A Foreign Object Detection Method using High Frequency Excitation for Wireless Power Transfer

A metal object detection (MOD) method based on impedance changes at high frequency (HF) is proposed for wireless power transfer (WPT) system. The impedance of the primary coil is found to be much more sensitive to metal object (MO) at self-resonant frequency, which is formed by self-inductance and its parasitic capacitance, than at operating frequency. Using this feature, a simple impedance detector for MOD utilizing HF excitation and current sensing is proposed. A series-series-compensated WPT system is used to verify this proposed MOD method, and experimental result shows small MOs can be detected effectively for offline as well as online WPT situations. Further more, by using circuits of HF-coupling, the impact on the WPT system itself is very limited, and additional detector coils for MOD covering the primary side are eliminated.