Improving the Signal-to-Noise Ratio of Eddy Current Thickness Gauges Used to Measure Metal Coatings on Moving Test Objects

Abstract The article focuses on the generalized application of the method developed by the authors to improve the signal-to-noise ratio based on Taguchi's robust parameter design to the entire class of surface eddy current thickness gauges for non-magnetic metal coatings, applied to metal non-ferromagnetic and weakly magnetic substrates of moving planar objects, without physically eliminating the effects of noise factors. In the specific examples presented, robust configurations of eddy current probes have been found, namely, their corresponding technical modifications, which are characterized by the maximum possible signal-to-noise ratios and ensure the minimization of the variance of the output signals directly selected from them, which were disturbed by all the uncontrolled noise factors, without additional processing. For robust design, a mathematical model of a probe over a moving object with a coating was used, which, together with the use of orthogonal arrays, allows Taguchi's designs of experiment to be created and implemented. As a result, the obtained data were used to evaluate design options using the “larger is better” quality loss function and the signal-to-noise ratios calculated on its basis, which made it possible to select the optimal combination of design and operating parameters for thickness gauges. The efficiency and reliability of the found optimal configurations of eddy current probes have been proven by confirmatory numerical calculations. Numerical-experimental Monte-Carlo method have demonstrated a significant reduction in the effect of various noises on signals of optimal thickness gauge designs with an evaluation of variations around their undistorted noise variants. Using variance analysis, a hierarchy of the influence on the characteristics of the signals of probes by their design and operating parameters has been established. This made it possible to formulate technical requirements for the accuracy of manufacturing designs and conditions for maintaining the operating modes of probes.