Determining the Effective Design of Eddy Current Probes with Compensation Features for Measuring Coating Thickness on Moving Objects

Abstract

The publication contains the results of a study on selecting the best design among known self-differential and self-nulling properties of eddy current probes for intended measuring the thickness of metal coatings on planar moving test objects. This selection is determined by the level and sensitivity of the output information signal. The study specifically considered designs employing rectangular excitation coils, which generate both electromagnetic fields perpendicular to the object’s surface and fields oriented parallel to it, the latter being referred to as tangential fields. Rectangular excitation coils are capable of generating an almost uniform distribution of eddy currents within the measurement object. The probe designs under analysis are characterized by the orthogonal arrangement of the pick-up coils with respect to the excitation coil. Both rectangular and cylindrical pick-up coils were investigated. Mathematical models for a range of probe designs were selected to calculate the information signals, and dedicated software was developed for this purpose. The software has been verified and its adequacy established by comparing it with original data on signal dynamics at varying excitation frequencies and movement speeds of the objects being measured. For this purpose, data from experiments published by other researchers were utilized. Information on non-magnetic and weakly magnetic substrates, as well as metals used as surface coatings, was employed in the design selection process. These data enabled numerical modeling to predict probe output signals as a function of coating thickness variations. Calculations were performed for a moving object composed of two layers with distinct electrophysical properties, one of which represented the coating. As a result of the model calculations, the design featuring orthogonal rectangular coils was selected, as it demonstrated the highest signal performance among the options considered.