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Inversion of Distance and Magnetic Permeability Based on Material-Independent and Liftoff Insensitive Algorithms Using Eddy Current Sensor

Eddy current sensors can be used to test the characteristics and measure the parameters of the conductive samples. As the main obstacle of the multifrequency eddy current (MEC) sensor, the liftoff distance affects the effectiveness and accuracy of the measurement. In this article, a material-independent algorithm has been proposed for the restoration of the liftoff distance when using the MEC sensor, which is based on the approximation under the thin-skin effect. Experiment testing on the performance of the proposed method is presented. The results show that from the dual-frequency inductance, the liftoff distance could be restored with a maximum error of 0.24 mm for the distance up to 12 mm. Besides, the derived liftoff distance is used for the inversion of the magnetic permeability. Based on a liftoff insensitive inductance (LII) feature, the magnetic permeability of steels can be inversed in an iterative manner, with an error of less than 0.6 % for the liftoff distance up to 12 mm.