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A power pick-up for an Inductively Coupled Power Transfer (ICPT) system is provided having a resonant pick up circuit. The natural frequency of the pick-up circuit may be varied by controlling the conductance or capacitance of a variable reactive in the resonant circuit. The load being supplied by t

A power pick-up for an Inductively Coupled Power Transfer (ICPT) system is provided having a resonant pick up circuit. The natural frequency of the pick-up circuit may be varied by controlling the conductance or capacitance of a variable reactive in the resonant circuit. The load being supplied by the pick-up circuit is sensed, and the effective capacitance or inductance of the variable reactive component is controlled to vary the natural resonant frequency of the pick-up circuit to thereby control the power flow into the pick-up to satisfy the power required by the load.

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1. An inductively coupled power transfer pick-up comprising: a pick-up resonant circuit comprising a capacitive element and an inductive element adapted to receive power from a magnetic field associated with a primary conductive path to supply a load, one of the capacitive element and the inductive

1. An inductively coupled power transfer pick-up comprising: a pick-up resonant circuit comprising a capacitive element and an inductive element adapted to receive power from a magnetic field associated with a primary conductive path to supply a load, one of the capacitive element and the inductive element comprising a controlled reactive element;a phase device configured to sense the phase of a voltage or current in the pick-up resonant circuit;a sensor configured to sense a power requirement of the load; anda controller configured to selectively tune or de-tune the pick-up resonant circuit in response to the load power requirement sensed by the sensor by selectively electrically connecting or disconnecting the controlled reactive element to or from the pick-up resonant circuit in each cycle of the voltage or current dependent on the sensed phase to vary the effective capacitance or inductance of the controlled reactive element of the pick-up resonant circuit to control the transfer of power to the pick-up resonant circuit dependant on the sensed load power requirement. 2. The inductively coupled power transfer pick-up as claimed in claim 1, wherein the controlled reactive element comprises a switching device configured to allow the controlled reactive element to be selectively electrically connected to the pick-up resonant circuit. 3. The inductively coupled power transfer pick-up as claimed in claim 2, wherein the controller is operable to control the switching device so that the apparent capacitance or inductance of the controlled reactive element is varied to thereby tune or detune the pick-up resonant circuit. 4. The inductively coupled power transfer pick-up as claimed in claim 3, wherein the controlled reactive element comprises a pick-up coil or is connected in parallel with the pick-up coil. 5. The inductively coupled power transfer pick-up as claimed in claim 2, wherein the controller actuates the switching device to allow the controlled reactive element to be electrically connected to or disconnected from the pick-up resonant circuit dependant on the sensed phase. 6. The inductively coupled power transfer pick-up as claimed in claim 5, wherein: the controlled reactive element comprises an inductor,the phase device senses a voltage in the pick-up resonant circuit, andthe controller is operable to switch the switching device to electrically connect or disconnect the inductor to or from the pick-up resonant circuit a predetermined time period after a sensed voltage zero crossing. 7. The inductively coupled power transfer pick-up as claimed in claim 6, wherein the controller is adapted to activate the switching device to connect the inductor to the pick-up resonant circuit after the predetermined time period following a voltage zero crossing has elapsed, and further adapted to allow the switching device to be deactivated when the voltage again reaches substantially zero. 8. The inductively coupled power transfer pick-up as claimed in claim 6, wherein the controller is capable of varying the predetermined time period between substantially 0 electrical degrees and substantially 180 electrical degrees. 9. The inductively coupled power transfer pick-up as claimed in claim 6 wherein the controller is capable of varying the predetermined time period between substantially 90 electrical degrees and substantially 150 electrical degrees. 10. The inductively coupled power transfer pick-up as claimed in claim 6 wherein the inductor is connected in parallel with a tuning capacitor of the pick-up resonant circuit. 11. The inductively coupled power transfer pick-up as claimed in claim 6, wherein the inductor comprising two terminals, and wherein the switching device comprises at least two controllable semiconductor switching elements, a respective semiconductor switching element being connected between each terminal and the pick-up resonant circuit. 12. The inductively coupled power transfer pick-up as claimed in claim 11 wherein each switching element comprises an anti-parallel diode connected thereacross. 13. The inductively coupled power transfer pick-up as claimed in claim 11 wherein the semiconductor switch elements comprises at least one of IGBT's, MOSFETS, MCT's, and BJT's. 14. The inductively coupled power transfer pick-up as claimed in claim 5, wherein: the controlled reactive element comprises a capacitor,the phase sensing device senses a voltage in the pick-up resonant circuit, andthe controller is operable to switch the switching device to electrically connect or disconnect the capacitor to or from the pick-up resonant circuit in a predetermined time period after a sensed voltage zero crossing. 15. The inductively coupled power transfer pick-up as claimed in claim 14, further comprising: a frequency sensing device configured to sense the frequency of the pick-up resonant circuit, andwherein the controller actuates the switching device to allow the capacitor to be electrically connected to or disconnected from the pick-up resonant circuit dependant on the sensed frequency to alter the natural resonant frequency of the pick-up resonant circuit. 16. The inductively coupled power transfer pick-up as claimed in claim 14, wherein: the phase sensing device senses the frequency of the pick-up resonant circuit; andthe controller actuates the switching device to allow the capacitor to be electrically connected to or disconnected from the pick-up resonant circuit dependant on the sensed frequency to alter the natural resonant frequency of the pick-up resonant circuit. 17. The inductively coupled power transfer pick-up as claimed in claim 14 wherein the controller is adapted to activate the switching device to disconnect the capacitor from the pick-up resonant circuit after the predetermined time period following a voltage zero crossing has elapsed. 18. The inductively coupled power transfer pick-up as claimed in claim 14 wherein the controller is capable of varying the predetermined time period between substantially 0 electrical degrees and substantially 90 electrical degrees. 19. The inductively coupled power transfer pick-up as claimed in claim 14 wherein the capacitor is connected in parallel with a tuning capacitor of the pick-up resonant circuit. 20. The inductively coupled power transfer pick-up as claimed in claim 19 wherein a capacitance of the capacitor is substantially equal to a capacitance of the tuning capacitor. 21. The inductively coupled power transfer pick-up as claimed in claim 14 wherein: the capacitor comprises two terminals, andthe switching device comprises two controllable semiconductor switching elements, a respective semiconductor switching element being connected between each terminal and the pick-up resonant circuit. 22. The inductively coupled power transfer pick-up as claimed in claim 21 wherein each switching element comprises an anti-parallel diode connected thereacross. 23. The inductively coupled power transfer pick-up as claimed in claim 21 wherein the semiconductor switch elements comprise at least one of IGBT's, MOSFETS, and BJT's. 24. The inductively coupled power transfer pick-up as claimed in claim 14, wherein the capacitor comprises the tuning capacitor of the pick-up resonant circuit. 25. The inductively coupled power transfer pick-up as claimed in claim 2, further comprising: a frequency sensing device configured to sense the frequency of the pick-up resonant circuit whereby the controller actuates the switching device to allow the controlled reactive element to be electrically connected to or disconnected from the pick-up resonant circuit dependant on the sensed frequency to alter the natural resonant frequency of the pick-up resonant circuit. 26. The inductively coupled power transfer pick-up as claimed in claim 2, wherein: the phase sensing device senses the frequency of the pick-up resonant circuit, andthe controller actuates the switching device to allow the controlled reactive element to be electrically connected to or disconnected from the pick-up resonant circuit dependant on the sensed frequency to alter the natural resonant frequency of the pick-up resonant circuit. 27. An inductively coupled power transfer system comprising: a power supply comprising a resonant converter to provide alternating current to a primary conductive path of the inductively coupled power transfer system;one or more inductively coupled power transfer system pick-up devices, each of said pick-up devices comprising a pick-up resonant circuit comprising: a capacitive element; andan inductive element adapted to receive power from a magnetic field associated with a primary conductive path to supply a load;a phase device configured to sense the phase of a voltage or current in the pick-up resonant circuit;a sensor configured to sense a power requirement of the load; anda controller configured to selectively tune or de-tune the pick-up resonant circuit in response to the load power requirement sensed by the sensor by selectively electrically connecting or disconnecting the controlled reactive element to or from the pick-up resonant circuit in each cycle of the voltage or current dependent on the sensed phase to vary the effective capacitance or inductance of the controlled reactive element of the pick-up resonant circuit to control the transfer of power to the pick-up resonant circuit dependant on the sensed load power requirement. 28. The inductively coupled power transfer system as claimed in claim 27 wherein the primary conductive path comprises one or more turns of electrically conductive material. 29. The inductively coupled power transfer system as claimed in claim 28 wherein the primary conductive path is provided beneath a substantially planar surface. 30. The inductively coupled power transfer system as claimed in claim 27 wherein the primary conductive path comprises at least one region about which there is a greater magnetic field strength than one or more other regions of the path. 31. The inductively coupled power transfer system as claimed in claim 27 wherein the primary conductive path comprises one or more lumped inductances or one or more distributed inductances. 32. The inductively coupled power transfer system as claimed in claim 27 wherein the primary conductive path is mounted adjacent to an amorphous magnetic material to provide a desired magnetic flux path. 33. The inductively coupled power transfer system as claimed in claim 27 wherein the pick-up resonant circuit comprises an amorphous magnetic material adjacent to the pick-up coil to provide a desired magnetic flux path. 34. The inductively coupled power transfer system as claimed in claim 27 wherein the pick-up resonant circuit is battery-free. 35. The inductively coupled power transfer system as claimed in claim 27 wherein the pick-up resonant circuit comprises a super-capacitor. 36. A method for controlling power drawn by an inductively coupled power transfer pick-up, the method comprising the steps of: sensing the phase of a voltage or current in a pick-up resonant circuit;sensing a power requirement of a load supplied by the pick-up resonant circuit; andselectively tuning or detuning the pick-up resonant circuit in response to the power requirement sensed by the sensor by selectively electrically connecting or disconnecting the controlled reactive element to or from the pick-up resonant circuit in each cycle of the voltage or current dependent on the sensed phase to vary the effective capacitance or inductance of a controlled reactive element of the pick-up resonant circuit to thereby control the transfer of power to the pick-up resonant circuit dependant on the sensed load power requirement. 37. A method as claimed in claim 36 wherein the step of tuning or detuning the pickup resonant circuit comprises the step of moving a resonant frequency of the pick-up resonant circuit toward or away from a tuned condition. 38. A method as claimed in claim 36 further comprising the step of sensing a frequency of a current or voltage in the pick-up resonant circuit. 39. A method as claimed in claim 38 further comprising the steps of: comparing the sensed frequency with a nominal frequency for the pick-up resonant circuit; andtuning or de-tuning toward or away from a nominal frequency dependant on the sensed load. 40. A method as claimed in claim 36 further comprising the step of: selectively switching the controlled reactive element into or out of the pick-up resonant circuit to the effective inductance or capacitance of the controlled reactive element to thereby tune or de-tune the pick-up resonant circuit. 41. A method as claimed in claim 40, further comprising the steps of: sensing a phase of a voltage; andelectrically connecting the controlled reactive element to the pick-up resonant circuit in a predetermined time period after a sensed voltage zero crossing. 42. A method as claimed in claim 40 further comprising the steps of: sensing the frequency of the pick-up resonant circuit; andactivating a switching device to electrically connect or disconnect the controlled reactive element to or from the pick-up resonant circuit dependant on the sensed frequency to alter the natural resonant frequency of the pick-up resonant circuit. 43. A method as claimed in claim 40 further comprising the steps of: comparing the sensed frequency with a nominal frequency; andvarying the predetermined time period to tune the pick-up resonant circuit toward or away from the nominal frequency. 44. A method as claimed in claim 40, further comprising the steps of: activating a switching device to connect the controlled reactive element to the pick-up resonant circuit after the predetermined time period following a voltage zero crossing has elapsed; andallowing the switching device to be deactivated when the voltage again reaches substantially zero. 45. A method as claimed in claim 40 further comprising the step of selecting the predetermined time period from a range between substantially 0 electrical degrees and substantially 180 electrical degrees. 46. A method as claimed in claim 40 further comprising the step of selecting the predetermined time period from a range between substantially 90 electrical degrees and substantially 150 electrical degrees. 47. A method as claimed in claim 40, further comprising the steps of: sensing a phase of a voltage; andelectrically disconnecting the controlled reactive element to the pick-up resonant circuit in a predetermined time period after a sensed voltage zero crossing. 48. A method as claimed in claim 47, wherein the controlled reactive element comprises a capacitor, and the predetermined time period is selected from a range between substantially 0 electrical degrees and substantially 90 electrical degrees.