用低变形传感器监测民用工程结构变形的一致性毕业设计外文翻译

1、 Low-coherence deformation sensors for the monitoring of civil-engineering structures D. Inaudia, A. Elamarib, L. Pfluga, N. Gisinb, J. Breguetb, S. Vurpillota “IMAC, Laboratory of Stress Analysis, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland GAP, Group of Applied Physics -Op

2、tical Seciion, Geneva University CH-1205 Geneva, Switzerland Rcccivcd 25 January 1993; in revised form 8 March 1994; accepted 25 March 1994 Abstract An optical-fiber deformation sensor with a resolution of 10 pm and an operational range of 60 mm has been realized. The system is based on low-coherenc

3、e interferometry in standard single-mode telecommunication fibers. It allows the monitoring of large structures over several months without noticeable drift. No continuous measurement is needed and the system is insensitive to variations of the fiber losses. This technique has been applied to the mo

4、nitoring of a 20 m X5 m X0.5 m, 120 ton concrete slab over six months. It is possible to measure the shrinkage of concrete and its elastic coefficient during pre-straining, giving reproducible results in good agreement with theoretical calculations and measurements performed on small concrete sample

5、s. This paper describes the optical arrangement and the procedures used to install optical fibers in concrete. Keywor&: Ikformation sensors; Civil-engineering structures 1. Introduction Both the security of civil-engineering works and the law require a periodic monitoring of structures. The methods

6、used for this purpose, such as triangulation, water levels or vibrating strings, are often of tedious application and require one or many specialized operators. This complexity and the resulting costs limit the frequency of the measurements. Furthermore, the spatial resolution is often poor and the

7、observation is usually restricted to the surface of the object. There is thus a real demand for a tool allowing an internal, automatic and permanent monitoring of structures with high accuracy and stability over periods typically of the order of 100 years for bridges. In this framework, fiber-optic

8、smart structures (i.e., structures with self-testing capabilities) are gaining in importance in many fields including aeronautics and composite material monitoring. This technology can be applied in civil engineering and in particular for the short- and long-time observation of large structures such

9、 as bridges, tall building frames, dams, tunnels, roads, airport runways, domes, pre-stressing and anchorage cables. The monitoring of such structures requires the development of a measuring technique with high accuracy,stability and reliability over long periods. It has to beindependent of variatio

10、ns in the fiber losses and adapted to the adverse environment of a building site. To reduce the cost of the instrumentation, it is furthermore desirable to use the same portable reading unit for the monitoring of multiple structures. We describe here asystem based on low-coherence interferometry res

11、ponding to all these requirements.2. Experimental arrangementThe measuring technique relies on an array of standard telecommunication optical fibers in mechanical contact with concrete. Any deformation of the host structure results jn a change in the optical length of he fibers. Each sensor line con

12、sists of two single-mode ibers: one measurement fiber in mechanical contact with the structure (glued or cemented) and a reference iber placed loose near the first one (in a pipe) in order to be at the same temperature. Since the measurement technique monitors the length difference beween these two

13、fibers, only the mechanical deformation will have an effect on the results while all other perurbations, such as thermally induced changes in the refractive index of the fibers,will affect the two in an identical way and cancel each another out. To measure the optical path difference between the two

14、 fibers, a low-coherence double interferometer in tandem configuration has been used (Fig. 1) l. The source is an LED (light-emitting diode) working around 1.3 pm with a coherence length L, of 30 pm and a rated power of 200 pW. The radiation is launched into a single-mode fiber and then directed tow

15、ard the measurement and the reference fibers by means of a 50:50 single-mode directional coupler. At the ends of the fibers two mirrors reflect the light back to the coupler, where the beams arc recombined with a relative delay due to the length difference AL, between the fibers, and then directed t

16、owards the second (reference) interferometer. The reference interferometer is of Michelson type with one of the arms ended by a mobile mirror mounted on a micromctric displacement table with a resolution of 0.1 pm and an operating range of 50 mm. It allows the introduction of an exactly known path d

17、ifFcrence AL, between its two arms. This fiber interferometer is portable and needs no optical adjustment after transportation. It has been developed by the GAP with the support of the Swiss PTT for optical cable testing 2. The intensity at the output of the reference inter- ferometer is measured wi

18、th a pig-tail photodiode and is then given by 3where zz,r is the effective refractive index of the fiber, zzg the group refractive index (about 1% higher than nefr in silica), A, the central vacuum wavelength of the light, zi, the autocorrelation function taking the spectral characteristics of the e

19、mission into account and AL the physical path difference between the two interfering paths. Further similar interference terms appear in Eq. (1) in the special cases when AL, L, or AL, L,. When the optical path difference between the arms in the reference interferometer corresponds to the one induce

20、d by the two fibers installed in the structure (within the coherence length of the source), interference fringes appear. Scanning AL, with the mirror of the reference interferometer it is possible to obtain AL = 0 either with AL, = AL, or with AL, = -AL, and thus two interference fringe packets as d

21、escribed by Eq. (1). The mirror position corresponding to AL, = 0 also produces an interference and is used as a reference. These three fringe packets arc detected by means of a lock-in amplifier synchronized with the mirror displacements. The mirror displacements and the digitalization of the lock-

22、in output are carried out by means of a portable personal computer. Since the reference signal is gcnerated separately and does not have a constant phase relation to the interference signal, only the envelope of the demodulated signal has a physical meaning and corresponds to the envelope of the fri

23、nge pattern. A lock-in plot showing the three typical peaks is shown in Fig. 2. Each peak has a width of about 30 pm. The calculation of its center of gravity determines its position with a precision better than 10 pm. This precision is the limiting factor of the whole measurement technique. Since A

24、L, is known with micrometer precision, it is possible to follow AL, with the same precision.Fig. 1. Experimental setup of the low-coherence double Michelson interferomctcr. D. Innudi et al. 1 Semors and Fig. 2. Typical fringe cnvclope as a function of the mirror position. The distance between the ce

25、ntral and the lateral peaks corresponds to the length difference between the measurement and the reference fibers mounted in the table. Any change in the length of the structure results in a change in the position of these peaks. Any change in the losses of the fibers will result in a change of the

26、height of the peaks. The central peak is fixed and used as a reference. The path difference AL, is proportional to the de-formation of the structure AL, with the relation between the two given by 4 where p is Poissons ratio and pij is the strain optic tensor (Pockcls coefhcients). The coefficient 5

27、takes into account the variation of the effective index neff in a fiber under strain. A degradation of one or both fibers (due to aging, for example) will result in a lower visibility of the fringes but will not affect its position. The information about the deformation of the structure is encoded i

28、n the coherence properties of light and not in its intensity as in the majority of the sensors applied to date in civil-engineering structures, mostly based on microbend losses and/or optical time-domain reflectometry (OTDR) techniques. Interference peaks resulting from reflections as low as -30 dB

29、of the source power can be detected by our system without phase modulators. By modulating the phase in one of the four arms of the two interferometers, one can increase the dynamic range of the device to more than 100 dB 5. Even if the polarization dispersion and bend-induced birefringence in the se

30、nsing fibers could reduce the visibility of the interference fringes or even split the fringe packets, none of those effects was observed in our experiment. No adjustment of polarization between the reference and the sensing arm was then necessary. A good mechanical contact between the measurement f

31、iber and the structure under test is fundamental. In this study a number of installation procedures have been tested and optimized for the different measurements (shrinkage, elasticity modulus, etc.). The mounting techniques can be divided into two main categories: full-length coupling and local cou

32、pling. During our tests five out of six optical fiber pairs with a 0.9 mm nylon coating, being mounted on the external face of a 20 m long plastic pipe and protected only with thin rubber bands (see Fig. 3(a), survived the concreting process. During the setting process the concrete envelops the fibe

33、r and realizes the desired mechanical contact. Those fibers showed a minor increase in the scattering losses and the appearance of small parasite peaks. The measurements on those fibers were consistent with the results obtained with other installation techniques (see below). It seems that for full-l

34、ength coupling the nylon coating transmits the structure deformations (extension and shortening) entirely to the fiber core. This installation technique is very promising when compared to the usual procedure, consisting of a pipe protecting the fibers during the pouring of concrete and being removed

35、 before the setting process begins. This second method seems more adapted to small samples than to full-scale structures. Eleven other fiber pairs were glued at the two ends of the table after removing locally the protective coating layers of the fibers (see Fig. 3(b). The silica fiber was ftxed wit

36、h epoxy glue to a metallic plate mounted on the end faces of the concrete structure. The gluing length was about 20 mm. A pre-strain (between 0.1 and 0.4%) has been given to those fibers during the gluing process to keep them under tension and allow the measurement of both expansion and shrinkage of

37、 the structure. This type of local coupling proved to be the most reliable, but was not adapted to following the deformation during the pre-stressing of the table because of the important surface deformations occurring during this operation. The problem has been overcome by gluing other fibers insid

38、e the pipes at about two meters from the surfaces, i.e., far from the force insertion region (see Fig. 3(c).Fig. 3. Schematic representation of three of the installation techniques used:(a) direct concreting of the measurement fiber mounted on a plastic pipe; (b) fiber glued at the table surface; (c

39、) fiber glued inside the pipe at 2m from the pipe ends.Fig. 4. Top and side views of the concrete table measured in the experiment and position of the sensing-fiber pairs A, B, C and D. Fibers A, B and C arc glued at the surface of the structure, while fiber D is glued inside a pipe, 2 m away from t

40、he surface of the slab. Twelve more fihcr pairs were installed, but are not shown for simplicity.To study the possible effect of creep in strained fibers 6, one fiber has been mounted on a mechanical support that allows the fiber to be tightened only at the time of the measurement. No difference bet

41、ween this fiberand those permanently strained has been observed over a period of six months, confirming the assumption that no creep occurs for fiber strains below 1%. Since the scanning range of the mirror is 5 mm, it was easy to cleave the 20 m long fibers within this margin. The Fresnel reflectio

42、n of the cleaved fibers combined with the high dynamic of the system allow a measurement of AL,. This value of AL, can than be used to correct the cutting and obtain pairs with length differences below 1 mm. Two ferrules were then installed on the fiber ends and mounted in front of a polished inox s

43、urface. Chemical silver deposition was also used to produce mirrors on the cleaved fiber ends.Fig. 6. Comparison between the measurements performed on the structure by optical fibers and the ones performed on 360 mm and 500 mm samples in a mechanical micrometer comparator. The measurement on the sam

44、ples was possible only during the first two months.3. ResultsSeveral long- and short-term measurements have been carried on a 20 m x 5 m x 0.5 m, 120 ton concrete slab intended to be used as a vibration-isolated base for optical analysis (in particular by holographic and speckle interferometry) of l

45、arge structures 7. This structure has been concreted indoors, allowing controlled environmcntal conditions and known concrete composition to be achieved. Samples have been prepared with the same material composition and are under permanent test for their mechanical properties (resistance, shrinkage

46、and elastic coefficient). This allows a direct comparison between the results on the full-scale structure and the samples. The table has been pre-strained 23 days after concreting in both length and width. It was possible at this time to measure the elastic coefficient of the material in full scale.

47、 Fig. 4 shows a schematic representation of the table and the position of the fibers referred to in the experimental results. At the time of writing, the table has been under test for six months. Over this period the shrinkage in the longitudinal direction (i.e., over 20 m) has been about 6 mm. We s

48、how in Fig. 5 the results of the measurements for three (glued) fibers over 175 days. The table has a T profile (Fig. 4). It is evident from Fig. 5 that the fibers mounted near the borders of the table, i.e., were the thickness is smaller, registered a larger shrinkage, as expected according to the

49、concrete theory. Adjacent fibers give consistent results independently of the installation technique. No difference has been noticed between the fibers under permanent tension and those loosened between the measurements, suggesting that no creep of glass fibers occurred. The shrinkage measured with

50、the fiber system has been compared during the first two months with the results obtained with a mechanical comparator mounted on two samples of 360 mm and 500 mm, respectively. The observed deformations have been scaled to 20m and are compared in Fig. 6 to the results obtained with fibers B and C. V

51、ery good agreement is found between the two measurements. A theoretical comparison between the experimental results and the Swiss civil engineering standards has also been carried out. The experimental data and the standards are in agreement within f 10%. A more accurate simulation including the phy

52、sico-chemical properties of the concrete used is under development. The table was pre-stressed 23 days after concreting. The five steel cables running over the length of the table and the forty cables running over its width were stretched with a force of 185 kN (18.5 Tons) each. The fibers glued to

53、the surface and those in direct contact with concrete over the whole length measured an expansion of the table instead of the expected shrinkage. This is due to the important surface deformations occurring near the force-insertion points, i.e., near the pre-stress heads that were placed near the fib

54、er ends. Fiber D glued inside the plastic pipe at 2m from each end was not subject to these local effects and measured a shortening of 0.23 mm. The theoretical calculation based on an elastic coefficient of 30 kN/mm2 gives a shortening of 0.28mm at the borders and 0.19 mm at the center of the table.

55、 Since fiber D was placed in an intermediate position, the experimental value can be considered to be in good agreement with the theory.4. Conclusions A new deformation sensor adapted to the monitoring of civil-engineering structures has been proposed. it is based on low-coherence interferometry in

56、standard lowcost telecommunication fibers. The resolution of the measurements is 10 pm, the operational range is 60mm and the stability has been tested over six months without noticeable drift. The reading unit is compact and portable, needing no optical alignment before the measurements. It is cont

57、rolled by a portable personal computer, which is also responsible for the data trcatment. The same reading unit can be used to monitor multiple fiber lines by simple manual unplugging. This technique is furthermore practically insensitive to increased losses due to degradation of the fibers. A test

58、study has been carried out on a 20m 5m X 0.5m concrete slab, giving consistent results when compared to other measurement techniques based on samples or to concrete theories. It was possible to follow concrete shrinkage over six months (the cxper- iment will continue for about five years) and to mea

59、sure the elastic coefficient on the full-scale structure. Different fiber-installation techniques adapted to the measurement of various parameters have been tested in building-site conditions. This technique appears very promising for the mon-itoring of civil-engineering structures such as bridges,

60、dams and tunnels, allowing internal, automatic and permanent monitoring with high precision and stability over long periods.AcknowledgmentsThe authors are indebted to R. Passy and R. Delez for their assistance, encouragement and helpful dis-cussion. We acknowledge the IMM Institute in Lugano (Switze