Lift-off invariance (LOI) is getting much attention from researchers in the field of electromagnetic nondestructive evaluation (ENDE) because, at the LOI point, eddy-current signals for different lift-offs intersect and the signal amplitude is independent of the lift-off variance. We discuss our ongoing research into LOI, starting from an overview of the state-of-the art of pulsed eddy-current testing (PEC) systems and their use in the elimination of the lift-off effect. We have investigated LOI characteristics with respect to variation in the configuration of the PEC probe in a theoretical study, implemented by extended truncated region eigenfunction expansion (ETREE) modeling. We found that: 1) the LOI occurs when the first-order time derivative of the magnetic field signals are acquired from Hall sensors; 2) an LOI range instead of a single LOI point, when multiple lift-offs are introduced through both experimental and theoretical studies. The LOI range varies with the Hall sensor position in the probe assembly and the conductivity of the samples under inspection, which are important parameters for the design and development of PEC systems. Based on this understanding, we investigated new approaches using theoretical computation and multiple lift-off, or magnetic sensor arrays, for conductivity and lift-off estimation. Our study can be extended for design and development of multipurpose eddy-current sensor systems for surface form measurement and defect detection .
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