Eddy-current reflection probes: Theory and experiment

A general ΔZ theory of reflection-type eddy-current probes was recently developed for new types of robotic proximity sensors. In this paper, the theory is applied to the characterization of surface-connected flaws. Flaw signals were calculated using a finite-difference implementation of the ΔZ theory developed at Stanford University. A special air-core eddy-current reflection probe was fabricated at the National Institute of Standards and Technology (NIST) and used to obtain flaw signals for a number of rectangular-shaped electrical-discharge-machined (EDM) notches in aluminum. An automatic impedance analyzer was used to measure flaw signals as the probe was scanned over the length of the flaw. Experimental results were then compared to predictions of the theory.