GB2440571A

GB2440571A - Drive for an inductive coupling with a changing magnetic field direction

Info

Publication number: GB2440571A
Application number: GB0615268A
Authority: GB
Inventors: John Robert Dunton; Julian Fells; Pilgrim Beart; Jonathan Pearson
Original assignee: Splashpower Ltd
Current assignee: Splashpower Ltd
Priority date: 2006-08-01
Filing date: 2006-08-01
Publication date: 2008-02-06
Also published as: GB0615268D0

Abstract

An apparatus for, or method of, generating an electromagnetic field in a region comprises first and second coils 101, 102. The coils 101, 102 are configured so that if the coils are individually driven they would generate an electromagnetic field crossing the said region in predetermined but different respective directions. In use only the first coil 101 may be directly driven by an alternating current signal and the second coil 102 is indirectly driven by being inductively coupled to the first coil 101. The inductive coupling between the coils 101, 102 may relate to transformer or particular winding arrangements. The said coils 101, 102 generate an electromagnetic field across the said region with a direction which moves over time. Alternatively, both coils may be driven by the same drive signal but due to the different response characteristics of the respective coils a phase shifted signal is developed such that the coils generate an electromagnetic field across the said region with a direction which moves over time. The response characteristic may be the resonant frequency of a coil where one coil resonates above and the other below the frequency of the drive signal such that leading and lagging signals are produced. A further alternative may use two frequency generators which provide two different frequency signals via a drive arrangement to the coils. Each coil has a different response characteristic to the different frequency signals such that a rotating magnetic field is generated in the said region. The apparatus or method may be used in a contact-less power transfer system which allows a portable device to be recharged when placed in any arbitrary position and orientation upon such an inductive coupler.

Description

IMPROVEMENTS RELATING TO CONTACT-LESS POWER TRANSFER

Many of today's portable devices incorporate secondary" power cells which can be recharged, saving the user the cost and inconvenience of regularly having to purchase new cells. In some of these devices, the cells are recharged via inductive coupling rather than direct electrical connection. Examples include the Braun Oral B Plak Control power toothbrush, the Panasonic Digital Cordless Phone Solution KX-PH1 5AL and the Panasonic multi-head men's shavers ES7O/40 series. Chargers which use inductive charging remove the need to have open electrical contacts hence allowing the adaptor and device to be sealed and used in wet environments.

The above examples all require the device needing power to be precisely aligned to the charger in some way. This requirement is inconvenient as the user has to spend time aligning the device in the correct position and orientation. To increase the ease of use, several chargers allow some freedom in how the device is placed on the charger.

These chargers consist of a pad onto which devices may be placed in any position and/or orientation to be recharged. One such example uses an array of spiral coils as described in international application PCT/AU2003100721, filed on 10th June 2003.

Another such example is described in the applicants' granted UK Patent Publication No. GB2399226, the entire content of which is incorporated herein by reference, and is illustrated in Fig. 1 of the accompanying drawings. The pad I consists of a rectangular slab of ferrite core 10, around which is wound a coil 20. A second coil 30 is wound around the slab 10, so that the windings of the first coil 20 are perpendicular to the windings of the second coil 30. When sinusoidal current is applied to one of the coils, a magnetic field is generated in a direction parallel to the coil axis (perpendicular to the coil windings) in a horizontal plane above the surface of the pad. The portable device contains a module 50 consisting of a thin core of magnetically permeable material 60 and a single coil wound around it 70. When this module is positioned above the surface of the pad, with a component of the magnetic field along the coil axis, energy is transferred from the pad to the module. In order to allow the module to be rotated in the plane above the pad, both coils on the pad 20, 30 are driven with high power sinusoidal current generators 25 and 35. These would each typically require a sinusoidal voltage reference 26, 36 and a high current driver 27, 37. The sinusoidal current generators 25 and 35 have a phase offset of 900 between them, so that the resolved component in magnetic field rotates in a horizontal plane above the pad. In this way, the module 50 experiences an alternating magnetic field regardless of its orientation.

The prior art system of Fig. 1 is very effective at allowing devices to receive power, regardless of their position and orientation on the pad. However, a disadvantage is that two high current signal sources are required, 25, 35, one for each of the coils. The high current drivers 27, 37 are expensive as they must be able to supply a large current to an inductive load. This increases the overall cost and size of the system. Further, the system requires electronics to maintain the phase angle between the two sources to be 900 apart.

It is therefore an object of the present invention to remove the need for two high power current sources.

According to a first aspect of the invention there is provided a power transfer unit for transferring power by electromagnetic induction to a portable device, the portable device being separable from the power transfer unit, the power transfer unit comprising at least two coils, wherein the first coil is driven by an alternating current source and the second coil is not driven by a current source; wherein energy is coupled from the first coil to the second coil; wherein both coils generate a magnetic field. This has the advantage that only a single drive is required, reducing cost, complexity and size.

The first coil may have a coil axis that is substantially orthogonal to the second coil.

One or other or both coils may be wound around or be in proximity to a magnetic core.

There may be more than two coils, each with a coil axis in a different direction.

The coils may have axes parallel to each other. There may be more than two coils with parallel axes. The coils may be arranged in the same horizontal plane. The coils may be arranged in different horizontal planes.

The first coil may be coupled to the second coil by means of a transformer. The transformer may have a coupling coefficient less than 1. The coupling coefficient may be significantly less than 1. The coupling coefficient may be approximately 0.5.

The first coil axis may be orthogonal to the second coil axis. The first coil may be close to but not quite orthogonal to the second coil axis. The coupling between the first coil and the second coil may be magnetic. The second coil of the power transfer unit may be resonant. The first coil and the second coil may be resonant at substantially the same frequency. The second coil may be connected to one or more reactive elements to make it resonant. These features have the advantage of increasing the power transfer from the first coil to the second coil.

The axis of the first coil may not be identical with the axis of the second coil. The axis of the second coil may be almost, but not exactly orthogonal to the axis of the first coil.

These features have the advantage that the two coils will be able to provide a field which allows coupling between the two coils, and the resultant field is able to supply current to the module in all orientations.

The first coil or the second coil or both coils may be wound such that the turns on the second face are not parallel to the turns on the first face. The first coil or the second coil or both coils may be wound such that the turns on a first face are substantially parallel to one another and the turns on a second face are substantially parallel to one another, but that the turns on the second face are not parallel to the turns on the first face. This has the advantage of allowing the turns to be easily wound and to give control over the angle of the resultant magnetic field.

The current flowing in the second coil alone in the absence of a secondary device may be insufficient to generate a magnetic field that would couple adequate power to the secondary device; wherein the presence of the secondary device increases the coupling between first and second coils, such that the second coil generates a field that does couple adequate power to the secondary device. This has the advantage that it is not necessary to misalign the two coils from orthogonality to such an extent that there are some orientations of secondary device that couple significantly less power than other orientations.

Coupling between the first and second coil may take place via a magnetic field in a different direction to the field used for power transfer. This field may be orthogonal to the field generated for power transfer. Elements of metal or magnetic material may be added to disrupt the field symmetry and allow coupling. There may be a plurality of coils each having a different coil axis. There may be coupling between the coils by means of additional elements, such as a transformer, capacitance or an electrical network introducing a phase shift. The capacitance could be discrete capacitors or it could be the capacitance of the coil windings or some other component in the system.

According to a second aspect of the invention there is provided a power transfer system for transferring power by electromagnetic induction to a portable device, the system éomprising: a power transfer unit and a portable device, separable from the power transfer unit; the power transfer unit comprising: at least two coils; wherein the first coil is driven by an alternating current source and the second coil is not driven by a current source; wherein energy is coupled from the first coil to the second coil; wherein both coils generate a magnetic field. This has the advantage that only a single drive is required, reducing cost, complexity and size.

According to a third aspect of the invention, there is provided a method of transferring power by electromagnetic induction from a power transfer unit to a portable device, separable from the power transfer unit, comprising the steps of: providing at least two coils; driving the first coil with an alternating current; and coupling energy from the first coil to the second coil. This has the advantage that only a single drive is required, reducing cost, complexity and size.

According to a fourth aspect of the invention there is provided a power transfer unit for transferring power by electromagnetic induction to a portable device, the portable device being separable from the power transfer unit, the power transfer unit comprising: at least two coils; an electrical current generator; wherein the first coil is resonant at one frequency and the second coil is resonant at a second frequency different from the first frequency; wherein the electrical current generator drives both coils. This has the advantage that only a single drive is required, reducing cost, complexity and size.

An alternating current signal may be applied to the two coils with a frequency that is above the frequency of the first coil and below the frequency of the second coil. The two coils may provide a differential phase of approximately 900. The first coil may provide a phase lag of approximately 45 . The second coil may provide a phase lead of approximately 45 . The first coil may provide a phase lag other than 45 and the second coil may provide a phase lead other than 45 such that the differential phase is approximately 90 .

According to a fifth aspect of the invention there is provided a power transfer system for transferring power by electromagnetic induction to a portable device, the system comprising: a power transfer unit and a portable device separable from the power transfer unit; the power transfer unit comprising: at least two coils; an electrical current generator; wherein the first coil is resonant at one frequency and the second coil is resonant at a second frequency different from the first frequency; wherein the electrical current generator drives both coils.. This has the advantage that only a single drive is required, reducing cost, complexity and size.

According to a sixth aspect of the invention there is provided a method of transferring power from a power transfer unit by electromagnetic induction to a portable device separable from the power transfer unit, comprising the steps of: providing at least two coils; providing a driver for generating an electrical signal; adapting the first coil to be resonant at one frequency and adapting the second coil to be resonant at a second frequency different from the first frequency; applying the electrical signal to both coils.

This has the advantage that only a single drive is required, reducing cost, complexity and size.

According to a seventh aspect of the invention there is provided a power transfer unit for transferring power by electromagnetic induction to a portable device, the portable device being separable from the power transfer unit, the power transfer unit comprising: at least two coils, an electrical driver for providing current to each coil wherein the electrical driver may provide at least first and second frequencies, wherein the first frequency is different from the second frequency. This has the advantage that only a single drive is required, reducing cost, complexity and size.

The first and second frequency may be present at the same time. The first and second frequency may be combined onto a single electrical signal and fed through a single amplifier. The first and second coil may be resonant at different frequencies. The first coil may be resonant at the first frequency and the second coil may be resonant at the second frequency. Filters may be used to separate the first and second frequency.

The first and second frequency may be present at different times. The drive may provide a signal which is swept in frequency over time. The drive may be varied in frequency between the resonant frequencies of the first and second coil. The drive may be switched in frequency between the first and second frequency. The drive may be swept in frequency between the first and second frequency in a continuous or quasi continuous manner.

According to a eighth aspect of the invention there is provided a power transfer system for transferring power to a portable device, the power transfer system comprising: a power transfer unit and a portable device separable from the power transfer unit; the power transfer unit comprising: at least two coils, an electrical driver for providing current to each coil; wherein the electrical driver may provide at least first and second frequencies; wherein the first frequency is different from the second frequency. This has the advantage that only a single drive is required, reducing cost, complexity and size.

According to a ninth aspect of the invention there is provided a method of transferring power by electromagnetic induction to a portable device, the method comprising the steps of: providing at least two coils; driving each coil with a different frequency or varying the frequency applied to each coil,. This has the advantage that only a single drive is required, reducing cost, complexity and size.

The two frequencies may be present at the same time or at different times.

According to an embodiment of a tenth aspect of the present invention, there is provided apparatus for generating an electromagnetic field in a region of interest, comprising: first and second field-generating coils, each configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said second field-generating coil, wherein said first and second field-generating coils are connected such that, in use, the first said coil is driven by a coil-driving means and the second said coil is substantially undriven, whereby a magnetic field having a moving direction over time is generated in said region of interest.

It is to be understood that such apparatus generates a field having a moving direction over time by driving the first coil, and arranging the second coil such that it is undriven, the first coil magnetically coupling to the second coil during use. The field is not generated by alternately driving the first and second coils. That is, the second field-generating coil is preferably connected such that, in use, it is always substantially undriven.

Preferably, the predetermined direction of said first field-generating coil is substantially perpendicular to the predetermined direction of said second field-generating coil.

Preferably, the direction of the generated magnetic field rotates over time.

The apparatus preferably further comprises magnetic coupling means arranged, when the apparatus is in use, to magnetically couple the two coils together. The coupling means may comprise a transformer having a primary winding connected to one of said coils, and a secondary winding attached to the other one of said coils. The coupling means may comprise at least a third coil. The coupling means may comprise magnetic material. The coupling means may comprise amorphous metal.

Preferably, the first and second coils have substantially the same resonant frequency; and, in use, the first coil is preferably driven by an electrical drive signal comprising an alternating-current signal having said resonant frequency.

According to an embodiment of an eleventh aspect of the present invention, there is provided apparatus for generating an electromagnetic field in a region of interest, comprising: first and second field-generating coils having different signal response characteristics from one another and each configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said second field-generating coil; and a coil-driving means connected to both of said first and second coils and operable to output substantially the same electrical drive signal to both of said coils, whereby a magnetic field having a moving direction over time is generated in said region of interest.

The predetermined direction of said first field-generating coil is preferably substantially perpendicular to the predetermined direction of said second field-generating coil.

The first and second field-generating coils preferably have different frequency response characteristics from one another. For example, the first and second field-generating coils may have different resonant frequencies from one another. In that case, the electrical drive signal preferably comprises a signal having an intermediate frequency that is between said different resonant frequencies. Such an intermediate frequency is preferably substantially mid-way between said different resonant frequencies.

In the case that the first and second field-generating coils have different resonant frequencies from one another, the electrical drive signal may have a frequency spectrum which varies over time, and/or may comprise a signal having both of said different resonant frequencies. For example, the signal may be a two tone signal.

Apparatus embodying the aforementioned tenth and eleventh aspects of the present invention may be a power transfer unit operable to employ the generated magnetic field to transfer power inductively to a separate power-receiving unit. Apparatus embodying the aforementioned tenth and eleventh aspects of the present invention may further comprise a power transfer surface, wherein said first and second field-generating coils are arranged such that a central axis of each of those coils is substantially parallel to said power transfer surface. Such apparatus may further comprise a core, wherein said first and second field-generating coils are wrapped around said core.

According to an embodiment of a twelfth aspect of the present invention, there is provided a method of generating an electromagnetic field in a region of interest, comprising: applying a particular electrical drive signal to a first one of two field-generating coils, each said coil being configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said second field-generating coil; and not applying such an electrical drive signal to the second one of said coils, such that the first said coil is driven and the second said coil is substantially undriven, whereby a magnetic field having a moving direction over time is generated in said region of interest. Preferably, such an electrical drive signal is never applied to the second one of said coils.

According to an embodiment of a thirteenth aspect of the present invention, there is provided a method of generating an electromagnetic field in a region of interest, comprising: applying substantially the same electrical drive signal to each of first and second field-generating coils, the first and second said coils having different signal response characteristics from one another and each being configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said second field-generating coil, whereby a magnetic field having a moving direction over time is generated in said region of interest.

According to an embodiment of a fourteenth aspect of the present invention, there is provided a system comprising a first apparatus, being apparatus according to the aforementioned tenth and eleventh aspects of the present invention, and a second apparatus comprising a powerreceiving coil which, when the second apparatus is present in the region of interest, is adapted to couple with said generated field to enable the second apparatus to receive power inductively from the first apparatus.

For a better understanding of the present invention and to show how it may be carried into effect, reference shah now be made, by way of example only, to the accompanying drawings, in which: Fig. 1 is a prior art system, showing a power transfer unit and a portable device Fig. 2 shows the physical layout of the present invention Fig. 3 shows the electrical equivalent circuit of the Fig. 2 system Fig. 4 shows the present invention using transformer coupling Fig. 5 shows a transformer having a coupling of 0.5 Fig. 6 shows a coil wound around a ferrite core in which the coil axis is skewed.

Fig. 7 shows two coils wound a ferrite for two orthogonal axis, but with each coil slightly skewed.

Fig. 8 shows the two coils of Fig. 7 wound on the same ferrite coil in an embodiment of the present invention.

Fig. 9 shows simulation results for the embodiments of Fig. 4 and Fig. 8.

Fig. 10 shows coupling taking place by means of an angled strip of amorphous metal.

Fig. 11 shows an embodiment in which there are two orthogonal coils, each with a slightly different resonant frequency Fig. 12 shows the amplitude and phase response of the embodiment of Fig. 11 Fig. 13 shows an embodiment in which the two coils are driven at different frequencies.

Fig. 14 shows voltage waveforms for the voltage received in a portable device for different physical orientations.

Fig. 2 shows the physical layout of the present invention. There are two coils 101, 201 wound around a ferrite slab 100, such that they are nominally orthogonal to one another. The first coil 101 is driven by a sinusoidal current generator 203 in series with a capacitor 204. The sinusoidal current generator 203 is shown as a sinewave voltage reference 311 coupled to an electrical transconductance amplifier 312, though equally a fully integrated high current sinusoidal oscillator could be used. The electrical amplifier 312 may be a pulse-width modulation (PWM) amplifier. There maybe feedback from the current in the primary coil to the electrical amplifier in order to achieve a constant current amplitude. The capacitor is chosen to be resonant with the inductance of the coil to present a low impedance to the sinusoidal current generator 203. The second coil 201 is only electrically connected to a capacitor 205. Capacitor 205 is chosen to be resonant with the coil 201 at the frequency of the current generator 203. Also shown in Fig. 2 is a portable device, 300, comprising a coil 301 wound around a core 305. The coil 301 is electrically connected to a resonant capacitor 302 and a load 303.

Fig. 3 shows the electrical equivalent circuit of Fig. 2. The two coils 101, 201 form the primary side of a transformer. The first coil 101 is driven by the sinusoidal current generator 203, with a series resonant capacitor 204. The second coil is electrically connected to its own resonant capacitor 205. When a portable device 300 is placed in proximity to the pad, the secondary part of the transformer is formed. The secondary device has a coil 301, a capacitor 302 and a load 303. The load 303 incorporates a diode bridge for performing full wave rectification a capacitor for smoothing, a means for regulating the power and an element requiring power such as a rechargeable battery (not shown).

The two coils 101, 201 are coupled magnetically in such a way that energy is transferred from the driven coil 101 to the undriven coil 201. At resonance, the current in coil 201 will be offset by 900 to the current in coil 101. The circuit is therefore able to deliver current to both coils 101, 201 with a phase offset of 90 . The coils 101, 201 in turn generate a rotating magnetic field in a horizontal plane above the ferrite slab.

When a portable device 300 is placed with its coil axis in this plane, it will couple to the rotating field and power will be transferred to the load 303.

Fig. 4 shows an embodiment in which the two coils 101,201 are coupled by means of a transformer, 250. The transformer couples energy from the first coil 101 to the second coil. It is necessary to ensure that both coils are maintained at resonance. Therefore the additional inductance of the transformer must be taken into account when determining the capacitor values. The degree of coupling between the two circuits is also a parameter which must be controlled. By using a fixed inductor, this coupling factor can be kept constant and can be repeatable from one device to the next. Good performance is achieved with a coupling factor of approximately 0.5. One method of fabricating a transformer with this relatively low coupling factor is shown in Fig. 5. The transformer comprises a ferrite E-l core, 251, a primary winding 252 wound round one leg and a secondary winding round around the middle leg 253. Simulation results for this arrangement are shown in Fig. 9a. Very good results are obtained with a transformer having a coupling factor of 0.55, inductance lOpH, when used with coil inductances of 55 pH and resonant capacitors of l7OnF. The current in the first coil 401 is of the same amplitude and 90 out of phase with the current in the second coil 402.

When a portable device is placed on the unit, the presence of the secondary coil coupled to the primary coil may cause a shift in the reactance seen by the primary circuit. Therefore it may be necessary to take this into account when selecting the capacitor values.

In another embodiment, the coupling between the first coil 101 and the second coil 210 is achieved by misaligning the coil windings so that they are not perfectly orthogonal.

The coupling between the two coils does not actually need to be thathigh when the coils are at resonance. At resonance the only impedance in the circuit around the second coil is the resistance of the windings. Thus even with a low coupling of around 5% it is possible to transfer half the power to the second coil.

Fig. 6 shows a coil 101 wound around a ferrite core 100, such that the coil axis is skewed. The coil 101 is a continuous conductor divided into segments lOla through lOlq sequentially for illustrative purposes only. The segments lOla, lOlc, lOle, lOig, lOu, 101k, lOim 1010 and lOlq are against the top face of the core 100 and the segments lOib, bid, lOif, lOlh, lOlj, lOll, lOin and lOip are against the bottom face. Typically there would be many more turns than illustrated. Also shown for illustration purposes is a line perpendicular to the segments on the top face 103 and a line perpendicular to the segments on the bottom face 104. The resulting magnetic field is indicated by 102 and it is located in a plane above the surface of the core, in a direction parallel to the effective coil axis. The magnetic field is misaligned from the 103 by an angle 0. The drawing has been exaggerated for illustration and typically 9 would be smaller than the example shown. This drawing shows that by asymmetrically winding the coil on the core, it is possible to misalign the resulting magnetic field direction. In practice the presence of the magnetic core will tend to skew the direction of the magnetic field such that it is not solely determined by the coil axis.

Fig. 7 shows coils wound in different orientations and their resulting magnetic field direction. Fig. 7(a) shows a ferrite core 100, with a coil 101 wound in a similar manner to Fig. 6. Fig. 7(b) shows the magnetic field direction for the coil of Fig. 7(a) to be 1.50 off the y-axis. Fig 7(c) shows the same core 100, but with a coil 201 wound in a sense approximately orthogonal to the coil 101. This coil has the bottom windings parallel to the core edges rather than the top windings, though this is not critical for the invention.

It is not necessary for any segments of the coil to be parallel to the core edges. Fig. 7(d) shows the magnetic field direction for the coil of Fig. 7(c) to be 1.50 off the x-axis.

Fig. 8 shows an embodiment of the present invention. Fig. 8(a) shows the physical layout. There is a core 100, a first coil 101 and a second coil 201. The fist coil 101 is driven with an alternating current source 203 in series with a capacitor 204. There is a capacitor 205 connecting the two ends of the second coil 201. Fig. 8(b) shows the resultant magnetic field vectors 102, 202 have a mutual angle of 930 between them.

Alternating current is applied to the first coil 101 at its resonant frequency (determined by the inductance of the coil and the series capacitor 204). This coil generates an alternating magnetic field in a direction along 102. A component of this field is in a direction along 202, given by cos(93) 5%. This field couples to the second coil 201.

At resonance, the current in the second coil 201 will be 900 out of phase with the current in the first coil 101. The second coil will generate a field in a direction along 202, which is close to being orthogonal with the field of the first coil 101. Owing to the phase offset between the signals, the resultant magnetic field direction rotates through a circle over time. The portable device has a thin magnetic core, around which is wrapped a single coil. When the device is placed on the pad to receive power, the axis of the coil is parallel to the surface of the pad. The rotating field on the surface of the pad allows such a portable device to couple to the magnetic field, regardless of its orientation with respect to the pad.

Fig. 9(b) shows simulation results for the device of Fig. 8 with a 5% coupling between the first coil 101 and the second coil 201. The current in the first coil 403 is of the same amplitude as the current in the second coil 404. The two currents are close to being 900 out of phase.

Mutual angles other than 93 may be used between the first and second coils, for example 91 , 92 , 940, 95 , 100 or larger or smaller or intermediate values may be used in order to gain advantage.

Fig. 10 shows an alternative embodiment in which coupling between the two coils is facilitated using a thin strip of amorphous metal. The amorphous metal is placed at an angle on the underside of the ferrite below the coil windings. As the angle of the strip with respect to the coil windings is varied the amount of coupling is varied. The angle of the strip can therefore be trimmed to give exactly the coupling coefficient required.

Alternatively the size, shape of the amorphous metal or its proximity to the pad could be varied. The amorphous metal could be part of the pad.

In order to facilitate coupling between the first and second coils in the pad, other methods may be employed as an alternative to or in addition to the methods described above. For instance an extra set of windings may be wound on the pad. The axis of these extra windings may be at angle that is neither orthogonal nor parallel to the axes of either the first or second coils. These extra windings may be coupled to a reactive element (for an example a capacitor) in order to bring them into resonance at the operational frequency. The extra windings may or may not be used to couple to the portable device. The material itself may be of a circular, square, rectangular or other shape.

Another embodiment would be to introduce elements of magnetic material in proximity to the first and second coils into to facilitate coupling. This magnetic material may or may not have a coil or coils wound around it and such coils may or may not be connected to a reactive element such as a capacitor. The magnetic material may be located away from where the portable devices would be present, for instance on the underside of the pad. Another embodiment would be to modify the geometry of the pad, so that it was not a uniform shape to facilitate coupling.

The presence of the portable device on the secondary side of the transformer has the effect of changing the inductance of the primary side. This in turn changes the resonant frequency of the primary coils (determined by the coil inductances and their series capacitors). As the load resistance is reduced, to supply a higher current, the effective inductance of the primary coils is reduced, thereby increasing the resonant frequency.

It is advantageous to operate at a frequency above the off-load resonant frequency.

This way, as the load demands more current, the resonant peak moves closer to the frequency of operation to meet this demand.

Alternatively, the load-current itself may be dynamically adjusted to change the inductance of the primary coils. As more portable devices are added, shifting the resonant frequency, the load current of each device can be altered to maintain the primary coils at resonance.

In a further embodiment there are three coils, with an angle of 60 separating the axis of one coil with respect to each of the other two coils. These coils may be wrapped around a circular core. Each coil may be connected to a reactive element. The coils may each have different reactive elements, such that they have slightly different resonant frequencies to obtain the appropriate phase angle in their respective currents.

With a three phase system, the three signals may be generated by one of the many circuits used to drive three-phase motors. This scheme may be extended to any number of coils.

In a further embodiment, coupling between coils takes place through the vertical fields that are generated by each coil. The vertical field is generated at the corners of each coil because the array of conductive elements is not infinite in extent. It is possible that the vertical field from one set of conductors can couple to the field in the other set of conductors. With perfect symmetry this will not occur. However by adding metallic or magnetic elements in proximity to the vertical field region, it is possible to disrupt the

symmetry to facilitate field coupling.

In a further embodiment, the coupling between the coils takes place capacitively. The capacitance may be the result of the physical structure, such as capacitance between the coil windings or additional discrete capacitors may be used. Alternatively the coils could be coupled together electrically, but via an electrical network which introduces the required phase shift. Such a network could comprise inductors, capacitors and resistors.

It may be desirable to use a number of coupling methods described in combination in order to achieve the desired coupling coefficient.

An alternative solution to removing one of the electrical driver circuits is shown in Fig. 11. Here there is a high power sinusoidal voltage generator 610 (though a current source could be used) connected to the first coil 101 in series with a capacitor 601 and a resistor 603. The signal generator 610 comprises a sinusoidal voltage source 611 and a high power electrical amplifier 612. The second coil 201 is also electrically connected to the signal generator 610, in series with capacitor 604 and resistor 606.

The values are chosen such that the first coil 101 is resonant (with capacitor 601) at a frequency slightly lower than the signal generator 610 and the second coil 201 is resonant (with capacitor 604) at a frequency slightly higher than the signal generator 610. The electrical responses are illustrated in Fig. 12. The current amplitude response of the first coil is shown by 701 and the current amplitude of the second coil is shown by 702. The operating frequency is marked by an arrow 704. The phase of the current leads the voltage at frequencies below resonance and current lags the voltage above resonance. The operating frequency 704 is above the resonant frequency of the first coil 701, at a point such that the current through coil 101 lags the voltage source by 45 . Conversely, the operating frequency 704 is below the resonant frequency of the second coil 702, at a point such that the current through coil 201 leads the voltage source by 45 . The differential phase between the current in the first coil 101 and the current in the second coil 201 is illustrated by 703. At the operating frequency, there is a 900 phase difference between the currents in the coils 101,201, as required. Further, at the operating frequency the amplitudes are the same. There is a reasonable tolerance around the operating frequency where the phase difference is close enough to 90 for adequate operation. It may desirable for the phase difference at the operating frequency to be chosen such that it is greater than 90 to improve the overall tolerance on operating frequency. The purpose of the resistors 603 and 606 is merely to broaden out the resonant peak, such that there is a more gradual change in phase and amplitude with frequency. In practise the resistance of the coil windings may be sufficient, without the need for extra resistors.

An alternative solution to removing one of the high power electrical driver circuits is shown in Fig. 13. In this embodiment, the two coils 101, 201 are driven at different frequencies. The two frequencies are generated by a dual frequency source 801. This comprises two digital sine-wave generators 802 at frequency w1 and 803 at frequency w2. These are combined with combiner 804 and converted into an analogue signal with a digital to analogue converter 805. The analogue signal is amplified via a high power amplifier 806. The high power amplifier 806 is electrically connected to the first coil 101 with capacitor 810 in series. It is also connected to the second coil 201 with capacitor 812 in series. The capacitor and inductor values are chosen such that the combination of 101 and 810 is resonant at frequency w1 and the combination of 201 and 812 is resonant at angular frequency w2. As a result, the first coil 101 will predominantly have alternating current at frequency w1, whilst the second coil 201 will predominantly have current at angular frequency w2. Driving the coils at two different frequencies provides an alternative to driving the coils in quadrature. It is not possible to combine two quadrature signals onto a single output, because the result will simply be the vector sum. However, it is possible to combine two different frequencies onto a single output.

The resultant vector will still rotate, such that a device may be recharged, regardless of its rotational orientation.

If a current Asin(w1t) is applied to the first coil and current Asin(w2t) is applied to the second coil, then the voltage that is seen across the coil of the portable device will be V = -k(cosOcosa.1t +sin9cosa2t) where 0 is the angle of the device coil axis with respect to the first coil axis and k is a constant. The voltages seen across the coil of the secondary device are shown in Fig. 14 for different values of 0. At 0=00, the secondary coil simply sees a voltage at the same frequency, w1 as the current applied the first coil 101, shown 901. At 0=900, the secondary coil sees frequency w2, applied to the second coil, shown 902. However at 0=45 the voltage is J' =_2kcos(' _W2) os101 +W2) This waveform 903 has a frequency equal to the average of the frequencies w1 and w2, with a lower frequency envelope at half the difference in frequencies. The higher frequency signally is fully modulated by the envelope, such that its amplitude is reduced zero at the minima. The voltage from the secondary coil would typically by rectified by a full-wave or half-wave rectifier and smoothed by a capacitor to produce a near steady voltage signal. With the dual frequency approach, the smoothing capacitor needs to be larger, because it is determined not by w1 or w2, but by half the difference frequency.

At intermediate angles, 904. the higher frequency is no longer the average of the two frequencies, but closer to one or the other. The amount of modulation is less, such that the amplitude is no longer reduced to zero.

There is a trade-off in the choice of frequencies w1 and ui2. The two frequencies need to be sufficiently far apart, to enable the coil resonant responses 701 and 702 to be sufficiently separated that one frequency is predominantly directed to the first foil and the other frequency is predominantly directed to the second coil. They also need to be reasonably separated to minimise the size of the smoothing capacitor in the regulator, as this can have a considerable contribution to the cost and size of the power receiving module. However, both frequencies must lie within the resonance of the coil in the portable device as the device must be able to receive power at both frequencies. It may be necessary to have a broader, lower Q resonance in the coil within the secondary device. Alternatively it may be better to have as high Q as possible and use additional filters on the inputs to the coils 101, 201. These could be passive LC filters.

Instead of generating the two frequencies simultaneously in the generator 801, a single frequency may be generated which is varied over time. The frequency could be continuously or quasi-continuously swept from w1 to w2 and back again. By making the resonances of the responses 701, 702 overlapping, the proportion of power distributed to each coil would vary with time, such that the resultant angle varies with time. In this way power can be delivered regardless of the orientation of the secondary device.

Although this embodiment shows pure sinusoidal tones, this does not have to be the case. Instead, a broadband noise source could be applied, so that a range of frequencies are present simultaneously. Equally a source that emits frequency that varies pseudo-randomly on a short timescale could be used. There is a wide range of random and deterministic signals that could be used. Either digital or analogue techniques can be used to generate the signal.

Although the embodiments of the invention have shown the applicants horizontal field power transfer system, all embodiments of the invention could equally be applied to any structure requiring different phases of current in different coils. In particular there are other power transfer systems that use a vertical coil system, such as that described in PCT/AU2003/00721, filed on 10th June 2003. This system uses arrays of flat coils to generate a vertical field above the surface. It also uses multiple layers of coil arrays to improve the uniformity of the field. The invention could be applied to such a system to provide different phases of current to different layers or to different groups of coils or to individual coils.

In all the embodiments of the invention, the signals applied to each coil may be current or voltage signals. They may be pure sinusoidal tones, square wave, filtered square waves, or other periodic functions. They may digitally or analogue generated.

Claims

CLAIMS: 1. Apparatus for generating an electromagnetic field in a

region of interest, comprising: first and second field-generating coils, each configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said second field-generating coil, wherein said first and second field-generating coils are connected such that, in use, the first said coil is driven by a coil-driving means and the second said coil is substantially undriven, whereby a magnetic field having a moving direction over time is generated in said region of interest.

2. Apparatus according to claim 1, wherein said second field-generating coil is connected such that, in use, it is always substantially undriven.

3. Apparatus according to claim 1 or 2, wherein the predetermined direction of said first field-generating coil is substantially perpendicular to the predetermined

direction of said second field-generating coil.

4. Apparatus according to any preceding claim, wherein the direction of the

generated magnetic field rotates over time.

5. Apparatus according to any preceding claim, further comprising: magnetic coupling means arranged, when the apparatus is in use, to magnetically couple the two coils together.

6. Apparatus according to claim 5, wherein said magnetic coupling means comprises a transformer having a primary winding connected to one of said coils, and a secondary winding attached to the other one of said coils.

7. Apparatus according to claim 5 or 6, wherein said magnetic coupling means comprises at least a third coil.

8. Apparatus according to any one of claims 5 to 7, wherein said magnetic coupling means comprises magnetic material.

9. Apparatus according to any one of claims 5 to 8, wherein said magnetic coupling means comprises amorphous metal.

10. Apparatus according to any preceding claim, wherein: said first and second coils have substantially the same resonant frequency; and in use, said first coil is driven by an electrical drive signal comprising an alternating-current signal having said resonant frequency.

11. Apparatus for generating an electromagnetic field in a region of interest, comprising: first and second field-generating coils having different signal response characteristics from one another and each configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said second field-generating coil; and a coil-driving means connected to both of said first and second coils and operable to output substantially the same electrical drive signal to both of said coils, whereby a magnetic field having a moving direction over time is generated in said region of interest.

12. Apparatus according to claim 11, wherein the predetermined direction of said first field-generating coil is substantially perpendicular to the predetermined direction of

said second field-generating coil.

13. Apparatus according to claim 11 or 12, wherein the direction of the generated

magnetic field rotates over time.

14. Apparatus according to any one of claims 11 to 13, wherein said first and second field-generating coils have different frequency response characteristics from one another.

15. Apparatus according to any one of claims 1110 14, wherein: said first and second field-generating coils have different resonant frequencies from one another; and said electrical drive signal comprises a signal having an intermediate frequency that is between said different resonant frequencies.

16. Apparatus according to claim 15, wherein said intermediate frequency is substantially mid-way between said different resonant frequencies.

17. Apparatus according to any one of claims 11 to 14, wherein: said first and second field-generating coils have different resonant frequencies from one another; and said electrical drive signal has a frequency spectrum which varies over time.

18. Apparatus according to any one of claims 1110 14, wherein: said first and second field-generating coils have different resonant frequencies from one another; and said electrical drive signal comprises a signal having said different resonant frequencies.

19. Apparatus according to any preceding claim being a power transfer unit operable to employ the generated magnetic field to transfer power inductively to a separate power-receiving unit.

20. Apparatus according to any preceding claim, further comprising a power transfer surface, wherein said first and second field-generating coils are arranged such that a central axis of each of those coils is substantially parallel to said power transfer surface.

21. Apparatus according to any preceding claim, further comprising a core, wherein said first and second field-generating coils are wrapped around said core.

22. A method of generating an electromagnetic field in a region of interest, comprising: applying a particular electrical drive signal to a first one of two field-generating coils, each said coil being configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said

second field-generating coil; and

not applying such an electrical drive signal to the second one of said coils, such that the first said coil is driven and the second said coil is substantially undriven, whereby a magnetic field having a moving direction over time is generated in said region of interest.

23. A method according to claim 22, comprising never applying such an electrical drive signal to the second one of said coils.

24. A method of generating an electromagnetic field in a region of interest, comprising: applying substantially the same electrical drive signal to each of first and second field-generating coils, the first and second said coils having different signal response characteristics from one another and each being configured so that if driven individually the coil generates an electromagnetic field having field lines which extend generally in a predetermined direction across said region of interest, said predetermined direction of said first field-generating coil being substantially different from said predetermined direction of said second field-generating coil, whereby a magnetic field having a moving direction over time is generated in said region of interest.

25. A system comprising a first apparatus, being apparatus as claimed in any one of claims I to 21, and a second apparatus comprising a power-receiving coil which, when the second apparatus is present in the region of interest, is adapted to couple with said generated field to enable the second apparatus to receive power inductively from the first apparatus.

26. Apparatus for generating an electromagnetic field substantially as hereinbefore described with reference to the accompanying figures 2 to 14.

27. A method of generating an electromagnetic field substantially as hereinbefore described with reference to the accompanying figures 2 to 14.

28. A system for transferring power by electromagnetic induction substantially as hereinbefore described with reference to the accompanying figures 2 to 14.