Inversion of lift-off distance and thickness for non-magnetic metal using eddy current testing

Xiaobai Meng Meng*, Mingyang Lu, Wuliang Yin, Abdeldjalil Bennecer, Katherine Kirk

*Corresponding author for this work

Research output: Contribution to JournalArticlepeer-review


For the electromagnetic eddy current (EC) testing, various methods have been proposed for reducing the lift-off error on the measurement of samples. In this article, 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 nonmagnetic conductive plates. The algorithm is based on a sample-independent inductance (SII) feature. That is, under high working frequencies, the inductance is found to be 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 nonmagnetic conductive samples using an iterative method. Considering the EC 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 - aluminum, 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.
Original languageEnglish
Pages (from-to)1 - 8
Number of pages8
JournalIEEE Transactions on Instrumentation and Measurement
Early online date24 Nov 2020
Publication statusE-pub ahead of print - 24 Nov 2020


  • Eddy current sensor
  • lift-off measurement
  • thickness measurement
  • non-destructive testing
  • sample-independence
  • Electrical and Electronic Engineering
  • Instrumentation
  • nondestructive testing (NDT)
  • sample independence
  • Eddy current (EC) sensor


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