wheel system and fastening/removing dynamometers should be designed. ... The exciter frame structure with sliding support wheel system, allowing fastening ...
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Prototype of exciter for railway bridge testing W. Skoczynski, A. Roszkowski Wroclaw University of Technology, Institute of Production Engineering and Automation, Wroclaw, Poland
J. Bien, J. Zwolski Wroclaw University of Technology, Institute of Civil Engineering, Wroclaw, Poland
ABSTRACT: The aim of this study was to develop a prototype of exciter that enables inspection of railway bridges dynamic parameters. A previously built inertial exciter was adopted and a special frame supporting system that enabled transferring the excitations from the exciter to the rails was designed and produced. The system was equipped with special tensometric bowshaped dynamometers that allow to measure the excitation force components during bridge test. The laboratory tests proved the exciter fulfilled all functional demands, i.e. it allowed to measure permanently the excitation forces components, to attach temporarily the wheel system for the transport purposes and to raise/lower the system for attaching/detaching wheels and fastening/removing dynamometers. The field test pointed out the weak points of the supporting frame structure and enabled to improve its functionality. The efficiency of the developed device and its applicability in modal tests of railway bridge was also confirmed. 1 INTRODUCTION One of the way for determining dynamic characteristics of bridges is employment a group of methods called “measured – input tests” (Farrar et al., 1999). This kind of method needs an efficient source of excitation to ensure a controllable and measurable force amplitude at frequencies that cover the natural frequencies range of the bridges intended to test. Reasonable design should include a proper exciter mounting to the bridge, excitation force measurement, equipment and facilities for simple transportation. A challenging task of this work was to build a smart machinery fulfilling these requirements that could be used for railway bridges testing. The aim of this study was to develop a prototype of the exciter that enables to inspect railway bridges dynamic parameters. A prototype of an exciter intended to use in tests of railway bridges dynamic performances has been designed and produced at Wroclaw University of Technology (Skoczynski et al., 2006). The principle of operation of this inertial exciter is based on the appearance of the centrifugal force of rotating unbalanced masses. Considering adopted excitation method the input forcing function could be measured during the tests. It needed to design and produce a special frame with dynamometers transferring the excitations to the rails. Concerning the new construction the following assumptions were made: • It should be possible to permanently measure the excitation force components during the railway bridges inspections. • It should enable to locate the exciter in various positions in relation to the rails. • The temporary fastening of the road wheels for the transport purposes from its unloading place to the investigation plant should be provided. • Raising and lowering system of the truck with the exciter for attaching/detaching the road wheel system and fastening/removing dynamometers should be designed. The exciter was tested both in laboratory as well in field conditions. Some results of these tests were also presented in this study.
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2 EXCITER CONSTRUCTION A previously built inertial exciter (Bien et al., 2002) was adopted in this work to new structure intended to testing railway bridges dynamic performances. A special frame supporting system that enabled transferring the excitations from the exciter to the rails was designed and produced (Fig.1). The frame consisted of two placed lengthwise channel irons connected with crossbars. The spacing between channels corresponded to the exciter base width. Threaded holes made at the top part of each channel iron enabled fastening of exciter to the frame with screws in three different placements: symmetrically according to the rails, and above each of them in such a way, that the resultant excitation force laid at the symmetry center of the rail. The crossbars made of flat iron and connecting two placed lengthwise channel irons at their bottom served as a base to fasten dynamometers (Fig.2). The role, which played two transversal channel irons placed bellow the frame, was dual. They supported four screws allowing raising and lowering the frame with the exciter and allowing to temporarily fastening driving system. exciter's body
position 1
position 2
position 3
frame rail
detachable axle with driving wheels screw allowing to rise and lower the frame
Figure 1. The exciter frame structure with sliding support wheel system, allowing fastening the exciter in three different positions (Skoczynski et al., 2006).
Figure 2. The exciter supporting frame construction set on two bow shaped dynamometers together with raising/lowering screws (Skoczynski et al., 2006)
Components of the excitation forces were measured using three designed and produced tensometric bow shaped dynamometers (Fig.3a). These dynamometers could be detached from the frame with the exciter for the transport purposes. The frame’s construction enabled raising on the railway bridge using four screws, what allowed to detach the wheel system and fasten all dynamometers. After lowering the frame with the exciter and the dynamometers should rest on the rails. The whole construction was supported at three points, and bullets applied at the dynamometer – rail contact points allow for stabile support of the exciter frame (firm of contact to the rail and disabling additional torsion of the dynamometer base). To attach the dynamometers to the rails dedicated screw clamping rings were constructed (Fig.3b). They prevented the frame
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with the exciter from moving during the tests (falling off them due to vibrations). The rings were designed in such a way, that they did not cause bending the dynamometers and stretching them at places were the strain gauges were stuck on.
A
A A
A
Figure 3. The tensometric bow shaped dynamometers: a) scheme, b) view with screw clamping rings
Dynamometers were equipped with measuring tensometers stuck in pairs at a stretched and compressed part of the bail, and electrically arranged in a full bridge configuration. The dynamometers were calibrated at the laboratory, where they were loaded with static forces ranging from 0 to 10kN every 1kN. The tensometers were fed with use of the three-channel measuring bridge HBM KWS 523C. Such solution allowed every tensometric dynamometer to be operated independently in a separate measurement channel with individually chosen gains. Two detachable axles, which consisted of a tube with wheels mounted on it, established the driving system of the exciter supporting frame. Ball bearings were mounted in the developed wheel system. Such a solution allowed to transport not only for the frame with the exciter alone, but also with the whole accompanying equipment. 3 LABORATORY TESTS OF THE EXCITER A series of tests of the exciter mounted on the supporting frame were conducted at the laboratory. The whole structure with dynamometers was fixed to the rails, which were screwed with clamps to the base plate made of cast iron (Fig. 4). The tests were performed for three different exciter placements on the supporting frame: in its middle, and at both its edges. At the utmost positions of the exciter the resultant excitation force acted in the symmetry axis of the rail. The aim of these tests was to determine components of the excitation force at the support points of the frame while applying excitations of frequencies in the range 1–23 Hz, as well as to check whether at any of these frequencies there occur whole system resonance with any of the excitation force component, what could be expressed by swinging of the system.
Figure 4. View of the exciter with the supporting frame at the laboratory plant
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At first the excitation force components at each dynamometer for different exciter placements and increasing frequencies were analyzed. As actual force values their RMS values on period of 20 seconds were taken into account. Figures 5 and 6 show dependencies of the force components on the exciter placements. One can see that the components relations are logical. In the case of the exciter placed over two of dynamometers their indications (No. 1 and 2, see Fig. 5) are very similar up to the frequency of 14 Hz, when the difference between them can be observed with the largest difference value at 18 Hz. This results from the fact, that this is the lowest resonant frequency of the frame with the exciter at which vibrations resulting from the swinging of the frame on the elastic bow shaped dynamometers can be observed. The force component acting on the second rail (dynamometer No. 3, compare Fig. 5) is much smaller than the excitation force components acting directly below the inertial exciter. The frequency of this component differs from the frequency of excitations and probably is equal to one of the higher components of resonant frequencies.
Figure 5. Dependence of the excitation force components (RMS values) on the excitation frequency for the exciter mounted at the edge of the supporting frame above the dynamometers no. 1 and 2
When the exciter was mounted at the edge of the supporting frame above the single dynamometer (Fig. 6), the force acting on that dynamometer (No. 3) dominates the others. The characteristics of excitation force components dependence on the excitation frequency show no influence of the resonant vibration of the supporting frame with the exciter on the force components values. So the placement of the exciter over the single dynamometer seems to be the best. Moreover it displays additional advantage i.e. during railway bridges dynamical characteristics measurements it is enough to record only the single force component acting directly below the exciter, since the other components values are more then twenty times smaller. Because for the frequency of 18 Hz the resonant vibration of the supporting frame with the exciter in one of its position were observed, the free vibration of this construction were also checked. From the dynamic point of view the tested object could be considered as an elastic beam with large reduced mass (the exciter) racked on three elastic supports (the bow shaped dynamometers). The most important information about this system was, if this system did not suffer from free vibration in the range of the applied excitation frequencies. So the supporting frame was subjected to impact excitations in three directions, while the free vibration of the exciter placed in three positions on the frame were measured with use of triaxial piezoelectric accelerometer. It was placed on the exciter flange in its symmetry plane. Additional two transducers were placed on the transversal channel irons placed bellow the frame and the exciter. They were very useful to detect swinging of the whole structure. Signals were recorded and analyzed by the multi-channel analyzer with built-in Fourier transform function. The lowest natural frequencies of tested exciter structure were estimated and presented in the Table 1.
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Figure 6. Dependence of the excitation force components (RMS values) on the excitation frequency for the exciter mounted at the edge of the supporting frame above the dynamometers No. 3 Table 1. The lowest natural frequencies of the supporting system with the inertial exciter mounted on the frame for impact excitations realized in different directions Direction of impact excitation Investigated system case Horizontally, along Horizontally, laterally Vertically the frame Position 1: (Exciter mounted above the two dynamometers) 27,4 Hz 18,8 Hz 63,3 Hz Position 2: (Exciter mounted in the middle of the frame)
29,3 Hz
12,4 Hz
42,3 Hz
Position 3: (Exciter mounted above the single dynamometer)
27,5 Hz
6,6 Hz
36,1 Hz
First of all the lowest resonant frequencies of the supporting frame for all excitation directions and exciter positions were analyzed (Tab. 1). Their highest values were reported for vertical excitations and they were equal to 42-63 Hz, depending on the exciter placement. For the impact excitations put lengthwise the frame the lowest resonant frequencies stayed almost steady irrespectively of the exciter placement and they were approximately equal to 27 Hz. It was to observe that for the free vibration of the whole system, appearing vertically and horizontally along the frame, the lowest natural frequencies lied out of the range of excitation frequencies, thus the resonance vibration during exciter operation could not occur. For the horizontal system excitations across the frame the lowest frequencies were observed. All of them lying within the operating range of the exciter for all its placements. These frequencies varied from 18.75 Hz to 6.63 Hz, where the highest frequency occurred for the placement of the exciter above the two dynamometers, and the lowest while it was placed above the single dynamometer. Together with them the mode shape resulting from the frame swinging on the flexible dynamometers was noticed. When the exciter placement was moved from the place above the two support points to the place above the single support point, the lowest natural frequency decreased. This frequency was mainly influenced by the placement of large mass of the exciter, what resulted in considerable distance of the system gravity center upwards, above the frame support surface. Small dynamometers spacing, imposed by the frame dimensions and their relatively large flexibility, resulting from the application of a set of strain gauges for the force measurements purposes, caused that the natural frequency of the exciter supporting system related to its swinging considerably decreased in comparison with the case of the system firmly (without dynamometers) mounted on the rails (compare with Table 1). When the exciter was
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moved away from the two-points support place the natural frequency was decreased by increasing participation of the frame torsional flexibility in the supporting system total flexibility. One can ask if in the presence of one natural frequency within the range of the exciter applicable excitation frequencies the system can be utilized to test bridge structures. The answer seems to be plain when one looks again at the exciter characteristics displaying dependence of the excitation force components on the excitation frequencies (see Fig. 5-6). Visible increase of the excitation force components, accompanying to the supporting system resonance, can be noticed only for the exciter placement on the edge above the two dynamometers (compare Fig. 5). In the other case of the analyzed exciter placements (see Fig. 6) the influence of the system resonance cannot be observed in the force components acting to the rail. Of course also in these cases the resonant vibration appear, but they influence to the excitation force components could be neglected in comparison with forces generated by the exciter. When one consider the practical point of view, the exciter positioning above the single dynamometer, due to the domination of the force component measured by this sensor over the two others, seams to be the most profitable. The resonant vibration appearing at this position resulting from the system swinging does not influence this dynamometer signals due to its one-point supporting. In this case the recording of the excitation force during the bridge structures tests would be the easiest and straight, since only the one measurement channel is required. The next step of analysis lead to evaluate of a frequency distortion and a stabilization of excitation force amplitude while an actual exciter parameters were adjusted. Before the analysis to be made, all time sequences of each force component were filtered by low-pass Cauer filter. A special algorithm for checking of exciting frequency deviations was prepared. The approved sample resolution is corresponded to 0.125 Hz. No frequency deviations were observed using above resolution. It can be concluded the real exciting frequency was stable for each adjusted frequencies. The laboratory tests proved, that actual excitation frequencies are stable with accordance to desired values. The maximal percentage deviations of significant components of the excitation force were smaller than 5% for all the exciter placements. By a significant component we mean the measured components which participation in the resultant force is grater than 20%. In other words, only for the exciter placed at the frame edges, the analysis does not include measurements of the dynamometers mounted on the rail above which there was no the exciter. For these dynamometers large amount of noise component in measured signals was observed due to small real force signal values. It should not be regarded as force fluctuation. 4 FIELD TESTS OF THE EXCITER The aim of tests at the site was to check the supporting system functionality and the whole system ability to force resonant vibration of a railway bridge. The field tests were conducted in two stages. The first test consisted of checking the system functionality whereas the second was focused on the check of the exciter ability to force the resonance of the bridge. At first the evaluation was performed of the exciter wheel system, the frame’s raising/lowering system, the dynamometers attaching method and the method of fastening the exciter to the frame. During transporting the supporting frame with the exciter at the test site it turned out that the original wheels of the frame weren’t working properly. Since there was no possibility of direct approaching the inspected bridge with a car the frame and the exciter together with the inverter and the power generator were put on the frame and moved to the site. Due to this fact the frame wheels were loaded by force two times bigger than in the case of transport of the supporting frame with the exciter only. Despite of lubricating the wheels bearings with grease they seized quite quickly and really huge forces with the engagement of many people had to be applied to have the truck pulled to the goal destination. It was the reason that the original wheels had to be changed. Newly developed wheels (Fig. 7a) with larger diameter and with ball bearings replaced the old type having the sliding bearings. They were designed to carry higher payloads. The functionality of 4 bolts was also examined which enable to raise and to lower the frame during the preparation of the bridge test. The bolts worked impeccably. Application of special bolt with spherical recess placed on cross-sills or directly on sleepers enabled steadfast support-
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ing of the whole structure on the ground. The usefulness of the bolted clamping rings applied to fasten the dynamometers to the rails was also confirmed. They enabled fastening the system to the rails through the dynamometers. The connection between the supporting frame and the rails did not open during all the tests. During the site tests the exciter was moved along the frame and mounted in three different positions. The method of the exciter fastening – with use of four bolts – was unqualified. However significant force has to be applied to shift the exciter along the frame and it is quite difficult to place the flange notches over the holes. To facilitate the action a simple construction with a crank (Fig. 7b) was designed and implemented. The crank can be put on both sides of the frame. It drives a roller on which climbing cord catching the exciter is reeled. The climbing cord was applied, since an alternate steel cord has a tendency to tangle after unrolling and more over it can hurt operator’s hands. Shifting the exciter on the supporting frame with the use of climbing cord displayed the crank system helpfulness arising in reducing the direct operator’s effort needed to change the place of the exciter on the frame.
Figure 7. New developed constructions after the preliminary field tests: a) wheel to transport exciter with ball bearings b) the crank for shifting the exciter on the supporting frame
The next tests concerned checking the possibility of stable amplitude and frequency generation by the exciter while the mobile generator as a power supply was used. Both the excitation parameters were checked at each of three exciter placements, using the same method of their estimation as in the case when the exciter was driven by a stationary power supply. The site tests proved again, that actual excitation frequencies are stable with accordance to desired values. The excitation amplitude varied slightly from the nominal value. Regarding the exciter ability to force railway bridges resonant vibration some tests on the real object were performed. The bridge was a steel two-girder structure with open deck and its static scheme was simply supported span with length of 15.0 m. The exciter was mounted in the middle of the supporting frame, placed on the railway bridge in quarter of the span and fasten to the rails (Fig. 8). It generated excitation force with an adjusted frequency that was changed step by step in the range from 3 to 24 Hz with resolution of about 0,3 Hz. Four transducers were used to measure bridge response. A set of frequency characteristics was shown in Figure 9. Amplitudes of the transmittance for all channels were calculated on the basis of averaged cross- and autospectra.
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Figure 8. Exciter system fastened to the rails on the railway bridge
0.45
15.11 Hz
0.50 A1 A2
0.35
A3
0.30
9.20 Hz
2
Accelerance [m/Ns ]
0.40
0.25 0.20 0.15 0.10 0.05 0.00 0
5
10 Frequency [Hz]
15
20
Figure 9. Accelerance functions for 3 response sensors obtained in the Stepped Sine Test
For each detected resonance a special procedure was employed to detect the resonant frequency (Bien et al., 2006). In its vicinity a fine step of sine excitation was applied to determine the accelerance more precisely to be able to estimate more reliable values of the structure modal parameters. To enable application of a standard method for modal parameters estimation (e.g. Circle Fit, Line Fit, Rational Fraction Polynomial and others) equal frequency spacing of the transmittance have to be ensured. In Figure 10 an application of Circle Fit method is shown for the first mode around 9.20 Hz.
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Absolute values of receptance
-4
10
abs(H)
max=9.1951 Hz
-5
10
-5
Imaginary
0
x 10
6
8
10 12 Frequency [Hz]
Circle fit
-2 -4 -6
fr=9.1014 Hz -5
0 Real
5 -5
Structural damping
4
14
16
18
20
Damping plot 0.05
η=0.02722 0 3
2 Before resonance
1 4
2
0 After resonance
x 10 Figure 10. The resonance frequency identified by Circle Fit method
5 CONCLUSION The exciter together with a new construction of the supporting frame, adopted to its fastening to rails, fulfill all the functional assumptions. It allows to permanently measure components of the excitation forces during the railway bridge inspections. The exciter system enables to temporarily attach the wheel system for the transport purposes and raise/lower the system for attaching/detaching wheels and fastening/removing dynamometers. Additionally the exciter can be mounted in three different positions on the frame allowing torsional excitations of bridges. A set of three tensometric dynamometers were designed and produced. They allow to permanent measure of the excitation force components during the railway bridges inspections. Placement of the exciter above the single dynamometer seems to be the best solution, since in this case there is no influence of resonant vibration of the supporting frame with the exciter on the excitation force components values measured by the two other dynamometers. These values can be neglected in comparison with the single force component values acting directly below the exciter, thus during measurements of the railway bridges dynamical characteristics it is enough to record only the single force course at one point. The inertial exciter with the supporting system allows obtaining an excitation force with a stable frequency. The amplitude deviations are not larger then 5% of nominal value. The deviations get smaller when the excitation frequency and resulting excitation force increase. Switching the power supply from stationary to mobile does not influence the system performance. The excitation force for frequencies in the range of 1–23 Hz contains the base harmonic signal with frequency set by an operator as well as the harmonic frequencies and measurement noise. Usually the amplitudes of the latter components were significantly smaller than the base component and thus their influence on the resultant force could be neglected. The lowest resonance frequencies of typical railway bridges cover the range exciter adjustable frequencies. However, due to the ratio of the harmonic components amplitude to the base component amplitude and the value of the latter, the exciter is useful to excite bridge vibration of the frequency above 5 Hz. The aim of performed field tests was to check the supporting system functionality and the whole system ability to force railway bridges resonant vibration. They pointed out weak points of the exciter structure what allowed to remove them and improve its functionality. The other
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tests performed on the real railway bridge confirmed the efficiency of the developed device and its ability to excite resonances. The determined resonant frequencies and modes can be very helpful in technical condition evaluation of tested bridges. Conclusions of both laboratory and field tests will enable preparation of a special guideline devoted to forced vibration tests of railway bridge structures, their modal analysis and damage detection based on changes of modal parameters. Future tests will be focused on problems related to damage detection sensitivity. A special algorithm will be develop to detect damages of bridges. Some practical implications of developed exciter can be concluded. The exciter is a real system that can be applied to perform dynamic behavior testing of railway bridges. It allows permanently excitations of railway bridges and measurement the excitation forces components of in the range of 1–23 Hz with resolution of 0,008 Hz. 6 REFERENCES Bien J., Krzyzanowski J., Poprawski W., Skoczynski W. & Szymkowski J. (2002). Experimental study of bridge structure dynamic characteristics using periodic excitation, Proceedings of the 2002 ISMA, Vol.II, P. Sas, B. Van Hal (Ed.), Katholieke Universiteit Leuven, Departement Werktuigkunde, Leuven, p. 555-562 Bien J., Rawa P., Zwolski J., Krzyzanowski J., Skoczynski W. & Szymkowski J. (2006). System for monitoring of steel railway bridges based on forced vibration tests. In: Bridge maintenance, safety, management, life-cycle performance and cost. Ed. by Cruz P.J.S., Frangopol D.M., Neves L.C., Taylor and Francis, London, p. 853-854, Farrar C.R., Duffey T.A., Cornwell P.J. & Doebling S.W. (1999). Excitation methods for bridge structures. Proceedings of the 17th Int. Modal Analysis Conference-IMAC, v.1, Kissimmee 1999 February 8-11, Pub. SEM, Bethel, p. 1063-1068 Skoczynski W., Krzyzanowski J. & Bien J. (2006) New solution of shaker's supporting structure intended to railway bridges testing. In: DAAAM International Scientific Book 2006. Ed. B. Katalinic. Vienna, p. 569-582
The work was co-funded by the European Commission within the Sixth Framework Programme Project No. TIP3-CT-2003-001653 entitled Sustainable Bridges - Assessment for Future Traffic Demands and Longer Lives.
