Controlling inductive power transfer systems

A method of controlling inductive power transfer in an inductive power transfer system comprising a primary unit having a primary and electrical drive circuitry connected to the primary for applying electrical drive signals thereto so as to generate an electromagnetic field, the system including at least one portable device separable from the primary unit and having a secondary adapted to couple with the primary to receive power inductively without direct electrical conductive contacts, said method comprising:

• transferring power inductively from the primary unit to the at least one portable device;
• providing inductively received power from the secondary to a load of the at least one portable device, wherein the at least one portable device is capable of selectively connecting the secondary to the load;
• receiving communication from the primary unit;
• based on communication from the primary unit, isolating the load from the secondary;
• and
• operating the primary unit to determine presence of a parasitic load in proximity to the primary unit.

An inductive power transfer system including a primary unit having a primary and electrical drive circuitry connected to the primary for applying electrical drive signals thereto so as to generate an electromagnetic field, the system also including at least one portable device separable from the primary unit and having a secondary adapted to couple with the primary to receive power inductively without direct electrical conductive contacts, the inductive power transfer system comprising:

• a control unit operable to control the electrical drive circuitry to control application of the electrical drive signals to the primary;
• and
• a measurement unit operable to take a first measurement in a first measurement period and a second measurement in a second measurement period, wherein a first load of the portable device is imposed on the primary unit in the first measurement period, and wherein a second load of the portable device is imposed on the primary unit in the second measurement period;
• wherein the control unit is operable, in dependence upon the first measurement and the second measurement, to detect presence of a parasitic load in proximity to the primary.