Impedance calculation of arbitrary-shaped thin-walled coils for eddy-current testing of planar media

Eddy-current testing is used in a variety of fields, so it is important to analyze the coil impedance. Previous studies on impedance calculation have mainly focused on circular and rectangular coils. This paper presents a general method to evaluate the impedance of an arbitrary-shaped thin-walled coil facing with a planar media. The impedance formulas are deduced by using second order vector potential (SOVP) method. In the formulas, the coil function, which is in double-integral form, is defined. The impedance calculation for arbitrary-shaped coils can be represented by the corresponding coil function. The coil functions for common coils such as circular, triangular, rectangular and trapezoidal coils are also derived in this paper. Finally, the impedance and impedance change of various shaped coils above an aluminum plate are measured, and compared with calculated values. The results indicated that they have a good agreement.