Patent Publication Number: US-2018035776-A1

Title: Hair styling appliance

Description:
FIELD OF THE INVENTION 
     This invention relates to hair styling appliances, in particular low voltage, for example battery operated devices. 
     BACKGROUND TO THE INVENTION 
     There are a variety of apparatus available for styling hair. One form of apparatus is known as a straightener which employs plates that are heatable. To style, hair is clamped between the plates and heated above a transition temperature where it becomes mouldable. Depending on the type, thickness, condition and quantity of hair, the transition temperature may be in the range of 160-200° C. 
     A hair styling appliance can be employed to straighten, curl and/or crimp hair. 
     The temperature range required, user expectations with regard to the time to heat-up, thermal control, and other factors combine to drive existing hair styling appliances to employ mains power for the heater(s). 
     In WO2014/001769 and GB2503521 to the present applicant, a hair styling appliance including a battery power source for at least one heater is taught. 
     The inventors have realised that further improvement in the use of a battery power source is possible. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a hair styling apparatus comprising: a plurality of heater electrodes which heat one or more hair styling heaters, the plurality of heaters comprising a first subset and a second subset; a power source for powering the plurality of heater electrodes and a controller configured to control powering of the plurality of heater electrodes from the power source, wherein, in a first mode of operation, the controller is configured to control the power source so that the first and second subsets of the plurality of heaters are not simultaneously powered. The power source may be a battery source. Alternatively, the power source may be mains power. 
     Typically, in this first mode of operation, the controller controls power delivery so that power is delivered to the first and second subsets in a time interleaved manner, preferably multiple times per second. Interleaving the power delivery in this way offers a number of advantages. For example, if the first subset of heater electrodes is associated with a first hair styling heater and the second subset of heater electrodes is associated with a second hair styling heater, then the controller can control the heating of the first and second hair styling heaters so that they are both heated, at the same time, to respective desired operating temperatures (which may be the same). Similarly, if the first and second subsets of heater electrodes are associated with different parts of one hair styling heater then the controller can control the heating of the different parts of the heater so that they are both heated, at the same time, to a desired operating temperature. This is possible as such hair styling heaters (and other kinds of heaters) normally have a relatively high thermal inertia so that they do not cool down quickly once power is removed from the heater electrodes. 
     Interleaving of the driving of the electrodes in this manner also reduces the current drawn from the power source. When the power source is a battery, reducing the current draw is important as this reduces the energy lost in the internal resistance of the battery: P=I 2 R, where I is the current drawn and R is the internal resistance. Hence operating in this first mode of operation provides the most efficient heating of the heaters. Of course, in other modes of operation heaters from the first and second subsets may be powered simultaneously, for example, if the load is very high. 
     The controller may be configured to select the first and second subsets so that a total current drawn by each of the first and second subsets is below a predetermined current threshold. This is particularly useful for a battery power source because keeping the current draw below the threshold may prevent the battery from overheating (due to the above described I 2 R losses). The predetermined current threshold may be equivalent to a multiple (e.g. 1.5) of the current draw for a single heater electrode. 
     In other words, the controller is configured to control powering of the heater electrodes from the power source. The first mode of operation of the controller (there may only be the one mode) comprises limiting the total number of heater electrodes that may be simultaneously powered such that a predetermined current limit is not exceeded. The fact a current limit is imposed means that the controller is configured to prevent all the heater electrodes being powered at the same time. This current limit may be deemed a nominal current draw. 
     The plurality of heater electrodes may be divided into discrete subsets. Thus no heater electrodes are in both the first and second subsets. Alternatively, the first and second subsets may have some (but not all) heater electrodes in common. It will be appreciated that there may be more than two subsets of electrodes, e.g. three or even four, depending on the overall number of heater electrodes within the apparatus. Each subset may comprise one or more electrodes. 
     The controller may be configured to switch the power source between the first and second subsets, for example multiple times per second. This may be done, for example, to maintain the heater at a desired operating temperature. The controller may comprise a heating cycle in which it cycles through all of the subsets of powerable heater electrodes, determining if power may need to be applied. If not, the controller may opt to retain power to the currently powered subset, switch to another, or opt to power none of the heater electrodes if heater plates (or zones on a heater plate) are at a preferred operating temperature. The switching frequency between each subset may be in the order of tens, hundreds or thousands of cycles per second. Typically the heating cycle will have a period of between 100 μs and 500 ms. The first and second subsets may be powered in anti-phase, i.e. one subset is off when the other subset is on. In such anti-phase operation there may however be periods in which none of the electrodes are powered. 
     The controller may be configured to alternate between the first mode of operation and a second mode of operation in which the first and second subsets of the plurality of heater electrodes are simultaneously powered. For example, simultaneous heating may take place during the initial heat up from power on. In this second mode of operation, the nominal current draw is exceeded temporarily. When operating in this second mode, the controller may switch the power to both subsets of heater electrodes such that there are: 1) overlapping periods in which power is simultaneously supplied to heater electrodes in the first and second subsets; 2) periods in which power is supplied to heater electrodes of just one of the first and second subsets; and 3) periods in which no power is supplied to heater electrodes of the first and second subsets. The controller may control the duration of the overlapping periods so that they reduce with time from, for example, an initial switching on time. The controller may reduce the duration of the overlap either in response to a sensed condition or based on pre-stored data defining the switching sequence. 
     The controller may be configured to switch from the second mode of operation to the first mode of operation in response to a control signal. The control signal may be that a predetermined amount of time in the second mode of operation has elapsed. The predetermined amount of time may be based on predetermined characteristics of the battery power source and may be a few seconds. The controller may then continue to operate in the first mode of operation for a further period of time (for example 30 seconds or more) after which periods of operating in the second mode may be possible. 
     Where the power source is a battery source, the hair styling apparatus may further comprise a battery temperature sensor which senses the temperature of the battery source and which sends a battery temperature sense signal to the controller. In some embodiments a battery temperature sensor may be integrated into the hair styling apparatus, however in other embodiments the battery power source may comprise an integrated temperature sensor having a connection coupleable to the battery temperature sense input. 
     The battery temperature sense signal may be compared to a battery temperature threshold and the control signal may be generated by the controller or at the sensor when the battery temperature sense signal is greater than the battery temperature threshold. Alternatively, the control signal may be generated when the battery temperature is increasing at a rate such that a threshold value is predicted to be exceeded. The battery temperature threshold may be in the range of 60 to 80 degrees C., more preferably 70 degrees C. 
     There may also be temperature sensor sensing ambient temperature. Said control signal may be generated by the controller or the sensor when the sensed ambient temperature is below a threshold ambient temperature. The ambient temperature threshold may be in the range of 25-35 degrees C., more preferably 25 or 33 degrees C. 
     In other words, the controller may be configured to limit a duration in which the subsets of heater electrodes are simultaneously powerable by the power source. This may also be dependent on the battery temperature or ambient temperature. 
     The hair styling apparatus may comprise a first arm having a first contacting surface and a second arm having a second contacting surface, wherein the arms are moveable between a closed position in which the first and second contacting surfaces are adjacent and an open position in which the first and second contacting surfaces are spaced apart. The first arm may comprise a first hair styling heater having a plurality of heater electrodes. The first arm comprises a first hair styling heater and the second arm comprises a second hair styling heater and each hair styling heater comprises at least one heater electrode. In this arrangement, the plurality of electrodes comprises at least one on each arm. A plurality of electrodes includes two electrodes. Where there are only two electrodes, e.g. one on each arm or a single heater with two electrodes, the first subset may comprise the first electrode and the second subset may comprise the second electrode. 
     The hair styling apparatus may further comprise a touch sensitive switch configured to enable or disable the hair styling apparatus. It will be appreciated that the touch sensitive switch can be used on its own as a separate invention as well as in conjunction with the different powering modes of operation. 
     For an apparatus having a pair of arms as described above, the touch sensitive switch may be located on the first or second contacting surface. When the arms are in the closed position, the touch sensitive switch may be deactivated to prevent unintended activation of the switch. In use, a user may activate the touch sensitive switch by pressing on or otherwise contacting said touch sensitive switch for at least a predetermined duration of time. This activation may be determined by the switch or the controller and the controller may enable or disable the hair styling apparatus responsive to said determining. 
     The or each heater may comprise a heater plate which is mounted on a thermally insulating support structure. It will be appreciated that the thermally insulating support structure can be used on its own as a separate invention as well as in conjunction with the different powering modes of operation and/or touch sensitive switch. 
     The heater plate may comprise at least one recess which cooperates with a corresponding projection on the thermally insulating support structure. The recess and projection may be L-shaped. Other mechanisms for mounting the heater plate on the support may be used. The thermally insulating support structure may be resiliently mounted within an arm of the hair styling apparatus. For example, a spring mechanism may be used. Such a resilient mounting allows the heater plate to move relative to the casing of the arm during styling, allowing the plates to retain contact with varying thicknesses and changes in the profile of hair clamped between opposing pairs of styling surfaces on the heater plates. 
     According to another aspect of the invention there is provided a method of controlling a hair styling apparatus comprising a plurality of heater electrodes which heat one or more hair styling heaters, the plurality of heaters comprising a first subset and a second subset; the method comprising: controlling powering of the heater electrodes so that the first and second subsets of the plurality of heaters are not simultaneously powered. 
     According to another aspect of the invention there is provided a controller for a hair styling appliance, wherein the controller configured is configured to implement the method described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: 
         FIG. 1  shows a first example of a hair styling appliance in which embodiments of the invention may be employed; 
         FIG. 2  shows a schematic block diagram of a hair styling appliance of the type illustrated in  FIG. 1 ; 
         FIG. 3  shows a plan view of an embodiment of a hair styling heater for use in the hair styling appliance of  FIG. 1 ; 
         FIGS. 4 a  and 4 b    show timing diagrams illustrating example duty cycles of heating electrodes driven, for example, by the control system in  FIG. 2 ; 
         FIG. 5  shows a variant of the schematic block diagram of  FIG. 2 ; 
         FIGS. 6 a  to 6 d    show timing diagrams illustrating example duty cycles of heating electrodes driven, for example, by the control system in  FIG. 5 ; and 
         FIG. 7  shows a cross sectional view of the hair styling appliance. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  depicts an example of a typical hair straightener  1 . The hair straightener  1  includes first and second arms each comprising an arm member  4   a,    4   b  and heatable plates  6   a,    6   b  coupled to heaters (not shown) in thermal contact with the heatable plates. The heatable plates are substantially flat and are arranged on the inside surfaces of the arms in an opposing formation. During the straightening process, hair is clamped between the hot heatable plates and then pulled under tension through the plates so as to mould it into a straightened form. The hair straightener may also be used to curl hair by rotating the hair straightener 180° towards the head prior to pulling the hair through the hot heatable plates. 
     Also shown in  FIG. 1  is touch sensitive switch  5070  which is used to power the hair styling appliance on and off. The switch may be implemented as a capacitive touch switch comprising an electrode placed behind the plastic casing of the hair styling appliance. This obviates the need for a mechanical switch. In variants a resistive touch sensitive switch may also be used, or a piezo touch switch. As shown in  FIG. 1 , the touch sensitive switch  5070  is positioned on the inside of an arm facing the other arm. This means that the switch can only be pressed when the arms are spaced apart to prevent a user accidentally touching the switch and unintentionally turning the hair styling appliance on. Further, should the hair styling appliance be placed in luggage, such as a handbag, if a user rummages around in the bag for an item, it also prevents any accidental pressing of the switch. As a further safety mechanism, in embodiments the switch may also be deactivated (or the power supply configured to prevent activation of the appliance) when the arms of the hair styling appliance are closed together. 
     It will be appreciated that a hair straightener is just one example of a hair styling appliance and a skilled person would implement the various embodiments of the invention without difficult into other hair styling appliances such as a “crimping iron” for crimping hair or a hair styling appliance for curling hair. 
       FIG. 2  shows a block diagram of a power/control system  500  for a hair styling appliance incorporating a heater  300 . The system comprises a low voltage power supply  504  deriving power from a 12v lithium ion battery  505  and/or a mains power supply input  502 , which is used to charge the battery  505  via an AC to DC converter  503  which may be external or internal to the appliance. Power supply  504  may be configured to provide approximately 100 watts per heater; the heater resistance when hot may be selected accordingly—for example at 12v a current in the range 5-10 amps may be delivered to a heater with a resistance in the range 1-2 ohms. The resistance may be scaled accordingly as the design voltage increases or decreases (changing as the inverse square of the voltage). 
     Power from power supply  504  is provided to a power control module  514 , which in turn powers the one or more heaters  516 . Power control module  514  may employ one or more power semiconductor switching devices to provide pulse width modulation control of the (DC) voltage from power supply  504  to heaters  516 . Thus a high percentage on-time duty cycle may be employed during the initial, heating phase and afterwards the on-time duty cycle may be reduced and controlled to control the temperature(s) of the heaters  516 . 
     Power from power supply  504  is also provided to a microcontroller/control means  506  coupled to non-volatile memory  508  storing processor control code for a temperature control algorithm, and to RAM  510 . The skilled person will appreciate that any of a wide range of different control algorithms may be employed including, but not limited to, on-off control and proportional control. Optionally the control loop may include a feed-forward element responsive to a further input parameter relating to the hair styling appliance, for example to use the operation of the apparatus to improve the temperature control. An optional user interface  512  is also coupled to microcontroller  506 , for example to provide one or more user controls and/or output indications such as a light or audible alert. The output(s) may be employed to indicate, for example, when the temperature of the heating plate has reached an operating temperature, for example in a region 140° C.-185° C. 
     Microcontroller  506  is also coupled to one or more optional temperature sensors such as thermistors  340 . However, as previously mentioned, the temperature of a heating element may be sensed from its resistance and thus embodiments of the system include a current sense input  515  to microcontroller  506  sensing the current provided to a heater, for example via a current-sense resistor connected in series with the electrode. A predetermined calibration of resistance against temperature for an electrode may be stored in non-volatile memory  504  and in this way the printed track may be employed as a temperature sensor. 
     In the illustrative embodiment of  FIG. 2 , the touch sensitive switch  5070  is shown coupled to the low voltage PSU  504  to remove the need for the microcontroller to be permanently powered up. When off, the low voltage PSU  504  monitors for a change in capacitance of the touch sensitive switch indicating that a user has pressed the switch. The low voltage PSU then powers up the hair styling appliance. When on, the low voltage PSU  504  again monitors for a change in capacitance of the switch, then powering down the hair styling appliance. To power up and down, it may be necessary for a user to press on the touch sensitive switch for a minimum period of time before the PSU fully registers the press as a valid request to power on/off. This eliminates any accidental power up, or when styling, any accidental touching of the touch sensitive switch. A user may, for example, be required to press the touch sensitive switch for two, three, four or five seconds, or longer. In variants two touch sensitive switches may be used: one for turning the styling appliance on, another for turning the styling appliance off. 
     Providing such a touch sensitive switch on a hair styling appliance provides several advantages. Firstly, for battery powered products, if the hair styling appliance is carried around in luggage, it prevents the appliance being accidentally knocked on by other items in the luggage. Further, it also improves the aesthetic appearance of the product, eliminating the need for additional components on the surface of the hair styling appliance. 
     In this embodiment the touch sensitive switch is shown coupled to the low voltage PSU. In variants such a switch may be coupled to the microcontroller, although in such variants it will then be appreciated that the microcontroller may then need to be permanently powered to permit detection of a press of the touch sensitive switch. In other variants a dedicated circuit may also be used. 
     This touch sensitive power switch may be applied to any of the embodiments described herein and also as an adaption to otherwise standard devices. 
     Each heater plate may be powered by a heater electrode. Depending upon the thickness of the heater plate, lateral conductivity within the plate may not be sufficient to give the desired results with a single heater electrode. Accordingly, an example of a heater plate is illustrated in  FIG. 3  which may form the heatable plates  6   a,    6   b  of the hair straightener of  FIG. 1 . The heater plate  300  may be provided with a plurality of separately controllable heating zones  300   a, b,  each with a respective electrode  330   a, b  and thermistor  340   a, b.  Connections to these are brought out, for convenience, to one edge of the heater plate; a broadened track region  332  is provided for the electrode further from the connection point to reduce heating in the connection path. Each of the electrodes is provided with a separate control loop controlled by the temperature sensed by the respective thermistor. In embodiments more than 3 zones may be provided. 
     The heater used in the various embodiments described herein may be formed as described in WO2014/001769 and GB2503521 which are incorporated by reference. Thus, the heater may comprise an aluminium heater plate of thickness of order 1 mm, bearing a plasma electrolytic oxide (PEO) coating of aluminium oxide of thickness less than 100 μm, for example in the range 5-15 μm. 
     The hair styling appliance comprises a plurality of electrodes. As shown in  FIG. 1 , there may two heater plates, each with their own electrode and thus there are two electrodes. Alternatively, only one arm of the appliance shown in  FIG. 1  may comprise a heater plate but this heater plate comprises at least two, possibly more electrodes. There may also be multiple heater plates each having multiple electrodes. 
     The power to the plurality of heater electrodes may be independently controlled. For example, for an appliance having two arms, each with a heater and one heater electrode for each heater,  FIGS. 4 a  and 4 b    show the Voltage against Time for each heater electrode. In this example illustration, the microcontroller  506  uses pulse width modulation (PWM) power control to control the supply of power to the two heater electrodes using the power control block  514 . In pulse width modulation power control, the “on time” of the power signal within a sequence of PWM periods (here labelled ‘d’ and also referred to in other parts of this document as heater cycles) is varied in order to vary the amount of power delivered to each heater electrode. Typically, the PWM period / heater cycle may be between 100 μs and 500 ms. It will be observed from  FIGS. 4 a  and 4 b    that initially both heater electrodes are powered simultaneously until the desired operating temperature is reached. A high percentage on-time duty cycle within any given period ‘d’ may be employed during the initial heating phase; afterwards the on-time duty cycle may be reduced for each heater so as to retain the hair styling heater at a desired operating temperature. Each heater may be controlled independently to stabilise the temperature of each heater at the desired operating temperature. Accordingly, the draw is not exactly the same for both heater electrodes. Nevertheless, it can be observed from  FIGS. 4 a  and 4 b    that there may be sustained periods of a maximum current draw as a result of both heaters being powered simultaneously. 
     When powered by battery, a high current draw, such as from driving both heaters simultaneously, may lead to the battery power source heating up to an unacceptably high temperature. This may be exacerbated if both heaters are simultaneously driven for extended periods of time. The current draw when simultaneously driving the heaters, combined with a desire to conceal the battery power source means that heat dissipation may become an important factor in the construction of such a hair styling appliance. 
       FIG. 5  shows a variant of the schematic block diagram of  FIG. 2  with modified microcontroller/control means and power control module  514   b.  The reference numbers in common are used in both systems and thus any description applies equally to both. 
     The microcontroller switching control signal  708 , labelled ‘temp/power control’ in  FIG. 5  may comprise multiple outputs, one for each heater power switch to be controlled. The embodiment shown in  FIG. 5  comprises two heater electrodes, one on each of the two heater plates. Two outputs, one to activate the first heater electrode via the first power switch  702  and the second to activate the second heater electrode via the second power switch  704  are present on the power control module. In  FIG. 5 , further heater electrodes may also be driven. These may be present, for example, in a multi-zoned heater variant. Dotted arrow lines to the heaters  516  show optional connections to such additional heater elements. 
     In variants the switching control signal may be a serial data connection or encoded such that the control system can scale to independently control multiple heater elements. This may be particularly useful in embodiments having multiple heating zones (two or three per heater for example) and where the number of outputs from the microcontroller may be limited. Alternatively the microcontroller may have multiple outputs, one for each power switch. The optional decode block  706  in the power control module  514   b  decodes the signal received from the microcontroller and splits this out into separate drive signals to activate the power switches  702  and  704 . In variants incorporating multiple heating zones on each heater plate the signal may be decoded into more outputs, one for each zone. 
     Battery power source  505  in  FIG. 5  may further incorporate a battery temperature sensor  5050 , such as a thermistor. The battery temperature sensor  5050  provides a battery temperature sense signal  5051  coupled to the microcontroller  506 . The battery temperature sense signal may be factored into the temperature control algorithm and powering of the heater plates. It will be appreciated however that such a feature is optional. 
     In the embodiment shown in  FIG. 5 , the battery temperature sensor many also be used as part of the safety shutdown  520 . As set out above, the styling appliance may incorporate one or more safety shut down circuits  520  coupled to the one or more heater electrodes and/or temperature sensors  340  to monitor the heater temperature and electronically shut down the power supply to the heater should overheating be detected. In embodiments, this may be extended to also prevent overheating of the battery. In embodiments safety shut down circuit  520  controls a guard transistor  522 , as illustrated a power MOSFET, which removes power from the power control block on detection of a potential fault. Guard transistor  522  may be provided either before or after power control block  514   a.  In normal operation this device is always on; the device may be selected such that when power is removed from the transistor it switches off, thus failing safe, for example by employing an enhancement-mode device. Such control and safety shut down is applicable to all the embodiments described herein. 
     The battery temperature sensor  5050  (or another battery temperature sensor) may additionally or alternatively be used to control power on/off of the hair styling appliance. The generated signal B Tsense    5051  is fed into the microcontroller  506  and may be used to provide battery temperature information for use in a safety mechanism to shut down the styling appliance, or stop power delivery to one or more heater elements/plates if the battery temperature exceeds a battery threshold temperature. Thus, the microcontroller/control means may be arranged such that power is only supplied to the heating elements/plates when the temperature sensed by the battery temperature sensor is below a battery threshold temperature. In embodiments, this threshold temperature may be a value in the range of 60-100° C., for example 70° C. However it will be appreciated that the operational threshold temperature may be dependent on the particular construction (packaging, chemical formulation for example) of the particular battery used. Techniques such as active cooling of the battery pack, or heat transfer means such as a heat sink, may often be insufficient to retain the battery pack within its preferred safe operating range. 
     Following deactivation of the heater plates, the control system may prevent the hair styling appliance from being used again until the battery temperature has fallen below either the battery threshold temperature at which power down was previously initiated, or below a lower ‘reactivation’ temperature which would be set to a temperature below the battery threshold temperature. 
     Another technique that may be used to prevent heat build-up in the battery power source is to slow the rate of heating by throttling the maximum current delivered or using higher resistance heating elements. However, adopting such a technique may mean that the temperature of a heater plate cannot be changed very rapidly, which may lead to a poor transient response. 
     Further, an ambient temperature sensor, such as temperature sensor  5060  in  FIG. 5  may be used to monitor the ambient temperature (i.e. the temperature surrounding the hair styling appliance) and prevent power delivery to the heater plates if an ambient temperature threshold is exceeded. An ambient temperature sense signal A Tsense    5061  is then generated and fed into the microcontroller  5061 . In embodiments, this ambient threshold temperature may be a value in the range of 25-35 degrees C., for example 25 degrees C. or 33 degrees C. Such ambient temperature sensing may be used as a further safety mechanism to protect against overheating of the hair styling appliance. 
     This is particularly useful in warmer environments in which the battery which may heat up too fast (such as outside in hot climates, or in a hot indoor environment). A user may then be preventing from turning on the hair styling appliance until the ambient temperature has reduced. Thus, the microcontroller/control means may be arranged such that power is only supplied to the heating elements/plates when the sensed ambient temperature is below an ambient threshold temperature. 
     In one or both instances above, if either the ambient or battery threshold temperatures are exceeded; visual or audio feedback may be provided to the user to indicate that the device has entered a safety mode or indicate the temperature status. 
     In a first control mode the microcontroller/control means in  FIG. 5  is configured to lower the maximum current draw by operating the heaters in anti-phase. This means that in an embodiment having two heater electrodes (one for each heater), only one heater element may be powered at a time in the embodiment in  FIG. 5 . Operating in anti-phase, there may also be periods where both heaters electrodes are off, such as when both heater plates are at a desired operating temperature. Given a current draw of ‘I’ for one heater arranged to heat an entire heater plate, when two heaters are powered simultaneously to heat two heater plates, the current draw may be approximately ‘2I’. Using this convention, the maximum current draw is limited to the current draw for driving one heater (i.e. ‘I’, the current draw for one heater arranged to heat an entire heater plate). This means that the controller may be configured to prevent all (both in the embodiment of  FIG. 5 ) heater electrodes being powered at the same time. 
       FIGS. 6 a  and 6 b    show a graph for each heater in an appliance having two heaters and operating according to the preceding paragraph. As before, the microcontroller  506  uses pulse width modulation power control to control power delivery to the two heaters. It will be observed from  FIGS. 6 a  and 6 b    that both heater electrodes are now not powered at the same time within each heating cycle (here labelled ‘c’). The dotted lines between the Figures show instances of one heater starting or stopping—note there is no overlap. In a heating cycle (‘c’), each of the heaters below temperature are powered in a sequence which may be fixed or determined by the microcontroller, but only one at a time. As before, the heating cycle may be between 100 μs and 500 ms and thus the controller rapidly switches the delivery of the power between the two heaters such that, as far as the user is concerned, both heaters appear to be heating up simultaneously. 
     As will be apparent to those of ordinary skill in the art from  FIG. 5 , the controller  506  controls this delivery of power to the first and second heaters by generating control signals  708  that cause the decode/drive enable unit  706  to open and close the switches  702  and  704 . In some cases, the control signals  708  may be directly used to control the switching of the switches  702  and  704 . 
     The microcontroller may implement a control algorithm configured to allocate equal percentages of a heating cycle ‘c’ to each heater, for example 50% of the time. Typically this may be the case when a user powers on the appliance to heat both heater plates to the desired operating temperature evenly and as fast as possible. However, in the event that one heater plate heats up slower than the other, a higher portion of time in any given period/heating cycle ‘c’ may be allocated to the cooler heater plate. Furthermore, in the event one heater plate cools faster than the other when placed about a quantity of hair, the microcontroller, in response to a temperature dependent sense signal, may act accordingly to allocate a higher portion of heating time in any one heating cycle to power the heater in the cooler heating plate. 
     In some embodiments of the hair styling appliance, there may be multiple heating zones on each heater plate, as shown in  FIG. 3  for example and also shown in GB2477834, herein incorporated by reference. Each heating zone may comprise a separate heater electrode arranged to heat a portion of the heater plate. In such embodiments, it may then be permissible to simultaneously heat multiple heating zones in many different configurations. The controller may therefore be configured to prevent all the heater electrodes distributed across one or more plates being powered at the same time (or for only short periods of time). 
     As previously discussed, we generally consider a current draw ‘I’ to correspond to the current draw necessary to power a heater electrode heating an entire heater plate. Thus, in embodiments having multiple heating zones on a heater plate, one electrode in each zone may be considered to draw (for the purposes of comparison only), a portion of current draw ‘I’. In an embodiment having two heating zones on each heater plate, i.e. four heating zones in total, each zone may be considered to draw a current of 0.5I (presuming the resistances are generally the same). A maximum preferred current draw ‘I’ may therefore correspond to powering two zones simultaneously. Any two zones: both on the same plate, or one on each plate may be simultaneously powered. This means that in the event a quantity of hair is placed on only one section of the heater plates, such that only one zone needs to be powered to retain the desired operating temperature, then opposing zones on two heater plates may be simultaneously powered whilst staying within the preferred current draw limit (‘I’) to prevent the battery source overheating. 
     It will be appreciated that the maximum preferred current draw to prevent the battery source overheating may not be ‘I’, it may instead be higher or lower than this, Therefore, in some embodiments it may then be possible to power different combinations and numbers of heating zones simultaneously without the battery source overheating. By way of example, in an embodiment having two heater zones on each of two heater plates, given a preferred maximum current draw of ‘1.5I’, it may then be possible to power three heater zones simultaneously whilst staying within the preferred current draw limit. 
     Table 1 below shows exemplary combinations of the maximum zones that may be powered at any one time. The ‘nominal current draw’ column provides examples of the nominal current draw limit, defined in multiples of the current draw of one heater arranged to heat an entire heater plate. Accordingly, for the purposes of this illustrative example, the current draw of two heater electrodes, each heating half of a heater plate, is deemed the same as one heater element powering an entire heater plate. It will however be appreciated that in practice the current draw may be different. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Zones per 
                 Number of plates  
                 Nominal 
                 Zones powered at 
               
               
                 plate 
                 in appliance 
                 draw ‘I’ 
                 any one time 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 2 
                 1 
                 1 
               
               
                 2 
                 2 
                 1 
                 2 
               
               
                 3 
                 2 
                 1 
                 3 
               
               
                 1 
                 2 
                 1.5 
                 1 
               
               
                 2 
                 2 
                 1.5 
                 3 
               
               
                 3 
                 2 
                 1.5 
                 3 
               
               
                   
               
            
           
         
       
     
     Returning now to the embodiments shown in  FIG. 5  having one heater electrode in each of two plates, in a second control mode the microcontroller may allow periods of overlap in which both heaters are powered simultaneously to heat up both heater plates at the same time. Given the nominal preferred current draw of ‘I’, limited periods of a higher current draw may be permitted, so long as these higher current draw periods are interleaved with rest periods in which the nominal preferred current draw is not exceeded. So as to prevent overheating, the duration of overlap may be limited by the microcontroller/control means. The microcontroller/control means may be configured to limit this overlap to a predetermined duration within a fixed period of time based on predetermined characteristics of the battery. The microcontroller may permit, for example, simultaneous heating to only take place during the initial heat up from power on, then revert to the first mode of operation. In other words, the controller may be configured to limit a duration in which the two heater electrodes are simultaneously powerable by the power source. This may also be dependent on the battery temperature or ambient temperature. 
     In an enhancement to the second control mode the overlap control may be variable, being controlled, for example, in response to feedback from a battery temperature sense signal  5051  as depicted in  FIG. 5 . In this variant, the microcontroller may then actively monitor the temperature of the battery source, controlling the permissible overlap in which the nominal preferred current draw may be exceeded in response to the temperature of the battery source. This may be useful to allow both heaters (based on an embodiment have one heater element in each heater plate) to be driven simultaneously from cold at power on, with the microcontroller then disabling any overlap in heating once the heaters are first up to temperature. 
     By way of example,  FIGS. 6 c  and 6 d    show a graph for each heater in an appliance having two heaters where overlap is permitted. In the first phase, the battery temperature is within the preferred operating range and so the microcontroller is configured to operate in the second control mode with periods in which both heaters are heated simultaneously. In the second phase, the battery temperature sensor may sense the temperature approaching (or exceeding) a threshold temperature which results in the microcontroller changing to the first control of operation in which the heaters are powered in anti-phase. The microcontroller may then optionally return to the second control mode when the temperature of the battery source drops. The dotted lines between the Figures show regions of overlap in heaters being powered in the first phase. 
     The second technique may also be implemented for embodiments having multiple heating zones on one or more of the heater plates. Incorporating the second technique, the microcontroller may then permit various combinations of zones to be heated simultaneously as previously described, with periods in which the nominal preferred current draw is exceeded by powering further heating zones for a limited period of time. This means that the controller may be configured to limit a duration in which at least two or more of the heater electrodes are simultaneously powerable by the battery power source. 
       FIG. 7  shows a cross-sectional view of an illustrative embodiment of an arm  700  of a hair styling appliance. The arm  700  comprises an outer casing  712  to which other components of the hair styling appliance are secured. A heater element  704  is positioned on heater plate  702  to form a hair styling heater assembly. The hair styling heater assembly is then retained on the arm by the use of a thermally insulating support structure  714 . 
     The heater plate  702  comprises a styling surface  715  on one side that contacts the hair to be styled during use. On the other side of the heater plate two L-shaped recesses  709   a,    709   b  provide sockets for securely fixing the hair styling heater assembly to the thermally insulating support structure  714 . 
     The thermally insulating support structure  714  is formed from insulating material and may, for example, be constructed from a similar material to the casing. The support structure  714  comprises a pair of L-shaped projections  708   a,    708   b  arranged to fit into the  709   a,    709   b  recesses in the heater plate  702  and couple the heater plate and support structure together. To allow the projections to fit into the recesses, they may have a small degree of flex such that then can snap-fit into the recesses, thereby securely fixing the heater plate and support structure together. It will be appreciated however that other means for coupling the hair styling heater assembly and the support structure are possible, and the example shown in  FIG. 15  is purely illustrative of one way of doing so. 
     To secure the support structure  714  to the casing, sprinted members  710   a  and  710   b  are used. These are secured at one end to the casing and at the other end to the support structure. In the illustrative embodiment shown in  FIG. 15 , compression springs are used which bias the heater assembly and support structure away from the arm. These allow the heater plate to move relative to the casing during styling, allowing the plates to retain contact with varying thicknesses and changes in the profile of hair clamped between opposing pairs of styling surfaces on the heater plates. It will be appreciated that various other arrangements may be used that provide allow for movement of the heater plates. 
     This heater assembly arrangement provides several advantages:
         1. Firstly, it reduces the width of the outer casing needed to retain the hair styling heater assembly as no retaining lugs or fixings are now needed at the sides of the heater assembly.   2. Secondly, with no protrusions extending to one or more sides of the heater plate  702 , the widest part of the heater plate is the styling surface  715 . Such an arrangement is particularly advantageous during manufacturing as it allows the heater plates to be closely packed, with no or minimal gap between them. This allows a large number of styling surfaces to be screen printed, as if they were one large surface, improving the efficiency of the printing process.       

     The skilled person will appreciate that the techniques we have described above may be employed for a range of hair styling appliances including, but not limited to, a hair straightener, a hair crimping device, and a hair curler. The skilled person would also appreciate that features from many of the embodiments are interchangeable and not limited to the specific embodiment they are described in relation to. 
     No doubt many other effective alternatives will occur to the skilled person. It will 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 spirit and scope of the claims appended hereto.