Geodetect is an innovative geotextile-based monitoring system, developed for the measurement of strain and deformation of earthwork structures reinforced with geosynthetics. This paper presents the background and the results of the development and the practical experience acquired in real time jobs with this new technology.
The disposal of high radioactive waste in underground repository sites is a complex and complicated topic due to the strong heat generation. At the Mont Terri URL (Underground Research Laboratory), which is located in a potential clay host rock in Switzerland, a special heater test is currently performed to investigate the heat impact on the clay behavior. A fiber optic measurement system has been developed in order to measure the temperature evolution around the heat sources that have a maximum temperature of 100°C as well as the induced rock deformation due to the thermal expansion of the clay. The system as well as some first results will be described.
We present a polarimetric and interferometric fiber-optic sensor systems employing highly birefringent fibers for stress monitoring in civil engineering applications. Both systems are designed to be used with concrete-embeddable pressure c ells, optimized for stress measurement inside a concrete medium and equipped with a direct read-out of its inside pressure using an internally incorporated all-fiber sensor. In its present form the polarimetric system contains up to four space division multiplexed sensors. An overall system accuracy, including temperature drift in a wide range of temperatures is better than 1% at pressures up to 40 MPa. A concept of coherence multiplexing was applied in the interferometric system. We built and tested the system composed of three serially connected temperature-compensated pressure sensors.
A fiber optic pore water pressure sensing system for structural monitoring of a large dam at L'eau d'heure in Belgium by M.R.H. Voet, B. Verwilghen. The concept and performances of a fiber-optic pore water pressure sensor is discussed for a specific geotechnical application. Monitoring of experimental conditions in the underground inspection galleries and drainage holes underneath the dam is a first aspect and construction and operation of the artificial water dam is the second aspect of structural monitoring. The monitoring of engineering, geotechnical, and operational parameters (total pressure, pore water pressures, waterflow rates, etc.) are essential in these test and site survey programs. Comparison with parallel hydraulic and pneumatic measuring systems has been conducted. Performance of a distributed fiber optic network survey system, including eight sensors interconnected in a ring structure up to 64 sensors, is discussed. The fiber optic interrogating method (time domain multiplexing/frequency domain multiplexing) is described and typical results are presented.
In this paper we discuss two particular topologies of fiber-optic sensors developed recently at the Laboratory of Optoelectronics at UQAH, based on pressure induced birefringence effects occurring in HB fibers, and both in temperature-compensating configurations. We present a thorough analysis of the performance of our first industrial prototype of fiber-optic stress cell developed around the concept of polarimetric pressure sensor. This prototype is based on all- fiber configuration and uses a Corning highly birefringent fiber for sensing, for temperature compensation and for transmission of light signal; a 3M polarizing fiber is also employed as a distributed polarizer. The leading input and output HB fibers are connectorized to the semiconductor laser diode and to the portable electronic detection unit. Metrological data presented in this paper show an overall accuracy of the system better than 1% of full scale in the pressure range up to 21 MPa, including stability, repeatability and temperature drift in the range of about 35 degree(s)C. Both pressure hysteresis effects and sensitivity of the leading fibers to external perturbations are negligible, and the excellent power budget of the sensor allows for remote operation up to several kilometers. The paper presents also a novel configuration of the white-light interferometric pressure sensor based entirely on HB fibers and employing electronic scanning to improve mechanical stability of the receiving interferometer, where the white-light interference pattern registered by the CCD detector is transversally shifted with pressure. We present metrological evaluation of the laboratory prototype of this sensor for the pressures up to 40 MPa for the temperatures from 10 to 35 deg. C. We also evaluate the application of coherence multiplexing for networking this type of sensors.
KEYWORDS: Sensors, Fiber optics sensors, Composites, Optical fibers, Signal processing, Space operations, Control systems, Temperature metrology, Optical arrays, Detector arrays
The rapid growth and implementation of fiber optic sensor devices have evolved to the stage of demonstration and in-situ applications, whereas normally pressure and temperature parameters are measured. New photonic space opto- electronic systems could sense in a distributed way impact, damage assessment and pressure variations in structural integrity testing of spacecraft and platforms. The system consists of several loops (up to 8 per control unit) of fiber optic sensors or cables buried or implemented in smart structures or complex materials in wings or structures. Signal processing has been performed by FDM/TDM techniques to interrogate and to demultiplex unambiguously the fiber optic sensors. The control units could be placed locally or centrally in a network configuration to interrogate up to 256 loops or single sensors.
Recent developments of stability control in mines, essentially based on Ge-doped Fibre Bragg Gratings
(FBG) are reported including results about the different aspects of the system : accurate characterisations of FBG,
sensor network topology and multiplexing method, user interface design and sensor packaging.
In this paper we propose replacing a widely used but often difficult and cumbersome technique of hydraulic evaluation of stress in concrete materials with a new fiber-optic measurement device, which has all inherent advantages of fiber-optic sensors. The sensing element of the device consists of a highly birefringent (HB) polarization-maintaining optical fiber. The stress inside it induced by external pressure modulates the polarization state of the output light signal at the detection end of the system. The all-fiber instrumentation system of the sensor consists of a semiconductor pigtailed laser, input and output HB optical fibers, an analyzer and a computer-controlled synchronous detection system. A specially designed leadthrough integrated with the sensor head allowed us to insert the sensor inside a pressure pad filled with oil or alternatively with mercury. For calibration purposes, the pressure cell was placed inside a large pressure chamber designed to simulate the real environment. Characterization of the device for hysteresis, selectivity and sensitivity was performed for pressures up to 70 bar and for ambient temperatures. The described sensor is simple, cost-effective, safe in explosive environments and well adapted for stress monitoring in the large-scale structures.
This paper reports how to choose the modulation frequencies of a time domain multiplexed optical fiber sensor network in order to obtain accurate measurement of the sensors' transmission factor. The signals are obtained by the interrogation method of distributed optical sensors with a multiplex of several modulation frequencies. Using simulation programs, the influence of the different system parameters was investigated. Shift invariance can be obtained with an IQ detector instead of the single real synchronous detector.
KEYWORDS: Sensors, Fiber optics sensors, Signal attenuation, Multiplexing, Modulation, Signal processing, Multimode fibers, Signal detection, Calibration, Frequency division multiplexing
Intensity modulation, induced by a modified microbending pressure
sensor is considered as a possible transduction mechanism for detecting
environmental changes.
Implementations of these kind of sensors into multiple sensor systems
is based on simple and reliable technology with multimode fiber,
couplers, a reference and a multiplexing technique.
A frequency domain technique is presented to interrogate unambiguously
time division multiplexed sensors.
Line neutrality is obtained by a referencing method.
Torsion experiments with multimode fibers in a microbend pressure
transducer application, to improve the sensor attenuation, are
reported.
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