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

Abstract

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.

Determination of Surface Crack Orientation Based on Thin-Skin Regime Using Triple-Coil Drive–Pickup Eddy-Current Sensor

“…8 (b), the angle of the surface crack is retrieved. Since the inductance change dominates under high frequencies (compared to the resistance) [14,[46][47][48], the imaginary part in Fig. 8 (b) is used for the orientation retrieval.…”

Conductivity Classification of Multi-Shape Nonmagnetic Metal Considering Spatial Position Drift Effect with a Triple-Coil Electromagnetic Sensor

“…Although there is relatively little literature on the classification of multi-shape metals, researchers have developed theoretical models of electromagnetic induction in metals of different shapes to study qualitatively and quantitatively the impedance change. These theoretical models have been applied to thickness, conductivity, and magnetic permeability measurements [ 15 , 16 ]. Dodd and Deeds explained an analytical model of an infinite half-plane for an axially symmetric eddy-current problem [ 17 ]; Du et al, implemented a metal classification method for planar metals using real and imaginary trajectories of mutual inductance change [ 18 ]; Liu et al, considered the case of tilted planar metals leading to lift-off variations and extracted the characteristic slope of the normalized inductance trajectory on the complex plane to achieve the classification of tilted planar metals [ 19 ]; Yin et al, designed a coaxial triple-coil sensor and a compensation algorithm to eliminate the effect caused by lift-off on the peak frequency of the imaginary part of inductance change [ 20 ].…”

A Novel Conductivity Classification Technique for Nonmagnetic Metal Immune to Tilt Variations Using Eddy Current Testing

“…In this respect, nonmagnetic metals cannot be classified during the recovery process using magnetic deflection forces like ferromagnetic metals [6], but different types of nonmagnetic metals have different conductive properties. Therefore, eddy current testing is very suitable for detecting the electromagnetic properties of metals due to the advantages of non-contact, non-destructive, high adaptability and fast detection speed [7][8][9]. Currently, there are three main techniques for eddy current testing: single-frequency eddy current (SEC) testing, multifrequency eddy current (MEC) testing, and pulsed eddy current (PEC) testing [10][11][12][13][14][15].…”