Fast Measurement of the Coating Thickness and Conductivity Using Eddy Currents and Plane Wave Approximation

An analytical formula for the electromagnetic field generated by a current-carrying cylindrical coil above a half-infinite conductive plate has been studied. The theoretical analysis showed that when the radius of the coil was much larger than the penetration depth, the electromagnetic wave at the plate surface could be approximated as a plane wave, and the real part of the plane wave impedance would not be affected by lift-off changes. Using normalized plane wave impedances as theoretical values and normalized coil impedances measured by swept-frequency eddy current technique as experimental values, a novel approach is proposed to rapidly and simultaneously solve the coating thickness, coating conductivity and substrate conductivity of a conductive plate with ultra-thin metallic coatings by Levenberg-Marquardt algorithm. The experimental results show that the maximum relative measurement errors of the proposed method for single- and double-coated plates are less than 8% and 18%, respectively, and the measurement time is less than 1 s, which is suitable for fast parameter detection.