Engineering Structures 28 (2006) 648–659 www.elsevier.com/locate/engstruct Fiber Bragg grating sensors for structural health monitoring of Tsing Ma bridge: Background and experimental observation ✩ T.H.T. Chan a,∗ , L. Yu a,d , H.Y. Tam b , Y.Q. Ni a , S.Y. Liu b , W.H. Chung b , L.K. Cheng c a Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, HungHom, Kowloon, Hong Kong, PR China b Photonics Research Centre, Department of Electrical Engineering, The Hong Kong Polytechnic University, HungHom, Kowloon, Hong Kong, PR China c TNO (TPD), Delft, The Netherlands d Blasting and Vibration Department, Changjiang River Scientific Research Institute, 23 Huangpu Street, Wuhan, Hubei 430010, PR China Received 16 February 2005; received in revised form 15 September 2005; accepted 16 September 2005 Available online 27 October 2005 Abstract The rapid expansion of the optical fiber telecommunication industry due to the explosion of the Internet has substantially driven down the cost of optical components, making fiber optic sensors more economically viable. In addition, the rapid development of fiber-optic sensors, particularly the fiber Bragg grating (FBG) sensors offers many advantages and capability that could not be achieved otherwise. In the past few years, fiber Bragg grating sensors have attracted a lot of interest and they are being used in numerous applications. This paper describes the FBG sensors developed for structural health monitoring, and were installed on Hong Kong’s landmark Tsing Ma bridge (TMB), which is the world longest (1377 m) suspension bridge that carried both railway and regular road traffic. Forty FBG sensors divided into three arrays were installed on the hanger cable, rocker bearing and truss girders of the TMB. The objectives of the study are to investigate the feasibility of using the developed FBG sensors for structural health monitoring, via monitoring the strain of different parts of the TMB under both the railway and highway loads as well as comparing the FBG sensors’ performance with the conventional structural health monitoring system — Wind and Structural Health Monitoring System (WASHMS) that has been operating at TMB since the bridge’s commissioning in May 1997. The experimental observations in this project show that the results using FBG sensors were in excellent agreement with those acquired by WASHMS. c 2005 Elsevier Ltd. All rights reserved. Keywords: Structural health monitoring; Fiber Bragg grating (FBG); Tsing Ma bridge (TMB); Strain 1. Introduction Fiber photosensitivity is the main phenomenon involved in writing Bragg gratings into the core of a fiber, which was first demonstrated by K.O. Hill et al. in 1978 at Canadian Communications Research Centre (CRC), Ottawa, Ont., Canada [1]. However, pioneering work at the United Technology Research Centre in fabricating fiber Bragg gratings in a fiber core through its side, which was a significant milestone for in-fiber Bragg grating (FBG) sensors, was published eleven years later by G. Meltz et al. in 1989 [2]. This side-writing technique creates a Bragg grating ✩ It is hereby stated that content of this paper has not been published elsewhere and it has not been submitted for publication elsewhere. ∗ Corresponding author. Tel.: +852 2766 6061; fax: +852 2334 6389. E-mail address: [email protected] (T.H.T. Chan). directly in the fiber core using a holographic interferometer illuminated with a coherent ultraviolet (UV) source. Since then, many methods have been found to increase the refractive index by improving both the UV exposure method, and the photosensitivity of the fiber core [3]. Transverse holographic methods are particularly useful for producing the modulated UV since they can easily create FBG sensors with both the desired spectral response and at any position along the fiber [4]. In 1993, an advanced FBG production technique was reported [5,6], which involved the use of an optical phase mask to generate interference fringes. Similar techniques for FBG production are used in most current FBG sensor work. Earlier research on FBG sensors concentrated on the fundamentals of the reliability of these sensors in terms of their strain sensitivity, repeatability, signal demodulation techniques, and the general characterization of their performance [7–9]. These studies have shown encouraging results, underlining the potential of FBG 0141-0296/$ - see front matter c 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2005.09.018
aDepartment of Civil and Structural Engineering, The Hong Kong Polytechnic University, HungHom, Kowloon, Hong Kong, PR Chinab Photonics Research Centre, Department of Electrical Engineering, The Hong Kong Polytechnic University, HungHom, Kowloon, Hong Kong, PR China
c TNO (TPD), Delft, The Netherlandsd Blasting and Vibration Department, Changjiang River Scientific Research Institute, 23 Huangpu Street, Wuhan, Hubei 430010, PR China
Received 16 February 2005; received in revised form 15 September 2005; accepted 16 September 2005Available online 27 October 2005
Keywords: Structural health monitoring; Fiber Bragg grating (FBG); Tsing Ma bridge (TMB); Strain
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1. Introduction
Fiber photosensitivity is the main phenomenon involvin writing Bragg gratings into the core of a fiber, whiwas first demonstrated by K.O. Hill et al. in 1978Canadian Communications Research Centre (CRC), OttOnt., Canada [1]. However, pioneering work at the UniteTechnology Research Centre in fabricating fiber Bragg gratin a fiber core through its side, which was a significmilestone for in-fiber Bragg grating (FBG) sensors, wpublished eleven years later by G. Meltz et al. in 19[2]. This side-writing technique creates a Bragg grat
✩ It is hereby stated that content of this paper has not been publielsewhere and it has not been submitted for publication elsewhere.
directly in the fiber core using a holographic interferomeilluminated with a coherent ultraviolet (UV) source. Sinthen, many methods have been found to increase the refraindex by improving both the UV exposure method, andphotosensitivity of the fiber core [3]. Transverse holographimethods are particularly useful for producing the modulaUV since they can easily create FBG sensors with both thedesired spectral response and at any position along the[4]. In 1993, an advanced FBG production technique wreported [5,6], which involved the use of an optical phase mato generate interference fringes. Similar techniques for FBproduction are used in most current FBG sensor work. Earesearch on FBG sensors concentrated on the fundamentthe reliability of these sensors in terms of their strain sensitivrepeatability, signal demodulation techniques, and the genercharacterization of their performance [7–9]. These studies havshown encouraging results, underlining the potential of FB
sensors for strain monitoring. The measurand versatilitythe unique advantages offered by FBG sensors have resin their use in a wide range ofsectors such as in advanceaircraft and space vehicles [10,11], marine [12] and medicalscience [13]. Recently, it has been demonstrated that FBsensors have great potential for a wide range of applicatiowhere quasi-distributed measurements of physical paramsuch as strain, pressure, vibration, temperature, ultrasound,magneticfield and high-g acceleration are required [14,15].Comprehensive reviews of recent progress in the applicatof FBG are available elsewhere in the literature [16–18].
These research works have shown that FBG senhave several inherent advantages over conventional elecsensors such as small size, light weight, non-conductivity, fresponse, resistance to corrosion, higher temperature capabilitand immunity to electromagnetic noise and radio frequeinterference. The distinct advantages of FBG sensors over othtypes of fiber optic sensors are their multiplexing capability awavelength-encoded measurand information. A single stringoptical fiber can accommodate up to many tens of FBG sensThe measurand information is encoded in the wavelenwhich is an absolute parameter and thus FBG sensor sysare less susceptible to signal amplitude fluctuations. Anoattractive feature of FBG sensors is their inherent abilityserve asboth the sensing element and the signal transmismedium which opens new possibilities in the field of reliabremote structural health monitoring [19,20]. In this regard,application of this new technology would have a significaimpact on the health and efficiency of civil infrastructuresystems [21]. However, the practical applications of this kindsensor to real civil engineering structures have not been widadopted although the progress of fiber optic health monitois impressive [22,23].
Different types of fiber Bragg grating (FBG) sensofor simultaneous strain and temperature measurementfor temperature independent strain measurement havedeveloped by The Hong Kong Polytechnic Univers[24–29]. Fiber-laser-based wavelength-division-multiplex(WDM) FBG sensor interrogation techniques and a broadblight source-based multiplexed FBG sensor interrogasystem have also been devised. The system can perform botstatic and dynamic strain measurement (up to a samplingof 52 Hz), and allows one to read the measurement datlocations tens of kilometers away from the monitoring sAfter a series of successful laboratory tests and verificatthe system has been installed in different parts of the TsMa bridge [30] and compared with the results obtained ba sophisticated long-term monitoring system, known as WAnd Structural Health Monitoring System (WASHMS), whicwas devisedand implemented by the Highways Departmeof Hong Kong SAR Government to monitor the structuhealth and conditions of the three cable-supported bridin Hong Kong, including the Tsing Ma bridge, the TinKau bridge and the Kap Shui Mun bridge. This on-structuinstrumentation WASHMS system consists of a total of ab800 sensors of different types permanently installed onthree bridges, including strain gauges, GPS position sen
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accelerometers, level sensors, temperature sensors and win-motion sensors. Because the WASHMS was implementedseveral years ago, it has not benefited from the newly develooptical fiber sensor technology. In order to investigatefeasibility of using the developed FBG sensors for structhealth monitoring, a field test was carried out in May 20in which a number of such FBG sensors were installedthe Tsing Ma bridge to conduct real time and full scameasurements. The results were assessed and comparethe conventional strain gauges obtained from the WASHMS.
This paper first introduces the background of fiber Bragrating sensor technology and then follows by the high-spdemultiplexing/interrogation system for FBG sensor arraFBG sensor fabrication and package units. In this paper, thinstallation and experimental setup in the Tsing Ma bridgealso described, and the preliminary observation results onstructural health monitoring of the Tsing Ma bridge using tFBG sensor units are presented, analyzed and compared withe conventional strain gauges employed in the WASHMSthe Tsing Mabridge. Some conclusions are made, and furtmeasurements and discussionsin detail will be reported inanother subsequent paper [31].
2. Background of fiber Bragg grating sensor technology
In the field of fiber-optic sensors (FOSs) the FBG sensare one of the most exciting developments in recent years. Thave a unique property and many advantages over other Fdue to their quasi-point sensing and multiplexing capabilThe most important advantage of an FBG sensor is thatmeasurand is encoded directly in terms of the wavelenwhich is an absolute parameter and does not suffer frodisturbances of the light paths [32]. Hence, the output signais independent of the intensity of the source, and lossesthe connecting fibers and couplers. Furthermore, each oreflected signals will have a unique wavelength and caneasily monitored; an array of wavelength-multiplexed FBsensors may thus be implemented for simultaneous mulmeasurements using a single fiber. A typical FBG is a perioperturbation of the refractive index in the fiber core as showFig. 1.
2.1. FBG-based sensing principle
A FBG is a periodic structure, which is written intosegment of germanium-doped single-mode fiber in whichperiodic modulation of the core refractive index (RI) is formalong the fiber length by exposure of the core to a spapattern of UV light at 197 or 248 nm wavelengths. Whlight within a fiber passes through a FBG, multiple Fresreflections take place along the entire length of the gratingto the variations in refractive index. Constructive interferebetween the forward wave and the contra-propagating lwave occurs when the wavelength of the propagating lighthe fiber doubles the grating pitch, i.e. the Bragg (or phmatching) condition is satisfied. This leads to narrowband bareflection of light. The reflected wavelength is known as
Fig. 1. A periodic perturbation of the refractive index in the fiber core.
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wheren is the effective refraction index of the fiber core aΛ the period of the index modulation. Bothn andΛ dependon temperature and strain, consequently the Bragg waveleis sensitive to both strain and temperature. A FBG is tan intrinsic fiber-optic sensor. Light that does not satisfyBragg condition passes through the FBG with very low loas shown inFig. 1 [33]. The changes of index created in FBGare relatively permanent and FBGs are sensitive to a numof physical parameters. Thus, by monitoring the resulchanges in reflected wavelength, FBG sensors can be usa variety of sensing applications to measure physical quantifor example, strain, temperature, pressure, ultrasound,magnetic field, force and vibration. Each of the reflected signwill have aunique wavelength and can be easily monitored, tachieving multiplexing of the outputs of multiple sensors usa single fiber. However, the FBG central wavelength will varywith the change of these parameters experienced by theFor an applied longitudinal strain change of�ε, the wavelengthshift, �λBS, is given by
�λBS = λB(1 − ρε)�ε, (2)
whereρε is thephotoelastic coefficient of the fiber, given by
ρε = n2
2[ρ12 − ν(ρ11 − ρ12)], (3)
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where ρ11 and ρ12 are the components of the fiber-opstrain tensor andν the Poisson’s ratio. For the silica fibethe wavelength–strain sensitivities of 800 nm and 1.55 µmFBG sensors have been measured as∼0.64 pm/µε and∼1.15 pm/µε respectively [7,34]. For a temperature change�T , the corresponding wavelength shift,�λBT , is given by
�λBT = λB(α + ζ )�ε, (4)
where α and ζ are the thermal expansion coefficient athermo-optic coefficient of the fiber material respectively. Fothe silica fiber, the wavelength–temperature sensitivities800 nm and 1.55 µm FBG sensors have been measured wvalues of∼6.8 pm/◦C and∼13 pm/◦C respectively [7,34].The refractive index and the grating period, and subsequethe Bragg wavelength vary with changes in axial strain�ε andtemperature�T . The shift in the Bragg wavelength in responto strain and temperature is given by
�λB
λB= (1 − ρε)�ε + (α + ζ )�T . (5)
2.2. FBG multiplexing technique
The primary advantage of FBG sensors is their capabfor multiplexing operation. In most practical applications, FBsensors need to be multiplexed in order to achieve qudistributed measurements and potentially to compete withconventional electrical or other types of optical sensors. A laarray of FBG sensors may be addressed by a single so
and detector using one or a combination of the four standtechniques: time-division multiplexing (TDM), spatial-divisiomultiplexing (SDM), frequency-division multiplexing (FDMand wavelength-division multiplexing (WDM). In principle, thmultiplexing techniques which are suitable for conventiofiber-optic sensors can alsobe applied to FBG sensors [35].
Fig. 2 shows the schematic of a scanning filter FBGinterrogation system based onWDM techniques. A broadbansource is used to interrogate a chain of wavelength-multipleFBG sensors. A quasi-distributed chain of FBG sensors wouthen be multiplexed in the wavelength domain to sepathe individual sensor signals. WDM interrogation systetypically require highly reflective FBG sensors each operatin a distinct wavelength window. It is one of the mostraightforward techniques for FBG elements in which the Fsensors in the network are arranged such that their operationaregions do not overlap. The maximal number of FBG senalong a single fiber that could be interrogated by a WDinterrogation unit depends on the optical bandwidth of the ligsource, operation range of each FBG sensor and the turange of the tunable filter. The typical number of FBG sensthat can be dealt with by a WDM interrogation unit is aboutand is mainly limited by the light source’s optical bandwidThe FBG sensors would be assigned an operating waveleband wide enough to cover the measurand-induced waveleshif t without overlap [36]. Commercial FBGinterrogationsystems based on scanning Fabry–Perot (F–P) filtersavailable from several companies. Spatial-division multiplex(SDM) is a simple technique that involves the splittingthe interrogation light, typically using a fused fiber-couplto several stringsof optical fiber. The reflected light fromthe FBGs of each fiber is either routed to an equal numof wavelength measurement units or time-shared by a siwavelength measurement unit with the help of an optiswitch. The FBG-reflected signal from each string of fiber dnot interfere and thusidentical FBGs (i.e. FBGs with the samBragg wavelength) can be used in each fiber. This techncombined with the WDM technique can increase the numbeFBG sensingpoints significantly.
Fig. 3shows the schematic diagram for a FDM interrogationsystem [36]. A wavelength controlled laser source is usedaddress a chain of identical FBG sensors. The sourcea periodically modulated chirp in frequency that is smalthan the line width of the gratings. The reference signal frthe first coupler interferes with the reflected FBG signaIf the FBG sensors operate at different frequencies,signals from each can be separated using switching electroSuccessful demultiplexing relies on no overlap betweenFBG modulation frequencies.
Fig. 4 shows that schematic of a TDM interrogation systwhere narrow optical pulses were launched into an optical fibecontaining many low-reflection FBG sensors with virtuaidentical Bragg wavelengths. Light takes about 10 nspropagate one round trip along 1 m of optical fiber atherefore the separation between adjacent sensors mustgreater than 1 m for interrogating pulses of 10 ns wiIndividual sensors are distinguished by measuring the tim
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flight of signals returning to the interrogation unit. A singoptical pulse consists of many wavelength components butsensors reflect only the wavelength component that mattheir Bragg wavelength. Normally, the wavelength shift of thsensor is determined by using a linear optical filter whconverts wavelength shift to optical power variation. TDsystems utilize identical, low reflectivity FBG sensors (typi4% reflection) all operating in the same wavelength windHowever, the signal-to-noise ratio of TDM systems is lowthan that of WDM systems and therefore the performanceTDM systems is generally not as good as that of WDM systeThe TDM approach is able to multiplex a large number of Fsensors without the need for wavelength selective componThe wavelength measurement accuracies demonstrateWDM interrogation systems and TDM interrogation systeare typically 1 pm and 10 pm, respectively. This is equivalentemperature and strain measurement accuracies of 0.1 ◦C (1 ◦C)
and 1µε (10 µε), respectively, for WDM (TDM) interrogationunits.
The FBG sensor technology has been undergoing rdevelopment since its first demonstration by Morey in 1[7]. This is because the FBG sensors have many advancompared to other optical and electrical sensors. All theaforementioned features make FBG sensors very suitablhealth monitoring for civil structures, such as bridges, tunnand dams. FBG sensors can be employed in existing structuresby attaching the sensors onto their surfaces. New struccan be monitored by embedding FBG sensors in them duconstruction without affecting the structural integrity becausof the small size of optical fiber. The firstfield application ofFBG sensorsfor bridge health monitoring was demonstrated1995, by embedding the FBG sensors into the concrete giof a new road bridge in Calgary, Alberta, Canada [37].
3. High-speed demultiplexing/interrogation system forFBG sensor arrays
In the past few years, the FBG sensor is demonstratebe very suitable for strain sensing. An important advanof a FBG sensor is that it can be easily surface-mouto or embedded in different structures. The FBG sensorslightweight, small and can even be embedded and integin a composite structure. Furthermore, FBG sensorsvery suitable for simple wavelength multiplexing so thasingle detection unit can be used to interrogate many Fsensors in one fiber. A newdemultiplexing/interrogationsystem(DEMINSYS) for FBG sensor arrays has been developedTNO TPD [38]. This potentially low cost system combineshigh readout frequency (about 20 kHz) for all the FBG senchannels with absolute measurement, and has a detection limitof lessthan 1 pm. The detection limit of the DEMINSYS fodynamic signal is verified with a tunable laser (New Fo6200). The wavelength of the tunable laser is modulateda frequency of 600 Hz and the amplitude is 0.6 pm (p–The spectrum of the DEMINSYS output signal showed tthe600 Hz modulation can clearly be observed in the spectand the amplitude of 0.6 pm (p–p) is found to be about 25
Fig. 2. Schematic of a scanning filter FBG interrogation system based on WDM.
Fig. 3. Schematic of a FDM interrogation system.
Fig. 4. TDM interrogation system of multiple FBG sensors.
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higher than the noise level. Therefore detection of a subwavelength shift with a readout frequency of about 20 kHzfeasible.
The DEMINSYS is designed for Structural HeaMonitoring for aerospace applications, but is also suitablecivil structures. Other specifications of the DEMINSYS ashown inTable 1.
For this field experiment the maximum readout frequehas been used for noise reduction by averaging to achievepm resolution. Since the strain gauge signal in the WASHhas a maximum detection frequency of about 20 Hz, the Fsensors can also provide information of high-frequency eveDuring the measurements, theDEMINSYS is located in thebridge close to the FBG sensors. Vibration of the bridge
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cause disturbances to the DEMINSYS. This can be verifiedusing a reference FBG sensor.
4. FBG sensor fabrication and package units
Standard telecommunication single mode fibers (CorninSMF28) wereused to fabricate FBG sensors. In order noweaken the mechanical strength of the FBG, theouter coatingof the fiber was removed by soaking a short length inwarm acid bath instead of using a mechanical stripper. Aftethe FBG inscription, this short un-coated FBG was annealeTo facilitate the installation process while maintainingstraightness of the FBG, the FBG was mounted on niti(an acronym for Nickel Titanium Naval Ordnance Laboratostrips with thickness of∼7.5 µm (0.0029′′) and dimensions
No. of input fibers No. of FBG sensors No. of fiber input Resolution FBG channel spacing Wavelength ran
4 32 (8 per input fiber) 4 Better than 1 pma(∼1 µε) About 4 nm 1530–1565 nm
a Depending on sampling frequency.
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Fig. 5. Packaged FBG strain gauges boxed in ABS enclosures with two 3fiber outlets (inset shows the FBG sensors epoxied on nitinol sheets in anafter baking at 80◦ C for 5 h).
of 6 × 110 mm, which were cleaned with high concentratisopropanol to remove grease stains. Nitinol is a rotemperature super elastic metal which is corrosion resisand can withstand 8% elongation without deformation. TFBG–nitinol sheet combo was sandwiched and pressed togethusing two Teflon sheets to minimize the thickness as wellthe evenness of epoxy between the FBG and the nitinol shFig. 5 shows a packaged FBG strain gauge boxed in ABenclosures with two 3 mm fiber outlets. The inset ofFig. 5shows the packaged FBG sensors epoxied on nitinol sheean oven after baking at 80◦C for 5 h.
In order to protect the packaged FBG sensor from moisand dust, weather proof ABS enclosures measuring 12×80× 60 mm that complied with IP65 specifications were usThese enclosures are similar to the WASHMS resistive straingaugeenclosures in terms of functionality and appearanceare in use by the Highways Department of Hong Kong SATo attach the packaged FBG sensor to the ABS enclosurrectangular opening was cut from the bottom of the enclosso that the sensor can be attached to the structure throughopening. The sensor was then connected to more rigid 3single mode optical fiber cables and led out from the enclosthrough two stress relieving boots as shown inFigs. 5and6.In the case where an extra FBG is needed for temperareferencing inside the enclosure, this FBG was connectethe strain sensing FBG before the 3 mmcable is connectedThis temperature referencing FBG must be free from stresavoid cross-sensitivity of strain and temperature [39]. Fig. 6shows the insideof the enclosure. It can be seen that the FBis protected with a nitinol backing sheet which is attached tthe FBG with high strength epoxy. The FBG sensor andnitinol sheet are attached to the steel using a two-part ep
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Fig. 6. Inside of the FBG sensor enclosure.
which is suitable for the application. The FBG is led out frothe enclosure by fusion splicing the FBG to two 3 mm optifiber cables that are much stronger than the 250µm bare fiber. Inorder to maintain consistent glue thickness and pressure amthe sensors, a pressure plate which is supported with a sprinthe back is installed at the inner side of the enclosure lid. Tpressure plate exerts pressure evenly on the FBG sensorthe lid is fixed with screws.
5. Installation and experimental setup in the Tsing Mabridge
5.1. WASHMS of the Tsing Ma bridge
The Tsing Ma bridge (TMB) is the longest suspension bridcarrying both highway and railway traffic. The bridge hasdouble deck configuration with the expressway on the updeck and the railway below. The upper deck carries a dual thlane carriageway and the sheltered lower deck containstracks of railway and two single emergency roadways that allolimited traffic to use the crossingduring periods of very strongwind. Structurally, the deck section of the Tsing Ma bridgehybrid arrangement combining both truss and box forms [40].Two longitudinal trusses to the full depth of the deck atm centers act in conjunction with the steel orthotropic deof the upper and lower carriageways to provide the vertbending stiffness. Plan diagonal bracings at the upper and llevels enable the trusses to provide lateral bending stiffnCrossframes of Vierendeel form are provided at 4.5 m cenwith every fourth frame being supported from suspenders.bridge was commissioned on May 22, 1997. The monitorsystem, WASHMS, has been operating since then for the TMBThe structural health monitoring system for the TMB comprisensors such as accelerometers,strain gauges, displaceme
Fig. 7. Strain gauge layout of the WASHMS installed on the Tsing Ma bridge.
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transducers, level sensors, anemometers, temperature sensoand weigh-in-motion sensors, installed permanently onbridges, and the data acquisition and processing system.strain gauges were installed on bridge-deck sections to meathe strain of members as shown inFig. 7.
5.2. Installation and experimental setup
In this test, three different strategic locations: (1) hangecables, (2) rocker bearing, and (3) supporting structure osection (Chainage 23488) of lower deck respectively as shin Fig. 8, were chosen to install FBG sensors. Three photostaken after the FBG sensors are installed on these locatioshown inFig. 9, in which someFBG sensors were installenearby the resistive strain gauges in use such that comparbetween conventional sensors and FBG sensors can be maaddition, strain-free FBG sensors were also used to measurtemperature at different sensing points to provide temperatucompensation for the measurements.
The FBG DEMINSYS unit developed is a complesystem with the capability to perform multiple optical FBsensor measurements and provide a major advancemenmechanical and temperature sensing applications. The unprovides a rapid measurement rate of hundreds of FBG senon several fibers. It includes an optical light source usedilluminate the FBG sensors, and an optical detector to meathe reflected optical signals on eachsensor. An external portablePC provides the on-line calibration data display/storage ofFBG sensors under test. The system resolution and accuare 1 pm and 10 pm respectively. The sampling rate ofsystem is adjustable and can be increased up to 20 kHz.Fig. 10briefly shows the experimental setup of the interrogation sysused to measure the wavelength of FBG sensors. The syincluded a broadband light source operating around 1550and a wavelength detection module based on a sensitive chcoupled device (CCD) array. Three strands of fibers, in whup to 21 FBG sensors were serially connected, were instaon the hanger cables, rocker bearing, supporting structuresection (Chainage 23488) of lower deck and suspender carespectively as shown inFigs. 8and9, respectively. To attachthe FBG sensors firmly on the structure, a small area of
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protective paint was removed. Attention was paid duringremoval of paint to avoid damaging the steel beneath. Beattaching the packaged FBG sensor, thatarea was cleaned witclear water and isopropanol. Two types of adhesive glues wused to attach the enclosure and FBG sensors on the steecuring cyanoacrylate glue was applied on the outer rim ofenclosure base and, on the inner rim of the enclosure andsensors, slow curing two-part weather proof epoxy was usA combination of the two glues at different positions providfast attachment of the sensor package in place while allowingthe much stronger epoxy bonding to cure slowly. The lidthe enclosure is fixed with four captive screws and the joinfinished with tongue and groove with neoprene gasket.
6. Measurement and observation based on FBG sensorunits
FBG sensors for strain measurement are excellent candifor structural health monitoring of the Tsing Ma bridge.field trial with the DEMINSYS system described inSection 3,a FBG sensor array system, was carried out on Hong Konlandmark Tsing Ma bridge in 2003. The experiment wcarried out with a FBG interrogator based on a scannoptical bandpass filter that provides a sampling speed of u70 sample/s as well as with a high-speed interrogation syste(DC to ∼20 kHz for all channels simultaneously) basedCCD. Forty FBG sensorsdivided into 3 arrays were installeon different parts of the bridge (hanger cable, rocker and tgirders), as shown inFig. 8. The goal of this field trial is tomonitor the strain of the different parts of the bridge unrailway load and highway load. Various measurements wperformed including an overnight measurement of about 2with a sampling frequency of about 500 Hz. The measuremresults will reveal the dynamic responses of the Tsingbridge, which will be measured by the FBG sensors during tpassage. The results of the FBG sensor were also comwith existing conventional strain gauges.
6.1. Temperature and strain measurement
The interrogation system was switched on continuoufor 24 h to monitor the structure and temperature cha
Fig. 8. Forty FBG sensors installed on the Tsing Ma bridge to measure temperature and strain at (1) hanger cable, (2)rocker bearing, and (3) truss girders of sectionChainage 23488.
.
Fig. 9. FBG sensors installed on (1) hanger cable, (2) rocker bearing, and (3) truss girders of section Chainage 23488
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of the bridge.Fig. 11 shows the wavelength variations of asingle sensor that incorporated both temperature referencand strain FBG sensors over a 10 min period.Fig. 11(a) and(b), respectively, show the wavelength change of the strfree temperature measuring FBG and the strain measuFBG which was affected by both thermal and strain effeConsidering that the FBG is mounted on steel material whicsubjected to both thermal and mechanical effects [39], Eq. (5)
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�λB
λB= (1 − ρε)ε + (1 − ρε)(α�T ) + ξ�T (6)
where λB is the Braggwavelength, ε is the strain, otherparameters have the same meanings as before. Thereforactual strain response of the sensing point can be resolveoffsetting the thermal effect using the temperature referen
Fig. 10. Experimental setup of FBG interrogation system.
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FBG. Fig. 11(c) gives a comparison of the strain responsesthe sensingpoint measured by both the FBG sensor and tconventional strain gauge over 10 min. At about 320, 420 a560 s, three trains passed through the bridge. Meanwhile, thovershoots were clearly shown by the interrogation systand conventional strain measurements. The experimentalshow that the FBG results are in good agreement with the dobtained by the installed WASHMS resistive strain gauges.
6.2. Hanger cable measurement
Six FBG sensors are mounted on the hanger cables. Oof the FBG sensors mounted on a hanger cable to theWan tower was monitored with the high-speed DEMINSYSinterrogation system. Via another input fiber, an athermpackaged FBG sensor in a vibration-isolated case is uas reference. For the hanger cable measurement, the reafrequency is set to 0.106 ms. The measured histories fromtwo FBG sensors are shown inFig. 12, where anarbitrary off-set of−40 micro strain is applied to the reference FBG histofor display purposes: the upper time history is from the FBsensor on the hanger cable, the lower one from the referenFBG sensor.
It shows that train passages att = 307 s andt = 377 s canclearly be detected by the FBG mounted on the hanger caFrom the signals, train running directions can be deducedas in an opposite direction. In the middle of the measuremhistory, the passage of heavy traffic can also be noticedcorresponds to the signal att = 195 s.
6.3. Rocker bearing measurement
Various measurements with the FBG sensors mounted onrocker bearings (as shown inFigs. 8 and 9) were performedincluding an overnight measurement of about 20 h withsample time of 2.1 ms; the data from strain gauges on the rocwere also logged simultaneously. The corresponding histresults of a one-hour track are compared inFig. 13, where theupper time history is from one conventional strain gauge, athe lower from one FBG sensor respectively.
In the Tsing Ma bridge construction, the rocker bearingsused to support and hold the deck. Therefore, the loading on
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Fig. 11. Wavelength variations and comparison of strain responses meaby FBG sensor (dot) and WASHMS strain gauge (line) respectively.
rocker is very complex and the strain depends strongly onposition of the sensor. Despite the FBG sensors and the sgauges are not mounted on exactly the same location orocker the signals of the 2 types of sensors, as shown inFig. 13,are found to be very similar and the train passages can clbe detected.
Fig. 12. Hanger cable tension measurement history.
Fig. 13. Comparison between FBG (lower) and conventional (upper) strain gauges installed on rocker bearings.
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6.4. Truss girder measurement
An array of 21 FBG sensors was mounted on differlocations of Chainage 23488 (as shown inFigs. 8and9). TheFBG sensors are placed close to the existing strain gauges foptimal comparison. InFig. 14, the upper history from a FBGsensor is compared with the signal of the lower correspondstrain gauge. The sampling time of the FBG sensor is 0.0ms. A moving average filter of 10 points is applied to the datathe FBG sensor and the detection bandwidth of the FBG senis reduced to about 2 kHz. The FBG sensor signal has alsarbitrary offset for display purposes. Although the sensorsnot located at exactly the same location, a great resemblhas been found.
Zooming in the signal reveals that the noise in the FBsensor signal is mainly caused by a 13 Hz component andbe reduced by appropriate filtering or placing the DEMINSoutside the bridge. The resolution is found to be about 1 pmcan be reduced by further averaging. The effect of a mov
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average filter of 5000 points is shown inFig. 15. Using suchfiltering, sub-pm/µε resolution can be achieved.
7. Conclusions
The background of fiber Bragg grating sensor technogy was briefly introduced. The high-speed demultipleing/interrogation system forFBG sensor arrays and the FBsensor package units were developed. A field trial with FBsensor arrays for measurement on a hanger cable, rocker bings and truss girder of the Tsing Ma bridge was performsuccessfully. The application of FBG sensors and interrogasystem to monitor the dynamic strain on Hong Kong’s landmaTsing Ma bridge has been demonstrated. The FBG packtechnique was proposed to apply for structural health moning applications. It can clearly and correctly detect the dynamstrain responses of the bridge induced by the passage of ton the bridge. The measurement result of the interrogationtem was in excellent agreement with those obtained by retive strain gauge measurements. The FBG sensor system o
Fig. 14. Comparison between FBG (upper) and conventional strain gauge (lower) installed on a truss girder of section Chainage 23488 of the Tsing Ma bridge.
Fig. 15. Comparison between FBG (upper) and conventional strain gauge (lower) histories after filtering data inFig. 14.
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many advantages over traditional resistive strain gauges. Tinclude remote sensing, ease of installation, non-corrosivelower maintenance cost. This shows that FBG sensor techogy is a good alternative for civil and structural dynamic strmonitoring.
Acknowledgements
The support provided by the HK Highways Departmethe HK Research Grants Council, the Hong Kong PolytechUniversity Research Grants (with nos. G-YD20 and G-YXand the National Natural Science Foundation of China (wgrant no. 50378009) is gratefully acknowledged.
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