application (iii) the sensing performance of customized sensor protection package BOTDR sensor that manufactured involving layers of fiber reinforced ...
Customization and calibration of BOTDR sensors for underground structural health monitoring Wai Lun Sham*a, Yaowen Yanga, Mulay Pravina School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
a
ABSTRACT In recent years, it is becoming more common to use fiber optic sensors (FOS) in the structural health monitoring (SHM) sector, especially in the civil engineering field. A number of surface-mountable sensor system for FOS have been developed in the past years, the recent development of Brillouin Optical Time Domain Reflectometry (BOTDR) was a great evolution towards the SHM system development, it inspired the new edge of FOS SHM system. Different from the traditional monitoring instruments, it provides distributed, long distance, real-time, interference free and high accuracy/precision measurement data. It is now possible to achieve “continuous” measurement data and this SHM technique is applicable in area that is inaccessible. The research aims to solve the problems which exist in the convergence measurement using the conventional measuring methods, however, there is still a gap between the lab experiments and field applications. Limited research has been conducted on how to maximize its possible applications due to its brittle and fragile material nature. A number of additional considerations for a successful pairing of these two must be taken into account for successful field applications. This article provides a short review on underground monitoring techniques and FOS SHM systems. The focuses is on examine (i) the feasibility and effectiveness of different BOTDR sensors installation methods (ii) the suitable commercially-available o sensing cable for underground application (iii) the sensing performance of customized sensor protection package BOTDR sensor that manufactured involving layers of fiber reinforced composites. This research serves a bridge in between the technology advancement to the creation of a structure health monitoring system with practical application, numerical simulation and theoretical analysis aspects, and also to provide the insights into the mechanisms of BOTDR. Keywords: BOTDR, optical fiber, structural health monitoring, underground structure, rock cavern
1. INTRODUCTION Civil structures are ubiquitous in every society, irrespective of geographical location, culture and economical development. It is hard to imagine the world without bridges, buildings, roads, tunnels and railways. Structures can affect human, economical, cultural and aesthetic aspects of the society, thus, good design, excellent construction and safe development of structures are the goals of structural engineering. Structural health monitoring (SHM) is a process designed to provide accurate and in-time information relating to structural performance and condition. New materials, new construction technologies and new structural systems are increasingly being used, and it is necessary to increase knowledge about their on-site performance, to control the design, to verify performance, and to create and calibrate numerical models (Bernard, 2000). Even though quite a number of new SHM methods are available in the industries, especially those specifically used for strain and temperature detection purposes, the industries are still using traditional equipments like strain gauges with data logger and load cell to monitor the defects in structures. These traditional methods have their own limitations like limited distance for the sensors to measure strains along the large structure, and the long tangling wires from the gauges to the data logger may bring obstructions to the surrounding. The emergence of optical fiber sensors has brought about various solutions to overcome the limitations of traditional sensing methods, especially in terms of large scale distributed monitoring of structures. Distributed sensing techniques based on the Brillouin scattering – Brillouin optical time-domain analysis (BOTDA) and the Brillouin optical timedomain Reflectometer (BOTDR), a distributed optical fiber strain sensor based on Brillouin scattering, is used in this project. BOTDR can measure strain and temperature along an optic fiber. The BOTDR’s particular advantage is that it
Health Monitoring of Structural and Biological Systems 2011, edited by Tribikram Kundu, Proc. of SPIE Vol. 7984, 79842A · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.880345
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can measure continuous strain or temperature along several tens of kilometers by accessing only one end of the sensing fiber. This capability is extremely useful for health monitoring and diagnosis of long and large infrastructures (B.Shi, 2004). This technology development has attracted lots of attention in recent years, as they have the capabilities of providing flexibility and accurate measurement of strain and temperature at any point along the fiber optic sensor. This special sensing technique is not only applicable to buildings, other structures like tunnels, underground caverns, bridges and even landslide area have been installed with such sensors for displacement, object movement or temperature sensing purposes. However, such promising and practical sensing technique still has several obstacles in its application; the high cost and unreliability associated with sensor selection, installation and failure. Selection of suitable fiber optic cable as the sensor is a tedious process, as different material of fiber optic cable shows different strength, surface bonding characteristic and sensitivity to strain and temperature. Reliable packaging technique for fiber optic cable is critical to ensure reliable sensing in structures. Another main concern is the installation method for the sensor. Rough construction conditions or harsh environments have posed great challenge to determine an effective and sound installation method for the fiber optic sensor. All these challenges have motivated the current study. 1.1 Current Practice Monitoring of underground or high rise civil structures during construction and operation should be real-time and continuous, and preferably in an automatic way. Reliable data should be available to the engineers and designers at minimum time intervals. A very quick feedback loop will then allow engineers and designers to modify and reinforce the structures based on the monitoring data. The current monitoring of structures especially underground structures is mainly based on existing conventional monitoring instrumentation (e.g. load cells or strain gauges or LVDTs) with electrical output and reading units, and is usually conducted manually at specific periods (not in a continuous way). For example, in the Mindef UAF cavern in Singapore, borehole extensometers and convergence tapes were monitored by conventional Electric Strain Gauges (ESGs) and monitoring data were acquired manually at specific periods. It was found that the existing instruments of ESG type lack consistency, and their reliability tends to diminish with time. Moreover, inaccurate results were to observe on convergence tapes due to the ventilation in the underground environment. Vibrating wire strain gauges (VWSGs) seem to offer a better solution but are too costly and prone to mechanical noise. Furthermore, ESGs and the VWSGs warrant long cables for data transmission which renders these sensors susceptible to electro-magnetic interference (EMI), which further contaminates the measurements. Therefore, it is desirable to develop such a system, operating in an automatic and continuous mode, for monitoring of structure, especially for underground structures. It is important to have accurate and real time monitoring on the safety assessment, those evaluations are carried out by engineers trained in visual inspection, which sometime can be inaccurate due to personal experience differences on the safety condition assessment generated by this practice. To increase the inspection efficiency and accuracy, various sensors have been developed and being demonstrated in the field. Among many sensors being used fir civil structural monitoring, fiber optic sensors which has been widely developed and used in the aerospace, automotive and defense industry, are one of the most promising candidates due to their features of durability, stability, small sizes and insensitivity to external perturbations, with conceptual models developed for the sensor location, signal transmission and central processing of information for simple structural systems (Choi et al., 1990; Nee, 1990; Spyrakos et al., 1990; Wu, 1990). Laboratory and field experimentation with frame structures and bridges have shown promise for identification of system behavior and critical parameter benchmarking (Lu and Askar, 1990; Agbabian and Masri, 1988; Beliveau and Huston, 1988; Biswas et al., 1989), which makes them ideal for the long-term continuous health monitoring system for civil structures (Tennyson et al., 2000; Culshaw, 2004). Fiber optical sensors, such as Bragg grating, long gauge or Fabry Perot type of sensors are point sensors that provide local stress information at pre-determined specific points. However, for SHM the cracks and displacement locations are often unknown in advance, and thus it will be difficult for point sensors to be placed at the right point to be close to “potential” crack, deformation or buckling locations. Because of the single point detection, it is unlikely to accurately correlate the strains at the different locations to the status of the structures, as the critical location may change after some event. Point sensors may lose information about the change of critical location, which is directly related to SHM in which the structural strain monitoring is required to assess the safety of the structures. This could be achieved with the distributed fiber sensors to cover the large areas of the civil structures and to access the safety and the status of the
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structure. This kind of sensor has the advantages of a long sensing range and capability of providing the strain or temperature at every spatial resolution over the entire sensing fiber imbedded in or attached on the structures by using the fiber itself as the sensing medium. Spatial information along the length of the fiber can be obtained through Optical Time Domain Reflectometry (OTDR), measuring pulse propagation times for light traveling in the fiber. The length covered by the optical pulse is the spatial resolution. This allows for continuous monitoring of the structural strain in a distributed fashion, which can be obtained by the distributed Brillouin scattering or Rayleigh scattering based distributed sensing as it provides long sensing length for the measurement of the temperature or strain. Currently there are two Brillouin scattering configurations commercially available: (a) Brillouin backscattering ( ANDO, 2005) in which a single source of light is used as the probe, and (b) stimulated Brillouin (Omnisens, 2005) in which a pump laser light wave enters one end of the fiber and a counterwave probe enters the other end. The interaction between the pump and the probe singles maximizes the information available from the scattering, and hence increases significantly the accuracy of the strain estimate (Figure 1). The backscattering Brillouin configuration has a certain advantage over the stimulated approach in case a fiber breaks, as only one end of the fiber is required to obtain measurement.
Figure 1 – Time and Frequency Analysis of Fiber
Therefore, to further enhance the applicability of such an advantageous technology development, this The research will focus on examine (i) the feasibility and effectiveness of different BOTDR sensors installation methods (ii) the suitable commercially-available o sensing cable for underground application (iii) the sensing performance of customized sensor protection package BOTDR sensor that manufactured involving layers of fiber reinforced composites. This research serves a bridge in between the technology advancement to the creation of a structure health monitoring system with practical application, numerical simulation and theoretical analysis aspects, and also to provide the insights into the mechanisms of BOTDR.
2. EXPERIMENTAL TESTING The usual sensors installation method by adhering the sensors directly onto the structure surface is not applicable for underground tunnels and caverns, with non-façade surface finishes (N. Hiroshi et. al., 2001). Alternatively, the authors have chose and investigated in the clamping method which found to be practically suitable as an installation method for
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underground structure monitoring application. This method of installation is then further applied to investigate three different BOTDR sensors of different protection package. The three sensors are: a)
Fiber optic sensing cable with tight buffer – thin and soft coating in nature, this sensor is usually used in laboratory testing.
b)
Loose tube tight buffer sensor with Kevlar layer and PE jacket coating – soft and flexible coating, this sensor is usually used for temperature sensing.
c)
Special stain sensing cable – taught coating with layers of tightly glued FRP jacket, this sensing cable is specially developed with requested specification from the research group.
2.1 Experiment and Set-up The main focus of this experiment is to compare the strain measured using the BOTDR sensing method with the data obtained from the strain gauges fixed along the specimen, under the compressive load applied to the universal column steel specimen as shown in Figure 2 below. Results of different types of optic fiber cable will then be compared and discussed. The experiment setup consisted of 6 main components, namely the DiTest STA100/200 Fiber Optic Brillouin Analyzer, Optical Fiber Sensors, strain gauges and LVDT connected to an Analogue Data Logger System, Compressive Strength Test Machine and the test specimen. Before the setting up, plastic clamps (Figure 3), which are to be used to hold the optic fiber cable onto the test specimen, marking will be done to ensure that the cable is clamped at the correct position when it is installed onto the side of the web of the specimen. During the marking of the cable, a pre-tension of 0.2% of the 3m tested length of optic fiber cable is taken into consideration and marked on the cable. This is to ensure that it has already been strained with a known percentage for easier calculation and comparison in the later stages. Once the installation process was completed, the specimen, with the strain gauges and the optic fiber cable attached to it, will be mounted onto the compressive strength test machine. Compressive loading of 50kN increment was applied to the specimen from 0kN to 250kN. At 0kN, both the readings from BOTDR and strain gauges were recorded first as the initial readings before any load was applied. At every 50kN interval, both readings from the BOTDR and the strain gauges were recorded. A minimum of 2 readings were taken for each interval so as to minimize random error and obtain a reliable and consistent result with high accuracy.
Compression Testing Machine
3.6m long UC specimen
Figure 2 – Compressive strength test machine with loaded specimen
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Clamper installed on specimen
Figure 3 – Clamper fabricated and installed on specimen
2.2 Result and Discussion Figure 4 shows that the micro-strain readings of the strain gauges are quite consistent and uniform as the shape of the graph of each location are quite similar along the deviation in micro-strain readings increment for every additional load of 50kN. We can conclude that as the compressive loading increases, the strain gauges readings tend to deviate and have a higher inconsistency. However, this inconsistency does not really affect the reliability of the strain gauges values as the deviation is considered negligible and minimal. This shows that the strain gauges values are reliable to be used for comparison with the strain values from the BOTDR sensing cables.
Micro-strain (Strain Gauges) VS Load Micro-strain (μe)
0
0 0
100
200
-100
300
0.2 0.75
-200 -300
1.3 1.5 Load (kN)
1.7
Figure 4 – Micro-strain VS Load at different locations along the specimen length
Figure 5 below shows a plot of micro-strain values of the tight buffer sensing cable versus the strain gauges reading along the effective length of the cable for each individual load case. It shows that the micro-strain measurement obtained from BOTDR have a general shape and they are quite consistent along the effective length of the specimen the reading decrease as the compressive loading increases. A comparison with the strain gauges values is plotted against the BOTDR measurement since it is presumed that the conventional strain gauges measurement are reliable and have a high degree of accuracy in measuring micro-strains. The results show that the accuracy of the BOTDR measurement is considered acceptable since the deviation of its reading against the strain gauges results is consistence and within the ±10% range, the convergence of reading noticed at the beginning and ending section of the cable is believed due to the localized stress induced by the clip. However, the protection package is too fragile and not suitable for site application but good to serve as a control reference in laboratory testing.
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Tight Buffer Cable vs Strain Gauge Length (m) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
Micro-strain (um)
0
50kN - TB 100kN - TB 150kN - TB 200kN - TB 250kN - TB 50kN - SG 100kN - SG 150kN - SG 200kN - SG 250kN - SG
-50 -100 -150 -200 -250
Figure 5 – Micro-strain comparison of tight buffer cable vs Strain gauge results Figure 6 shows the measurement result of the tight buffer sensing cable with Kevlar layer and PE jacket coating, the author believes that due to the soft coating of the sensor protection package, the BOTDR measurement varies largely before and after the clamping point located at 1.5m length. The accuracy and consistence seems not satisfactory when it is compared against the strain gauge measurement.
Tight Buffer with Kevlar & PE Coating vs Strain Gauge Length (m) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
0
50 kN - K&PE 100kN - K&PE
Micro-strain (um)
-50
150kN - K&PE 200kN - K&PE
-100
250kN - K&PE -150
50kN - SG 100kN - SG
-200
150kN - SG -250
Figure 6 – Micro-strain comparison of tight buffer with Kevlar & PE coating cable vs Strain gauge results Figure 7 shows the comparison of BOTDR measurement result from the special strain sensing cable against the strain gauges measurement. Both measurements matched each other very well with slight variation. The special developed strain sensing cable shows high accuracy and consistence in terms of strain measurement application
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Special Strain Sensing Cable vs Strain Gauge 0.16
Length (m) 0.569 0.977 1.386 1.794 2.202 2.611
100kN - SS
0 Mirco-strain (um)
50kN - SS
150kN - SS
-50
200kN - SS -100
250kN - SS
-150
50kN - SG 100kN - SG
-200
150kN - SG -250
Figure 7 – Micro-strain comparison of special strain sensing cable vs Strain gauge results
3. CONCLUSION After a serious of experiments, the author can compare the performance of the three different BOTDR sensing cables (Figure 8). The tight buffer sensing cable, having good accuracy and consistence in strain measurement but the protection package is too fragile and soft, it is suitable for laboratory experimental testing but definitely not suitable for any site application. The tight buffer with Kevlar and PE jacket coating sensing cable, although it does not appear to suitable for capturing strain measurement but this protection package having a good resistance against moisture penetration , it can serves as a good BOTDR temperature sensor since no pre-tension of sensing cable is not require for temperature sensing during sensor installation. The special developed strain sensing cable shows good performance in strain measurement. The glued interconnected layers minimize the point location shift of the optic fiber core against the protection layers, this also ensures the exact location of the clamping point and measurement point as recorded along the fiber length. The protection package is also toughest of the three experimented sensing cable. This BOTDR sensing cable is therefore, recommended to be used for strain measurement in underground application.
Comparison of Micro-strain for all Teseted Sensors @ 200kN Load 0.14 0.34 0.55 0.75 0.96 1.16 1.37 1.57 1.77 1.98 2.18 2.39 2.59 2.79 3.00 3.20 3.41
Length (m)
Micro-strain (um)
0
200kN - SG
-50 -100
200kN - TB
-150
200kN - K&PE
-200
200kN - SS
-250
Figure 8 – Micro-strain comparison of three cables vs Strain gauge results
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