Thickness and Permeability Estimation of Metallic Plates by Triple-Frequency Eddy-Current Testing With Probe Lift-Off

Eddy-current (EC) testing is one of the contactless nondestructive testing techniques for the physical property measurement (e.g., thickness, electrical conductivity, and magnetic permeability) of steel plates in various industrial applications. In this study, an estimation method of plate thickness and permeability is proposed using the triple-frequency coil inductance with probe lift-off. The functional relationship between the plate properties, probe lift-off, and characteristics of coil inductance is indicated by the simplified analytical model, which is employed for the initial guesses of optimization. The proposed method can decrease the influence of the local minimum on property estimation. Numerical simulations and experiments have been performed to evaluate the proposed method with various plate properties and probe lift-off. Compared with the estimation results without the appropriate initial guesses, the proposed method is more robust and accurate for various cases, with a relative estimation error smaller than 8%.