Patent ID: 7605496
Filing Date: 2009-10-20
Classification: G05F,H01F,H02J

Abstract:
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.