Simultaneous Estimation of Conductivity and Radial Eccentricity of Metallic Cylinders Using Eddy Current Testing

Metallic cylinders are extensively used across a range of industries. The inspection of their properties through eddy current testing (ECT) is crucial to ensure the desired performance of the piece in practical applications. This article proposes for the first time an analytical model for the mutual inductance variation of a coil pair encircling an eccentric metallic cylinder, applicable to 3-D asymmetric cases where vibration and wobble exist. The analytical solution is further simplified for faster calculation while maintaining high consistency with the complete model. Moreover, an inverse approach is proposed to simultaneously measure rod conductivity and its eccentricity from the center based on the simplified analytical model, exploiting the crossing frequency between the real and imaginary parts of the inductance spectra. A modified Newton–Raphson method is employed to reduce the estimation error further. Experiments are carried out using a multifrequency eddy current sensor to test different metallic specimens, the results of which validated the effectiveness of the analytical solution. Finally, the proposed inverse approach achieves high-accuracy estimations for both conductivity and eccentricity.