Sensorless Metal Object Detection Using Transmission-Side Voltage Pulses in Standby Phase for Dynamic Wireless Power Transfer

Abstract

In the Wireless Power Transfer (WPT) by magnetic field resonant coupling, the presence of a metal foreign object between two coils is dangerous due to overheating caused by induction heating. Most previous studies have focused on detection methods for the static WPT (S-WPT), which targets stationary electric vehicles (EVs). In the S-WPT, the power supply time is long, and foreign metal objects tend to heat up. However, the dynamic WPT (D-WPT) is expected to generate less heat than the S-WPT because of its short power supply time and long standby time. We studied the types and sizes of metals which must be detected in D-WPT and the appropriate detection methods for them. We propose a detection method by applying voltage pulses to the transmitter circuit and measuring the steady-state value of the transmitter current. By performing detection only during the standby time specific to D-WPT, the reference impedance can be made small and free from effects of misalignment with the power receiver coil. From the heating experiment, we determined that the metals which must be detected in the 20 kW D-WPT system were ferromagnetic and the minimum size was 50 mm × 50 mm. The detection experiment showed that the invasion of the metal objects which must be detected reduced the steady-state value of transmitter current by at least 20%. In conclusion, the proposed method can be implemented at a low cost and satisfy the required detection accuracy in the D-WPT.