Input parasitic metal detection

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

• providing a primary side with a tank circuit for transferring contactless energy;
• providing a secondary side including a secondary coil for receiving the contactless energy and a load in electrical communication with a load;
• changing the distance between the primary side and the secondary side;
• changing the load of the secondary side;
• for a plurality of distances between the primary side and the secondary side and for a plurality of loads, measuring at least one 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 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 circuit parameter on the primary side in the tank circuit, and the at least one circuit parameter on the secondary side;
• determining the coefficients using the measured circuit parameters on the secondary side and the measured circuit parameters on the primary side;
• and storing the coefficient in the indicative power transfer system for use in predicting whether unaccounted for losses during operation are present.

A primary unit, having a tank circuit and switching circuit, operable to generate an electromagnetic field for transferring power to at least one secondary device, separate from the primary unit, and adapted to couple with the field when the secondary device is in proximity to the primary unit so that power is received inductively by the secondary device from the primary unit without direct electrical conductive contacts therebetween, the primary unit comprising:

• a sensor for measuring a characteristic of input power in the primary unit;
• a sensor for measuring a characteristic of power in the tank circuit of the primary unit;
• a receiver for receiving information from the at least one secondary device;
• a controller programmed for:
• estimating power consumption in the inductive power transfer system as a function of at least the measured characteristic of power in the tank circuit of the primary unit;
• comparing the measured characteristic of input power in the primary unit, the information from the at least one secondary device, and the estimated power consumption to determine there is an unacceptable amount of parasitic metal present in proximity to the primary unit;
• and
• restricting or stopping the inductive power transfer from the primary unit in response to a determination that the unacceptable amount of parasitic metal is present in proximity to the primary unit.