Real-time health monitoring of engineering structures is crucial for improving structural safety, extending the lifespan of structures, and reducing maintenance costs. In this paper, based on the strain monitoring data from distributed fiber optic sensors, a deformation reconstruction algorithm is established to obtain structural deformation information from strain measurements. An experimental test is conducted on a reinforced concrete plate subjected to explosive loads. Distributed fiber optic strain sensors are embedded in the upper and lower surfaces of the plate, and the deformation of the concrete plate under different blast conditions is evaluated by varying the explosive yield. The comparison between the reconstructed deformation results and the actual state of the structure (obtained through high-precision laser scanning) shows that the error of the structural deformation monitoring system based on fiber optic sensors is less than 3mm.
This paper aims to realize the structural safety monitoring of protective engineering by conducting experimental research on the application of fiber optic grating sensors in high-speed penetration and damage monitoring. The study designs a miniaturized flexible fiber optic grating strain sensor for dynamic strain measurement during high-speed penetration and damage of the target body by projectiles, and verifies its measurement performance by comparing it with strain gauges. The experimental results demonstrate that the dynamic strain measurement results of the fiber optic grating strain sensor during high-speed penetration and damage are consistent with those of the strain gauges and the actual situation. The fiber optic grating strain sensor can achieve dynamic strain measurement during high-speed penetration and damage processes. Moreover, the variance of strain measurement results from the fiber optic grating strain sensors is much smaller than that of the strain gauges. This study validates that fiber optic grating strain sensors have the advantages of convenient installation, suitability for embedding, no need for electrical power, safety against explosions, immunity to electromagnetic interference, and strong environmental adaptability. They provide comparable results to strain gauges for measuring highspeed dynamic strains while overcoming the challenges associated with strain gauge installation difficulties, susceptibility to damage during embedding, high grounding requirements, poor environmental adaptability, and high surface bonding requirements. Therefore, fiber optic grating strain sensors are an important new technological approach that can replace strain gauges in future safety and health monitoring of protective engineering projects during wartime.
In order to measure the structural strain inside concrete under the impact of explosions and other strong shock loads, a flat-type multi-parameter distributed fiber optic sensor was developed. In this experiment, we employed distributed fiber optic sensing technology by deploying optical fibers at typical positions on the explosive model to monitor the real-time strain distribution of the structure under explosive impacts. Simultaneously, fiber optic grating sensors were embedded in the structure to accurately measure the strain parameters at specific locations. The experimental results demonstrate that the embedded distributed fiber optic sensors can accurately measure the internal strain of the concrete structure. The measured pit diameter on the target surface was 41cm, which is very close to the actual damage range (diameter of 37cm). The measured damage level also aligns with the actual damage level of the target. The fiber optic grating sensors, packaged with glass fiber-reinforced polymer (GFRP), exhibited stable measurements around 298με, which is almost identical to the distributed fiber optic measurement of strain at the center of the target, which was 253με. This technology overcomes the limitations of traditional measurement methods and is suitable for testing dynamic stress and strain under explosive impacts and in harsh environments with moisture and electromagnetic interference. It has excellent prospects for application.
To meet the high-precision positioning requirements of dam health monitoring,this paper designs a high spatial resolution distributed Brillouin dam health monitoring scheme based on DPP-BOTDA (Differential Pulse Pair Brillouin Optical Time Domain Analysis) technology.Due to the large pulse width and strong pulse energy of the two differential pulses used in the Brillouin system, the high-precision measurement of Brillouin frequency shift can be achieved; Moreover, the difference in pulse width between the differential pulse pair is small, which can achieve high spatial resolution and meet the high-precision positioning and measurement requirements for dam health monitoring. The rise and fall time of the pulse signals is a key factor affecting the spatial resolution of DPP-BOTDA systems. To achieve centimeter level spatial resolution, this paper designs a SOA (semiconductor optical amplifiers) narrow pulse driving circuit that can generate pulse signals with rise and fall times less than 1 ns (ps level pulse edge). An experimental setup for the DPP-BOTDA system is built. The SOA narrow pulse driving circuit mentioned above is used to generate pulse signals with pulse widths of 48ns and 50ns, respectively, for differential Brillouin tests. By analyzing the spatial resolution test curve of the system with the differential pulse signals demodulation, it can be concluded that the system can achieve a spatial resolution of 0.2m. It meets the high spatial resolution requirements for dam health monitoring and is of great significance in the field of distributed Brillouin fiber optic sensing applications.
KEYWORDS: Demodulation, Temperature metrology, Spatial resolution, Pulse signals, Frequency response, Data processing, Time-frequency analysis, Signal detection
Traditional Brillouin optical time domain reflectometer (BOTDR) generally uses frequency scanning to obtain the Brillouin gain spectrum. And the measurement speed of the sweeping frequency (SF) method is slow, usually on the order of minutes. In this paper, the mechanism of rapid BOTDR measurement based on sloped-assisted (SA) technology is analyzed, the measurement frequency of SA-BOTDR is theoretically calculated, and the system scheme is proposed and designed. Through experimental research, the temperature measurement range of the system is 25°C~70°C at the end of the 203m sensing fiber. The temperature measurement accuracy is 1.38°C, the spatial resolution is 1.21 m, and the measurement frequency is 11.49 Hz.
The pumped-storage power station is affected by external forces and environmental erosion during its long-term service, thus causing serious safety accidents and economic losses. In order to avoid casualties and reduce economic losses, longterm health monitoring of the power station is necessary. Optical fiber sensor is one potential technology for the pumpedstorage power station structural health monitoring (SHM) by measuring distributed strain and temperature. In this paper, the dam construction site of the Liaoning Qingyuan Pumped Storage station is installed with the optical fiber sensors to monitor its temperature change during concrete cure process. The distributed temperature at four sides of the panel are recorded and compared. Temperature change due to curing process as well as energy interchange between the top and bottom of the panel is revealed by the optical fiber measurements. It is therefore verified that fiber optical sensor is complement for dam temperature monitoring and is well suited for the identification of energy interchange insides the dam panels.
The application of distributed Brillouin optical fiber sensing technology in dam health monitoring is mainly studied in this paper. In the dam health monitoring system, the key technology is real-time deformation monitoring. Firstly, the principle of Brillouin optical time domain analysis (BOTDA) is introduced. A long-term on-line monitoring scheme of dam strain is proposed based on the distributed Brillouin sensing technology. The scheme includes cable layout and strain test. In the test, a 200m single-mode optical cable is selected. Firstly, the center frequency shift data of the whole optical cable is measured, when the displacement load is set to 0 mm, which is used as the test reference of strain variation. Then, the 2m length of the middle part of the optical cable is stretched to perform large amplitude tensile tests and shrinkage tests. The displacement loads are 2mm, 4mm, 6mm, 8mm and 10mm respectively. Next, small amplitude tensile tests and shrinkage tests are performed. The displacement load is 0.5mm, 1mm, 1.5mm and 2mm respectively. Analysis of test data shows that, the strain measurement range of the scheme can reach ± 4500με, the resolution of the scheme can reach 20 με. The results prove that the distributed fiber Brillouin sensor system can realize the long-term on-line distributed measurement of dam strain, which meets the requirements of the dam strain monitoring. It provides an effective means for dam health monitoring and protection, and a strong support for large-scale structural health monitoring with distributed Brillouin optical fiber sensing.
In this paper, we introduce an optimal peak search strategy of Brillouin gain spectrum (BGS) and denoising method of central wavelength of BGS based on Brillouin optical time domain analysis (BOTDA) system. The system utilizes both up shifted and down shifted continuous-wave light to generate Brillouin scattering light, while using the amplified pulsed light to boost the signal by stimulated Brillouin scatting effect. In order to obtain an accurate strain change of fiber under test (FUT), the peak of each Brillouin scattering spectrum must be accurate and precise. In addition, the intensity of shifted light should be stable as well. Therefore, we propose a novel peak search algorithm of BGS of a BOTDA system, which keeps the right central wavelength even if the scan step of the frequency is in large condition. Besides that, we also proposed a time domain denoising method to enhance the signal to noise ratio. We also implement static experiments to verify our proposed scheme. Strain/temperature tests were taken out evaluate the performance of the BOTDA. Results showed that the resolution of BOTDA system could reach 13με/0.65°C at 1 km fiber cable.
KEYWORDS: Global Positioning System, Safety, 3D scanning, Laser applications, Optical fibers, Optical sensing, Data modeling, Data acquisition, Sensors, Laser scanners
Deformation monitoring is an important content of dam safety monitoring. This article firstly introduces the origin and development process of dam safety monitoring. Dam deformation monitoring is then reviewed. Artificial dam deformation monitoring technologies including tension line method, alignment method, forward intersection method, traverse method, positive and negative vertical line method, geometric leveling method and hydrostatic leveling method, as well as automatic dam deformation monitoring technologies including measuring robot, photogrammetric technology, global positioning system (GPS), three-dimensional laser scanning technology and fiber optic sensing technology are discussed by comparing their principles, advantages and disadvantages. Main problems of current dam deformation monitoring procedure are summarized. Based on that, the development trends in dam deformation monitoring are finally predicted.
The application of distributed optical fiber sensing technology in nuclear island safety monitoring is mainly studied in this paper. The anti-radiation ability of the system is an important index. The distributed optical fiber system in this paper is designed with a special anti-radiation optical fiber. The temperature of power supply cables and other facilities in the nuclear island containment can be distributed monitored in full period in real time. Radiation tests are carried out to validate the anti-radiation performance of the system. 60Co is used as a gamma ray radiation source to generate continuous pulses with an average energy of 1.25 MeV. The rate of radiation dose is 1800 Gy/h and the total radiation dose is 1950 Gy. The anti-radiation ability of anti-radiation optical fiber and common optical fiber is compared. It is proved by tests that common optical fiber sensor has great fiber loss in radiation environment, about 0.5dBm, and the loss is decreasing continuously. Anti-radiation optical fiber has little fiber loss in radiation environment, about 0.12 dBm, and the loss tends to be saturated. During the radiation test, the temperature measurement performance of the distributed optical fiber sensing system using anti-radiation optical fiber and the one using common optical fiber is tested in real time. The results show that the anti-radiation distributed optical fiber sensing system performs well in the whole process, which meets the requirements of temperature monitoring for 1339.2 Gy total radiation dose in the refueling cycle of nuclear island.
In this paper, we introduce an optimal bias voltage searching strategy and maintaining method in BOTDA system based on dual sideband modulation. The system utilizes both up shifted and down shifted continuous wave light to generate Brillouin scattering light, while using the amplified pulsed light to boost the signal by stimulated Brillouin scatting effect. In order to obtain a clean Brillouin shifted curve along the fiber under test, the probe light must be in good signal to noise ratio. In addition, the intensity of shifted light should be stable as well. Therefore, we propose a novel control method of frequency shift in sideband modulation of a BOTDA system, which keeps the first order frequency-shifted light at its maximum signal to noise ratio and minimum light intensity fluctuation. We also implement static experiments to verify our proposed scheme. Strain/temperature tests were taken out to evaluate the performance of the BOTDA. Results showed that the resolution of BOTDA system could reach 13με/0.65°C at 50 km fiber cable with spatial resolution of 0.5m.
An optical model to simulate the distributed fiber optical sensor based on spontaneous Brillouin spectrum is derived. The reliability of this model is validated with experimental measurements. Using this analytical expression, parametric studies are conducted to investigate impacts of key factors including fiber loss, signal to noise ratio, bandwidth and scanning step on the optical fiber sensor measurement error. The simulation results exhibit good agreement with previous published calculation results. Applying this novel model into the data interpretation, measurement error of distributed fiber optical sensor based on spontaneous Brillouin scattering can be better controlled.
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