Inductive power transfer system and method

1. A foreign object detection apparatus comprising a control adapted to detect the presence of a foreign object on or adjacent to a primary magnetic structure of an Inductive Power Transfer (IPT) system, the control configured to place a secondary magnetic structure of the IPT system under a no-load condition and detect the no-load current of the secondary magnetic structure to detect the presence of the foreign object, wherein at least one of:

the apparatus is adapted to measure the phase and magnitude of the no-load current in the secondary magnetic structure to determine the lateral offset of the primary and secondary magnetic structures;

the control is configured to detect a foreign object on or adjacent the primary magnetic structure from a reduction in the no-load current in the secondary magnetic structure; or

the apparatus is configured to detect VAR currents in the primary magnetic structure, estimate a component of the detected VAR currents that is attributable to misalignment of the primary magnetic structure and the secondary magnetic structure, and determine that the foreign object is on or adjacent the primary pad when the detected VAR currents deviate from an expected range of VAR currents.

2. The apparatus as claimed in claim 1 wherein the control is adapted to selectively reduce power transferred by the system on detection of the foreign object.

3. The apparatus as claimed in claim 1 wherein the apparatus is adapted to measure the phase and magnitude of the no-load current in the secondary magnetic structure to determine the lateral offset of the primary and secondary magnetic structures.

4. The apparatus as claimed in claim 1 wherein the control is configured to detect the foreign object on or adjacent the primary magnetic structure from a reduction in the no-load current in the secondary magnetic structure.

5. The apparatus as claimed in claim 1 wherein the control is configured to detect the foreign object on or adjacent the primary magnetic structure from changes in both the no-load current of the secondary magnetic structure and current supplied to the primary magnetic structure when the secondary magnetic structure is in the no-load condition.

6. The apparatus as claimed in claim 1 wherein the control is configured to detect the foreign object on or adjacent the primary magnetic structure from changes in the no-load current of the secondary magnetic structure.

7. The apparatus as claimed in claim 1 wherein the apparatus is configured to detect VAR currents in the primary magnetic structure, estimate a component of the detected VAR currents that is attributable to misalignment of the primary magnetic structure and the secondary magnetic structure, and determine that the foreign object is on or adjacent the primary pad when the detected VAR currents deviate from an expected range of VAR currents.

8. A foreign object detection apparatus comprising a control adapted to detect the presence of a foreign object on or adjacent to a primary magnetic structure of an Inductive Power Transfer (IPT) system, the control configured to place a secondary magnetic structure of the IPT system under a no-load condition and detect the no-load current of the secondary magnetic structure to detect the presence of the foreign object, wherein the control detects the foreign object when the secondary is in the no-load condition by at least:

determining an expected rate of power transfer;

determining the actual rate of power transfer; and

comparing the expected rate of power transfer with the actual rate of power transfer.

9. The apparatus as claimed in claim 8 further comprising determining if the actual rate of power transfer is less than the expected rate of power transfer by more than a predetermined amount.

10. The apparatus as claimed in claim 8 wherein the apparatus is configured to compare relative electrical characteristics of the primary magnetic structure and the secondary magnetic structure, when the secondary magnetic structure is in the no-load condition, to determine relative position between the primary magnetic structure and the secondary magnetic structure.

11. The apparatus as claimed in claim 8 wherein the apparatus is configured to compare the magnitude of current in the primary magnetic structure to the magnitude of current in the secondary magnetic structure, and determine the separation between the primary magnetic structure and the secondary magnetic structure from the relative magnitude of current in the primary magnetic structure and secondary magnetic structure.

12. The apparatus as claimed in claim 8 wherein the control is configured to detect a foreign object on or adjacent to the primary magnetic structure from an increase in the current supplied to the primary magnetic structure when the secondary magnetic structure is in the no-load condition.

13. The apparatus as claimed in claim 8 wherein the control is configured to detect a foreign object on or adjacent the primary magnetic structure from changes in current supplied to the primary magnetic structure when the secondary magnetic structure is in the no-load condition.

14. The apparatus as claimed in claim 8 wherein the apparatus is configured to short circuit a parallel tuned pick-up coil of the secondary magnetic structure to place the secondary magnetic structure in the no-load condition.

15. The apparatus as claimed in claim 8 wherein the apparatus is configured to measure AC current in at least one pick-up coil of the secondary magnetic structure prior to rectification.

16. A method of operating an Inductive Power Transfer (IPT) system comprising a primary magnetic structure and a secondary magnetic structure to detect the presence of a foreign object adjacent to the primary magnetic structure, the method comprising placing the secondary magnetic structure under a no-load condition and detecting the no-load current of the secondary magnetic structure to detect the presence of the foreign object, wherein the method further comprises at least one of:

determining that a foreign object is on or adjacent the primary magnetic structure from a reduction in the no-load current in the secondary magnetic structure; or

measuring the phase and magnitude of no-load current in the secondary magnetic structure, and estimating the relative position of the secondary magnetic structure and the primary magnetic structure from the measured no-load current.

17. A method as claimed in claim 16 further comprising:

determining an expected rate of power transfer;

determining the actual rate of power transfer; and

comparing the expected rate of power transfer with the actual rate of power transfer.

18. The method as in claimed in claim 17 further comprising determining if the actual rate of power transfer is less than the expected rate of power transfer by more than a predetermined amount.

19. The method as claimed in claim 16 wherein the method comprises measuring the phase and magnitude of no-load current in the secondary magnetic structure, and estimating the relative position of the secondary magnetic structure and the primary magnetic structure from the measured no-load current.

20. The method as claimed in claim 16 wherein the method comprises comparing the magnitude of current in the primary magnetic structure to the magnitude of current in the secondary magnetic structure, and determining the separation between the primary magnetic structure and the secondary magnetic structure from the relative magnitude of current in the primary magnetic structure and the secondary magnetic structure.

21. The method as claimed in claim 16 comprising determining that the foreign object is on or adjacent the primary magnetic structure from the reduction in the no-load current in the secondary magnetic structure.

22. The method as claimed in claim 16 comprising determining that the foreign object is on or adjacent the primary magnetic structure from an increase in current supplied to the primary magnetic structure when the secondary magnetic structure is in the no-load condition.

23. The method as claimed in claim 16 comprising determining that the foreign object is on or adjacent the primary magnetic structure from changes in both the no-load current of the secondary magnetic structure and current supplied to the primary magnetic structure when the secondary magnetic structure is in the no-load condition.

24. The method as claimed in claim 16 wherein the method comprises shorting a pick-up coil of the secondary magnetic structure to place the secondary magnetic structure in the no-load condition.

25. The method as claimed in claim 16 wherein the method comprises detecting an AC current induced in a pick-up coil of the secondary magnetic structure when the secondary magnetic structure is under the no-load condition.

26. A method comprising periodically shorting the coil of a resonant secondary pad, while the secondary pad is receiving power from a primary pad, to detect the presence of a foreign object on or adjacent the primary pad, wherein the method comprises detecting at least one of a reduction in short circuit current in the coil secondary pad, or an increase in current supplied to a coil of the primary pad when the secondary coil is shorted.

27. The method of claim 26 , wherein the method comprises measuring an induced AC current in the coil of the secondary pad when the coil is shorted.

28. The method of claim 26 , wherein the method comprises measuring VAR currents in the primary pad, and determining that a foreign object is on or adjacent the primary pad from deviation of the measured VAR currents from an expected VAR current.

29. The method of claim 28 , wherein the method comprises determining a component of the VAR currents measured in the primary pad that is attributable to misalignment of the primary pad and the secondary pad.

30. The method of claim 26 , wherein the method comprises detecting a reduction in short circuit current in the coil secondary pad.

31. The method of claim 26 , wherein the method comprises detecting an increase in current supplied to a coil of the primary pad when the secondary coil is shorted.