Showing papers on "Laser Doppler vibrometer published in 2022"

Finite Element Modeling and Experimental Characterization of Piezoceramic Frequency Steerable Acoustic Transducers

Abstract: This paper presents a novel implementation of Frequency Steerable Acoustic Transducers (FSATs) for guided waves (GWs) inspections. Unlike the conventional phased array systems, which demand a large number of transducers and complex wiring, FSATs benefit from inherent directional properties for generation and sensing ultrasonic guided waves, resulting in significant hardware simplification and cost reductions for GWs-based systems. The FSATs exploit the frequency-dependent spatial filtering effect, leading to a direct relationship between the direction of propagation and the frequency content of the transmitted signal. The proposed Piezoceramic FSAT is fabricated through patterning the spiral electrodes on a Piezoelectric (PZT) plate bonded on a supporting aluminum plate. In light of the need to investigate the influence of the generated stiffness caused by the PZT plate on the coupling behavior between the FSAT and the aluminum plate, Finite Element (FE) simulations were carried out. Furthermore, experimental validations were conducted using a Scanning Laser Doppler Vibrometer (SLDV) to highlight the directional capabilities of such devices. The results show that the generation of directional GWs in a host structure is substantially improved thanks to the high actuation strength of Piezoceramic materials employed in the proposed transducer.

“Singing” Multilayer Ceramic Capacitors and Mitigation Methods—A Review

Abstract: Multilayer Ceramic Capacitors (MLCC) have a major role in modern electronic devices due to their small price and size, large range of capacitance, small ESL and ESR, and good frequency response. Unfortunately, the main dielectric material used for MLCCs, Barium Titanate, makes the capacitors vibrate due to the piezoelectric and electrostrictive effects. This vibration is transferred to the PCB, making it resonate in the audible range of 20 Hz–20 kHz, and in this way the singing capacitors phenomenon occurs. This phenomenon is usually measured with a microphone, to measure the sound pressure level, or with a Laser Doppler Vibrometer (LDV), to measure the vibration. Besides this, other methods are mentioned in the literature, for example, the optical fiber and the active excitation method. There are several solutions to attenuate or even eliminate the acoustic noise caused by MLCC. Specially designed capacitors for low acoustic levels and different layout geometries are only two options found in the literature. To prevent the singing capacitor phenomenon, different simulations can be performed, the harmonic analysis being the most popular technique. This paper is an up-to-date review of the acoustic noise caused by MLCCs in electronic devices, containing measurements methodologies, solutions, and simulation methods.

Optical laser microphone for human-robot interaction: speech recognition in extremely noisy service environments

Abstract: Domestic robots are often required to understand spoken commands in noisy environments, including service appliances' operating sounds. Most conventional domestic robots use electret condenser microphones (ECMs) to record the sound. However, the ECMs are known to be sensitive to the noise in the direction of sound arrival. The laser Doppler vibrometer (LDV), which has been widely used in the research field of measurement, has the potential to work as a new speech-input device to solve this problem. The aim of this paper is to investigate the effectiveness of using the LDV as an optical laser microphone for human-robot interaction in extremely noisy service environments. Our robot irradiates an object near a speaker with a laser and measures the vibration of the object to record the sound. We conducted three experiments to assess the performance of speech recognition using the optical laser microphone in various settings and showed stable performance in extremely noisy conditions compared with a conventional ECM. GRAPHICAL ABSTRACT

Modal analysis and tension estimation of stay cables using noncontact vision‐based motion magnification method

Abstract: This paper describes a study on modal analysis and tension estimation of stay cables by vision‐based measurement under ambient condition. Microvibration of stay cable is captured by video camera, and the amplitude is amplified using phase‐based video motion magnification method. From the sequences of cable images, spatial displacements of the cable are extracted via discretized centroid searching method. Modal parameters of the cable, namely, natural frequency, damping ratio, and mode shape, are identified by dynamic mode decomposition method from cable displacement responses. Furthermore, tension of the stay cable is estimated based on the identified natural frequencies by minimizing an error function of an approximate relationship between frequency and tension iteratively. The technique is verified in the laboratory‐scale experiments and implemented to full‐scale measurement of medium size and long‐span cable‐stayed bridges. To compare the accuracy and efficacy of the vision‐based method, noncontact vibration measurement using laser Doppler vibrometer was also conducted. The results demonstrate that the proposed vision‐based vibration measurement techniques can estimate modal parameters and tension of the stay cables accurately in a noncontact and distant measurement under ambient condition. The proposed method offers an alternative of effective and accurate vibration measurement of stay cables using only motion of the cables recorded by video camera.

Detection of Internal Defects in Concrete and Evaluation of a Healthy Part of Concrete by Noncontact Acoustic Inspection Using Normalized Spectral Entropy and Normalized SSE

Abstract: Non-destructive testing, with non-contact from a remote location, to detect and visualize internal defects in composite materials such as a concrete is desired. Therefore, a noncontact acoustic inspection method has been studied. In this method, the measurement surface is forced to vibrate by powerful aerial sound waves from a remote sound source, and the vibration state is measured by a laser Doppler vibrometer. The distribution of acoustic feature quantities (spectral entropy and vibrational energy ratio) is analyzed to statistically identify and evaluate healthy parts of concrete. If healthy parts in the measuring plane can be identified, the other part is considered to be internal defects or an abnormal measurement point. As a result, internal defects are detected. Spectral entropy (SE) was used to distinguish between defective parts and healthy parts. Furthermore, in order to distinguish between the resonance of a laser head and the resonance of the defective part of the concrete, spatial spectral entropy (SSE) was also used. SSE is an extension of the concept of SE to a two-dimensional measuring space. That is, based on the concept of SE, SSE is calculated, at each frequency, for spatial distribution of vibration velocity spectrum in the measuring plane. However, these two entropy values were used in unnormalized expressions. Therefore, although relative evaluation within the same measurement surface was possible, there was the issue that changes in the entropy value could not be evaluated in a unified manner in measurements under different conditions and environments. Therefore, this study verified whether it is possible to perform a unified evaluation for different defective parts of concrete specimen by using normalized SE and normalized SSE. From the experimental results using cavity defects and peeling defects, the detection and visualization of internal defects in concrete can be effectively carried out by the following two analysis methods. The first is using both the normalized SE and the evaluation of a healthy part of concrete. The second is the normalized SSE analysis that detects resonance frequency band of internal defects.

Miniaturization of Laser Doppler Vibrometers—A Review

Abstract: Laser Doppler vibrometry (LDV) is a non-contact vibration measurement technique based on the Doppler effect of the reflected laser beam. Thanks to its feature of high resolution and flexibility, LDV has been used in many different fields today. The miniaturization of the LDV systems is one important development direction for the current LDV systems that can enable many new applications. In this paper, we will review the state-of-the-art method on LDV miniaturization. Systems based on three miniaturization techniques will be discussed: photonic integrated circuit (PIC), self-mixing, and micro-electrochemical systems (MEMS). We will explain the basics of these techniques and summarize the reported miniaturized LDV systems. The advantages and disadvantages of these techniques will also be compared and discussed.

Research on improvement of defect detection accuracy by resonance judgment for noncontact acoustic inspection method by acoustic irradiation-induced vibration

Abstract: Noncontact acoustic inspection methods using acoustic irradiation-induced vibration and laser Doppler vibrometer that can perform defect exploration from a distance are being studied. This method has the feature that it can measure a wide range of measurement objects such as composite materials used for aerospace as well as concrete structures such as tunnels and bridges without contact. From the experimental results, it was found that the increase in noise level due to the decrease in the return light of the laser due to the condition of the measurement surface causes a decrease in the estimation accuracy of defect exploration. Therefore, it has been clarified that the detection accuracy of the defect position can be improved by devising a resonance judgement process for discriminating the signal and noise due to the resonance of the defect portion.

Wavefield Analysis Tools for Wavenumber and Velocities Extraction in Polar Coordinates

Abstract: Experimental characterization of Lamb waves in plate-like structures overcomes the intrinsic limits of a priori semianalytical finite element simulations, where material inaccuracies and nonidealities cannot be easily considered. Unfortunately, the experimental extraction of guided wave dispersion curves, and especially their polar representation along different directions of propagation at a given frequency, is not trivial. In nonisotropic materials, such analysis is a key aspect for a reliable and robust characterization of the behavior of waves. In this work, by exploiting scanning laser Doppler vibrometer measurements with narrowband excitation, two different signal processing methods for the extraction of the wavenumber polar representation at the excitation frequency are investigated and characterized. The first method is based on a distance regularized level set (DRLSE) algorithm, widely used in image processing and computer vision but, to the best of the author's knowledge, never used in the Lamb waves' field. The second method is based on the 2-D sparse wavenumber analysis which exploits the wavefield sparse representation in the wavenumber domain. With a precise and reliable extraction of the wavenumber characteristic in the k -space, the polar representations at the excitation frequency of phase and group velocities can be estimated. The former, by exploiting the well-known wavenumber-frequency relation, the latter, instead, by computing numerical derivative among wavenumbers at multiple frequencies. The methodology has been validated on three different composite plates with different degrees of nonisotropy properties. The results show the effectiveness of the two methods, highlighting the advantages and disadvantages of both.

High-resolution 3D phased-array imaging of fatigue cracks using piezoelectric and laser ultrasonic system (PLUS)

Abstract: This paper reports the effectiveness of a novel imaging system, piezoelectric and laser ultrasonic system (PLUS), for the three-dimensional (3D) imaging of fatigue cracks with a high-resolution. The PLUS combines a piezoelectric transmitter and the two-dimensional (2D) mechanical scanning of a laser Doppler vibrometer, enabling the 2D matrix array with an ultra-multiple number of receiving points for 3D phased array imaging. After describing the principle and 3D imaging algorithm of PLUS, we show the fundamental 3D imaging capability of the PLUS in a flat-bottom-hole specimen with varying the number of receiving points under a fixed large receiving aperture. We then demonstrate that the PLUS with 4275 receiving points (i.e. 75 × 57) achieves high-resolution 3D imaging of a fatigue crack with a high signal-to-noise ratio, providing the outline of the fatigue crack geometry. We also discuss the effectiveness of the ultra-multiple receiving points for suppressing grating lobes and random noise.

Temporal-offset dual-comb vibrometer with picometer axial precision

Abstract: We demonstrate a dual-comb vibrometer where the pulses of one frequency-comb are split into pulse pairs. We introduce a delay between the two pulses of each pulse pair in front of the sample, and after the corresponding two consecutive reflections at the vibrating sample surface, the initially introduced delay is cancelled by a modified Sagnac geometry. The remaining phase difference between the two pulses corresponds to the change in the axial position of the surface during the two consecutive reflections. The Sagnac geometry reduces the effect of phase jitter since both pulses propagate through nearly the same optical path (in opposite directions), and spurious signals are eliminated by time gating. We determine the amplitude of a surface vibration on a surface-acoustic-wave device with an axial precision of 4 pm. This technique enables highly accurate determination of extremely small displacements.

Laser vibrometer-rangefinder based on self-sweeping fiber laser

Abstract: A vibrometer-rangefinder based on principles of coherent optical frequency-domain reflectometry (C-OFDR) is experimentally demonstrated. A self-sweeping ytterbium-doped fiber laser, which does not require any spectrally selective elements and drivers for wavelength tuning, with a sweeping range of 1056-1074 nm is used as a tunable source of probe radiation for the C-OFDR measurements. We demonstrate the possibility of measuring target vibrations in the frequency range from 2 Hz to 5 kHz with an amplitude of down to ∼5 nm at a distance of up to ∼13 m. The maximum measurable vibration frequency is limited by the instability of the self-sweeping laser parameters in the time domain and is estimated as ∼7.5 kHz.