Method of Two-Parameter Signal Processing from Thickness Gauge Eddy-Current Measuring Transducers of Non-Conductive Coatings on Non-Magnetic Metal Bases

The well-known existing amplitude-sensitive eddy-current method for measuring the thickness of a non-conductive coating on a non-magnetic base metal does not meet modern requirements for measurement accuracy, in particular, when measurement result is highly influenced by a change in the specific electrical conductivity of the base metal. The purpose of this study is to increase the reliability of thicknessmeasurements of non-conductive coatings on non-magnetic base metals by taking into account (suppressing) the influence of the base metalspecific electrical conductivity. To achieve this goal, a three-winding transformer compensated eddy-current probe was used. Its output signal is represented in a complex form by real and imaginary components on a complex plane, which makes it possible to implement the amplitude-phase-sensitive method of eddy-current non-destructive testing. The developed method of two-parameter signal processing based on the algorithm for determining point belonging to a polygon is described. The implementation of the latter is realized by a two-dimensional calibration of the thickness gauge with several reference bases with different specific electrical conductivity, displayed on the complex plane in the form of a fan-shaped grid. The specified algorithm determines which section of this grid the eddy-current probe signal belongs to, while the order number of the determined section reflects both the coating thickness and the specific electrical conductivity of the base metal as independent quantities. The method of selecting the number of calibration points is described, based on choosing the optimal interval between them according to the criteria of probability ofmeasurement result distribution. The non-conductive coating thickness simulating system has been developed, connecting by two-way communication with a graduated thickness gauge and implementing its automatic calibration. The test results of the applied method for measuring the thickness of a non-conductive coating on a non-magnetic base metal are presented, that showed a decrease in the deviation of the coating thickness measurementresults under the influence of the base metal specific electrical conductivity by two orders of magnitude.