Double-plate calibration for coil capacitance effect and application in high-frequency eddy-current testing

In high-frequency eddy-current testing, the coil impedance is often seriously affected by the capacitance effect, while the existing calibration method only corrects the capacitive couplings within the coil itself and ignores the coupling between the coil and the conductor. Combined with the equivalent circuit, a correction method for both the capacitive couplings is proposed using single-plate calibration. Furthermore, considering the skin effect at high frequencies, which leads to the calculation error of coil impedance, a modified impedance is introduced in the equivalent circuit and the double-plate calibration is presented. Using the two proposed methods and the existing calibration method, the experimental values of the coil impedance were respectively corrected when the coil was on a single-coated plate (titanium coating and TiAl6V4 titanium alloy substrate), and compared with the theoretical values between 1 and 100 MHz. The results show a sequentially improved correction effect of the existing calibration method, single-plate calibration, and double-plate calibration. Combined with the Levenberg–Marquardt method, the double-plate calibration was applied to the parameter measurement of the conductive plate with tens of micrometer coating. Through analyzing the sensitivity function of each tested parameter, we employed the inductance to calculate the substrate conductivity at hundreds of kilohertz and the resistance to calculate the coating thickness and conductivity at tens of megahertz. The parameters of three single-coated plates with coating thicknesses of 23 μm, 32 μm and 52 μm were respectively tested, and the results show that the relative measurement errors of all parameters are less than 15%. (paper)