Power transmitter unit

1. A power-transmitter-unit comprising:

a power-transmitting-coil for wirelessly providing power to a power-receiver-unit, and a resonant-capacitor connected to the power-transmitting-coil, such that together they define an LC circuit, wherein the LC circuit comprises a first-end and a second-end;

a controller configured to define a foreign-object-detection-mode of operation, wherein:

in the foreign-object-detection-mode of operation: the first-end of the LC circuit is connected to the second-end of the LC circuit such that the LC circuit is short-circuited and defines a closed-LC-circuit; and

the controller is configured to receive a coil-current-signal that is representative of a current through the closed-LC-circuit;

wherein the controller is further configured to:

process the coil-current-signal that was received during the foreign-object-detection-mode of operation and a capacitance of the resonant-capacitor, in order to determine a resistance of the closed-LC-circuit; and

provide an indication that a foreign object has been detected if the determined resistance of the closed-LC-circuit is greater than a resistance-threshold value.

2. The power-transmitter-unit of claim 1 , further comprising:

a power circuit for selectively providing power to the LC circuit; and

wherein the controller is further configured to define a power-transmission-mode of operation, wherein in the power-transmission-mode of operation:

the power circuit is configured to provide a potential difference across the first-end and the second-end of the LC circuit.

3. The power-transmitter-unit of claim 1 , further comprising an analogue-to-digital (ADC) configured to provide the coil-current-signal as a sequence of digital samples that are representative of the current through the closed-LC-circuit.

4. The power-transmitter-unit of claim 3 , wherein the ADC is configured to provide one or more digital samples for each period of the current through the closed-LC-circuit.

5. The power-transmitter-unit of claim 3 , wherein the ADC is configured to apply a sampling rate that satisfies a Nyquist sampling rate for the power-transmitter-unit.

6. The power-transmitter-unit of claim 3 , wherein the ADC is configured to provide digital samples that have a constant time period.

7. The power-transmitter-unit of claim 1 , wherein the controller is configured to apply an auto-regressive modelling algorithm to the coil-current-signal in order to determine the resistance of the closed-LC-circuit.

8. The power-transmitter-unit of claim 1 , wherein the controller is configured to apply a least squares method to the coil-current-signal in order to determine the resistance of the closed-LC-circuit.

9. The power-transmitter-unit of claim 1 , wherein the controller is configured to disable a power-transmission-mode of operation if the determined resistance of the closed-LC-circuit is greater than the resistance-threshold value.

10. The power-transmitter-unit of claim 1 , wherein the controller is configured to periodically switch between a power-transmission-mode of operation and the foreign-object-detection-mode of operation.

11. The power-transmitter-unit of claim 1 , wherein:

the power-transmitter-unit comprises a plurality of power-transmitting-coils for wirelessly providing power to the power-receiver-unit, wherein each power-transmitting-coil is connected to a corresponding resonant-capacitor to define a corresponding LC circuit, and wherein each corresponding LC circuit comprises a first-end and a second-end of a corresponding LC circuit; and

in a power-transmission-mode of operation:

the power circuit is configured to provide a potential difference across the respective first-ends and second-end of the plurality of corresponding LC circuits;

in the foreign-object-detection-mode of operation:

the power circuit is configured to not provide the potential difference across the respective first-ends and second-end of the plurality of corresponding LC circuits;

for each of the plurality of corresponding LC circuits in turn:

the first-end of the corresponding LC circuit is connected to the second-end of the corresponding LC circuit such that the corresponding LC circuit is short-circuited and defines a corresponding closed-LC-circuit; and

the controller is configured to receive a corresponding coil-current-signal that is representative of a current through the corresponding closed-LC-circuit; and

wherein the controller is further configured to:

process the corresponding coil-current-signals in order to determine a resistance of the corresponding closed-LC-circuits; and

provide the indication that the foreign object has been detected if the determined resistance of any of the corresponding closed-LC-circuits is greater than an associated resistance-threshold value.

12. The power-transmitter-unit of claim 11 , wherein a single capacitor provides functionality of the resonant-capacitor of a plurality of the corresponding LC circuits.

13. The power-transmitter-unit of claim 11 , wherein a single capacitor provides functionality of the resonant-capacitor of all of the corresponding LC circuits.

14. A method of detecting a foreign object, the method comprising:

connecting a first-end of an LC circuit that comprises a resonant-capacitor to a second-end of the LC circuit such that the LC circuit is short-circuited and defines a closed-LC-circuit; and

receiving a coil-current-signal that is representative of a current through the closed-LC-circuit;

processing the coil-current-signal and a capacitance of the resonant-capacitor in order to determine a resistance of the closed-LC-circuit; and

providing an indication that a foreign object has been detected if the determined resistance of the closed-LC-circuit is greater than a resistance-threshold value.