Simultaneous Measurement of Radius and Coating Thickness of Finite Length Metal Rods by Eddy Current Surface Probes

This article proposes an efficient inverse method for simultaneous measurement of radius and coating thickness of a finite length metal rod using the output signal of a surface eddy current (EC) probe. The proposed method adopts a physical model for iterative solution of the inverse problem. It utilizes a hybrid conjugate gradient (CG) optimization algorithm in the inversion process and adopts an efficient forward problem solver to obtain the predicted probe output signal in each iteration. The solution of the forward problem is based on the truncated region eigenfunction expansion method and uses the even/odd parity solution decomposition to treat the governing boundary value problem. The main feature of the proposed inversion method is its robustness in the presence of noise regardless of the choice of the initial values. Experimental and simulation results are provided to demonstrate the validity of the proposed methods for solving the forward and inverse problems. It has been shown that the rod radius and coating thickness can be measured with an error less than 5% of the true value when the probe is located at the half distance from the edge of the rod.