A Novel Triple-Coil Electromagnetic Sensor for Thickness Measurement Immune to Lift-Off Variations

Lift-off variation causes errors in eddy-current measurement of metallic plate thickness. In this paper, we designed a triple-coil sensor operating as two coil pairs and in a multifrequency mode. It is found that the difference in their peak frequencies (the frequency when the imaginary part of the inductance reaches peak) is linearly proportional to the plate thickness but virtually immune to lift-off variations. Mathematical derivation, simulation, and experimental results verified the validity of the methodology.