Enrico Primo Tomasini

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.

A Noncontact Approach for the Evaluation of Large Artery Stiffness: A Preliminary Study

BACKGROUND The time from carotid to femoral pulse wave propagation (pulse transit time (PTT)) is required to estimate the carotid-femoral pulse wave velocity (PWV), a reliable index for evaluating large artery stiffness. METHODS In this work, we propose a novel, noncontact laser-based technique, named optical vibrocardiography (VCG), for evaluating PTT from synchronous recordings of the motion of the skin on the neck and the groin. These measurements, which have been demonstrated to be related to the radial displacement of the underlying blood vessels, were performed on 14 healthy subjects. As validation, applanation tonometry was performed to determine PTT between the carotid and the femoral artery. RESULTS PTT evaluated by VCG was not different from applanation tonometry (74.86 +/- 8.63 ms vs. 75.85 +/- 8.61 ms, P = 0.377). CONCLUSIONS Our preliminary results demonstrate that laser-based noncontact measurement in young healthy volunteers is feasible, and yields PTTs that are equivalent to those measured using arterial applanation tonometry. Its clinical application can overcome limitations inherent to a contact method like arterial tonometry.

An optical measurement method for the simultaneous assessment of respiration and heart rates in preterm infants

Many conventional medical monitoring devices, while not technically invasive, are nevertheless obtrusive insofar as they require contact with the patient. This obtrusiveness sometimes poses problems in daily clinical practice. The need for contact with electrodes or transducers is particularly relevant in the case of patients recovering in intensive care units where continuous monitoring is required, in turn requiring continuous direct transducer contact for prolonged periods. Among the many physiological parameters commonly acquired, the respiratory and the cardiac rates of the patients are of primary importance. Typically these two parameters are measured respectively using spirometry and electrocardiography (ECG), both involving obtrusive measurement systems requiring contact with the patient with an air conduit and electrodes. This paper presents an optical measurement method for the simultaneous assessment of respiration and heart rates based on the measurement of the chest wall movements, associated with inspiratory/expiratory activities of the lungs and by the mechanical pumping action of the heart. The measurement method has been adapted for use with preterm infants and it has been applied to 55 patients recovering in a Neonatal Intensive Care Unit. The method is based on the use of a laser Doppler vibrometer (LDVi) pointed at the left, ventral thoracic surface of the patient. LDVi-based measures of respiration and heart rate have been simultaneously acquired for each patient, in parallel with the same quantities acquired using conventional reference instrumentation (flow-meter and ECG) for comparison purposes. Results show that for respiration rate, differences with respect to the spirometer data are <3%, while for the cardiac rate they are <6% with respect to ECG data. The method proposed in this paper has the advantage of requiring no contact with the patient. Moreover, it supports, by means of a single instrument, the simultaneous measurement of respiration and heart rates, thus reducing the burden of the number of electrodes, transducers, and other instrumentation that must be applied to the patient--a consideration that is particularly important in the Neonatal Intensive Care Unit. In addition to the measurement of respiration rate, we also describe the sensitivity of the LDVi method in detecting key respiration events (irregular inspiration/expiration cycles, apneas, and hiccups) which are relevant to clinical monitoring.

Carotid artery pulse wave measured by a laser vibrometer

Early diagnosis of occlusive arterial diseases demands a device which sensitivity is able to resolve small differences between normal and abnormal pressure and flow pulse patterns. The present paper reports the use of a laser doppler vibrometer (LDV) in preliminary test for registration the pressure pulse. The measurements were performed in the right carotid artery of four volunteers to observe presumed typical pressure pulse patterns. LDV was used in single point mode to obtain the pressure waves in two distinct physiological situations: breathing and in apnea.

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.

Non contact measurement of heart and respiration rates based on Kinect

Heart Rate (HR) and Respiration Rate (RR) are considered among the most useful biomedical signals to be observed from a subject in order to evaluate his/her health conditions. HR and RR are routinely monitored in patients recovered in hospitals and eventual variations of these quantities need to be measured and reported. Today HR and RR are measured with standard methods: electrocardiography (ECG) and spirometry (SP). Both this methods need to be in contact with the subject and require the presence of expert personnel to be correctly operated. Consequently, their use is limited to hospitals or ambulatory environments and their diffusion in domestic environments is rare. In this paper we present a novel method for the measurement of HR and RR without contact on a subject. The proposed method is realized by means of a Kinect™ Device (KD). The KD is a widely-diffused multi-sensors device based on a depth-sensor, a camera-sensor and 4 microphones. In our work it has been used in conjunction with a special processing algorithm to calculate the HR and RR values. In order to measure HP and RR 10 healthy subjects were observed with the proposed method and with reference methods (ECG and a SP). Results from tests show that the standard deviation of the residuals (difference between the ECG or SP data and the corresponding measurements obtained by KD) are 6% and 9.7% for HR and RR values respectively. Therefore the proposed measurement method, based on the use of KD, could be used for the home-monitoring of HR and RR values in healthy subject without the presence of experts or clinicians.

Analogical model for mechanical vibrations in flue organ pipes inferred by independent component analysis

Several experiments have been performed to investigate the mechanical vibrations associated with an organ pipe. The measurements have been made by using laser Doppler vibrometry, a well-known not-invasive optical measurement technique that is very widely used in structural dynamics. The recorded signals are analyzed by using a well-established decomposition method in time domain, i.e., independent component analysis. Asymptotic dynamics methods to recognize low-dimensional dynamic system associated with this wave field is then considered. The full-toned recorded signals appear decomposed into three independent components. The independent components are nonlinear due to the fractal dimension of the attractor. These results for the mechanic vibrational field are compared with those of the acoustic one. It is interesting to note that the two fields have many common characteristics. Finally, a low-dimensional dynamic system that reproduces the main characteristics of the mechanical wave field in the time and frequency domains is introduced.

Hand-arm vibration measurement by a laser scanning vibrometer

Measurement of hand-arm transmitted vibration is a relevant issue for human health and safety. Many measurement problems occur with conventional transducers. In this paper, new measurement techniques are proposed. The methodology is based on a laser scanning vibrometer. Those techniques can be applied in laboratory tests and also to perform field tests on hand-guided vibrating tools, vehicles, and machines. Results of tests performed on different subjects with their hand on three laboratory test devices, designed according to ISO standards, are presented. Comparison of simultaneous measurements performed by the vibrometer and by conventional techniques (accelerometer) has been carried out. Further work has been done in order to test film sensors for measurement of the contact force between the hand and the vibrating surface. First results are here illustrated of film sensor characterisation and mechanical impedance measurements at some points of hands of different subjects, obtained by the laser Doppler vibrometer and by the film sensor.