This paper presents a remote state monitoring system designed for and installed on the Hongcaofang Crossroad Bridge in Chongqing, China. In this system, three kinds of sensor, one of which is new, are installed in the bridge to periodically collect strain and deflection information. To control the operation of the sensors, a local computer is integrated in the pier of the bridge. The local computer processes the data from sensors, records processed results, and sends the state information to a host computer through the local Public Service Telephone Network (PSTN) using an ordinary modem. At the other terminus, the host computer receives and analyzes the data, stores the history information, queries the health state, and extracts abnormity information regarding the bridge. With the interconnect technology available through the PSTN, real time state information can be obtained on command in the monitoring room far from the bridge. This on-line monitoring system operated on the Hongcaofang Bridge for over two years. This paper reviews some of the more important results regarding both the strain and the two-dimensional deflection of the bridge, and discusses the experience gained thus far.
This paper briefly describes a health monitoring system designed for use on the Dafosi Bridge, the largest cable-stayed bridge across the Yangtze River in western China. The system can be divided into two major components, one for measurement, and one for control and data processing. The measurement system itself includes four sensing subsystems relating to: 1) fiber optic strain sensing, 2) displacement sensing, 3) temperature sensing, and 4) dynamic measurements. The control and data processing system consists of three subsystems: 1) a local computer, 2) a communication subsystem, and 3) a host computer. Sensor outputs are pre-processed locally and sent to the host computer at the management center via the Internet. The system design and implementation are reviewed, and the results of data from two sensing subsystems are presented.
The theoretical description of linearly polarized (LP) modes in weakly guiding fibers includes an amplitude parameter which relates to source intensity and input conditions. As such, this parameter has direct bearing on the amount of power carried by individual modes. This paper derives the mathematical relationship between the power in the first two LP modes and their respective amplitude parameters. In subsequent experiments, this parameter is employed in conjunction with a measurement of the amplitude parameter to yield the modal power distribution in a dual mode fiber. Alternatively, if the power distribution is known by empirical means, the amplitude parameters can be determined to allow for accurate visualization of the propagating modes. This work allows for a better understanding of the operation of dual mode fibers in preparation for structural sensing.
In this paper we report both theoretical and experimental evidence that equal MPD is extremely unlikely in few mode fiber, and next to impossible to achieve in highly multimode fiber. In one approach, we simply add together the intensity distributions of the various possible modes for a given V number fiber, and compare with what is observed in the laboratory under a variety of conditions. This is done for the incoherent case using a one hundred meter length of a similar fiber. In another method, Gloge's theory for the number of propagating modes is used to determine the approximate MPD in a highly multimode fiber. Both techniques point to the conclusion that the most likely distribution of power among the modes is one that decreases exponentially with mode, despite various mode-scrambling efforts.
An optical waveguide sensing method is proposed for monitoring the corrosion of steel in reinforced concrete structures. The method depends on the absorption of light in the waveguide by a metallic cladding which is applied at a particular location. When this cladding is exposed to a corrosive environment, it degrades, and the optical waveguide output increases. This sensing principle is analyzed, as is an electrochemical method proposed to lay the metal film onto the waveguide. In order to establish a reliable procedure for electroplating an Fe-C alloy film onto fused silica, we first investigated the use of a planar waveguide substrate, and electroless plating of silver onto SiO2 was introduced as the electroplating cathode. After analysis, an optical fiber version of the corrosion sensor was then fabricated using the electrochemical method. The sensor was intensively corroded with HNO3, NSS, and simulated concrete pore solutions. The experimental results are presented and to some degree coincide with theory.
With the rising cost of building and replacing civil infrastructure systems, as well as increased demands placed on such system, has come an elevated concern about monitoring their health. This requires the development of smart sensors suitable for real-time and in-situ measurement of concrete-based structures. Many smart sensor have been investigated to meet this requirement in the past several years. This paper describes some of the recent research activity undertaken at the Center for Intelligent Structures at Chongqing University regarding both optical sensor and monitoring strategies aimed at civil infrastructure damage assessment and integrity evaluation. Several experiments and sensor applications are also briefly described.
A method for detecting interstory drift and chord drift in buildings is proposed and demonstrated based on the motion of a laser generated cross hair in free space. The beam is projected form ceiling to floor onto an arrangement of four position sensitive detectors, each with two separate outputs. Every lateral displacement and relative angle between ceiling and floor gives a unique set of detector voltages, so that as floors shift with respect to one another due to wind and earthquake loads, voltages vary, the new beam axis location is determined, and the 3D interstory drift, velocity, and acceleration can be obtained. Experimental results show excellent linearity and sensitivity of roughly 10 microns for lateral displacement and 0.02 degrees for relative rotation between ceiling and floor.
This paper reports the research of a Fiber Optic Corrosion Sensor (FOCS) fabricated by electroplating an Fe-C alloy film onto an optical fiber core within the sensing region. Fabrication of the sensing film involves removal of the cladding, metallization of the optical fiber core, and electroplating of the Fe-C alloy layer. The initial results show that the sensor output power increases by about 35 percent when the film is corroded with the sensor of the film passivated, and that the time taken to rise to maximum output power is prolonged by almost 3 times, compared with that for non-passivated film. These result demonstrate the feasibility of using optical fiber corrosion sensors for monitoring corrosion of steel in civil structures.
A new method is proposed for measuring interstory drift, the shifting of floors relative to one another when a building undergoes wind or earthquake loading. A laser crosshair is projected from one story to the next, and onto a set of photodetectors which sense changes in the position of the projected light. This paper reports the theory of operation and a quasi-static verification of the method using micropositioning stages to provide input displacements. Lateral positions, including translational and rotational components are calculated from the photodetector outputs, and show excellent agreement with input displacements. The overall performance of the sensor system is extremely linear and predictable, and appears robust enough for field deployment.
A temperature sensor is constructed in which light strikes the boundary between a glass (BK7) corner cube reflector and a small volume of liquid (water). The liquid is contained in a metallic housing, which is bonded to a test structure. As the structure changes temperature, the indices of refraction of both the liquid and the glass vary, and the amount of light reflected out of the corner cube changes. A full three-dimensional vector analysis of the corner cube sensor is presented in conjunction with the Fresnel equations and the refractive index temperature dependence of both BK7 and water. Experimental results show the sensor output to be roughly linear with temperature for the range between 30 and 90 degrees Celsius.
A novel optical fiber sensor implementation for monitoring structural displacement and load is explored. This new sensor makes use of simple coupling losses between fibers in a hollow core sleeve, and as such, offers variable sensitivity, ruggedness, and design flexibility. Preliminary test results show promise towards the goal of making practical and cost effective fiber optic sensors for field use in structural monitoring. In addition to hardware tests, a computer simulation was written to model sensor performance, and critical design parameters were identified.
A prototype optical fiber sensor for monitoring corrosion on large steel structures has been designed and tested with favorable results. The sensor works by pulling a multimode fiber into a tight bend and securing it with a `corrosion fuse.' When the fuse corrodes, it eventually breaks and allows the fiber to straighten. The resulting difference in optical intensity emerging from the fiber is measurable using an OTDR or other optical detector. Initial experiments were carried out to determine the effect of bending fibers in a small radius and showed the feasibility of the device. Following, tests were performed on three in-line sensors in a simulated corrosive atmosphere and showed that this cheap and easily implemented monitoring scheme could be used to infer the presence of corrosion at different locations, and/or the degree of corrosion at a single location.
In this paper a straightforward method is proposed to determine the MPD within a given multimode fiber. The method combines Gloge's mode loss theory with new assumptions about the MPD, as well as a simple experiment which involves measuring the amount of power which survives a series of increasingly sharp bends. Using this technique, the most likely MPD within a fiber can be derived before and after the addition of a sensor.
A smart structure based on fiber optic sensor technology is used to provide nondestructive evaluation of fluids. Acoustic Resonance Spectroscopy techniques are used to extract fluid signatures using the proposed smart structure. These signatures are then presented to a nonparametric classifier either in their raw format or in a processed format based on autoregressive modeling techniques. Three types of nonparametric fluid classifiers are examined in this study. The performances of these classifiers are compared and the effects of autoregressive modelling on the classification performance are examined.
A method for measuring lateral displacements in structures is proposed based on the motion of a Gaussian optical beam. A sensor is designed and constructed using the free space output of a single mode optical fiber, which well approximates the Gaussian intensity distribution, and is tested in both quasi-static and dynamic displacement modes. Good agreement between the experimental data and the predicted output is obtained for small displacements of the size expected in real buildings. It is envisioned that this sensor could serve as the input to an active control system used to stabilize smart buildings experiencing earthquake and wind loads.
The applicability of optical fiber modal domain sensors to impact monitoring in aircraft components is studied. This work pulls together a number of scattered ideas from previous research programs onto one working platform, and includes the incorporation of lead-in and lead-out fibers, integrated spatial filtering, a novel fiber termination technique, and an inertial microbender to enhance mode coupling effects. Impacts to both graphite/epoxy composite and metallic specimens were carried out using a unique test apparatus constructed to accommodate a wide variety of measurement conditions. Test results suggest that modal domain sensors can offer quantitative information about the energy associated with a particular impact, as well as its location. Various sensor configurations and signal processing options are discussed, and recommendations for optimized sensing given.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.