Correction of eddy current measurements to obtain accordance with simulation results Dieter Joneit a , Michael Nadler b , Henning Heuer a a Fraunhofer Institute for Nondestructive Testing Dresden, Germany b HTS GmbH Coswig, Coswig, Germany article info Article history: Received 22 August 2013 Received in revised form 4 November 2013 Accepted 26 December 2013 Available online 4 January 2014 Keywords: Eddy current testing Impedance measurement Simulation abstracts Forecasting an eddy current inspection task using simulation software is relevant for a better under- standing of new problems. Simulation results have to be validated to prove how good the simulation is in compliance with the reality. A comparison of the simulated results and the direct output of an ET device very often lead to significant discrepancies. It can be shown that the reason for these discrepancies lies in the testing equipment itself and how these measurement errors can be corrected. Examples illustrate the strong correlation between simulation results and corrected measured values. Crown Copyright & 2014 Published by Elsevier Ltd. All rights reserved. 1. Introduction Forecasting of an inspection task by simulation software is needed for better understanding of new problems and for choosing most suitable probes. VIC3D by Victor Technologies and CIVA by CEDRAT are well known software, which are optimized exactly for these purposes. Simulation results have to be validated to prove how good the simulation is in compliance with reality. Using impedance analy- zers or network analyzers the measured values show this good compliance [1–5]. Nevertheless the accuracy of simulation can be improved [9] and it will be done further. Using a typical eddy current testing system, the measurement results may be completely different and without any accordance to the simulation results. However, we have to use ET systems for practical purposes. Why do they not display similar values? What are the reasons for these large discrepancies? As it will be shown below, the reason for these discrepancies lies in the testing equipment itself. The displayed measurement results contain phase errors that occur in the ET instruments and are not related to the effects of the eddy current in the material to be analyzed. 2. Structure of an eddy current testing instrument In order to understand the reasons for the problems and to find solutions we have to look briefly at the structure and principles of an eddy current instrument as shown in the simplified circuit diagram (Fig. 1). The measurement frequency is generated by an oscillator powered by a coil driver to the excitation voltage U E and placed on the excita- tion coil. The coil current I L flows through the w windings of the excitation coil and results in a magnetic flux φ: φ ¼ I L  w ð1Þ In the w windings of the receiving coil, the changes of the magnetic flux induce the receiving voltage U R : U R ¼ w  dφ dt ð2Þ It will be gained using a certain gain factor G and transferred to the demodulator to generate the real and imaginary part of the measurement result. The demodulation is made by the reference signal. This technique is also defined in the standard DIN EN ISO 15548-1 for ET systems. It will be shown that even this detail is the reason for the measurement errors. Eddy current systems usually present the measured values in units of voltage, or even simpler as percentage of screen height or ADC values. In this case, these voltages U have to be calculated from the displayed values M by gain G.(U max is the maximal voltage, M max the maximal value to be displayed.) U ¼ M U max M max e G=20 ð3Þ Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ndteint NDT&E International 0963-8695/$ - see front matter Crown Copyright & 2014 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2013.12.010 NDT&E International 62 (2014) 167–171