STATIC AND DYNAMIC DISPLACEMENT OF BRIDGES MEASURE USING DIGITAL CAMCORDER

Transcription

1 Monography Reprocity Resume, The Silesian University of Technology, Gliwice 2008, s Zbigniew OPILSKI 1 Marek SALAMAK 2 STATIC AND DYNAMIC DISPLACEMENT OF BRIDGES MEASURE USING DIGITAL CAMCORDER 1. Introduction Measurement of bridge spans displacements during static and dynamic tests can be performed by many different methods. Most often used are traditional techniques, using precise leveling, dial or inductive gauges. Precise levelling does not allow sufficient accuracy at longer sight lines and is very time-consuming. Closing the level circuit can take even fifteen minutes, which is as long as a time interval between following deflection readings at static tests. This method is completely inappropriate for dynamic tests. Dial and inductive gauges have sufficient accuracy and inductive gauges can be used for dynamic tests as well. Nevertheless both devices require stable reference basis. If using of under-bridge space is not possible, it is practically impossible to perform measurements. Even if we do not have access to the under-bridge area, we have to use at higher heights cables bringing the measurement to the level of the base. Such measurements will always contain mistakes and disturbances connected with measurement method (flexibility of cable, wind influence on the cable, base vibrations etc.). In result of this we will record dynamic characteristic of the system bridge-cable, not of the bridge itself. Such problems can be avoided when we use modern optical techniques. 2. Optical systems for displacement measurement Optical systems for displacement measurement used in bridges are: photogrammetry, laser techniques and visual techniques. Photogrammetry because of its characteristics is rarely used. The two other techniques are much more popular. Laser measurement apparatus is able to pro- 1 Dr inż., Instytut Fizyki Politechniki Śląskiej w Gliwicach, zbigniew.opilski@ polsl.pl 2 Dr inż., Katedra Dróg i Mostów Politechniki Śląskiej w Gliwicach, marek.salamak@polsl.pl

2 duce such coherent and strong beam of laser light that it is possible to assume rectilinearity of light propagation. Laser transmitter is attached to a stable base in proximity of tested object. Transmitted beam is directed precisely towards the receiver located on the bridge. Photosensitive matrix of the receiver is able to register the laser dot and determine the displacement of the receiver plane towards emitted beam of light. Description of laser measurements can be found in publication [1]. Using a camcorder for bridge tests was in the focus of interest of scientists even in Poland. In the 90 s in the Instytut Budowy Dróg i Mostów (Institute for Roads and Bridges Construction) dr Olaszek has built a measurement set for measuring of bridge spans displacement which was successfully used for many static and dynamic tests of various bridge objects [1]. Similar works were performed in many well-known centers abroad, e.g. on University in Sheffield [3]. Possibilities of video and computer devices are changing very rapidly. New digital image recording technologies, faster and more efficient data transfer buses and newer, more efficient algorithms of image analysis emerged. Simultaneously costs of these advanced technologies are still lower. For this reason, authors aiming at designing an easy-to-use, universal and cheap measuring set for static and dynamic tests of bridges, have elaborated such device, the basic of which should be commonly available, medium-class digital camcorder. 3. Construction of the measuring system and software The measuring system consists of a standard digital camcorder recording the image on a magnetic tape, type Digital 8. Authors have used a digital camcorder with a lens with 20x zoom. A movie recorded on a magnetic tape is then stored into computer hard-disc drive in *.avi format. This stage can be then realized on several ways, e.g.: If the camcorder is equipped in one analog video output only, then the computer should be equipped with a frame-grabber card (or a TV-card) enabling the image acquisition and its storage on a hard-disc. Currently camcorders and computers are equipped in a fast serial bus FIREWIRE (IEEE 1394). This enables easy transfer of recorded movie to the computer hard disc drive. Further necessary movie processing is performed by means of special software. The idea of program operation consists in selecting of a graphic object on one movie frame this should be a master connected with tested object. Then this object should be searched on all other frames. During this search its location (in pixels) towards one of frame s corners is determined. Such obtained measurement results can be then of course recalculated into real position of the master. Scaling coefficients are used for this, which can be determined knowing the real magnitude of some elements on the illustration. The time basis is reconstructed on the basic of information concerning the fact, how many frames are saved by the camera within one unit of time. Working in the PAL standard, the camera is saving 25 images per second, that is one image every 40 ms. Procedure of master searching on the screen enables to determine its location with much better accuracy than a pixel, which has been shown in point 4. To perform such measurement, following conditions must be fulfilled: camera must be located on a tripod, which would be immovable towards the bridge (camcorder stability has crucial influence on the measurements accuracy),

3 recommended, although not necessary, is securing of graphical master to the tested object; master (masters) should be in contrast colours and unrepeatable within the scope of analyzed image. Illustration 1 Program menu content An image analysis program has been created in LabVIEW environment. It enables: to select the sequence of frames subject to further analysis, to point the searched master, to determine scaling coefficients and use them, to present and describe measurements results on computer hard disc drive. Illustration 1 presents the program menu view. Except of this presented one, additional windows will appear, containing, among others, searched master and following movie frames with selected searching result, as presented on illustration 2. Determined coordinates Found master Illustration. 2 Fragment of the image with selected searching result

4 On presented illustration the role of searched master fulfills a natural image element in this case this is the fragment of balustrade of a footbridge. Program enables to trace measured results. Illustration No. 3 presents the trajectory Y(X), and location of the master in the function of time Y(t), X(t). Described program is the only non-standard element of the measuring system, because the camcorder used is a standard, non-professional camcorder. Illustration 3 Presentation of results. Left diagram presents results in the shot Y(X). Diagrams on the right present from top to bottom: Y(t), X(t), α(t) (here not active). 4. Laboratory tests Several tests have been performed, the aim of which was checking the possibilities of the measuring system and estimating of its accuracy. Tests have been performed in laboratory conditions, measuring vertical vibrations of a horizontal beam in system presented on illustration 4. A steel beam has been attached on the right side. To its left, free end, an inductive displacement sensor has been attached. This sensor has recorded vibrations for comparative reasons. Light shields have also been attached to the beam that are masters with printed geometric figures. A horizontal and vertical line marked on each sample was 180 mm long. The highest amplitude of vibrations has been recorded on the square, much lower amplitude was recorded on the wheel attached close to the axis of rotation. The triangle was not attached to the vibrating beam at all and was serving as a reference point. Several 20 seconds long movie sequences at various distances of camcorder from vibrating beam have been recorded. This distance has been changed within the limit from 10 to 60 meters. Vibrations registered from the distance of 10 m has been presented on illustration 5 and from the distance of 60 m on illustration No. 6.

5 Comparison of the last two drawings leads to a conclusion that the image of vibrations of square from drawing 5 are overlaid with camera vibrations visible on drawing 6. To reduce to minimum possibility of camcorder stimulation, a remote controller has been used for its switching on and off. During this experiment the camera has been located on a standard tripod, which turned out as not rigid enough. For determining of noise level of the measuring system the measurement results of the triangle location (which according to made assumption - should not vibrate) have been used. Measurements have been performed for direction X and Y. Determined standard deviations of a single measurement have been presented in Table 1. From comparisons of presented results comes that measured measurement uncertainty are larger for X direction (horizontal) than for Y direction (vertical). This fact results probably from construction of the camera tripod used. Very small measurement uncertainty determined for the distance of 10 m and given in pixels is to be commented. It results from the method of master location determining. Observed master image size was 324x350= pixels. In the described method, all elements of the master are used for its location on following movie frames. In other known methods detection of border location of a contrast element of the image is performed, for this reason, only these image elements take parts in its determining, which are located close to the mentioned border. Inductive sensor Fixing of beam Vibrating beam Illustration 4 View of the measuring stand with vibrating beam Assuming a linear increase of measurement uncertainty as the distance is increased, the measuriement uncertainty (for the X-axis direction) for the distance of 60 m should be ca. σ=0,16 pixel or σ=0,25mm. But the measured values are larger. This is due to the fact that during both measurements the same, maximal magnification of the camera lens has been used. For this reason the size of master (seen via the CCD converter of the camcorder) calculated in pixels varies substantially each time and in the second case was 56x60=3360 pixels. To keep the measurement accuracy it would be necessary to use a lens with larger magnitude or a larger master, so that the size of the picture measured in pixels would remain constant.

6 Y [mm] Time Czas [s] [s] Illustration 5 Square vibrations registered from the distance of 10 m Y [mm] Time [s] -15 Czas [s] Illustration 6 Square vibrations registered from the distance of 60 m The system is adapted for performing differential measurements consisting in determining of master coordinates as a difference between the location of a moving master (square) and a stationary master (triangle) recorded at the same time. Because camcorder vibrations are transferred in the same manner on vibration of both samples, subtracting their coordinates will allow compensating camcorder vibrations. Table 1. Setting-up of measured meaurement uncertainties. Standard deviation of a sigle measurement σ distance 10m (324x350) distance 60m (56x60) [pixel] [mm] [pixel] [mm] direction 0,026 0,041 0,28 0,6 X direction Y 0,023 0,033 0,15 0,63 Because the optical master (square) and inductive displacement sensor are fixed in various places of the beam, it is impossible to compare directly recorded displacements. Nevertheless it is possible to determine the resonance frequency as well as logarithmic damping decrement from both measurements. From optical measurements results that f o =5,550 (5,565) (5,534) Hz and logarithmic damping decrement is λ=0,042. Contact measurement by means of induction sensor indicates that these values are adequately f o =5,565 Hz and λ=0,041.

7 5. Conclusions In presented work have been presented first laboratory measurements results collected by means of a measurement system consisting of a standard amateur camcorder and appropriate software. The purpose of these tests is evaluation of quality of collected results and their suitability for measurements of static and dynamic bridge structures. On the basic of performed measurements we can say that it is advisable to use masters having the axial symmetry in case of elements making a revolution as one of the movement components. Performed tests have also shown necessity of using the reference masters allowing compensation of camcorder vibrations. Literature [1] Bień J., Rawa P., Laserowe pomiary statycznych I dynamicznych przeemieszczeń dużych konstrukcji mostowych, Inżynieria i Budownictwo 3-4/2002 [2] Olaszek P., Trials of computer-vision method application in bridge investigation, International Bridge Conference Warsaw 94, Warszawa 1994 [3] Patsias S., Staszewski W.J., Damage Detection Using Optical Measurements and Wavelets, Structural Health Monitoring, Vol. 1(1): , 2002 [4] Weseli J., Radziecki A., Salamak M., Silarski A., Badanie i ocena cech dynamicznych podwieszonej kładki dla pieszych usytuowanej w sąsiedztwie stadionu sportowego, Inżynieria i Budownictwo 3-4/2002. STATIC AND DYNAMIC DISPLACEMENT OF BRIDGES MEASURE USING DIGITAL CAMCORDER Summary The paper presents the application of the digital camcorder for measuring static and dynamic displacements of bridges. Different optical techniques are described and compared. The results of laboratory dynamic identification of simple cantilever beam are presented. These results are compared to natural frequencies and logarithmic decrement obtained using traditional contact method with induction gauges.