Estimation of Unknown Parameters of the Equivalent Electrical Model During an Eddy Current Test

Abstract

This article presents a method to estimate unknown parameters in an eddy current detection system: the magnetic coupling coefficient $k$ between the probe coil and the surface of a conductive sample and also a constant $C$ dependent on the material and geometrical properties of the conductive target. These parameters are defined from the analysis of the electrical equivalent model of the measurement system. Thereby, the liftoff from an ECT probe can be estimated during a scanning procedure by estimating $k$ . The presented method depends on the input impedance of the inductive probe stimulated by a power source with an appropriate range of frequencies. The self-inductance as well the intrinsic resistance of the probe needs to be measured in advance. While the input impedance is monitored, the magnetic coupling coefficient $k$ and constant $C$ are continuously estimated with a recursive least squares method. A theoretical analysis is presented and supported by experiments conducted with three different conductive metal samples: aluminum, copper, and brass. The results presented good agreement with the reference and estimated parameters $C$ and $k$ as well as the liftoff between the probe and the conductive target.