Papers by Keyword: Laser Doppler Vibrometry

Abstract: The development of modern technologies requires new materials and technologies prepared for specific technical applications. Aluminum is one of these materials, it can be welded and anodized, which gives them anticorrosive characteristics. The determination of mechanical properties (Young’s modulus and shear modulus) is of great importance from both scientific and practical points of view. Most of the known methods for determination of the Young’s modulus and shear modulus are sample destructive and base on measuring a force (energy) necessary for break the sample. This paper presents a nondestructive method for the determination of the elastic properties of welded aluminum plates by vibration tests and laser measurements using Doppler velocimeter. A Fast Fourier Transform algorithm is used for processing the sampled signals.

Abstract: Micro-Electro-Mechanical Systems (MEMS), also known as micromechatronic devices, integrate on the same chip (substrate) both micromechanical structures and microelectronics components. Microcantilevers are miniaturized beams clamped at one end and with the other end suspended freely outwards. They can be used as resonant structures in nano/micro mass detectors, allowing a quantitative assessment of the (substance) mass attached to these devices. An accurate modal analysis makes possible to estimate the sensitivity of the cantilevers or their ability to detect minimum frequencies shifts induced by the substance absorption. In order to obtain a high sensitivity, the structures must present high resonant frequencies (usually bending or torsion), in close correlation with a small equivalent mass. This paper deals with the vibration testing, modeling and simulation of a silicon rectangular microcantilever, micromachined through MEMS technologies. The results of analytical calculations and numerical computation by finite element analysis (FEM) have been compared with those measured through Laser Doppler Vibrometry (LDV) method using MSA-500 system from Polytec.

Abstract: A circular high electron mobility transistor (C-HEMT) prepared on the AlGaN/GaN membrane surface has been investigated and its potential for pressure sensing has been already demonstrated. The key issue in the design process of such heterostructure based MEMS sensors is the stress engineering. This way we can scale the sensor performance, measured pressure range and sensitivity. Especially, the knowledge of the exact value of the residual stress in membranes (caused by deposition process) helps us to optimize the sensing devices. In this work, the residual stress determination method in gallium nitride circular shaped membrane is reported. It is shown that resonant frequency method using Laser Doppler Vibrometry (LDV) for membrane vibration measurement seems to be an appropriate technique to determine the residual stress in micro-scale membranes. Circularly shaped AlGaN/GaN micro-membranes are excited by acoustic short time pulse. The decay oscillating motion of the membrane is recorded by oscilloscope. By FFT spectral analysis of the signals the resonance frequencies are obtained. For the sample studied, the natural frequency mode resonance peak is used to define the residual stress level. To verify the observed stress in investigated membranes, prestressed modal analysis in finite element method (FEM) code ANSYS is performed. The stress extracted from the measured frequency is taken as an initial stress state of the modelled membrane. Experimentally obtained shock spectra are compared with that computed by FEM simulation.

Abstract: Laser Doppler Vibrometry (LDV) is a well established technique able to accurately measure vibration velocity of any kind of structure in remote, i.e. non-intrusive way, this allowing to overcome the problem of mass loading, typical of contact sensors as accelerometers and strain-gauges, which has strong influence in case of lightweight structures. Moreover, the possibility of driving automatically the laser beam, by means of moving mirrors controlled with galvanometer servo-actuators, permits to perform scanning measurements at different locations with high spatial resolution and reduced testing time and easily measure the operational deflection shapes (ODS) of the scanned surface. The exploitation of the moving mirrors has allowed to drive the laser beam in a continuous way making it to scan continuously over the structure surface and cover it completely. This way of operation, named Continuous Scanning LDV, permits to perform full-field measurements, the LDV output carrying simultaneously the time-and spatial-dependent information related to the structural vibration. A complementary strategy making use of the LDV coupled with moving mirrors is the so called Tracking LDV, where the laser beam is driven to follow a moving object whose trajectory must be known a priori or measured during operation (e.g. via an encoder in the case of rotating structures). In this paper some applications of the Tracking Laser Doppler Vibrometry (TLDV) and Continuous Scanning Laser Doppler Vibrometry (CSLDV) will be described they concerning, specifically modal and vibrational analysis of rotating structures.

Abstract: Rapid assessment of large, short-period structures is extremely important for establishing structural integrity. This paper demonstrates experimental non-contact detection of consistent frequency peaks from the ambient vibration of a range of large buildings. Long distance, remote Laser Doppler Vibrometry is employed to estimate the dominant response frequencies of these large building structures from their ambient vibration. These dominant frequencies were reproducible in the frequency domain. The results demonstrate potential field applications of this method in a number of important applications. Such applications include model-free and rapid assessment or monitoring of historical structures, strategically important structures, lifelines, assessment and monitoring of structures such as nuclear facilities and rapid evaluation of large scale structures following disasters. Empirical formulas specified in codes do not cover such special structures and experimental determination of periods employing the method proposed may thus become essential.

Abstract: Non-contact optical/laser measuring techniques are very attractive in many engineering applications. The paper demonstrates examples related to structural health monitoring. Various methods based on strain, vibration and ultrasound measurements are presented together with relevant references. Applications examples utilise in-plane and out-of-plane measurements taken by 1-D and 3-D laser Doppler vibrometers.

Abstract: MEMS devices, Micro Electro-Mechanical Systems, are electrical and mechanical systems with characteristic dimensions on the order of microns. Since these systems have moving mechanical parts, characterization of their dynamics, including their modal parameters, is highly desirable. This paper describes the validation of an existing implementation of the Stochastic Subspace Identification (SSI) algorithm, called MACEC, for experimental modal analysis of a micro-cantilever switch. A white noise signal applied to the built-in electrostatic actuator in the switches excited a response measured using microscanning Laser Doppler Vibrometry (LDV). The modal parameters found using MACEC matched well those predicted by theory, thus validating this combination for experimental modal analysis of MEMS structures.

Abstract: Due to increased product requirements, usual conditions for materials cannot be applied anymore. To reduce energy consumption, lightweight constructions are demanded but they cannot stand necessary mechanical stress. Therefore, local reinforced elements are applied for instance. According to actual research in material sciences, it is prerequisite to verify the position of this strengthening. Various physical principles can be used for non-destructive testing of this effect. In this article, an approach based on acoustic signals is presented. On the one hand a strategy could be to do comparative studies between nominal and sample and on the other to predetermine the place just by acoustic parameters like eigenmodes. In preliminary test, vibration characteristics of these innovative materials (e.g. bake-hardening plate and sandwich structure) are determined and reveal capabilities for acoustic methodologies. So, this approach could be a promising tool to localize reinforced elements.