Controlling inductive power transfer systems

A primary unit, for use in an inductive power transfer system, the primary unit comprising:

• a primary coil;
• electrical drive circuitry connected to said primary coil for applying electrical drive signals thereto so as to generate an electromagnetic field;
• a control unit operable to cause a circuit including said primary coil to operate, during a measurement period, in an undriven resonating condition in which the application of said drive signals to said primary coil by said electrical drive circuitry is suspended so that energy stored in said circuit decays over the course of said period;
• and
• a decay measurement unit operable to take one or more measures of such energy decay during said period;
• said control unit being further operable, in dependence upon said one or more energy decay measures, to control the electrical drive circuitry so as to restrict or stop inductive power transfer from the primary unit.

A method of controlling inductive power transfer in an inductive power transfer system comprising a primary unit, having a primary coil to which electrical drive signals are applied to generate an electromagnetic field, and also comprising at least one secondary device, separable from the primary unit and having a secondary coil adapted to couple with said field when the secondary device is in proximity to the primary unit so that power can be transferred inductively from the primary unit to the secondary device without direct electrical conductive contacts therebetween, which method comprises:

• causing a circuit including said primary coil to operate, during a measurement period, in an undriven resonating condition in which the application of said drive signals to said primary coil is suspended so that energy stored in the circuit decays over the course of said period;
• taking one or more measures of such energy decay during said period;
• and
• restricting or stopping inductive power transfer from the primary unit in dependence upon said one or more energy decay measures.