GB2625113A

GB2625113A - Heating system

Info

Publication number: GB2625113A
Application number: GB2218358.6A
Authority: GB
Inventors: James Bernard Smith Daniel
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2022-12-07
Filing date: 2022-12-07
Publication date: 2024-06-12
Also published as: GB202218358D0; WO2024121743A1

Abstract

A heating system 200, e.g. for a haircare appliance, comprises an induction heating assembly 102 having at least one driving coil 108a – 108e and configured to generate a varying magnetic field, an induction heating plate 104 spaced apart from the driving coil and heatable by penetration with the varying magnetic field, and one or more repeater coils 210a – 210e arranged between the driving coils and the induction heating plate, the repeater coils being electrically isolated from the driving coils. Use of the repeater coils may enhance the influence of the magnetic field generated by the driving coils, thereby extending the range of the magnetic field. Alternatively or in addition, one or more magnetically permeable elements (510a – 510e; figure 5) which may comprise ferrite particles, may be arranged between the driving coils and heating plate to guide magnetic flux therebetween.

Description

Intellectual Property Office Application No GI32218358.6 RTM Date:1 June 2023 The following term is a registered trade mark and should be read as such wherever it occurs in this document:

METGLAS

Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo

HEATING SYS IEM

Technical Field

The present invention relates to induction heating, and may find particular use in an induction heating system such as a haircare appliance for straightening or curling hair.

Background

Induction heating is a process whereby an electrically conducting object is heated by electromagnetic induction in which a varying/alternating magnetic field is produced. The magnetic field penetrates the electrically conductive object, and induces eddy currents within the object.

These eddy currents flow through the object and heat the object via Joule heating. In some examples, the object may also be ferromagnetic, such that additional heat is generated by magnetic hysteresis.

Summary

According to a first aspect, there is provided a haircare appliance comprising an induction heating assembly having a driving coil and configured to generate a varying magnetic field, an induction heating plate spaced apart from the driving coil and heatable by penetration with the varying magnetic field, and a repeater coil arranged between the driving coil and the induction heating plate, the repeater coil being electrically isolated from the driving coil.

Incorporating a repeater coil between the driving coil and induction heating plate improves magnetic energy transfer across the gap between them. This may allow the gap to be larger for the same power applied to the driving coil and/or may allow the power applied to the driving coil to be reduced thereby improving overall efficiency of the appliance and reducing heat build-up in the driving coil. This can be useful in applications where the induction heating plate is designed to be moveable with respect to the induction heating assembly and incorporates a compliant material to control this movement. The addition of the compliant material may increase the size of the gap and the repeater coil can address the reduced magnetic energy transfer which would otherwise occur.

The repeater coil may be electrically coupled to a capacitor to form a resonant circuit having a resonant frequency. The induction heating assembly may be configured to generate the varying magnetic field at the resonant frequency of the resonant circuit.

This use of a resonant circuit significantly increases the magnetic energy transfer across the gap when the varying magnetic field is driven at the resonant frequency.

The induction heating plate may be moveable relative to the induction heating assembly. A compliant material arranged between the induction heating assembly and the induction heating plate.

The use of a flexible induction heating plate allows the haircare appliance to more effectively accommodate and heat different thicknesses of hair. The compliant material may return the induction heating plate to its original position when the hair is removed.

In one example, the repeater coil may be embedded in the compliant material. In another example, the repeater coil may be fixed to the induction heating plate. The repeater coil may be constructed in a film which is bonded to the induction heating plate.

The haircare appliance may comprise a plurality of repeater coils, each repeater coil being electrically isolated from the driving coil. The repeater coils may each be associated with respective heating zones of the induction heating plate, allowing separate heating control of these zones Each repeater coil may be electrically coupled to a respective capacitor to form a respective resonant circuit having a respective resonant frequency. Heating to different parts of the induction heating plate may be controlled by driving the varying magnetic field at one or more of the respective resonant frequencies.

In one example, the induction heating assembly is configured to generate the varying magnetic field at two or more of the respective resonant frequencies. This may be done by driving a single driving coil at a sequence of resonant frequencies or using by driving multiple driving coils at different resonant frequencies.

In an example, the haircare appliance comprises a compliant material arranged between the induction coil and the induction heating plate and at least partially encompassing a magnetically permeable element having a lower magnetic permeability than the compliant material.

The addition of a magnetically permeable element helps guide the magnetic flux between the driving coil and the inductive heating plate which adds to the benefits of the repeater coil by further improving magnetic energy transfer across the gap between them. This may allow the gap to be even larger for the same power applied to the driving coil and/or may allow the power applied to the driving coil to be further reduced thereby improving overall efficiency of the appliance and reducing heat build-up in the driving coil.

The magnetically permeable element may be configured to guide the varying magnetic field between the driving coil and the induction heating plate. This may be implemented by aligning a long element axially between the driving coil and a region of the inductive heating plate.

The compliant material may encompass a plurality of magnetically permeable elements, for example ferrite particles. This effectively dopes the compliant material with a predetermined density of magnetically permeable particles to create one or more paths between a driving coil and the induction heating plate. The path may have a higher density of magnetically permeable elements than other parts of the compliant material.

The path may comprise a tapering cross-section towards the driving coil. The geometry of the path may be adapted to reduce leakage flux.

The inductive heating assembly may comprise a receiving coil configured to receive the varying magnetic field, and wherein the magnetically permeable elements may be configured to guide the varying magnetic field between the induction heating plate and the receiving coil.

A double pole coil arrangement may improve the magnetic flux through the inductive heating plate and reduce flux leakage.

The inductive heating assembly may comprise an array of driving coils and the magnetically permeable elements may be arranged in respective paths between the driving coils and the induction heating plate.

An array of driving coils enables controlled heating of the inductive heating plate across different zones.

The inductive heating assembly may comprise an array of receiving coils each corresponding to a respective driving coil and configured to receive the varying magnetic field. The magnetically permeable elements may be configured in respective paths between the receiving coils and the induction heating plate.

In an example, at least one said path is shared between two driving coils and/or two receiving coils.

Sharing of paths of magnetically permeable elements may simplify manufacturing.

The haircare appliance may comprise a screening coil spaced apart from the driving coil on the opposite side from the induction heating plate.

The use of a screening coil may reflect any of the varying magnetic field generated behind the driving coil back towards the inductive heating plate.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Figures IA and 1B are schematic diagrams of a heating system comprising a flexible induction heating plate, according to an example; Figures 2A and 2B are schematic diagrams of a heating system comprising repeater coils, according to an example; Figures 3A and 3B are schematic diagrams of a partial heating systems comprising resonant repeater coil configurations, according to examples; Figures 4A and 4B schematic diagrams of partial heating system assemblies illustrating flux pathways in single and double horizontal pole coil configurations according to examples; Figures 5A and 5B are schematic diagrams of a heating system comprising magnetically permeable elements, according to an example, Figure 6 is a schematic diagram of a partial induction heating system comprising regions of ferrite particle doped compliant material, according to an example; Figures 7A show a sectional view of an induction heating system having tapered magnetically permeable elements, according to an example, and Figure 7B illustrates a magnetic flux simulation for the system of Figure 7A; Figures 8A -8B illustrate side sectional views of induction heating systems having alternative arrangements of magnetically permeable elements according to examples; and Figure 9 is a schematic diagram of a induction heating plate having insulating boundaries between regions on the heating surface, according to an example.

Detailed Description

Figures IA and 1B are schematic diagrams of a heating system 100 comprising an induction heating assembly 102 and an induction heating plate 104. In this example, the induction heating plate 104 comprises a single plate or heating target, but in other examples may include one or more thin metallic plates or heating targets. The induction heating plate 104 of this example takes the form of a flexible heating plate. The induction heating plate 104 is moveable and is arranged in a first configuration in Figure IA and a second configuration in Figure 1B.

When the induction heating assembly 102 generates or is supplied with a high frequency alternating current, the induction heating assembly 102 generates an alternating/varying magnetic field that penetrates the induction heating plate 104. The magnetic field induces eddy currents within the electrically conductive induction heating plate 104 which causes the induction heating plate 104 to heat up. In this example, the induction heating assembly 102 comprises an induction coil assembly 106 comprising one or more driving coils 108a -108e and the induction coil assembly 106 is supplied with the high frequency current to generate the magnetic field. As will be discussed in more detail below, the induction coil assembly 106 has a top side that faces the induction heating plate 104, and a bottom side that faces away from the induction heating plate 104. The induction heating plate 104 is spaced apart from the one or more driving coils 108a 108e and is heatable by penetration of the varying magnetic field generated by the one or more driving coils.

To generate and supply the high frequency current, the induction heating assembly 102 comprises a drive circuit 130. The drive circuit 130 is used to provide and control the current flow through the induction coil assembly 106. The alternating current provided to the induction coil assembly 106 by the drive circuit 130 is at a particular frequency, known as the drive frequency. As will be well understood, an induction coil forms part of an induction system that can be driven to resonance, and the induction system therefore has an associated resonant frequency. The induction system includes the induction heating assembly 102 and at least part of the induction heating plate 104. As will be discussed in more detail below, when the drive frequency matches the resonant frequency of the induction system, the induction heating plate 104 can be heated most effectively. Movement of the induction heating plate 104, or regions of the induction heating plate 104, relative to the induction heating assembly 102 may cause the resonant frequency of the induction system to change. The drive circuit may be configured to adjust the drive frequency to match changes in the resonant frequency of the induction system, for example by adjusting the drive frequency towards the maximum power supplied to the drive circuit. In other examples the drive frequency may be fixed or only partially adjustable.

In this particular example, the induction heating plate 104 is flexible such that a force applied to the induction heating plate 104 causes the induction heating plate 104 to move/flex. In Figure 1A, regions of the induction heating plate 104 are arranged in a first position and the induction heating plate 104 is substantially flat and unflexed. The induction heating plate 104 is spaced apart from the one or more driving coils 108a -108e at a first distance 132. In Figure 1B, a particular region of the induction heating plate 104 is arranged in a second position in which the region has been moved, bent or flexed towards the one or more drive coils 108c. This particular region of the induction heating plate 104 is therefore closer to the induction heating assembly 102 in the second position when compared to the first position, and is arranged at a second distance 134 away from the driving coil 108c. The second distance 134 is smaller than the first distance 132.

The induction heating plate 104 or regions of the induction heating plate 104 can move from the first position to the second position upon application of a force 136 by an entity 138. In this example, the entity is a volume of hair 138. Upon removal of the hair 138, and therefore the force 136, the induction heating plate 104 is configured to return to the first position depicted in Figure 1A. One or more biasing members 140, such as a layer of compliant material arranged between the induction heating assembly 102 and the induction heating plate 104, may urge the induction heating plate 104 back towards the first position. The induction heating plate 104 is therefore biased towards the first position, in this example. A moveable or flexible induction heating plate 104 finds particular use in heater system to control the level of heating of the induction heating plate 104.

In a first example, the heater system 100 is configured such that when a region of the induction heating plate 104 is arranged in the first position (Figure 1A), the region is heated to a lower temperature than when the region arranged in the second position (Figure 1B). To achieve this, the drive circuit 130 drives the induction coil assembly 106 at a substantially constant drive frequency as the region of the induction heating plate 104 moves between the first and second positions.

As mentioned above, the induction system has an associated resonant frequency. In some examples, the heating system 100 is made up of a plurality of induction systems with different resonant frequencies, where each induction system includes the induction heating assembly and different flexed/unflexed regions of the induction heating plate 104. Figure 1B shows a first region 104a of the induction heating plate 104 spaced further apart (distance 132) from the driving coils 108a -108e and a second region 104b of the induction heating plate 104 spaced closer (distance 134) to the driving coils. These regions 104a, 104b form first and second induction systems having different respective resonant frequencies. The system of the second region 104b may have the same or a similar resonant frequency to the first region 104a when the inductive heating plate 104 is not flexed or bent, as illustrated in Figure 1A. The resonant frequency of an inductive system associated with a particular region 104a, 104b of the inductive heating plate 104 will therefore depend on the distance between that region and the (nearest) driving coil 108a -108e.

If the drive frequency of the drive circuit 130 remains constant and is selected to correspond to the closer distance 134 corresponding to region 104b in Figure 1B, the second region 104b will be heated resonantly when it is located in the position shown in Figure 1B because the drive frequency substantially matches the resonant frequency of the second region 104b in this position. However, for the more spaced apart first region 104a in Figure 1B, the resonant frequency associated with this region does not match the drive frequency such that reduced or even no heating occurs. Thus, the drive frequency can be selected such that the region 104b is heated non-resonantly when arranged in a first position (corresponding to Figure 1A) and is heated resonantly when arranged in a second position (corresponding to Figure IB). This also applies for other regions of the inductive heating plate such that the smaller the difference between the drive frequency and the resonant frequency of the region, the greater the region is heated.

Accordingly, the drive frequency may be selected such that the temperature of the induction heating plate 104 in each region is relatively low when arranged in the first position shown in Figure 1A. The temperature may be below a threshold temperature for example. The temperature may be at a level to avoid serious burns, should a user accidentally touch the induction heating plate 104. The temperature may be at a level to reduce the likelihood of nearby objects being burnt, melted or set on fire, should the device come into contact with the object. For example, the temperature may be below the combustion temperature of common household objects, such as clothing, wood or carpet. The induction heating plate 104 temperature in this unflexed "default" position can be predetermined by a manufacturer by choosing a particular drive frequency. It will be appreciated that in some instances, as a first region such as 104a moves, a second region such as 104b may experience a slight displacement, but the change in resonant frequency of the second induction system in this situation may be small or negligible.

The compliant material 140 may comprise a layer of compressible material such as silicone or rubber for example. Metamateri al s may additionally or alternatively be used. In other examples, separated sections of compliant material may be provided along the inductive heating plate and these may be separated by air gaps. In order to provide some functionality such as effective hair straightening and/or curling, the change in distance between a first position in which the inductive heating plate is flat or unflexed and a second position in which a region of the inductive heating plate is maximally flexed may be large, requiring a thick layer or section of compliant material between the inductive heating plate 104 and the induction heating assembly. However, thicker layers of compliant material reduce the efficiency of energy transfer and therefore the inductive heating effect. High efficiency is desirable in battery powered appliances such as hair straighteners to extend the functional effectiveness of the appliance between charges. Low efficiency is undesirable as it may require higher currents to be driven through the driving coils in order to achieve a wanted heating. This may in turn result in excessive heat build-up in the induction heating assembly and the appliance itself In an example, the induction coil assembly 106 may also comprise a number of screening coils 109a -109e. Each screening coil may be electrically isolated and arranged underneath a respective driving or induction coil 108a -108e. Simple circular driving coil arrangements may generate a varying magnetic field both above and below the driving coils 108a 108e. The screening coil or coils 109a -109e reflect the varying magnetic field emanating from below the driving coils to enhance the combined field above the driving coils. In other examples, nonferrous material such as an aluminium plate may be employed or a more complex driving coil arrangement may be used.

In an example, one or more varying magnetic field enhancers 110 are provided between the one or more driving coils 108a -108e and the inductive heating plate 104. In the examples of Figure 1A and 1B, five driving coils 108-a -108e are shown and each is associated with a varying magnetic field enhancers 110, though for simplicity only four are shown.

Examples of varying magnetic field enhancers will be described in more detail below, but may include one or more repeater coils located between the one or more driving coils and the inductive heating plate 104. The of each repeater coil may form a resonant circuit with a capacitor. The or each repeater coil may be embedded in the compliant material 140 and/or be located between the compliant material and the inductive heating plate 104. Another example implementation of the varying magnetic field enhancers 110 includes one or more magnetically permeable elements. These elements may be implemented by doping part of the compliant material with ferrite particles. Different numbers of varying magnetic field enhancers HO may be used compared with the number of driving coils. The varying magnetic field enhancers 110 may include a mix or a combination of repeater coils and magnetically permeable elements.

The varying magnetic field enhancers 110 improve the geometry of the varying magnetic field between the one or more driving coils 108a -108e and the inductive heating plate 104 which increases the energy transferred to the plate 104. Improved energy transfer improves the efficiency of the inductive heating function which may reduce waste heat generation and/or improve battery life.

Figures 2A and 2B are schematic diagrams of a heating system 200 in which the varying magnetic field enhancers of Figures lA and 1B are implemented as repeater coils 210a-210e. Common features retain the numbering of Figures 1A and 1B. In an example, the repeater coils may be embedded within the compliant material 140 between the driving coils 108a -108e and the inductive heating plate 104. A repeater coil 210a-210e may be paired with a respective driving coil 108a -108e, although a non-one-to-one relationship may exist between the driving and repeater coils. The repeater coils 210a -210e may be located close to the induction heating plate, for example within the upper half or at the upper surface of the compliant material 140.

The or each driving coil, the or each repeater coil and the inductive heating plate form one or more electromagnetic systems which deliver power from the driving coils to the inductive heating plate. The repeater coils 210a -210e effectively enhance the influence of the varying magnetic field generated by the driving coils 108a -108e. The effect of this is to enhance the range of the varying magnetic field so that a greater thickness of compliant material 140 may be used between the driving coils and the induction heating plate. This in turn allows for greater displacement of the induction heating plate 140 which improves the control of heating applied to hair 138 as well as improving the heating induction efficiency. Improving induction heating efficiency reduces the power required for heating which improves battery power on battery powered devices and also reduces heating within the appliance due to waste heat generation. This effect also allows the MMF (magnetomotive force) or current applied to driving coil to be reduced for the same distance between the driving coil and induction heating plate.

One or more of the repeater coils 210a -210e are electrically isolated from the driving coils 108a -108e so that they are only influenced by the varying magnetic field generated by the driving coils. The repeater coils may be electrically isolated from any other components, including other repeater coils.

Alternatively, in one example, one or more repeater coils 210a -210e may be electrically coupled with a respective capacitor to form a resonant circuit 230 having a resonant frequency. The resonant frequency will be dependent on the inductance of the repeater coil and the capacitance of the coupled capacitor. A driving coil 108e may be driving at the resonant frequency of the resonant circuit 230 of its paired repeater coil 210e. This further enhances the above noted varying magnetic field enhancing effects. As the resonant circuit 230, driving coil 108e and induction heating plate 140 form an electromagnetic system, the resonant frequency of this system may varying depending not only on the resonant frequency of the resonant circuit 230, but also on factors such as the (changing) distance between the repeater coil 210e and the driving coil 108e and the distances between the repeater coil 210e and the induction heating plate 104 and other repeater coils. These distances may vary as the compliant material is compressed or decompressed by deflection of the induction heating plate. The induction heating assembly may be arranged to monitor the resonant frequency of the electromagnetic system and adjust this towards the nominal resonant frequency of the resonant circuit 230 in order to optimise power transfer to the induction heating plate. This may be implemented by monitoring the power drawn by the driving coil 108e, and adjusting the frequency at which this is driven to maximise this power.

Whilst the above example has been described with respect to driving coil 108e and repeater coil 210e, it will be appreciated that the same configuration and control approach may be employed so other pairs of driving coils 108a 108d and repeater coils 2I0a 210e, In some examples the resonant circuits 230 formed by the other pairs of driving coils and repeater coils may be configured with the same or different resonant frequencies. Having different resonant frequencies allows for improved control over zoned heating as any flux from a varying magnetic field at a resonant frequency of a wanted repeater coil that reaches a neighbouring repeater coil at a different resonant frequency will be attenuated. This also allows for improved control of arrangements having different numbers of repeater and driving coils. For example, a single driving coil may be used with multiple repeater coils each having a resonant frequency tuned to a different resonant frequency. This will allow the driving coil to control which part of the inductive heating plate is heated by changing the frequency at which it is driven to the resonant frequency associated with the repeater coil adjacent the wanted heating zone. Repeater coils which are non-resonant may provide additional heating to the plate when driven.

Figure 3A and 3B illustrate two examples of implementing a repeater coil with a capacitor to form a resonant circuit. These figures may show only part of a larger system with a plurality of repeater coils. In Figure 3A, a repeater coil 310 is embedded within compliant material 340 located between a driving coil and an induction heating plate 304. The repeater coil 310 is electrically coupled with a capacitor 315 also embedded within the compliant material. In an alternative arrangement, the capacitor may be positioned outside the compliant material and connected to the repeater coil 320 via connectors extending to an edge of the compliant material. The repeater coil 310 and the capacitor 315 form a resonant circuit 330 having a resonant frequency.

In Figure 3B, the repeater coil 310 and electrically coupled capacitor 315 are formed within a thin flexible film, for example of plastic material, and which is sandwiched between the compliant material 340 and the induction heating plate 304. The thin flexible film may be bonded to the rear of the induction heating plate 304. In an alternative arrangement, the capacitor 325 may be surface mounted to the induction heating plate 304 at the edge of the film so that only the repeater coil is manufactured within the film.

In an alternative example, the repeater coil and capacitor may be bonded directly to the rear of the induction heating plate, and wire used to electrically couple them. Compliant material 340 may then be sandwiched between the modified induction heating plate and the driving coil.

In an alternative example, the compliant material 140 may be omitted so that an airgap exists between the driving coils 108a -108e and the induction heating plate 104. The induction heating plate 104 may be spaced apart from the driving coil or coils by compliant members or components such as springs which urge the induction heating plate into the position of Figure lA but allow deflection such as to the position of Figure 1B. The repeater coil or coils may also be positioned using compliant members or components, or it may be attached to the inductive heating plate 104 between it and the driving coil or coils.

The repeater coils 210a -210e, 320 and the driving coils 108a -108e may be any suitable coil arrangement, such as circular, rectangular, spiral wound and may have any suitable number of windings or turns. Figure 4A illustrates a flux pattern for a single horizontal pole driving and repeater coil configuration. In this example, representative flux 440 extends from the driving coil 408 through the repeater coil 410 and through the induction heating plate 404. The flux returns around and back into the underside of the driving coil 308. The presence of the repeater coil 410 extends the flux 440 upwards towards the induction heating plate 404 more than would be the case without the repeater coil.

Figure 4B illustrates a representative flux pattern 450 for a double horizontal pole driving and repeater coil configuration. The flux extends from the driving coil 408-1 through the repeater coil 410 and through the induction heating plate 404. The flux 450 returns to a receiving coil 408- 2 which is electrically coupled with the driving coil 408-1 with the flux alternating in direction between the two coils 408-1, 408-2 with the varying magnetic field. The double horizontal pole configuration may result in less leakage flux and therefore greater power transfer efficiency to the induction heating plate 404. In an example, two or more repeating coils may be used; for example there may be a repeating coil paired with the driving coil 408-1 and another repeating coil paired with the receiving coil 408-2.

A plurality of the configurations of Figure 4A and/or 4B could be used in the heating appliance to enable different heating zones. It should also be noted that either or a combination of these configurations could be used in the examples described above, for example in Figures 1A, 1B, 2A, 2B, 3A, 3B.

Figures 5A and 5B are schematic diagrams of a heating system 500 in which the varying magnetic field enhancers of Figures IA and 1B are implemented as magnetically permeable elements 510a-510e. Common features retain the numbering of Figures IA and 1B. In an example, the magnetically permeable elements are at least partially encompassed by the compliant material 140. Each magnetically permeable element 510a -510e may be a single object comprising a material such as ferrite or each magnetically permeable element 510a -510e may comprise a plurality of smaller objects separated by the compliant material or air. Some of these objects may extend to the edge of the compliant material such that they are not fully encompassed by it. In one example, parts of the compliant material 140 are doped with magnetically permeable particles such as ferrite powder. The doped region may include a threshold density of magnetically permeable particles. The compliant material may include regions having different levels of doping.

In some examples, doping may range from 20 -50% of ferrite particles by volume for magnetically conductive rubber.

Each magnetically permeable element 510a -510e may be located between a respective driving coil 108a-108e and the inductive heating plate 104. The magnetically permeable elements are configured to guide varying magnetics fields between the driving coils the induction heating plate. This flux guide effect reduces leakage flux and the increased relative permeability allows the flux to travel a greater distance for the same applied MIVIF to the driving coils. Therefore the power transfer efficiency is increased per gap or distance between the driving coils and the induction heating plate. Increased efficiency reduces battery drain on battery powered devices and also reduce the build-up of excessive heat caused by coil losses.

The magnetically permeable elements form paths between the driving coils and the inductive heating plate with the paths having a higher density of magnetically permeable elements or material than other part of the compliant material. A path may have a constant cross-section or may have a tapering cross section towards the driving coil where it is smallest. Other geometries may alternatively be used.

The magnetically permeable elements 510a -510e may be formed of any magnetically permeable material having a permeability u (Him) greater than the surrounding (undoped) compliant material 140. In the example of doping, the magnetically permeable material may be mixed with the compliant material 140. Examples of magnetically permeable material include ferrite, iron, Metgl as 2714A.

In some examples, the compliant material 140 may be formed in discrete beams separated by an airgap for example, with a beam extending between a driving coil and the induction heating plate.

Figure 6 illustrates an example inductive heating arrangement using a double horizontal pole driving and repeater coil configuration. A representative flux path is also illustrated. A compliant material 640 is sandwiched between a driving coil 608-1 and receiving coil 608-2 configuration and an induction heating plate 604. Two regions 610-1, 610-2 doped with ferrite particles are located between the driving coil 608-1 and the induction heating plate 604 and the receiving coil 610-2 and the induction heating plate. Flux 660 generated from the driving coil 6081 extends through the first doped region 610-1 to the induction heating plate 604 and back through the second doped region 610-2 to the receiving coil 608-2. This flux path 660 is extended and also well controlled compared with a flux path without the two doped regions which will not extend as far and will include greater flux leakage. It will be understood that the polarities of the driving coil 608-1 and receiving coil 608-2 will reverse with the changing direction of current through the configuration as the varying magnetic field changes polarity, and the flux path will reverse. In another example. A single horizontal pole configuration may be used, such as that illustrated in Figure 4A. In this case a single doped region may extend between the driving coil and induction heating plate. Alternatively, two additional doped regions may extend on wither side of the first doped region to guide the return flux.

Figure 7A illustrates a section through an example inductive heating system 700 comprising a plurality of driving coils 708-1 and receiving coils 708-2, an inductive heating plate 704 and a plurality of magnetically permeable elements 710. The magnetically permeable elements may be ferrite particle doped sections of compliant material located between the coils and the induction heating plate. This allows the induction heating plate to be moved closer to the coils which may result in increased inductive heating. An airgap may be provided between the coils and inductive heating plate and in between the magnetically permeable elements 710. The system may also include screening coils 709.

In an example, the flux paths between adjacent pairs of driving and receiving coils may share a magnetically permeable element 710. This is illustrated in Figure 7B which shows a magnetic flux simulation for the system of Figure 7A. High flux concentrations can be observed between and above the coils. The flux flows up one magnetically permeable element and back down an adjacent one. It can be seen that the flux pattern is well controlled to efficiently transfer power from the coils to the inductive heating plate and minimises leakage flux to other areas which may cause unwanted heating The magnetically permeable element need not have an especially high permeability and compliant material such as silicone relatively lightly doped with ferrite may be sufficient to significantly improve the power transfer efficiency. High frequency varying magnetic fields can be used to enhance the flux guiding characteristics of the magnetically permeable elements even when these have a low permeability, not much higher than the surrounding air or undoped flexible material. Example frequencies may include 800kHz and 2MIlz.

Figures 8A -8B illustrate section views through example inductive heating systems 800A 800D, each comprising an inductive heating plate 804, undoped compressible material 840 (unshaded) and doped compressible material 810 (shaded) as well as driving coils 808. The undoped compressible material may be rubber or silicone for example, and the doped compressible material may comprise the same material but with ferrite particles dispersed throughout, for example in the range 20 -50% by volume. A rigid ferrite back plate 842 may optionally be provided below the driving coils as illustrated in Fig. 8D. This reduces flux below the coils which might otherwise induce unwanted heating in nearby parts of the system.

Fig. 8A shows an arrangement in which the doped compressible material 810 is positioned between the driving coils 808 and the inductive heating plate 804 to provide a path having a higher density of magnetically permeable elements which improves power transfer between the driving coils and the heating plate as well as reducing leakage flux. The even number of paths provides forward and return pathways for the flux generated by the coils 808.

Fig. 8B is a similar arrangement to that of Fig. 8A, with additional doped compressible material below the driving coils. This further improves the magnetic circuit for directing the flow of flow between the driving coils and the inductive heating plate and additionally reducing the amount of leakage flux that might otherwise be directed below the driving coils.

Fig. 8C is similar to Fig. 8B but additionally includes tapered portions of the doped compressible material paths, with the tapering widening nearer the induction heating plate. This may further improve flux flow and reduce leakage flux.

Fig 8D is similar to Fig. 8A but replaces the lower horizontal portion of doped compressible material with a ferrite plate which helps the flux flow back down and around the coils and upwards towards the inductive heating plate whilst reducing stray or leakage flux below the plate 842. Whilst the use of repeater coil and magnetically permeable element examples of varying magnetic field enhancers 110 have been described largely separately, these examples could be combined. For example, an induction heating system may comprise a magnetically permeable element 610-1, 610-2 located between a driving (and receiving) coil and an induction heating plate. One or more repeater 410 coils may also be included between the magnetically permeable element and the induction heating plate. In other examples, some driving coils of an array may be paired with magnetically permeable elements and other driving coils in the same array may be paired with repeater coils. Further, some driving coils may be paired with both. Various configurations may be implemented to achieved desired characteristics of an induction heating system.

Whilst the described examples could be implemented in various applications of induction heating such as cooking, they may be employed separately or combined in a haircare appliance such as a hair straightening or a hair curling appliance.

Figure 9 is a perspective view of an example hair straightening appliance 900 comprising a first arm 902a and a second arm 902b, which are joined together at one end by a hinge 906. A power supply cable 908 extends away from the hinged end of the hair straightening appliance 900.

In other examples, the hair straightening appliance 900 comprises an internal battery power source, such that the power supply cable 908 is omitted.

Each arm 902a, 902b comprises a induction heating plate 904 located towards the end of the arm furthest away from the hinge 906. Inside each arm is an induction heating assembly to heat the induction heating plate 904. Any of the induction heating assemblies described herein may be used as the induction heating assembly to heat the induction heating plate 804. Figure 8 shows the hair straightening device 900 in an open position where the induction heating plates 804 are spaced apart. The induction heating plates 904 are arranged to contact each other when the first and second arms 902a, 902b are brought together by a user into a closed position. The induction heating plates 904 comprise a hair contacting surface which contacts hair, in use. Hair that is to be straightened is trapped between the two induction heating plates 904 and heat is transferred to the hair from the induction heating plates 904.

In applications such as induction cooking, compliant material may be omitted and instead a guide material may be used which comprises one or more magnetically permeable elements. This guide material could be a non-compliant material that is thermally compliant, such as plastic, rubber, ceramics or meta-materials. Ferrite particles or other magnetically permeable elements may be dispersed or doped through the guide material to improve magnetic power transfer.

The above examples are to be understood as illustrative. Further examples are envisaged.

Any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the examples, or any combination of any other of the examples. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

A haircare appliance comprising: an induction heating assembly having a driving coil and configured to generate a varyingmagnetic field;an induction heating plate spaced apart from the driving coil and heatable by penetration with the varying magnetic field; a repeater coil arranged between the driving coil and the induction heating plate, the repeater coil being electrically isolated from the driving coil 2. The haircare appliance of claim 1, wherein the repeater coil is electrically coupled to a capacitor to form a resonant circuit having a resonant frequency.
3. The haircare appliance of claim 2, the induction heating assembly configured to generate the varying magnetic field at the resonant frequency of the resonant circuit.
4. The haircare appliance of any one preceding claim, wherein the induction heating plate is moveable relative to the induction heating assembly.
5 The haircare appliance of any one preceding claim, comprising a resilient material arranged between the induction heating assembly and the induction heating plate.
6. The haircare appliance of claim 5, wherein the repeater coil is embedded in the resilient material.
7. The haircare appliance of any one of claims Ito 5, wherein the repeater coil is fixed to the induction heating plate.
8. The haircare appliance of any one preceding claim, comprising a plurality of repeater coils, each repeater coil being electrically isolated from the driving coil.
9. The haircare appliance of claim 8, wherein each repeater coil is electrically coupled to a respective capacitor to form a respective resonant circuit having a respective resonant frequency.
10. The haircare appliance of claim 8, wherein the induction heating assembly is configured to generate the varying magnetic field at two or more of the respective resonant frequencies.
11 The hair care appliance of any one preceding claim, wherein the induction heating assembly comprises a plurality of driving coils each configured to generate a respective varying magnetic field.
12. The haircare appliance of claim II when dependent on claim 10, each driving coil configured to generate the respective varying magnetic field at a respective resonant frequency of a respective resonant circuit.
13. The haircare appliance of any one preceding claim, wherein the inductive heating assembly comprises a receiving coil configured to receive the varying magnetic field.