Inductive power transfer

An inductive power transfer system including a wireless power transmitter and at least one secondary unit, the wireless power transmitter being operable to transmit power wirelessly to one or more of a) the at least one secondary unit and b) a foreign object located in proximity to the wireless power transmitter, said inductive power transfer system comprising:

• a secondary coil configured to receive power transmitted wirelessly from a primary coil of the wireless power transmitter;
• a regulator operably coupled to said secondary coil, said regulator configured to regulate power drawn from said secondary coil and to provide regulated power to a load such that power drawn from said secondary coil is at or above an operating threshold of said regulator;
• processing circuitry configured to predetermine a rate for changing an electrical drive signal during a measurement phase;
• driver circuitry operably coupled to said primary coil, said driver circuitry operable to drive said primary coil to transmit power sufficient to operate the regulator at or above the operating threshold, said driver circuitry operable to drive said primary coil so that, in the measurement phase, a magnitude of an electrical drive signal supplied to said primary coil changes from a first value by the predetermined rate to a second value, wherein power transmitted at said first value is sufficient to operate the regulator at or above the operating threshold and power transmitted at said second value is sufficient to operate the regulator at or above the operating threshold;
• and
• sensor circuitry configured to sense an electrical characteristic of one or more of the driver circuitry and the primary coil, said sensor circuitry configured to generate a first measurement of said electrical characteristic at said first value and to generate a second measurement of said electrical characteristic at said second value,
• wherein the sensor circuitry is further configured to detect, based on a difference between said first measurement and said second measurement, presence of one or more of a) the at least one secondary unit including the regulator and b) the foreign object located in proximity to said wireless power transmitter.

A detection method for an inductive power transfer system, the inductive power transfer system including a wireless power transmitter operable to transmit power wirelessly to one or more of a) at least one secondary unit located in proximity to the wireless power transmitter and b) a foreign object located in proximity to the wireless power transmitter, the at least one secondary unit including a regulator with an operating threshold, the detection method comprising:

• predetermining a rate for changing an electrical drive signal during a measurement phase;
• driving the wireless power transmitter to transmit power sufficient to operate the regulator at or above the operating threshold and so that, during the measurement phase, a magnitude of an electrical drive signal supplied to a primary coil of the wireless power transmitter changes by the predetermined rate from a first value to a second value, wherein power transmitted at the first value is sufficient to operate the regulator at or above the operating threshold and power transmitted at the second value is sufficient to operate the regulator at or above the operating threshold;
• sensing an electrical characteristic of the wireless power transmitter at the first value to generate a first measurement;
• sensing the electrical characteristic of the wireless power transmitter at the second value to generate a second measurement;
• and
• detecting, based on a difference between the first measurement and the second measurement, presence of one or more of a) the at least one secondary unit including the regulator and b) the foreign object located in proximity to the wireless power transmitter.