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A Foreign Object Detection Method Based on Variation of Quality Factor of Detection Coil at Multi-frequency

Due to the contactless feature of the WPT (Wireless Power Transfer) system, the metal foreign objects could enter the coupling charging area possibly and be exposed in the high power alternating magnetic field. Heated by the eddy current, the metal foreign objects would ignite the inflammable materials and cause fire which has become the most potential safety hazard of WPT system. To solve this problem, this paper proposed a foreign object detection (FOD) method based on the quality factor variation of the detection coil at multi-frequencies. The proposed method could calculate the quality factor of the detection coil in real time through measuring the resonant capacitance voltage during the GA power transmission. Compared to traditional FOD methods, the proposed method could utilize the resistance variation of detection coil reasonably and preliminary realize the material identification of metal foreign object. By configuring different excitation frequencies the method could accurately identify the ferromagnetic metal foreign object which has the hazard of sharp temperature rise to WPT system. A 3.3kW FOD system prototype, which satisfied the standard SAEJ2954, was established to verify its feasibility. The experiment results showed that the error between the calculated quality factor of the proposed method and the actual measurement of instrument was less than 10% during power transmission. And the ferromagnetic and non-ferromagnetic metal foreign object with same size could be distinguished.