Showing papers on "Laser Doppler vibrometer published in 2010"

Distributed Vibration Sensor Based on Coherent Detection of Phase-OTDR

Abstract: We developed a distributed vibration sensor by using heterodyne detection and signal processing of moving averaging and moving differential for the phase optical time domain reflectometry system. The broadband acoustic frequency components generated by pencil-break vibration have been measured and identified in location by our distributed vibration sensor for the first time. Pencil break measurement is a standard technique to emulate the acoustic emission of cracks in concrete or steel bridges for early crack identification. The spatial resolution is 5m and the highest frequency response is 1 kHz, which is limited by the trigger frequency of data acquisition card. This new sensing system can be used for vibration detection of health monitoring of various civil structures as well as any dynamic monitoring requirement.

Adaptive self-mixing vibrometer based on a liquid lens

Abstract: A self-mixing laser diode vibrometer including an adaptive optical element in the form of a liquid lens (LL) has been implemented and its benefits demonstrated. The LL arrangement is able to control the feedback level of the self-mixing phenomenon, keeping it in the moderate feedback regime, particularly suitable for displacement measurements. This control capability has enabled a remarkable increase in the sensor-to-target distance range where measurements are feasible. Target vibration signal reconstructions present a maximum error of λ/16 as compared with a commercial sensor, thus providing an improved working range of 6.5 cmto265 cm.

Spatially encoded multibeam laser Doppler vibrometry using a single photodetector

Abstract: A laser Doppler vibrometer with single photodetector is introduced to measure the vibration on multiple points of target simultaneously. A 2 x 5 beam array with various frequency shifts is generated by three acousto-optic devices, illuminating different points on a vibrating object. The reflected beams interfere with a reference beam on a high-speed photodetector, and the signal is amplified and digitized with a rate of 500 megasamples/s. To extract vibration information of different points, the carrier frequencies of each beam are elaborately designed so that they can be separated from cross-talk regions in the spectrum. The experimental results are compared with that from a commercial single-point vibrometer, and the comparison shows that it is possible to do a precise measurement on multiple points simultaneously using a single photodetector.

Amplitude saturation of MEMS resonators explained by autoparametric resonance

Abstract: This paper describes a phenomenon that limits the power handling of MEMS resonators. It is observed that above a certain driving level, the resonance amplitude becomes independent of the driving level. In contrast to previous studies of power handling of MEMS resonators, it is found that this amplitude saturation cannot be explained by nonlinear terms in the spring constant or electrostatic force. Instead we show that the amplitude in our experiments is limited by nonlinear terms in the equation of motion which couple the in-plane length-extensional resonance mode to one or more out-of-plane (OOP) bending modes. We present experimental evidence for the autoparametric excitation of these OOP modes using a vibrometer. The measurements are compared to a model that can be used to predict a power-handling limit for MEMS resonators. (Some figures in this article are in colour only in the electronic version)

Feasibility of Using Lateral Mode Coupling Method for a Large Scale Ultrasound Phased Array for Noninvasive Transcranial Therapy

Abstract: A hemispherical-focused, ultrasound phased array was designed and fabricated using 1372 cylindrical piezoelectric transducers that utilize lateral coupling for noninvasive transcranial therapy. The cylindrical transducers allowed the electrical impedance to be reduced by at least an order of magnitude, such that effective operation could be achieved without electronic matching circuits. In addition, the transducer elements generated the maximum acoustic average surface intensity of 27 W/cm2. The array, driven at the low (306-kHz) or high frequency (840-kHz), achieved excellent focusing through an ex vivo human skull and an adequate beam steering range for clinical brain treatments. It could electronically steer the ultrasound beam over cylindrical volumes of 100-mm in diameter and 60-mm in height at 306 kHz, and 30-mm in diameter and 30-mm in height at 840 kHz. A scanning laser vibrometer was used to investigate the radial and length mode vibrations of the element. The maximum pressure amplitudes through the skull at the geometric focus were predicted to be 5.5 MPa at 306 kHz and 3.7 MPa at 840 kHz for RF power of 1 W on each element. This is the first study demonstrating the feasibility of using cylindrical transducer elements and lateral coupling in construction of ultrasound phased arrays.

Comparison between vibration and stator current analysis for the detection of bearing faults in asynchronous drives

Abstract: This study deals with the application of vibration and motor current spectral analysis for the monitoring of rolling bearings damage in asynchronous drives. Vibration measurement is widely used to detect faulty bearings operations. However, this approach is expensive and cannot always be performed, while electrical quantities such as the machine stator current are often already measured for control and detection purposes. Signal processing methods and global indicators associated with bearing fault detection of vibration measurements are recalled. Compared to these methods, an automatic detector based on vibration spectral energy extraction is then proposed and its performances are discussed. Moreover, load torque measurements underlines that bearing faults also induce mechanical load torque oscillations. Therefore a theoretical stator current model in case of load torque oscillations is used to demonstrate the presence of phase modulation (PM) on stator currents. Frequency behaviour of the related sideband components is strongly investigated for monitoring purposes. Thus, a fault detector using the extraction of spectral energy of stator current is proposed to detect damaged bearings. This detector is then compared to the one defined on vibration signals.

Depth and breakthrough detection for laser machining

Abstract: A system comprises a working laser beam, a sensing laser beam, first and second optical elements, an optical sensor, an aperture and a controller. The first optical element generates a coaxial beam from the working laser beam and the sensing laser beam. The second optical element focuses the coaxial beam onto a workpiece, such that the working laser beam machines the workpiece and the sensing laser beam reflects from the workpiece. The optical sensor senses an intensity of the reflected sensing beam. The aperture determines a focus position by translating along the reflected sensing beam, such that the reflected intensity is maximized. The controller determining a machining parameter of the working laser beam, based on the focus position.

Analysis of the quality factor of AlN-actuated micro-resonators in air and liquid

Abstract: The quality factor (Q) of different modes of self-actuated aluminium nitride beams operated in air and liquid media is analyzed applying different techniques. In air, both optical and electrical techniques are used. In the case of the optical approach, characterization is done with a laser Doppler vibrometer. The values for the Q factors are deduced straight forward from both the frequency spectrum and the transient response, while electrical impedance measurements need a more careful evaluation with two different fitting methods to obtain the Q factor of the vibration modes. In water, only optical measurements, more sensitive than the electrical measurements, could be used to determine the Q factor of selected modes. In addition, the results of finite element model analysis were compared to the experimental data, showing an excellent agreement regarding the modal shape and the resonance frequency of the microresonators.

The Mechanical Response of Shape Memory Alloys Under a Rapid Heating Pulse

Abstract: Shape memory alloy (SMA) actuators are very promising due to their large strain and work-output, but are considered to be very slow due to their cooling rate. In this article, we explore the capabilities of a fast one-directional actuation mode based on one-occasional rapid Joule heating of SMA elements. For this purpose, a unique experimental system has been developed that applies a high-voltage electric pulse to a detwined NiTi wire and measures the resulting displacement due to the martensite to austenite phase transformation. The electric pulse is tuned to produce variable temperature jump of up to 160°C within a risetime of few microseconds. One end of the wire is clamped while the displacement of the other end is monitored both by a laser doppler vibrometer and by an optical encoder that measures the displacement of a grating device. Analysis of experimental results reveals a dead time delay between the electric pulse and the wire’s response, which is attributed to the austenite nucleation time and which determines the limit on the fastest possible shape memory actuation. Further analysis reveals relations between the dead weight used and the average acceleration and maximal velocity obtained. In particular, the maximal velocity correlates to a constant kinetic energy delivered by the wire, which suggests a constant integrand over the stress-strain curve regardless of the dead-weight used. A comparison of actuation performances demonstrates that our actuation experiments are significantly advantageous over other fast actuation methods in almost every actuation aspect reviewed. This demonstrates the great potential of SMA for applications that require high speeds and large displacements one-occasional actuation.

Wavelocker for Improving Laser Wavelength Accuracy in WDM Networks

Abstract: The present invention includes novel techniques, apparatus, and systems for optical WDM communications. Various wavelocker apparatus and methods are disclosed that measure the frequency offsets between signal lasers and reference lasers. The measured offsets are used to adjust the signal laser frequencies to meet their target frequencies. The absolute accuracy of the reference laser frequency is improved by measuring the absorption of the reference laser by a gas cell with known fixed absorption lines versus the reference laser frequency. Apparatus and methods are disclosed to cover scenarios in which the reference laser polarization is aligned with the signal lasers, as well as those in which the reference laser polarization is not aligned with the signal lasers. The wavelocker apparatus may or may not be located at the same network site as the signal lasers.

Fluid-Structure-Acoustics Interaction of the Flow Past a Thin Flexible Structure

Abstract: The acoustic field resulting from the interaction of a thin flexible structure with a turbulent flow was investigated both numerically and experimentally. Two different model configurations were considered: in one a flexible plate acted as a moving wall in a turbulent boundary layer, and in the other the flexible plate was located in the wake of a square cylinder. The fully coupled simulation of the fluid—structure—acoustics interaction was based on a partitioned approach employing two different simulation codes: a finite-volume flow solver of second-order accuracy in space and time and a finite-element structural-mechanics and acoustics solver. A code coupling interface was used for the exchange of data between the different discretizations. The experiments were performed in an acoustic wind tunnel employing microphone measurements of the sound pressure level. Detailed flow measurements were carried out using laser Doppler anemometry and three-component hot-wire anemometry. The flow-induced vibration of the flexible structure was measured with a laser-scanning vibrometer. Experimental and numerical results characterizing the flow field, the structural vibration, and the generated sound are presented.

Fast scanning cavity offset lock for laser frequency drift stabilization.

Abstract: We have implemented a compact setup for long-term laser frequency stabilization. Light from a stable reference laser and several slave lasers is coupled into a confocal Fabry–Perot resonator. By stabilizing the position of the transmission peaks of the slave lasers relative to successive peaks of the master laser as the length of the cavity is scanned over one free spectral range, the long-term stability of the master laser is transferred to the slave lasers. By using fast analog peak detection and low-latency microcontroller-based digital feedback, with a scanning frequency of 3 kHz, we obtain a feedback bandwidth of 380 Hz and a relative stability of better than 10 kHz at timescales longer than 1 s, a significant improvement on previous scanning-cavity stabilization systems.

Optical system with movable lens for ophthalmic surgical laser

Abstract: An eye-surgical laser system includes a laser source, to generate a laser beam, an XY scanner, to scan a focal spot of a received laser beam in an XY direction essentially transverse to an optical axis of the laser system, and a lens group, disposed in the optical path between the laser source and the XY scanner, to receive the laser beam generated by the laser source, to precompensate an aberration of the laser beam, and to forward the precompensated laser beam to the XY scanner, the lens group having a movable lens, movable in a Z direction along an optical axis.

Laser light source device and image display apparatus

Abstract: A laser light source device has a laser light source for emitting a laser beam, a laser driver for driving the laser light source, a light-collecting lens for collecting the laser beam and reflecting a part of the laser beam, an optical sensor for receiving a reflected laser beam which has been reflected by the light-collecting lens and outputting a detection signal corresponding to intensity of the reflected laser beam, and a controller for controlling driving of the laser light source by the laser driver in accordance with the detection signal, and the light-collecting lens is disposed in a eccentric-rotated state so that an optical axis of the light-collecting lens has a tilt with respect to a central ray of the laser beam being incident on the light-collecting lens.

Comparative study of autodyne and heterodyne laser interferometry for imaging.

Abstract: For given laser output power, object under investigation, and photodiode noise level, we have theoretically compared the signal-to-noise ratios of a heterodyne scanning imager based on a Michelson interferometer and of an autodyne setup based on the laser optical feedback imaging (LOFI) technique. In both cases, the image is obtained point by point. In the heterodyne configuration, the beating between the reference beam and the signal beam is realized outside the laser cavity (i.e., directly on the detector), while in the autodyne configuration, the wave beating takes place inside the laser cavity and therefore is indirectly detected. In the autodyne configuration, where the laser relaxation oscillations play a leading role, we have compared one-dimensional scans obtained by numerical simulations with different lasers' dynamical parameters. Finally, we have determined the best laser for LOFI applications and the experimental conditions for which the LOFI detection setup (autodyne interferometer) is competitive compared to a heterodyne interferometer.

Research on laser Doppler velocimeter for vehicle self-contained inertial navigation system

Abstract: An idea of using laser Doppler velocimeter (LDV) to measure the velocity for the vehicle inertial navigation system was put forward. The principle of measuring its own velocity with laser Doppler technique was elaborated and reference-beam LDV was designed. Then Doppler signal was processed by tracking filter, frequency spectrum refinement and frequency spectrum correction algorithm. The result of theory and experiment showed that the reference-beam LDV solved the problem that dual-beam LDV cannot be used for measuring when the system was out of focus. Doppler signal was tracked so that signal-to-noise ratio was improved, and the accuracy of the system was enhanced by the technology of frequency spectrum refinement and correction. The measurement mean error was less than 1.5% in velocity range of 0–30 m/s.

Theoretical and experimental analysis of the electromechanical behavior of a compact spherical loudspeaker array for directivity control

Abstract: Sound directivity control is made possible by a compact array of independent loudspeakers operating at the same frequency range. The drivers are usually distributed over a sphere-like frame according to a Platonic solid geometry to obtain a highly symmetrical configuration. The radiation pattern of spherical loudspeaker arrays has been predicted from the surface velocity pattern by approximating the drivers membranes as rigid vibrating spherical caps, although a rigorous assessment of this model has not been provided so far. Many aspects concerning compact array electromechanics remain unclear, such as the effects on the acoustical performance of the drivers interaction inside the array cavity, or the fact that voltages rather than velocities are controlled in practice. This work presents a detailed investigation of the electromechanical behavior of spherical loudspeaker arrays. Simulation results are shown to agree with laser vibrometer measurements and experimental sound power data obtained for a 12-driver spherical array prototype at low frequencies, whereas the non-rigid body motion and the first cavity eigenfrequency yield a discrepancy between theoretical and experimental results at high frequencies. Finally, although the internal acoustic coupling affects the drivers vibration in the low-frequency range, it does not play an important role on the radiated sound power.

Dynamic behavior of the circular membrane of an electrostatic microphone: Effect of holes in the backing electrode

Abstract: Today, several applications require using electrostatic microphones in environments and/or in frequency ranges, which are significantly different from those they were designed for. When low uncertainties on the behavior of acoustic fields, generated or measured by these transducers, are required, the displacement field of the diaphragm of the transducers (which can be highly nonuniform in the highest frequency range) must be characterized with an appropriate accuracy. An analytical approach, which leads to results depending on the location of the holes in the backing electrode (i.e., depending on the azimuthal coordinate) not available until now (regarding the displacement field of the membrane in the highest frequency range, up to 100 kHz), is presented here. The holes and the slit surrounding the electrode are considered as localized sources described by their volume velocity in the propagation equation governing the pressure field in the air gap (not by nonuniform boundary conditions on the surface of the backing electrode as usual). Experimental results, obtained from measurements of the displacement field of the membrane using a laser scanning vibrometer, are presented and compared to the theoretical results.

Frequency Compensation for a Self-Mixing Interferometer

Abstract: Laser interferometry is a well-established technique that is widely used in industrial and laboratory environments to measure displacement, velocity, vibration, and distance. Recently, a new technique based on self-mixing (or feedback) interferometry has been introduced. The operation of this technique is based on a feedback loop controlling the laser pump current. The characterization of the vibrometer shows that the closed-loop performance is frequency dependent in view of the wavelength modulation (WM) dynamics. In this context, this paper addresses the measurement of the frequency response of the WM in a laser diode, for application in a self-mixing interferometer. The WM is derived by the measurement of the interferometric phase, whereas the frequency response is estimated by means of frequency-domain model identification techniques. The estimated frequency response is eventually used to design a frequency compensator for the vibrometer.

A comparison of 1D and 3D laser vibrometry measurements of Lamb waves

Abstract: This paper compares and contrasts one-dimensional (1D) and three-dimensional (3D) scanning laser Doppler vibrometer (LDV) measurements of Lamb waves generated by lead zirconate titanate (PZT) transducers. Due to the large cost and capability differences between the previously mentioned systems, this study is provided to highlight differences between these systems. 1D measurements are defined here as measurements of only out-of-plane velocities which are well-suited for studying anti-symmetric Lamb wave modes. 3D measurements provide both in- and out-of-plane velocities, which are especially important when studying both symmetric and anti-symmetric Lamb wave modes. The primary reason for using scanning LDVs is that these systems can make non-contact, accurate surface velocity measurements over a spatially-dense grid providing relatively high resolution image sequences of wave propagation. These scans can result in a clear understanding of Lamb waves propagating in plate-like structures and interacting with structural variations.

A piezoelectric linear ultrasonic motor with the structure of a circular cylindrical stator and slider

Abstract: A piezoelectric linear ultrasonic motor is proposed, with a cylindrical stator and slider structure. The length and diameter of the motor are about 10 and 1.5 mm, respectively. The stator consists of two piezoelectric ceramic (PZT) tubes connected by a thin film metallic glass (TFMG) pipe. The stator is designed based on theoretical analyses and finite element method (FEM) simulation. The traveling wave propagation is obtained in the FEM simulation under the proper geometrical sizes, suitable boundary conditions and driving voltage signals. The trajectories of particles on the TFMG pipe are elliptical motion. In the experiment, a 25 µm thick TFMG pipe is fabricated using the rotating magnetron sputtering technique and the vibration characteristics of the stator are measured by a laser Doppler vibrometer (LDV) system. Bidirectional motion of the slider is observed around 600 kHz, the maximum velocity is near to 40 mm s − 1 at 50 Vp–p for the loose slider and the maximum output force is 6 mN at 70 Vp–p for the tight slider.

Health monitoring of pipeline girth weld using empirical mode decomposition

Abstract: In the present paper the Hilbert–Huang transform (HHT), as a time-series analysis technique, has been combined with a local diagnostic approach in an effort to identify flaws in pipeline girth welds. This method is based on monitoring the free vibration signals of the pipe at its healthy and flawed states, and processing the signals through the HHT and its associated signal decomposition technique, known as empirical mode decomposition (EMD). The EMD method decomposes the vibration signals into a collection of intrinsic mode functions (IMFs). The deviations in structural integrity, measured from a healthy-state baseline, are subsequently evaluated by two damage sensitive parameters. The first is a damage index, referred to as the EM-EDI, which is established based on an energy comparison of the first or second IMF of the vibration signals, before and after occurrence of damage. The second parameter is the evaluation of the lag in instantaneous phase, a quantity derived from the HHT. In the developed methodologies, the pipe's free vibration is monitored by piezoceramic sensors and a laser Doppler vibrometer. The effectiveness of the proposed techniques is demonstrated through a set of numerical and experimental studies on a steel pipe with a mid-span girth weld, for both pressurized and nonpressurized conditions. To simulate a crack, a narrow notch is cut on one side of the girth weld. Several damage scenarios, including notches of different depths and at various locations on the pipe, are investigated. Results from both numerical and experimental studies reveal that in all damage cases the sensor located at the notch vicinity could successfully detect the notch and qualitatively predict its severity. The effect of internal pressure on the damage identification method is also monitored. Overall, the results are encouraging and promise the effectiveness of the proposed approaches as inexpensive systems for structural health monitoring purposes.