Multi-frequency method of control for eddy current thickness measurement

When eddy-current thickness measurement is carried out, one of the disturbing factors leading to an error in determining the thickness of the conductive coating on a conducting ferromagnetic or non-ferromagnetic substrate are the variations of the electromagnetic parameters of the coating and the substrate observed when the transducer moves from point to point along the surface of the controlled product, when moving from one product to another, at presence of  heat treatment or other thermal effects on the controlled product after coating. The paper presents the results of experimental studies of the influence of variations in the electromagnetic parameters of a conducting ferromagnetic substrate on the phase of the emf, introduced into the superimposed transducer. It is shown, when the minimum influence of such variations on the specified phase is achieved. As a result, it was suggested to use the multi-frequency method to reduce the influence of variations of electromagnetic parameters on the accuracy of determining the coating thickness during application of the phase control method. It consists in the fact that the frequency of the excitation current of the transducer, mounted on the monitored product, is discretely reduced from a certain maximum to a certain minimum frequency during measurements. At the high frequency, the specific electric conductivity of the coating material is taken into account, with decreasing frequency, such a value is determined when the electromagnetic parameters of the substrate begin to affect the phase formation. Then, using the calibration curve obtained from samples from the same coating material and substrate as the controlled article and having a known coating thickness, the desired coating thickness on the product to be tested is determined.

thickness gauge, eddy current method, conductivity, conductive coating, layer thickness, thickness measurement error

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