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Chapter 3.3 Laser Doppler Vibrometry Enrico Esposito Department of Mechanics, Polytechnic
Contents 3.3.2 Optical
Sensors for Vibration Measurements 3.3.3 Laser
Doppler Vibrometers
3.3.4.2 Dynamic
Characterization of Large Scale Structures 3.3.6.3 Providers and Useful Websites 3.3.8 Appendix:
Features of Signals in the Time Domain During the last years the growing importance of
the correct determination of the state of conservation of artworks has
been stated by all personalities in care of Cultural Heritage. 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/technician and the typical diagnostic process is accomplished
mainly through manual and visual inspection of the structure. For this
reason, many innovative optical techniques have been tried and applied
to this issue and in these pages we will show some examples regarding
the use of the laser Doppler vibrometer (LDV); for further detail on
other techniques and also a comparison of features and results, please
refer to the publication of Tornari et al [1]. The basic idea behind the employment of LDV is
to substitute human senses and contact sensors with measurement systems
capable of remote acquisition and, if necessary, of remote structural
excitation: surfaces are very slightly vibrated by mechanical and
acoustical actuators, while a laser Doppler vibrometer means the
objects measuring surface velocity and producing 2D or 3D maps. Think,
for example, of a fresco with delaminated areas: where these defects
occur, velocity is higher than neighbouring areas so defects can be
easily spotted by a LDV. Laser vibrometers also identify structural
resonance frequencies thus leading to a complete characterization of
these defects, and this holds true also for massive structures, like
towers, buildings, churches. Laser Doppler Vibrometers, or better Scanning
Laser Doppler Vibrometers (SLDV), have been applied to different types
of movable or decorative artworks, like frescoes, icons, mosaics,
ceramics, inlaid wood and easel painting, with different degrees of
success, but always showing an impressive list of important advantages:
— no remarkable intrusivity, — remote measurements, — ample frequency response, — high sensibility, — portability. Moreover all existing systems are completely PC
controlled and this allows digital data storage and easy data transfer
to other applications like software packages for structural and modal
analysis, and to spreadsheets applications like Excel or Matlab. The application to historical buildings is more
recent [2] and still limited but looks promising and will be the
subject of much research in the immediate future. Of course there still
exist a lot of difficulties, mainly related to the non-optically
collaborative surfaces of tested structures and the necessity of
working at great distances to get data that can be considered
representative of the examined object. These two factors work one
against the other, and this makes the application of SLDV mainly a
“prototype” application yet, but already exist
situations where this is not the case anymore [3]. Also we must not forget other problems, like
instrument isolation from ground vibrations and the realization of
special excitation techniques but it has been already demonstrated the
capability of the LDV to acquire non-intrusively vibrational data on
not-treated surfaces up to 10-15 meters, a real asset when dealing with
large structures. Regular monitoring of important parameters related to
the state of conservation of these huge objects, like frequencies of
resonance, is thus possible with no external intervention on the
structure and may be performed quickly and with a high degree of
accuracy.
3.3.2 Optical Sensors for Vibration Measurements When we say “optical
sensors”, we mean an immense variety of instruments, devices
and systems. Just think of such different instruments like infra red
thermal cameras or a Bragg grating strain sensor. Even to measure
vibrations we may have such different solutions like laser based, LED
based, fibre based sensors; not to mention the physical scale of such
sensors, going from the micro scale, e.g. optical MEMS accelerometers,
to relatively “immense” laser Doppler vibrometers.
If we confine ourselves to laser based instrumentation, we may mention
full field techniques (holography,
shearography, ESPI, for
example) or focused beam ones (laser Doppler vibrometers). The advantages of optical sensors are
outstanding and their use is spreading more and more, day after day.
Think for example to optical fibres sensors: their solid state design
is resistant to vibration, unaffected by electromagnetic interference
(nor do they create additional EMI), and, because the light source can
be located far away from explosive materials, do not run the risk of
sparking an explosion. They also offer superior multiplexing
capabilities, thanks to the possibility of having multiple sensors in a
single fibre line. Also, fibre optic sensors fall into a variety
of sensor types: chemical, temperature, strain, biomedical, electrical
and magnetic, rotation, vibration, displacement, pressure, and flow.
Many of these categories were developed by military organizations
during the nineties. These sensors are extremely effective at creating
"smarter" structures, widely used nowadays for chemical sensing
(especially in the petrochemical industry), transportation, building
and structural monitoring, and biomedical. However, fibre sensors must be placed in
contact or closed to the object to be measured, and so they maybe not
used in many occasions, where the objects cannot be reached or are
impossible to modify, e.g. a fresco in a church. For cases like these, instrumentation based on
a laser beam used as a probe is much more suited, and we will deal
exclusively with these devices in the following of this publication.
Major advantages of such instruments rely not only in this absence of
invasivity, but also in their high sensibility and in their capacity of
acquiring detailed data in the terms of space, time, and frequency.
Many of these systems are still quite expensive, but their contribution
to solve design, production process, or quality control problems is
invaluable. More specifically we will deal with focused
laser beam instruments, laser Doppler vibrometers. We will avoid
detailed mathematical description of involved theory, preferring a more
intuitive approach to make this matter more palatable to a wider range
of learners. The scanning version of the LDV may
automatically and accurately measure point-by-point surface velocities
using interferometric techniques and a couple of galvanometric driven
mirrors steering the laser beam. In this way it is possible to scan a
grid of acquisition points acquiring response spectra and time
histories of the velocity of each point; these data are then processed
and presented as 2D or 3D colour maps. Modern SLDVs may scan 100
points/second for a total number of more than 100.000 points working
with a maximum frequency in the range of some tens of MHz, and with a
lower limit of less than a Hertz. Full-scale highest range is typically
10 m/s with lower ranges in the order of 1 mm/s, corresponding to a
displacement of some tens of nanometres. These features make the SLDV an ideal
instrument in applications where it is impossible or very difficult to
use standard vibration measuring devices, such as accelerometers.
Accelerometers will load the examined structures and may even damage
the delicate surface of precious objects. Moreover, to perform an
accurate vibrational analysis it would require to employ many
transducers or to move one all around the tested piece and in both
cases time and cost would rise considerably.
3.3.3
Laser Doppler
Vibrometers As already mentioned, vibrometry is opening new
possibilities with respect to traditional contact (e.g. accelerometers,
strain gages) and non-contact (e.g. triangulation or reflection sensor,
proximity) techniques. The standard Laser Doppler Vibrometer is a
non-contact velocity transducer working on the principle of measuring
the Doppler frequency shift of a laser beam scattered from a moving
target by means of an interferometer. The on-board electronics converts
the Doppler signal to an analog voltage proportional to the
instantaneous velocity of the target. The combination of an interferometer with two
moving mirrors driven by galvanometric actuators makes it possible to
direct the laser beam to the desired measurement points. Such an
instrument, named the Scanning Laser Doppler Vibrometer (SLDV), can
quickly perform a series of velocity measurements on a grid of points
over the structure under test. These techniques are effectively used in
structural dynamic testing, biological and clinical diagnostics,
fluid-structure interaction, on-line monitoring of industrial plants,
acoustics, and fault detection, to quote only a few. Furthermore, the
coupling of laser vibrometers and scanning systems seems to open up new
possibilities, e.g. in the field of measurements in tracking mode on
moving objects. A well established application, first developed
by the Department of Mechanics, regards the employment of the SLDV for
non-invasive and non-contact diagnostics of works of art. From this
application an important research field, regarding diagnostics of civil
structures by SLDV, started recently and led to the creation of an
inter-departmental spin-off company. Given a strong structural
resemblance with multi-layer artworks (e.g. frescos), an important
activity where SLDVs are commonly employed, is the fault detection in
composite materials, with important rewards in the fields of
aeronautics and aerospace.
3.3.3.2
Basic Working Principles As its own name says, this type of vibrometer
uses the Doppler effect to remotely acquire vibration velocities:
surface vibrations induce a Doppler frequency shift on the impinging
laser beam, and this shift is linearly related to the velocity
component in the direction of the laser beam. In this way we have
established a linear connection between laser beam frequency variations
and velocity values. The obstacle facing us now is that Doppler shifts
are usually very small when compared to the laser fundamental
frequency, typically 1 part out of 108; the only way to appreciate such
small quantities is to use interferometry, so that high frequency
oscillations are combined and reduced to much lower values that can be
dealt with by standard electronics. A scheme of a vibrometer
incorporating a Mach-Zender interferometer is presented in Fig. 3.3.1. The light from the laser is split into a
“reference beam” and an “object
(measurement) beam” by beam splitter BS1. The object beam
passes through beam splitter BS3 and is focused to a point on the
vibrating object by the lens. The backscattered light is diverted by
BS3 towards BS2. At BS2, the backscatter from the object mixes together
(interferes) with the frequency shifted reference beam. The mixing
process causes any frequency difference, due to the Doppler effect,
between object and reference beam to show up as an intensity modulation
at BS2. Vibrometers usually employ He-Ne lasers, so we have a Doppler
frequency shift (fD) of about 3.16 kHz for each
mm/s (see (2)).
Finally, the optical signal is converted to an electrical signal by
photo detectors PD1 and PD2. The use of two detectors minimises noise
and drift. We will not demonstrate it here, but the resultant signal at
the output of the operational amplifier is (not considering the
frequency contribution due to the Bragg cell):
where A is
the amplitude of the laser beam. We may also describe the process in another
way: surface displacement varies the optical path difference between
the two laser beams and this results into a phase lag varying with
object vibration velocity, v.
So we have a time varying phase
difference corresponding to an instantaneous frequency component that
follows v.
This frequency shift is equal to the Doppler shift (fD) and
we know from basic physics that fD depends on v and source wavelength
(λ):
Substituting (2) into (1) we see that s(t) is a
Frequency Modulated (FM) signal, and, by demodulating this FM signal,
it is possible to obtain the amplitude of v; however we still lack the
information on the direction of the surface velocity because of the
cosine function. To solve this problem there exist two solutions, based
on electro-mechanical-optical shifting of the frequency of the
reference beam by a Bragg cell (like in Figure 1), or on electronic
manipulation of the recombined beams. Most diffused SLDVs have a maximum velocity
range of 10 m/s, with a frequency upper limit of 200 kHz, a resolution
of about 1 μm/s and a base accuracy in the order of 1%-2% of RMS
reading. Laser power is less than 1 mW, so that no special safety
measures are required, but nevertheless also with such low power levels
working distances of some tens of meters are possible with a spatial
resolution of 1 mm. All SLDV systems are governed by an industrial PC
and results are stored in digital formats like BMP and JPG images, AVI
movies, or UFF and TXT text data files. Moreover, results maps may be
superimposed on images of the structure recorded by the internal CCD
camera that always equips SLDVs.
In Fig. 3.3.2 a single point LDV laser head is
shown, not including mirrors to move the laser beam, and a scanning one
on top of each other to let you compare dimensions. In the same
picture, in the photo at right, we have a complete SLDV system, where
you may see the controlling unit (B), the general management PC (A),
the laser scanning head (M) and also a signal generator (C). The laser
head contains the laser tube, the focusing system, the interferometer,
the demodulation electronics, the scanning mirrors and the CCD camera.
The control unit main task is to interface the laser head with the PC,
so it contains the electronics to control the scanning mirrors and the
demodulation electronics (if not present in the laser head). Lastly,
the management PC is in charge of data acquisition by A/D boards, data
storage on hard disk, general system control and also hosts the data
acquisition/analysis software.
Generally speaking, the application of SLDV to
the problem of structural defects detection is based on the well
established principle that it is possible to evaluate the structural
state of an object from the examination of its vibrations. The
procedure of defects identification is thus basically configured as the
analysis of the signal obtained from the sample façade (see
the Appendix for some notes on signal analysis). We may have RMS
analysis, to obtain a point-to-point average value of the sample
surface velocity, or FFT analysis, to get spectral information on each
scanned point. There also exists a third approach, the so called
LOCK-IN (or Fast Scan, depending on the commercial name used by
different manufacturers); in this case we employ a single frequency
excitation and we rapidly acquire maps of the resulting vibrations in
terms of amplitude and phase. The LOCK-IN usually follows and RMS and
FFT study and is used to trace much more detailed maps. Structural
excitation is done in many ways, including sound waves, shakers, impact
hammers, ambient sources (traffic, wind, micro seismic activity). In
Fig. 3.3.3 an example of a measurement set-up is shown, used to test
ventilated walls, a particular type of facing where stone plates are
not directly glued to the building surface, but hang by steel cramps
that keep them at a distance of about 2-3 cm from the surface,
and also frescos, mosaics and other types of artworks. The test set up is thus composed by an SLDV
system, an excitation system using loudspeakers, plus additional
instrumentation such as a sound meter to monitor emitted sound level.
In the same figure we show an example of the results obtained in situ,
where, thanks to the coloured scale on the right, we may appreciate
that the plate in the lower left angle is clearly detached from its
support.
In the second example reported in Fig. 3.3.4,
the results obtained by measuring a turbine blade are demonstrated. In
(a) you may see the frequency response of the blade, with resonance
peaks clearly standing out at 1006 Hz, 1663 Hz, 3313 Hz and 4200 Hz, in
(b) and (c) isolines and colour map representation of the blade
vibrating at 1663 Hz.
In the preceding chapter, we mentioned
accelerometers, quite a simple device generally based on a
piezoelectric crystal that will measure the vibrations of the body to
which it is attached. The drawbacks of such approach are quite evident:
even if modern accelerometers may be smaller than 0.5 cm3
and weight less than 0.5 g, they will possibly alter light object
movements. Moreover, sometimes it is not possible to fix or glue even
these micro instruments to an object for a number of reasons: high
temperature, precious items, fragile items, and hostile environments. To solve these problems we repeatedly mentioned
the use of the laser vibrometer: for example, when you aim a SLDV at a
fresco and make the painting (very slightly) vibrate, the resulting map
will immediately spot areas where the fresco is crumbling to the floor.
The reason is that areas where the painting has detached from the heavy
substrate wall will vibrate more than well-bonded ones; the laser beam
will measure on these areas higher velocity values that will be clearly
individuated on the output maps. The same principle can be applied to
icons and other types of artworks, whenever and wherever you may think
of using vibrations as a diagnostic tool. The starting point of SLDVs measurements are
the object vibrations and to make artworks vibrate we usually use
acoustic waves emitted by standard and horn loudspeakers. The induced
vibration levels are very low, comparable to those normally found in
churches and museums and much lower than those induced by a restorer's
knock. Joining SLDVs and acoustic excitation we have thus assembled a
completely non-invasive measurement system, capable of remote
acquisitions and fully transportable for field use. The problems we
mentioned with accelerometers are solved because we have no need to
touch the artwork and the influence of the laser beam on the artwork
surface is completely negligible. For massive structures acoustic excitation may
not be useful, so other devices capable of developing a greater
excitation level must be used; our experience is concentrated on road
compactors, because they are easily found in every construction yard,
develop a repeatable pulsed impacting force, are cheap and easy to
operate. On the opposite side, small objects (icons)
cannot be adequately excited by loudspeakers, because the small defects
they may show are not put into vibration by the relatively long
wavelength acoustic waves. For this scope, piezoelectric actuators have
been developed. Mechanical impedance of ceramic piezo composites (PZT)
is much higher than that of a sound wave and applying small cylindrical
disks (10 x 1 mm) to specimens greatly reinforces mechanical coupling;
for delicate items like, e.g. icons, harmless bee wax is commonly used.
A custom designed amplifier is employed with a flat undistorted linear
response up to 20 kHz and a maximum output of 1 kV. In the following we will present applications
to frescos, icons and buildings diagnostics to better illustrate the
concepts introduced in this section.
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 present the development of 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 contact free
investigation of detachments and delaminations has been implemented. At present, structural diagnostics of these
works of art fully relies on the expertise of the restorer and the
typical diagnostic process is accomplished mainly through manual and
visual inspection of the object surface. The restorer knocks on the
surface and then senses the surface vibrations with his fingertips
while listening to the induced sound. The response to these stimuli
allows him to identify and characterise the defects. The most important
limitations of this technique are the non-objective nature, the poor
repeatability and the high cost. First attempts to translate this technique into
a more systematic approach can be found in Esposito4 and Mannaioli [5].
The basic idea was that defects could be identified as high mobility
areas resonating at some specific frequencies; restorer's knuckles were
substituted with impact hammers and his fingertips with accelerometers.
Characterisation of defects could be done by finite element analysis of
suitable models. Moreover Esposito employed acoustical excitation of
structures, a first step towards remote measurement techniques.
Castellini, Paone and Tomasini [6-8] introduced the laser Doppler
vibrometer as the remote sensing device substituting the
accelerometers. A fully functional set-up employing acoustical
excitation was developed thus completing a remote measurement system. Based on our previous experience we have
defined a general measurement procedure, consisting of two different
stages, leading to defects identification and characterisation. As
already mentioned, the first scan on the work of art is done by white
noise excitation of the work of art and measuring the RMS value (see
Appendix for a definition of RMS) of surface vibration by the scanning
laser Doppler vibrometer; the result is a point by point map of the
surface RMS velocity. The process may be very quick and the detached
areas show as higher velocity ones. At this stage it is very important
to look at the signal spectrum to note the presence of high amplitude
structural vibrations (see “A” in the frequency
spectrum in Fig. 3.3.5) that could mask the useful part of the signal,
i.e. small amplitude/high frequency local surface vibrations induced by
a defect, for example an hidden void (see “a” in
Fig. 3.3.5). In general the signal from the laser head should be
carefully high-pass filtered because of the common presence of very low
frequency/high amplitude vibrations (for example those induced by a
nearby road loaded with heavy traffic). In some very difficult cases a
notch filter is needed to eliminate very high amplitude narrow band
interfering signals, an example being acoustic standing waves in the
measurement environment. After defects localisation it is possible to
investigate the associated spectrum by pointing the laser, for example,
at the centre of the detached regions. Employing a so-called Fast
Fourier Transform-(FFT)-analyser, resonance frequencies are identified
by examining acquired frequency spectra and subsequent scans are
executed looking at surface vibrations at these same single frequencies
(for example “a” again in Fig. 3.3.5). Signal to
noise ratio is greatly improved if compared with the one of a RMS scan
although measurement time grows in a similar way. Resonance frequencies
are those frequencies where the system response is at its maximum and
are also needed if one wants to study a model of the work of art by
Finite Element analysis (see for example Reference 22 in the
Literature). Not all vibrometers have separated RMS and FFT modules, so
very often only the second scan is performed, and the RMS maps are then
derived by post-processing the spectral data.
In the following pages we will present some
examples of in-situ investigations. First test case regards the fresco
of the apse of the Orvieto Cathedral, examined in November 1998. In
Fig. 3.3.6 you may look at the measurement set up installed on existing
scaffoldings, comprising two SLDVs and two sets of loudspeakers for
excitation.
The company on charge of the ongoing
restoration work, TECHNIRECO srl, supplied to the
A second example is reported in Fig. 3.3.8
where we made a mosaic picture of the combined results of the two
SLDVs; the first one uses a green/red velocity scale the second one a
so-called “rainbow” colour scale, but the final
result is the same, i.e. red or reddish colours represents areas in
danger.
The
In Fig. 3.3.10 an RMS scan conducted on a XVII
century icon is presented. A piezo has been used as the excitation, and
a clear representation of cracks and delaminations is given in the same
figure.
SLDVs may also be used for monitoring purposes.
In the example below, we have documented the progress of a restoration
work done by a Greek expert on an icon of the same age of that in Fig.
3.3.11. Note how the area surrounding the head is completely detached
from the support and how, after some consolidation work has been done,
the same area presents a very low vibration level, thus demonstrating
that the damage had been recovered.
3.3.4.2 Dynamic Characterisation of Large Scale Structures The material of this chapter has been supplied
by Artemis srl, a spin-off company of the A SLDV has been used to initiate the dynamical
characterization of a 17 m high tower in the small town of
As we can see in the photos of the actual setup
(Fig. 3.3.13), the SLDV head has been positioned on a tripod mounted on
an industrial elevator, so that it has been possible to measure at
about 15 m from the ground keeping the laser head at the same distance
from the wall; this makes measurement far more comparable that changing
the tilt angle of the head, and also helps in maintaining a good back
diffusion from the surface to it. The single point system has never
been moved from its position and the same has been for the compactor. The walls of tower have been restored some
years ago, substituting the first layer of old bricks with new ones. To
verify the quality of the intervention a series of measurements using
forced vibrations have been done on bricks of the two different ages,
closely spaced to avoid doubts data interpretation arising from
measuring bricks in distant locations. The time responses reported
below (Fig. 3.3.14) show the different velocity vibrations of two
bricks belonging to the new and old masonries respectively. We see that
the new masonry is quite unstable with respect to the surrounding
construction, this meaning that the applied layer of bricks has not
adhered to the substrate.
Also frequency responses have been acquired to
compare measurements obtained using both forced and natural excitation
of the structure. Fundamental resonances below 10 Hz are quite
comparable, while the use of the compactor allows us to put in evidence
frequency peaks also at higher frequencies (Fig. 3.3.15).
Scans of an area including sections of both new
and old masonry have been conducted and we can observe in Fig. 3.3.16,
how also this measurement confirms the low degree of adhesion of the
former to the tower structure.
In this short document we presented in a very
simple way the basics of the operational principles of laser Doppler
vibrometers, as applied to the diagnostics and characterization of
artworks and in general Cultural Heritage artefacts. Laser Doppler vibrometers are capable of
examining a wide range of structures, ranging from small ones, like
icons, to huge ones like buildings, towers, churches. This is made
possible by their broad frequency and dynamic responses, unattainable
by any contact sensor. SLDVs may acquire data from both large and short
distances and this make a difference with other non intrusive sensors,
and also store a great amount of data, consisting of both temporal and
spectral records. Power and experienced users will appreciate the
flexibility given by this instrument, which may become the front end of
complex data analysis procedures [9], while normal users will be able
to supply in a very short time comprehensive reports on the
conservation state of examined artworks, including automatically
produced images and even AVI animations for effective presentations. There still remains room for further
improvements, because we illustrated some problems linked to big
structures that must be solved, and also fresco examination could be
perfectioned by lowering the noise floor of the instrument: it is not
possible to paint them by, for example, white tempera, to increase the
working distance, as it is common to do in many fields like automotive
and aeronautics. However, the SLDV represents now a well
consolidated investigation technique in the field of Cultural Heritage
and EU research projects should be able to deliver in a short time
instruments that are simpler to use and finely tailored to this very
special application [10]. Also overseas the SLDV has demonstrated its
attitude to frescos diagnostics, having being selected for an
extensive, successful test on the decorated walls of the
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measurement of damage by a laser scanning vibrometer. SPIE
International Symposium on Non-destructive Evaluation Techniques for
Aging Infrastructure & Manufacturing, San Antonio ( P. Castellini, G.M. Revel, E.P. Tomasini: Laser
Doppler Vibrometry: a Review of Advances and Applications. The Shock
and Vibration Digest 30-6 (1998) pp. 443-456 P. Castellini, E. Esposito, N. Paone, E.P.
Tomasini: Non-invasive measurements of damage of frescoes paintings and
icon by Laser Scanning Vibrometer: experimental results on artificial
samples and real works of art. 33rd International Conference on
Vibration Measurement by Laser Techniques, Ancona (I), 16-19 June 1998,
pp. 439-448 P. Castellini, E. Esposito, N. Paone, E.P.
Tomasini: Non-invasive measurements of damage of frescoes paintings and
icons by Laser Scanning Vibrometer: a comparison of different exciters
used with artificial samples. Fifth International Conference on Optics
Within Life Sciences OWLS V – Biomedicine and Culture in the
Era of Modern Optics and Lasers, Heraklion, Crete, Greece, 13-16
October 1998. Springer, pp.174-178 P. Castellini, E. Esposito, F. Miandro, N.
Paone, C. Santolini, E.P. Tomasini: Non-invasive measurements of
structural damage by Laser Scanning Vibrometer: an experimental
comparison among different exciters. 17th International Modal Analysis
Conference - IMAC XVII P. Castellini, E. Esposito, N. Paone, E.P.
Tomasini: Non-invasive measurements of structural damage by Laser
Scanning Vibrometer: an experimental comparison among different
exciters. SPIE International Symposium on Non-destructive Evaluation
Techniques for Aging Infrastructure & Manufacturing, Newport
Beach (USA), 3-5 March 1999, pp. 304-315 P. Castellini, E. Esposito, N. Paone, E.P.
Tomasini: Non-invasive measurements of damage of frescoes paintings and
icon by Laser scanning Vibrometer: experimental results on artificial
samples using different types of structural exciters. VI International
Conference on Non-destructive Testing and Microanalysis for the
Diagnostics and Conservation of the Cultural and Environmental
Heritage, Roma, 17-20 May 1999, pp 185- 198 (see also http://www.ndt.net/article/v04n12/tomasini/tomasini.htm E.P. Tomasini, F. Piazza, E. Esposito, M.
Possanzini: Non-destructive diagnostics of layered structures: advanced
signal analysis algorithms applied to vibrometric data. Proceedings of
18th International Modal Analysis Conference - IMAC XVIII, San Antonio
(USA), 7-10 February 2000, pp. 1604-1610 P. Castellini, E. Esposito, N. Paone, E.P.
Tomasini: Non-invasive measurements of damage of frescoes paintings and
icons by Laser Scanning Vibrometer: experimental results on artificial
samples and real works of art. Measurement 28-1, 07/2000, pp. 33-45 P. Castellini, E. Esposito, V. Legoux, M.
Stefanaggi, E.P. Tomasini: On field validation of non-invasive Laser
Scanning Vibrometer measurement of damaged frescoes: experiments on
large walls artificially aged. Journal of Cultural Heritage, 09/00,
Elsevier, P. Castellini, E. Esposito, B. Marchetti, N.
Paone, E.P. Tomasini: New applications of Scanning Laser Doppler
Vibrometry (SLDV) to non-destructive diagnostics of artworks: mosaics,
ceramics, inlaid wood and easel painting. Proceedings of Lasers in the
Conservation of Artworks – LACONA IV, Paris 11-14 September
2001, pp. 203-206 Y. Fujino, K. Kaito, M. Abe: Detection of
Structural Damage by Ambient Vibration Measurement Using Laser Doppler
Vibrometer. Proceedings of SPIE'S 6th Annual International Symposium on
NDE for Health Monitoring and Diagnostics, Newport Beach, California,
2001, Proceedings of SPIE Volume 4337 J.S. Popovics, H. Wiggenhauser: Non-contact
Laser Vibrometer Wave Sensing on Concrete. 15th ASCE Engineering
Mechanics Conference, International Workshop on “Advanced
Sensors, Structural Health Monitoring and Smart Structures”, M. Abe, D.M. Siringoringo: Laser Doppler
Vibrometers for Operational Modal Analysis and Structural Health
Monitoring”, http://www.mita.sd.keio.ac.jp/news/workshop/proceedings/Abe.pdf M. De Grassi, S. Copparoni, E.P. Tomasini, E.
Esposito: Quality control and programmed maintenance of ventilated wall
facings: pathologies surveying with laser vibrometry. International
Workshop on Management of Durability in the Building Process,
Proceedings CD, Politecnico di Milano, Milano (I), 25–26 June
2003 S. Okamoto, R. Nanba: Wind Induced Vibration
Analysis of Roof Tiles. 22nd International Modal Analysis Conference
– IMAC XXII, 26-29 January 2004, P. Castellini, G.M. Revel, L. Scalise:
Measurement of Vibrational Modal Parameters using Laser Pulse
Excitation Techniques. Measurement, Elsevier Science Ltd., J.F. Vignola, J. Bucaro, B. Lemon, G.W. Adams,
A.J. Kurdila , B. Marchetti, E. Esposito, E.P. Tomasini, H.J. Simpson,
B.H. Houston: Locating Faults in Wall Paintings at the E. Esposito, P. Castellini, N. Paone, E.P.
Tomasini: Laser signal dependence on artworks surface characteristics:
a study of frescoes and icons samples. LACONA V, Osnabrueck (D), 15-18
September 2003. Book of Abstracts pp.137-139. Published in: K. Dickman, C. Fotakis, J.F.
Asmus (Eds.) Lasers in the Conservation of Artworks – LACONA
V, Springer, Berlin-Heidelberg 2005, ISSN 0930-8989, ISBN
3-540-22996-5, pp. 327-332, H.H. Nassifa, M. Gindyb, J. Davisa: Comparison
of laser Doppler vibrometer with contact sensors for monitoring bridge
deflection and vibration. NDT&E International 38 (2005)
213–218 W. Wei, W. Kragt, A. Visser: Non-Contact
Measurement of Vibrations in Paintings Using Laser Doppler Vibrometry.
art’05 - 8th International Conference on "Non Destructive
Investigations and Micronalysis for the Diagnostics and Conservation of
the Cultural and Environmental Heritage", A. Agnani, E. Esposito, M. Feligiotti: Damage
characterisation in artworks by Finite Element Method simulations: an
application to delaminations in frescos. art’05 - 8th
International Conference on "Non Destructive Investigations and
Micronalysis for the Diagnostics and Conservation of the Cultural and
Environmental Heritage", A. Agnani, A. del Conte, E. Esposito, B.
Naticchia: Applicazione integrata di sistemi di misura non distruttivi
per la caratterizzazione di murature monumentali. IDN 50, CD
proceedings of Conferenza Nazionale sulle Prove non Distruttive
Monitoraggio Diagnostica - 11°Congresso Nazionale
dell’AIPnD, Milano 13-15 October 2005, ISBN 88-89758-02-3 T. Miyashita, H. Ishii, Y. Fujino, A. Shoji, M.
Seki: Clarification of the Effect of High-Speed Train Induced
Vibrations on a
3.3.6.3 Providers and Useful Websites Polytec GmbH Polytec-Platz 1-7 D-76337 Waldbronn T: + 49 (72 43) 6 04 0 F: + 49 (72 43) 6 99 44 W: http://www.polytec.com/eur/158_321.asp?highlightSubMenu=Vibrometers ONO-SOKKI vibrometers 1-16-1 Hakusan Midori-ku Yokohama 226-8507 Japan T: +81-45-935-3976 F: +81-45-930-1906 (Overseas Div.) W: http://www.onosokki.co.jp/English/hp_e/products/keisoku/s_v/lv1700.html MetroLaser T: (949) 553-0688 F: (949) 553-0495 W: http://www.metrolaserinc.com/ Brimrose 19 Loveton Circle Baltimore Maryland 21152-9201 USA T: 410-472-7070 F: 410-472-7960 W: http://www.brimrose.com/laser_instrument.html Brüel & Kjær DK-2850 Nærum Denmark T: +45 4580 0500 F: +45 4580 1405 W: http://www.bkhome.com/bk_home.asp W: http://www.bkhome.com/bk_template1.asp?spid=207&ctid=6
Enrico
Esposito Department
of Mechanics via Brecce Bianche I - 60131 Ancona Italy
Features
of Signals in the Time Domain In
time domain we may individuate some important characteristics of the
signal that help technicians to quickly evaluate vibrations effects: —
Average value, —
Peak value, —
RMS value, —
Crest factor. They
can be defined as follows:
While
peak value has an immediate meaning for most people, RMS must be
explained a little bit. Think of a varying signal that causes some
power to be dissipated on a load; the signal can be of any kind,
because the meaning of RMS is completely general. For a given load we
may think of a DC signal that will develop the same power on it: the
RMS value of a signal is by definition that DC value; so we may think
of the RMS as an average value in terms of power, rather than the
ordinary average value that is calculated in terms of amplitude. If
you are interested in signal analysis, there exist many publications on
these topics, but we will direct you to the following one, that is
freely downloadable from the site of Agilent: “The
Fundamentals of Signal Analysis”, Application Note 243, 1999,
http://www.modalshop.com/techlibrary/Fundamentals%20of%20DSP.pdf. A
good reference on digital techniques and other relevant topics such as
“leakage” and “windowing” is
from the site of Analog Devices: “Mixed-Signal and DSP Design
Techniques”, 2003, http://www.analog.com/library/analogDialogue/archives/39-06/mixed_signal.html. Also
consult the site of National Instrument, http://zone.ni.com/devzone/conceptd.nsf/webmain/C045A890751303A6862568650061EA98. For
some “flashy” animation visit the
Educator’s Corner at Agilent’s site: http://www.educatorscorner.com/index.cgi Other
useful reference addressing specifically mechanical vibrations: “The Fundamentals
of Modal Testing”, Agilent Tech. App. Note 243 – 3,
2000, http://www.modalshop.com/techlibrary/Fundamentals%20of%20Modal%20Testing.pdf. |
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