New laser vibration sensor detects buried objects from moving vehicle

Sensor is about to improve antipersonnel mine detection and non-destructive testing.

The detection of landmines can be a slow and difficult process. Detecting them from a moving vehicle would make the process faster, but at the expense of precision.

At the Laser Congress organized by the Optical Society (OSA), from 29 September to 3 October 2019 in Vienna, Austria, researchers from the University of Mississippi in the United States will present a new sensor based on the laser that effectively detects buried objects, even when the detector is inside. movement. This new device offers a significant improvement over existing technologies, which can not be used on the fly and lose accuracy in the presence of external sound sources or vibrations.

Doppler laser vibrometers (LDVs) combined with excited vibrations in the soil have shown promise for the detection of landmines and other buried objects, but their sensitivity to environmental vibrations implies that they must be exploited from a stable and special platform. The device, called a laser multi-beam differential interferometric sensor (LAMBDIS), offers comparable detection capabilities, but is much less sensitive to motion, allowing it to be used on board a vehicle in motion. movement.

Vibration imaging of an object buried with LAMBDIS. Credit: V. Aranchuk, University of Mississippi

"The persistent plague of anti-personnel mines poses a serious challenge for the rapid and accurate interrogation of large areas from moving vehicles," said lead researcher Dr. Vyacheslav Aranchuk. "Our new device overcomes this challenge by using a series of laser beams and then combining their signals to create a fast detection scheme robust enough to compensate for movement and other" noise "that could overwhelm other techniques. LAMBDIS allows to measure vibration fields with high sensitivity, while having a low sensitivity to the whole body movement of the object, or to the sensor itself, which allows to operate from 39, a moving vehicle. "

Measurements without reference beam

To detect buried objects, LDVs are used in conjunction with an audio source such as a speaker or seismic source such as a mechanical shaker. Sound or seismic waves vibrate the ground. The LDV can detect subtle differences in the pattern of vibration when an object is buried, provided that the detector is stationary and the environment is sufficiently free of vibration.

The operation of traditional LDVs is based on the interference of light reflected by an object with a reference beam internal to the LDV. As a result, the movement of the LDV itself may cause the LDV signals to be significantly higher than the signals caused by the vibration of an object and that it is impossible to distinguish them.

In the new work, the researchers used a linear array of 30 laser beams directed at the interrogated area.

Optical elements, including a receiver lens and a shear interferometer, are used to combine light reflected from different ground points on a photodetector array (PDA), which generates interference signals on the PDA outputs. . The frequency of the signals is proportional to the speed of vibration between the illuminated points due to the Doppler effect. The processing of the PDA signals reveals vibrations between the light spots on the surface.

"Unlike LDVs, the LAMBDIS does not use an internal reference beam, but detects a Doppler shift using the interference of light reflected from different points of the object," Aranchuk said. "Due to the absence of a reference beam, the Doppler frequency caused by the movement of the sensor is virtually identical for all reflected beams and is automatically subtracted from the interference signals. As a result, LAMBDIS has a very low sensitivity to the movement of the sensor itself, while having a high sensitivity to the relative vibration between the points of the object. "

Successful field tests

The researchers reported that the LAMBDIS device had worked well in a wide range of conditions during laboratory and field trials. LAMBDIS was able to detect buried objects at a distance of 7.5 to 20 meters and in a vehicle traveling at 3.8 meters per second, with results comparable to those of a stable LDV mounted on the platform. The researchers tested the device with both airborne and seismic sound sources and at different scanning angles, suggesting that it could provide accurate results in various real-life conditions.

In addition to detecting landmines, light commercial vehicles are commonly used to inspect automobiles and aircraft components, to evaluate the vibration of bridges and structures, to calibrate equipment and to study materials, as well as in dental and biomedical applications. LAMBDIS could benefit such applications in cases where noise or environmental movements impede the use of LDV devices.

Recognition

This research is sponsored by the Department of Marine, Office of Naval Research, under number ONR N00014-18-2489.