Input parasitic metal detection

A method of designing an inductive power transfer system with power accounting comprising:

• providing a primary side with a tank circuit;
• providing a secondary side,
• wherein the secondary side comprises a secondary coil and a load,
• wherein the secondary coil is arranged to receive contactless energy from the primary side;
• changing the distance between the primary side and the secondary side;
• changing the load;
• for a plurality of distances between the primary side and the secondary side and for a plurality of loads, measuring at least one primary circuit parameter on the primary side in the tank circuit during the transfer of contactless energy;
• for a plurality of distances between the primary side and the secondary device and for a plurality of loads of the secondary side, measuring at least one secondary circuit parameter on the secondary side during the transfer of contactless energy;
• selecting a formula to describe power consumption in the system during the transfer of contactless energy based on a plurality of coefficients, the at least one primary circuit parameter, and the at least one secondary circuit parameter;
• determining the coefficients using the at least one secondary circuit parameter and the at least one primary circuit parameter;
• and
• storing the coefficient in the inductive power transfer system for use in predicting whether unaccounted for losses during operation are present.

A primary circuit comprising:

• a tank circuit;
• a switching circuit,
• wherein the primary circuit is arranged to generate an electromagnetic field,
• wherein the primary circuit is arranged to couple to the secondary device using the electromagnetic field,
• a sensor for measuring an input characteristic of input power in the primary circuit;
• a sensor for measuring a tank characteristic of power in the tank circuit;
• a receiver circuit, wherein the receiver circuit is arranged to receive secondary device information from the at least one secondary device;
• a controller circuit,
• wherein the controller circuit is programmed to estimate power consumption in the primary circuit in combination with the secondary device as a function of at least the tank characteristic;
• wherein the controller circuit is programmed to compare the input characteristic, the secondary device information, and the estimated power consumption to determine when there is an unacceptable amount of parasitic metal present in proximity to the primary circuit;
• and
• wherein the controller circuit is programmed to restrict or stop the inductive power transfer from the primary circuit in response to a determination that the unacceptable amount of parasitic metal is present in proximity to the primary circuit.