Showing papers on "Laser Doppler vibrometer published in 2005"

Comparison of laser Doppler vibrometer with contact sensors for monitoring bridge deflection and vibration

Abstract: This paper compares results from dynamic live load tests using the non-contact laser Doppler vibrometer (LDV) system with those from two types of contact sensors. Bridge girder deflections and vibrations are simultaneously measured using a linear variable differential transducer (LVDT)-cable system (deflection) and geophone sensors (velocity), both attached to the girders, and the LDV, equipped with displacement and velocity signal decoders. Live load tests are performed on a 3-span continuous unit of the 9-span Doremus Avenue Bridge Replacement Project using 5-axle trucks of known weight and configuration. The Doremus Avenue Bridge is a composite steel slab-on-girder construction. Bridge response is compared for two of the 10 girders. Overall, the LDV measurements of deflection and velocity compare very well with those recorded by the contact sensors and may be used as an alternative to the two systems. Other advantages and disadvantages are also highlighted.

Modal vibration analysis of a metal plate by using a laser vibrometer and the POD method

Abstract: The proper orthogonal decomposition (POD), also known as Karhunen–Loeve expansion, is applied to the modal vibration analysis of a metal plate. The metal plate was subject to vibrations with an electrodynamical shaker in a range of frequencies from 100 to 5000 Hz. The data were obtained from the measurements with a laser vibrometer. The plate vibration measurements were used to calculate the eigenfunctions and the eigenvalues. It was found that a large fraction of the total energy of the vibrations is contained within the first four POD modes. The essential features of the vibrations are thus described by only the first four eigenfunctions.

Directly modulated laser optical transmission system

Abstract: An optical transmitter for generating a modulated optical signal for transmission over dispersive fiber optic links in which a broadband analog radio frequency signal input is applied to a modulation circuit for directly modulating a semiconductor laser with the analog signal input. The transmitter may further include a temperature sensor in proximity to the laser and a negative feedback control circuit coupled to the temperature sensor for adjusting the temperature of the laser in response to an output characteristic of the laser, such as linearity.

Full-field vibrometry with digital Fresnel holography

Abstract: A setup that permits full-field vibration amplitude and phase retrieval with digital Fresnel holography is presented Full reconstruction of the vibration is achieved with a three-step stroboscopic holographic recording, and an extraction algorithm is proposed The finite temporal width of the illuminating light is considered in an investigation of the distortion of the measured amplitude and phase In particular, a theoretical analysis is proposed and compared with numerical simulations that show good agreement Experimental results are presented for a loudspeaker under sinusoidal excitation; the mean quadratic velocity extracted from amplitude evaluation under two different measuring conditions is presented Comparison with time averaging validates the full-field vibrometer

Fibre optic sensors for delamination identification in composite beams using a genetic algorithm

Abstract: Fibre Bragg grating (FBG) sensors associated with a genetic algorithm (GA) were used to detect and identify the size and location of delaminations in composite beams. A theoretical beam model is implemented into the GA for on-line delamination parameter searching. The objective function of this vibration-based delamination detection problem in the GA is defined as the sum of squared ratios of the differences between the shifts of eigenvalues of a delaminated beam measured by the FBG sensors and calculated from the theoretical beam model to the eigenvalues of an intact beam measured by the FBG sensors in the first three vibration modes. The principle of the FBG sensors for vibration detection is briefly discussed in this paper. A laser vibrometer and an accelerometer are utilized to compare the results measured from the FBG sensors. The reliability of using the FBG sensors for delamination detection is highlighted. Different delamination sizes and locations in spanwise and thickness-wise directions of the beams are simulated to demonstrate the feasibility of using the GA for the detection of delamination in the composite beams.

Method For Laser Machining And Laser Device With Laser Power Controlled As a Function Of The Laser Motion

Abstract: A method and device ( 1 ) for laser machining vehicle bodies or body parts ( 2 ) uses a laser beam ( 14 ) that is guided from a laser source ( 13 ) to a remote laser tool ( 15 ) on a robot hand by a guiding device ( 16 ). The robot ( 4 ) maintains the laser tool ( 15 ) in a suspended manner over the tool ( 2 ), at a focal length (F) and at a contact free distance and guides it along a machining path ( 30 ). The laser beam ( 14 ) is deviated, by movement of the hand axis (IV, V, VI), about a variable deviation angle (α), and the laser source ( 13 ), whose power is variable, is controlled according to the movement of the laser beam. The beam deviation of the hand axis (IV, V, VI) can be superimposed on an offset movement of the robot ( 4 ).

Investigation of the phase velocities of guided acoustic waves in soft porous layers

Abstract: A new experimental method for measuring the phase velocities of guided acoustic waves in soft poroelastic or poroviscoelastic plates is proposed. The method is based on the generation of standing waves in the material and on the spatial Fourier transform of the displacement profile of the upper surface. The plate is glued on a rigid substrate so that it has a free upper surface and a nonmoving lower surface. The displacement is measured with a laser Doppler vibrometer along a line corresponding to the direction of propagation of plane surface waves. A continuous sine with varying frequencies was chosen as excitation signal to maximize the precision of the measurements. The spatial Fourier transform provides the wave numbers, and the phase velocities are obtained from the relationship between wave number and frequency. The phase velocities of several guided modes could be measured in a highly porous foam saturated by air. The modes were also studied theoretically and, from the theoretical results, the experimental results, and a fitting procedure, it was possible to determine the frequency behavior of the complex shear modulus and of the complex Poisson ratio from 200 Hz to 1.4 kHz, in a frequency range higher than the traditional methods.

Method for driving a laser scanner

Abstract: What is disclosed is a method for driving a laser scanner, comprising a beam deflection system for a laser beam emitted by a laser light source, wherein a minimum distance to be observed is determined in dependence on the laser power of the laser light source. During the measurement the detected object distance is compared with this minimum distance, and the laser light source is deactivated and/or the laser power is reduced if the detected object distance is less than the minimum distance. There is moreover disclosed a laser scanner for implementing this method.

CO2 detection using carbon nanotube networks and micromachined resonant transducers

Abstract: A CO2 sensor using single-walled carbon nanotubes (CNT) and a microelectromechanical system (MEMS) resonator is demonstrated. The MEMS transducer comprises a membrane driven into transverse vibrations by means of Lorentz forces. A downshift in the resonant frequency of the device is measured by a laser vibrometer when changes in the stress state of the CNT film∕membrane structure are caused by adsorption of CO2 onto the nanotubes. The sensor has shown excellent sensitivity, linearity, and recovery over a broad range of concentrations (0–15vol%). In comparison to resistive, dielectric, and gravimetric CNT transducers, this sensor displayed remarkable intrinsic selectivity in the presence of interferences.

Development of non-contact scanning vibration measurement system for real-scale structures

Abstract: In order to rationalize vibration monitoring in structures, this paper addresses a non-contact scanning vibration measurement system employing laser doppler vibrometers. Generally, in the case of measurement for concrete members or dirt-adhering steel members, monitoring by laser doppler vibrometers is extremely difficult due to low laser reflectance. In this study, it is experimentally verified that the return or reflected laser beam quantity depends on subtle differences in the surface condition of a measurement object. Therefore, by searching the vicinity of the insufficient measurement point for the optimum point which provides a maximum reflected laser beam quantity, a remarkable improvement of measuring accuracy for real-scale structures can be achieved. As an example, vibrations of steel girders and reinforced concrete deck of actual bridges are measured, so as to verify the measuring accuracy of the developed system, and eigen local mode shapes of the members are able to be identified.

Sound field measurements based on reconstruction from laser projections

Abstract: In this paper, we describe some new sound field measurement methods by using a laser Doppler vibrometer (LDV). By irradiating the reflection wall with a laser, we can observe the light velocity change that is caused by the refractive index change from the change in air density. It means that it is possible to observe the change of the sound pressure. We measured a sound field projection on a 2D plane using a scanning laser Doppler vibrometer (SVM) which can visualize a sound field. And we made a 3D sound field reconstruction from some 2D laser projections based on computed tomography (CT) techniques. We made the reconstructed image for the sound field near the loudspeaker or in the room.

Demonstration of synthetic aperture imaging ladar

Abstract: The spatial resolution of a conventional imaging LADAR system is constrained by the diffraction limit of the telescope aperture. The purpose of this work is to investigate Synthetic Aperture Imaging LADAR (SAIL), which employs aperture synthesis with coherent laser radar to overcome the diffraction limit and achieve fine-resolution, long range, two-dimensional imaging with modest aperture diameters. This paper details our laboratory-scale SAIL testbed, digital signal processing techniques, and image results. A number of fine-resolution, well-focused SAIL images are shown including both retro-reflecting and diffuse scattering targets. A general digital signal processing solution to the laser waveform instability problem is described and demonstrated, involving both new algorithms and hardware elements. These algorithms are primarily data-driven, without a priori knowledge of waveform and sensor position, representing a crucial step in developing a robust imaging system. These techniques perform well on waveform errors, but not on external phase errors such as turbulence or vibration. As a first step towards mitigating phase errors of this type, we have developed a balanced, quadrature phase, laser vibrometer to work in conjunction with our SAIL system to measure and compensate for relative line of sight motion between the target and transceiver. We describe this system and present a comparison of the vibrometer-measured phase error with the phase error inferred from the SAIL data.

Method and system for operating a laser self-modulated at alkali-metal atom hyperfine frequency

Abstract: The present invention provides a method and apparatus for making atomic clocks or atomic magnetometers as self-modulated laser systems based on the physics of push-pull optical pumping. An atomic vapor cell is required to be in the laser cavity. With proper conditions, spontaneous push-pull optical pumping can occur inside the laser cavity. This causes the laser beam to be modulated at hyperfine-resonance frequency. With a fast photodetector, the modulated laser signal can be converted into the electrical signal, which serves as the atomic clock ticking signal or magnetometer signal. The self-modulated laser system does not use any local oscillator and the microwave circuit to lock the oscillator frequency to the hyperfine-resonance frequency, and therefore can consume less power and become more compact than conventional systems. This invention will benefit applications of time measurements and magnetic-field measurements.

Self-mixing laser range sensor

Abstract: A laser range finder includes a laser and a photosensitive element. The laser projects a beam onto a target surface. A backscattered portion of the laser beam returns to the laser from the target surface and enters the laser emitting cavity. Variations in laser output power are detected by the photosensitive element, which provides a “beat” signal to a frequency detection circuit. The frequency detection circuit includes a difference frequency analog phase locked loop (DFAPLL) providing a purified form of the beat signal. The frequency of the purified beat signal is determined and used to calculate distance to the target surface.

Characterization for vision science applications of a bimorph deformable mirror using phase-shifting interferometry

Abstract: The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.

Preliminary Assessment of Remote Photoelectric Excitation of an Actuator for a Hearing Implant

Abstract: Sound modulated infrared (IR) light, as an alternative signal (and energy) transmission method, was evaluated for remote powering of a hearing implant via an implanted photodiode receiver. One human cadaveric temporal bone was used to test this concept by locating an IR source (LED) in the ear canal 4 mm away from the eardrum and a photodiode in the middle ear cavity. The photodiode output was directly used to drive a piezo-ceramic disc actuator which was located externally for facilitating the test. The displacement frequency response of the actuator was measured by a laser vibrometer. It was found that, at a LED current of 8.6 mA, the actuator displacement was about 22.4 nm, equivalent to a stapes displacement from about 91 dB sound-pressure-level (SPL) stimulation at the eardrum. The IR light transmittance through the eardrum (compared to through an air path) was about 67%. The LED optical power was 0.77 mW (its maximal irradiance was about 3.95 mW/cm2) which is a safe to the human body. Current consumption can be reduced by a factor of 3-5 by using a more efficient LED and photodiode

Measurement of dynamic and static radiation force on a sphere.

Abstract: Dynamic radiation force from ultrasound has found increasing applications in elasticity imaging methods such as vibro-acoustography. Radiation force that has both static and dynamic components can be produced by interfering two ultrasound beams of slightly different frequencies. This paper presents a method to measure both static and dynamic components of the radiation force on a sphere suspended by thin threads in water. Due to ultrasound radiation force, the sphere deflects to an equilibrant position and vibrates around it. The static radiation force is estimated from the deflection of the sphere. The dynamic radiation force is estimated from the calculated radiation impedance of the sphere and its vibration speed measured by a laser vibrometer. Experimental results on spheres of different size, vibrated at various frequencies, confirm the theoretical prediction that the dynamic and static radiation force on a sphere have approximately equal magnitudes [G. T. Silva, Phys. Rev. E 71, 056617 (2005)].

Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane

Abstract: A technique for the calibration of hydrophones using an optical method is presented. In the method, a measurement is made of the acoustic particle velocity in the field of a transducer by use of a thin plastic pellicle that is used to reflect the optical beam of a laser vibrometer, the pellicle being acoustically transparent at the frequency of interest. The hydrophone under test is then substituted for the pellicle, and the hydrophone response to the known acoustic field is measured. A commercially available laser vibrometer is used to undertake the calibrations, and results are presented over a frequency range from 10 to 600 kHz. A comparison is made with the method of three-transducer spherical-wave reciprocity, with agreement of better than 0.5 dB over the majority of the frequency range. The pellicle used is in the form of a narrow strip of thin Mylar©, and a discussion is given of the effect of the properties of the pellicle on the measurement results. The initial results presented here show that the method has the potential to form the basis of a primary standard method, with the calibration traceable to standards of length measurement through the wavelength of the laser light.

Systems and methods for laser material manipulation

Abstract: A laser material manipulation system is provided for material processing, such as laser ablation, laser deposition and laser machining. The system includes a laser for emitting laser pulses and a laser imaging device having an array of controllable imaging elements. The laser imaging device receives the laser pulses emitted from the laser, forms a laser image through the controllable imaging elements, and projects the laser image onto a target material which is to be processed. The projected laser image processes the material according to a desired pattern. The laser can be a femtosecond laser. The laser imaging device can be a liquid crystal display (LCD) or a digital micromirror device (DMD). An SEM can be used for monitoring the material distribution and dynamically adjust the laser image according to the monitor result.

System and method for interferometric laser photoacoustic spectroscopy

Abstract: A system of using an interferometer, in combination with a laser, and a detector to determine absorptive characteristics of a material under test. The operation of the interferometer allows for determination of the wavelength of the laser beam and for determining relative changes in the wavelength of the laser beam. A method for using a laser source and an interferometer to determine characteristics of a material under test in accordance with the present invention is also provided.

Laser architectures for coherent short-wavelength light generation

Abstract: Several methods are disclosed for the generation of coherent short-wavelength electromagnetic radiation through optical nonlinear frequency mixing means. The invention involves several stages of efficient nonlinear frequency conversion to shift the output of high-power infra-red fiber-lasers into the vacuum ultraviolet (VUV). The described laser source architecture is designed around non-critically phase-matched (NCPM) sum-frequency mixing (SFM) interactions in the nonlinear crystal CLBO. The NCPM interaction is an optimum condition for bulk frequency conversion of cw radiation because it allows tight focusing of the input laser radiation without Poynting vector walk-off, thereby increasing the non-linear drive significantly. The sub-200-nm output wave is generated from SFM of a long-wave IR laser field and a short-wave UV laser field. The long-wave laser beam may be derived directly from a rare-earth-doped fiber laser, whereas the short-wavelength UV beam is provided as the fourth frequency harmonic of a second rare-earth-doped fiber laser system.

Control of vortex-induced non-resonance vibration using piezo-ceramic actuators embedded in a structure

Abstract: Closed-loop control of vortex-induced non-resonant vibration of a flexible square cylinder is experimentally investigated in this paper. Piezo-ceramic actuators were embedded inside the cylinder to cause an oscillation of the cylinder surface, which subsequently altered the fluid?structure interaction. Experiments were conducted in a wind tunnel at Reynolds numbers, Re, of 2800 and 8000. Two typical control schemes, i.e.?Y_control and u+Y_control, were deployed using feedback signals from structural vibration Y and combined Y and fluctuating flow velocity u, respectively. The control effects on the structural vibration and flow were assessed using a laser vibrometer, an optical fiber Bragg grating sensor, hot wires and particle image velocimetry. Experimental results show that both vortex shedding from the cylinder and the vortex-induced non-resonant vibration were effectively suppressed. The best control effects were observed with u+Y_control in use; the root mean square values of Y, the structural strain rate ?y along the lift direction and u, i.e.?Yrms, ?y,rms and urms, and the circulation ? dropped at Re = 8000 by 58%, 52%, 53% and 88%, respectively, compared with the unperturbed case. It was found that the control effectively modified the nature of the fluid?structure interaction by changing the in-phase fluid?structure synchronization at all dominant frequencies into anti-phase interaction, accounting for the suppression in both vortex shedding and structural vibration.

Multi-beam laser Doppler vibrometry for acoustic landmine detection using airborne and mechanically coupled vibration

Abstract: 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.

Laser package with digital electronic interface

Abstract: Optical transmission components, systems, and packages where the package includes a common housing containing a laser for transmission of an optical signal, a photodetector optically coupled to the laser for monitoring the laser transmission, and a laser driver electrically coupled to the laser for providing a drive current to the laser. The optical package may be a TO-Can package, the laser may be a vertical cavity surface emitting laser (“VCSEL”), and the laser driver may be an AC modulation laser driver, where a bias current is supplied to the laser from external to the optical transmission component package. An external bias source may be used for providing a bias current to the laser. A temperature sensor located in the laser driver may be used to control operational parameters of the laser.

Vibration power flow: A critical review

Abstract: Prediction and measurement of vibration power flow in structures are both important for locating vibration sources and sinks and for confirming vibration propagating paths. From the point of view of noise and vibration control in industries, the method is very important for practical reasons. Consequently, it is essential to address the state of the art of the method, its effectiveness and limitations. Vibration power flow methods or structural intensity techniques are reviewed and compared in this paper. There is a particular focus on flexural waves, but in-plane waves are also considered. Both experimental and numerical methods are put forward. Likely future trends of structural intensity are also discussed. The anticipated areas of interest include complex structures such as build-up edges, stiffeners, and modified geometry of beams and plates.

Laser measuring method and laser measuring system

Abstract: A laser measuring method in a laser measuring system, which comprises a rotary laser system for projecting a laser beam by rotary irradiation and at least one photodetection system having at least one photodetector for receiving the laser beam, comprising a step of emitting at least two fan-shaped laser beams by the rotary laser system, at least one of the fan-shaped laser beams being tilted, a step of receiving the laser beams at least at three known points by the photodetection system, a step of obtaining elevation angles with respect to the rotary laser system based on photodetection signals which are formed when the photodetector receives the laser beam, and a step of measuring an installing position of the rotary laser system based on elevation angles and position data at the three known points.

A study on the performance of bending mode piezoelectric accelerometers

Abstract: The method of eigenfunction expansion is proposed for obtaining an analytical expression for the frequency response of a bending mode piezoelectric accelerometer system. A set of approximate formulae derived relating the sensitivity with system parameters such as length, width, thickness and the seismic to beam mass ratio. Furthermore, the proposed model was experimentally validated using a laser vibrometer system configured for linear displacement measurements.

Accuracy analysis of the Fabry-Perot etalon based Doppler wind lidar

Abstract: A direct detection Doppler wind lidar with the dual Fabry–Perot technique has been built in Hefei, China, and the system is described in this paper. A dual, air-spaced Fabry–Perot etalon is designed as the frequency discriminator and the transmission characteristics are measured with a stabilized cw laser and a pulse laser. The experimental data are found to be in agreement with the designed parameters. The Doppler shift is measured by taking the ratio of the transmitted intensities from the dual etalon. Analysis of the accuracy of Doppler measurement is made by considering the factors of the number of iterations in converting the measured ratio to the Doppler shift, the laser intensity fluctuation and the signal-to-noise ratio. In the sufficient signal-to-noise ratio the velocity accuracy of the Doppler lidar system is estimated to be better than 0.5 m/s.