Pulsed eddy current thickness measurement using phase features immune to liftoff effect

Conventionally, peak value and peak time are extracted from pulsed eddy current (PEC) response as features for thickness measurement. However, they suffer from liftoff variations. In this work, the phase of spectral PEC response from a Hall sensor are proposed to serve as robust features for thickness evaluation. The presented novel features are immune to liftoff effect, because phase signals remain nearly constant against liftoff variations. An analytical model was formulated, and simulations were performed to uncover the physics of the characteristics of the phase feature and establish the relationship between the phase feature and sample thickness. Experiments were carried out to validate the proposed phase feature. Eventually, the proposed phase feature was evaluated for accurate thickness measurement and some key factors were discussed.