Two approaches to coil impedance calculation of eddy current sensor

Abstract

Eddy current (EC) sensor detects measurand by the change of coil impedance, which is introduced by coupling the magnetic energy between coil and measurement target. Therefore, it is of great significance for both the forward and inverse eddy current problems to study on the approaches to coil impedance calculation. In the current paper, two approaches are presented to calculate coil impedance of EC sensor. First, the theoretical calculation model of coil impedance is simplified as an air-cored cylindrical coil, over an infinite measurement target. And the integral expression of coil impedance is derived according to the sensor operation principle and electromagnetic field theory. Then, the integral expression is expanded as series form in cylindrical coordinate system in order to compute the coil impedance easily, and computed using MathematicaTM. Next, based on the finite element modeling theory, two different finite element models of eddy current measurement system are built. The infinite boundary in mode1 is truncated, while in model 2, it is simulated completely by an infinite element. Furthermore, the appropriate location where the air domain is truncated at is determined. Finally, the coil impedance and the magnetic distribution of eddy current testing system are obtained by finite element method, respectively. By comparison, the results from the theoretical calculation approach and finite element method are in very good agreement, and the accuracy of two approaches is validated with each other.