Detecting foreign objects in wireless power transfer systems

1. A method of performing foreign object detection in a wireless power transmission system, the method comprising:

performing a plurality of FOD (Foreign Object Detection) measurements at different times;

determining whether the plurality of FOD measurements converge; and

when the plurality of FOD measurements converge, determining whether to enable or disable wireless power transfer based on one or more FOD measurements of the plurality of FOD measurements.

2. The method of claim 1 , wherein determining whether the plurality of FOD measurements converge comprises determining a minimum, average and/or median of the plurality of FOD measurements.

3. The method of claim 1 , further comprising discarding one or more FOD measurements of the plurality of FOD measurements and determining whether or not to disable wireless power transfer based on one or more other FOD measurements of the plurality of FOD measurements.

4. The method of claim 1 , wherein a number of the plurality of FOD measurements is based upon whether the plurality of FOD measurements converge.

5. The method of claim 1 , wherein the plurality of FOD measurements are Q-factor measurements or self-resonant frequency measurements.

6. An apparatus for performing foreign object detection, the apparatus comprising:

circuitry configured to:

perform a plurality of FOD (Foreign Object Detection) measurements at different times;

determine whether the plurality of FOD measurements converge; and

when the plurality of FOD measurements converge, determine whether to enable or disable wireless power transfer based on one or more FOD measurements of the plurality of FOD measurements.

7. The apparatus of claim 6 , wherein the circuitry comprises:

a drive circuit configured to energize a matching network and transmit coil of a wireless power transmitter; and

a controller configured to perform the plurality of FOD measurements at least in part by controlling the drive circuit, process the plurality of FOD measurements to obtain a FOD measurement result and determine whether to enable or disable wireless power transfer based on the FOD measurement result.

8. The apparatus of claim 7 , wherein the controller is configured to process the plurality of FOD measurements at least in part by determining a minimum, average and/or median of the plurality of FOD measurements.

9. The apparatus of claim 7 , wherein the controller is configured to discard one or more FOD measurements of the plurality of FOD measurements and determine whether or not to disable wireless power transfer based on one or more other FOD measurements of the plurality of FOD measurements.

10. The apparatus of claim 6 , wherein a number of the plurality of FOD measurements is based upon whether the plurality of FOD measurements converge.

11. The apparatus of claim 6 , wherein the plurality of FOD measurements are Q-factor measurements or self-resonant frequency measurements.

12. A method of performing foreign object detection in a wireless power transmission system having a wireless power transmitter and a wireless power receiver, the method comprising:

receiving, by the wireless power transmitter, a reference value from the wireless power receiver, the reference value being a reference Q-factor or self-resonant frequency;

the wireless power transmitter determining an expected Q-factor or self-resonant frequency for the wireless power transmitter-receiver pair based on the reference value and one or more characteristics of the wireless power transmitter;

the wireless power transmitter setting a foreign object detection (FOD) threshold based on the expected Q-factor or self-resonant frequency for the wireless power transmitter-receiver pair;

the wireless power transmitter performing an FOD measurement in the presence of the wireless power receiver to obtain an FOD measurement result;

the wireless power transmitter determining whether to enable or disable wireless power transfer based on a comparison of the FOD measurement result and the FOD threshold; and

the wireless power transmitter performing wireless power transfer to the wireless power receiver when the determination is made to enable wireless power transfer.

13. The method of claim 12 , wherein the setting of the FOD threshold is based on a stored relationship between Q-factors or self-resonance frequencies for the transmitter-receiver pair and FOD thresholds.

14. An apparatus for performing foreign object detection in a wireless power transmission system having a wireless power transmitter and a wireless power receiver, the method comprising:

circuitry configured to:

receive a reference value from the wireless power receiver, the reference value being a reference Q-factor or self-resonant frequency;

determine an expected Q-factor or self-resonant frequency for the wireless power transmitter-receiver pair based on the reference value and one or more characteristics of the wireless power transmitter;

set a foreign object detection (FOD) threshold based on the expected Q-factor or self-resonant frequency for the wireless power transmitter-receiver pair;

perform an FOD measurement in the presence of the wireless power receiver to obtain an FOD measurement result;

determine whether to enable or disable wireless power transfer based on a comparison of the FOD measurement result and the FOD threshold; and

perform wireless power transfer to the wireless power receiver when the determination is made to enable wireless power transfer.

15. The apparatus of claim 14 , wherein the circuitry is configured to set the FOD detection result based on a stored relationship between Q-factors or self-resonance frequencies for the transmitter-receiver pair and FOD thresholds.

16. A method of performing foreign object detection (FOD) prior to initiating wireless power transfer from a wireless power transmitter to a wireless power receiver, the method comprising, by the wireless power transmitter:

performing a first FOD measurement;

determining whether the first FOD measurement is within a range for initiating wireless power transfer;

making a first attempt to communicate with the wireless power receiver when the first FOD measurement is determined to be in range for initiating wireless power transfer,

performing a second FOD measurement when the first attempt to communicate with the wireless power receiver is successful;

determining whether the second FOD measurement is within a range for initiating wireless power transfer;

making a second attempt to communicate with the wireless power receiver when the second FOD measurement is determined to be in a range for initiating wireless power transfer; and

initiating wireless power transfer when the second attempt is successful.

17. An apparatus for performing foreign object detection (FOD) prior to initiating wireless power transfer from a wireless power transmitter to a wireless power receiver, the apparatus comprising:

circuitry configured to control the wireless power transmitter to:

perform a first FOD measurement;

determine whether the first FOD measurement is within a range for initiating wireless power transfer;

make a first attempt to communicate with the wireless power receiver when the first FOD measurement is determined to be in range for initiating wireless power transfer,

perform a second FOD measurement when the first attempt to communicate with the wireless power receiver is successful;

determine whether the second FOD measurement is within a range for initiating wireless power transfer;

make a second attempt to communicate with the wireless power receiver when the second FOD measurement is determined to be in a range for initiating wireless power transfer; and initiate wireless power transfer when the second attempt is successful.