Eddy current detection of embedded steel nails using differential coils and TMR sensor with large lift off tolerance

Insulation boards are indispensable components for external wall insulation in the construction industry, where they are typically secured using steel fasteners. The precise localization of these nails embedded within wall structures under high lift-off conditions poses a significant technical challenge. To enhance detection sensitivity and robustness, this study develops a hybrid probe integrating a tunnel magnetoresistance (TMR) sensor with differential coils. The sensor configuration comprises two vertically aligned concentric coils, with the TMR element strategically positioned within the lower coil to optimize magnetic flux detection. This research systematically investigates the impacts of the coupling gap, lift-off interference, and the TMR sensor’s installation coordinates on the overall detection efficacy. Strong agreement between numerical simulations and experimental results validates the feasibility of detecting steel nails at a substantial lift-off distance of 100 mm, demonstrating performance superior to that of conventional eddy-current sensors. Furthermore, a signal peak matching method is proposed to fuse the differential coil and TMR signals, achieving a signal-to-noise ratio (SNR) enhancement of up to 28.2 dB. Finally, an intelligent evaluation framework based on convolutional neural networks (CNNs) is implemented to assess the installation integrity of the fasteners, providing a reliable solution for construction quality inspection.