A Shared Method of Metal Object Detection and Living Object Detection Based on the Quality Factor of Detection Coils for Electric Vehicle Wireless Charging

Abstract

High power alternating magnetic fields can cause lesions and pathological changes in living object bodies entering the wireless charging area. Also, as for metal objects, the eddy effect can cause the security risks such as fire and ignition hazards. Hence, metal object detection (MOD) and living object detection (LOD) are the necessary functions for a wireless power transfer (WPT) system. A shared method of MOD and LOD based on the quality factor of detection coils for electric vehicle wireless charging was proposed in this paper to realize the dual functions by the same detection coil array. Firstly, the effect of metal objects and living objects on the detection coil quality factor was analyzed in detail. Secondly, a method of real-time calculation of quality factor was proposed to detect whether the foreign object exists. Then, a distinction algorithm based on the phase variation trend of resonant capacitor voltage was designed to classify the type of foreign objects. Finally, a 3.3 kW experimental prototype, including a foreign object detection (FOD) system, was established to verify the validity. The advantage of the quality factor as the core indicator for FOD was analyzed in detail. Under the condition of an iron wafer, aluminum wafer, and beef chunk placed on the detection coil, the quality factor calculation results of the detection coil in contrast to those without foreign objects decreased by 80.8%, 50.5%, and 69.9%, respectively. Meanwhile, the phase difference of the excitation relative to the resonant capacitor voltage changed -61.3°, -74.6°, and 49.4°, respectively.