Thickness measurement using liftoff point of intersection in pulsed eddy current responses for elimination of liftoff effect

Pulsed eddy current (PEC) responses would intersect at a point for a nonferrous plate when only liftoff distance is varied. In this paper, the behaviors of liftoff point of intersection (LOI) points due to a plate with varying conductivity and thickness were investigated and interpreted in physics by means of simulations and experiments. LOI points result from the invariance of the phase spectrum of PEC responses to liftoff effect, and the time of LOI points would increase with an increase of phase spectrum. Subsequently, the signatures of LOI points were compared with the peak heights and time to peaks of PEC signals and their derivatives. The results demonstrate that the signatures of LOI points possess the comparable performances in terms of sensitivity, linearity and measurement range. Moreover, the signatures of LOI points are immune to liftoff variations in nature, which shall enhance the accuracy and reliability of PEC evaluation.