Systems, methods, and apparatus for foreign object detection loop based on inductive thermal sensing

1. An apparatus for detecting a presence of an object located in a power transfer area of a wireless power transmitter, the apparatus including:

a first inductive sense circuit and a second inductive sense circuit, each including a sense coil configured to inductively sense the presence of the object based on an electrical characteristic of the sense coil that varies as a function of a temperature of the object when the object is exposed to an alternating magnetic field generated by the wireless power transmitter in the power transfer area at an operating frequency of the wireless power transmitter,

each of the first and second inductive sense circuits comprising a location for measuring the electrical characteristic and configured to reduce a disturbance voltage measured at the location; and

a controller configured to:

schedule an exposure cycle for the alternating magnetic field generated by the wireless power transmitter,

transmit a signal to the wireless power transmitter to control the exposure cycle for the alternating magnetic field,

measure the electrical characteristic of each sense coil over time based on measuring at least one of a voltage and a current signal at the location of each of the first and second inductive sense circuits,

suppress effects of the disturbance voltage at fundamental and harmonic frequencies of the wireless power transmitter by filtering to achieve a target detection sensitivity of the apparatus,

determine a first correlation result between the measured electrical characteristic and the exposure cycle for the first sense circuit based on comparing the measured electrical characteristic with the exposure cycle over at least a portion of a duration of the exposure cycle and determine a second correlation result between the measured electrical characteristic and the exposure cycle for the second sense circuit based on comparing the measured electrical characteristic with the exposure cycle over at least a portion of a duration of the exposure cycle, and

determine the presence of the object based on a comparison of the first correlation result with the second correlation result.

2. The apparatus of claim 1 , wherein each measured electrical characteristic comprises one or more of a current, a voltage, an equivalent resistance, equivalent inductance, a resonant frequency, a complex resonant frequency, a dampening factor, a Q-factor, an impedance, or a phase when the inductive sense circuit is driven by a sinusoidal signal source.

3. The apparatus of claim 1 , wherein each measured electrical characteristic comprises an impulse response of the corresponding inductive sense circuit when the inductive sense circuit is driven by a pulse signal source, or a response of the corresponding inductive sense circuit when the inductive sense circuit is driven by any of a voltage or current time function.

4. The apparatus of claim 1 , wherein the first and second inductive sense circuits each comprise a plurality of inductive sense circuits, each comprising a respective sense coil and a respective location.

5. The apparatus of claim 4 , wherein the controller is further configured to multiplex measurements of the measured electrical characteristic of the respective sense coils, to compare each of a plurality of measured electrical characteristics with the exposure cycle to determine a plurality of first and second correlation results between the measured electrical characteristic and the exposure cycle for each of the respective first and second sense circuits.

6. The apparatus of claim 5 , wherein the controller is further configured to determine the presence of the object based on the first correlation results and at least one of the second correlation results.

7. The apparatus of claim 1 , wherein the controller is further configured to determine the presence of the object when a difference between the first correlation result and the second correlation result exceeds a threshold.

8. The apparatus of claim 1 , wherein the controller is further configured to compare a derivative of each electrical characteristic over at least a portion of time of the exposure cycle with the exposure cycle over the same period of time.

9. The apparatus of claim 1 , wherein the controller is further configured to receive a feedback signal from the wireless power transmitter generating the alternating magnetic field indicating at least one of a timing schedule for the exposure cycle and electrical characteristics of a power signal used to generate the alternating magnetic field during the exposure cycle.

10. The apparatus of claim 1 , wherein the first and second inductive sense circuits and the controller are positioned within the wireless power transmitter.

11. The apparatus of claim 1 , wherein the controller is further configured to control the wireless power transmitter to start with an uncritical magnetic field level in an initial exposure cycle and to successively increase the level of the alternating magnetic field in following exposure cycles if no object is detected.

12. The apparatus of claim 1 , wherein the disturbance effects suppressed by the filtering comprise linear disturbance effects.

13. A method for detecting a presence of an object located in a power transfer area of a wireless power transmitter, comprising:

sensing the presence of the object based on an electrical characteristic of a sense coil of first and second sense circuits that varies as a function of a temperature of the object when the object is exposed to an alternating magnetic field generated by the wireless power transmitter in the power transfer area at an operating frequency of the wireless power transmitter;

reducing a disturbance voltage measured at a location for measuring the electrical characteristic of the respective sense circuit at the operating frequency of the wireless power transmitter;

scheduling an exposure cycle for the alternating magnetic field;

transmitting a signal to the wireless power transmitter to control the exposure cycle for the alternating magnetic field;

generating the alternating magnetic field based upon the exposure cycle;

measuring the electrical characteristic of each sense coil over time based on measuring at least one of a voltage and a current signal at the location of each of the first and second inductive sense circuits;

suppressing effects of the disturbance voltage at fundamental and harmonic frequencies of the wireless power transmitter by filtering to achieve a target detection sensitivity;

determining a first correlation result between the measured electrical characteristic and the exposure cycle for the first sense circuit based on comparing the measured electrical characteristic with the exposure cycle over at least a portion of a duration of the exposure cycle and determining a second correlation result between the measured electrical characteristic and the exposure cycle for the second sense circuit based on comparing the measured electrical characteristic with the exposure cycle over at least a portion of a duration of the exposure cycle, and

determining the presence of the object based on a comparison of the first correlation result with the second correlation result.

14. The method of claim 13 , wherein each measured electrical characteristic comprises one or more of a current, a voltage, an equivalent resistance, equivalent inductance, a resonant frequency, a complex resonant frequency, a dampening factor, a Q-factor, an impedance, or a phase when the inductive sense circuit is driven by a sinusoidal signal source.

15. The method of claim 13 , wherein each measured electrical characteristic comprises an impulse response of the corresponding inductive sense circuit when the inductive sense circuit is driven by a pulse signal source, or a response of the corresponding inductive sense circuit when the inductive sense circuit is driven by any of a voltage or current time function.

16. The method of claim 13 , wherein measuring an electrical characteristic of each sense coil comprises measuring the electrical characteristic based on measuring at least one of a voltage and a current signal at the location of each of a plurality of first and second sense circuits and multiplexing measurements of the electrical characteristic of each sense coil.

17. The method of claim 13 , wherein filtering comprises suppressing linear disturbance effects.

18. The method of claim 16 , wherein comparing the measured electrical characteristics over time with the exposure cycle comprises determining a plurality of first and second correlation results between the measured electrical characteristic and the exposure cycle for each of the respective first and second sense circuits.

19. The method of claim 18 , wherein determining the presence of the object comprises determining the presence of the object based on the first correlation results and at least one of the second correlation results.

20. The method of claim 13 , wherein determining the presence of the object comprises determining the presence of the object when a difference between the first and second correlation result exceeds a threshold.

21. The method of claim 13 , wherein comparing each measured electrical characteristic with the exposure cycle comprises comparing a derivative of each electrical characteristic over at least a portion of time of the exposure cycle with the exposure cycle over the same period of time.

22. The method of claim 13 , further comprising receiving a feedback signal from the wireless power transmitter generating the exposure magnetic field indicating electrical characteristics of a power signal used to generate the exposure magnetic field during the exposure cycle.

23. The method of claim 13 , further comprising controlling the wireless power transmitter to start with an uncritical magnetic field level in an initial exposure cycle and to successively increase the level of the alternating magnetic field in following exposure cycles if no object is detected.