Lift-off variation causes errors in eddy current measurement of metallic plate thickness. In this paper, we have developed an algorithm which can compensate this variation and produce an index that is linked to the thickness but virtually independent of lift-off. This index, termed as the compensated peak frequency, can be obtained from the measured multi-frequency inductance spectral data using the algorithm we developed in this paper. This method has been derived through mathematical manipulation and verified by both simulation and experimental data. Accuracy in thickness measurements at different lift-off proved to be within 2%.
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