LabVIEW-Based data acquisition system for eddy current testing probe

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LabVIEW-Based data acquisition system for eddy current testing probe

Meziane Hamel, Mouloud Zaidi

Laboratoire de modélisation numérique des phénomènes électromagnétiques et composants

Université de Tizi-Ouzou h_meziane@hotmail.com

Farid Hocini, Hassane Mohellebi

Laboratoire de modélisation numérique des phénomènes électromagnétiques et composants

Université de Tizi-Ouzou mohellebi@yahoo.fr

Abstract—This paper presents the implementation aspects of a LabVIEW-Based data acquisition system intended for an eddy current testing (ECT) probe. The goal of this work is to design and develop a data acquisition system that can be used for automated non destructive testing tests and measurements applications. The system deals with a pancake type coil, placed above a flat copper plate with a crack. The probe coil moves along the crack length direction and is connected to a National Instruments (NI) data acquisition board. The system developed under this work has the capability to acquire the impedance of the coil. The experimental results are compared with LCR-meter measurements.

Index Terms—Data acquisition, eddy current testing, LabVIEW, sensor, impedance.

I. INTRODUCTION

The ECT is an attractive non-destructive electromagnetic method. It is used in all industries that are dealing with pieces made of an electrically conducting material. It can operate in a production line, in an installation under operation, and at maintenance time. This popularity and this diversity are due to a number of technical characteristics including speed, high sensitivity, and feasibility of implementation on complex structures with adaptable sensors [1]. The cracks and defects are real threats for reliability of a structure, as they can rapidly grow to cause failures of structural integrity, to prevent these failures ECT probes are used as a predictive approach to maintain the safety of the structures [2]. The exploited electromagnetic feature is the variation of impedance of the sensor [3].

LabVIEW is the graphical development environment for creating flexible and scalable test, measurement, and control applications rapidly and at minimal cost. With LabVIEW, engineers and scientists interface with real-world signals, analyze data for meaningful information, and share results and applications [4]. Regardless of experience, LabVIEW makes development fast and easy for all users. LabVIEW programs are called virtual instruments (VI), because their appearance and operation imitate physical instruments, such as oscilloscopes and multimeters. Every VI uses functions that

manipulate input from the user interface or other sources and display that information or move it to other files or other computers [5].

In this paper we developed a LabVIEW program for data acquisition from ECT-probe.

II. EXPERIMENTAL SETUP

The ECT system deals with a pancake type coil, placed above a copper plate with a crack [6]. The probe coil has axis- symmetric shape and moves along the crack length direction, it’s characterized by the following parameters: inner radius 2.35 mm, outer radius 4.5 mm, number of turns 170. The tested sample is a cooper plate of 4 mm thickness, 60 mm width and 110 mm length. The crack having the width of 2 mm and 12 mm length at 1.3 mm depth from the sample surface. A LabVIEW-Based data acquisition program was developed to continuously monitor the variation in the impedance of the coil and is observed on the computer screen.

Fig. 1. Test bench.

III. SIGNAL PROCESSING

The excitation current is produced by a function generator.

The voltage signals V₁, V₂ obtained from the ECT probe are applied to the analog inputs of the NI PCI-6281 16-bit

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multifunction board working with a maximum acquisition rate of 625 kHz.

The scan starts from the middle of the crack. For each scan position of the ECT probe the output signals are acquired. The estimation of the signal amplitude is obtained using LabVIEW program, thus the variation of impedance is calculated and will be analyzed for crack and flaw detection and characterization.

The calculated impedance is stored together with the coordinate of the corresponding probe localization of the ECT probe.

Fig. 2. Eddy current testing system.

Figure 3 and figure 4 show the program blocks in LabVIEW

Fig. 3. Block diagram.

Fig. 4. Front panel.

IV. RESULTS AND DISCUSSION

The ECT-probe is driven by a sinusoidal excitation current with different frequencies 50 kHz, 100 kHz, 150 kHz, in order to monitor the sensitivity of superficial crack. Signals for impedance variation are shown in the following figures.

Fig. 5. Variation of impedance according to displacement (f = 50 KHz).

0 2 4 6 8 10 12 14 16 18

34 34.5 35 35.5 36 36.5 37 37.5 38

Position (mm)

Impedance (Ohm)

Plate with crack

LabVIEW

&

Computer display Function

Generator

NI PCI- 6281 V₁

V2

Shunt

Sensor

LCR-meter results Data acquisition results

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We observe that the impedance variation is all the more important as the frequency is high. The operating frequency chosen for the inspection have a significant effect on the eddy current probe response. Frequency selection affects both the phase relationship and the relative signal strength of the response from the flaw. Choosing the proper frequency is critical to acquiring optimal resolution between flaw signals and noise contributors from the material under test. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity.

V. CONCLUSION

In this paper a practical approach concerning eddy current non destructive testing using a pancake probe was presented. A Data acquisition system has been implemented using a PCI card. The application software for the system was developed using graphical language LabVIEW.

LabVIEW provides the flexibility of integration of data acquisition software/hardware for test and measurement applications. A good agreement between the experimental results and LCR-meter measurements was obtained.

REFERENCES

[1] A. Ferrigno, M. Laracca and M. Molinara, “Crack shape reconstruction in eddy current testing using machine learning systems for regression”, instrumentation and measurement , IEEE transactions on, vol 57, pp. 1958-1968, September 2008.

[2] L.Udpa and S.S. Udpa, “neural Networks for the classification of non-destructive evaluation signals”, IEE Proceedings-F, vol 138, pp 201-205, February 1991.

[3] M. Tanaka, “Finite element model of natural crack in eddy current testing problem”, IEEE Transaction on Magnetic, Vol.37, pp 3125-3128, September 2001.

[4] National Instruments Corp,”Getting started with LabVIEW”, April 2003.

[5] D. Jianjun and T. Tonghui, “The reconstruction of crack in surface based on virtual instrument”, International conference on Information engineering and computer science, pp 1-4, December 2009.

[6] P. Burascano, E. Cardelli, “Numerical analysis of eddy current non destructive testing JSAEM benchmark problem #6 cracks with different shapes”, Proc ENDE-V Springer, pp 333-340, June 2000.

0 2 4 6 8 10 12 14 16 18

66 67 68 69 70 71 72 73

Position (mm)

Impedance (Ohm)

0 2 4 6 8 10 12 14 16 18

96 98 100 102 104 106 108

Position (mm)

Impedance (Ohm)

LCR-meter results Data acquisition results