Thickness Measurement of Metallic Plates With an Electromagnetic Sensor Using Phase Signature Analysis

This paper presents a simple model that can predict the phase signature of eddy current sensors over nonmagnetic metallic plates. In contrast to the magnitude of the eddy current signal, which generally significantly decreases with an increase in liftoff, the phase signature appears to be virtually immune to liftoff variations and, therefore, can be more reliably used in field eddy current testing. A parameter that encapsulates the coil geometry factors can be obtained through calibration with samples of known properties. This geometry parameter can then be used in the simple model to predict the phase signature of this sensor over any plate with any thickness. The use of this new model provides a fast and accurate method to measure plate thickness. Measurements and finite-element (FE) simulations made for two sample coils next to copper and aluminum plates of various thicknesses verified the theory and the proposed method.