Metal Object Detection using Magnetic Field Change for Wireless EV Chargers

Abstract

Demand and interest of wireless power has increased for industry and the general public. Among the various wireless power systems inductive power transfer (IPT) is becoming mainstream. One problem of IPT is that overheating of metal objects caused by an eddy current may cause ignition, resulting in fire or damage to the equipment. Therefore, it is necessary detects the metal between the transmitter and the receiver of the wireless power system in terms of safety. A metal object detection (MOD) circuit using magnetic field change to ensure safe operation of wireless EV chargers is explained in this paper. The proposed MOD circuit was experimentally verified using spatial symmetry sensing coils and a 2cm x 2cm metal object, where its output voltage was changed from 200 mV to 1V. The MOD circuit can detect multiple metal objects without dead zone on the all area of Tx pad and is suitable for high power wireless EV chargers. Each chapter is structured as follows. Chapter 1 introduces various methods for performing MOD. It introduces and analyzes the advantages and disadvantages of system parameter change, power transmission situation analysis, usage of external equipment and magnetic field change detection method. Through analysis, the optimal method for detecting metal is derived. and a influence of metal objects under an alternating magnetic field is described with respect to various thicknesses and types of metal objects. Chapter 2 describes two types of sensing coils to sense the magnetic field changes caused by the metal object. The non-overlapped sensing coil reduces the number of layers required to completely eliminate the dead zone, which is an area where metal cannot be detected, compared to conventional methods. The second coil is designed to completely eliminate the dead zone with only one layer using spatial symmetry. Chapter 3 explains how to process signals acquired from sensing coils. The proposed MOD circuit is composed of a variable resistor, a low pass filter, and a precision rectifier with amplification for accurate data acquisition. The circuit works best in various environments such as misalignment using digital elements. The fabricated equipment can be attached to the existing Tx pad. The MOD system can detect 4cm2 size of metal object in all area of sensing coil The proposed MOD circuit with minimized number of components and MOD sensing coil array without dead zone are designed. The proposed MOD system ensures safe operation of wireless EV chargers and is ready for commercialization.