Equivalent-Circuit-Based Design of Symmetric Sensing Coil for Self-Inductance-Based Metal Object Detection

A symmetric sensing coil design is proposed and adapted for the self-inductance-based metal object detection (MOD) of wireless electric vehicle (WEV) charging. Compared to conventional non-symmetric sensing coils, the proposed sensing coil provides symmetric detection sensitivity and ease of manufacturing, pertaining blind-zone-free characteristics. Moreover, it is found that the design of the proposed symmetric sensing coil is very different from that of the symmetric sensing coil for conventional induced voltage sensing (IVS) based MOD method due to different principles of MOD methods. Therefore, an analysis based on a newly proposed equivalent circuit, is used to design a high sensitivity symmetric sensing coils, considering many aspects such as metal object covering, metal object size, and mutual inductance between sensing coils. Based on the analysis, an optimized sensing coil design created through a finite element method simulation is proposed to achieve highest sensitivity with the lowest number of sensing coils. It can be readily applied to a cost-effective MOD system that can be used for high-power WEV charging. Experiments on a metal object 20 mm $\times20$ mm in size with a 20Arms Tx current verified high detection sensitivity of 38% without any blind zones.