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Integration of GPS, Accelerometer and Optical Fiber Sensors for Structural Deformation Monitoring

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Monitoring the response of structures, especially tall buildings, under severe loading conditions is an important requirement for the validation of their design and construction, as well as being a maintenance concern. Traditionally such response has
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  Integration of GPS, Accelerometer and Optical Fibre Sensors for Structural Deformation Monitoring Xiaojing Li, Gang-Ding Peng, Chris Rizos, Linlin Ge, Yukio Tamura and Akihito Yoshida University of New South Wales, Sydney NSW 2052, Australia Tokyo Polytechnic University, 1583, Iiyama, Atsugi, Kanagawa, 243-0297, Japan Email: xj.li@unsw.edu.au  BIOGRAPHY Xiaojing Li and Gang-Ding Peng are a PhD candidate and an Associate Professor respectively in the School of Electrical Engineering & Telecommunications, the University of New South Wales (UNSW), Australia. Linlin Ge and Chris Rizos are a Research Fellow and a Professor respectively in the School of Surveying and Spatial Systems at UNSW. Yukio Tamura and Akihito Yoshida are a Professor and Research Assistant respectively in the Department of Architecture, Tokyo Polytechnic University, Japan. ABSTRACT An integrated system comprising GPS, accelerometer, and optical fibre sensors has been proposed to monitor structural deformation in order to assess the integrity of the structure. The GPS and accelerometer sensors have been installed on a 108m tall steel tower, and data have been collected during Typhoon No. 21 on 1 October 2002, at 10Hz and 20Hz rates respectively. The wind induced deformation has been analysed in both time and frequency domains. In the frequency domain, both the GPS and accelerometer results show strong peaks at 0.57Hz, although GPS measurements are noisy in the low frequency end. On the other hand, the result of a series of indoor experiments shows that the optical fibre Bragg grating (FBG) sensors have demonstrated excellent performance with respect to sensitivity, linearity, repeatability and dynamic range. 1. INTRODUCTION The security of civil engineering works demands periodic monitoring of the structures in question. The deformations of a structure are the most relevant parameter to be monitored. Moreover, buildings can be damaged by earthquakes, typhoons, strong winds, or even terrorist attacks. Many damaged buildings (especially steel ones) cannot be simply inspected by eye because there may be no major visible damage to the surface of fire-protection material covering structural members (Yamakawa et al., 1999). Therefore, it is important to have a monitoring system built into the major structure. To some extent, the safety monitoring of tunnels and bridges face the same challenges as in the case of buildings. For structural deformation monitoring the most important requirement is strain measurement. The continuously measured strain provides valuable information concerning the integrity of the structure, the stiffness of the structural members, and the load level. Damage induces changes in the load carrying behaviour. Meanwhile, the strain sensors should satisfy the need for long-term stability of the output data achieved by a quasi-calibration-free measuring system. Hence conventional electrical resistance strain gauges appear to be not suitable for long-term measurements, due to the almost unavoidable “drift” of the strain readings. In general, until recently, monitoring the dynamic response of civil structures for the purpose of assessment and mitigation of damage has relied on measurements by accelerometers deployed on the structure of interest. Studies conducted on such data records have been useful in assessing structural design procedures, improving building codes and correlating the response of the structure with the damage caused. However, an integration process is normally required to arrive at the relative static displacements, and thus they cannot offer online solutions. In contrast, the Global Positioning System (GPS) can measure directly the position coordinates (e.g. Parkinson & Spilker, 1996), hence providing an opportunity to monitor, in real-time, the dynamic characteristics of the structure to which the GPS antennas are attached. Preliminary studies have proved the technical feasibility of using GPS to monitor dynamic structural response due to winds, traffic,  earthquakes and similar loading events (Ashkenazi & Roberts, 1997; Tamura et al., 2002). Although GPS offers real-time solutions, it has its own limitations. GPS can only give us the overall deformation of the building and thus little will be revealed regarding the location of the actual deforming position. Moreover, GPS can only sample at a rate of up to 20Hz (Trimble, 2003). On the other hand, more and more fibre optic sensors have  been investigated and applied in civil engineering structures. The Fibre Bragg Grating (FBG) is one of the most  promising new technologies in fibre optics for strain and temperature measurements, because such FBG-based sensors have many advantages over conventional electronic sensors. They are characterised by a very good long-term stability and a high reliability, in addition to all the general advantages of glass fibre-based sensors, such as electromagnetic insensitivity, small size, and the possibility to distribute several sensors in one fibre. By embedding the optic fibre sensors into a structure to obtain useful information both during the construction phase and in the long term can be a very efficient means of monitoring the structure’s health. In other words, FBG sensors can be embedded into concrete, glued or welded to a steel structural member, so that we can have the structure’s “inner” deformation monitored. Therefore GPS and FBG are complementary, and the authors propose to integrate them with the traditionally used accelerometer, in order to address the application of structural deformation monitoring. The rest of this paper is organised as follows: the second  part will introduce the principle of the FBG sensor, an experiment to explore its linearity and repeatability, and the experimental results from a low-cost FBG strain measurement system. The third part will present an analysis of data collected using GPS and accelerometer sensors while monitoring the deformation of a 108m tall steel tower in Japan subjected to the stress of the Typhoon No. 21 that struck on 1 October 2002. The fourth part will discuss the integration of FBG, GPS and accelerometer sensors; and finally we conclude with a summary of the research findings. 2. THE OPTIC FIBRE BRAGG GRATING SENSOR 2.1 The FBG sensor The Bragg grating in the fibre can be formed by exposure of the core of single-mode glass fibre to an intense optical interference pattern, which effectively creates a single axis strain and temperature sensor on the core of the fibre. The Bragg grating resonance is the centre wavelength of light reflected from the Bragg grating, and depends on the effective index of refraction of the fibre core, as well as the  periodicity of the grating. Both the effective index of refraction and the periodic pitch between the grating planes will be affected during strain and temperature changes. Strain affects the Bragg response directly through the expansion or contraction of the fibre of grating elements and through the strain-optic effect. There are various schemes for detecting the Bragg wavelength shift, which is extremely sensitive to the applied strain (Othonos, 1997). The Bragg wavelength of a grating is given by: Λ= n  B 2 λ   (1) where Λ is the grating periodic spacing and n  is the effective index of the fibre core. The shift in the Bragg grating central wavelength due to strain l l  ∆  and temperature T  ∆  change can be expressed by: T T nT nl l nl n  B  ∆∂Λ∂+∂∂Λ+∆∂Λ∂+∂∂Λ=∆ )(2)(2 λ   (2) The strain effect on the optic fibre is represented by the first term in Eq. (2), and can be presented as: ε λ λ  )1( e B B  p −=∆  (3) where ε  is the applied strain, and the e  p  is an effective strain-optic constant which is defined as: )]([2 1211122  p pv p n p e  +−=  (4) 2.2 Testing the performance of FBG as a strain sensor for structural deformation measurement In order to ensure that the FBG can be efficiently used as a strain sensor for structural deformation monitoring, we have setup an experimental system to study the FBG characteristics, as shown in Figure 1. A length of optic fibre with a FBG was glued on two fixtures at points A and B on a slide-track, in order to measure the average strain over length AB. The difference between the two fixtures is that  point A is moveable along the track controlled either by turning the rocker arm or running a stepper motor while B is firmly fixed. A broadband light from the light source can  be injected into the fibre and then reflected by the FBG. The reflected light will be wavelength shifted if there is strain applied on the FBG according to Eq. (3), and passed through the 3dB coupler to be detected by the high- precision OSA (Optical Spectrum Analyzer Agilent 86140B), which determines the peak wavelength of the reflected Bragg signal. The reflected peak shifts to higher or lower wavelength following the axial strain and temperature change. Hence, through measuring the central Bragg wavelength the related strain can be evaluated. The spectrum analysis result can also be recorded by the built-in computer (PC) of the OSA.  PCRocker Arm ABOptical FibreLight sourceOSA3dB coupler   Figure 1 Strain measurement setup. The central wavelength of the tested FBG is 1555.191nm on the morning at 10am on the day of testing. The OSA auto-measure reference level was -20.12dBm, and the reflected power of the signal from the FBG was -27.99dBm. The slide rocker arm was turned to move fixture A at 0.1mm per step, in order to stretch or release the fibre and apply strain on the FBG. The data were recorded and  plotted as green/grey in Figure 2 (Case A). After the above experiment was completed, an air conditioner was started running to lower the temperature in the laboratory. After 4 hours the experiment was repeated. The recorded data were plotted as blue/black in Figure 2 (Case B). This time the central wavelength of the same tested FBG was at 1555.184nm, which is a 0.007nm shift from the previous case. Also the OSA auto-measure reference level was -20.17dBm, and the reflected signal  power was -28.01dBm. Comparing the two data sets collected under different room temperatures, the strain vs. wavelength shift relationship (the linear coefficient) remained unchanged, although the central Bragg wavelength was changed because of the room temperature change. Figure 2 Measured strain vs. wavelength at two different room temperatures. The factor e  p  (Kersey et al., 1997) has a numerical value of about 0.22. Therefore the measured strain response at constant temperature can be found by theoretical calculation. In our experiment, we produced a maximum strain of 1245.158µ ε  with the Bragg central wavelength shift of 1.313nm at 1555.191nm (Case A). Thus, the theoretical wavelength shift was 1.510nm from Eq. (3), and the actual wavelength shift 1.313nm. The difference  between the theoretical  Bt  λ  ∆  and the actual  Bp λ  ∆  per 1000µ ε  is: ε λ λ  /)(1000  Bp Bt   ∆−∆=∆  =1000(1.51-1.313)/1245.158 =0.158 (nm) and similarly for Case B, of 1555.184nm Bragg central wavelength, the theoretical wavelength shift of 1.678nm with applied strain of 1383.509µ ε , but the actual wavelength shift being only 1.472nm. Hence, the difference  per 1000µ ε  is 0.149nm. The strain-optic efficiency  ρ  in the experiment is given by: )/(1005.1 158.1245313.1 3 µε ε λ  ρ  nm  B  − ×==∆=  for Case A, and for Case B: )/(1006.1 509.1385472.1 3 µε  ρ  nm − ×==  The results have shown us that the FBG sensor has a very good linearity and repeatability with respect to the applied strain, under different temperature conditions. 2.3 Low cost strain measurement system Because strain change has been modulated as wavelength shift, and almost all the photo-electrical detectors (PD) are only sensitive to optical intensity (NOT the wavelength), the straightforward approach of detecting the wavelength shift by using an OSA is a truly expensive option. In order to develop a cost-effective FBG deformation monitoring system it is crucial to design a low-cost, yet sensitive, demodulation scheme so that strain induced wavelength shift can be effectively converted into optical intensity change on the detector. Figure 3 shows such a demodulation scheme designed at UNSW. The system consists of three parts. The first is the strain sensing sub-system consisting of a FBG sensor between A and B in an optic fibre, a 3dB coupler, and a broadband source. The second part is the tracking sub-system consisting of a FBG filter, piezo-electric transducer (PZT), a 3dB coupler, two PDs and an oscilloscope. The third part is the control and data acquisition sub-system consisting of a PC, an analog-to-digital converter (ADC), and a stepper  motor. The whole system has been developed as a real-time data acquisition, signal processing, and analysis system. For the optical portion of the system, light generated by the  broadband source is injected into the fibre through the coupler C, and then modulated and reflected by the FBG. The reflected light passes through coupler C and is split into two parts by coupler at D: one is directly detected by PD at E, and another passes through FBG filter and detected by PD at F. For the electrical portion of the system, a PC will control the stepper motor to move fixture B on the slide-track in order to generate strain change on the FBG. The output from PD at E will remain constant because it is NOT sensitive to wavelength shift. On the other hand, the output from PD at F will change with the strain change because wavelength shift introduced at the FBG sensor will damage the alignment between the reflection spectrum of the FBG sensor and the transmission spectrum of the FBG filter (more details later). After the ADC, the PC will be able to  pickup this misalignment caused by the strain change on the FBG sensor and then control the PZT to apply the same strain change to achieve a new alignment between the two spectrums. Hence, the strain change at the FBG sensor will  be tracked by the PZT. PDPCStepper Motor  ABOptical FibreBroadband source3dB coupler PDOscilloscopeFBG Sensor FBG Filter  ADCPZTCh1Ch2CDEF  Figure 3 The low cost FBG strain measurement system. The measurement is based on the applied strain into the sensing FBG, which can be adapted to direct concrete embedding or surface mounting in a real structure, and the output optical intensity change (in the Figure 2 dots block). The efficient strain measurement gauge is the length  A l   of the fibre. In the system, a pair of 1555nm FBGs have been used one as the sensor and another as the filter. The oscilloscope caught the voltage signals coming from the  photo detectors. The reflected Bragg wavelength of the sensing FBG is modulated by applied strain, and split into the filter and reference legs. In the filter leg, signal was coupled with the transmitted Bragg wavelength of the filtering FBG then detected by a photo detector, picked up  by Ch1 of the oscilloscope. The reference leg gives us the strain modulated signal intensity level presented in voltage through Ch2. The reflected Bragg spectrum for the sensing FBG is shown in Figure 4, and the central wavelength is 1555.182nm with bandwidth of 0.515nm. Meanwhile, the transmitted Bragg spectrum for the filtering FBG is shown in Figure 5, with the central wavelength at 1555.135nm and  bandwidth of 0.405nm. The initial strain-free condition can give us a minimum voltage from Ch1, because the reflected signal is exactly aligned to the transmitted signal and is attenuated at its maximum. If the two spectrums move away from each other, the output voltage from Ch1 will increase. However, if there is no overlap between them anymore, the output voltage from Ch1 will NOT change even if the strain applied on the sensing FBG continues to increase. Figure 4 Reflection spectrum of the sensing FBG. Figure 5 Transmission profile of the filtering FBG.   Now we set the voltage difference between Ch1 and Ch2 to zero in this stain-free condition, i.e.: 0 21  =•−=∆ ChCh V C V V   C is equivalent to the maximum attenuation of the FBG filter. Because strain causes the central Bragg wavelength of the sensing FBG to shift, the voltage difference will be no longer zero if the sensing FBG is driven to stretch or release  by the stepper motor. The non-zero voltage V  ∆  will be detected by the PC, and used to control the PZT to stretch or release the filtering FBG in order to induce a compensating wavelength shift. When the two FBGs’ Bragg central wavelengths are aligned again, the output voltage becomes zero. The strain deduced from the PZT is  precisely the strain applied to the sensing FBG. The reflected and transmitted Bragg signals have quite different amplitudes due to 95% of the injected light being reflected. And they all have their own slightly different  bandwidth although they are paired up. The reflection and transmission Bragg spectrums will have an overlapping range. In order to analyse the accuracy of the system, two types of data were collected. The first one is to read the applied strain with respect to unit output voltage change. Another is to read the output voltage with respect to unit displacement applied. Figures 6 and 7 show the readings. Y1 and Y3 are when the sensing FBG was stretched, and Y2 and Y4 when it is released gradually. In Figure 8 we combine the results from Figures 6 and 7. Moreover, an independent dataset obtained by using a smaller scale reading is shown in the black curve (y5). From Figure 8 what we can see is that the FBG strain measurement system does have very good performance in terms of reliability and repeatability. 00.20.40.60.811.21.40100200300400500600700Voltage (v)    A  p  p   l   i  e   d   M   i  c  r  o  n   S   t  r  a   i  n Data with unit voltage change in large scaleY1Y2  Figure 6 Strain applied on the sensing FBG with respect to unit output voltage change. 00.20.40.60.811.21.40100200300400500600700Data with Unit displacementVoltage (v)    A  p  p   l   i  e   d   M   i  c  r  o  n   S   t  r  a   i  n   Y3Y4   Figure 7 Output voltage with respect to unit displacement applied on the sensing FBG. 00.20.40.60.811.21.40100200300400500600700Voltage (v)    A  p  p   l   i  e   d   M   i  c  r  o  n   S   t  r  a   i  n Data AnalysisY1Y2Y3Y4y5  Figure 8 Three sets of measurements superimposed together. The data reading error by using the oscilloscope can be as much as half of the smallest division of the scale used. In the data reading, 0.04v is the smallest division, the voltage difference between Ch1 and Ch2 with the 40mv output is induced by applying 69.17µ ε  strain. Thus, the minimum detectable stain will be 34.59µ ε . As can be seen from Figure 8 the output voltage dynamic range is 0-1.25v. When the total offset occurred (i.e. the spectrum overlap  between sensing and filtering FBGs becomes zero), it  becomes a constant of 1.25v. So that through a low cost ADC of 12 bit resolution, a stain resolution of up to 0.264µ ε  can be achieved. 3. THE GPS & ACCELEROMETER SENSORS FOR DEFORMATION MONITORING 3.1 Data collected At the top of an actual 108m high steel tower in Japan, a GPS antenna together with accelerometers and an anemometer were setup. Another GPS antenna was setup on the top of a 16m high rigid building as a reference point
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