Systems, methods, and apparatus for detecting ferromagnetic foreign objects in a predetermined space

1. An apparatus for detecting a presence of an object in a predetermined space of a wireless power transfer system, the apparatus comprising:

an inductive sense circuit comprising an electrically conductive structure configured to generate a magnetic field at a sense frequency when driven with a sense signal to thereby inductively sense the presence of the object, a capacitor electrically connected to the electrically conductive structure and having a capacitance configured to tune the inductive sense circuit for a resonance substantially at the sense frequency, and a measurement port configured for measuring an impedance of the inductive sense circuit;

a signal source coupled to the inductive sense circuit and configured to generate the sense signal at the sense frequency;

a sensor electrically connected to the measurement port and configured to sense an electrical characteristic at the measurement port and to provide a sensor output signal indicative of the electrical characteristic;

a detector circuit electrically connected to the sensor and configured to perform narrowband filtering and detection of the sensor output signal at the sense frequency, and to provide a detector output signal indicative of the electrical characteristic; and

a controller electrically connected to the signal source and to the detector circuit, the controller configured to:

control the signal source and the detector circuit including the sense frequency,

receive the detector output signal,

measure an impedance indicative of the impedance of the inductive sense circuit based on the received detector output signal when the electrically conductive structure is driven by the sense signal at the sense frequency,

tune the sense frequency to a frequency providing an extremum of a magnitude of the measured impedance,

determine a phase error as a phase angle of the impedance as measured at the frequency of the extremum,

correct the measured impedance for the phase error by shifting the phase angle of the measured impedance by an amount of the determined phase error to produce a corrected impedance value,

detect a potential interference signal coupled into the electrically conductive structure based on a level of the received detector output signal in absence of the sense signal,

determine that the level exceeds a threshold,

shift at least one of the sense frequency or an operating frequency of the wireless power transfer system in a manner to reduce the potential interference signal while substantially remaining at the resonance of the sense circuit, and

determine the presence of the object based on a change in the corrected impedance value.

2. The apparatus of claim 1 , wherein the object is a metal object.

3. The apparatus of claim 1 , wherein the change in the corrected impedance value comprises a resistive component and a reactive component.

4. The apparatus of claim 1 , wherein

the magnitude and the phase angle of the measured impedance are represented as a phasor, and

shifting the phase angle of the measured impedance to produce the corrected impedance value comprises rotating the phasor of the measured impedance by the amount of the determined phase error.

5. The apparatus of claim 1 , wherein the wireless power transfer system is operated at a frequency below 150 kHz.

6. The apparatus of claim 5 , wherein the sense frequency is greater than 1 MHz.

7. The apparatus of claim 1 , wherein the sense signal has a sinusoidal waveform.

8. The apparatus of claim 1 , wherein the electrically conductive structure and the capacitor are connected in series.

9. The apparatus of claim 8 , wherein the inductive sense circuit further comprises a shunt inductor connected in parallel with a series combination of the electrically conductive structure and the capacitor, the shunt inductor configured to attenuate a low frequency signal component induced into the electrically conductive structure by the wireless power transfer system when active.

10. The apparatus of claim 1 , wherein the sensor is one of a voltage sensor and a current sensor configured to sense one of a voltage and a current at the measurement port.

11. The apparatus of claim 1 , wherein shifting the phase angle of the measured impedance comprises performing a phase zeroing by rotating a phasor.

12. A method for detecting a presence of an object in a predetermined space of a wireless power transfer system, the method comprising:

applying a sense signal to an inductive sense circuit using a signal source, thereby generating a magnetic field at a sense frequency, wherein the inductive sense circuit comprises an electrically conductive structure, a capacitor electrically connected to the electrically conductive structure and having a capacitance configured to tune the inductive sense circuit for a resonance substantially at the sense frequency, and a measurement port configured for measuring an impedance of the inductive sense circuit;

sense an electrical characteristic at the measurement port by a sensor electrically connected to the measurement port and provide a sensor output signal indicative of the electrical characteristic,

in a detector circuit electrically connected to the sensor, perform narrowband filtering and detection of the sensor output signal at the sense frequency, and provide a detector output signal indicative of the electrical characteristic, and

in a controller electrically connected to the signal source and the detector circuit:

controlling the signal source and the detector circuit including the sense frequency,

receiving the detector output signal,

measuring an impedance indicative of the impedance of the inductive sense circuit based on the received detector output signal when the electrically conductive structure is driven by the sense signal at the sense frequency,

tuning the sense frequency to a frequency providing an extremum of a magnitude of the measured impedance,

determining a phase error as a phase angle of the impedance as measured at the frequency of the extremum,

correcting the measured impedance for the phase error by shifting the phase angle of the measured impedance by an amount of the determined phase error to produce a corrected impedance value,

detecting a potential interference signal coupled into the electrically conductive structure based on a level of the received detector output signal in absence of the sense signal,

determining that the level exceeds a threshold,

shifting at least one of the sense frequency or an operating frequency of the wireless power transfer system in a manner to reduce the potential interference signal while substantially remaining at the resonance of the sense circuit, and

determining the presence of the object based on a change in the corrected impedance value.

13. The method of claim 12 , wherein the object is a metal object.

14. The method of claim 12 , wherein the change in the corrected impedance value comprises a resistive component and a reactive component.

15. The method of claim 12 , wherein

the magnitude and phase angle of the measured impedance are represented as a phasor, and

shifting the phase angle of the measured impedance to produce the corrected impedance value comprises rotating the phasor of the measured impedance by the amount of the determined phase error.

16. The method of claim 12 , wherein sensing at least one electrical characteristic at the measurement comprises sensing one of a voltage and a current at the measurement port.

17. The method of claim 12 , wherein shifting the phase angle of the measured impedance comprises performing a phase zeroing by rotating a phasor.