A metal object detection system with multilayer detection coil layouts for electric vehicle wireless charging

Non-radiative inductive power transfer is one of the most studied and commercially applied wireless charging technologies, where the magnetic field is employed as the medium for power transfer. In the wireless charging of electric vehicles, the strong magnetic field will heat up any metal items falling in the charging area due to eddy current induced in the metal objects, causing hazards like fire. Metal object detection (MOD) is necessary for the market penetration of inductive power transfer technology. This paper aims to improve the performance of systems that detect metal objects based on inductance variations. Two novel multi-layer detection coil layouts are proposed, which can not only cover the entire charging area without blind spots but can also be decoupled from the transmitter and receiver to minimize the influence of the magnetic field that is used for power transfer. Two mixed resonant circuits are proposed and proven to have better performance than parallel and series resonance. The impacts of the detection coil layer, trace width, and turn-number are investigated. The test results indicate that the MOD system can detect one-cent coins at various positions of the detection coil printed circuit board, and can also detect various inductance variations without blind spots in the processing circuit.