Measurement of Lift-Off Distance and Thickness of Nonmagnetic Metallic Plate Using Pulsed Eddy Current Testing

Eddy current testing (ECT) is an efficient method to estimate the thickness of sample but it is affected by other parameters information including electromagnetic properties and lift-off. In the previous researches, the lift-off point of intersection (LOI) feature has been found in pulsed eddy current (PEC), which can overcome the fluctuation of lift-off. In this paper, another feature, i.e. conductivity point of intersection(CPI), is proposed for non-coaxial T-R sensor in PEC. Specifically, the response signals of different sample conductivity almost intersect with each other at the certain time, which can be regarded immune to the conductivity of sample. The analytical solution and numerical solution are conducted, and the results indicate that the intersection point magnitude is mainly related with parameters of sensor and thickness of samples. Considering the elements related with conductivity in the Hessian matrix exists ill-conditioning, it will produce the magnification of measurement error under iterative inversion process. By referring to the CPI feature, the thickness of the sample and lift-off of sensor can be accurately inverted based on the modified Newton–Raphson method. The experiments are carried out to verify the proposed method, and the error is only 2.9%.