Scanning Laser Doppler Vibrometry Research Articles

Characterisation of bone following ultrasonic cutting

Ultrasonic surgery is an increasingly popular technique for cutting bone, but little research has investigated how the ultrasonic tip oscillations may affect the cuts they produce in bone. The aim of this investigation was to evaluate the oscillation and cutting characteristics of an ultrasonic surgical device. A Piezosurgery 3 (Mectron, Carasco, Italy) ultrasonic cutting system was utilised with an OP3 style tip. The system was operated with the tip in contact with porcine bone samples (loads of 50 to 200g) mounted at 45° to the vertical insert tip and with a water flow of 57ml/min. Tip oscillation amplitude was determined using scanning laser vibrometry. Bone surfaces defects were characterised using laser profilometry and scanning electron microscopy. A positive relationship was observed between the magnitude of tip oscillations and the dimensions of defects cut into the bone surface. Overloading the tip led to a reduction in oscillation and hence in the defect produced. A contact load of 150g provided the greatest depth of cut. Defects produced in the bone came from two clear phases of cutting. The structure of the bone was found to be an important factor in the cut characteristics following piezosurgery. Cutting of bone with ultrasonics is influenced by the load applied and the setting used. Care must be used to prevent the tip from sliding over the bone at low loadings.

Performance analysis of continuous tracking laser Doppler vibrometry applied to rotating structures in coast-down

In this paper a performance analysis of the so-called tracking continuous scanning laser Doppler vibrometry (TCSLDV) exploited in coast-down has been performed. This non-contact measurement system is able to scan continuously over a rotating surface during coast-down and to determine vibration operational deflection shapes (ODSs) and natural frequencies in short time, i.e. the temporal extent of the coast-down. The method is based on a laser Doppler vibrometer (LDV) whose laser beam is driven to scan continuously over the whole rotor surface synchronously with its rotation, so that the LDV output is modulated by the structure's ODSs. This technique has a full-field nature that enables it to measure simultaneously the time and spatial dependence of the vibration in a unique measurement. However, the TCSLDV presents some criticalities in practical applications, especially when applied to rotary transient and fast processes. In fact, if the vibration is transient and decays very fast, then the laser beam could not have had the time to scan the complete structure surface and the modulation of the ODS could be partial. An analytical model reproducing a representative experiment has been developed in order to evaluate the sensitivity of results to testing conditions. The laser beam trajectory in both the fixed and rotating reference systems has been synthesized showing its dependence on experimental parameters as the rotation speed variation during coast-down. It has been demonstrated the decrease in speed induces the deformation of the laser trajectory influencing the LDV output time history, spectrum and consequently the recovered ODS.

Damage detection strategies based on propagation of guided elastic waves

This paper demonstrates the effectiveness of certain structural health monitoring (SHM) strategies based on propagation of guided elastic waves within thin-walled shell structures. This is realized based on results of numerical and experimental investigations obtained by the use of the spectral finite element method (SFEM) together with laser scanning vibrometry (LSV) and subsequent application of two different integral-based indices for damage quantification, these being the integral mean value (IMV) and the root mean square (RMS). Numerical tests by SFEM were carried out for a square plate with through-hole damage, a cracked fuselage section with two stiffeners and a cracked wing section skin, all made out of aluminium. Experimental measurements by LSV included tests on a square aluminium plate, composite plate and a composite stabilizer of a PZL W-3A helicopter, all with simulated damage. During numerical and experimental investigations damage sensitivity of both IMV and RMS indices was tested by considering various displacement components, signal timescales or weighting factors. A certain practical method for quick construction of RMS damage maps based on experimental measurements by LSV was also presented.

Modelling locust tympanal systems: understanding the link between biomechanics and neurophysiology

Modelling locust tympanal systems: understanding the link between biomechanics and neurophysiology

Modal response and sound radiation from a hammered dulcimer

The sound radiation of the hammered dulcimer has been investigated. The dulcimer studied is a 16/15 fixed-top instrument of typical size with Baltic birch (laminated plywood) soundboard and back. To determine the instrument body resonances, the dulcimer was driven at the treble and bass bridges with a shaker. Accelerometers were used to obtain the resonance frequencies of the soundboard and back, and a microphone was placed inside the instrument to obtain the cavity resonances. The individual resonance peaks found were further investigated using scanning laser Doppler vibrometry and near-field acoustical holography. Preliminary results show that there is little modal response of the instrument at the fundamental frequencies of the lowest notes of the dulcimer. In addition, the vibration coupling to the back plate through the internal bracing causes it to serve as a second soundboard. Lastly, the holography results indicate significant radiation from the sound holes at some frequencies, which may contradict the commonly held notion that the dulcimer sound holes are largely decorative.

Synchronized vibrations measurements at multiple locations using a single continuously scanning laser doppler vibrometer. Applications to non-contact sensing of human body vibrations

Laser Doppler vibrometry (LDV) is a non-contact method to measure surface velocity. Typical LDV configurations uses one fixed laser (He-Ne) beam at a specific point and orientation on a surface under test. A continuously scanning laser Doppler vibrometry (CSLDV) technique using a single laser beam performing quickly a long scan was developed to measure surface velocity at each laser position during its scan. This fast CSLDV can then replace several fixed LDVs. This technique is especially advantageous for sensing human body natural vibrations (typically below 100 Hz) at multiple locations (e.g., along small muscles) as it does not require traditional skin-mounted sensors (e.g., accelerometers array), which eliminates mass artifacts and the set-up time to attach those sensors. Experimental measurements were conducted using a CSLDV with a 200 Hz linear scan rate, over scan lengths up to 5 cm, to measure low-frequency vibrations (f<100 Hz) on gel samples which mimic human soft tissues. Validations of the CSLDV measurements were done using an array of several fixed LDVs distributed along the same scan line. The effects of speckle noise on CSLDV measurements will be quantified. Applications of this CSLDV technique for active and passive elastography measurements will be presented.

Damage detection strategies for aircraft shell–like structures based on propagation guided elastic waves

Damage of aircraft structural elements in any form always present high risks. Failures of these elements can be caused by various reasons including material fatigue or impact leading to damage initiation and growth. Detection of these failures at their earliest stage of development, estimation of their size and location, are one of the most crucial factors for each damage detection method. Structural health monitoring strategies based on propagation of guided elastic waves in structures and wave interaction with damage related discontinuities are very promising tools that offer not only damage detection capabilities, but are also meant to provide precise information about the state of the structures and their remaining lifetime. Because of that various techniques are employed to simulate and mimic the wave–discontinuity interactions. The use of various types of sensors, their networks together with sophisticated contactless measuring techniques are investigated both numerically and experimentally. Certain results of numerical simulations obtained by the use of the spectral finite element method are presented by the authors and related with propagation of guided elastic waves in shell–type aircraft structures. Two types of structures are considered: flat 2D panels with or without stiffeners and 3D shell structures. The applicability of two different damage detection approaches is evaluated in order to detect and localise damage in these structures. Selected results related with the use of laser scanning vibrometry are also presented and discussed by the authors.

Experimental and Theoretical Analysis of Lamb Wave Generation by Piezoceramic Actuators for Structural Health Monitoring

Piezoceramic transducers, acting as actuators and sensors, are attractive for generation and reception of Lamb waves in Structural Health Monitoring (SHM) systems. To get insight into the source-mechanisms of Lamb waves, the vibrations of piezoceramic actuators are analyzed for the free and bonded state of the piezoceramic by analytical and finite element (FEM) calculations. Mode shapes and spectra of piezoceramic actuators and Lamb wave fields are experimentally recorded by scanning laser vibrometry. The analytical solutions for bending modes are shown to be valid for large diameter-to-thickness-relations of a free piezoactuator (D/H > 10) only. For thicker piezoceramics, a FEM-solution gives better results. Calculated frequencies for radial modes of vibration are confirmed by 3-D-laser-vibrometry and measurements of electrical impedance. The bonded case of a piezoactuator exhibits a broad resonance peak resulting from the strong coupling between radial and bending modes. The assumption that optimal excitation of Lamb modes occurs for a matching of the wavelengths to the diameter of the piezoceramic holds only for thin ceramics. Otherwise the distinct modes of out-of-plane and in-plane vibrations control the excitation of the Lamb modes more than the wavelength matching.

Laser sensing techniques for acoustic detection of buried mines.

Acoustic detection of buried mines is based on excitation of vibrations in the ground with airborne sound and measuring vibration of the ground surface by using a laser interferometric sensor. The principle of detection uses the fact that buried mines have mechanical impedances that are smaller than that of soils. That makes the soil above a mine vibrating with magnitude higher than the surrounding soil. A laser sensor creates a vibration image of the ground by measuring vibration in many points on the surface. The presence of a buried mine can be detected by an abnormality in the vibration image. The method has shown excellent performance in field tests. One limiting factor of the method is a long measurement time due to low frequency of vibrations. Different laser interferometric techniques have been investigated to reduce the time of detection. The paper discusses application of single‐beam scanning laser Doppler vibrometry (LDV), multiple‐beam LDV, and speckle‐interferometry based methods for acoustic detection of mines.

Investigation of continuous scanning laser Doppler vibrometry for non-contact measurement of linear and angular surface deflections.

Laser Doppler vibrometry (LDV) is a non-contact technique for sensing surface vibrations. Traditionally LDV uses one or several fixed beams to measure vibrational velocity of specific points and orientations. Measurement of angular velocity requires at least two beams. Instead, we develop a technique, continuous scanning laser Doppler vibrometry (CSLDV), using a single laser beam continuously sweeping the area of interest using a scanning mirror. Linear scans allow the measurement of normal and angular velocities while circular scans allow the measurement of normal and two angular velocities. We validated the CSLDV technique for measuring low frequency (less than 50 Hz) broadband vibrations of gel samples which mimics natural vibration of human body. Such system could potentially be used to monitor multiple DOF of the skin surface for haptic or remote control applications. There is a disadvantage of using CSLDV which is speckle noise that is generated when coherent light source is reflected back from an optically rough surface. We will discuss the effects of scan lengths, scanning frequency, target to sensor distance, and the excitation amplitude on the performance of CSLDV.

Quantify Resonance Inspection With Finite Element-Based Modal Analyses

Resonance inspection uses the natural acoustic resonances of a part to identify anomalous parts. Modern instrumentation can measure the many resonant frequencies rapidly and accurately. Sophisticated sorting algorithms trained on sets of good and anomalous parts can rapidly and reliably inspect and sort parts. This paper aims at using finite element-based modal analysis to put resonance inspection on a more quantitative basis. A production level automotive steering knuckle is used as the example part for our study. First, the resonance frequency spectra for the knuckle are measured with two different experimental techniques. Next, scanning laser vibrometry is used to determine the mode shape corresponding to each resonance. The material properties including anisotropy are next measured to high accuracy using resonance spectroscopy on cuboids cut from the part. Then, the finite element model of the knuckle is generated by meshing the actual part geometry obtained with computed tomography. The resonance frequencies and mode shapes are next predicted with a natural frequency extraction analysis after an extensive mesh size sensitivity study. The good comparison between the predicted and the experimentally measured resonance spectra indicates that finite element-based modal analyses have the potential to be a powerful tool in shortening the training process and improving the accuracy of the resonance inspection process for a complex production level part. The finite element-based analysis can also provide a means to computationally test the sensitivity of the frequencies to various possible defects such as porosity or oxide inclusions, especially in high stress regions that the part will experience in service.

Development of a novel ultrasonic motor resonator using topology optimization

The use of topology optimization in the design of a novel stator for an ultrasonic motor (USM) is investigated. The design challenge is to produce a stator, with two resonant modes whose frequencies are in a ratio of 1:2. When driven together, these modes result in a contact point trajectory in a figure of eight shape. As a result, only one electronic amplifier is required to drive the proposed device. In contrast traditional travelling wave USM, with elliptical contact point trajectories, require two modes with equal resonant frequencies to be driven 90° out of phase, and therefore require two amplifiers, one for each mode. To achieve a suitable stator design, a slightly unconventional topology optimization problem formulation is proposed, in which the objective function is to minimize the amount of material with intermediate density, while satisfying a constraint related to the frequency ratio of selected resonant modes. The planar design produced using the optimization procedure was refined using a detailed three dimensional finite element analysis. A prototype of the proposed stator design was manufactured and experimentally characterized. Scanning laser vibrometry measurements from two positions were used to measure the figure of-eight motion. Finally, the stator was fitted with a preloaded slider to form a simple linear motor demonstrator which was characterized experimentally. The prototype motor produced a slider speed of 14 mm/s reversibly and a maximum force of 50 mN.

A laser scanning vibrometer for the monitoring of stiffness and density defects in fibre-reinforced plastics

Laser scanning vibrometry (LSV) is a non-contact technique for precise measurements of elastic wave parameters. In particular, the measurements of the wave velocity and dissipation deliver information on a material's stiffness and imperfections. In this paper, LSV was applied to monitor the stiffness and density of defects in composite laminate specimens cured at various exposures to microwave radiation. The specimens were supported by nylon wires and excited by a loudspeaker. Its driving frequency was swept and the frequency response of the specimens was measured by a laser vibrometer. The frequency shift of the fundamental flexural mode due to stiffness variation was measured as a function of the microwave exposure. To evaluate the dissipation factor, which is related to the density of defects, a short pulse was used for acoustic excitation. The temporal decay of the vibrations (reverberation) was measured by LSV and interpolated with an exponential function. The results obtained enable monitoring of the deterioration of the composite properties with the increase in the microwave exposure.

The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films

Abstract Aluminium nitride (AlN) reactively sputter-deposited from an aluminium target is an interesting piezoelectric thin film material with high CMOS compatibility. A good c -axis orientation is essential for obtaining high piezoelectric coefficients. Therefore, the influence of different sputtering conditions on the microstructure of AlN thin films with a typical thickness of about 500 nm was investigated. In this study it is demonstrated that highly c -axis oriented AlN thin films can be deposited on nominally unheated (1 0 0) silicon substrates, most preferentially when using a pure nitrogen atmosphere. The degree of c -axis orientation increases with higher nitrogen concentration and with decreasing the sputtering pressure, whereas the influence of plasma power on the microstructure was found to be negligible. A low sputtering pressure is also useful for minimizing the amount of oxygen contaminations in the deposition chamber and hence for reducing the incorporation of impurities into the AlN films. Intrinsic stress values of AlN thin films were determined by wafer bow measurements and were found to be between −3.5 and 750 MPa depending on choice of deposition parameters. Finally, the piezoelectric coefficients d 33 and d 31 were determined experimentally by laser scanning vibrometry in conjunction with a theoretical model. Effective values in c -axis oriented 500 nm films with FWHM of 0.33° are 3.0 and −1.0 pm/V. For a film of 2.4 μm thickness, values of 5.0 and −1.8 pm/V were measured, which are near the bulk values.

Higher-order eigenmodes of qPlus sensors for high resolution dynamic atomic force microscopy

The time response of tuning-fork based sensors can be improved by operating them at higher eigenmodes because a measurement takes at least one oscillation cycle in dynamic force microscopy and the oscillation period of the second eigenmode is only about one sixth of the fundamental mode. Here we study the higher-order eigenmodes of quartz qPlus sensors [Bettac et al., Nanotechnology 20, 264009 (2009); Giessibl and Reichling, Nanotechnology 16, S118 (2005); Giessibl, Appl. Phys. Lett. 76, 1470 (2000); and Giessibl, Appl. Phys. Lett. 73, 3956 (1998)], their equivalent stiffness, and piezoelectric sensitivity, while paying special attention to the influence of the mass and rotary inertia of the sensing tip which is attached to the end of the qPlus quartz cantilever. A combination of theoretical modeling and scanning laser Doppler vibrometry is used to study the eigenmodes of qPlus sensors with tungsten tips. We find that the geometry of tungsten tips can greatly influence the shape, equivalent stiffness, and piezoelectric sensitivity of the second eigenmode of the quartz cantilever. At a critical tip length it is possible to theoretically achieve infinite equivalent stiffness and infinite piezoelectric sensitivity when the tip becomes a perfect node of vibration and beyond this critical tip length the second eigenmode loses its vibration node and the trajectory of the tip reverses with respect to the beam curvature. The findings have major implications for optimizing tip geometry for high-resolution imaging with qPlus sensors using higher eigenmodes.

Accurate measurement of the piezoelectric coefficient of thin films by eliminating thesubstrate bending effect using spatial scanning laser vibrometry

One of the major difficulties in measuring the piezoelectric coefficientd33, f for thin films is the elimination of the contribution from substrate bending. We show bytheoretical analysis and experimental measurements that by bonding thin film piezoelectricsamples to a substantial holder, the substrate bending can be minimized to a negligiblelevel. Once the substrate bending can be effectively eliminated, single-beam laser scanningvibrometry can be used to measure the precise strain distribution of a piezoelectric thinfilm under converse actuation. A significant strain increase toward the inside edge of thetop electrode (assuming a fully covered bottom electrode) and a corresponding strain peakin the opposite direction just outside the electrode edge were observed. These peaks werefound to increase with the increasing Poisson’s ratio and transverse piezoelectriccoefficient of the piezoelectric thin film. This is due to the non-continuity of theelectric field at the edge of the top electrode, which leads to the concentration ofshear stress and electric field in the vicinity of the electrode edge. The measuredd33, f was found to depend not only on the material properties such as the electromechanicalcoefficients of the piezoelectric thin films and elastic coefficients of the thin film and thesubstrate, but also on the geometry factors such as the thickness of the piezoelectricfilms, the dimensions of the electrode, and also the thickness of the substrate.

Vibrational characteristics of Balinese gamelan metallophones

A study of the eight metallophone pairs from a Balinese gamelan semara dana has been conducted. Acoustical recordings of metallophone bars being struck were used to examine ratios of overtone frequencies to the fundamental. Results showed large variability in the number and ratios of overtones present. Scanning laser Doppler vibrometry measurements made on several bars also revealed great variability in mode shapes present. The distribution of prominent overtones and their modal shapes do not appear to match those of Western metallophones. Notably, the overall gamelan metallophone characteristics are quite dissimilar to the glockenspiel, which disagrees with previous studies.

Array measurement and imaging of sound transmission through the lungs.

Non‐contacting scanning laser Doppler vibrometry (SLDV) was used to measure torso surface motion at an array of points caused by sound introduced into the lungs in human subjects. The SLDV serves as a gold standard as it does not have the complexity of surface contact dynamics. Additionally, a sensor array integrated into a compliant mesh fabric chair was constructed and tested as a less expensive and faster means of acquiring similar acoustic array data. Silicone gel‐coupled, piezoelectric contact sensors for the detection of subsurface acoustic sources at the skin surface were used in the array. Array measurements were processed to determine the response at each measurement point as a function of frequency up to 1.6 kHz. Acoustic images were generated indicating the amplitude level at each point relative to the amplitude measured at the suprasternal notch, which served as the reference input. The acoustic measurements illustrated the complexity of sound transmission in the human lung and chest. Expected attenuation over the scapulae and below the diaphragm was observed. There were other significant variations as a function of location and frequency; measurement results are discussed. [Work supported by NIH Grant # EB008373.]

Vibrational characteristics of Balinese gamelan metallophones.

A study of the 16 metallophones from a Balinese gamelan has been conducted. Acoustical recordings of metallophone bars being struck were used to examine ratios of overtone frequencies to the fundamental. Results showed large variability in the number and ratios of overtones present. Scanning laser Doppler vibrometry measurements made on several bars also revealed great variability in mode shapes present. The distribution of prominent overtones and their modal shapes do not appear to match those of Western metallophones. Notably, the overall gamelan metallophone characteristics are quite dissimilar to the glockenspiel, which disagrees with previous studies. [This study was supported by the National Science Foundation through its Research Experiences for Undergraduates program.]

Five-axis scanning laser vibrometry for three-dimensional measurements of non-planar surfaces

A novel system for directing the beam of a single-point laser Doppler vibrometer (LDV) to measure three-dimensional velocity components of non-planar targets has been developed. A description of this measurement system is presented, along with a discussion of relative merits as compared with conventional scanning LDV systems. Two data sets are presented. In the first, sample measurements of a bulk solid are compared with an analytic model as verification of the measurement technique. In the second, measurements of the surface of sand over a buried anti-tank landmine are presented as a demonstration of the full capability of the system.