Weak signal detection signifying foreign metal objects in wireless power transfer systems

To ensure the safe and stable operation of wireless power transfer (WPT) systems, metal foreign object detection is one of its essential functions. In this paper, a system is designed for detecting and locating small metal foreign objects such as iron clips, coins, and screws. First, simulations are conducted to analyze the mutual inductance between different detection coils and metal foreign objects. The detection coils are determined to have a trapezoidal shape, and the structural parameters, including wire diameter and spacing, are optimized. Next, a dual-channel lock-in amplifier is designed based on the correlation detection principle, and complementary upper computer software is developed to process the output signals from the detection coils. A 5 mV voltage increment test is performed on the conditioning circuit, and the system measurement error is within 5%. Finally, a metal foreign object detection experimental platform is constructed. Through multiple experiments without foreign objects, the maximum value of the digital quantity corresponding to the induced voltage change of the coil is established as the detection threshold for the system. Detection experiments are conducted on common small metal foreign objects such as iron clips, coins, and screws. Results show that the digital quantity change for iron clips can reach up to four times the detection threshold, and the smallest screw exhibits a digital quantity change exceeding 1.2 times the detection threshold. This indicates that the system can effectively detect small metal foreign objects. In addition, the use of orthogonally stacked dual-layer detection coils enables effective positioning of foreign objects.