The tight tolerances imposed on photonic components for standardized integration into DWDM systems requires that some form of temperature stabilization must be incorporated. Frequently this involves the use of traditional temperature sensors integrated into a classic control-feedback scheme involving thermo-electric coolers. While easy to implement, such a scheme is hardly novel and may result in an inefficient use of electrical power. A technique using flexible thermal tape configured into packages applicable for thermal stabilization of DWDM compliant laser diode sources and tunable filters is described.
While fiber optic Bragg grating sensors have emerged as a viable commercial product and principle component within numerous fiber optic components, they possess the inherent inflexibility that once written, the nonstressed grating spacing is fixed. An alternative method of fabricating Bragg gratings where a photosensitive fiber core material (similar to that used in photogrey sunglasses) exists at the grating site along the fiber has been examined. As opposed to a conventional grating writing method using lateral illumination of the fiber core, in this instance the diffraction rating is written via a deliberate intensity variation in the light which is injected into the fiber. The slight difference in refractive index between the photogrey section of the core and the regular fiber causes an internal Fabry-Perot resonator cavity to be established. By using an intensity-modulated high power laser, the illuminating modulation pattern reflects back and forth within this cavity establishing a standing wave pattern. Different patterns may be launched into the fiber resulting in a grating spacing which is variable. This standing wave pattern effectively illuminates the photogrey section nonuniformly with the high power portions of the standing wave pattern causing more darkening - thereby in essence creating the Bragg diffraction grating. Removal of this illumination source results in a grating that fades away yielding a re-writable component. An examination of this type or re-writable component will be reported along with its suitability for 100- and 5-GHz DWDM applications.
This paper gives an overview of the primary issues of structural health and evaluation monitoring for civil structures, such as bridges, dams, buildings and roadways, and role that fiber optic sensors play in the monitoring efforts. Some of the quantities that need to be measured are displacement, velocity, acceleration, jerk, force, stress, strain, temperature, fracture, moisture, fatigue, and chemical state, i.e. corrosion. Fiber optic sensors have the capability to measure most, if not all, of these quantities. Fiber optic sensors exploit a variety of physical principles through which physical quantities are measured. The particular types of fiber sensors that will be discussed in this paper are: intensity-based, modal domain interferometric, Bragg grating, white light interferometric, and Brillouin backscatter. The operating principles and application results from field and laboratory studies are presented.
A novel antenna, known as a Good Impedance Matching Antenna (GIMA), has been developed for use in Ground Penetrating Radar (GPR) NDE of concrete structures. The requirements of a useful GPR antenna are that it provides sufficient penetrating depth in the concrete with sufficient resolution to determine the location and magnitude of the defects, such as deteriorations and delaminations. The GIMA antenna is designed to have a self-defined aperture that minimizes impedance mismatching at the aperture. This unique feature allows the antenna to be used in various frequency bands. The tested frequency range is from 500 MHz to 16 GHz. The antenna provided a high penetrating depth (more than 330 mm) and the sufficient resolution of the image that can recognize cracks up to 1 mm thick, with a radiation coefficient of about 99%. This paper will introduce the theory behind the design, as well as present experimental results. The characteristic parameters of the antenna, such as impedance matching status represented by SWR and Smith chart, as well as the power radiated using return loss. In addition, the aperture reflection is determined via the time-domain air shot reflections. Also presented are the results of the resolution test and penetrating test using concrete slabs.
A cantilever beam and fiber Bragg grating is used to measure acceleration. The cantilever induces strain on the grating resulting in a Bragg wavelength modification which is subsequently detected. The output signal is insensitive to temperature variations and for a temperature change from -20 degrees Celsius to 40 degrees Celsius, the output signal fluctuated less than 5% without any temperature compensation schemes. Because the sensor does not utilize expensive and complex demodulation techniques it is potentially inexpensive. For the experimental system a linear output range of 8 g could be detected.
FIber optic sensors have repeatedly been shown to provide measurement capabilities of parameters within such reinforced concrete structures. Development of a fiber optic chloride sensor capable of being embedded within a roadway or bridge deck is reported. Once the specific chemistry of the fiber optic sensor was developed and tested, multiple iterations occurred in order to result in a sensor system capable of being embedded into roadway bridges. During the summer and fall of 1997, a total of 64 fiber optic sensors were then embedded into 3 roadway bridges which spanned rivers in northern Vermont. Installation details as well as results are presented.
The use of fiber optic sensors for the internal state measurements of large civil structures has been increasing in recent years. In many instances, sensors are embedded into the reinforced concrete structure is an attempt to measure a single parameter of interest. Installation and preliminary measurements obtained from a suite of fiber optic sensors which were embedded into a 67 m steel truss bridge spanning the Winooski River in Vermont (USA) are presented.
Researchers throughout the world have successfully developed embedded long-lived silica-based fiber optic sensors for the performance and health monitoring of reinforced concrete structures. We have examined the user of polymer optical fiber (POF) as the base material for fiber optic sensors which are to be embedded into concrete structures. Such POF sensors hold the possibility for considerably reduced expense both in terms of actual component costs and, perhaps more importantly, in the relative ease with which such sensor may be embedded into concrete structures. Results of our laboratory and field studies, where POF sensors were embedded into an in-service railroad bridge, are discussed.
The use of chloride-based deicing agents to help clear US highways of roadway hazards leads to associated chemical related problems. Fouling of local rivers and streams due to runoff of the water borne chlorides is significant and has contributed to local ordances are attempting to force state agencies to reduce, if not eliminate, the use of these chlorides. With respect to the corrosion aspects of chloride application, cracks that occur in the roadway/bridge pavement allow water to seep into the pavement carrying the chloride to the rebar with the resultant increase in corrosion. The costs of this corrosion are considerable and have led to the widespread use of chloride/water impermeable membranes on roadways and especially within bridges. Fiber optic sensor have repeatedly been shown to provide measurement capabilities of parameters within such reinforced concrete structures. Development of a fiber optic chloride sensors capable of being embedded within a roadway or bridge deck is reported.
Speckle based fiber optic sensors have been developed and used in laboratory and field settings for a number of years. In the case of speckle-based fiber optic vibration sensor, optical signal processing of the field emerging form a multimode fiber has been performed through the use of intensity modulating masks. In such cases, portions of the speckle pattern are not allowed to illuminate a photodetector thereby performing forms of statistical selection of modal patterns. While this technique is robust, it does inherently reduce the sensor's operational dynamic range, and signal-to-noise ratio, due to the simple fact that a portion of the light field is not used. By contrast, it is possible to use phase-contrast enhancing techniques found in microscopy to phase-modulate the fiber's speckle pattern. The performance of a multimode fiber optic sensor using these techniques is compared with the more traditional intensity-modulating fiber optic sensor.
While fiber optic Bragg gratings sensors have emerged as a viable commercial product they possess the inherent inflexibility that once written, the nonstressed gratin spacing is fixed. We have begun to explore an alternative method of fabricating Bragg sensors - one where you have a single fiber optic Bragg grating sensor, but where you are able to write your own grating prior to parameter measuring with the broadband light source. In this alternative, a photosensitive fiber core material exists at the FBGS sensing 'site' along the fiber. THe diffraction grating is written via a deliberate intensity variation in the light which is injected into the fiber. The slight difference in refractive index between the photogrey section of the core and the 'regular' fiber causes an internal Fabry-Perot resonator cavity to be established. The intensity-modulated high power laser bit stream reflects back and forth within this cavity establishing a standing wave pattern. This pattern may be varied, and hence the grating spacing is variable, by changing the high power laser's bit pattern. This standing wave pattern effectively illuminates the photogrey section nonuniformly with the high power portions of the standing wave pattern causing more darkening - thereby in essence creating the Bragg diffraction grating.
The problems associated with the application of chloride-based deicing agents to roadways and specifically bridges include chemical pollution and accelerated corrosion of strength members (especially rebar) within the structure. In many instances, local ordinances are attempting to force state agencies to reduce, if not eliminate, the use of these chlorides (typically at the cost of increased driving hazards). With respect to the corrosion aspects of chloride application, cracks that occur in the roadway/bridge pavement allow water to seep into the pavement carrying the chloride to the rebar with the resultant increase in corrosion. In response to this problem, particularly in high roadsalt usage areas, a chloride/water impermeable membrane is placed above the rebar matrix so if/when roadway cracking occurs, the roadsalts won't be able to damage the rebar. Such a membrane is costly -- and the question of its in-service performance is questionable. In a joint effort between the University of Vermont and the Vermont Agency of Transportation, we are developing fiber optic chloride detectors which are capable of being embedded into the rebar-concrete roadway under this membrane. The sensing mechanism relies on spectroscopic analysis of a chemical reaction of chloride and reagents (which have been coated onto the ends of fibers). Laboratory results of these detectors and a usable system configuration are presented.
The construction phase of a building is one in which the structure is rather vulnerable to damage and/or collapse. The potential for human and economic loss, as well as the potential for reduced construction costs, presents an opportunity for developing construction structural load and response monitoring systems that can possibly be effective in reducing the frequency of collapses and/or damage. This paper presents the results of a study in which such systems were developed and applied to slab-style concrete building construction. The shoring systems that are used to support fresh concrete floor slabs are instrumented with strain gage based load cells to measure the loads during construction operations. The loads on the shores have been measured during the construction of three different buildings. This information is being used for construction procedure code development. The next phase of this research is to apply the sensing system so that it can provide an early warning for potential collapses of the partially- built structure. Mechanical models of the structural system indicate that load monitoring of individual shoring members is probably insufficient because the major structural collapses are due to a global instability. It appears that a combination of strategically-placed load, temperature and deflection monitoring instrumentation combined with a real-time analysis of the data may be necessary. The design and use of such systems are discussed.
Routine acquisition of data from instrumented civil structures need not require onsite presence. We have developed a sensor interrogation method which utilizes the Internet global computer network as the information conduit from sensor(s) to user. It is therefore possible for the data monitoring to be performed at a remote location with the only requirements for data acquisition being Internet accessibility. Such a system may prove quite advantageous when a single user, or small group, is required to acquire and analyze data from several instrumented structures which are geographically very separated. In this developed system, it is possible to remotely acquire raw sensor data as well as realtime video/audio images of the current status at the instrumented structure.
In this paper we report the development of a fiber optic corrosion sensing system that complements and/or surpasses the capabilities of conventional corrosion sensing techniques. The sensing system is based on evanescent wave phenomena and in the configured sensor allows for the detection of general corrosion on and within materials. Based on the authors' experience installing may kilometers of fiberoptic sensors into large civil structures such as multistory buildings, hydroelectric dams, and railway/roadway bridges, we are (currently) embedding these sensors into bridge test members -- limited structures that are being subjected to accelerated corrosion testing conditions. Three Vermont Agency of Transportation bridges, one in a low salt use region, one in a medium salt use region, and the third in a high salt use region, are being selected and will be instrumented with these embedded fiber optic corrosion sensors. Monitoring of chloride penetration and rebar corrosion status will be measured during the course of a longitudinal study. The specific sensing mechanism and design for robustness (to allow survival of the embedding process during repaving of the bridges) are discussed and laboratory and initial field results are presented.
The effective guarding of machines against human injury is an enormous problem already with many available solutions. Guarding procedures generally fall into one of three classes: (1) Physical guards which prevent the placing of limbs and personnel in potential crush points. (2) Those devices which require the removal of the human from the device so that they can be activated, e.g. switches for activation. (3) Sensor, such as light sheets which shut down the machinery when the presence of an object, such as a hand, is detected in a hazardous position. We have developed a novel machine guarding technique based on the measurement of electromagnetic field distortion. There are no physical connections between the operator and the machine. The machine guarding is accomplished by merely sensing operator presence/absence through the capacitance change caused by the operator themself. As such we see few easy ways in which such a machine guard may be deactivated.
An optical technique has been developed whereby two angles and linear displacement can be simultaneously measured in a non-contact manner. The method depends upon the usage of a diffraction grating with linear variation of period along its length. The grating is attached to a structure at a point of interest while all other system components are placed at a remote location. Evaluation of this measurement technique has been demonstrated on a laboratory- based structure which simulated conditions found at deep trench (or tunnel) walls or bracing systems. In a construction site configuration, this sensor allows the user to determine if the walls are undergoing structural deformation. In addition, the magnitude of deformation may be measured and alarm conditions may be monitored. Experimental results obtained using this technique are presented and compared with theory.
The advances in the area of fiber optic sensors have led to applications in various niche areas. Within the past few years, researchers have seemingly led the way in the application of such fiber optic sensors within the civil engineering arena. Specifically, various large civil structures have had differing types of fiber optic sensors installed within and upon these structures leading to measurements not previously available. A review of this `smart structures' research is presented in this paper.
The requirements of sensor monitoring associated with instrumented civil structures poses potential logistical constraints on manpower, training, and costs. The need for frequent or even continuous data monitoring places potentially severe constraints on overall system performance given real-world factors such as available manpower, geographic separation of the instrumented structures, and data archiving as well as the training and cost issues. While the pool of available low wage, moderate skill workers available to the authors is sizable (undergraduate engineering students), the level of performance of such workers is quite variable leading to data acquisition integrity and continuity issues - matters that are not acceptable in the practical field implementation of such developed systems. In the case of acquiring data from the numerous sensors within the civil structures which the authors have instrumented (e.g., a multistory building, roadway/railway bridges, and a hydroelectric dam), we have found that many of these concerns may be alleviated through the use of an automated data acquisition system which archives the acquired information in an electronic location remotely accessible through the Internet global computer network. It is therefore a possible for the data monitoring to be performed at a remote location with the only requirements for data acquisition being Internet accessibility. A description of the developed scheme is presented as well as guiding philosophies.
Over the past 25 years in the United States there have been more than 85 collapses of structures under construction that have been directly attributable to formwork failure. Sensing systems and techniques applicable to the monitoring of construction site shoring and scaffolding are designed and implemented with preliminary systems being used in the field and in the laboratory. Such a sensor network can provide significant information about the load distribution on shoring systems. This information can allow dangerous situations to be quickly identified so that corrective action can be taken. Furthermore, the load data acquired with this system can be used to formulate improved construction codes that enhance construction work safety. Laboratory proof-of-concept experiments as well as actual field site measurements presenting the in-service use and capabilities of an intelligent shoring system are described in this paper.
Fiber optic sensors were embedded into a 7.5 MW hydroelectric dam during its construction phase. Power generation began at the facility during the spring of 1993 as did initial verification of the on-line use of certain key embedded sensors. Reliability information, as well as structural vibration signatures for the dam, has been obtained under various operational regimes. This information as well as the initial perspectives from builders, owners, users, and regulatory officials regarding embedded sensors is presented.
Installation of fiber optic sensors into large civil structures entails either embedding or attaching the sensors to some structural member. While we have performed sensor installations on numerous types and sizes of civil structures, this process is frequently quite labor- and time-intensive -- both of which can potentially conflict with construction site practices. In instances where it is desirable to install many sensors in close proximity, it may prove very beneficial to prefabricate panels of sensors and then install the panel into the structure. However, issues such as the effect of the panels on the structural integrity of the building may arise. In this paper we discuss this matter as well as the panel interconnect and sensor interrogation issues. Examples of prefabricated sensor panels designed for installation into a new wing of the Medical Center Hospital of Vermont are then presented.
During the past number of years, research has been underway in Vermont investigating development and implementation of smart civil structures. During the course of this effort, numerous fiber optic and conventional sensor techniques and designs have been developed and tested. Laboratory studies have led to field testing at numerous sites and types of installations ranging from residential homes, conventional multi-story buildings, pedestrian footbridges, interstate highway road surfaces and railway bridges to high performance structures such as a hydroelectric dam. A description of the efforts in sensor design as well as multiplexing and multi-use sensors and systems with particular emphasis on smart civil structures applications is given.
The increasing complexity of large space structures further emphasizes the need for multi-use single element spacecraft components and sensors. Sensing of the vibrations of components and/or sections of a large space structure is necessary for ultra-high precision pointing and tracking as well as general platform stability. At the same time, the quest for a method of detecting spacecraft impacts with micrometeorites or debris continues. In this paper a method for using a single optical fiber for simultaneous impact detection and vibration sensing is described. Laboratory studies of these techniques using composite material panels are presented.
Smart structures technology is being increasingly applied to civil structure applications. In particular, development of health monitoring for bridge structures is of considerable importance. In order to explore the possibility of developing such a system, an investigation was carried out on a scale model steel bridge element using an attached sensor system consisting of two point sensors (piezoelectric accelerometers) and one integrating sensor (fiber optic modal sensor). The model element was selectively configured to produce the equivalent of a number of damage conditions. For each condition, it was physically perturbed. The sensor outputs were then used as inputs to a neural net which then provided an estimate of structural damage. A reasonable correlation between net output and actual damage indicated that this type of health monitoring system offers potential for practical application on full scale bridge structures.
A new method for measuring spatial coherence in which optical fibers replace the traditional slits in Young's experiment is developed. The beam is sampled by the fiber ends, and the phase difference that causes the fringes is introduced by translating one fiber along its long axis by a few wavelengths. The technique has several advantages over the slit method: The fibers can be moved to arbitrarily chosen points, as opposed to removing and replacing sets of slits, giving greater flexibility in measurement as well as easing alignment problems. This method also does not rely on diffraction, and therefore enables measurement of much dimmer beams and dimmer points on beams. One can take as many data points as desired, thus tracing the shape of the coherence function accurately, whereas with slits one is restricted to the number of slit sets available. Finally, one does not have to assume beam symmetry. Although the magnitude of the spatial coherence function is not accurately reproduced, this can be corrected for, and the ability to reproduce the shape of the beam remains a useful measurement tool. We report, in particular, measurements of spatial coherence of a single-stripe diode laser, as well as a HeNe laser.
Fiber optic cables have long since held the promise of providing low cost, widespread sensing capabilities. The use of fiber optic sensors within a large civil structure could allow for multiple sensing capabilities providing information as to the health of a structure. The Stafford Emerging Technologies Research Complex is a five-story, 65,000 square foot building currently under the final phases of construction on the campus of the University of Vermont. Over the course of the eight months approximately seventy fiber optic sensors have been installed within the concrete frame work of the building. The intrinsic and extrinsic fiber sensors are comprised of various types of singlemode and multimode cables. Since this project is the first major installation of it's kind, very little was known as to what techniques should be implemented to maximize fiber survivability. While installing the sensor network at the Stafford building site many lessons have been learned that would aid in future fiber installations. The techniques developed while installing fiber optic sensors are presented.
The area of embedded sensors for performance and health monitoring of critical structures such as dams represents a logical extension of earlier efforts in applications of fiber optic sensors. We have been most fortunate in having a 7.5+ MW hydroelectric dam beginning construction on the Winooski River here in Vermont. Based on our work in embedding sensors into the Stafford Building, the idea of embedding sensors into the concrete superstructure of a hydroelectric dam seemed most noteworthy. We are modifying photoelastic (or polarization) based fiber optic pressure sensors while allowing the multiplexing of up to 10 sensors onto each of separate multimode fibers. The modifications entail varying the physical packaging of the sensor's components for better meshing with the dam's rebar-concrete configuration. The individual sensors will be interrogated via optical frequency domain (chirped) techniques to provide a total of 50 discrete pressure readings along the dam's 15 m (high) by 160 m (long) surface. A number (probably 12, chosen because of materials costs) of fewmode and multimode embedded fiber optic vibration sensors are also being developed for embedding in the immediate area surrounding the hydroelectric turbines. We will then be able to determine the dam structure's frequency response as the turbines are subjected to varying electrical and water loads. By relying on our prior experience with using embedded sensors for communications and sensing, we will attempt to also analyze the multiplexed upstream- surface embedded fibers to determine if we can also use those fibers to perform vibration studies. The results and/or plans for this project will also be presented.
We have examined various fiber sensor multiplexing techniques, e.g., frequency-, time-, coherence-multiplexing, in an attempt to ascertain the method best suited for interrogation of multiple sensors scattered throughout a modern civil structure. Based on our embedded fiber sensor results conducted at the Stafford Biotechnology Complex at the University of Vermont, a 65,000 square foot, multistory reinforced concrete structure, where more than fifty single- mode and multimode fiber optic sensors have been embedded into the structure, we have determined that in many instances a radio telemetry method of interrogating the sensors is optimal. Many real-world factors such as architectural details, lighting, power, and HVAC design requirements influence the overall nature of the use of multiplexed fiber sensors in civil structures. In instances where we have multiplexed intensity-modulating fiber sensors onto a single transmit/receive fiber, radio telemeterized command and data acquisition from the fiber sensor `network' may be achieved. The development of the interrogation of the multiplexed fiber optic sensors is presented, as are experimental results obtained from fiber optic vibration sensors.
Although the concept of using a single optical fiber for simultaneous sensing and communications is not new, surprisingly little experimental work has been reported. Initial studies combining a single analog tone frequency and modal domain vibration sensing were first reported only within the past few years. We describe an extension of that work that includes the demonstration of simultaneous high data rate digital optical communications and linear displacement sensing on a single multimode optical fiber. The data transmission consisted of digital pseudorandom bit sequences at various standard telecommunication speeds. Measurement of variations in the data stream bit error rate allowed determination of the position of a simple in-line fiber optic displacement sensor. Experimental results and system configurations are given for both static and dynamic displacements. Finally, the use of communication ranging techniques to determine in-line sensor location is described.
A noncontact method of monitoring structural displacement is demonstrated. The technique uses a lightweight diffraction grating with a variable period that is attached to the structure of interest. The position of the grating is monitored optically and linear displacement information can be separated from the effects of simultaneous rotary displacement. The technique is demonstrated for both static and dynamic measurements.
It is shown that both impact localization and impact magnitude determination can be achieved in rigid structures with embedded fiber-optic sensors. Impact-generated acoustic signals were detected by means of statistical mode fiber-optic vibration sensors that were embedded in a polymer matrix composite material panel. The 1-sigma error in inferred vs actual impact location, determined via relative-timing measurements, was found to be of about 4 cm.
An RF optical modulation technique for multiplexing and selfreferencing a number of fiber optic intensity sensors is described. The optical transducers are incorporated into recirculating optical fiber loops connected in parallel between transmit and receive optical fibers. A linear RF ramped optical signal is coupled into the system and the detected optical signal is electronically mixed with the input. Beat signals are produced in the frequency domain in the form of pulse trains that characterize the output of each sensor module. The relative magnitudes of the frequency components are insensitive to varying optical loss characteristics between the sensor modules and the signal processing location. The theoretical basis of the technique is presented and experimental results are given.
3 March 2003 | San Diego, California, United States
SC292: Fiber Optic Smart Civil Structures
The revolutions in the fiber optic telecommunication and optoelectronic industries have enabled the development of fiber optic sensors that offer a series of advantages over conventional electrical sensors. This course will provide an overview of applications of such sensors within the civil structures arena which results in fiber optic smart civil structures.