Controlling inductive power transfer systems

1. A method of controlling inductive power transfer in an inductive power transfer system comprising a primary unit operable to generate an electromagnetic field and at least one secondary device, separable from the primary unit, and adapted to couple with said field when the secondary device is in proximity to the primary unit so that power can be received inductively by the secondary device from the primary unit without direct electrical conductive contacts therebetween, which method comprises:

receiving, in the primary unit, information from the secondary device relating to a power required by the secondary device or, if there is more than one secondary device, a combined power required by the secondary devices;

determining in the primary unit, in dependence upon the received information, whether there is parasitic metal in the electromagnetic field by determining if there is a substantial difference between a power drawn from the primary unit and a power required by the secondary device or, if there is more than one secondary device, a combined power required by the secondary devices;

following such determination of parasitic metal in the electromagnetic field, restricting or stopping the inductive power supply from the primary unit;

employing, when carrying out said determination, first compensation information relating to losses in the primary unit itself so as to compensate for said losses;

employing, when carrying out said determination, second compensation information relating to a parasitic load imposed on the primary unit by the secondary device so as to compensate for said parasitic load of the secondary device;

wherein the secondary device communicates its said second compensation information directly to said primary unit or communicates to the primary unit other information from which the primary unit derives said second compensation information;

wherein the secondary device communicates its said second compensation information or its said other information to the primary unit by varying a load imposed by it on the primary unit; and

wherein the secondary device has a dummy load, representative of its said parasitic load, which it imposes on said primary unit to vary the load imposed by it on the primary unit.

2. A method of controlling inductive power transfer in an inductive power transfer system comprising a primary unit operable to generate an electromagnetic field and at least one secondary device, separable from the primary unit, and adapted to couple with said field when the secondary device is in proximity to the primary unit so that power can be received inductively by the secondary device from the primary unit without direct electrical conductive contacts therebetween, which method comprises:

receiving, in the primary unit, information from the secondary device relating to a power required by the secondary device or, if there is more than one secondary device, a combined power required by the secondary devices;

determining in the primary unit, in dependence upon the received information, whether there is parasitic metal in the electromagnetic field by determining if there is a substantial difference between a power drawn from the primary unit and a power required by the secondary device or, if there is more than one secondary device, a combined power required by the secondary devices;

following such determination of parasitic metal in the electromagnetic field, restricting or stopping the inductive power supply from the primary unit; and

wherein the electromagnetic field is generated by a primary coil driven by an electrical drive unit, electrical power for the drive unit is supplied from a power supply of the primary unit to a power input of the drive unit, and the power drawn from the primary unit is measured by temporarily disconnecting the power supply and detecting a change at said power input during the disconnection.

3. A method as claimed in claim 2 , further comprising storing energy in an energy storage unit connected to said power input so that power can continue to be supplied to said power input whilst said power supply is disconnected.

4. A method of controlling inductive power transfer in an inductive power transfer system comprising a primary unit operable to generate an electromagnetic field and at least one secondary device, separable from the primary unit, and adapted to couple with said field when the secondary device is in proximity to the primary unit so that power can be received inductively by the secondary device from the primary unit without direct electrical conductive contacts therebetween, which method comprises:

receiving, in the primary unit, information from the secondary device relating to a power required by the secondary device or, if there is more than one secondary device, a combined power required by the secondary devices;

determining in the primary unit, in dependence upon the received information, whether there is parasitic metal in the electromagnetic field by determining if there is a substantial difference between a power drawn from the primary unit and a power required by the secondary device or, if there is more than one secondary device, a combined power required by the secondary devices; and

following such determination of parasitic metal in the electromagnetic field, restricting or stopping the inductive power supply from the primary unit, wherein the electromagnetic field is generated by a primary coil, and the method further comprises:

causing a circuit including said primary coil to operate, during a measurement period, in an undriven resonating condition in which the application of drive signals to the 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 employing said one or more measures to measure said power drawn from the primary unit.