Metal Object Detection System with Parallel-mistuned Resonant Circuits and Nullifying Induced Voltage for Wireless EV Chargers

In this paper, a metal object detection (MOD) system, a kind of foreign object detection (FOD), which is based on mistuned resonant circuits and utilizes variation of self-inductance of a sensing pattern, is newly proposed for wireless electric vehicle (EV) chargers. The sensing pattern that consists of multiple loop coil sets is mounted on the transmitting (Tx) pad of an EV charger, where a loop coil set has two coils connected in series with opposite polarity to cancel out the induced voltage generated by the Tx coil. Variation of self-inductance of the loop coil set is detected by a parallel-resonant circuit, driven by a current source and operating at near 1 MHz. To increase the detection sensitivity of the proposed MOD system, instead of an exact resonant frequency, a mistuned operating frequency near the -3dB point is utilized for the parallel-resonant circuit. Through simulations and experiments, it is found that the proposed MOD system detects not only horizontal but also standing upright metal objects. A prototype MOD system, operating at 85 kHz to satisfy the standard J2954, was fabricated to verify its feasibility. The results showed that output voltage change of the proposed MOD system becomes 22.7 % for a piece of aluminum foil of 3 x 3 cm(2) and 40.9 % for 100 Korean Won coin, respectively.