Thickness Measurement of Metallic Plates with an Electromagnetic Sensor Using hase Signature Analysis

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