Liftoff-Insensitive Conductivity Mapping Using a Self-Resonant Eddy-Current Probe

This study proposes a novel approach for contactless conductivity measurement of nonmagnetic metal sheets for a production line. The system uses a self-resonant eddy-current (EC) sensor as the conductivity probe, of which the sensitivity is enhanced by minimizing the parasitic resistance in the resonant circuit. The output responses at various liftoff distances are nonlinear curves unique to the sample’s conductivity. The quaternary polynomial regression is utilized to establish the analytic approximation function to depict the correlation between the phase and the logarithmic magnitude for standard samples. The phase–magnitude relationships are generally monotonic and useful for conductivity conversion using the standard data. The polynomial interpolation is employed to establish the phase–conductivity relations based on the standard samples. The self-developed computer software can instantaneously convert the probe output into the conductivity of the metallic material under test. The experimental findings point to the feasibility of the suggested method that can perform conductivity measurement with a relative error of less than 3% at a liftoff distance of less than 1 mm, indicating the practicability of the proposed technique in conductivity mapping for nonferrous metals.