Lift-Off Invariant Footprint Control in Pulsed Eddy Current Probes Using Finite Plate Modeling

Pulsed eddy current (PEC) is widely used in the metal industry for corrosion detection or thickness inspection due to its rapid response, strong penetration, and rich time–frequency information. Because PEC measurements average the remaining metal thickness over the probe footprint, accurately evaluating and controlling the footprint is essential for improving spatial resolution. This work considers a practical probe comprising a circular excitation coil and a tunnel magneto-resistance (TMR) receiver. The probe footprint is the effective spatial region of the specimen that contributes to the measured PEC signal, we develop an explicit analytical PEC model for finite plates based on inverse Laplace transform, markedly improving efficiency compared with the traditional inverse Fourier transform. Building on this model, a footprint evaluation method that gradually increases the plate radius and monitors the changes in the detection signal is proposed. The expansion of the footprint with lift-off and its variation with probe size are quantified, revealing a second-order exponential growth with increasing lift-off and a linear dependence on probe size. Based on the obtained signal growth rates related to the metal plate radius, heatmap of the probe footprint was generated, quantifying the spatial resolution of the probe. The method is validated experimentally, with an error below 5%. Finally, analysis of the interplay among probe size, lift-off, and footprint yields a strategy to maintain a constant footprint within a specified lift-off range by appropriately adjusting the probe size.