Showing papers on "Laser Doppler vibrometer published in 2015"

Quantitative measurements of electromechanical response with a combined optical beam and interferometric atomic force microscope

Abstract: An ongoing challenge in atomic force microscope (AFM) experiments is the quantitative measurement of cantilever motion. The vast majority of AFMs use the optical beam deflection (OBD) method to infer the deflection of the cantilever. The OBD method is easy to implement, has impressive noise performance, and tends to be mechanically robust. However, it represents an indirect measurement of the cantilever displacement, since it is fundamentally an angular rather than a displacement measurement. Here, we demonstrate a metrological AFM that combines an OBD sensor with a laser Doppler vibrometer (LDV) to enable accurate measurements of the cantilever velocity and displacement. The OBD/LDV AFM allows a host of quantitative measurements to be performed, including in-situ measurements of cantilever oscillation modes in piezoresponse force microscopy. As an example application, we demonstrate how this instrument can be used for accurate quantification of piezoelectric sensitivity—a longstanding goal in the electromechanical community.

Highly Nonlinear Wave Propagation in Elastic Woodpile Periodic Structures

Abstract: In the present work, we experimentally implement, numerically compute with, and theoretically analyze a configuration in the form of a single column woodpile periodic structure. Our main finding is that a Hertzian, locally resonant, woodpile lattice offers a test bed for the formation of genuinely traveling waves composed of a strongly localized solitary wave on top of a small amplitude oscillatory tail. This type of wave, called a nanopteron, is not only motivated theoretically and numerically, but is also visualized experimentally by means of a laser Doppler vibrometer. This system can also be useful for manipulating stress waves at will, for example, to achieve strong attenuation and modulation of high-amplitude impacts without relying on damping in the system.

Damage identification for composite structures using a cross-correlation reverse-time migration technique:

Abstract: This article presents a reverse-time migration technique to image damage by cross-correlating forward and backward propagating wavefields in composite structures using flexural wave signals. First,...

Noncontact fatigue crack visualization using nonlinear ultrasonic modulation

Abstract: This paper presents a complete noncontact fatigue crack visualization technique based on nonlinear ultrasonic wave modulation and investigates the main source of nonlinear modulation generation. Two distinctive frequency input signals are created by two air-coupled transducers and the corresponding ultrasonic responses are scanned using a 3D laser Doppler vibrometer. The effectiveness of the proposed technique is tested using aluminum plates with different stages of fatigue crack formation such as micro and macro-cracks. Furthermore, the main source of nonlinear modulation is discussed based on the visualization results and the microscopic images.

The Young's modulus of high-aspect-ratio carbon/carbon nanotube composite microcantilevers by experimental and modeling validation

Abstract: This paper reports the Young's modulus of a carbon nanotube (CNT)-reinforced carbon/CNT (C/CNT) composite microcantilevers measured by laser Doppler vibrometer and validated by finite element method. Also, the microfabrication process of the high-aspect-ratio C/CNT microcantilever arrays based on silicon micromolding and pyrolysis is presented in detail. With the in-plane natural resonant frequencies of the microcantilevers measured by a laser Doppler vibrometer, a single degree of freedom (SDoF) model based on Euler-Bernoulli (E-B) beam theory is used to calculate the Young's modulus of this composite. To figure out whether this SDoF model can be applied to these composite microcantilevers, the finite element (FE) simulation of these microcantilevers was performed. The Young's modulus of C/CNT composite microcantilevers fabricated by the pyrolysis process at 600 °C is 9391 MPa, and a good agreement between the results from experiments and FE simulation is obtained.

Precision density and viscosity measurement using two cantilevers with different widths

Abstract: We introduce a novel method for fast measurement of liquid viscosity and density using two cantilevers with different geometries. Our method can be used for real-time monitoring in lab on chip systems and offer high accuracy for a large range of densities and viscosities. The measurement principle is based on tracking the oscillation frequencies of two cantilevers with a phase-locked loop (PLL) and comparing with reference measurements with a known fluid. A set of equations and a simple algorithm is developed to relate the density and the viscosity to the frequency shifts of the cantilevers. We found that the effect of the density and the viscosity can be well separated if cantilevers have different widths. In the experiments, two Nickel microcantilevers (widths 25 μm and 100 μm, length: 200 μm, thickness: 1.75 μm) were fully immersed in the liquid and the temperature was controlled. The actuation was using an external electro-coil and the oscillations were monitored using laser Doppler vibrometer. Thus, electrical connections to the cantilevers are not required, enabling measurements also in conductive liquids. The PLL is used to set the phase difference to 90° between the actuator and the sensor. Calibration measurements were performed using glycerol and ethylene glycol solutions with known densities and viscosities. The measurement error with the new method was lower than 3% in density in the range 995–1150 kg/m 3 and 4.6% in viscosity in the range 0.935–4 mPa.s. Based on the signal-to-noise ratio, the minimum detectable difference in the viscosity is 1 μPa.s and the density is 0.18 kg/m 3 . Further improvements in the range and the accuracy are possible using 3 or more cantilevers with different geometries.

Multi-point vibrometer based on high-speed digital in-line holography

Abstract: This paper describes a digital holographic setup based on in-line holography and a high-speed recording to get a multipoint vibrometer. The use of a high-speed sensor leads to specificities that enable the in-line configuration to be used. The case of transient vibrations is investigated through a full simulation of the holographic process. The simulation shows that the first instants are critical since distortion may occur, resulting in errors in the phase measurement. Experimental results are provided by exciting an aluminum beam with a transient signal. A comparison with the velocity measured by a pointwise vibrometer is provided. Frequency response functions are extracted and the experimental results confirm the ability of the method to provide full-field contactless measurements at the high-speed time scale evolution of the vibration.

Polysilicon thin film piezoresistive pressure microsensor: design, fabrication and characterization

Abstract: Polysilicon based pressure sensors use a silicon dioxide layer for isolation of piezoresistors from bulk. This helps in reducing the leakage current compared to the p---n junction isolation in silicon piezoresistors. They are also more cost effective than silicon-on-insulator (SOI) based sensors for high temperature applications. This paper reports the design, fabrication process and characterization of a polysilicon piezoresistive pressure sensor with wet bulk micromachined diaphragm. Novel meander shaped polysilicon piezoresistors are placed at optimized locations, found using finite element method (FEM) simulations, to experience high stress. The effect of clamping conditions of the diaphragm on the piezoresistors placement is shown through FEM simulations and the piezoresistor shapes are designed to keep the metal lines outside the diaphragm structure for better reliability. After fabrication and dicing, the mechanical characterization of the sensor is performed using laser doppler vibrometer (LDV) for determining the first mode resonance frequency and transient response of the sensor diaphragm. A first mode resonant frequency of 306.6 kHz and a response time of 0.56 ms are obtained. The sensor is then packaged inside a customized jig and tested with pressure load for determining the static and temperature characteristics of the sensor in the pressure range of 0---30 Bar. The sensor is tested at three different temperatures, viz. ?5, 25 and 55 °C. A sensitivity of 3.35---3.73 mV/Bar, non-linearity of less than 0.3 %, and a hysteresis of less than 0.1 % are obtained for all the test temperatures.

Baseline-free damage visualization using noncontact laser nonlinear ultrasonics and state space geometrical changes

Abstract: Damage often causes a structural system to exhibit severe nonlinear behaviors, and the resulting nonlinear features are often much more sensitive to the damage than their linear counterparts. This study develops a laser nonlinear wave modulation spectroscopy (LNWMS) so that certain types of damage can be detected without any sensor placement. The proposed LNWMS utilizes a pulse laser to generate ultrasonic waves and a laser vibrometer for ultrasonic measurement. Under the broadband excitation of the pulse laser, a nonlinear source generates modulations at various frequency values due to interactions among various input frequency components. State space attractors are reconstructed from the ultrasonic responses measured by LNWMS, and a damage feature called Bhattacharyya distance (BD) is computed from the state space attractors to quantify the degree of damage-induced nonlinearity. By computing the BD values over the entire target surface using laser scanning, damage can be localized and visualized without relying on the baseline data obtained from the pristine condition of a target structure. The proposed technique has been successfully used for visualizing fatigue crack in an aluminum plate and delamination and debonding in a glass fiber reinforced polymer wind turbine blade.

Laser vibrometry as a diagnostic tool for detecting wood-boring beetle larvae

Abstract: Wood-boring insect pests, such as the invasive Asian longhorned beetle (ALB, Anoplophora glabripennis), are difficult to detect because larvae mine inside deciduous trees, logs or wood packing material. Currently, only visual survey methods are used, which are mostly unable to detect the presence of wood-boring insects. Bioacoustic detection, however, exploits sounds and vibrations produced by larvae during feeding and other movements inside the wood. Bioacoustic detection methods require mounting of the sensors, which can be complicated, time consuming and may even damage the surface of the tested material. Laser vibrometry avoids all these problems as vibrations produced by the larvae are detected via the laser beam. We used a portable digital laser vibrometer to detect the activity of mining ALB larvae within poplar logs. Three types of pulses were recorded: the broadband pulses lasting 1–2 ms were the most frequent, with frequency maxima between 8 and 13 kHz. Less frequent were the low and the high frequency pulses, covering frequency bands between 4 and 7, and 9 and 20 kHz, respectively. The signal-to-noise ratio across the whole frequency range (0–22 kHz) of the laser vibrometer was around 35 dB. We show that laser vibrometry can be successfully employed as a non-destructive diagnostic tool for detecting infestations by the wood-boring beetles.

Resonant ultrasound spectroscopy for materials with high damping and samples of arbitrary geometry

Abstract: Resonant ultrasound spectroscopy (RUS) is a powerful and established technique for measuring elastic constants of a material with general anisotropy. The first step of this technique consists of extracting resonance frequencies and damping from the vibrational frequency spectrum measured on a sample with free boundary conditions. An inversion technique is then used to retrieve the elastic tensor from the measured resonance frequencies. As originally developed, RUS has been mostly applicable to (i) materials with small damping such that the resonances of the sample are well separated and (ii) samples with simple geometries for which analytical solutions exist. In this paper, these limitations are addressed with a new RUS approach adapted to materials with high damping and samples of arbitrary geometry. Resonances are extracted by fitting a sum of exponentially damped sinusoids to the measured frequency spectrum. The inversion of the elastic tensor is achieved with a genetic algorithm, which allows searching for a global minimum within a discrete and relatively wide solution space. First, the accuracy of the proposed approach is evaluated against numerical data simulated for samples with isotropic symmetry and transversely isotropic symmetry. Subsequently, the applicability of the approach is demonstrated using experimental data collected on a composite structure consisting of a cylindrical sample of Berea sandstone glued to a large piezoelectric disk. In the proposed experiments, RUS is further enhanced by the use of a 3-D laser vibrometer allowing the visualization of most of the modes in the frequency band studied.

Sensor for measuring an external magnetic field

Abstract: In general, techniques of this disclosure are directed to a sensor for measuring an external magnetic field. The sensor an optical cavity, a laser medium which together with the optical cavity has a laser threshold, a laser pump, and a radio-frequency (RF) drive applied to the laser medium, such that the laser threshold varies with a change in the external magnetic field. The RF drive may be applied to the laser medium at or around a particular resonance frequency which varies depending on the external magnetic field, such that depending on the value of the external magnetic field, the RF drive induces transitions between at least two states of the laser medium, each state causing a different laser threshold in an intensity of a laser output. Further, the intensity of the laser output may provide a measurement of the value of the external magnetic field.

Dynamic Strain Measurements on Automotive and Aeronautic Composite Components by Means of Embedded Fiber Bragg Grating Sensors.

Abstract: The measurement of the internal deformations occurring in real-life composite components is a very challenging task, especially for those components that are rather difficult to access. Optical fiber sensors can overcome such a problem, since they can be embedded in the composite materials and serve as in situ sensors. In this article, embedded optical fiber Bragg grating (FBG) sensors are used to analyze the vibration characteristics of two real-life composite components. The first component is a carbon fiber-reinforced polymer automotive control arm; the second is a glass fiber-reinforced polymer aeronautic hinge arm. The modal parameters of both components were estimated by processing the FBG signals with two interrogation techniques: the maximum detection and fast phase correlation algorithms were employed for the demodulation of the FBG signals; the Peak-Picking and PolyMax techniques were instead used for the parameter estimation. To validate the FBG outcomes, reference measurements were performed by means of a laser Doppler vibrometer. The analysis of the results showed that the FBG sensing capabilities were enhanced when the recently-introduced fast phase correlation algorithm was combined with the state-of-the-art PolyMax estimator curve fitting method. In this case, the FBGs provided the most accurate results, i.e., it was possible to fully characterize the vibration behavior of both composite components. When using more traditional interrogation algorithms (maximum detection) and modal parameter estimation techniques (Peak-Picking), some of the modes were not successfully identified.

Smartphone laser beam spatial profiler.

Abstract: A simple, low-cost, portable, smartphone-based laser beam profiler for characterizing laser beam profiles is reported. The beam profiler utilizes a phosphor silica glass plate to convert UV light into visible (green) light that can be directly imaged onto an existing smartphone CMOS chip and analyzed using a customized app. 3D printing enables the ready fabrication of the instrument package. The beam's diameter, shape, divergence, beam quality factor, and output power are measured for two UV lasers: a CW 244 nm frequency-doubled Ar ion laser and a pulsed 193 nm ArF exciplex laser. The availability of specialized phosphor converters can extend the instrument from the UV to the near infrared and beyond, and the smartphone platform extends the Internet of Things to map laser beam profiles simultaneously in different locations.

An intelligent procedure for watermelon ripeness detection based on vibration signals

Abstract: In this paper, an efficient procedure for ripeness detection of watermelon was presented. A nondestructive method was used based on vibration response to determine the internal quality of watermelon. The responses of samples to vibration excitation were optically recorded by a Laser Doppler (LD) vibrometer. Vibration data was collected from watermelons of two qualities, namely, ripe and unripe. Vibration signals were transformed from time-domain to frequency-domain by fast Fourier transform (FFT). Twenty nine features were extracted from the FFT amplitude and phase angle of the vibration signals. K-nearest neighbor (KNN) analysis was applied as a classifier in decision-making stage. The experimental results showed that the usage of the FFT amplitude of the vibration signals gave the maximum classification accuracy. This method allowed identification at a 95.0 % level of efficiency. Hence, the proposed method can reliably detect watermelon ripeness.

Operational and defect parameters concerning the acoustic-laser vibrometry method for FRP-reinforced concrete

Abstract: Acoustic-laser vibrometry, a non-contact method for nondestructive testing, was studied by altering operational and defect parameters to determine their effects on measured signatures and system performance. The method detects delamination and voids in fiber-reinforced polymer reinforced concrete by vibrating the material with an acoustic excitation and measuring the vibration signature with a laser vibrometer. The operational parameters studied were excitation sound pressure level, laser signal, angle of incidence, and dwell time. The defect parameters studied were aspect ratio, size, and curvature. This study was undertaken to understand the method׳s phenomenology and to provide fundamental knowledge for an operational field system.

Frequency control of tunable lasers using a frequency-calibrated λ-meter in an experiment on preparation of Rydberg atoms in a magneto-optical trap

Abstract: A new technique is proposed and applied to study the frequency drift of an external-cavity semiconductor laser, locked to the transmission resonances of a thermally stabilised Fabry–Perot interferometer. The interferometer frequency drift is measured to be less than 2 MHz h-1. The laser frequency is measured using an Angstrom wavemeter, calibrated using an additional stabilised laser. It is shown that this system of laser frequency control can be used to identify Rydberg transitions in ultracold 7Li atoms.

Self-mixing thin-slice solid-state laser Doppler velocimetry with much less than one feedback photon per Doppler cycle.

Abstract: The drastic shortening of a photon lifetime as compared with normal TEM00 operations has been shown to be associated with the formation of annular mode operations in a thin-slice Nd:GdVO4 laser with tilted laser diode end pumping. The 15 dB enhancement of the signal-to-noise ratio, owing to the shortened photon lifetime, has been demonstrated in the self-mixing laser Doppler velocimetry experiment in comparison with the TEM00 operations, where the minimum intensity feedback rate from a target to the laser for successful measurements was estimated to be -123 dB, which corresponds to 0.007 photon per Doppler cycle.

Experimental Study of the Applicability of the Remotely Positioned Laser Doppler Vibrometer to Rock-Block Stability Assessment

Abstract: This paper examines a new method for evaluating the stability of rock blocks on slopes using a remotely positioned Laser Doppler Vibrometer (LDV). A series of experiments using physical models were conducted to evaluate the validity of this new method. Based on the experimental studies, the applicability of LDV was examined by comparing results with a conventional seismometer measurement. To examine the quantitative correlations between vibration properties and the stability of a rock block, the effects on the vibration properties of the size of the rock block, the initial block position, the slope incline, and the type of ground surface were studied. The experimental results showed that LDV measurements agreed with conventional seismometer measurements. There was also a good correlation between vibration properties and rock-block stability. On the other hand, it was found that for a boulder on tightly compacted ground, the application of block stability assessment by tonometry was difficult when measuring microtremors or sloppy vibration due to nearby vehicle traffic. Furthermore, numerical analysis of the slope model was carried out to examine the validity of the model experiment and application of the suggested technique. The results of the analysis demonstrated that the suggested technique was effective for application to stability monitoring of a block and evaluation of the effect of stability measures.

Fabrication of capacitive acoustic resonators combining 3D printing and 2D inkjet printing techniques.

Abstract: A capacitive acoustic resonator developed by combining three-dimensional (3D) printing and two-dimensional (2D) printed electronics technique is described. During this work, a patterned bottom structure with rigid backplate and cavity is fabricated directly by a 3D printing method, and then a direct write inkjet printing technique has been employed to print a silver conductive layer. A novel approach has been used to fabricate a diaphragm for the acoustic sensor as well, where the conductive layer is inkjet-printed on a pre-stressed thin organic film. After assembly, the resulting structure contains an electrically conductive diaphragm positioned at a distance from a fixed bottom electrode separated by a spacer. Measurements confirm that the transducer acts as capacitor. The deflection of the diaphragm in response to the incident acoustic single was observed by a laser Doppler vibrometer and the corresponding change of capacitance has been calculated, which is then compared with the numerical result. Observation confirms that the device performs as a resonator and provides adequate sensitivity and selectivity at its resonance frequency.

Elastic–Plastic Wave Propagation in Uniform and Periodic Granular Chains

Abstract: We investigate the properties of high-amplitude stress waves propagating through chains of elastic–plastic particles using experiments and simulations. We model the system after impact using discrete element method (DEM) with strain-rate dependent contact interactions. Experiments are performed on a Hopkinson bar coupled with a laser vibrometer. The bar excites chains of 50 identical particles and dimer chains of two alternating materials. After investigating how the speed of the initial stress wave varies with particle properties and loading amplitude, we provide an upper bound for the leading pulse velocity that can be used to design materials with tailored wave propagation.

Comparative Study of Laser Doppler Vibrometer and Capacitive Air‐coupled Transducer for Ultrasonic Propagation Imager and the New Development of an Efficient Ultrasonic Wavenumber Imaging Algorithm

Abstract: Damage detection techniques using guided waves have been studied for decades with very few successful real-world applications. The recent development with the full wavefield technique using the Ultrasonic Propagation Imager (UPI) is one of those few exceptions. In this paper, we study two non-contact sensors: the laser Doppler vibrometer and the capacitive air-coupled transducer in the context as the sensing modules for the UPI. The aim of this paper is to provide a comprehensive study for optimisation of the two sensors, as well as a comparison between them for use in the UPI. First, the parameters for laser ultrasonic measurement of each sensor were studied: surface treatment, measurement angle and stand-off distance in the case of the laser Doppler vibrometer and measurement angle, lift-off distance and bias voltage in the case of the capacitive air-coupled transducer. Two optimised sensors were then compared in terms of their ability to detect damages using the UPI. Also, in this paper, we presented the ultrasonic wavenumber imaging (UWI) algorithm with the new development towards an efficient implementation. The uniqueness of the UWI algorithm with the capability of damage size estimation makes this algorithm very attractive for the future study with full wavefield signal processing.

Defect-detection algorithm for noncontact acoustic inspection using spectrum entropy

Abstract: In recent years, the detachment of concrete from bridges or tunnels and the degradation of concrete structures have become serious social problems. The importance of inspection, repair, and updating is recognized in measures against degradation. We have so far studied the noncontact acoustic inspection method using airborne sound and the laser Doppler vibrometer. In this method, depending on the surface state (reflectance, dirt, etc.), the quantity of the light of the returning laser decreases and optical noise resulting from the leakage of light reception arises. Some influencing factors are the stability of the output of the laser Doppler vibrometer, the low reflective characteristic of the measurement surface, the diffused reflection characteristic, measurement distance, and laser irradiation angle. If defect detection depends only on the vibration energy ratio since the frequency characteristic of the optical noise resembles white noise, the detection of optical noise resulting from the leakage of light reception may indicate a defective part. Therefore, in this work, the combination of the vibrational energy ratio and spectrum entropy is used to judge whether a measured point is healthy or defective or an abnormal measurement point. An algorithm that enables more vivid detection of a defective part is proposed. When our technique was applied in an experiment with real concrete structures, the defective part could be extracted more vividly and the validity of our proposed algorithm was confirmed.