Sensor arrangement for a foreign object detection device

A foreign object detection device for a primary part of a wireless power transfer system for transferring power over an air gap to a nearby receiver, comprising:

• a sensor arrangement, comprising:
• a current input and a current output;
• a plurality of detection cells, each comprising a sense coil including a winding spirally wound in a plane and having a plurality of turns, wherein the sense coils of the detection coils are arranged in one or more layers, and the sense coils of a particular layer are arranged in a matrix-like structure with multiple rows and multiple columns;
• a plurality of input leads and one or more output leads, wherein each detection cell is connected between one of the input leads and one output lead, and each detection cell is connected to a different combination of input and output leads, and the input lead and the output lead of at least one sense coil is routed through the matrix-like structure to which the at least one sense coil belongs close to each other such as to reduce an area parallel to said matrix-like structure and enclosed by said input and output leads;
• and
• an input selection circuit adapted to selectively establish an electrical connection between the current input and one or more of the input leads;
• a stimulus circuit for generating a predetermined stimulation signal, connected to the current input of the sensor arrangement;
• a measurement unit, adapted to sense an electrical signal applied to said current input, the measurement unit comprising an analogue to digital converter for acquiring sampling data representing the sensed electrical signal, and
• a signal processing unit configured to determine time response data of a selected detection cell based on the sampling data acquired by the measurement unit.

A primary part for a wireless power transfer system for transferring power over an air gap to a nearby receiver, configured to generate a magnetic field in an active area at a transmission frequency, the primary part comprising a sensor arrangement comprising:

• a current input and a current output;
• a plurality of detection cells, each comprising a sense coil including a winding spirally wound in a plane and having a plurality of turns;
• a plurality of input leads and one or more output leads, wherein each detection cell is connected between one of the input leads and one output lead, and each detection cell is connected to a different combination of input and output leads, and
• an input selection circuit adapted to selectively establish an electrical connection between the current input and one or more of the input leads;
• wherein the sense coil of at least one detection cell comprises an outer coil section and an inner coil section arranged inside the outer coil section, and a first distance between an outermost turn of the inner coil section and an innermost turn of the outer coil section is at least twice a largest distance between two turns of the outer coil section, and
• wherein the sense coils of the detection cells are arranged in a way that a sensing area defined by the sense coils at least partly covers the active area.