A magnetic field reinforced excitation structure for enhanced motion-induced eddy current defect detection

Copper and aluminum foils, serving as current collectors in lithium-ion batteries, are manufactured by continuously rolling thick plates to the desired thin-foil thickness. Once defects occur during the rolling of thick plates, they inevitably propagate into the foils, thereby compromising production quality and further deteriorating the electrochemical performance and safety of batteries. Conventional detection methods based on a single permanent magnet or DC coil often generate weak responses to small defects, making them unsuitable for high-speed manufacturing. To overcome this limitation, this study proposes a magnetic-field-reinforced excitation structure and establishes a theoretical model to analyze the effects of magnet spacing and lift-off distance on magnetic field characteristics. Finite element simulations are employed to investigate their influence on motion-induced eddy current (MIEC) signals. Experimental results show that the proposed structure enhances defect signal amplitudes by 59.9–92.7 % compared with single magnet configurations, with particularly significant improvements for small defect sizes and short lift-off distances. This work provides an effective method for high-speed defect detection and quality control of copper and aluminum foils in battery manufacturing.