Bridge Health Monitoring

Transcription

1 Bridge Health Monitoring with Fiber Optic Sensors White Paper Cleveland Electric Laboratories 11/6/2008

2 Disclaimers The information contained in this document is the proprietary and exclusive property of Cleveland Electric Laboratories except as otherwise indicated. No part of this document, in whole or in part, may be reproduced, stored, transmitted, or used for design purposes without the prior written permission of Cleveland Electric Laboratories. The information contained in this document is subject to change without notice. The information in this document is provided for informational purposes only. Cleveland Electric Laboratories specifically disclaims all warranties, express or limited, including, but not limited, to the implied warranties of merchantability and fitness for a particular purpose, except as provided for in a separate software license agreement. Privacy Information This document may contain information of a sensitive nature. This information should not be given to persons other than those who are involved in the evaluation of the products and services of Cleveland Electric Laboratories. Bridge Monitoring with Fiber Optic Sensors.doc Page 2 of 35

3 Table of Contents Introduction... 4 FBG Basics... 7 FBG Multiplexing FBG Strain Sensors FBG Accelerometers FBG Sensor Instrumentation FBG Sensor Software FBG Sensor Systems Summary Appendix Bridge Monitoring with Fiber Optic Sensors.doc Page 3 of 35

4 Introduction The August, 2007 collapse of the I35 bridge in Minneapolis has heightened concern about the need to critically evaluate and monitor the condition of the nation s 600,000 bridges. As many as 25% of these structures have been classified as deficient, including some of Ohio s 42,000+ bridges. This document discusses technologies that can substantially help you to evaluate and monitor such bridges. A need for enhanced bridge monitoring The Ohio Department of Transportation (ODOT) has the responsibility to inspect nearly 15,000 bridges once each calendar year. Its bridgeinspection manual 1 details a variety of inspection locations, equipment, and evaluation criteria. Doubtless some of these inspections will always require direct, on-site human judgment. However, the visual and manual hands-on nature of routine inspections limits the ability of bridge inspectors to detect hidden weaknesses or impending failures an ability further dependent on the training, skills, and judgment of individual inspectors. In Minnesota, routine inspections were inadequate to predict the I35 bridge failure. Closer to home, ultrasonic metal-thickness tests apparently not routine during earlier Ohio inspections 2,3 detected unexpectedly thin gusset sections on Cleveland s Inner Belt Bridge. Corrosion had reduced the thickness of ½ gussets to as little as1/8, resulting in the need for unanticipated repairs. 4 Using present methods, the inspection frequency for 15,000 bridges is necessarily time and budget limited. Yet unexpected deterioration can occur between annual inspections. Even more frequent inspections with traditional tools and methods still cannot directly evaluate the effects of dynamic loading conditions neither at the time of the inspection nor during normal traffic conditions. Dynamic stress on a member cannot be estimated directly without strain measurements, and abnormal vibrational modes cannot be detected without accelerometers or high-speed strain measurements Ohio Department of Transportation, Manual of Bridge Inspection, November, Ibid, pages 90-92, section 7.2, Inspection Tools and Equipment. However, new, multi-point ultrasonic technology will make such inspections more effective and perhaps more routine. See Scott Varner, Futuristic technology helps keep bridges safe. ODOT Transcript newsletter, July/August 2008 Laura Johnston, Bridge undergoes safety repairs, Cleveland Plain Dealer, May 23, Bridge Monitoring with Fiber Optic Sensors.doc Page 4 of 35

5 Enhanced monitoring with fiber optic sensors Of course, a complete solution to all of these problems would be the development of non-deteriorating bridges. A less desirable but more realistic solution would be smart structures on bridges that report their problems to us. Reality and the Second Law of Thermodynamics prohibit perfect bridges. But proven, presently available fiber-optic technology can at least partially convert new and existing bridges to smart structures. Placing a network of fiber-optic strain sensors on a bridge s fracturecritical structural members, or at least on its especially vulnerable members, enables these structural members to report even alarm their problems in either of the following ways: Continuously and automatically, without anyone visiting the bridges, using fixed instrumentation at the bridges and internet/satellite communications to a central remote monitoring site(s). Multiple bridges can be monitored simultaneously from anywhere in the world. Periodically, to a portable instrument that an inspector can connect to the sensor network quickly and easily say at peak traffic times. In this way one inspector can check many bridges relatively frequently with a single portable instrument Fiber-optic strain sensors on these structural members can continuously or periodically report dynamic loading conditions, both in the deck and superstructure. This capability applies both to new and already-deteriorated bridges. Engineers can initially evaluate existing load capabilities, such as strains and calculated stresses before and after driving a known load(s) onto the bridge. Such information provides baselines against which future deterioration is judged. Fiber-optic accelerometers and strain sensor systems on these members, in combination with a routinely supplied Fast Fourier Transform (FFT) analysis routine, 5 can continuously or periodically display structural frequency spectra, thereby alerting engineers to abnormal resonance characteristics under various loads. Fiber optic strain sensors excel in structure- integrated, real-world, toughenvironment, long-term monitoring applications. They can be tack welded, cemented, or bolted to structural members even welded to rebar and/or embedded in concrete. 5 See FBG Sensor Software on page 26. Bridge Monitoring with Fiber Optic Sensors.doc Page 5 of 35

6 But why, specifically, use fiber-optic sensors? Established, proven technology. Fast, accurate & field-reliable. Fatigue and drift resistant. Immune to electromagnetic interference. Resistant to corrosion, chemicals, water, and lightning. Tolerant of low and high temperatures (-40 to +150 C). Ideal for long-range and remote monitoring. The variable wavelength not variable amplitude sensor signals from FBG fiber-optic sensors can travel long distances, even kilometers, without error. Small, light weight, and easy to install. Surface-mountable or embeddable into structures Installable and monitorable in multiples on one optical fiber. (>100 sensors/fiber in some cases). Each sensor on the fiber is wavelengthspecific, and so is implicitly identifiable and multiplexable. This capability greatly simplifies and streamlines installation, cabling, and instrumentation, as illustrated below: Cabling and instrumentation for 400 wired strain sensors Field cabling and instrumentation for 640 multiplexed fiber-optic strain sensors, inside conditioned environmental enclosure. What type of fiber-optic sensors to use? FBGs (Fiber Bragg Gratings) are the most common type of fiber-optic sensors for structural health monitoring. FBG technology is well established. The first commercial FBGs became available in 1995, and many manufacturers now supply them. Cleveland Electric Labs monitors structural health using FBG sensors and interrogates these sensors converts their signals to usable data using the best instrumentation available. Bridge Monitoring with Fiber Optic Sensors.doc Page 6 of 35

7 FBG Bas sicss FBG introduction FBGs are, effectively, optical filters that are created integrally within the tiny cores of optical fibers, using powerful lasers. When broad-band infrared light enters one end of an FBG-containing fiber, a very narrow band of wavelengths is missing from the light transmitted out the other end: Input wavelength Output (transmitted) wavelength However, the missing light not lost but rather reflected back to the input end of the fiber: λ Bragg Reflected wavelength The peak of this reflected band is called the Bragg wavelength, λ Bragg. Because they reflect narrow bands of wavelengths, FBGs were first used in fiber-optic communications and are still used for separating and redirecting wavelength-specific signals. However, users soon realized that fiber deformation and temperature affect the Bragg wavelength in a useful way for sensor applications (and presumably in a less-than-useful way for many communications applications). As a result, FBGs are used today as the active elements of a wide variety of sensors, including strain sensors, accelerometers, pressure sensors, and temperature sensors. Bridge Monitoring with Fiber Optic Sensors.doc Page 7 of 35

8 FBG material the single-mode optical fiber An FBG is fabricated on a single-mode optical fiber, a cross-section of which is illustrated below: When light enters the fiber nearly parallel to its axis the norm it travels long distances almost 100% in the core, with minimal distortion: The light internally bounces off the cladding, because 1) the cladding refractive index (n d-cladding ) is less than the core refractive index (n d-core ) and 2) per Snell s law, the internal angle of the light relative to the cladding is less than the critical angle of reflection. The fiber s single-mode property helps the many light waves going into and reflected out of the FBG to stay in phase during their trip through the fiber. As a result, FBG sensor signals can travel kilometers with minimal loss or distortion. FBG construction Bridge Monitoring with Fiber Optic Sensors.doc Page 8 of 35

9 FBG function The diagram above illustrates, in a simplified way, how an FBG is constructed. A very short section of fiber is irradiated with uniformly spaced beams of light from a powerful laser usually an ultraviolet laser. The irradiated areas, shown as yellow rectangles, undergo internal electronic and/or physical changes. In turn, these changes modify the refractive index of the irradiated areas, causing optical discontinuities The spacing between the refractive-index discontinuities is tiny, on the order of 500 to 550 nanometers roughly half of a millionth of a millimeter and there are thousands of these deliberately-created discontinuities. Each time light in the fiber encounters the FBG s refractive index discontinuities, it reflects very slightly. For most wavelengths of light inside the fiber, these thousands of minute reflections ultimately become out of phase with each other and tend to cancel. However, at in-fiber wavelengths approaching twice the spacing of the refractive-index discontinuities the FBG s grating period, Λ the reflections are in-phase and interfere constructively. That is, they reinforce each other as illustrated below: NOTE: Here and subsequently in this document, the symbol λ stands for the wavelength of light. Subscripts on λ such as Laser in λ Laser and Bragg in λ Bragg designate particular wavelengths. Each in-phase reflection subtracts strength from the transmitted waves and adds strength to the reflected waves shown above, in green, getting progressively thicker as they move toward the detector. These thousands of tiny individual reflections add up to a strong combined reflection when λ Laser = λ Bragg = 2n d Λ. NOTE: The index of refraction term n d in the equation above corrects for the fact that the wavelength of light inside the fiber the wavelength that the FBG Bridge Monitoring with Fiber Optic Sensors.doc Page 9 of 35

10 sees is shorter than the wavelength outside the fiber the wavelength coming from the laser, because: The speed of light through a material is inversely proportional to its n d. n d-air 1 but n d-fiber 1.5, so light slows by ~1/1.5 after it enters the fiber. The wavelength of light inside the fiber is directly proportional to its speed, so reducing its speed by ~1/1.5 shortens its wavelength by ~1/1.5. FBG interrogation and data logging λ Bragg λ Bragg As illustrated above, an FBG interrogator: Dynamically changes the wavelength of light transmitted to the FBG by scanning a tuneable laser Simultaneously records/ analyzes the reflected power of light from the FBG by evaluating the detector signal. As the wavelength of light transmitted to the FBG approaches λ Bragg, the power of light reflected from the FBG rises to trace out a peak. In a slow-to-moderate-speed FBG interrogator, the instrument dynamically memorizes all scan data and post-scan processes it in software to detect λ Bragg. In a high-speed FBG interrogator, the instrument dynamically analyzes the scan data in real time, using hardware techniques to detect λ Bragg at sampling rates as high as 2000 Hz. Bridge Monitoring with Fiber Optic Sensors.doc Page 10 of 35

11 Scanning for bridge-health data Scanning for FBG spectra For continuous monitoring, the interrogator, in combination with an internal or external computer, does the following: Dynamically analyzes for and records the precise numerical value of λ Bragg for each sensor, as described above. Compares these λ Bragg values with warning and alarm thresholds. Optionally outputs these results to a data-acquisition computer(s), locally and/or remotely. Optionally reports these results on a computer monitor in various ways, including tables, plots, and data icons that are superimposed on a bridge photo at the sensor locations. These reporting modes apply to one or many FBGs on a fiber. For more information, refer to FBG Sensor Software on page 25. For periodic monitoring, the actions are very similar. However, a user only periodically connects a portable interrogator to the sensors, temporarily stores the data within the instrument, and later uploads it to a central PC. As needed, slow-to-moderate-speed interrogators in combination with internal or external PCs can report an entire scan(s) as a spectrum of reflected-power vs. wavelength. In this mode, the interrogator memorizes all wavelength and reflected power data during the scan and displays the results as a full-range spectrum. See the example below. High-speed interrogators, which sample as fast as 2 khz, focus primarily on rapid, hardware-driven peak detection. These instruments provide only a rough graphical summary of the spectrum. Bridge Monitoring with Fiber Optic Sensors.doc Page 11 of 35

12 FBG Multiplexing Interrogation of multiple FBGs on one fiber A huge strength of FBG sensing technology is the implicit ability to place and interrogate many FBGs on a single fiber. Within reason, multiplying the number of FBGs on a fiber simply multiplies the number of λ Bragg peaks! See below. So long as the peaks don t crowd or overlap each other, a single standard interrogator scan can precisely find and report the distinct λ Bragg for every peak. 6 This technique is called Wavelength-Division Multiplexing (WDM). 6 Note further that many FBG interrogators can rapidly interrogate multiple fibers at once. Bridge Monitoring with Fiber Optic Sensors.doc Page 12 of 35

13 In combination with a computer, the interrogator can numerically log multiple λ Bragg values on a fiber. See the spreadsheet segment below: Further, the software that CEL provides can report the results in more user-friendly ways, such as illustrated below 7 : Placement of the multiple FBGs on one fiber Multiple FBGs integrally written on a fiber Some applications utilize many FBGs written on a continuous length of fiber sometimes as part of the of fiber fabrication process. Spool containing an FBG array: A multitude of FBGs laser-written at discrete points 7 For more information about this reporting method, see FBG Sensor Software on page 20. Bridge Monitoring with Fiber Optic Sensors.doc Page 13 of 35

14 Such FBG-laden fibers can be enclosed in armored sheaths and used, for example, to sense temperatures at multiple locations over long distances. Multiple FBGs sequentially spliced to a fiber However, for most strain and vibration monitoring applications, the FBGs are mounted within discrete sensors, which in turn are anchored to structural monitoring points e.g. girders, gussets, etc. on a bridge. The FBGs in each sensor are then field-spliced 8 sequentially to an armored fiber, illustrated in green in the diagram below: Again, the final result is many FBGs on a single fiber. Advantages of multiple FBGs on one fiber A user typically can monitor up to 40 FBG strain sensors per fiber 9 which can be up to a few kilometers long. This multiplexing capability greatly simplifies cabling, instrumentation, general installation, and maintenance. 8 9 NOTE: CEL s experts proof-test all splices for 100 kpsi tensile strength. For strain sensors measuring a maximum strain of 2500 με using a 160nm-range interrogator. Larger numbers of FBG strain sensors per fiber are possible when measuring maximum strains of less than 2500 με. For ambient temperature sensors, interrogation of more than100 FBGs per fiber is possible if all temperatures change similarly. Bridge Monitoring with Fiber Optic Sensors.doc Page 14 of 35

15 FBG Strain Sensors FBG strain-sensor principle In an FBG strain gage, structural strain results in FBG strain: Tensile strain stretches the FBG and increases its period, Λ the distance between yellow lines in our FBG illustrations. Therefore, tensile strain also increases λ Bragg, because λ Bragg, = 2n d Λ. 10 λ Bragg λ Bragg Compressive strain reduces the stretch of the pre-stretched FBG 11 and thereby decreases its period, Λ.Therefore, compressive strain likewise decreases λ Bragg Stretching also decreases the refractive index slightly. In a practical FBG strain gauge, the fiber is pre-loaded. Otherwise, compressive strain would buckle the FBG. Bridge Monitoring with Fiber Optic Sensors.doc Page 15 of 35

16 FBG strain-sensor strain response Analogous to the gage-factor definition for a resistive strain gage ΔR/R Δ = F G, Resistive ε the gage-factor, F G, for an FBG strain gage is defined as Δλ Bragg /λ Brag gg = F G, FBG ε For most FBG strain gages, F G, FBG 0.78 FBG strain-sensor temperature response More generally, FBG sensors respond to temperature as welll as to strain: The curve below for an unstrained FBG strain sensor mounted on steel illustrates the need for temperaturee compensation: FBG strain-sensor temperature compensationn A simple way to temperature-compensate an FBG strain sensor follows: 1. Mount a second identical sensor sometimes called a dummy sensor on an unstrained piece of same substrate material. Locate it next to the primary sensor so that it experiences the same temperatures. Thereafter, when monitoring strain, subtract (Δλ Brag gg /λ Bragg ) Unstrained for the unstrained sensor from (Δλ Bragg /λ Bragg ) Straine ed for the strained sensor. This straightforward calculation isolates the strain value as follows: Bridge Monitoring with Fiber Optic Sensors.doc Page 16 of 35

17 Trivial algebraic manipulation then yields: Alternative temperature-compensation approaches involve independent measurements of nearby temperatures and corrections to strain-sensor responses with appropriate equations built into the monitoring software. FBG strain-sensor hardware Tack-weldable/bondable FBG strain sensor CEL s team typically uses this sensor for strain monitoring of metal structural members: trusses, beams, girders, gussets, cables, plates, etc. The above sensor needs no calibrations for many structural applications: A stable gage-factor calibration is supplied with each unit. The F G, FBG tolerance between sensors is ±5% over its T range. Bridge Monitoring with Fiber Optic Sensors.doc Page 17 of 35

18 Tack-weldable/bondable FBG temperature-compensation T sensor This sensor provides the temperature data needed for calculation-based (vs. dummy-strain-sensor-based) temperature-compensation approaches. Multi-mountable, environmentally sealed FBG strain sensor This versatile sensor can be embedded in new concrete, welded to rebar, welded or bolted to structural members, or grouted into existing concrete or masonry. It contains an integral temperature-compensation T sensor. Bridge Monitoring with Fiber Optic Sensors.doc Page 18 of 35

19 FBG strain and T sensor installation The figure below illustrates installation of the tack-weldable/bondable strain and temperature sensors shown on pages 17 and 18. Bridge Monitoring with Fiber Optic Sensors.doc Page 19 of 35

20 FBG Accelerometers FBG strain gauges can respond fairly quickly to dynamic strains. Therefore, with limitations, they can provide useful structural vibration information. However, smartstructure modal analysis capabilities require FBG accelerometers. FBG accelerometer principle In an FBG accelerometer, acceleration of a mass coupled to an FBG creates a force that strains the FBG and shifts λ Bragg. A simplified accelerometer, shown below, consists of an FBG anchored to the monitored structure at one end and attached to a floating mass m at the other: FBG λ Bragg λ Bragg As structural accelerations a move the anchored end of the FBG, they likewise accelerate attached mass m but only by applying a force F through the FBG, as required by Newton s Second Law: F = ma. That is, the required force F equals the mass m to be accelerated times the acceleration a. We know mass m and can calculate force F from the resulting strain on the FBG as observed from the shift in λ Bragg. Therefore, we can calculate acceleration a from Newton s Second Law: a = F/m. Bridge Monitoring with Fiber Optic Sensors.doc Page 20 of 35

21 FBG accelerometer hardware This section shows three commercial FBG accelerometers. The model shown immediately below, with two pigtails, can directly join a multitude of other FBG sensors on the same optical fiber, such as FBG strain and temperature sensors. The other two models, with single pigtails, can as well join other sensors on a single fiber using passive optical couplers. Frequency range: 0 to 40 Hz. Acceleration range: 20 m 2 /sec (~2 G) Temperature range: 10 to 60 C. Weight: 33 gm. Frequency range: 0 to 1 khz; Acceleration range: 15 G. Temperature range: -20 to 80 C. Weight: 25 gm. Specifications are not yet finalized for this relatively-high frequency, high-value unit (shown mounted on a Bruel & Kjaer calibration system). This accelerometerr will be commercially available in January, FBG accelerometer modal analysis The software that CEL provides includes an FFT (Fast Fourier Transforoutine for examining vibration frequencies and amplitudes. For more information, refer to FBG Sensor Software on page 25. Bridge Monitoring with Fiber Optic Sensors.doc Page 21 of 35

22 FBG Sen nsor Instrumentation You can monitor your bridges continuously with fixed instrumentation or periodically with portable instrumentation. CEL can assist you with both continuous and periodic monitoring systems. Each system features optical equipment made by Micron Optics recognized internationally as the world s most respected manufacturer of FBG sensor instrumentation. The sectionss below briefly describe some of this equipment. Interrogators for continuous monitoring Slow-to-moderate speed fixed-location FBG interrogator Sampling rate as fast as 10 Hz with each scan referenced to a stable, zero-temperature-coefficient wavelength standard. 80 nm standard range (1510 to 1590 nm); 160 nm optional range. For multiplexed monitoring of 4 fibers 16 fibers with expansion module, each fiber potentially containing 40 FBGs For strain sensors measuring a maximum strain of 2500 με using a 160nm-rangare possible when measuring maximum s trains of less than 2500 με. For ambient temperature sensors, interrogation of interrogator. Larger numbers of FBG strain sensors per fiber more than 100 FBGs per fiber is possible if all temperatures move similarly. Bridge Monitoring with Fiber Optic Sensors.doc Page 22 of 35

23 High speed fixed-location FBG interrogator Sampling rate as fast as 2000 Hz with each scan referenced to a stable, zero-temperature-coefficient wavelength standard. 80 nm standard range (1510 to 1590 nm); 160 nm optional range. For multiplexed monitoring of 4 fibers 16 fibers with expansion module, each fiber potentially containing 40 FBGs. 13 Fixed-location compute r Interrogator-matched industrial grade PC for data logging, manipulation/analysis, display, and communication One of two available models 13 For strain sensors measuring a maximum strain of 2500 με using a 160nm-range interrogator. Larger numbers of FBG strain sensors per fiber are possible when measuring maximum s trains of less than 2500 με. For ambient temperature sensors, interrogation of more than 100 FBGs per fiber is possible if all temperatures move similarly. Bridge Monitoring with Fiber Optic Sensors.doc Page 23 of 35

24 Interrrogators for perio odic monitor m ring h For sampling at 1 Hz. A similar mo odel sample es at 1000 Hz. h For multiplexed monito oring of one fiber at a tim me, each po otentially containing g dozens of FBGs. F Comp plete with inttegral monito or and preinstalled software. Field d instr umenttation systems Th he photo below shows fiber-optic insstrumentatio on systems in nstalled on the Chiapas Bridge B in Novvember (top) and readied r for installation on o a second bridg ge (bottom).. The heat-an nd humiditycontrolled instrumenta ation cabinet at left contains a highspeed or, a interrogato 16-channel a expander, an industrial PC, P a satellite mo odem, and a satellitecommunica ations router. Bridge Monitoring M with w Fiber Op ptic Sensorss.doc Page 24 of 35 3

25 FBG Sensor Software CEL supplies all bridge-monitoring systems with powerful, user-friendly Enlight Pro software for FBG-data logging, display, and analysis. This section only highlights only a few of the software s many features. 14 For additional information, refer to: %20software%20-% pdf. Some Key User Interface Capabilities Display of bridge conditions at sensor locations on a bridge photo A particularly user-friendly feature of the software is the ability to graphically display bridge conditions right where they are measured on a photo of the bridge. The pictures and text below illustrate the placement of such indicators and the functions of each type. Full indicator Gage indicator LED indicator Full indicator Displays the first ten characters of the sensor name, the current reading for the sensor numerically, the current reading for the sensor graphically, and the sensor s alarm state by color: the graphical bar turns green for useracceptable conditions, yellow for warninglevel conditions, and red for alarm-level conditions. Gage indicator The full circumference of the dial represents the full range of the sensor, the green arc represents the user-acceptable part of the range, the yellow arc represents the warning-level part of the range, and the pointer position indicates the current sensor value. If the current sensor value falls outside of the warning range, the entire background turns red to indicate an alarm condition. LED indicator Simply indicates sensor-location conditions by color User-acceptable: green, Warning: yellow, and Alarm: red. 14 All screen captures shown in the FBG Sensor Software section have been excerpted from the Enlight Pro user guide and related publications, Copyright 2008 Micron Optics, Inc. Bridge Monitoring with Fiber Optic Sensors.doc Page 25 of 35

26 Display of sensor conditions in multiple other ways on one screen The Sensors tab simultaneously displays a wealth of numerical and graphical information about all sensors that are connected to a selected interrogator channel. including warning and alarm highlights. In the figure just above, note the following: Highlights of a past below-minimum-warning history for the saddle_strain sensor, via yellow shading in the Alarm Min. cell. Highlights of an above-alarm present condition at the saddle_strain sensor, via red shading in the Current and Alarm Max. table cells and a red exclamation mark in the Name table cell. Bridge Monitoring with Fiber Optic Sensors.doc Page 26 of 35

27 Display of FFT (Fast Fourier Transform) spectra Meaningful modal analysis from FBG accelerometer data requires intensity vs. frequency spectra. The software s FFT view displays such a spectrum for any FBG that is connected to a high-speed interrogator: Internal Calculation Capabilities The software has real-time capabilities to calculate and display temperaturecompensated strain data via built-in equations. It also has real-time capabilities to calculate and display engineering-unit results from custom, user-entered equations. Data Logging Capabilities The software can rapidly stream data to disk as a text file potentially hundreds of times per second from a high-speed FBG interrogator. See the data sample below which includes real-time-calculated parameters: Bridge Monitoring with Fiber Optic Sensors.doc Page 27 of 35

28 FBG Sensor Systems All FBG sensor systems installed by CEL s experts include strain gauges and temperature-compensating devices that are attached to and/or embedded in your structures. The system may also include FBG accelerometers and other sensing devices, per your requirements (for example, fiber-optic displacement gages, fiber optic tilt gages, and corrosion sensors). All fiber-optic devices are connected by armored cables and, where appropriate, also high-tensile-strength multi-fiber optical cables and sealed optical junction boxes. All systems include the user-friendly software described previously under FBG Sensor Software on page 25. Periodic monitoring systems For periodic monitoring, sensor network cables terminate in low-loss FC/APC fiber optic connectors that are housed within a normally-locked, environmentally sealed enclosure. As needed, the user attaches these connectors to a CEL-supplied portable interrogator. The environmental enclosure is large enough to temporarily house and protect the portable interrogator during use in rainy or snowy weather conditions. The figure below generically illustrates such a system. Bridge Monitoring with Fiber Optic Sensors.doc Page 28 of 35

29 Continuous monitoring systems For continuous monitoring, sensor network cables terminate into an interrogator(s) and/or a channel expander(s) that are permanently installed in an environmentally controlled enclosure. The system typically includes an industrial PC and any required communications interfaces. Per client requirements, the system may also other types of sensors and instrumentation. Typically such a system will have some type of communications link to a remote site(s) an Internet connection, a satellite link, or both. Optionally (not shown) the system may have a solar collection panel, power conditioning modules, and storage cells for powerline-independent operation. The figure below generically illustrates such a system. Bridge Monitoring with Fiber Optic Sensors.doc Page 29 of 35

30 Summary Proven FBG fiber-optic sensor technology is today's best choice for structural health monitoring especially for bridge monitoring: An FBG is a stable, environmentally isolated array of reflectors within the core of an optical fiber. It reflects a sharply defined, sensor-specific wavelength of light that shifts in response to strain and temperature. FBG-based monitoring systems are: Stable, reliable, EMI-immune, environmentally tough, and implicitly labeled each FBG on fiber is wavelength-specific. Particularly suitable monitoring at many remote/widely-spaced locations (kilometers). Much simpler to install and maintain than electrical sensor systems. A single fiber supports many sensors. More durable than electrical sensor systems. FBG-based monitoring systems are easily retrofitted to existing structures and readily integrated into new structures. Sophisticated system software will log and display your data numerically and graphically: In traditional graphical and tabular formats. With numbers, graphs, and alarms displayed at each sensor location atop a photograph of your bridge. CEL's experts can do the job. They have applied their many years of fiber-optics experience to fiber-optic monitoring of the Chiapas bridge in November, 2008 and are scheduled to apply fiber-optic monitoring to the Vicksburg Mississippi and Rock Island Arsenal bridges in August, 2009.They'll apply this expertise to monitoring of your bridges as well. Bridge Monitoring with Fiber Optic Sensors.doc Page 30 of 35

31 Appendix Bridges Being Instrumented with FBG Sensors by Same Team That Will Instrument Yours Bridge Monitoring with Fiber Optic Sensors.doc Page 31 of 35

32 Chiapas Bridge in Mexico The Chiapas Bridge ("Puente Chiapas") was inaugurated in December It crosses the Nezahualcoyotl Lake part of the Malpaso Dam, in southern Mexico. It is 1,208 meters long and 10 meters wide and has two driving lanes. This connection between Las Choapas (Veracruz) and Raudales- Ocozocoautla (Chiapas) shortens travel time between Mexico City and Chiapas by 3.5 hours The solar-powered optical sensing system monitors 96 points over the full length of the bridge and transmits data by satellite. Bridge Monitoring with Fiber Optic Sensors.doc Page 32 of 35

33 Rock Island Arsenal Bridge This historic bridge was built in 1896 at the same location as an 1872 structure, using the same piers. Upper and lower decks carry rail and road traffic over the Mississippi River between Rock Island, Illinois and Davenport, Iowa. Automated structural-health above. Optical sensors will provide the primary monitoring of structural healthh and bridge life, in combination with other technologies. The system will monitoring will be applied to the swing truss highlighted feature satellite communications, as well as solar power to prove its feasibility for other bridge monitoring systems. Bridge Monitoring with Fiber Optic Sensors.doc Page 33 of 35

34 Vicksburg Mississippi Bridge This cantilever bridge carries Interstate 20 and US 80 across the Mississippi River between Delta, Louisiana and Vicksburg, Mississippi. Four main spans will be monitored using optical sensing technology as the main structural health technology. Solar energy will power all equipment. Satellite data communications will provide web access for viewing of analyses at will The spans will be monitored for strain, acceleration (for modal response), corrosion, temperature, and pier tilt. A weather station on the span will enable correlations between environmental and loading conditions. Bridge Monitoring with Fiber Optic Sensors.doc Page 34 of 35

35 Copyright 2008 Cleveland Electric Laboratories. All Rights Reserved. Cleveland Electric Laboratories logos, and trademarks or registered trademarks of Cleveland Electric Laboratories or its subsidiaries in the United States and other countries. Copyright 2008 Cleveland Electric Laboratories. All Rights Reserved. Other names and brands may be claimed as the property of others. Information regarding third party products is provided solely for educational purposes. Cleveland Electric Laboratories is not responsible for the performance or support of third party products and does not make any representations or warranties whatsoever regarding quality, reliability, functionality, or compatibility of these devices or products.