Gian Marco Revel

Laser Doppler vibrometry : A review of advances and applications

The use of laser Doppler vibrometry (LDVi) offers great potential for the improvement of the investigation capability of experimental vibration testing. Because of this, this technique is studied and applied with increasing interest in several industrial and scientific areas of research, including biomedical engineering. In particular, the traditional fields of vibration testing, such as damage detection, system identification, and model updating, benefit from the use of these novel techniques. In fact, they significantly extend measurement capabilities with respect to traditional accelerometers, as they allow remote, nonintrusive, high-spatial resolution measurements with reduced testing time and increased performances (bandwidth up to 200 kHz, velocity range of ±10 m/s, resolution of about 8 nm in displacement and 0.5 μm/s in velocity). In this work, the state of the art in LDVi technique is addressed, and the new instrument configurations, such as those for in-plane and rotational vibration measurements, are described. A review of the most innovative LDVi advances is presented with reference to recent publications, and the different methodologies are categorized according to their relative fields of application. Interesting results achieved by continuously controlling the movement of scanning LDVi mirrors are shown: measurements in tracking mode on rotating objects or continuous scanning for operational mode shapes determination are examples of these activities. Also, actual limits and fields of future research are discussed. The main limitations in LDVi instrumentation are actually represented by speckle effects and poor signal-to-noise ratio when measuring on low diffusive surfaces. For these problems, the research is continuing to develop, and important improvements are expected within the next few years.

Vibration measurements for diagnosis of structural defects on human teeth

In this paper, the authors propose and analyse a non-invasive measurement procedure for the diagnosis of structural defects on human teeth based on non-contact excitation and measurement of vibration by laser techniques. The tooth is excited in vibration by Nd:YAG laser pulses, below the ablation threshold, which induces local thermal expansion with consequent propagation of bending waves. Vibrations are then measured by Laser Doppler Vibrometry (LDV), which permits non-invasive and accurate measurements. The presence of structural defects, caused, for example, by caries, can be detected by variation in the dynamic behaviour of the tooth. The technique and the results of in-vitro tests on artificially defected human teeth are presented. The metrological problems connected with measurement repeatibility in different operating conditions are addressed. The effect of pulse energy level has also been measured, with the aim of determining the minimum energy level sufficient to achieve satisfactory signal-to-noise ratio in vibration measurement. In fact, if high-energy pulses are utilised, overheating or ablation on the tooth structure may be caused with serious consequences for the patient.

Measurement of stress?strain and vibrational properties of tendons

The authors present a new non-intrusive experimental procedure based on laser techniques for the measurement of mechanical properties of tendons. The procedure is based on the measurement of the first resonance frequency of the tendon by laser Doppler vibrometry during in vitro tensile experiments, with the final aim of establishing a measurement procedure to perform the mechanical characterization of tendons by extracting parameters such as the resonance frequency, also achievable during in vivo investigation. The experimental procedure is reported, taking into account the need to simulate the physiological conditions of the Achilles tendon, and the measurement technique used for the non-invasive determination of tendon cross-sectional area during tensile vibration tests at different load levels is described. The test procedure is based on a tensile machine, which measures longitudinal tendons undergoing controlled load conditions. Cross-sectional area is measured using a new non-contact procedure for the measurement of tendon perimeter (repeatability of 99% and accuracy of 2%). For each loading condition, vibration resonance frequency and damping, cross-sectional area and tensile force are measured, allowing thus a mechanical characterization of the tendon. Tendon stress–strain curves are reported. Stress–strain curves have been correlated to the first vibration resonance frequency and damping of the tendon measured using a single-point laser Doppler vibrometer. Moreover, experimental results have been compared with a theoretical model of a vibrating cord showing discrepancies. In vitro tests are reported, demonstrating the validity of the method for the comparison of different aged rabbit tendons.

Experimental and numerical investigation on structural effects of laser pulses for modal parameter measurement

In this paper it has been described part of the research devoted to the development of a complete non-intrusive experimental modal analysis procedure based on laser techniques both for excitation and for measurement. In particular, attention has been focused on the thermal effects generated by laser pulses on the excited structure. An analytical model of the energy exchange between the light pulse and the target surface is proposed together with a finite element model of thermal and mechanical behaviour of the structure under excitation. Both the models (analytical and numerical) have been experimentally validated by measuring the thermal and the vibration responses induced by the laser pulses. The experimental part of the study has been performed on a cantilever beam excited with laser pulses from an Nd : YAG source (532 nm, 100 mJ/pulse) using an high-speed infrared camera and a scanning laser Doppler vibrometer. Results from this work can be used to improve understanding concerning the features of laser excitation and to establish a mechanical equivalent system of forces and moments, useful in order to increase the accuracy in the measurements of modal parameters when laser pulses are used as excitation sources.

A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part I. Development of the technique and preliminary validation

The selective intensity technique is a powerful tool for the localization of acoustic sources and for the identification of the structural contribution to the acoustic emission. In practice, the selective intensity method is based on simultaneous measurements of acoustic intensity, by means of a couple of matched microphones, and structural vibration of the emitting object. In this paper high spatial density multi-point vibration data, acquired by using a scanning laser Doppler vibrometer, have been used for the first time. Therefore, by applying the selective intensity algorithm, the contribution of a large number of structural sources to the acoustic field radiated by the vibrating object can be estimated. The selective intensity represents the distribution of the acoustic monopole sources on the emitting surface, as if each monopole acted separately from the others. This innovative selective intensity approach can be very helpful when the measurement is performed on large panels in highly reverberating environments, such as aircraft cabins. In this case the separation of the direct acoustic field (radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by traditional techniques. The first aim of this work is to develop and validate the technique in reverberating environments where the location and the quantification of each source are difficult by traditional techniques. The reverberant field is clearly challenging also for the proposed technique, affecting the achievable accuracy, mainly due to the fact that coherence between radiated and reverberated fields is often unknown and may be relevant. Secondly, the applicability of the method to real cases is demonstrated. A laboratory test case has been developed using a large wooden panel. The measurement is performed both in anechoic environment and under simulated reverberating conditions, for testing the ability of the selective intensity method to remove the reverberation.

Structural damage assessment in composite material using laser Doppler vibrometry

In recent years, a great effort has been done to improve damage detection techniques in structures by using vibration measurements. This paper presents a case where a non-contact measurement system, a Scanning Laser Doppler Vibrometer, has been used to detect delaminations in a composite material plate. The diagnostic technique is the evolution of a methodology previously approached by the authors. An in-house made software has been produced for data acquisition and vibrometer control. The maps of the detected defects are presented, thus allowing the assessment of the performances of this methodology to detect damages. This analysis permitted to outline the main points to be improved in the future investigations.

Development and experimental evaluation of a thermography measurement system for real-time monitoring of comfort and heat rate exchange in the built environment

A measurement system based on infrared (IR) thermovision technique (ITT) is developed for real-time estimation of room thermal variations and comfort conditions in office-type environment as a part of a feasibility study in the EU FP7 project ‘INTUBE’. An IR camera installed on the ceiling allows thermal image acquisition and post-processing is performed to derive mean surface temperatures, number of occupants and presence of other heat sources (e.g. computer) through detecting algorithms. A lumped parameter model of the room, developed in the Matlab/Simulink environment, receives as input the information extracted from image processing to compute room exchanged heat rate, air temperature and thermal comfort (PMV). The aim is to provide in real time the room thermal balance and comfort information for energy-saving purposes in an improved way with respect to traditional thermostats. Instantaneous information can be displayed for the users or eventually used for automatic HVAC control. The system is based on custom adaptation of a surveillance low-cost IR system with dedicated radiometric calibration. Experimental results show average absolute discrepancies in the order of 0.4 °C between calculated and measured air temperature during a time period of a day. A sensitivity analysis is performed in order to identify main uncertainty sources.

Experimental investigation by laser ultrasonics for high speed train axle diagnostics

The present paper demonstrates the applicability of a laser-ultrasonic procedure to improve the performances of train axle ultrasonic inspection. The method exploits an air-coupled ultrasonic probe that detects the ultrasonic waves generated by a high-power pulsed laser. As a result, the measurement chain is completely non-contact, from generation to detection, this making it possible to considerably speed up inspection time and make the set-up more flexible. The main advantage of the technique developed is that it works in thermo-elastic regime and it therefore can be considered as a non-destructive method. The laser-ultrasonic procedure investigated has been applied for the inspection of a real high speed train axle provided by the Italian railway company (Trenitalia), on which typical fatigue defects have been expressly created according to standard specifications. A dedicated test bench has been developed so as to rotate the axle with the angle control and to speed up the inspection of the axle surface. The laser-ultrasonic procedure proposed can be automated and is potentially suitable for regular inspection of train axles. The main achievements of the activity described in this paper are: – the study of the effective applicability of laser-ultrasonics for the diagnostic of train hollow axles with variable sections by means of a numerical FE model, – the carrying out of an automated experiment on a real train axle, – the analysis of the sensitivity to experimental parameters, like laser source – receiving probe distance and receiving probe angular position, – the demonstration that the technique is suitable for the detection of surface defects purposely created on the train axle.

A novel approach for features extraction in physiological signals

The authors have investigated a novel processing technique, which allows to measure possibly relevant features in the ECG (Electrocardiogram) signal according to the morphology of its waveform. The aim of this work is to prove its efficacy in the assessment of the subjects Heart Rate (HR) and to broaden its use to signals coming from different biomedical sensors (based on optical, acoustical and mechanical principles) for the computation of HR. The analysis technique proposed for the identification of the main feature (R-peak) in ECG signal provides results that are comparable to those obtained with traditional approaches. The approach has also been applied to other signals related to blood flow, such as PCG (Phonocardiography), PPG (Photoplethysmography) and VCG (Vibrocardiography), where standard algorithms (i.e. Pan & Tompkins) could not be widely applied. HR results from a measurement campaign on 8 healthy subjects have shown, respect to ECG, a deviation (calculated as 2σ) of ±3.3 bpm, ±2.3 bpm and ±1.5 bpm for PCG, PPG and VCG. Future work will involve the extraction of additional features from the previous signals, with the aim of a deeper characterization of them to better describe the subjects health status.

Exploiting continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation methods for noise source identification

This paper proposes the use of continuous scanning laser Doppler vibrometry (CSLDV) in time domain correlation techniques that aim at characterizing the structure-borne contributions of the noise emission of a mechanical system. The time domain correlation technique presented in this paper is based on the use of FIR (finite impulse response) filters obtained from the vibro-acoustic transfer matrix when vibration data are collected by laser Doppler vibrometry (LDV) exploited in continuous scan mode (CSLDV). The advantages, especially in terms of source decorrelation capabilities, related to the use of CSLDV for such purpose, with respect to standard discrete scan (SLDV), are discussed throughout the paper. To validate this approach, vibro-acoustic measurements were performed on a planetary gear motor for home appliances. The analysis of results is also supported by a simulation.