Giant magnetoresistance-based eddy-current sensor - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 5, SEPTEMBER 2001

3831

Giant Magnetoresistance-Based Eddy-Current Sensor Teodor Dogaru and Stuart T. Smith

Abstract—The purpose of this paper is to introduce a new eddycurrent testing technique for surface or near-surface defect detection in nonmagnetic metals using giant magnetoresistive (GMR) sensors. It is shown that GMR-based eddy-current probes are able to accurately detect short surface-breaking cracks in conductive materials. The self-rectifying property of the GMR sensor used in this study leads to a simplified signal conditioning circuit, which can be fully integrated on a silicon chip with the GMR sensor. The ability to manufacture probes having small dimensions and high sensitivity (220 mV/mT) to low magnetic fields over a broad frequency range (from dc up to 1 MHz) enhances the spatial resolution of such an eddy-current testing (ECT) probe. Experimental results obtained by scanning two different probes over a slotted aluminum specimen are presented. General performance characteristics are demonstrated by measurements of surface and subsurface defects of different sizes and geometries. Dependence of the sensor output on orientation, liftoff distance, and excitation intensity is also investigated. Index Terms—Crack detection, eddy-current testing, giant magnetoresistance (GMR), magnetic sensors.

I. INTRODUCTION

I

NCREASED research activity on nondestructive testing has been motivated by the need for precise evaluation of cracks and flaws for the assessment of the expected life of mechanical components. Along with a variety of methods that include dye penetrants, X-ray, and ultrasonic testing, eddy-current testing (ECT) is also commonly used for detecting defects such as fatigue cracks, inclusions, voids, etc., in conductive materials. Eddy currents are induced in a specimen as a result of the application of an alternating magnetic field. A cylindrical coil can be used to produce an excitation magnetic field that induces circular currents constrained to a region near to the surface of the specimen under test. In the presence of defects, which act as a high resistance barrier, the eddy-current flow is perturbed. As a result of this defect, a “leakage” magnetic field is produced. Such field perturbations are usually detected as an impedance change in the exciting coil [1]. To enhance the sensitivity and spatial resolution of the measurement, more advanced techniques based on the separation of the excitation and detection elements have been developed. Inductive sensors of reduced dimensions [2], [3] have been proposed for detection. In recent years, electromagnetic methods for eddy-current inspection have attracted increasing attention. Electromagnetic sensors, based on either Hall effect, anisotropic magnetoresis-

Manuscript received April 29, 2000; revised March 28, 2001. T. Dogaru is with the Department of Electrical Engineering, The University of North Carolina at Charlotte, Charlotte, NC 28223 USA. S. T. Smith is with the Center for Precision Engineering, The University of North Carolina at Charlotte, Charlotte, NC 28223 USA. Publisher Item Identifier S 0018-9464(01)07983-3.

tance (AMR) [4], [5], giant magnetoresistance (GMR) effect [6], [7], or SQUID have been successfully used for crack detection. Among these, the magnetoresistive sensors offer a good tradeoff in terms of performance versus cost. They have small dimensions, high sensitivity over a broad range of frequency (from hertz to megahertz domains), low noise, operate at room temperature, and are inexpensive. It has been demonstrated that the magnetoresistive probes perform better than conventional probes for low-frequency applications, i.e., when detecting deep buried flaws [6]. This is because the electromagnetic sensors are sensitive to the magnitude of the magnetic field. In the case of inductive-based probes, the output voltage is proportional to the rate of change of the magnetic field, therefore, their sensitivity is reduced at low frequency. Although their sensitivities are comparable, GMR sensors have better directional property than AMR sensors. Both types of sensors detect the component of the magnetic field vector along their sensitive axis. In the case of GMR sensors, fields applied perpendicularly to the sensitive axis have negligible effect on their output. In contrast, the sensitivity of AMR-based probes is lowered by a field perpendicular to the sensitive axis, which, at high values, can even “flip” the sensor response [4]. This property is particularly important in the coil–crack interaction problems, where the electromagnetic field has a complex three-dimensional (3-D) geometry. The directional property of GMR sensor can be used in a difficult problem encountered in NDE, detection of edge cracks [7]. It is shown that, by properly orienting the sensitive axis, the probe will be insensitive to the edge. Additionally, the presence of the edge enhances the sensitivity and resolution of the GMR probe to cracks initiating perpendicular to this edge. Most research using magnetoresistive sensors has been focused in the direction of deep flaw detection, a critical problem in the inspection of aircraft structures, such as riveted multilayers. This paper approaches a different problem, that of detecting small defects, such as short cracks at the surface or near the surface of a conductive specimen. This can be of particular importance in different applications, such as detecting fatigue cracks in early stages of development. It is envisaged that these two problems are optimally addressed using different probe designs. In general, in order to obtain a high penetration depth of the field and eddy currents into the material, large coils producing low-frequency excitation fields are required [8]. At the same time the resolution of the probe is limited by the coil diameter, and the only way to enhance the resolution is to reduce the coil dimensions. Therefore, in the case of deep flaw detection, only relatively large defects are reliably detected. Currently, inductive probes based on small pickup coils are used for surface crack detection with high resolution. However,

0018–9464/01$10.00 © 2001 IEEE

3832


(a)

(b) Fig. 1. Schematic diagram indicating probe assembly and coordinate system. (a) Cross section of the probe assembly utilizing a cylindrical coil surrounding the sensor. (b) Cross section of the probe assembly with flat coil placed on the sensor package.

the reduction of the coil diameter leads to the decrease in sensitivity of the these probes. This is because the induced voltage depends proportionally to the magnetic flux intercepted by the pickup coil, which in turn is proportional to its cross-section area [5]. This paper demonstrates that GMR-based probes are capable of detecting short surface-breaking cracks with high accuracy. The technique is based on the detection of the magnetic field in a direction parallel to the specimen surface, rather than in the perpendicular direction [6]. This method leads to a very simple self-nulling probe, comprising a small, flat excitation coil with the sensing element centrally located, without the need of auxiliary coils or circuitry to cancel the excitation field at the sensor location. The design also takes advantage of the planar geometry of the GMR sensor. Placing the coil between the sensor and the specimen surface, the liftoff of the sensor is lower than in the case of the vertical sensor configuration. Most importantly, the coil diameter can be dramatically reduced in the parallel configuration by using a flat spiral coil design. With this probe configuration, a better spatial resolution of crack detection is achievable. Another major benefit arises from the ability to integrate GMR manufacture with silicon planar technology. In principle, the coil can be directly patterned and deposited on the top of the GMR sensor. Signal conditioning circuitry may also be integrated with the sensor-coil system, leading to very compact eddy-current systems. Additionally, the planar technology enables the design of multisensor arrays to reduce inspection times in industrial applications.

Another characteristic of the GMR sensor that makes it unique among the existing transducers used in ECT is its self-rectifying property. Being a unipolar device, i.e., positive output for both positive and negative applied field, GMR sensors can simplify the signal conditioning circuit of the ECT transducer by eliminating the need for synchronous detection. This paper presents the geometric design and an experimental performance evaluation of a GMR sensor-based eddy-current probe. Two different probe designs, both based on the same excitation and detection configuration, are proposed. The first design can be used for detecting relatively large surface and subsurface defects, the other for detecting small surface flaws. II. PRINCIPLE OF OPERATION The main components of the eddy-current probes comprise either a relatively large cylindrical coil or a flat spiral “pancake-type” coil with the GMR sensor located on the coil axis. Probe geometry is shown in Fig. 1(a) and (b), while dimensions of the coils used in the experiments of this paper are given in Table I. The GMR sensor consists of four thin-film resistors in a Wheatstone bridge configuration with two of them being magnetically shielded and acting as dummy resistors. This sensor has been produced on a silicon substrate and this is, in turn, housed in a standard in-line package. The sensing axis of the GMR probe is coplanar with the surface of the specimen. The excitation field on the coil axis, being perpendicular to the sensing axis of the GMR films, has no effect on the sensor. In

DOGARU AND SMITH: GMR-BASED EDDY-CURRENT SENSOR

3833

TABLE I GEOMETRIC PARAMETERS OF THE COILS

Fig. 2. Block diagram indicating key components of eddy-current testing system.

a defect-free specimen, because of the circular symmetry of the induced eddy currents, these will produce no effect on the sensor output. In the presence of a defect, output signal from the sensor is produced only by the perturbation of the eddy-current flow path. A block diagram of the measurement system is represented in Fig. 2. The output of the sensor, which is an amplitude-modulated and rectified signal, is low-pass filtered. In this way, the dc component of the signal, being proportional to the amplitude of the field created by the flaw, is immediately extracted.

Fig. 3. General response of GMR sensor to a field applied in the direction of its sensing axis (reproduced from data sheets provided by NVE).

GEOMETRY

OF

TABLE II FLAWS MACHINED INTO THE SURFACE ALUMINUM PLATE

OF THE

III. GMR SENSOR CHARACTERISTICS In all subsequent experiments, a GMR bridge sensor manufactured by NVE company was used. The size of the sensor, which is deposited on silicon substrate, is 0.44 by 3.37 mm, although the dimensions of the active area of the bridge (the two sensitive resistors) is about 100 by 200 m in the middle of the layout. A typical magnetoresistance characteristic of this kind of sensor, from which the symmetric unipolar action is apparent, is shown in Fig. 3. In this figure, the double value near the origin of the magnetoresistance characteristic is due to the hysteresis effect. In general, the hysteresis affects the shape of the rectified signal, introducing high-frequency harmonics in the output response to a sinusoidal field. Because the signal is low-pass filtered to extract the mean value, these harmonics are not present in the output. Such a distortion will also have a small effect on the mean value, however, the linear relation between the amplitude of the excitation field and dc output will be maintained. Measurements on the sensors used in these experiments revealed a dc sensitivity of 130 mV/mT for 5-V voltage applied to the bridge. Because the output is linear with bridge voltage, the sensitivity can be expressed in the more general form as 26 mV/V mT . The AC sensitivity is defined as the ratio between the dc component of the sensor output voltage (of the full-rectified signal) and the rms value of the sinusoidal applied field. For fields below saturation, this value was found to be 110 mV/mT for 5 V applied to the bridge (22 mV/V mT ) and was observed to be constant over the whole range of frequencies from dc to 100 kHz. Saturation field for the GMR sensor occurs

at around 2 mT while the linear range is between 0 and 1.5 mT. Other characteristics of the GMR as provided by the manufacturer include a temperature range of operation from 50 C to 125 C and a bridge offset of between 4 and 4 mV, nonlinearity of the magnetoresistance characteristic is 1.5% and hysteresis is around 3.5%. IV. EXPERIMENTAL SETUP An experimental benchmark was developed to assess the feasibility of the GMR eddy-current probe as an effective flaw detector. Various calibrated defects were machined into the surface of an aluminum plate by using end-milling cutters. The dimensions of the defects are given in Table II. A second aluminum plate having cracks of fixed length (15 mm) and width (0.5 mm) but varying depths of 0.25, 0.5, 1.0, 2.0, and 4 mm was produced. In addition, an infinitely long crack of varying width was simulated by abutting two plates of aluminum separated by Mylar sheets of constant thickness (0.1 mm). The adjacent surfaces of the two plates were very finely ground to create a good contact in the absence of separating sheets. In this way, a “zero” width crack was simulated. A schematic diagram of the experimental setup is shown in Fig. 4. A sinusoidal current source provides a current through the coil of controlled amplitude (up to 3 A) and frequency (between 1 and 100 kHz). When not specified, the frequency used in the experiments described below is 30 kHz and the amplitude of the current through the coil is 1 A. At this frequency, the

3834


Fig. 4. Block diagram of experimental setup for scanning of specimens containing geometric flaws.

skin depth for aluminum is approximately 0.5 mm. The ECT probe is scanned over the surface of specimen in and directions by using a coordinate measuring machine (CMM). A computer program was used to set the scan area and velocity. This program could also be used to adjust the liftoff distance between specimen surface and the lower face of the probe. When not specified, the liftoff between the surface of the specimen and the underside of the probe was 0.1 mm. During measurements, the sensor’s output signal was amplified ( 20) and filtered by a second-order low-pass filter with a cutoff frequency of 10 Hz using a Stanford Research Systems SR560 low-noise preamplifier. The input across the two arms of the sensor bridge network was supplied with a dc voltage of 10 V. A data acquisition program written in Labview collects data from the output of the filter via a National Instruments ATMIO 16 16 bit analog-to-digital converter. For each scanning cycle, samples of filtered signal were collected at a sampling rate specified by the user. Time domain is converted into space domain taking into account the relation between number of samples, sampling rate, scanning velocity, and scanning length. Finally, for visualization of these results, 3-D maps representing the output voltage of the sensor (amplified and filtered) as a function of – displacements are plotted. V. RESULTS First, the probe using the large cylindrical excitation coil [see Fig. 1(a)] was scanned over the 15-mm surface crack (Table II) with the GMR’s sensing axis perpendicular to the direction of the crack. The map obtained, Fig. 5(a), shows two symmetrical peaks that are located on each side of the crack. In this figure it is noticed that, besides the central maxima at either side of the crack, each peak has two shoulders. Locating these shoulders at the points that correspond to the minimum magnitude of the output slope, it is observed, for this particular case, that the distance between these coincides with the length of the crack. This can be more clearly seen in Fig. 5(b). Rotating the sensing axis, it is observed that the central region of both peaks reduces in magnitude while the amplitude of the shoulders varies [Fig. 6(a)]. In particular, for each pair of shoulders, one reduces while the other increases in magnitude. Simultaneously, as the sensing axis is rotated, the position of the high shoulders, which become peaks, move in an arc about the crack tip. Finally, with

(a)

(b) Fig. 5. Output from eddy-current sensor scanning a crack of length 15 mm and depth 2 mm with sensing axis perpendicular to crack orientation. (a) Map showing magnitude and contours of sensor output. (b) Output of sensor corresponding to scanning in a direction along the crack.

the sensing axis oriented along the crack, Fig. 6(b), only two peaks remain with a line drawn between the two being coincident with the line of the crack. The distance between these peaks is longer than the actual length of the crack. Being shorter than the diameter of the coil, the crack of 5-mm length was scanned by the probe with the sensing axis perpendicular to the crack. The result is shown in Fig. 7. In this case, what were shoulders for the long crack [see Fig. 5(a)] become pairs of peaks located on each side of the crack. The asymmetry in Fig. 7 is due to misalignments during measurement. There are three components of the measurement that can lead to the asymmetry errors: the coil, sensor, and specimen’s surface. Misalignment between each pair of these components is responsible for the asymmetry seen in Fig. 7 and can also cause an offset of the response when the probe is scanned over a defect-free specimen. Coil–sensor misalignment, which can be both positional and angular, can be minimized by measuring the output of the sensor when the coil is far away from the specimen surface


3835

(a) Fig. 7. Output after scanning a crack, of length 5 mm, that is shorter than the diameter of the excitation coil; sensing axis perpendicular to the crack.

(b) Fig. 6. Output from eddy-current sensor scanning a crack of length 15 mm and depth 2 mm. (a) Sensing axis at 60 to crack orientation. (b) Sensing axis parallel to crack orientation.

and aligning the system until the output becomes zero. Angular alignment between the coil and specimen’s surface depends on the fixturing of these in the scanning system. In practice, there are two sources producing a dc offset during measurement one is due to the misalignments mentioned above while the other is produced by intrinsic tolerances of the sensor resistors and cumulative offsets in signal conditioning electronics. The offset in all measurements has been numerically removed. For the measurement of short surface cracks, a second probe was manufactured in which a flat coil having the dimensions given in Table I was attached onto the surface of the GMR sensor package. Fig. 8 shows results when scanning the same 5-mm crack with sensing axis perpendicular to the crack orientation. In this case, the crack is longer than the mean coil diameter and produces a map that is similar to that of Fig. 5(a). To determine the spatial resolution of this second probe design, cracks of length 2 and 1 mm were scanned [Fig. 9(a) and (b)]. It can be seen that, in this case for which the crack is shorter than the mean coil diameter, the central peaks either side of the crack are no longer visible and, for the very short crack, the amplitudes of the peaks corresponding to the crack tips are re-

Fig. 8. Output when scanning a 5-mm crack using the flat coil probe design; sensing axis perpendicular to the crack.

duced, approaching the sensor noise. However, it is apparent that with further reduction in coil dimensions shorter cracks may be readily measured. To measure subsurface cracks, it is necessary to lower the excitation frequency to increase penetration of the field and increase the coil dimensions to reduce divergence of the field. Fig. 10 is the map of the output when scanning the 15-mm subsurface crack of Table II using the large coil design. In this case, the map has been obtained by scanning a crack on the reverse side of the specimen, often referred to as an “outer defect” measurement. For this measurement, the maximum sensitivity of the probe occurred at an excitation frequency of 1.5 kHz. This map is similar to the map of Fig. 5(a) with a 15 reduction in the amplitude of the peaks. Effects of liftoff, crack depth, crack width, and excitation intensity have been measured using the probe design based on the large coil. To study the influence of the separation between the lower surface of the probe and upper surface of the specimen, the 15- mm- long crack was scanned again while the liftoff distance

3836


(a) Fig. 10.

Scan of a subsurface crack of length 15 mm.

(b) Fig. 9. Scans of short cracks using flat coil design. (a) Crack of 2-mm length. (b) Crack of 1-mm length.

was varied between 0.1 and 0.4 mm at 0.1-mm increments. The effective liftoff of the GMR sensor is calculated as the sum of the distance from the specimen surface to the underside of the probe (which is the top of the sensor package) and the 0.5-mm distance between the top of the package and the silicon chip on which the thin-film sensor is deposited. Therefore, the effective liftoff of the sensor varied from 0.6 to 1 mm in this experiment. For flat pancake designs, the sensor liftoff would include, in addition, the thickness of the coil. The sensing axis was perpendicular to crack orientation. The shape of the maps obtained are similar to the one obtained in Fig. 5(a), but the amplitude of the symmetrical peaks on either side of the crack decreases as the liftoff distance increases. This dependence is displayed in Fig. 11 and decreases in an approximately exponential manner. It was observed that the distance between the two shoulders in direction [Fig. 5(b)] is constant and does not depend on the liftoff distance. Additional experiments were carried out to determine the effects of crack depth on the measured output. In these experiments, surface cracks of length 15 mm having depths of 0.25, 0.5, 1.0, 2.0, and 4 mm were measured. Measuring the output

Fig. 11. Peak amplitude as a function of liftoff distance between probe and specimen surface.

Fig. 12. Output from eddy-current sensor when scanning across the central region of cracks of length 15 mm and depths of 0.25, 0.5, 1.0, 2.0, and 4 mm.

as the probe is scanned across the central region of each crack results in the graph of Fig. 12. As might be expected, for shallower cracks, there is considerable reduction in the magnitude of the output signal. The effect of the crack width on the output of the sensor was studied using two precisely machined plates with ground edges


3837

filter, puted from

Fig. 13. Maximum amplitude response for an infinite crack as a function of crack width.

that were placed adjacent to each other to form a long crack. First measurements were performed with the two plates in contact. Ignoring the effects of surface finish and other manufacturing tolerances, this is considered to approximate an infinite crack with zero separation of the two surfaces. Subsequently, inserting Mylar sheets between the plates varied the crack width. Each sheet has a thickness of 0.1 mm. During experiments, the liftoff distance remained constant and the sensing axis was perpendicular to the crack direction. Amplitude of the peak as a function of crack width is plotted in Fig. 13. Results show that the output voltage increases slowly tending toward a maximum value of around 3 V for rather unrealistic crack widths of over 2 mm. Further enlarging this width, it was observed that saturation of the peak value occurs at around 10-mm separation and primarily depends on the coil diameter. For separations of greater than 10 mm, the sensor measures the edge influence of only one plate (half-crack). The most striking result is that there is a large output signal even when the plates are in electrical contact. This can be explained by the large resistivity of the contact in comparison to the bulk, which produces almost the same deviation of the eddy-current paths as for a crack of finite width. The large contact resistivity is probably due to the oxide deposited on the edges and imperfectly polished surfaces. Influence of the amplitude of the excitation current in the coil was also assessed. In these measurements, the 15-mm-long surface crack of Table II was scanned with the sensing axis perpendicular to the crack orientation. An approximately linear increase of the peak amplitude with excitation current was obtained. It was observed that the distance between shoulders [refer to Fig. 5(b)] remained constant (does not depend on the excitation current).

, the rms value of the detected field can be com-

where is the filtered output voltage of the sensor. The peak amplitude of the output when scanning a 15-mm-long crack is approximately 0.3 V [Fig. 7(a)], giving 68.2 T. The maximum output voltage recorded during the experiments was 3.7 V corresponding to an infinitely long, 10-mm-wide crack, which is effectively an edge. This voltage 840 T. corresponds to Two conclusions can be inferred. First, the GMR sensor is extremely sensitive, being able to discriminate magnetic fields of the order of 10 T. Also, in all previous experiments the GMR sensor was always operating in the linear region (maxwas 840 T) and does not saturate ( is 2 mT). imum VII. CONCLUSION Based on the above results and discussions, a number of important features of this sensor can be deduced. a) The eddy-current probe design allows the location of defects and approximate evaluation of the crack length. In addition, due to its reduced dimensions, the sensor gives the local map of the distribution of the tangential (to the specimen surface) component of the magnetic field resulting from the interaction between crack and magnetic field produced by the coil. The signal detected by the sensor does not contain the applied field. b) The signal conditioning circuit of the sensor is very simple, comprising only a differential amplifier and a low-pass filter. Because of the planar technology of the GMR sensors, the signal conditioning circuit can be integrated on the same chip with the sensor bridge. The integrated sensor has already been designed and will be the subject of another paper. Besides the reduced area, the integration provides an improved signal-to-noise ratio. c) The spatial resolution of the probe is limited by the dimension of the coil. The maximum crack length that can be detected is roughly equal to the mean coil radius. Considering the dimensions of the current GMR sensor, reducing the scale of this probe design presents no immediate technological challenges. d) Sensitivity is high (220 mV/mT ) at very low fields (10 T), and the characteristic of the sensor (sensitivity and saturation field) can be customized by choosing a proper structure of the GMR multilayer.

VI. EVALUATION OF THE DETECTED FIELD

ACKNOWLEDGMENT

The magnitude of the detected field can be estimated from the experimental results from measurements using the large coil 220 design. Knowing the ac sensitivity, , of the sensor ( mV/mT for 10-V bridge supply) and the amplification of the

The authors would like to thank R. Hocken for his generous support and encouragement. They would also like to thank Dr. K. Daneshvar for providing laboratory facilities and many useful comments and suggestions.

3838


REFERENCES

1

[1] S. K. Burke, “A benchmark problem for computation of Z in eddy current nondestructive evaluation (NDE),” J. Non-Destruct. Eval., vol. 7, pp. 35–41, 1988. [2] J. Pavo and K. Miya, “Optimal design of eddy current testing probe using fluxset magnetic field sensors,” IEEE Trans. Magn., vol. 32, pp. 1597–1600, May 1996. [3] M. Uesaka, T. Nakanishi, K. Miya, H. Komatsu, K. Aoki, and K. Kasai, “Micro eddy current testing by micro magnetic sensor array,” IEEE Trans. Magn., vol. 31, pp. 870–876, Jan. 1995. [4] B. Lebrun, Y. Jayet, and J. C. Baboux, “Pulsed eddy current application to the detection of deep cracks,” Mater. Eval., vol. 53, no. 11, pp. 1296–1300, 1995.

[5] E. S. Boltz and T. C. Tiernan, “New electromagnetic sensors for detection of subsurface cracking and corrosion,” Rev. Prog. Quant. Non-Destruct. Eval., vol. 17, pp. 1033–1038, 1998. [6] W. W. Ward, III and J. C. Moulder, “Low frequency, pulsed eddy currents for deep penetration,” Rev. Prog. Quant. Non-Destruct. Eval., vol. 17, pp. 291–298, 1998. [7] T. Dogaru and S. T. Smith, “Edge crack detection using a giant magnetoresistance based eddy current sensor,” Non-Destruct. Test. Eval., vol. 16, pp. 31–53, 2000. [8] Z. Mottl, “The quantitative relations between true and standard depth of penetration for air-cored probe coils in eddy current testing,” NonDestruct. Test. Int., vol. 23, pp. 11–18, 1990.