Showing papers on "Laser Doppler vibrometer published in 2017"

Two tandem cylinders of different diameters in cross-flow: flow-induced vibration

Abstract: This paper presents a systematic study of the cross-flow-induced vibration on a spring-supported circular cylinder of diameter placed in the wake of a fixed cylinder of smaller diameter . The ratios and are varied from 0.2 to 1.0 and from 1.0 to 5.5, respectively, where is the distance between the centre of the upstream cylinder to the forward stagnation point of the downstream cylinder. Extensive measurements are conducted to capture the cylinder vibration and frequency responses, surface pressure, shedding frequencies and flow fields using laser vibrometer, hot-wire, pressure scanner and particle image velocimetry techniques. Six distinct flow regimes are identified. It has been found that a violent vibration may erupt for the spring-supported cylinder, and its dependence on and is documented. A careful examination and analysis of the flow structure, along with the simultaneously captured pressure distribution around and vibration of the downstream cylinder, cast light upon the mechanisms behind this vibration and its sustainability. The roles of added mass, flow-induced damping and physical aspects in the process of initiating the vibration are discussed in detail.

Imaging of Barely Visible Impact Damage on a Complex Composite Stiffened Panel Using a Nonlinear Ultrasound Stimulated Thermography Approach

Abstract: Thermosonics, also known as ultrasonic stimulated thermography, is a rapid non-destructive evaluation technique that uses an infrared camera to visualise material defects by detecting the frictional heating at crack surfaces when a part under inspection is vibrated These vibrations are usually produced by an ultrasonic horn being pressed against the surface of the test sample, which result in uncontrolled generations of frequency components and excitation amplitude This makes thermosonics highly non-reproducible and unreliable This paper presents a novel thermographic method, here named as nonlinear ultrasound stimulated thermography, for the detection and imaging of real material defects such as impact damage on a complex composite stiffener panel This technique combines nonlinear ultrasonic techniques with thermography A nonlinear ultrasonic approach was used as signature for a reliable frequency-selective excitation of material defects, while an infrared camera was employed to reveal the damage location and severity A nonlinear narrow sweep excitation method was employed to efficiently excite the local resonance frequencies of the damaged region in order to give rise to the highest nonlinear harmonic response in the material leading to a high heat generation at the crack surface The experimental tests were carried out with a laser vibrometer in order to better understand the interaction of elastic waves with nonlinear scattering An ad-hoc nonlinear thermal-structural finite element and crack model was developed to study the heat generation caused by the movement of the crack surfaces when elastic waves with a particular frequency impinges on the crack interphase with good agreement with the experimental results The proposed new method allows to detect single and multiple barely visible impact damage in a quick, reliable and reproducible manner and overcomes the main limitations of classical thermosonics

Self-mixing birefringent dual-frequency laser Doppler velocimeter.

Abstract: A self-mixing birefringent dual-frequency laser Doppler velocimeter (SBD-LDV) for high-resolution velocity measurements is presented in this paper. The velocity information of the object can be accurately extracted from the self-mixing Doppler frequency shift of the birefringent light-carried microwave signal. We generate a virtual stable light-carried microwave by using a birefringent dual-frequency He-Ne laser which further simplifies the structure of the light source. Moreover, the optical configuration based on the laser self-mixing interference brings benefits of compact optical setup, self-alignment, and direction discriminability. Experimentally, we extracted the Doppler beat frequency signal by the low-frequency (millihertz) phase lock-in amplifier, measured the beat frequency precisely in time-domain with a low sampling rate and calculated the magnitude of velocity. Compared with the previous self-mixing LDV, the average velocity resolution of SBD-LDV is improved to 0.030 mm/s for a target with longitudinal velocity, benefiting from the high stability of light-carried microwave. It is of great meaning and necessity because it helps to provide an available velocimeter with high stability and an extremely compact configuration, making a potential contribution to the velocimetry in practical engineering application.

Fibre Bragg Gratings for the Monitoring of Wooden Structures

Abstract: The aim of this work was to develop and validate an experimental methodology suitable for analysing on-site the behaviour of fibre-reinforced wooden structures. The proposed measurement method is based on the application of fibre Bragg grating (FBG) strain sensors. An analysis of adhesive behaviour was performed preliminarily, which provided indications for choosing the type of adhesive and for the fibre bonding length in accordance with the volume of measurement. The first series of tests was carried out on wood samples to verify the coupling between the measuring sensor and the wood support when the latter is subject to mechanical stresses. The second investigation was done on site to test the behaviour of a historical wood floor before and after reinforcement by means of a series of tests performed using optical fibres with the Bragg grating. The optical fibre system measurements were compared to those obtained using a laser vibrometer, a measurement system of proven stability and precision. The comparison makes it possible to confirm the validity of the results and the reliability of the system for the monitoring of historic wooden structures.

Dynamic characterization of fabricated guided two beam and four beam cantilever type MEMS based piezoelectric energy harvester having pyramidal shape seismic mass

Abstract: Piezoelectric energy harvesters are gaining importance due to simplicity in design, operation and ability to power microelectronic sensor nodes in remote areas. A cantilever based piezoelectric energy harvester acts as a spring mass damper system having a resonance frequency tuned with ambient vibrations of low frequency. Maximum potential is generated when the resonance frequency of the energy harvester matches with frequency of ambient vibration. The highlight of this paper is a pyramidal shape seismic mass which results in a higher generated potential as compared to the square shaped seismic mass. This pyramidal shape is realized using tetra methyl ammonium hydroxide CMOS compatible 25 wt% wet etching. Two beam and four beam structures are front released using deep reactive ion etching which are fixed from both ends and having pyramidal shape seismic mass at the center. This paper presents a comparison of guided four beam and two beam cantilever type piezoelectric energy harvester fabricated using MEMS technology using a laser doppler vibrometer (LDV). Experimentally using LDV the resonance frequency for the fabricated guided two beam cantilever structure resonance frequency is 1971.9 Hz and for the guided four beam cantilever structure is 2540.6 Hz. Bandwidth for two beam cantilever structure is 40.6 Hz whereas for four beam cantilever structure bandwidth comes as 101.9 Hz. The maximum displacement measured for two beam cantilever structure at resonance frequency is 49.67 nm and for four beam cantilever structure maximum displacement is 32.17 nm. FEM (Finite Element Method) analysis for the guided two beam and guided four beam device is also performed and parameters such as resonance frequency, displacement, electric potential and von Mises stress are also reported and compared in this paper. Further the resonance frequency calculated analytically and the results obtained from FEM simulator and experimental results are also compared and it is found that they are in close agreement with each other.

A novel single driven ultrasonic elliptical vibration cutting device

Abstract: In order to obtain an ultrasonic elliptical vibration device with a simple structure, a novel single-driven ultrasonic elliptical vibration cutting (SDUEVC) device with a complex-beam horn (CBH) is introduced and investigated in this paper. Using the theory of mechanical vibration, a mathematical model of the CBH is described. We studied the vibration characteristic of the SDUEVC using the finite element method (FEM). Experiments were carried out to investigate the vibration characteristics of the SDUEVC device prototype using a vibrometer, an oscilloscope, and a data processing system. In addition, the cutting characteristic of the prototype was verified. Our experimental results show that, compared to conventional cutting (CC), the SDUEVC device can reduce the cutting force and surface roughness.

Propagation of a Finite‐Amplitude Elastic Pulse in a Bar of Berea Sandstone: A Detailed Look at the Mechanisms of Classical Nonlinearity, Hysteresis, and Nonequilibrium Dynamics

Abstract: We study the propagation of a finite-amplitude elastic pulse in a long thin bar of Berea sandstone. In previous work, this type of experiment has been conducted to quantify classical nonlinearity, based on the amplitude growth of the second harmonic as a function of propagation distance. To greatly expand on that early work, a non-contact scanning 3D laser Doppler vibrometer was used to track the evolution of the axial component of the particle velocity over the entire surface of the bar as functions of the propagation distance and source amplitude. With these new measurements, the combined effects of classical nonlinearity, hysteresis, and nonequilibrium dynamics have all been measured simultaneously. We show that the numerical resolution of the 1D wave equation with terms for classical nonlinearity and attenuation accurately captures the spectral features of the waves up to the second harmonic. However, for higher harmonics the spectral content is shown to be strongly influenced by hysteresis. This work also shows data which not only quantifies classical nonlinearity but also the nonequilibrium dynamics based on the relative change in the arrival time of the elastic pulse as a function of strain and distance from the source. Finally, a comparison is made to a resonant bar measurement, a reference experiment used to quantify nonequilibrium dynamics, based on the relative shift of the resonance frequencies as a function of the maximum dynamic strain in the sample.

Reduction of the Influence of Laser Beam Directional Dithering in a Laser Triangulation Displacement Probe.

Abstract: Directional dithering of a laser beam potentially limits the detection accuracy of a laser triangulation displacement probe. A theoretical analysis indicates that the measurement accuracy will linearly decrease as the laser dithering angle increases. To suppress laser dithering, a scheme for reduction of the influence of laser beam directional dithering in a laser triangulation displacement probe, which consists of a collimated red laser, a laser beam pointing control setup, a receiver lens, and a charge-coupled device, is proposed in this paper. The laser beam pointing control setup is inserted into the source laser beam and the measured object and can separate the source laser beam into two symmetrical laser beams. Hence, at the angle at which the source laser beam dithers, the positional averages of the two laser spots are equal and opposite. Moreover, a virtual linear function method is used to maintain a stable average of the positions of the two spots on the imaging side. Experimental results indicate that with laser beam pointing control, the estimated standard deviation of the fitting error decreases from 0.3531 mm to 0.0100 mm , the repeatability accuracy can be lowered from ±7 mm to ±5 μ m , and the nonlinear error can be reduced from ±6 % FS (full scale) to ±0.16 % FS.

Low insertion loss and high isolation capacitive RF MEMS switch with low pull-in voltage

Abstract: Micro electromechanical system (MEMS) shunt capacitive switches behave chiefly as a capacitor at both up and down states. Therefore, input-output matching for these switches is affected by varying the frequency. Although increasing the amount of capacitance at the up state reduces the actuation voltage, it deteriorates the RF parameters. This paper proposes a remedy for this problem in the form of two short high-impedance transmission lines (SHITLs) which are included at both ends of the MEMS switch. The SHITLs are implemented through adoption of a discontinued CPW transmission line. With this implementation, the switch can be modelled as a T circuit, neutralizing a capacitance behavior of MEMS switch at the up state. The paper also proposes an optimised fabrication process to realize a flat and planar bridge for the suggested RF MEMS switch. The measurement recorded by SEM and AFM shows that the proposed method significantly improves the planarity of the membrane. The RF and mechanical parameters of the fabricated switch are evaluated by Vector Network Analyser and Laser Doppler Vibrometer. At the up state, the switch has a return loss less than -20 dB in the entire frequency band (C-K). The isolation at the down state is better than 10 dB for the lower frequencies and increases to values better than 18 dB for the higher frequencies. The measured pull-in voltage is almost 20 V and the mechanical resonance frequency is 164 kHz.

High-speed noncontact acoustic inspection method for civil engineering structure using multitone burst wave

Abstract: The noncontact acoustic inspection method focuses on the resonance phenomenon, and the target surface is measured by being vibrated with an airborne sound. It is possible to detect internal defects near the surface layer of a concrete structure from a long distance. However, it requires a fairly long measurement time to achieve the signal-to-noise (S/N) ratio just to find some resonance frequencies. In our method using the conventional waveform "single-tone burst wave", only one frequency was used for one-sound-wave emission to achieve a high S/N ratio using a laser Doppler vibrometer (LDV) at a safe low power (e.g., He–Ne 1 mW). On the other hand, in terms of the difference in propagation velocity between laser light and sound waves, the waveform that can be used for high-speed measurement was devised using plural frequencies for one-sound-wave emission ("multitone burst wave"). The measurement time at 35 measurement points has been dramatically decreased from 210 to 28 s when using this waveform. Accordingly, 7.5-fold high-speed measurement became possible. By some demonstration experiments, we confirmed the effectiveness of our measurement technique.

Observation and analysis of the quality factor variation behavior in a monolithic fused silica cylindrical resonator

Abstract: Cylindrical resonators are commonly used in Coriolis vibratory gyroscopes (CVGs), which measure angular velocity through the precession of solid waves. Quality factor and its homogeneity are critical indicators of the resonator. In this paper, we present the observation and analysis of the Q factor variation behavior around the resonator’s axis of symmetry. We measured the resonator’s Q factor by the amplitude frequency response (AFR), with an acoustic source for excitation and a laser Doppler vibrometer (LDV) for detection. We then theoretically analyzed the Q variation behavior based on the two-dimensional mass-spring model. The experimental results were consistent with the theoretical calculation. By comparing experimental results with numerical calculations, we showed that the nature of this Q variation was due to the fact that when the excitation direction is misaligned with principle axes, for each detection point, the vibration is the superposition of two eigen-modes, and fitting with a one-dimensional oscillator would result in repetitive errors. This work is significant for understanding the mechanism of resonator’s Q factor variation behavior, as well as improving CVG’s performances.

Dynamics Modeling and Residual Vibration Control of a Piezoelectric Gripper During Wire Bonding

Abstract: A microgripper capable of accurately releasing or clamping the microgold wire for ultrasonic bonding is a key component in integrated circuit and light-emitting diode wire bonders. A novel gripper with a piezoelectric (PZT) stack and flexible mechanism is presented. A dynamic model of the PZT gripper is established, and a method of suppression control for residual vibration is proposed. The vibration characteristics of the PZT gripper are calculated using finite-element modeling (FEM) to obtain the natural frequency, vibration mode, and displacement. An input-shaping algorithm is applied to restrain the residual vibration caused by the flexible structure. The performance of the control algorithm is discussed and compared to the performance of the square, trapezoidal, and synthetic control methods. Three typical input-shaping filters—zero vibration, zero vibration differential, and extreme insensitivity—are compared, and the effects of frequency and damping on vibration suppression are discussed. In the experiment, a high-speed camera is used to track the displacement of the gripper, and the relationship between displacement and driven voltage is proven. Impedance and frequency are measured by an impedance analyzer, and the results agree with the values from the FEM. The residual vibration with the different driver control methods is recorded by a noncontact laser Doppler vibrometer. With the input-shaping control method, the residual vibration of the PZT gripper is reduced to 10%, and the settling time is reduced by 0.7 ms compared to the original vibration, demonstrating that the method improves the performance of the PZT gripper.

Beam shaping laser with controllable gain

Abstract: We propose a novel intra-cavity laser beam shaping technique based on the manipulation of the transverse gain profile in the laser crystal. The method allows controllable reshaping of a laser output beam into a desired beam profile. Two laser diodes were used to pump the crystal and to create a desirable pump beam profile. By independent manipulation of output powers of both pump diodes, we are able to perform a controllable operation of the laser output intensity profile.

Electro-optic dual-comb interferometer for high-speed vibrometry.

Abstract: Electro-optic frequency comb generators are particularly promising for dual-comb spectroscopy. They provide a high degree of mutual coherence between the combs without resorting to complex feedback stabilization mechanisms. In addition, electro-optic frequency combs can operate at very high repetition rates, thus providing very fast acquisition speeds. Here, we exploit these two features to resolve the rapid movement of a vibrating target. Our electro-optic dual-comb interferometer is capable of combining time-of-fight information with a more precise interferometric measurement based on the carrier phase. This fact, previously demonstrated by stabilized femtosecond frequency combs, allows us to increase the precision of the time-of-flight measurement by several orders of magnitude. As a proof of concept, we implement a fiber-based vibrometer that offers sub-nanometer precision at an effective acquisition speed of 250 kHz. These results expand the application landscape of electro-optic dual-comb spectroscopy to laser ranging and other remote sensing measurements.

Phase sensitive imaging of 10 GHz vibrations in an AlN microdisk resonator

Abstract: We demonstrate a high frequency phase-sensitive heterodyne vibrometer, operating up to 10 GHz. Using this heterodyne vibrometer, the amplitude and phase fields of the fundamental thickness mode, the radial fundamental, and the 2nd-order modes of an AlN optomechanical microdisk resonator are mapped with a displacement sensitivity of around 0.36pm/Hz. The simultaneous amplitude and phase measurement allow precise mode identification and characterization. The recorded modal frequencies and profiles are consistent with numerical simulations. This vibrometer will be of great significance for the development of high frequency mechanical devices.

Frequency and timing stability of an airborne injection-seeded Nd:YAG laser system for direct-detection wind lidar.

Abstract: We report on the design and performance of the laser deployed in the airborne demonstrator Doppler wind lidar for the Aeolus mission of the European Space Agency (ESA). The all-solid-state, diode-pumped and frequency-tripled Nd:YAG laser is realized as a master oscillator power amplifier (MOPA) system, generating 60 mJ of single-frequency pulses at 355 nm wavelength, 50 Hz repetition rate and 20 ns pulse duration. For the measurement of the Doppler frequency shift over several accumulated laser shots, the frequency stability of the laser is of crucial importance. Injection-seeding, in combination with an active cavity control based on the Ramp-Delay-Fire technique, provides a pulse-to-pulse frequency stability of 0.25 MHz measured at 1064 nm under laboratory conditions. This value increases to 0.31 MHz for airborne operation in a vibration environment that has been characterized by multiple acceleration sensors during different flight conditions. In addition, a pure Ramp-Fire setting was tested for comparison leading to a frequency stability of 0.16 MHz both in airborne operation and on ground. The laser cavity control electronics also have to provide a trigger signal for the lidar detection electronics, about 60 μs prior to the expected laser pulse emission and with high timing stability. An in-flight timing stability of below 100 ns was measured decreasing to 20 ns for a shorter pre-trigger time of 10 μs.

Automated Experimental Modal Analysis of Bladed Wheels with an Anthropomorphic Robotic Station

Abstract: Experimental modal analysis is challenging when the component has a highly three-dimensional shape, since a great number of measurement points are needed with accurate positioning. An anthropomorphic robotic station is proposed to automate this analysis, specifically on bladed wheels. This provides a reliable control of the spot location and of the beam orientation of a Laser Doppler Vibrometer. The modal frequencies were obtained along with the vibrational shapes and their spatial resolution was managed by exploiting the programming flexibility of the robotic station. The SAFE diagram was easily obtained by measuring a single point for each sector, and an extension of this diagram was demonstrated for the splitter blade wheels. The use of multiple measurement points, for each wheel sector, significantly improved the characterization of the modes having the same number of nodal diameters, hence the same shape coordinate on the SAFE diagram.

Real-time complex amplitude reconstruction method for beam quality M 2 factor measurement

Abstract: We present a real-time complex amplitude reconstruction method for determining the beam propagation ratio M2 of laser beams based on the transport of intensity equation (TIE). In this work, a synchronous acquisition system consisting of two identical CCDs is established. Once two beam intensity images at different cross-section positions along the optical axis are captured simultaneously by the system, the complex amplitude of the laser beam can be rapidly reconstructed using TIE algorithm. Then the beam intensity distribution at any section position along its propagation direction can be obtained by using angular spectrum (AS) theory. The beam quality M2 factor is therefore calculated utilizing the second-order moments and hyperbola fitting methods, which conform to the ISO standard. The suitability of this method is verified by the numerical analysis and experiments with the He-Ne and high-power fiber laser sources, respectively. The experimental technique is simple and fast, which allows to investigate laser beams under conditions inaccessible to other methods.