Metal Object Detection Using Harmonic Chirp Voltage for Dynamic Wireless Power Transfer

In wireless power transfer (WPT) systems using magnetic fields, foreign metal objects on the transmitter (Tx) coil can heat up due to induction heating, potentially posing risks to the human body and causing damage to surrounding equipment. This paper focuses on dynamic WPT (DWPT) systems for electric vehicles and proposes a cost-effective metal object detection mechanism that can identify only heat-generating metals without requiring any additional sensors. The proposed method applies a low-amplitude rectangular chirp voltage to the Tx circuit during the standby interval to analyze its resonant characteristics. The chirp signal is generated using high harmonics with a voltage step-down ratio of less than 1% and a frequency resolution of 4.2 Hz. The changes in self-inductance and internal resistance of the Tx circuit are extracted from the impedance frequency response based on the current response. This approach enables not only the detection of foreign metal objects but also the identification of their magnetic characteristics, a critical parameter in induction heating. Experimental validation confirms that the proposed method can detect 50mm × 50mm or larger ferromagnetic metal objects at an embedment depth of 80 mm, thereby satisfying the detection requirements for 20 kW-class DWPT systems.