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Integrated giant magnetoresistive transducer for eddy current testing Teodor Dogaru Stuart T. Smith University of North Carolina/Charlotte, NC28223, e-mail: tdogaru@uncc.edu Contact

ABSTRACT

Among electromagnetic methods for detecting cracks in conductive materials using eddy currents, that based on the giant magnetoresistance (GMR) effect offers promising characteristics. Besides high sensitivity to low magnetic field (comparable to anisotropic magnetoresistive sensors), GMR sensors present some distinct advantages versus other technologies when applied to eddy current testing (ECT). The unipolarity of the magnetoresistance characteristic eliminates the need for synchronous detection, leading to a simplified signal conditioning circuitry. Compatibility between GMR film technology and the standard CMOS planar manufacture enables the integration of the GMR sensor together with signal conditioning circuitry on the same silicon chip. This paper reports the full-custom design of the CMOS signal conditioning circuit to be used in the integrated GMR transducer for ECT. An eddy current probe comprising a GMR sensor and a pancake-type excitation coil placed on the top of the sensor is presented and experimental results from scanning calibrated cracks using this probe are included within the paper. Surface cracks as small as 2 mm in length were reliably detected.

Keywords: eddy current testing, giant magnetoresistive sensors, crack detection, signal conditioning

1. Introduction

Recent developments in magnetoresistive sensor technologies opened new perspectives in electromagnetic nondestructive evaluation of conductive parts and structures using eddy currents. Both anisotropic [1,2] and giant magnetoresistive sensors [3] have been successfully used for detecting surface or buried cracks and corrosion in airplane structures, such as turbine blades, multi-layers with fasteners etc. Among the advantages of the magnetoresistive probes as compared to conventional probes are the high sensitivity to low magnetic field over a broad range of frequencies, ranging from hertz to megahertz domains. The drawback of inductive probes (using detection coils) is that their output is proportional to the rate of change of the magnetic field rather than to the magnitude of the field, their sensitivity being limited at low frequency. Therefore detection of deeply buried defects is difficult with inductive probes. Another major advantage is the reduced dimensions of the magnetoresistive sensing elements, that allows the local mapping of the magnetic field created by the flaw. The sensitivity of the inductive probe, depending proportionally to the magnetic flux intercepted by the pick-up coil, decreases with the reduction of the coil diameter. Consequently a higher spatial resolution for crack detection is achievable using magnetoresistive sensors.

In this paper we demonstrate that a very compact giant magnetoresistance based eddy current probe, containing a small pancake coil and a GMR sensor, is capable of detecting very short surface breaking cracks in conductive materials. Because the GMR technology is compatible with the silicon planar technology, the signal conditioning circuit of the probe can be integrated together with the sensor on the same chip. The design of the ASIC signal conditioning is presented, together with simulation results. Results from testing the eddy current probe using this ASIC chip are shown towards the end of the paper. Because the dimensions of the signal conditioning circuit are approximately the same as the dimensions of the GMR sensor, an array of sensors can be readily manufactured.

2. Probe configuration

The probe geometry is shown in Figure 1. A pancake type excitation coil of 0.8 mm internal radius, 2.8 mm external radius, 0.5 mm height and containing 20 turns is placed on the top of a GMR sensor in bridge configuration. The effective area of the sensor, of approximately 50 x 200 microns, is placed on the axis of the excitation coil. In the measuring configuration, the coil is located between the sensor and the specimen surface. The GMR sensor, being a directional device, detects only the component of the magnetic field along its sensitive axis. The sensitive axis is oriented coplanar with the specimen surface, therefore the sensor is insensitive to the excitation field created by the coil, which, along its axis, is normal to the specimen surface. In addition, in the case of a defect-free specimen, because the circular symmetry of the eddy currents, the magnetic field vector at the sensor location is vertical, and consequently the sensor's output is zero. When a defect such as a crack is present at the specimen surface, the eddy currents will be perturbed, creating a magnetic field in tangential direction that can be detected by the sensor.

Fig 1: Schematic diagram indicating probe configuration

The sensor's characteristic is shown in Figure 2. It can be noticed that the sensor's output voltage is positive for both negative and positive magnetic field applied along the sensitive axis. By applying an alternating field to the sensor, as in eddy current technique, the output of the sensor will be a full-rectified signal. The self-rectifying property of the GMR sensor reduces the complexity of the signal conditioning circuitry, eliminating the need of synchronous detection. To extract the output voltage related to the flaw signature, the signal from the sensor has only to be amplified and low pass filtered.

Fig 2: General response of GMR sensor to a field applied in the direction of its sensing axis

3. Experimental results

The probe was scanned over an aluminum plate of dimensions 100 x 100 x 10 mm containing calibrated surface slots. A Starrett coordinate measuring machine was used for the scanning. A power amplifier provided a sinusoidal current of 1A rms at 60 kHz through the excitation coil. The signal from the probe was amplified (x20) and low pass filtered (f_c =30Hz) using a Stanford Research Systems SR560 Low noise preamplifier.

The maps obtained from a two-dimensional raster scan over a crack of 5 mm length, 1 mm depth and 0.5 mm width, are shown in Figures 3a and 3b. Notice that the crack length is greater than the coil's mean diameter. Figure 3a shows the result of the scan when the sensitive axis of the sensor was oriented perpendicular to the crack direction. In this case, a peak between two shoulders can be observed along each side of the crack. The peaks are symmetrical, and the crack can be located at the midway between them. Also the length of the crack is approximately equal to the distance between the shoulders. Figure 3b shows the result from the same crack, when the sensitive axis was parallel to the crack orientation. In this map, one can notice two pairs of peaks symmetrically disposed about the center of the crack, corresponding to each tip of the crack. All peaks are aligned with the crack axis, with the two peaks of higher amplitude being located outside the crack, and the two lower peaks being situated inside.

Fig 3a: Signature of a 5 mm long crack when sensitive axis is perpendicular to the crack direction

Fig 3b: Signature of a 5 mm long crack when sensitive axis is parallel to the crack direction

Fig 4: Signature of 2 mm long crack when sensitive axis is perpendicular to the crack direction

A short crack of 2 mm length, 1 mm depth and 0.5 mm width was scanned using the same probe. Notice that the crack length is approximately equal to the coil's mean radius. The result of the scan is shown in Figure 4. The sensitive axis was perpendicular to the crack orientation. It can be observed that two peaks, corresponding to the tips of the crack, are formed symmetrically on each side of the crack. The small asymmetry of the map is due to the misalignment of the probe with respect to the specimen surface during the measurement.

More experiments were performed on smaller cracks to determine the spatial resolution of the probe. Cracks of 1 mm length, 0.5 mm depth and 0.2 mm width could be detected, but in this case the noise become comparable to the crack signal. By using probes of different coil diameter, it was observed that the resolution increases with the reduction of the coil dimensions. The mean radius of the coil is roughly equal to the minimum length of the crack that can be resolved using that probe. Further reduction of the coil's dimensions to increase the spatial resolution for surface crack detection is under investigation.

4. ASIC design for an integrated eddy current transducer

The signal conditioning of this probe is simple due to the selfrectifying property of the GMR sensor. Miniaturization of the eddy current measurement system is possible, by integrating the signal conditioning as an ASIC chip. The GMR sensor can be deposited on the top of the ASIC, dramatically reducing the size of the probe. Furthermore, an array of sensors, each having its own signal conditioning circuit, can be manufactured on the same substrate. The arrays are very useful for reducing the inspection time. Also by orienting the sensor axis in different direction, the sensor arrays can provide more information about the geometry and size of defects in the specimen under inspection.

In this section the ASIC of the signal conditioning circuit is presented. The layout was designed in full custom technique, in order to minimize the area of the chip and to match the components.

A block diagram of the circuit is shown in Figure 5. The circuit contains a differential amplifier (OTA3), which converts the differential output of the bridge sensor into an asymmetrical signal, and a second order low pass filter (LPF), which extracts the low frequency component of the signal. In addition, three buffers (OTA1, OTA2, OTA4) are used for matching impedance at the input and at the output of the circuit. The components, including the filter, are based on CMOS operational transconductance amplifiers. In Figure 5, all the amplifiers (OTA1..OTA4) are identical. The amplification of the circuit is given by the ratio R₂/R₁ and was chosen equal to 2. The low pass filter contains a transconductance amplifier connected in a unity gain configuration with controlled poles [4]. The cut-off frequency of the filter was chosen to be approximately 2 kHz.

Fig 5: Block diagram of the GMR eddy current tranducer

Fig 6: Layout of the signal conditioning circuit for the GMR eddy current transducer

The layout of the circuit is shown in Figure 6. It was designed with L-edit layout editor using the Orbit 2 microns N-well CMOS technology. Dimensions of the circuit are 894 microns by 695 microns, which are comparable to the die size of the GMR sensor bridge. A postlayout simulation of the circuit is shown in Figure 7. This simulation displays the frequency response of the circuit (magnitude plot), as obtained by using Hspice analog simulator. The maximum attenuation is about 60 dB at 35 kHz.

Fig 7: Postlayout simulation showing the frequency response of the ASIC

The chip was manufactured through the Mosis service and tested on real crack measurements. To test its performance, the chip replaced the standard equipment (the Stanford low noise preamplifier and filter) used prior in the experiments. The result from a scan across a 15 mm long surface crack is shown in Figure 8.

Fig 8: Output voltage of the integrated transducer when scanning across the central region of a 15 mm long crack. Sensitive axis was perpendicular to the crack direction

5. Conclusions

A very compact eddy current probe was proposed for detecting surface cracks in conductive materials with high resolution. Cracks of 2 mm length have been reliably detected by using very small pancake excitation coils and GMR sensors. The GMR sensor detects the magnetic field created by the crack in a direction parallel to the specimen surface. An integrated analog chip as signal conditioning circuit for this sensor was designed and manufactured. Future developments include integration of the signal conditioning together with the GMR sensor and manufacturing of sensor arrays to reduce the inspection time in industrial applications.

References

1. W. Avrin , "Magnetoresistive eddy-current sensor for detecting deeply buried flaws", Review of Progress in QNDE, 15, pp. 1145-1150, 1996.
2. E.S. Boltz and T.C. Tierny, "New electromagnetic sensors for detection of subsurface cracking and corrosion", Review of Progress in QNDE, 17, pp.1033-1038, 1998.
3. T. Dogaru and S.T. Smith, "Detection of cracks near sharp edges by using a giant magnetoresistance-based eddy current probe", Proceedings of SPIE, 2000
4. R. Geiger and E. Sanchez-Sinencio, "Active filter design using operational transconductance amplifiers; a tutorial", IEEE Circ. and Dev. Mag., March 1985

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