High Sensitivity Magnetoresistive Sensors for DC and EMI Magnetic Field Mapping
The current magnetic sensors that are used to detect unexploded ordnance (UXO) and discarded military munitions (DMM) are bulky and require relatively large spacing (0.25 to 0.5 m). Some of these sensor systems are capable of dual-mode operation; however, they are only of limited frequency range (typical <20 kHz). Developing an advanced magnetics sensor array is important for classification of smaller (less than 37 mm projectiles), deep targets or multiple targets.
The objective of this project was to investigate a lightweight, low-power, compact, practical highsensitivity magnetic sensor system suitable for both DC and wide frequency band electromagnetic induction (EMI) magnetic field mapping based on solid-state magnetic tunneling junction (MTJ) devices with a sensitivity in a few tenth picotesla (~ 10-12 tesla) range.
The technical goals for the MTJ sensor were to:
- Develop three-axis vector capability
- Detect UXO targets to the depth of interest (1-2 meters)
- Characterize target shape
- Reduce false alarm rate
The researchers proved the feasibility of developing a magnesium oxide (MgO)-based MTJ sensor – its impulse response to a primary pulse field decaying from a peak field of 0.1 mT to 100 pT within 100 µs measurement of standard target response.
The MgO based MTJ with a magnetoresistance (MR) ratio as high as 250% has been achieved. This sensor has a sensitivity as high as 4703% per mT. The magnetic sensor only dissipates 1 mW of power while operating under 1 V.
It is evident that the tunneling magnetoresistance (TMR) sensor measures the expected target response from a simulated UXO object. The sensor shows response as early as 10 µs following the transmit pulse, but with less than perfect signal-to-noise ratio (SNR) on the edges of the test grid. This early time data could be useful for detection of small objects, provided they are within the inner (high SNR) region of the transmit coil. Through further improvements of the sensor, anti-pulse coil and possibly transient recovery of the transmitter it may be possible to improve the “edge” SNR of the early time data. At 300 µs, the signal matches the model with excellent agreement, SNR is > 200 and the background subtraction is well behaved.
A high bandwidth, low power, light weight, high sensitivity and low cost sensor can provide enhancements to the probability of detection and discrimination of small UXO items and for applications in difficult geology, terrain and complex ordnance and clutter distributions. The magnetic sensor can also be used in other applications such as vehicle surveillance and orientation control.
Points of Contact
Dr. Sy-Hwang Liou
University of Nebraska
SERDP and ESTCP
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