Title of Invention

A LASER BASED SYSTEM FOR VIBRATION ANALYSIS

Abstract A system for vibration analysis and in particular a laser based system for vibration analysis, two axis measurements of any time dependant structural system, preferably identification of deflection of long span bridges in both the horizontal and vertical directions specially for the spans which are inundated with water. The laser based system comprises optical means to generate laser cross; means for screening of said laser cross; means adapted to video record the whole deflection pattern of the laser cross at a required frame rate; multimedia means for image processing and generating vibration analysis reports. The system thus provides for a simple and cost-effective system / method of vibration analysis which would facilitate measurement of vibration of bridge structures with reasonable degree of accuracy. By way of the system / method it is possible to carry out vibration analysis especially vibration of bridge structures, which can be carried out any time during day or night thereby providing for flexibility of measurement of vibration analysis. The laser based system / method for vibration analysis is simple and cost effective and have reasonable accuracy and avoid some inherent limitation of presently known systems / methods of vibration analysis. The system / method provide for analysis especially to measure the vibration of bridge structure with good resolution. The system is cost-effective and involve inexpensive instrumentation to measure bridge vibration.
Full Text FIELD OF THE INVENTION
The present invention relates to a system for vibration analysis and in particular a laser based system for vibration analysis, two axis measurements of any time dependant structural system, preferably identification of deflection of long span bridges in both the horizontal and vertical directions specially for the spans which are inundated with water.
A load rating is a simplified measure of the capacity of an existing bridge to carry
live loads. There are known guidelines for load testing of bridges and the like
such as Indian Road Congress (Special Publication 37 - 1991) has laid down
specifications for load testing an existing bridge on the basis of Static deflection
caused by the rated vehicle. Static load test has its inherent deficiencies.
Nondestructive vibration characterization can be used to assess:
a) Loss of Prestress; b) Stiffness of the Girder and thereby its condition.
Various methods used for vibration analysis of bridges are -
a) Mechanical methods, b) Optical methods, c) Wireless systems and d) Laser
based methods as discussed hereunder:
A. MECHANICAL METHOD
Such as system basically uses Dial Gage ; LVDT ; Strain Gage. The mechanical indicators (dial and inductive indicators or accelerometers) and leveling and photogrammetric methods are most often used for these investigations. Access to the area under the investigated structure is necessary when dial or inductive indicators are used. Measurement under dynamic loads can be obtained by using inductive indicators. The accelerometer need not be connected to the reference point, but their application is difficult due to low bridge vibration frequency (usually up to 5 Hz). The leveling method makes it possible to carry out measurements only during the static loads but does not require access to the area under the span (bridges over rivers or railway lines). The use of photogrammetric methods is possible both under static and dynamic loads
Although targe structures such as buildings, tall masts, bridges often move by quite large amounts, their rate of movement is also very slow, so they only generate low' levels of acceleration. This makes the measurement of their

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responses (to loading from wind, vehicular traffic etc.) difficult when using currently employed methods.
Moreover, structures of this type do not make good subjects for accelerometry.
The principal reasons for this are:
i. It can be difficult or possibly even dangerous to mount the accelerometer
on the structure at the point to be monitored.
ii. Although such structures can move by quite large amounts, their rates of movements are usually very low (often less than 1 Hz), so that vibrations or movements of interest generate only low levels of acceleration.
iii. Especially with rail or road bridges, there is small amplitude but comparatively large frequency vibrations that although not structurally significant, can easily generate acceleration signals large enough so as to completely swamp or hide those due to movement of interest. Especially in rail bridges, any extraneous vibrations caused for example by small by small flats on the wheels of the train would only have to be 0.2 mm in amplitude at a frequency of just 1 Hz to generate 10 times the signals difficult to interpret, requiring at least careful filtering to remove the unwanted signals and/or use the waveform or spectral analysis techniques to extract the required information.
The main problem with measuring displacement is that whereas accelerometers are inertially referenced, displacement transducers (for example LVDT'S) make relative measurements, that is, they have one terminal connected to zero displacement or reference surface and the other connected (mechanically in the case of LVDT) to the moving surface. As such transducers do not generally operate over long distances of more than about 1 m and, with large structures there is unlikely to be any nearby reference surface, they can be used for this purpose.
Mechanical methods are also disclosed in Measurement (18) No. 2 89-99 (1996). Also, such methods are known from Earthquake Resistant Construction and Design, Savids (ED) Ambient Vibration Testing of Bridge Superstructures N. Haritos, D.N. Mai

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B. OPTICAL METHOD
Known optical methods include those disclosed in Measurement 12 (1993) 159-
170
Elsevier, an optical displacement monitor. - A.J. Pugh, E.A.W. Maunder,
M.R.Belmont.
Also disclosed in Measurement 25 (1999) 227-236 Elsvier, Investigation of
dynamic characteristics of bridge structures using a computer vision method
Piotr Olaszek
However, such methods needs station close by where the camera can be positioned. Measurements are possible only when there is no wind disturbance during daytime. Such systems also have problems of accuracy and results get affected by refraction error.
Moreover, this is also a relative measurement but, by the use of suitable optical system (i.e. telescope), measurements can be taken over long distances, with the instrument mounting platform serving as a measurement reference. However, these rely on the direct visual observation of markings placed on the object to be monitored, so do not give any indication of frequency or waveform and are of limited accuracy.
C. WIRELESS SYSTEM.
(i) Wireless system of vibration monitoring include is Global Bridge Monitoring using Wireless Radars, (ii) Bridge Deflection Measurement using Precision Differential Global Positioning System (GPS), (iii) Dual Band Infrared Imaging System for Bridge Deck Inspection.
These systems are very costly as they use radars and satellites to monitor the global deflection of bridge structures.
D) LASER BASED METHOD
Such methods include:

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i. Coherent Laser RADAR system which can provide 3-D measurement of
Bridge Deflection (NASA); ii. Condition monitor of Cable Stayed Bridge using LASER Vibrometer; iii. Laser Sensor based on Interferometry Technique.
Laser interferometers come in two basic flavours; range finding interferometers and flatness measuring interferometers. Both are usually based on a design popularized by Michelson (1880) and use a reflective target surface to return the laser beam to its source, combine with the reference beam and thereby generate interference fringes. Distance is determined by counting these interference fringes. The distance between each fringe center is the wavelength () of the laser light. Interpolation techniques are available to subdivide the integer fringe count thereby improving the resolution. (Resolutions of /128 are commonplace with /512 achievable in certain cases. For a HeNe laser (=633 nm), this translates to achievable resolutions of 5 nm and 1.25 nm respectively). In theory there is no upper limit to the distance these laser can measure providing the laser coherence length is longer than the distance to be measured and one has a big enough counter to accumulate the fringe count. Lower limits are governed by the wavelength of the laser light, how well you can resolve each interference fringe and interpolate the sub-fringe spacing. In the later case, the smoothness and curvature of the reflecting surface effectively limits the signal to noise ratio at the detector and thus determines how welt the fringe spacing can be accurately subdivided.
Laser Doppler Vibrometers are also known in the art. Laser Doppler Vibrometers (LDV) use conventional interferometer configurations and HeNe gas lasers but are modified to include a high frequency "acousto-optic" modulator (Bragg cell). This Bragg celt is inserted in one leg of the interferometer to frequency modulate the laser beam. The modulation frequency can be any RF frequency but is usually chosen to be between 40 MHz and 120 MHz, depending on design considerations. The laser beam is used a frequency modulated (FM) carrier and effectively increases the capability of a conventional interferometer by adding a velocity measuring device. If the laser is focused onto a moving object, the laser beam will experience a Doppler shift that is proportional to the velocity of the

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measurement object. Using decoding techniques similar to those found in high quality FM receivers, the instantaneous target velocity can be readily determined. Both interferometers and bibrometers use interference techniques to determine either the deflection or the velocity. For dynamic measurement of bridges, the case often dealt with is low frequency and high amplitude, tnterferometric techniques are not well suited in measurement of vibration analysis especially of bridges and the like as the no. of fringes produced are very large and it becomes difficult to track the fringe count if amplitude of vibration is appreciably large.
Also disclosed in US Patent No. 5, 408, 318 a laser interferometry for bridge while US Patent No. 5,881,470 discloses TRW new nearfield measurement involving vertical tower for two axis measurements
It would thus be evident from the above that the presently known systems / methods of vibration analysis have limitations of either being complex and expensive and / or operatable only during daytime and not at night. Apart from the same of the prior art systems as discussed above also suffered from accuracy problems.
OBJECT OF THE INVENTION
It is thus the basic object of the present invention to provide for a simple and cost-effective system / method of vibration analysis which would facilitate measurement of vibration of bridge structures with reasonable degree of accuracy.
Another object of the present invention is directed to provide for system / method of vibration analysis especially vibration of bridge structures, which can be carried out any time during the day or night thereby providing for flexibility of measurement of vibration analysis.
Yet another object of the present invention is directed to provide for a laser based system / method for vibration analysis which would be simple and cost effective and have reasonable accuracy and avoid some inherent limitation of presently known systems / methods of vibration analysis.

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Yet another object is directed to provide for laser based system / method of vibration analysis especially to measure the vibration of bridge structure with good resolution.
Yet further object is directed to provide for a system / vibration for vibration analysis which would be cost-effective and involve inexpensive instrumentation to measure bridge vibration.
Yet another object of the present invention is directed to provide for a system for vibration analysis to provide real time processing of data.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a
laser based system for vibration analysis comprising -
optical means to generate laser cross;
means for screening of said laser cross;
means adapted to video record the whole deflection pattern of the laser cross at a
required frame rate;
multimedia means for image processing and generating vibration analysis reports.
In the above system of the invention, optical means to generate said laser cross basically comprises of a laser source for generating laser beam, a laser beam splitter adapted to split the laser beam into two, means to individually edge diffract the two laser beams to generate laser cross. Vertical and horizontal rods can be used for such edge diffraction. Screen means attached to the soffit of the bridge girder provide for screening of the said generated laser cross. When the bridge vibrates, the CCD (charged coupled device) camera records the changes in the laser cross. The images thus captured are processed to find the deflection curve of the bridge online.
The above discussed system is effective and involve simple gadgets for measuring / analysis of the vibration especially vibration of bridges and the like.

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Importantly, the gadges / components used are easily available. There is no loss of light as no slit is used to produce the cross. Importantly, the quality of the optical system can be easily improved by using high quality quartz tubes and splitters.
In accordance with another aspect of the present invention there is provided a
method for carrying out vibration analysis using the system of the invention
comprising:
generating a laser cross using suitable optical means;
screening the laser cross generated on a screen positioned below the bridge
deck;
means to identify deflection of the bridge by monitoring the changes in the
intercept from centre of cross to screen edge;
recording the whole deflection pattern at a required frame rate converting video
files into number of images using a multimedia converter;
image processing of such frame to find out required distance.
The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to the non-limiting exemplary embodiments and the accompanying figures wherein:
Figure 1 is a schematic illustration of the system in accordance with the present
invention for vibration analysis especially for bridges, loads and the like.
Figure 2 is a flow diagram of the process of carrying out the vibration analysis
using the system of the invention of figure 1 above.
Figure 3 is a flow diagram of the image processing in accordance with the present
invention.
Reference is first invited to accompanying figure 1 which illustrates the present system of vibration analysis in accordance with the present invention. As illustrated in said figure, the system basically comprises of a source for generating laser beam (GLB) preferably obtained from 10 mw HeNe Laser Gun, Laser Beam Splitter (LBS) which is adapted to split the laser beam into two beams which are individually edge detracted by means of a horizontal Quartz

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Rod (HQR) and Vertical Quartz Rod (VQR) to get the laser cross on the screen (SC).
Screen means (SC) is preferably secured to the soffit of the bridge girder. When the bridge vibrates changes in the Laser Cross take place. CCD camera is provided to record changes in the laser cross and the images captured are processed to find the deflection curve of the bridge online.
Reference is now invited to accompanying figure 2 which illustrates by way of a flow diagram the method of vibration analysis using the system in accordance with the present invention. As illustrated in said figure, the method involves initially generating a laser beam from the laser source and thereafter converting a laser point into a laser cross using the optical arrangement discussed above. In particular, this is accomplished by means of splitting of the laser beam followed by edge detraction to lead to intersection of two laser lines to form desired cross to be projected on the screen. For the purpose of screening of the laser cross, the screen is provided below the bridge deck. As the bridge deflects the intercept from center of cross to screen edge changes and gives deflection of the bridge.
The CCD camera is used to video record the whole deflection online at a required frame rate.
The multimedia means are used for converting video files into a number of images. Subsequently the images are processed in a computer based system to find out the required distance.
Reference is further invited to accompanying figure 3 which details the image processing sequence in accordance with the present invention. As illustrated in said figure, for the purpose of image processing initially it is required to input the center of the laser cross, the x and y coordinates. Thereafter, the dimensions are provided of the image to be processed. Subsequently the CPU time function is started and reading is taken of one image file at a time. Threshold R.G.B values by taking a suitable % component of R,G,B in the image. Thereafter, the distance of the edge of the threshold cross from the center of the cross is

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identified and the corresponding distance is plotted on distance-time plot. Thereafter, the image files taken are deleted and when all image files are over the final analysis is completed. So long as some image files are left over, these are continued to be read and plotted as discussed herein before.
The above discussed system of the invention clearly provides for a simple and cost-effective method of analysis of vibration. Importantly, the system utilizes very simple gadgets which are easily available and can be assembled. Importantly, in the system there is no loss of light as there is no slit used to produce the cross. The quality of the optical system can be easily improved by using high quality quartz tubes.
The system and method of vibration analysis of the invention is thus based on laser and image processing, which are simple and cost-effective. Importantly, it is possible by way of the invention to carry out vibration analysis / measurements at any time including at night which was not possible using the conventional systems.

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We Claim:
1. A laser based system for vibration analysis comprising:
i. optical means to generate laser cross;
ii. means for screening of said laser cross;
iii. means adapted to video record the whole deflection pattern of the laser
cross at a required frame rate; iv. multimedia means for image processing and generating vibration analysis
reports.
2. A laser based system for vibration analysis as claimed in claim 1 comprising :
i. optical means to generate laser cross comprise of a laser source for
generating laser beam, a laser beam splitter adapted to split the laser beam into two;
ii. means to individually edge diffract the two laser beams to generate laser cross.
3. A laser based system for vibration analysis as claimed in anyone of claims 1 or 2 comprising vertical and horizontal rods are used for laser edge diffraction.
4. A laser based system for vibration analysis as claimed in anyone of claims 1 to 3 wherein said means for screening of the laser cross comprise screen means.
5. A laser based system for vibration analysis as claimed in anyone of claims 1 to 4 wherein said CCD camera is adapted to record the changes in the laser cross, means to process said images to find the deflection curve online.
6. A laser based system for vibration analysis as claimed in anyone of claims 3 to 5 comprising high quality quartz tubes and splitters.
7. A laser based system for vibration analysis as claimed in anyone of claims 1 to 6 wherein for measuring the deflection of a bridge said screen is mounted

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with respect to the soffits of the bridge girder for screening of the generated laser cross.
8. A laser based system for vibration analysis as claimed in anyone of claims 1 to 7 wherein said laser beam splitter adapted to split the laser beam into two beams which are individually edge defracted by means of a horizontal quartz rod and vertical quartz rod to get the laser cross on the screen.
9. A laser based system for vibration analysis as claimed in anyone of claims 1 to 8 wherein multimedia means is provided for converting video files into a number of images and said images are processed in a computer based system for analysis of the required distance.
10. A method of carrying out vibration analysis of a structure using the laser based system as claimed in anyone of claims 1 to 9 comprising:
i. generating a laser cross using suitable optical means;
ii. screening the laser cross generated on a screen positioned below the said
structure; iii. means to identify the deflection of the bridge by monitoring the changes in
the intecept from centre of cross to screen edge; iv. recording the whole deflection pattern at a required frame rate converting
video files into number of images using a multimedia converter; v. image processing of such frame to find out required distance;
11. A method as claimed in claim 10 wherein said source for generating laser beam used is preferably HeNe laser gun.
12. A method of vibration analysis as claimed in anyone of claims 10 to 11 comprising;
i. generating a laser beam from the laser source;
ii. converting a laser points into a laser cross using the optical arrangement comprising said means of splitting to laser beam followed

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by edge deflection to lead to intersection of two laser lines to form desired cross to be screened on the screen; iii. means to monitor the changes in the intercept from centre of cross to screen edge and generate the deflection pattern of the bridge.
13. A method as claimed in anyone of claims 10 to 12 wherein said step of image
processing comprises:
i. inputting the centre of the laser cross, X-Y cordinates;
ii. means for providing dimensions of the image to be processed;
iii. starting the CPU time function and reading one image file at a time;
iv. thresholding the R,G,B values by taking suitable % component of R.G.B in
the image; v. distance of the edge of the threshold cross from the centre of the cross is
identified and the corresponding distance is plotted on distance-time plot; vi. deleting the image files already taken and finally when no further image file
is left for processing completing the analysis.
14. A system for vibration analysis using laser based means and a method of
carrying vibration analysis substantially as herein described and illustrated
with reference to the accompanying figures.
Dated the 12th day of September 2001
A system for vibration analysis and in particular a laser based system for vibration analysis, two axis measurements of any time dependant structural system, preferably identification of deflection of long span bridges in both the horizontal and vertical directions specially for the spans which are inundated with water. The laser based system comprises optical means to generate laser cross; means for screening of said laser cross; means adapted to video record the whole deflection pattern of the laser cross at a required frame rate; multimedia means for image processing and generating vibration analysis reports.
The system thus provides for a simple and cost-effective system / method of vibration analysis which would facilitate measurement of vibration of bridge structures with reasonable degree of accuracy. By way of the system / method it is possible to carry out vibration analysis especially vibration of bridge structures, which can be carried out any time during day or night thereby providing for flexibility of measurement of vibration analysis. The laser based system / method for vibration analysis is simple and cost effective and have reasonable accuracy and avoid some inherent limitation of presently known systems / methods of vibration analysis. The system / method provide for analysis especially to measure the vibration of bridge structure with good resolution. The system is cost-effective and involve inexpensive instrumentation to measure bridge vibration.

Documents:


Patent Number 209239
Indian Patent Application Number 00519/CAL/2001
PG Journal Number 34/2007
Publication Date 24-Aug-2007
Grant Date 23-Aug-2007
Date of Filing 12-Sep-2001
Name of Patentee INDIAN INSTITUATE OF TECHNOLOGY
Applicant Address KHARAGPUR,PIN-721 302,W.B.INDIA
Inventors:
# Inventor's Name Inventor's Address
1 KUMAR MR.CHANDAN DEPT.OF CIVIL ENGG.,INDIAN INSTITUTE OF TECHNOLOGY,KHARAGPUR,PIN-721 302,W.B., INDIA
2 GHOSH DR.ASHOK KUMAR, DEPT.OF CUVIL ENGG.,INDIAN INSTITUATE OF TECHNOLOGY,KHARAGPUR,PIN-721 302,W.B., INDIA
PCT International Classification Number G 01 B 9/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA