temperature of the heater as the initial temperature of the heater.

10. The hair styling apparatus of any of claims 7 to 9, when dependent on claim 6, wherein the hair styling apparatus comprises a plurality of temperature sensors, the plurality of temperature sensor including the temperature sensor, wherein each temperature sensor of the plurality of temperature sensors is associated with a respective heater of the plurality of heaters, and wherein the controller is adapted to:monitor a temperature of each of the plurality of the heaters detected by a respective temperature sensor of the plurality of temperature sensors; anddetermine an amount of hair in contact with each hair styling zone of the hair contact surface based on the temperature of the respective heater of the plurality of heaters and the profile of the electrical characteristic of the respective heater of the plurality of heaters.

11. The hair styling apparatus as claimed in any preceding claim, wherein the hair styling apparatus further comprises a sensor arrangement adapted to generate one or more signals, each signal being indicative of a presence of hair at the hair contact surface, and wherein the controller is further adapted to:determine a proportion of the hair contact surface in contact with the hair and a proportion of the hair contact surface exposed to air based on the one or more signals;determine a thermal energy loss based on the proportion of the hair contact surface exposed to air; anddetermine an amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater and the thermal energy loss.

12. The hair styling apparatus as claimed in claim 11, wherein the sensor arrangement comprises a non-contact sensor.

13. The hair styling apparatus as claimed in any of claims 11 to 12, when dependent on either claim 6 or claim 10, wherein the sensor arrangement comprises a plurality of sensors, wherein each sensor of the plurality of sensors is associated with a respective heater of the plurality of heaters, adapted to generate a signal indicative of a presence of hair at a respective hair styling zone of the hair contact surface.

14. The hair styling apparatus as claimed in any preceding claim, wherein determining the amount of hair in contact with the hair contact surface comprises classifying the amount of hair in contact with the hair contact surface as one of a plurality of categories using a machine learning algorithm, wherein the machine learning algorithm takes the electrical characteristic profile of the heater as an input.

15. The hair styling apparatus as claimed in any preceding claim, wherein the electrical characteristic comprises one or more of: a power draw of the heater; a current draw of the heater; a voltage of the heater; and a resistance of the heater.

16. A computer-implemented method for determining an amount of hair in contact with a hair contact surface of a hair styling apparatus, the hair styling apparatus comprising a heater for heating the hair contact surface, the method comprising:monitoring an electrical characteristic of the heater; anddetermining an amount of hair in contact with the hair contact surface based on a profile of the electrical characteristic of the heater.

17. The computer-implemented method claimed in claim 16, wherein the method further comprises:determining a rate of change of electrical characteristic of the heater; anddetermining an amount of hair in contact with the hair contact surface based on the rate of change of electrical characteristic of the heater.

18. The computer-implemented method claimed in any of claims 16 to 17, wherein the method further comprises:determining an amount of energy delivered to the heater; anddetermining an amount of hair in contact with the hair contact surface based on the amount of energy delivered to the heater.

19. The computer-implemented method claimed in any of claims 16 to 18, wherein the method further comprises:monitoring the electrical characteristic of the heater over a monitoring period; andcalculating the amount of energy delivered to the heater based on the electrical characteristic of the heater over the monitoring period.

20. The computer-implemented method claimed in any of claims 16 to 19, wherein the method further comprises:determining a maximum amplitude of the electrical characteristic of the heater; anddetermining an amount of hair in contact with the hair contact surface based on the maximum amplitude of the electrical characteristic of the heater.

21. The computer-implemented method claimed in any of claims 16 to 20, wherein the hair styling apparatus further comprises a temperature sensor for detecting a temperature of the heater, and wherein the method further comprises:monitoring a temperature of the heater detected by the temperature sensor; anddetermining an amount of hair in contact with the hair contact surface based on the temperature of the heater and the profile of the electrical characteristic of the heater.

22. The computer-implemented method claimed in claim 21, wherein the method further comprises:determining an initial temperature of the heater;monitoring the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to a target temperature; anddetermining the amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater as the temperature of the heater changes from the initial temperature to the target temperature.

23. The computer-implemented method claimed in any of claims 16 to 22, wherein the hair styling apparatus further comprises a sensor arrangement adapted to generate one or more signals, each signal being indicative of a presence of hair at the hair contact surface, and wherein the method further comprises:determining a proportion of the hair contact surface in contact with the hair and a proportion of the hair contact surface exposed to air;determining a thermal energy loss based on the proportion of the hair contact surface exposed to air based on the one or more signals; anddetermining an amount of hair in contact with the hair contact surface based on the profile of the electrical characteristic of the heater and the thermal energy loss.

24. The computer-implemented method claimed in any of claims 16 to 23, wherein determining the amount of hair in contact with the hair contact surface comprises classifying the amount of hair in contact with the hair contact surface as one of a plurality of categories using a machine learning algorithm, wherein the machine learning algorithm takes the electrical characteristic profile of the heater as an input.

25. A computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the methods of claims 16 to 24.

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