Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

# Inversion of Lift-Off Distance and Thickness for Non-Magnetic Metal Using Eddy Current Testing

Version 1 : Received: 6 November 2020 / Approved: 9 November 2020 / Online: 9 November 2020 (11:11:44 CET)

How to cite: Meng, X.; Lu, M.; Yin, W.; Bennecer, A.; Kirk, K. Inversion of Lift-Off Distance and Thickness for Non-Magnetic Metal Using Eddy Current Testing. Preprints 2020, 2020110278 (doi: 10.20944/preprints202011.0278.v1). Meng, X.; Lu, M.; Yin, W.; Bennecer, A.; Kirk, K. Inversion of Lift-Off Distance and Thickness for Non-Magnetic Metal Using Eddy Current Testing. Preprints 2020, 2020110278 (doi: 10.20944/preprints202011.0278.v1).

## Abstract

For the electromagnetic eddy current testing, various methods have been proposed for reducing the lift-off error on the measurement of samples. In this paper, instead of eliminating the measurement error caused by the lift-off effect, an algorithm has been proposed to directly measure the lift-off distance between the sensor and non-magnetic conductive plates. The algorithm is based on a sample-independent inductance (SII) feature. That is, under high working frequencies, the inductance is found sensitive to the lift-off distance and independent of the test piece under an optimal single high working frequency (43.87 kHz). Furthermore, the predicted lift-off distance is used for the thickness prediction of the non-magnetic conductive samples using an iterative method. Considering the eddy current skin depth, the thickness prediction is operated under a single lower frequency (0.20 kHz). As the inductance has different sensitivities to the lift-off and thickness, the prediction error of the sample thickness is different from that of the lift-off distance. From the experiments on three different nonmagnetic samples – aluminium, copper, and brass, the maximum prediction error of the lift-off distance and sample thickness is 1.1 mm and 5.42 % respectively at the lift-off of 12.0 mm.

## Keywords

Eddy current sensor; lift-off measurement; thickness measurement; non-destructive testing; sample-independence.

## Subject

ENGINEERING, Automotive Engineering

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