Laser Doppler multi-beam differential vibration sensor for real-time continuous visualization of buried objects in multiple frequency bands.

Laser-acoustic detection of buried objects, such as landmines, uses elastic waves in the ground and a laser vibrometer to create a vibration image of the ground surface. A decision on the presence of a buried object is made by analyzing vibration images for multiple vibration frequencies. With traditionally used laser Doppler vibrometers, the vibration imaging data are saved to a computer memory to be analyzed, which increases the detection time. A novel laser multi-beam differential interferometric sensor (LAMBDIS) that provides simultaneous real-time visualization of vibration images for multiple frequencies has been developed. The sensor is capable of displaying vibration images for up to 32 frequency bands simultaneously in real time. The sensor employs a digital line-scan complementary metal-oxide semiconductor camera and field programmable gate arraysbased real-time signal processing and is capable of operating continuously by scanning an object with a linear array of laser beams. Performance of the sensor has been verified experimentally. The ability of the LAMBDIS for real-time continuous vibration imaging of the ground surface in multiple frequency bands for laser-acoustic detection of buried objects has been demonstrated.

Application of line-scan CMOS camera in multi-beam heterodyne differential laser Doppler vibration sensor.

The possibility of using a line-scan digital CMOS camera as a photodetector in a multi-beam heterodyne differential laser Doppler vibration sensor has been investigated. Application of the line-scan CMOS camera allows for selection of a different number of beams for a particular application in the sensor design, and for a compact design of the sensor. It was demonstrated that a limitation of the maximum measured velocity caused by the camera limited line rate can be overcome by selecting the beams separation on the object and the value of shear between images on the camera.

Laser Doppler multi-beam differential vibration sensor based on a line-scan CMOS camera for real-time buried objects detection.

Laser Doppler vibrometers (LDVs) traditionally used for ground vibration sensing in laser-acoustic detection of buried objects are limited to operation from a stationary platform due to their sensitivity to the motion of the LDV itself. In order to overcome this limitation a novel Laser Multi-Beam Differential Interferometric Sensor (LAMBDIS), has been developed. The LAMBDIS allows for measurements of vibration fields with interferometric sensitivity, while having low sensitivity to the motion of the sensor itself. The LAMBDIS described in this paper is based on a digital line-scan CMOS camera and FPGA based real-time signal processing. The principle of operation of the LAMBDIS employs the interference of light reflected from different points on the object surface illuminated with an array of laser beams. The Doppler shift induced by the sensor motion is canceled out thereby allowing for measurements from a moving vehicle. The ability of the LAMBDIS to detect buried objects in real time from a moving vehicle has been confirmed in field experiments.

Multi-beam heterodyne laser Doppler vibrometer based on a line-scan CMOS digital camera.

Multi-beam laser Doppler vibrometers (MB-LDVs) have an advantage over scanning single-beam laser Doppler vibrometers (LDVs) due to the reduction in measurement time and their ability to measure non-stationary and transient events. However, the number of simultaneously interrogated points in current MB-LDVs is limited due to the complexity of the electronic hardware, which increases with the number of measurement channels. Recent developments of high-speed line-scan CMOS cameras suggest that their use in MB-LDVs can reduce the hardware complexity and increase the number of measurement channels. We developed a MB-LDV based on a digital line-scan CMOS camera that simultaneously measures vibrations on a linear array of 99 points. The experimental setup and performance of the developed MB-LDV are discussed in this paper.

Analytical, computational, and experimental study of thermoviscous acoustic damping in perforated micro-electro-mechanical systems with flexible diaphragm.

An analytical model for the damping and spring force coefficients of micro-electro-mechanical systems (MEMS) with a flexible diaphragm is developed. The model is based on the low reduced-frequency method, which includes thermal and viscous losses as well as inertial and compressibility effects. Specifically, the solutions are derived for circular MEMS with a clamped diaphragm with both open-edge and closed-edge boundaries. The deflection function of the circular clamped diaphragm is incorporated into the thermoviscous acoustic (TA) formulation to take into account the effect of the flexibility of the diaphragm. TA finite-element analysis (FEA) is also used to develop a computational model. The analytical results are in good agreement with the FEA results for a wide range of parameters and frequencies. The significance of the effect of the flexibility of the diaphragm on damping for actual MEMS is demonstrated. Measurements of the damping coefficient of circular MEMS are conducted for experimental validation of the presented model. The small difference between the experimental results and the results from the model (less than 6%) validates the accuracy of the presented model. The proposed analytical model can be applied to MEMS with various geometries and boundary conditions.

Thermo-viscous acoustic modeling of perforated micro-electro-mechanical systems (MEMS).

An analytical model based on the low reduced-frequency method is developed for the damping and spring force coefficients of micro-electro-mechanical systems (MEMS) structures. The model is based on a full-plate approach that includes thermal and viscous losses and hole end effects, as well as inertial and compressibility effects. Explicit analytical formulas are derived for damping and spring forces of perforated circular MEMS with open and closed edge boundary conditions. A thermo-viscous finite-element method (FEM) model is also developed for the numerical solution of the problem. Results for the damping and spring coefficients from the analytical models are in good agreement with the FEM results over a large range of frequencies and parameters. The analytic formulas obtained for the damping and spring coefficients provide a useful tool for the design and optimization of perforated MEMS. Specifically, it is shown that for a fixed perforation ratio of the back-plate the radius of the holes can be optimized to minimize the damping.

Acoustic end corrections for micro-perforated plates.

Micro-perforated plates (MPPs) are acoustically important elements in micro-electro-mechanical systems (MEMS). In this work an analytical solution for perforated plates is combined with finite element method (FEM) to develop formulas for the reactive and resistive end effects of the perforations on the plate. The reactive end effect is found to depend on the hole radius and porosity. The resistive end effect is found to depend on hole radius only. FEM is also used to develop an understanding of the loss mechanism that corresponds to the resistive end effects. The developed models can be used in optimization studies of the MEMS and MPPs.

Theory of inert gas-condensing vapor thermoacoustics: propagation equation.

The theory of acoustic propagation in an inert gas-condensing vapor mixture contained in a cylindrical pore with wet walls and an imposed temperature gradient is developed. It is shown that the vapor diffusion effects in the mixture are analogous to the heat diffusion effects in the thermoacoustics of inert gases, and that these effects occur in parallel with the heat diffusion effects in the wet system. The vapor diffusion effects can be expressed in terms of the thermoviscous function F(lambda) used in the theory of sound propagation of constant cross-section tubes. As such, these results can be extended to any shape parallel-walled tube. The propagation equations predict that the temperature gradient required for onset of sound amplification in a wet-walled prime mover is much lower than the corresponding temperature gradient for an inert gas prime mover. The results of a measurement of the onset temperature of a simple demonstration prime mover in air with a dry stack and with a stack wetted with water provide a qualitative verification of the theory.

Theory of inert gas-condensing vapor thermoacoustics: transport equations.

The preceding paper [J. Acoust. Soc. Am. 112, 1414-1422 (2002)] derives the propagation equation for sound in an inert gas-condensing vapor mixture in a wet-walled pore with an imposed temperature gradient. In this paper the mass, enthalpy, heat, and work transport equations necessary to describe the steady-state operation of a wet-walled thermoacoustic refrigerator are derived and presented in a form suitable for numerical evaluation. The requirement that the refrigerator operate in the steady state imposes zero mass flux for each species through a cross section. This in turn leads to the evaluation of the mass flux of vapor in the system. The vapor transport and heat transport are shown to work in parallel to produce additional cooling power in the wet refrigerator. An idealized calculation of the coefficient of performance (COP) of a wet-walled thermoacoustic refrigerator is derived and evaluated for a refrigeration system. The results of this calculation indicate that the wet-walled system can improve the performance of thermoacoustic refrigerators. Several experimental and practical questions and problems that must be addressed before a practical device can be designed and tested are described.