·Table of Contents
·Methods and Instrumentation
Integrated giant magnetoresistive transducer for eddy current testingTeodor Dogaru
Stuart T. Smith
University of North Carolina/Charlotte, NC28223, e-mail: email@example.com
Keywords: eddy current testing, giant magnetoresistive sensors, crack detection, signal conditioning
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.
|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|
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.
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 . 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|
|© AIPnD , created by NDT.net|||Home| |Top||