IOMAC'13
5th International Operational Modal Analysis Conference
2013 May 13-15 Guimarães - Portugal
COMPARATIVE STUDY OF MODAL TESTING
TECHNOLOGIES: APPLICATION TO OPERATIONAL
AND FUNCTIONAL COMPONENTS IN BUILDINGS
Manuel Archila1, Carlos Ventura2, Yan Yang3
ABSTRACT
Modal tests of operational and functional components (OFC) in buildings are typically performed
using conventional devices such as accelerometers, which are mounted on physical systems to
measure their vibrations. This approach requires direct access to the system and placing the sensors at
discrete locations to perform the test. Other technologies that can be used for modal testing include
laser vibrometers and high-speed cameras. These are non-contact systems that operate without
mounting any devices on the physical system. The laser vibrometer is convenient to measure
vibrations on systems at a wide range of standoff distances and allows performing measurements at
discrete locations along the system. The high-speed camera captures frames of high resolution at a
high sampling rate; and is also convenient for measuring vibrations at standoff distances. This paper
present a comparison of modal identification results of a 3-mass model using MEMS accelerometers,
laser vibrometers and a high-speed camera. An additional test was performed on a fire sprinkler pipe
using the MEMS accelerometers and the laser vibrometers. The modal models from the different
modal tests are well correlated and confirm these different testing technologies are accurate and
suitable for modal testing of OFC in buildings. A wide range of systems can be tested using these
versatile technologies; these applications are reviewed and discussed in detail herein.
Keywords: Modal Testing Technologies, Triaxial Wireless Accelerometer, Laser Doppler
Vibrometer, High Speed Camera
1.
INTRODUCTION
Modal testing technology has been evolving during the last 50 years to this day. The advent of digital
technology in the past 30 years has enabled engineering community and researchers to acquire,
transmit and store motion measurements in more convenient ways. Some of the modern devices
available for modal testing include accelerometers, high-speed cameras and laser Doppler vibrometers.
These are versatile vibration measuring devices which have different applications and are studied
herein.
1
Graduate Research Assistant at University of British Columbia, marchila@civil.ubc.ca
Professor at University of British Columbia, ventura@civil.ubc.ca
3
Graduate Research Assistant at University of British Columbia, yanyang@civil.ubc.ca
2
M. Archila, C. Ventura, Y. Yang
This study is motivated by the need to use reliable testing technology to determine the vibration
characteristics of equipment used for the normal operation and function of buildings, commonly
named “operational and functional components” (OFC) in buildings. This information is useful in
earthquake engineering applications for evaluating the expected performance of OFC during severe
shaking. The information is also useful to determine what modifications need to be implemented for
each of these components to ensure their reliable performance during and after a severe earthquake.
For critical facilities, such as hospitals and emergency response centres OFC should be fully
functional and operable during or immediately after the earthquake in order to permit the full
operation of such facilities.
1.1. High-Speed Camera
The high-speed camera allows capturing pictures of moving objects at a high rate of frames per
second. The high-speed camera has been used to trace motions of machinery, projectiles, animal and
human in military, research, industry and broadcasting applications. The use of software for image
acquisition and processing allows engineers to perform high speed video analyses.
The advantages of high-speed camera vibration measurements are:
 Vibrations of physical systems that are rotating or have hot surfaces can be readily measured.
 It is convenient for measurements of objects that cannot be easily accessed.
 Tracing motion at multiple locations of the physical system can be done with little additional
expense.
 Modal properties of physical system that are light can be determined without additional mass
loading.
The disadvantages of high-speed camera vibration measurements are
 The high-speed camera may not be suitable for measuring vibrations of systems that undergo
motion in the perpendicular direction to the camera; additional cameras might be needed for
this application.
 A clear view of the physical system is needed to measure vibrations.
1.2. Laser Doppler Vibrometer
The laser Doppler vibrometer is a device that measures vibrations on a surface. It operates by aiming a
laser beam at the surface of the vibrating system. The amplitude and frequency of vibration are
determined by comparing the Doppler shift of the reflected beam with a reference beam. The
applications of laser Doppler vibrometer are diverse and include acoustic, automotive, structural
engineering for civil infrastructure and aerospace infrastructure.
The advantages of laser vibrometer measurements over conventional accelerometers are:
 Vibrations of physical systems that have hot surfaces can be readily measured.
 Vibrations of small components can be measured.
 It is convenient for measurements of objects that cannot be easily accessed.
 Modal properties of physical system that are light can be determined without additional mass
loading.
The disadvantages of laser vibrometer measurements are:
 A major disadvantage of the laser is that the beam needs to be reflected from the vibrating
surface of the physical system. In case the laser beam is not reflected properly from the
surface there will be need to reach out to the physical system and temporarily place a
reflective target.
 Also there must be a clear path for the laser beam to be directed and reflected. Any significant
drop in the power of the signal will render the measurement ineffective.
 This device is convenient to measure vibrations in the direction parallel to the laser beam but
is not be adequate for motions in the perpendicular direction.
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5th International Operational Modal Analysis Conference, Guimarães 13-15 May 2013
1.3. Wireless Triaxial MEMS Accelerometers
Electromechanical devices that measure acceleration are called accelerometers. The development of
micro-electromechanical systems (MEMS) accelerometers dates back to 1979. In the 1990’s MEMS
accelerometers were used to improve the safety airbag systems of vehicles in the automotive industry
[1]. Other engineering applications have been to measure vibrations of machines, and monitor
buildings, bridges, seismic activity and tilting.
The advantages of wireless accelerometers to measure vibrations are:
 Being compact they can be placed virtually in any location on the physical system.
 Because of their size and design they can be very light, thus the additional mass they impose
on the physical system in most cases can be negligible.
 Suitable to measure vibrations at discrete locations of large systems.
 They are convenient to measure vibrations in 3 orthogonal components.
The disadvantages of accelerometers measurements are:
 Access to the physical system is needed to mount the accelerometers and they must be firmly
attached to the system to provide accurate measurements.
2.
MODAL TESTING STUDIES
2.1. Modal Test on 3-Mass Model
A modal test was performed on a 3-mass model using the different devices described above. The
picture of the 3-mass model is shown in Figure 1. The comparison of modal parameters obtained from
each test is presented in Table 1.
2.1.1. High Speed Camera Test
Two modal tests were performed on the 3-mass model under shake table excitations. Two harmonic
excitations at 1 Hz and 10 Hz were applied. High speed camera captured the images throughout the
tests at a sampling rate of 1000 frames per second for 8 seconds. ProAnalyst® program [2] was used
to carry out the video processing and motion analyses.
The motion was traced at four different locations on the model as identified in Figure 1. The time
series of acceleration and Fast Fourier Transform (FFT) under the 1Hz harmonic excitation are shown
in Figures 2 and 3, respectively. Motion of the four nodes shown in Figure 2 resembles resonant
motion at 1Hz. The FFT in Figure 3 shows a peak at 0.98Hz. This evidence indicates that the
fundamental frequency of the 3-mass model firmly attached to the shake table is 0.98 Hz. Similar
results were obtained for the 10Hz excitation test, with the FFT having two peaks at 0.98Hz and 10Hz.
Figure 1 Photo of 3-mass model and screenshot of motion tracking feature in ProAnalyst
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M. Archila, C. Ventura, Y. Yang
Figure 2 Filtered acceleration time histories of four nodes: top, middle and bottom mass and base, and envelope
of acceleration at top mass
Figure 3 Fourier transform of acceleration at four nodes
2.1.2. Laser Doppler Vibrometer Test
The laser vibrometer was used to measure free vibration of the 3-mass model. These tests were
conducted by placing the 3-mass model on the floor and imposing an initial displacement at the free
end. Two laser heads were used to measure the vibrations; a reference laser beam was directed at the
free end of the element and a roving laser beam at lower intermediate locations. A sampling rate of
1200 samples per second was used during the test. Two separate tests under same conditions were
conducted. The FFT of the motion at the free end is shown in Figure 4, the peak occurs at a frequency
of 0.91Hz for both tests.
0.05
0.7
0.04
Test 1
0.6
0.03
0.02
0.5
Velocity (m/s)
Fourier Amplitude
Test 2
0.4
0.3
0.2
0.01
0
-0.01 0
1
2
3
4
-0.02
-0.03
0.1
-0.04
-0.05
0
0.1
1
Frequency (Hz)
10
Time (s)
Figure 4 FFT and time series of velocity at free end
OMA was used to determine modal properties from laser and accelerometer measurements [3]. The
modal parameters of interest were natural frequencies, mode shapes and damping ratios. The
Enhanced Frequency Domain Decomposition method available in the program ARTeMIS v. 4.1 [4]
was implemented for processing the data. The fundamental frequency obtained is 0.91Hz, the damping
ratio is 1.4%. The mode shape is shown in Figure 5.
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5th International Operational Modal Analysis Conference, Guimarães 13-15 May 2013
Figure 5 Mode shape obtained from laser vibrometer measurements
2.1.3. Wireless Triaxial Accelerometer Test
The wireless MEMS accelerometers were used to measure free vibration of the 3-mass model. Two
tests were conducted by placing the 3-mass model on the floor and imposing an initial displacement at
the free end. Four accelerometers were used to measure the vibrations along the system. Sampling
rates of 256 and 64 samples per second were used for test 1 and 2, respectively. The FFT of the motion
at the free end is shown in Figure 6, the peak occurs at a frequency of 0.91Hz for both tests. The
fundamental frequency as estimated using the Enhanced Frequency Domain Decomposition method
was 0.91Hz and the damping ratio 1.1%. The mode shape is shown in Figure 7, and is similar to the
mode obtained in Figure 5 with the laser vibrometer.
0.25
Test 1
Fourier Amplitude
0.2
Test 2
0.15
0.1
0.05
0
0.1
1
Frequency (Hz)
10
Figure 6 FFT of acceleration at free end
Figure 7 Mode Shape obtained from MEMS accelerometer measurements
The results of the case study on the 3-mass model are shown in Table 1. The results of the free
vibration test obtained with the laser vibrometer and MEMS accelerometer correlate well. The
diference in the fundamental frequency with respect to the high-speed camera test can be due to the
different boundary conditions. In the free vibration test the model was freely standing on the floor,
vibrations under this support condition would induce slight rotations at the base and shorten the
frequency (elongating the period) of the vibrating system. Whereas for the test done with the high5
M. Archila, C. Ventura, Y. Yang
speed camera the 3-mass model was firmly attached to the mini-shake table, constraining the rocking
of the base.
Table 1 Comparison of results of modal identification
Measuring Device
Fundamental Frequency
[Hz]
Damping Ratio
[%]
High-speed camera
0.98
-
Laser vibrometer
0.91
1.4
MEMS accelerometers
0.91
1.1
2.2. Case Study on Fire Suppression System Pipe
As an illustration of the use of the equipment described above for the evaluation of the dynamic
properties of OFC, a modal test was performed on a fire suppression system steel pipe to evaluate the
capabilities of these devices in a “real world” application. The goal was to determine the modal
properties of the pipe in the horizontal transverse direction. The high-speed camera was not used for
this test, only the laser vibrometer and a set of eight wireless MEMS accelerometers.
The pipe run was 7.75m long and 51mm in diameter. The pipe was supported at both ends by a
concrete masonry wall 20cm thick and by two intermediate rod hangers at a height of 2.3m above the
floor level. The pipe was tested using a roving impact. A photo of the test setup for the pipe is shown
in Figure 8a. All the measuring equipment was synchronized to simultaneously measure the response
of the pipe to the impact. A record of pipe vibration under the impact test obtained with the laser
vibrometer is shown in Figure 8b, the motion decays in less than 8 seconds.
Wireless Sensor
(a) Laser Vibrometer, Wireless Sensor and Pipe
(b) Pipe Vibration Recorded with Laser for Impact Test
Figure 8 Testing Equipment, Pipe Tested and Recording from Impact Test
Figure 9 shows the mode shapes in the transverse horizontal direction of the pipe obtained with the
laser vibrometer and the MEMS wireless accelerometers. The mode shapes for this pipe run are well
defined and resemble the mode shapes of a uniform beam with fixed boundary conditions. The mode
shapes from the laser vibrometer test were smoother than the mode shapes obtained with MEMS
accelerometers, this because the versatility of the laser vibrometer made easier to take more
measurements. In spite of the different resolutions used for both tests, the differences were not
significant for the first and second modes but for the third and fourth modes.
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5th International Operational Modal Analysis Conference, Guimarães 13-15 May 2013
7.75m
(a) First Mode
(b) Second Mode
(c) Third Mode
(d) Fourth Mode
Red: Using MEMS accelerometers Blue: Using Laser Vibrometer Black: Underformed Shape
Figure 9 Mode Shapes in horizontal transverse direction of pipe
The fundamental frequency for vibration in the transverse direction of the pipe was obtained as 6.50Hz
and 6.54Hz, with MEMS accelerometers and the laser vibrometer, respectively. Overall the results of
both tests were well correlated for the fundamental mode and higher modes.
The Modal Assurance Criterion (MAC) was used to further evaluate the correlation of the mode
shapes from both MEMS accelerometer and laser vibrometer. The 3D MAC between mode shapes
obtained with the MEMS wireless accelerometer and the laser vibrometer is shown in Figure 10. The
coefficients were 0.99, 0.96, 0.93, and 0.76 for modes 1, 2, 3 and 4, respectively. A coefficient equal
to one indicates that mode shapes are fully correlated whereas a value of zero indicates they are
uncorrelated. The difference in the resolution of measurements used during both tests was evidenced
by a low correlation in the fourth mode which inherently had a more complex pattern of deformation.
Figure 10 3D MAC from mode shapes obtained using laser vibrometer and MEMS accelerometers
3.
CONCLUSIONS
The modal tests on the 3-mass model and the steel pipe proved MEMS accelerometers, laser
vibrometer and high-speed camera provide similar modal parameters. It is concluded that the results of
experimental modal tests conducted in this study using different vibration measuring devices are well
correlated.
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M. Archila, C. Ventura, Y. Yang
The case study on the fire suppression system pipe proved that non-contact devices such as the laser
vibrometers were more convenient than the wireless accelerometers, because vibration measurements
over the pipe elevated 2.3m from the floor were easily performed without the need to reach out to the
physical system to place the sensor. Due the convenience of the laser vibrometer, a larger number of
measurements were taken when compared to the conventional measurements with accelerometers.
ACKNOWLEDGEMENTS
We would like to thank Alicia Figueira and Jose Centeno students at the University of British
Columbia that helped to conduct these tests. The financial support for this study was provided by the
Canadian Seismic Research Network (CSRN).
REFERENCES
[1] Eddy D.S., Sparks D.R. (1998) Application of MEMS technology in automotive sensors and
actuators. In: Proceedings of the IEEE 86(8): 1747-1755
[2] ProAnalyst program. (2012) Xcitex. United States of America
[3] Brincker R., Zhang L, Andersen P. (2000) Modal Identification from ambient responses using
frequency domain decomposition. In: Proc. 18th International Modal Analysis Conference –
IMAC San Antonio, Texas, USA, 625-630
[4] Artemis Extractor Pro 2008. (2006) Vers. 4.1. Denmark: Structural Vibration Solutions A/S
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