Development of Very Low Frequency Self-Nulling Probe for Inspection of Thick Layered Aluminum Structures

Deep flaw detection with giant magnetoresistive (GMR) based self-nulling probe

“…figure 4a were taken without any feedback. As was seen in previous work, the output signal is rectified due to the insensitivity of the GMR sensor to the direction of the applied field [3]. A significant drop in the output voltage is observed when the probe is placed on the conducting sample, although a relatively large output voltage is still observed.…”

“…In this design four GMR elements are arranged in a Wheatstone bridge configuration on an eight-pin integrated circuit. Figure 2 shows the calibration data for the GMR sensor displayed schematically in figure 1 [3]. This data was acquired with the sensor placed in the center of a Helmholtz pair driven by a precision current source.…”

“…Previous work has shown that at low frequencies the magnetic flux leakage around the bottom of the flux focusing lens results in a large background signal level [3,9]. In the current design the background signal is removed by applying a feedback signal at the same frequency as the drive source but 180° out of phase with sensor output to the feedback coil.…”

“…The use of giant magnetoresistive (GMR) sensors for electromagnetic nondestructive evaluation has grown considerably in the last few years [1][2][3][4]. Technological advances in the research and development of giant magnetoresistive materials has led to commercially available GMR sensors with many qualities well suited for electromagnetic NDE.…”

“…The incorporation of a commercially available GMR sensor into the Self-Nulling Probe has been explored in the past [3]. This research showed that the modification could be performed in a straightforward manner to greatly enhance the low frequency capabilities of the device.…”