Paolo Castellini

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.

The laser doppler vibrometer as an instrument for nonintrusive diagnostic of works of art: Application to fresco paintings

Abstract The purpose of this paper is to present the application of the Laser Doppler Vibrometer as a nonintrusive measurement instrument to detect shape and location of damage in fresco paintings. The paper discusses performance of the measurement instrument and outlines a measurement procedure capable of nonintrusive diagnostic of the state of conservation of works of art, based on frequency response analysis of wall surface vibration induced by acoustic excitation. The procedure for the measurement of wall surface vibration is described. A 2D map of surface vibration amplitude is produced by scanning the measurement point across the surface. This pseudo-image is then processed by image analysis techniques to determine the position and the shape of damaged areas. Experiments are performed on an artificial fresco with induced damage and on a real painting inside a church.

Non-invasive measurements of damage of frescoes paintings and icon by Laser Scanning Vibrometer: experimental results on artificial samples and real works of art.

Abstract Frescoes and icons show analogies in terms of defects, both present layer-to-layer detachments and delaminations and surface cracks; the aim of this work is to develop a diagnostic system for the measurement of the defects’ position and size. After initial measurement set-ups based on accelerometers and impact hammers a novel system based on laser vibrometers and acoustic stimulation of structures to allow full remote and contactless investigation of detachments and delaminations has been developed. This paper presents a measurement procedure based on two subsequent scans of the work of art: the first scan aims at finding the defects’ positions, the second one characterizes their resonant response. A new kind of exciter, namely piezo actuators, has been introduced in the measurement chain and its effectiveness in finding defects in icons will be demonstrated. In situ measurements on real frescoes have been performed in Senigallia (near Ancona) and Orvieto to assess the proposed technique and a sample of the obtained results will be presented.

Laser-based systems for the structural diagnostic of artwork: an application to XVII-century Byzantine icons

Laser diagnostic systems based on shearography, vibrometry, and holography principles were employed to investigate the potential implementation of laser techniques in art conservation structural diagnostic field. The employed techniques offered the required measuring variability to allow a first discrimination on complex diagnostic uncertainties encountered often in art conservation. The systems were tested and used to assess the structural condition of well-documented Byzantine icons with known structural problems. Defects and mechanical deformations were identified in various distances from the artworks by alternative employment of techniques. In this way simultaneous eligibility classification of the used systems was also obtained. Results of the on-field application on the XVII century Byzantine icons are herein presented with discussion on the prospects of the techniques in the diagnostic field of art conservation.

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.

New applications of Scanning Laser Doppler Vibrometry (SLDV) to non-destructive diagnostics of artworks: mosaics, ceramics, inlaid wood and easel painting

Abstract There exist many analytical methodologies and techniques to individuate the physical and chemical characteristics of artworks, but at present, their structural diagnostics mainly rely on the expertise of the restorer and the typical diagnostic process is accomplished mainly through manual and visual inspection of the object surface 〚1〛 . The basic idea behind the proposed technique is to substitute human senses with measurement instruments: surfaces are very slightly vibrated by mechanical actuators, while a laser Doppler vibrometer scans the objects measuring surface velocity and producing velocity amplitude and phase two-dimensional (2D) or 3D maps. Where a defect occurs velocity is higher than neighbouring areas, so defects can be easily spotted. Laser vibrometers also identify structural resonance frequencies thus leading to a complete characterisation of defects. This work will present the most recent results coming out of the application of scanning laser Doppler vibrometry (SLDV) to different types of artworks: mosaics, ceramics, inlaid wood and easel painting. Real artworks and samples realised on purpose have been studied using the proposed technique and different measuring issues resulting from each artwork category will be described.

On field validation of non-invasive laser scanning vibrometer measurement of damaged frescoes: experiments on large walls artificially aged

Abstract Past experiences demonstrated that the study of surface vibrations could be used to locate defect positions and sizes in frescoes. At present a non-invasive diagnostic system is under development and the aim of this work is to present the results obtained on large painted walls. After initial measurement set-ups based on accelerometers and impact hammers, a novel system based on laser vibrometers and acoustic stimulation was assembled. Full remote and contactless investigation of typical defects of frescoes, detachments, cracks and delaminations is thus possible with a very high accuracy. For the present investigation we employed a commercial scanning laser doppler vibrometer (SLDV) system, a horn loudspeaker and bass reflex enclosure to fully cover the audio frequency range. This paper will present experimental results gathered from large samples made by LRMH and compare them with those extracted by traditional investigation techniques.

Laser Doppler vibrometry on rotating structures in coast-down: resonance frequencies and operational deflection shape characterization

In rotating machinery, variations of modal parameters with rotation speed may be extremely important in particular for very light and undamped structures, such as helicopter rotors or wind turbines. The natural frequency dependence on rotation speed is conventionally measured by varying the rotor velocity and plotting natural frequencies versus speed in the so-called Campbell diagram. However, this kind of analysis does not give any information about the vibration spatial distribution i.e. the mode shape variation with the rotation speed must be investigated with dedicated procedures. In several cases it is not possible to fully control the rotating speed of the machine and only coast-down tests can be performed. Due to the reduced inertia of rotors, the coast-down process is usually an abrupt transient and therefore an experimental technique, able to determine operational deflection shapes (ODSs) in short time, with high spatial density and accuracy, appears very promising. Moreover coast-down processes are very difficult to control, causing unsteady vibrations. Hence, a very efficient approach for the rotation control and synchronous acquisition must be developed. In this paper a continuous scanning system able to measure ODSs and natural frequencies excited during rotor coast-down is shown. The method is based on a laser Doppler vibrometer (LDV) whose laser beam is driven to scan continuously over the rotor surface, in order to measure the ODS, and to follow the rotation of the rotor itself even in coast-down. With a single measurement the ODSs can be recovered from the LDV output time history in short time and with huge data saving. This technique has been tested on a laboratory test bench, i.e. a rotating two-blade fan, and compared with a series of non-contact approaches based on LDV: traditional experimental modal analysis (EMA) results obtained under non-rotating conditions by measuring on a sequence of points on the blade surface excited by an impact hammer, continuous scanning LDV measuring the ODS of the structure excited by an impact hammer modulating the laser output, tracking laser Doppler vibrometry (TLDV) operating at different rotation speeds under stationary conditions, tracking continuous scanning laser Doppler vibrometry (TCSLDV) operating at different rotation speeds under stationary conditions. EMA and TLDV have been performed over the same grid of points sufficiently dense to have ODSs with adequate spatial resolution, it requiring long measurement time. The application of different techniques allowed us to completely characterize the tested bladed rotor and to validate the continuous scanning application to transient rotator processes.

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.

Vibration measurements on blades of naval propeller rotating in water

A powerful technique for monitoring and analyzing mechanical systems is the application of laser Doppler vibration measurements. When the object to be analyzed rotates, the measurement becomes difficult. That is, when a steady laser beam is used, the presence of a tangential motion of surface determines a noisy disturbance. The speckle pattern produced by the roughness of the analyzed surface is moving with the surface itself, and this phenomenon is observed as a signal with a frequency depending on the rotational speed. For these reasons, the aim of the present work is to show the possibility of Eulerian approach to the vibration measurement of rotating objects. A Laser Scanning Vibrometer has been used for the tracking of the same point in the complete circular motion. The capabilities of this system are tested on the measurement of vibration map of blades of a model of naval propeller working in water: a comparison has been found with the static analysis, in water and air, of the same propeller. Some results are shown.