METHOD AND APPARATUS FOR MANIPULATING THE SHAPE OF HAIR

Abstract

An apparatus manipulates the shape of hair using dielectric heating. Typically,the apparatus includes opposing first and second electrodes respectively provided on first and second arms that are movable towards and away from one another. Drive circuitry supplies electrical energy to the first and second electrodes, to cause an alternating electric field to be produced in the vicinity of the electrodes in use, and thereby cause dielectric heating of hair placed between the electrodes in use. Sensing circuitry senses a change in coupling of energy from the alternating electric field to the hair during heating of the hair. Control circuitry controls the drive circuitry to vary the electrical energy supplied to the first and second electrodes in dependence upon the sensed change in coupling. A related method manipulates the shape of hair using dielectric heating.

Claims

1. A hair styling apparatus comprising: first and second arms, the first and second arms being movable towards and away from one another; first and second electrodes provided on the first and second arms respectively, such that the electrodes oppose one another; drive circuitry for supplying electrical energy to the first and second electrodes, to cause an alternating electric field to be produced in the vicinity of the electrodes in use, and thereby cause dielectric heating of hair placed between the electrodes in use; and control circuitry for controlling the drive circuitry to vary the electrical energy supplied to the first and second electrodes during heating of the hair.

2. (canceled)

3. Apparatus according to claim 1, further comprising sensing circuitry for sensing a change in coupling of energy from the alternating electric field to the hair during heating of the hair; and wherein the control circuitry is arranged to control the drive circuitry to vary the electrical energy supplied to the first and second electrodes in dependence upon the sensed change in coupling.

4. Apparatus as claimed in claim 3, wherein the sensing circuitry further comprises means for determining a frequency of the electrical energy at which better coupling of the alternating electric field to the hair takes place than with other frequencies; and wherein the control circuitry is further configured to control the drive circuitry to adjust the frequency of the electrical energy so as to be at or around the determined frequency.

5. Apparatus as claimed in claim 4, wherein the means for determining comprise means for sensing the current drawn by the electrodes in dependence on the frequency of the supplied electrical energy, and wherein the determined frequency is the frequency of the supplied electrical energy at which the magnitude of the sensed current is substantially at a peak.

6. Apparatus as claimed in claim 5, wherein the means for sensing the current drawn by the electrodes is configured to generate a feedback signal representative of the magnitude of the current drawn by the electrodes; wherein the control circuitry is configured to cause the drive circuitry to vary the frequency of the electrical energy such as to supply test signals to the electrodes at a plurality of different frequencies across a range of frequencies; wherein the control circuitry is configured to receive said feedback signal in respect of each of the plurality of frequencies and thereby determine the frequency of the electrical energy at which a peak in the sensed current is obtained; and wherein the control circuitry is configured to cause the drive circuitry to supply the electrical energy at or around the determined frequency for a period of time.

7. Apparatus as claimed in claim 6, wherein the control circuitry is configured to cause the drive circuitry to generate the test signal or test signals comprising the different frequency components whilst substantially simultaneously supplying electrical energy to the electrodes at the determined frequency to cause heating of the hair.

8. Apparatus as claimed in claim 7, wherein the test signal or test signals are at a low amplitude relative to the electrical energy supplied at the determined frequency.

9.-12. (canceled)

13. Apparatus as claimed in claim 1, wherein the control circuitry is configured to control the drive circuitry to vary a frequency of the electrical energy supplied to the first and second electrodes.

14. Apparatus as claimed in claim 13, wherein the control circuitry is configured to vary the frequency of the electrical energy using a frequency hopping technique across a range of frequencies or in a sweeping manner across a range of frequencies.

15. Apparatus as claimed in claim 13, wherein the control circuitry is configured to apply a test signal to the electrodes comprising a plurality of frequencies simultaneously.

16. Apparatus as claimed in claim 1, further comprising means for detecting whether the first and second arms are closed together and means for cutting off the supply of electrical energy to the electrodes if the first and second arms are not detected as being closed together.

17.-18. (canceled)

19. Apparatus as claimed in claim 1, wherein the first arm bears a first dielectric heating plate, and the second arm bears a second dielectric heating plate, the first dielectric heating plate incorporating the first electrode and the second dielectric heating plate incorporating the second electrode.

20. Apparatus as claimed in claim 19, wherein at least the first dielectric heating plate has a plastic outer surface which forms a contact surface for hair sandwiched between the plates during use.

21. Apparatus as claimed in claim 1, wherein: each of the electrodes comprises a first conductive region interdigitated with a second conductive region; the first conductive region of the first electrode opposes the first conductive region of the second electrode; the second conductive region of the first electrode opposes the second conductive region of the second electrode; and the drive circuit is configured to drive the first and second conductive regions of each electrode with drive signals that are substantially 180 degrees out of phase with one another.

22.-25. (canceled)

26. A method of styling hair using dielectric heating, the method comprising: placing hair between first and second electrodes provided on respective first and second arms of a styler apparatus, the electrodes opposing one another, and the first and second arms being movable towards and away from one another; supplying electrical energy to the first and second electrodes, to cause an alternating electric field to be produced in the vicinity of the electrodes, and thereby cause dielectric heating of the hair; and varying the electrical energy supplied to the first and second electrodes during the heating of the hair.

27. (canceled)

28. The method according to claim 26, further comprising sensing a change in coupling of energy from the alternating electric field to the hair during heating of the hair; and wherein said varying varies the electrical energy supplied to the first and second electrodes in dependence upon the sensed change in coupling.

29. The method as claimed in claim 26, wherein varying the electrical energy supplied to the first and second electrodes comprises varying the frequency of the electrical energy supplied to the first and second electrodes.

30. The method as claimed in claim 29, further comprising: determining the frequency of the electrical energy at which coupling of the alternating electric field to the hair takes place; and adjusting the frequency of the electrical energy so as to be at the determined frequency.

31. The The method as claimed in claim 30, wherein the determining comprises sensing the current drawn by the electrodes in dependence on the frequency of the supplied electrical energy, and wherein the determined frequency is the frequency of the supplied electrical energy at which the magnitude of the sensed current is substantially at a peak.

32. The method as claimed in claim 31, wherein sensing the current drawn by the electrodes comprises generating a feedback signal representative of the magnitude of the current drawn by the electrodes; and wherein the method further comprises: varying the frequency of the electrical energy such as to supply test signals to the electrodes at a plurality of different frequencies across a range of frequencies; receiving said feedback signal in respect of each of the plurality of frequencies and thereby determining the frequency of the alternating current at which a peak in the sensed current is obtained; and then supplying the electrical energy at the determined frequency for a period of time.

33. The method as claimed in claim 32, wherein the frequency of the electrical energy is varied using frequency hopping across the range of frequencies or in a sweeping manner across the range of frequencies.

34.-36. (canceled)

37. The method as claimed in claim 30, wherein the method further comprises: triggering a wide band test signal comprising a simultaneous plurality of constituent signals at different frequencies within a range of frequencies; determining, via frequency analysis, the frequency of a constituent signal that has been subtracted from the wide band signal as a result of that frequency signal having caused coupling of the alternating electric field to the hair; and supplying the electrical energy at the determined frequency for a period of time.

38.-41. (canceled)

42. The method as claimed in claim 28, further comprising detecting whether the first and second arms are closed together, and cutting off the supply of electrical energy to the electrodes if the first and second arms are not detected as being closed together.

43.-46. (canceled)

47. The method as claimed in claim 28, wherein each of the electrodes comprises a first region interdigitated with a second region; the first region of the first electrode opposes the first region of the second electrode; the second region of the first electrode opposes the second region of the second electrode; and the first and second regions of each electrode are driven out of phase with one another.

48.-72. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:

[0044] FIG. 1 illustrates a hair styler which employs dielectric heating;

[0045] FIG. 2 is a simplified schematic circuit diagram of a hair styler as in FIG. 1, to illustrate the presence of a variable frequency alternating current source to cause an alternating electric field to be produced in the vicinity of the electrodes, and current sensing means (e.g. an ammeter) to provide feedback control to the current source;

[0046] FIG. 3 illustrates an alternative configuration of the electrodes, wherein each electrode includes alternating interdigitated regions;

[0047] FIG. 4 illustrates possible electrical circuitry for use in the hair styler of FIG. 1;

[0048] FIG. 5 illustrates a plot of absorption level against frequency of applied alternating electric field, in respect of the dielectric heating of hair, and showing that the absorption peak is variable; and

[0049] FIG. 6 illustrates an alternative drive circuit to that of FIG. 2, the drive circuit of FIG. 6 incorporating a DC power supply and switching circuitry to repeatedly reverse the polarity of voltage applied to each of the electrodes in order to cause an alternating electric field to be produced in the vicinity of the electrodes.

[0050] In the figures, like elements are indicated by like reference numerals throughout.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0051] The present embodiments represent the best ways known to the applicants of putting the invention into practice. However, they are not the only ways in which this can be achieved.

[0052] Overview of Hair Styler Employing Dielectric Heating

[0053] FIG. 1 illustrates a hair styler 1 which employs dielectric heating. The hair styler 1 includes a first movable arm 4a and a second movable arm 4b, which are coupled together by a hinge mechanism 2. The first and second movable arms 4a, 4b oppose one another and are movable relative to one other by virtue of the hinge mechanism 2. Thus, the first and second arms 4a, 4b can be brought together, into a closed configuration, or moved apart, into an open configuration, by a user in use.

[0054] The first arm 4a bears a first dielectric heating plate 6a, and the second arm 4b bears a second dielectric heating plate 6b. The first and second dielectric heating plates 6a, 6b oppose one another and, in use, are brought together as the first and second arms 4a, 4b are brought together, or separated as the first and second arms 4a, 4b are moved apart.

[0055] The hinge mechanism 2 can incorporate any suitable means for allowing the first and second arms 4a, 4b to be moved relative to one other.

[0056] The hinge mechanism 2 also incorporates spring means configured to bias the first and second arms 4a, 4b into the open configuration, such that the user is required to apply pressure to the arms 4a, 4b to close them together (overcoming the effect of the spring means), and such that the arms 4a, 4b automatically open, under the effect of the spring means, once the pressure is removed. For example, the hinge mechanism 2 may incorporate a leaf spring or a coiled spring.

[0057] The hinge mechanism and the spring means can be one and the same. For example, the spring means itself can be used to couple the first and second arms 4a, 4b together, thereby avoiding the need to provide a separate mechanical hinge and simplifying the overall construction of the styler. For example, the first and second arms 4a, 4b may be formed in a unitary manner (e.g. from a plastics material) with a “U” shaped middle part provided between the first and second arms 4a, 4b, the “U” shaped middle part being able to resiliently flex to allow opening and closing of the heating plates 6a, 6b.

[0058] The electrical and electronic circuitry of the hair styler 1 is housed in the two arms 4a, 4b, with a switch 3 being provided on the first arm 4a to enable the styler 1 to be turned on or off, together with a light 5 to indicate whether the power is on. A sound can also be played by a sound generator (not illustrated) when the styler 1 is switched on and ready to use. Together, the switch 3, light 5 and sound generator (if included) form a user interface (21 in FIG. 4). In alternative embodiments the user interface may include additional components (such as, for example, further display means, to provide the user with more information on the operational status of the styler).

[0059] In use the hair is clamped between the two heating plates 6a, 6b and pulled through, in a manner similar to that of a standard styler. The heating plates 6a, 6b are pivoted such that they can freely tilt about an axis longitudinal to the body of the styler 1.

[0060] Electrodes for Causing Dielectric Heating

[0061] With reference now to FIG. 2, each of the heating plates 6a, 6b includes a respective electrode 25a, 25b for causing dielectric heating of the hair 10 (reference numeral 10 in FIG. 2 being used to denote a bundle of hair rather than a single strand). As illustrated schematically in FIG. 2, in this example a variable frequency alternating current source 12 is provided to drive the electrodes 25a, 25b. The alternating current applied to the electrodes 25a, 25b causes an alternating electric field to be produced in the vicinity of (e.g. between) the electrodes 25a, 25b. Energy from the alternating electric field is coupled to the hair 10, thereby causing heating of the hair. Maximum energy coupling occurs when the frequency of the alternating electric field matches the peak absorption frequency of the hair, and when there is impedance matching between the drive circuitry (i.e. the circuitry that supplies electrical energy to the electrodes 25a, 25b) and the electrodes/hair.

[0062] In order to match the output impedance of the drive circuitry to the capacitive impedance formed by the electrodes and the hair between the electrodes, the inventors have found that the output impedance of the drive circuitry should be relatively low, of the order of 1-10 ohms and preferably about 2 ohms.

[0063] Typical frequencies of operation of the alternating current source 12 (and thus the alternating electric field produced) are in the range of 10 MHz to 100 MHz, although our experimental tests have shown that frequencies in the range of 20 MHz to 40 MHz are ideal. These frequencies are well suited for consumer products since they have limited wave propagation (unlike microwaves) and hence do not present a risk to health or undesirable EMC (electromagnetic compatibility) effects.

[0064] The electrodes 25a, 25b may themselves form the respective plates 6a, 6b, or they may be incorporated within the plates 6a, 6b.

[0065] For example, each of the plates 6a, 6b may be formed of a conductive material (e.g. aluminium), such that the plates 6a, 6b themselves act as the electrodes 25a, 25b. If the plates 6a, 6b are formed of a conductive material then the outer surface of each of the plates (i.e. the opposing surfaces of the plates 6a, 6b, against which the hair comes into contact) are coated or covered in a non-conductive material to prevent a short circuit from occurring when the plates 6a, 6b are brought together in use. The non-conductive material may be a plastics material. Alternatively, if aluminium is used to form the electrodes, then a non-conductive coating can be created on the aluminium by anodising.

[0066] Alternatively, each of the plates 6a, 6b may be formed of a non-conductive material carrying a planar conductor as the respective electrode 25a, 25b. For example, the plates 6a, 6b may be formed of a ceramic with a copper clad backing, or plastic with insert moulded metal. Again, to prevent a short circuit from occurring during use, the plates 6a, 6b are configured such that the electrodes 25a, 25b cannot come into direct contact with one another when the plates 6a, 6b are brought together.

[0067] Since the electrodes 25a, 25b do not themselves heat up to any significant extent during use of the styler 1, the opposing surfaces of the electrodes 25a, 25b (against which the hair comes into contact) may be coated in a plastics material. Furthermore, the arms 4a, 4b and/or plates 6a, 6b which support the electrodes 25a, 25b may also be formed from a plastics material, since high thermal resistance is not a requirement. Indeed, the plates 6a, 6b typically only heat up to a temperature of about 70° C. when heating hair. Furthermore, it would appear that water is not evaporated when using the present method, and hence it is likely to require less energy than conventional styling techniques.

[0068] Thus, the styler 1 can be made using lower temperature materials than those used to make conventional stylers that employ resistive heating. Such lower temperature materials (e.g. plastics) are typically less expensive than metals to obtain and form.

[0069] The shape of the electrodes 25a, 25b may be rectangular, with straight sides, as illustrated schematically in FIG. 2. Other configurations of the electrodes 25a, 25b are possible. For example, as illustrated schematically in FIG. 3, each electrode 25a, 25b may include alternating interdigitated conductive regions of “positive” electrode and “negative” electrode, the interdigitated regions being arranged such that when the plates 6a, 6b are bought together, a “positive” electrode region of the first plate 6a opposes a “negative” electrode region of the second plate 6b, and a “negative” electrode region of the first plate 6a opposes a “positive” electrode region of the second plate 6b (as denoted by the “+” and “−” symbols in FIG. 3). Naturally, as those skilled in the art will appreciate, the use of the terms “positive” and “negative” in this context is merely to enable the constituent regions of each electrode 25a, 25b to be distinguished from one another; in practice the constituent regions will both be subjected to an alternating current, with the constituent regions being driven out of phase with one another. The use of interdigitated electrodes in this manner helps to “focus” the electric field onto the hair, providing enhanced coupling of the energy into the hair, reducing stray field lines, and also reducing potential radiofrequency emissions.

[0070] It should be appreciated that the illustration in FIG. 3 is merely schematic, and that, in practice, the interdigitated “fingers” of the “positive” and “negative” electrode regions may be much narrower than is illustrated, such that a plurality of interdigitated fingers span the width of a typical bundle of hair 10. Alternatively, the interdigitated fingers may be wider, e.g. as illustrated or wider still. Furthermore, the extent to which the interdigitated fingers pass alongside one another may be less than, or greater than, that as illustrated.

[0071] Electrical Circuitry

[0072] As illustrated schematically in FIG. 2, the styler 1 is provided with electrical circuitry configured to provide feedback control to the variable frequency alternating current source 12, such that the frequency of the alternating current is tuned to the peak absorption frequency of the hair as it changes during the heating process.

[0073] In a general sense, the feedback control provides means for varying the frequency of the alternating current supplied by the alternating current source 12, for determining which frequency of the supplied alternating current provides good coupling (preferably maximum coupling) of the alternating electric field (as produced in the vicinity of the electrodes 25a, 25b) to the hair, and for adjusting the frequency of the alternating current supplied by the alternating current source 12 so as to be at or around the determined frequency.

[0074] In view of the fact that the peak absorption frequency of the hair varies over time (e.g. as the moisture content of the hair decreases) and that the peak absorption frequency can also vary due to other factors such as the packing density of the hair, during use of the styler 1 the feedback control causes the frequency of the alternating current to be repeatedly tuned (or retuned) to the peak absorption frequency of the hair.

[0075] With the embodiment illustrated in FIG. 2, the feedback control is performed by current sensing means 14 (e.g. an ammeter or other means for measuring current) arranged to sense the current being drawn from the drive circuitry by the electrodes 25a, 25b. A feedback signal from the current sensing means 14, representative of the magnitude of the current being drawn from the drive circuitry, is used to control the alternating current source 12.

[0076] In broad terms the feedback control operates on the principle that, when the frequency of the alternating current provided by the variable frequency alternating current source 12 is tuned to the peak absorption frequency of the hair 10, such that the alternating electric field (as produced in the vicinity of the electrodes 25a, 25b) couples well to the hair, the magnitude of the current drawn from the drive circuitry by the electrodes 25a, 25b will be significantly greater than when the frequency of the alternating current is not tuned to the peak absorption frequency of the hair and coupling is not occurring or is not occurring to the same extent. For example, the magnitude of the current drawn from the drive circuitry during coupling may be around 2A whereas when the alternating current is not tuned to the peak absorption frequency of the hair, the current drawn may fall to around 20 mA.

[0077] Accordingly, the output from the current sensing means 14, as fed back to the current source 12, is used to control the frequency of the alternating current produced by the current source 12, and thereby tune the frequency of the alternating current to the peak absorption frequency of the hair 10. When the frequency of the alternating current is tuned to the peak absorption frequency of the hair (as at that point in time) energy from the alternating electric field (as produced in the vicinity of the electrodes 25a, 25b) is coupled to the hair 10.

[0078] The circuitry shown in FIG. 2 is somewhat simplified, in order to illustrate the principle of tuning the frequency of the alternating current that is applied to the electrodes, to achieve coupling of the alternating electric field with the hair.

[0079] FIG. 4 illustrates in more detail electrical circuitry 20 suitable for use in the above embodiment of the styler 1. The electrical circuitry 20 includes a user interface 21, microprocessor 22, FET (field effect transistor) signal generator 23, drive circuitry 24, power supply 26, current sensing circuitry 27, and the above-described electrodes 25.

[0080] The user interface 21 is as described above in relation to FIG. 1, and typically includes a switch 3 to enable the styler 1 to be turned on or off, and indicator means such as light 5 to indicate whether the power is on and whether the styler is ready to use.

[0081] The microprocessor 22 is programmed and configured to control the operation of the styler 1, including the tuning of the frequency of the applied alternating current to the peak absorption frequency of the hair.

[0082] The FET signal generator 23 is configured to receive electrical power from the power supply 26 and to provide an alternating voltage having a set frequency to the drive circuitry 24. The frequency of the alternating voltage provided by the FET signal generator 23 is controlled (or set) by the microprocessor 22.

[0083] In the presently-preferred embodiment the power supply 26 is a mains power supply, in which case the FET signal generator 23 is configured to down-convert the mains AC electricity from around 230-240V to around 50V AC, e.g. using a switch mode system as will be familiar to those skilled in the art. In an alternative embodiment the power supply 26 comprises one or more DC batteries or cells (which may be rechargeable, e.g. from the mains via a charging lead). This enables the styler 1 to be a cordless product. In such an embodiment the FET signal generator 23 is configured to up-convert the DC voltage from the batteries/cells to around 50V AC.

[0084] The drive circuitry 24 is configured to receive the alternating voltage from the FET signal generator 23 and to apply it across the electrodes 25 of the plates 6a, 6b. This causes a corresponding AC current to flow from the drive circuitry 24 into the electrodes 25.

[0085] In a presently-preferred embodiment the drive circuitry 24 includes a switch that is activated (e.g. closed) when the arms 4a, 4b have been brought together and the plates 6a, 6b are closed. The drive circuitry 24 is configured to only apply energy to the electrodes 25 when the plates 6a, 6b are closed and the switch has been activated, thus providing a safety feature to the styler 1. As those skilled in the art will appreciate, other detection means may be used instead of a switch for this purpose, such as an optical interlock arrangement, or electrical contacts that come together when the plates 6a, 6b are closed.

[0086] Current sensing circuitry 27 is coupled to the drive circuitry 24 (e.g. to an output of the drive circuitry 24), and is configured to sense the current output from the drive circuitry 24 and applied to the electrodes 25. An output signal from the current sensing circuitry 27, representative of the magnitude of this current, is fed back to the microprocessor 22. As discussed in relation to FIG. 2 above, when the frequency of the alternating current is tuned to the peak absorption frequency of the hair, the magnitude of the current being drawn from the drive circuitry 24 by the electrodes 25 will be significantly greater than when the frequency of the alternating current is not tuned to the peak absorption frequency of the hair. Accordingly, the output from the current sensing means 14, as fed back to the microprocessor 22, is used by the microprocessor 22 to control the frequency of the alternating voltage produced by the FET signal generator 23, and thereby tune the frequency of the alternating current to the peak absorption frequency of the hair 10.

[0087] As mentioned above, and as illustrated schematically in FIG. 5, the frequency and size of the absorption peak of the hair varies with the dampness of the hair, and also with the packing density of the hair. Hence the absorption frequency is tracked throughout the styling process to ensure optimal coupling of energy into the hair during the styling process. This can be achieved in a number of ways: [0088] Use of spread spectrum techniques to provide a wide band of active frequencies of the alternating current (e.g. between 20 MHz and 40 MHz) as applied to the electrodes 25. The microprocessor 22 causes the alternating voltage produced by the signal generator 23 to jump around in frequency (i.e. employing a frequency hopping technique, which may be performed according to a predetermined pattern or sequence, e.g. pseudo-randomly). For each frequency the current sensing circuitry 27 senses the current drawn from the output amplifier (not shown) of the drive circuitry 24 by the electrodes 25 and provides a signal to the microprocessor 22 that is representative of the magnitude of the sensed current. The frequency of the applied alternating current which produces a peak in the sensed current is determined by the microprocessor 22, and that frequency (or a nearby frequency) is then used for a period of time which typically will be between 10 ms and 1 s, before the search is repeated across the said band of frequencies. [0089] Use of a scanning signal across the range of active frequencies (e.g. from 20 MHz to 40 MHz, repeatedly). The microprocessor 22 causes the frequency of the alternating voltage produced by the signal generator 23 to be varied in a continuous (sweeping) manner across the range of active frequencies. For each frequency the current sensing circuitry 27 senses the current drawn from the drive circuitry 24 by the electrodes 25 and provides a signal to the microprocessor 22 that is representative of the magnitude of the sensed current. The frequency of the applied alternating current which produces a peak in the sensed current is determined by the microprocessor 22, and that frequency (or a nearby frequency) is then used for a period of time, before the search is repeated across the said band of frequencies. [0090] Use of two signals substantially simultaneously: a low amplitude test signal to determine or update the peak absorption frequency (e.g. using either frequency hopping or scanning as outlined above), whilst substantially simultaneously providing a main drive signal to the electrodes 25a, 25b at the most recently determined frequency to cause heating of the hair.

[0091] Modifications and Alternatives

[0092] Detailed embodiments have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. By way of illustration only some of these alternatives and modifications will now be described.

[0093] In the above embodiments the current used to drive the electrodes, to cause an alternating electric field to be produced in the vicinity of the electrodes, is supplied by a variable frequency alternating current source 12 or a variable frequency alternating voltage source such as FET signal generator 23. However, in alternative embodiments a DC source can be used, together with switching circuitry that repeatedly reverses the polarity of voltage/current applied to each of the electrodes, thereby causing an alternating electric field to be produced in the vicinity of the electrodes.

[0094] Such an arrangement is illustrated in FIG. 6, where DC voltage source 32 is coupled to high frequency switches 34 and 35. Switches 34 and 35 are each reversibly switchable between a terminal A and a terminal B, under the control of switch controller 36, and in synchronicity with one another. Terminal A of switch 34 and terminal B of switch 35 are both connected to electrode 25a, whereas terminal B of switch 34 and terminal A of switch 35 are both connected to electrode 25b. When the switches 34, 35 are both in position A (as illustrated), electrode 25a is connected to the positive terminal of the DC voltage source 32, and electrode 25b is connected to the negative terminal of the DC voltage source. Conversely, when the switches 34, 35 are both in position B, electrode 25a is connected to the negative terminal of the DC voltage source 32, and electrode 25b is connected to the positive terminal of the DC voltage source. In such a manner, the polarity of the voltage applied to each of the electrodes 25a, 25b can be repeatedly reversed, in order to cause an alternating electric field to be produced in the vicinity of the electrodes 25a, 25b. The timing of the switching events for the main drive signal is controlled by the microprocessor 22 as before. If a wide band test signal is to be applied (for tracking the best drive frequency to use), then the timing of the switching events can be determined, for example, by a PN (pseudo noise) code generator 38, which is configured to supply a PN code to the switch controller 36. As a further alternative to generating a wideband signal, an impulse generator 39 (illustrated in phantom) may be provided to control the position of the switches 34, 35. In this case, an impulse generated by the impulse generator 39 causes the switch controller 36 to quickly change the positions of the switches 34 and 35 to cause a short burst of alternating voltage to be applied to the electrodes 25. By analysing the current drawn by the electrodes 25 as a result of this short burst, the system can determine the optimum frequency at which to drive the electrodes for maximum energy coupling into the hair.

[0095] In the above embodiments the frequency of the supplied alternating current which causes the best energy coupling of the alternating electric field to the hair is determined. The analysis to make this determination can be performed in the time domain or in the frequency domain. In embodiments that sense the magnitude of multiple frequencies applied at the same time, this analysis is preferably done using frequency domain techniques (rather than trying to use time domain filtering techniques to separate the different frequency components).

[0096] For example, with reference back to FIG. 2, in place of an ammeter 14, a frequency domain analyser may be provided, to analyse the frequencies present in the wideband current applied to the electrodes 25. In more detail, the applied current may be analysed in the frequency domain by a frequency domain analyser (e.g. a processor running a Fast Fourier Transform (FFT) algorithm). The frequency domain analyser is configured to determine, via frequency analysis, the frequency of the current component that has the largest amplitude. That frequency (or a nearby frequency) is then identified as the frequency at which the electrodes 25 are then to be driven, and the drive frequency of the main drive signal is adjusted accordingly.

[0097] In the above embodiments sensing circuitry is provided which comprises means for determining a frequency of the electrical energy at which better coupling of the alternating electric field to the hair takes place than with other frequencies; and the control circuitry is configured to control the drive circuitry to adjust the frequency of the electrical energy so as to be at or around the determined frequency. However, in other alternative embodiments control circuitry may be configured to vary the electrical energy supplied to the first and second electrodes without the use of such sensing circuitry, and without determining during use the frequency of the electrical energy at which better coupling of the alternating electric field to the hair takes place. For example, the control circuitry may be configured to vary the electrical energy supplied to the electrodes according to a stored sequence of changes. Such a stored sequence of changes may be, for example, a factory-pre-set sequence of changes; different sequences of changes according to hair type or the kind of styling to be carried out may be pre-programmed into the device. Alternatively a typical sequence of changes may be determined or “learnt” by the device based on previous use (e.g. in respect of a particular user and their hair) and stored in a memory of the device. Whilst these techniques may not be as effective as the use of sensing circuitry and active feedback control to the drive circuitry, they nevertheless enable the electrical energy supplied to the electrodes to be varied during use, compensating for variation in the peak absorption frequency of the hair during styling.

[0098] A person skilled in the art 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.

[0099] 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 scope of the claims appended hereto.

[0100] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps.