The laser-acoustic detection of buried objects, such as landmines, is based on excitation of elastic waves in the ground and creating a vibration image of the ground surface by using a laser Doppler vibrometer (LDV). The technique provides high probability of detection and low false alarm rate. However, traditional LDVs require operation from a stable stationary platform due to their sensitivity to the motion of the vibrometer itself. Recently developed laser Doppler multi-beam differential vibration sensor has low sensitivity to the motion of the sensor itself, while measuring vibration velocity difference between points on the object with interferometric sensitivity. Low sensitivity to the sensor motion allows for vibration measurements from a moving vehicle. Two configurations of the developed sensor: the linear array and the 2D array sensors, are discussed in the paper. The linear array sensor measures velocity difference between points on the object illuminated with a linear array of 30 laser beams, and creates a vibration image of the object by scanning the array of beams in a transverse direction. The 2D array sensor employs an array of 34 x 23 laser beams and measures velocity difference between corresponding points on the object over the whole illuminated area simultaneously. Simultaneous measurements at all points allow for the fast recording of the vibration image of the area of interest, and makes possible calculation of the vibration phase and instantaneous velocity images. Description of the sensors and the experimental results are presented in the paper.
A laser multibeam differential interferometric sensor (LAMBDIS) was developed that provides measurement of vibration fields of objects with high sensitivity, while having low sensitivity to the whole-body motion of the object, or the sensor itself. The principle of operation of the LAMBDIS is based on the interference of light reflected from different points on the object surface illuminated with a linear array of laser beams. The Doppler shift induced by the sensor motion is approximately the same for all beams and is automatically subtracted from the measurements. The performance of the sensor for laser-acoustic detection of a buried object was experimentally investigated. The ability of LAMBDIS to detect buried objects from a moving vehicle has been demonstrated in field experiments.
Laser Doppler vibrometers (LDVs) have been successfully used for ground vibration imaging in acoustic detection of buried objects. LDVs operating from a stationary platform or from a moving platform with a beam looking down can provide high sensitivity vibration measurement of the ground. However, operation from a moving vehicle with laser beams looking forward induces Doppler shift in the LDV beam. This shift can be much greater than the modulation bandwidth of the LDV. The demodulation must allow for the shift either by increasing the processing bandwidth, or by tracking the Doppler shift. The former increases the LDV noise while the latter can result in complex LDV design and signal processing. We developed a novel Laser Multi Beam Differential Interferometric Sensor (LAMBDIS) which provides measurement of vibration fields of objects with high sensitivity, while having low sensitivity to the whole body motion of the object, or sensor itself. The principle of operation of the LAMBDIS is based on the interference of light reflected from different points on the object surface illuminated with a linear array of laser beams. The Doppler shift induced by the sensor motion is approximately the same for all beams and is automatically subtracted from the measurements. Scanning the linear array of laser beams in the transverse direction provides a vibration image of the surface. Performance of the sensor for vibration imaging of a buried object was experimentally investigated. The experimental results and description of the sensor are presented in the paper.
We developed a pulsed digital shearography system that utilizes the spatial phase-shifting technique. The system employs a commercial micropolarizer camera and a double pulse laser, which allows for instantaneous phase measurements. The system can measure dynamic deformation of objects as large as 1 m at a 2-m distance during the time between two laser pulses that range from 30 μs to 30 ms. The ability of the system to measure dynamic deformation was demonstrated by obtaining phase wrapped and unwrapped shearograms of a vibrating object.
Producing vibration images of buried landmines using a multi-beam laser Doppler vibrometer (MB-LDV) operating
from a stationary platform have been accomplished in the past. Detection from a continuously moving platform can
reduce the time of detection compared to stop-and-stare measurement. However, there is a speed limitation, imposed by
the required spatial and frequency resolution. NCPA proposed a concept of time division multiplexing (TDM) of laser
beams of a MB-LDV to overcome that speed limitation. The system, based on 16-beam MB-LDV, has been built and
experimentally tested at an Army test facility. Vibration velocity profiles of buried mines have been obtained at different
system speeds. Algorithms for speckle noise reduction in continuously moving MB-LDV signals have been developed
and explored. The results of the current data collection, recent past data collection as well as the results of the
effectiveness of speckle noise reduction techniques are presented.
Using laser Doppler vibrometers (LDVs) to find buried land mines has been shown to have a high probability of
detection coupled with a low probability of false alarms. Equally good results have been achieved using a 16-beam
LDV. Time division multiplexing (TDM) of this multiple-beam LDV has also been investigated as a means of increasing
the scanning speed and potentially allowing the sensor to move down the road at speeds faster than that allowed using
stop-and-stare LDVs. A moving platform induces Doppler shifts in the LDV beams that are not perpendicular to the
motion vector. This shift can be much greater than the modulation bandwidth of a stationary LDV signal; therefore, the
demodulation must allow for the shift either by increasing the processing bandwidth, which increases the system noise or
by tracking the Doppler offset and adjusting a band pass filter's center frequency. A method has been developed to track
the carrier frequency to compensate for the Doppler offset for each of the 16 channels caused by the moving platform
and then adjusting the center frequency of a digital band pass filter. This paper will present the basic filter structure and
compare the noise statistics from two different carrier tracking methods that were investigated.
Detection and identification of vehicles obscured by forest canopy is a particularly challenging military problem.
Imaging techniques, e.g. laser radar imaging a target through gaps in foliage, require extensive data, making this
approach processing-intensive and time-consuming. A new method for standoff detection of a vehicle obscured under
forest canopy by remotely sensing the vibration of foliage with a laser Doppler vibrometer (LDV) has been proposed.
The method uses the effect of the vehicle engine creating sound waves, which then travel through the air and then couple
into tree leaves, causing them to vibrate. The presence of a vehicle can be determined by the spectrum of the leaves'
vibrations. Experimental study has shown that vibration velocity of leaves excited by sound from a vehicle is high
enough to be reliably detected with a LDV. The vibrations of leaves excited with simulated vehicle acoustic stimuli and
a real vehicle were successfully measured with a LDV in the laboratory and in an outdoor environment. The effect of
wind on measurements have been studied and discussed in the current work.
Using Laser Doppler vibrometry (LDV) to find buried land mines has been shown to have a high probability of detection
coupled with a low probability of false alarms. Previous work has shown that is it possible to scan a square meter in
20 seconds, but this method requires that discrete areas be scanned. This limits the use of LDVs for land mine detection
to a confirmation role. The current work at the University of Mississippi has been to explore ways to increase the speed
of scanning to allow the sensor to move down the road at speed. One approach has been to look at the feasibility of using
multiple beams to look at the same spot, time division multiplexing, in order to build a time history over small ground
segments as each beam passes over the spot. The composite velocity signature built from each beam will provide a long
enough time series to obtain the necessary frequency resolution.
The method of buried landmine detection based on using elastic waves in the ground and a laser Doppler vibrometer (LDV) as a vibration sensor has shown excellent performance in field tests. To increase the speed of measurements, a multi-beam laser Doppler vibrometer (MB-LDV) was developed. The system is based on a heterodyne interferometer and is capable of simultaneously measuring the vibration of the ground at 16 points over a span of 1 m with a velocity resolution of less than 1 µm/s. Both digital in-phase and quadrature (I&Q) and analog phase-locked loop (PLL) demodulation have been used for signal processing. The MB-LDV can create a velocity image of the ground surface either in "stop-and-stare" mode or in a continuously scanning mode. The continuously scanning operation results in an increased velocity noise floor due to speckle noise. The speckle noise floor increases with the increase of the speed of the laser beam and can degrade the velocity image of a mine. To overcome the effects of speckle noise, the excitation source must provide a ground vibration velocity higher than the velocity noise floor of the vibrometer. The MB-LDV has been tested at landmine test lanes and shows the ability to detect buried landmine within a one-square-meter area in a time of less than 20 s.
The multi-beam laser Doppler vibrometer (MB-LDV) has been successfully used for acoustic landmine detection in field experiments at an Army test site. Using the MB-LDV in a continuously scanning mode significantly reduces the time of the measurement. However, continuous motion of a laser beam across the ground surface generates noise at the vibrometer output due to dynamic speckles. This speckle noise defines the noise floor and the probability of detection of the system. This paper studies the origins of speckle noise for a continuously scanning LDV. The structure of the speckle field exhibits points of phase singularity that normally coincide with signal dropouts. The signal dropouts and phase singularities can cause spikes in the demodulated velocity signal, which increase the noise in the velocity signal. The response of FM demodulators to input signals causing spikes in the LDV output are investigated in this paper. Methods of spike reduction in the LDV signals have been developed and experimentally investigated.
Several experiments have demonstrated the potential of Laser Doppler Vibrometry, in conjunction with acoustic-toseismic coupling or mechanical shakers, for the detection of buried landmines. For example, experiments conducted by The University Of Mississippi and MetroLaser, Inc. have shown the ability to scan a one square meter area in less than 20 seconds with a 16-beam multi-beam LDV (MB-LDV), and find the landmines under a variety of soil conditions. Some critical requirements for this technology are to reduce the measurement time, increase the spatial resolution, and reduce the size of the systems. In this paper, MetroLaser presents data from three optical systems that help achieve these requirements: 1) A Compact MB-LDV, 2) A two dimensional, or Matrix Laser Doppler Vibrometer (MX-LDV), and 3) A Whole-field Digital Vibrometer (WDV). The compact MB-LDV produces a 1-D array of beams, which may be scanned over the target surface with a scanning mirror. The size of the new, compact MB-LDV system has been reduced to approximately 17" x 11" x 9", thus enhancing its capability for field applications. The MX-LDV, to be developed in 2006, produces a 16x16 array of beams over a one meter area, allowing the ground velocity of the entire area to be measured in a single measurement. The WDV uses a camera-based interferometry system to take a snapshot of the ground vibration over a one meter square area with very high spatial resolution. Field tests for this system are scheduled for mid-2006.
Acoustic-to-seismic coupling-based technology using a multi-beam laser Doppler vibrometer (LDV) as a vibration sensor has proved itself as a potential confirmatory sensor for buried landmine detection. The multi-beam LDV simultaneously measures the vibration of the ground at 16 points spread over a 1-meter line. The multi-beam LDV was used in two modes of operation: stop-and-stare, and continuously scanning beams. The noise floor of measurements in the continuously scanning mode increased with increasing scanning speed. This increase in the velocity noise floor is caused by dynamic speckles. The influence of amplitude and phase fluctuations of the Doppler signal due to dynamic speckles on the phase locked loop (PLL) demodulated output is discussed in the paper. Either airborne sound or mechanical shakers can be used as a source to excite vibration of the ground. A specially-designed loudspeaker array and mechanical shakers were used in the frequency range from 85-2000 Hz to excite vibrations in the ground and elicit resonances in the mine. The efficiency of these two methods of excitation has been investigated and is discussed in the paper. This research is supported by the U. S. Army Research, Development, and Engineering Command, Night, Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.
Recent work in acoustic landmine detection has shown that many landmines exhibit a multi-mode vibration pattern. To fully map the vibration pattern of these modes requires spatial resolutions on the order of millimeters. An optical technique that lends itself to such vibration sensing is an electronic speckle pattern interferometer (ESPI). In this work the double-pulse ESPI system has been used for the vibration measurement of the ground surface. The principle of method is based on recording two specklegrams of the object with two laser pulses synchronized with the vibration peak and the vibration valley respectively. The 2D vibration amplitude spatial distribution is obtained by subtracting two specklegrams and processing the received correlation fringe pattern. The experimental setup uses a mechanical shaker to excite vibrations in the ground to significantly increase the vibration amplitudes at the spot of interest and a laser Doppler vibrometer to detect the resonant frequency of the mine. Experimental results are presented from laboratory experiments. The spatial maps of the vibrating ground over buried antitank and antipersonnel landmines are studied. The effect of the vibration of a granular material like sand on the speckle decorrelation is discussed. This material is based upon work supported by the U. S. Army Communications-Electronics Command Night Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.
Acoustic-to-seismic coupling technology using an LDV as a vibration sensor has proved itself as a potential confirmatory sensor for buried landmine detection. One of the most important objectives of this technology is to increase the speed of measurements over traditional point-by-point scanning LDVs. A moving cart that uses 16 LDVs as well as a continuously-scanning single beam LDV have recently been demonstrated to increase the speed of detection. Recently a multi-beam LDV simultaneously probing 16 positions on the ground has been developed and successfully used for landmine detection. In this work, we report on a continuously-scanning multi-beam LDV as a confirmatory sensor for acoustic landmine detection. The multi-beam LDV simultaneously illuminates the ground in 16 points spread over a 1 meter line. A scanning mirror moves all 16 laser beams across the line. The system enables scanning a 1 meter square area in a much shorter time than previous scanning techniques. This material is based upon work supported by the U. S. Army Communications-Electronics Command Night Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.
The coupling of airborne sound into roadways and desert soils has been significantly investigated for the purposes of locating buried antitank (AT) landmines. However, there has been relatively little acoustic-to-seismic (A/S) coupling data collected for the purpose of buried antipersonnel (AP) landmine detection. A/S coupling landmine research has typically been accomplished with a low frequency sound source radiating pseudo-random noise in the frequency range 0f 80-300 Hz and a scanning single beam laser Doppler vibrometer (LDV) with a 10 cm beam spacing. The single beam LDV is operated in a serial data collection procedure resulting in long scan times. Recently, a data collection platform that uses 16 LDVs has been used to detect buried AT landmines. In the work reported here, this data collection platform is used to scan a significant number of AP landmines. For this purpose, the LDV beam spacing is reduced to 3 cm and the upper frequency of the sound source is increased to 2 KHz.
This paper discusses the performance and experimental results of a multiple beam laser Doppler vibrometer designed to locate buried landmines with the laser-acoustic technique. The device increases the speed of landmine detection by simultaneously probing 16 positions on the ground over a span of 1 meter, and measuring the ground velocity at each of these positions. Experimental results are presented from controlled laboratory experiments as well as from landmine test lanes at the University of Mississippi. In the mine lanes, the multiple beam system is raised to a height of 2.5 meters with a forklift, with the 16 beams spread over a 1 meter line along the mine lane. A motor system then allows the 16 beams to be translated across the mine lane, enabling the system to scan a 1 x 1 meter area in a much shorter time than with previous scanning techniques. The effects of experimental parameters such as platform motion, angle of incidence, speckle dropout, and system depth-of-field will be presented and discussed.
The use of a laser Doppler vibrometer (LDV) to sense the acoustic-to-seismic coupling ratio for buried landmine detection has previously been demonstrated. During these experiments, the LDV is mounted on a fixed platform and the beam moves continuously across the ground. Experiments show that fixed mounted LDV can achieve scanning speeds up to 3.6 km/h for successful detection of buried landmines in outdoor ground. The problems associated with taking a fixed-mount, scanning LDV and transitioning to a mobile system involve such issues as vehicle vibration, additional Doppler bandwidth due to vehicle speed, speckle noise, and sample time vs. spatial averaging. This paper presents the results of field tests with the moving platform on U.S. Army mine lanes showing that many of these issues can be overcome with an appropriately designed moving platform. The testing involved scanning different types of mines at varying depths and different speeds. Different aspects of the experiment are also discussed.
This paper discusses the development and performance of a multi-beam laser Doppler vibrometer specifically designed to locate buried landmines with a laser-acoustic technique. The device aims at increasing the speed of landmine detection with this technique by at least one order of magnitude. The present system is capable of simultaneously probing sixteen positions on the ground over a span of one meter, and of measuring the ground velocity at each of these positions with a velocity resolution of about 1 micrometers /s. This architecture could also be scaled to a larger number of beams or into two dimensions. The present system uses a low (100 kHz) carrier frequency, which enables digital signal processing in a simple architecture. This paper also discusses a numerical model to simulate and predict the performance of the multi-beam vibrometer. In particular, the model attempts to address issues associated with speckle dropout, signal/noise, and maximum scanning velocity.
Laser Doppler vibrometers are widely used to obtain velocity information from vibrating targets. Doppler shift of the light scattered from a vibrating target is detected by using heterodyning with a reference beam. To obtain high spatial resolution and high intensity of the backscattered light and to produce speckles of large size at a photodetector plane the target is placed in the waist of a probe laser beam. At that target position the curvatures of the wavefronts of the reference beam and the scattered light can be different. The difference in the wavefront curvatures result in mismatching of wavefronts of the heterodyning beams and decreasing in the Doppler signal amplitude.
A differential interferometer is used to measure the difference in motion of two points of the diffuse surface. The disadvantage of a differential interferometer is a very small amplitude of the interference signal. The heterodyning with a strong reference beam can be used for optical amplification of weak backscattered light to overcome this disadvantage. In that case the system of two separate reference beam interferometers is used instead of a differential interferometer. In this paper the comparative study of the signal-to-noise ratio of a differential interferometer and of a system of two reference beam Michelson type interferometers is carried out. The investigation is carried out on the assumption that the laser of the same power is used in both interferometers. It is found that when the shot noise is dominant the signal-to-noise ratio of a differential interferometer is practically the same as the signal-to-noise ratio of a reference beam Michelson type interferometer. When the thermal noise is dominant the signal-to-noise ratio of the reference beam interferometer greatly exceeds the signal-to- noise ratio of the differential interferometer for a typically rough surface.
In this paper the heterodyning detection of the Doppler signal of dynamic speckles by spaced photodetectors is studied theoretically. The investigation is carried out on the assumption that the laser field scattered by the diffuse object is the Gaussian speckles and the signal processing is implemented by the maximum amplitude channel section. It is shown that the detection probability of Doppler signal by spaced photodetectors increases with the increase of detection probability PD1 when only one photodetector is used. The spaced detection is practically effective for PD1 approximately greater than 0.85. The detection probability value close to the theoretically attainable maximum value takes place when 4-5 spaced photodetectors are used.
A novel laser doppler sensor for measuring transverse vibration and angular velocity of rotating cylindrical objects is proposed. The sensor has a sensitivity to only one component of the transverse vibration and it is insensitive to other movements of the object. The optical structure of the sensor is based on two differential type laser doppler velocimeters. The principle of operation of the proposed sensor is theoretically described on the basis of laser doppler anemomentry and the general expressions for vibratory displacement and angular velocity is obtained. The block diagram of the signal processing system is proposed.
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