Quasi-distributed Fiber Bragg Grating Research Articles

Experimental Study on CRTS III Ballastless Track Based on Quasi-distributed Fiber Bragg Grating Monitoring

Experimental Study on CRTS III Ballastless Track Based on Quasi-distributed Fiber Bragg Grating Monitoring

The Quantitative Detection System and Method for Natural Ice Covering of Composite Insulators by Quasi-Distributed Fiber Bragg Gratings

The Quantitative Detection System and Method for Natural Ice Covering of Composite Insulators by Quasi-Distributed Fiber Bragg Gratings

The Detecting System and Method of Quasi-Distributed Fiber Bragg Grating for Overhead Transmission Line Conductor Ice and Composite Insulator Icing Load

Transmission lines icing is destructive to the operation of the power grid. There is a lack of methods to monitor overhead transmission line conductors and insulators for icing load simultaneously. In this paper, we implanted six optical fibers with three quasi-distributed FBGs each into the interface of a 110kV composite insulator and built an icing detection system, and proposed three ways for measuring overhead transmission lines icing load. A method for quasi-distributed FBG to differentiate between the measurement of running insulator umbrella and conductor icing load was proposed and validated. The effect of the number of implanted optical fibers on the measurement accuracy was studied. The results show that the relative error of simulated insulator umbrella skirt icing load measurement is -7.3%∼-2.7%. The 200m-span running conductor insulator umbrella skirt icing load measurement relative error is -11.5%∼-3.8%; the relative error of conductor ice measurement is -16.5%∼9.0% and absolute error is -0.83∼1.5mm. The composite insulator with an optical fiber embedded can accurately warn of serious icing on the conductor, with two can achieve the conductor icing measuring absolute error of about ±1mm. As for accurately measuring both the umbrella skirt and the conductor icing, it is necessary to implant six optical fibers.

Monitoring shear deformation of sliding zone via fiber Bragg grating and particle image velocimetry

Monitoring shear deformation of sliding zones is of great significance for understanding the landslide evolution mechanism, in which fiber optic strain sensing has shown great potential. However, the correlation between strain measurements of quasi-distributed fiber Bragg grating (FBG) sensing arrays and shear displacements of surrounding soil remains elusive. In this study, a direct shear model test was conducted to simulate the shear deformation of sliding zones, in which the soil internal deformation was captured using FBG strain sensors and the soil surface deformation was measured by particle image velocimetry (PIV). The test results show that there were two main slip surfaces and two secondary ones, developing a spindle-shaped shear band in the soil. The formation of the shear band was successfully captured by FBG sensors. A sinusoidal model was proposed to describe the fiber optic cable deformation behavior. On this basis, the shear displacements and shear band widths were calculated by using strain measurements. This work provides important insight into the deduction of soil shear deformation using soil-embedded FBG strain sensors.

Legendre Polynomials-Based Displacement Field Reconstruction of Plate-Type Structures With Quasi-Distributed FBG Sensors

Structural deformation estimation is of great interest for many engineering fields, such as aerospace structures, antenna panels, and so on. During operation, however, the direct measurement of global displacements is often difficult, due to the limits of installation space and electromagnetic interference. This article proposed a displacement field reconstruction method for deformed plate-type structures using discrete curvature measurements and a polynomial superposition approach. Specifically, the displacement field is expressed as a linear superposition of the Legendre polynomial basis functions, where the weight coefficients are calculated using the discrete curvatures captured by a quasi-distributed fiber Bragg grating (FBG) sensor array and the given boundary conditions. This method overcomes the drawback of error accumulation in other curvature-based methods due to the use of recursion operations. In addition, the curvature fitting and the determination of the number of polynomial basis functions were also presented to obtain the best reconstruction quality. Simulations and experiments have been conducted to validate the proposed method. The simulation results show that the displacement reconstruction precisions are more than 99% for two types of complex deformations. Experiments were carried out on an aluminum plate and a phased-array antenna panel, respectively. The results show that the reconstructed displacements of the reference points match well with those measured displacements, and the reconstruction precision exceeds 96%. This proposed curvature-based approach has high potential to global displacement monitoring of plate-type structures used in aerospace and antenna industrial fields.

Cascade Edge Filter for Demodulation of Quasi-Distributed FBG Sensor

The realization of multiple demodulation sche- mes of fiber Bragg grating (FBG) sensor is important for its engineering application. Here, we proposed a new edge filter that is formed by cascading dispersion compensation fiber (DCF) and no-core fiber (NCF) structure between two single-mode fibers (SMFs) for demodulation of quasi-distributed FBG sensor. The edge filter can be suitable for FBG with different center wavelengths by changing its structure parameters to achieve a wide range of FBG demodulation. In order to verify the performance of the edge filter, a demodulation system for quasi-distributed FBG sensor is successfully designed by introducing an optical time-domain reflectometer (OTDR), the wavelength shift of FBG caused by the change of temperature and strain is converted into power change through the edge filter, so as to realize temperature and strain demodulation. In the experiment, two FBG sensors separated by 500 m were set at the end of a 3000-m optical fiber for temperature and strain monitoring. The results show that the temperature sensitivity is 0.23 dB/°C from 50 °C to 100 °C, and the strain sensitivity is 0.01658 dB/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \epsilon $ </tex-math></inline-formula> from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$669.6\,\mu \epsilon $ </tex-math></inline-formula> . The edge filter for FBG demodulation has the advantages of high sensitivity, adjustable structural parameters, low cost, and independent multipoint/multiparameter demodulation.

Quasi-distributed FBG interrogation based on coherent wavelength-to-time mapping using mode-locked laser and pseudo-random barcode

A microwave photonic demodulation scheme for multiplexing fiber Bragg grating (FBG) sensing system based on coherent wavelength-to-time (WTT) mapping is proposed and demonstrated experimentally in this paper. With the help of mode-locked laser (MLL) source and dispersion element, the broadband pseudo-random barcode obtained from fiber random grating can be mapped into time domain to fulfill high-speed interrogation for FBGs. The shifts of FBG center wavelength will be transformed into the variations of filtered pseudo-random barcode, and finally converted into the time location changes of the cross-correlation peaks with narrow full-width half-maximum (FWHM), which is helpful to realize high-accuracy and high-resolution FBG demodulation utilizing oscilloscope. The experiment results exhibit good demodulation linearity and a strain interrogation sensitivity of 0.0302 ns/με in cascaded sensing network with a repetition rate of 20 MHz, corresponding to a strain measurement resolution of 2.05 με. Besides, the impacts of system noises and light source line-shapes on the demodulation performance are studied numerically, and the relevant solutions are proposed and verified respectively.

Method of quasi-distributed interface fiber Bragg gratings monitoring dry band arc on the moist pollution layer of composite insulators

Arc discharge on the insulator surface poses a threat to the safe operation of electrical power system. However, effective monitoring technology for arc discharge is lacking. In this paper, a 10 kV composite insulator with 3 × 4 quasi-distributed fiber Bragg gratings (FBGs) embedded at the insulator core-housing interface was developed to monitor dry band arc on the moist pollution layer. The temperature coefficient of interface FBGs was obtained by the temperature calibration test. The interface temperature variation exposed to the instantaneous arc with arc duration of 67 ms and the short-lived arc with arc duration of 2.1 s was observed. Results show that owing to the occurrence of short-lived arc, a new phenomenon was observed: interface temperature drops sharply first, then increases rapidly. Three criteria of ED (Euclidean Distance, i.e. the measuring standard for the otherness of interface temperature of different axial positions), the sharp drop of interface temperature and interface temperature rise are proposed to monitor dry band arc. Based on the three criteria, interface FBGs can effectively monitor the instantaneous arc and the short-lived arc. This paper provides a beneficial method for discharge monitoring and flashover warning of composite insulators.

Research into using a fiber Bragg grating sensor group for three-dimensional in situ stress measurement

Abstract. The observation and estimation of the deep crustal stress state is a key and difficult problem for in situ stress measurement. Using a borehole wall strain gauge based on the overcoring stress-relieving method is one of the main methods of in situ stress measurement. In this paper, a strain-sensing array based on fiber Bragg grating (FBG) is designed by using the main structure of the classical hollow inclusion cell, and its layout scheme on the hollow inclusion is studied. According to the layout scheme, the in situ stress inversion algorithm of hole wall strain to stress is deduced. Following this, the triaxial loading and unloading experiment platform is built, and the calibration experiment for the FBG strain sensor is designed. Finally, Abaqus finite element software is used to simulate the in situ stress measurement process of the overcoring stress relief. The FBG strain values of each measurement direction before and after the overcoring process are extracted, and the stress inversion equation is used to carry out the stress inversion. The comparison of the inversion results proved that the FBG strain sensor group is feasible and reliable. The quasi-distributed FBG sensor module designed in this paper can invert the three-dimensional in situ stress by measuring the hole wall strain, which places a theoretical and experimental foundation for the development and application of an FBG hole wall strain gauge. It makes up for the deficiency of the existing hole wall strain gauge based on a resistance strain gauge, provides direct and accurate observations for hole wall strain measurement, and has important practical value for the development of in situ stress measurement technology.

Quasi-distributed fiber-optic in-situ monitoring technology for large-scale measurement of soil water content and its application

Accurate acquisition of in-situ soil water content is of great significance for mastering the water content distribution of unsaturated soil and understanding soil engineering properties. The actively heated fiber-optic (AHFO) method has recently become a research focus because of the capability to realize distributed and real-time measurement of soil water content. However, most reported AHFO systems have been coupled to distributed temperature sensing (DTS) which does not allow for remote measurements with high spatial resolution. In this paper, a new AHFO method based on the quasi-distributed fiber Bragg grating (AHFO-FBG) technique is proposed for the large-scale in-situ measurement of soil water content. An AHFO-FBG in-situ monitoring system is developed which consists of a sensor module, heating module, data acquisition and transmission module, and data processing module. The established system is applied to a loess slope in Yan'an City, Shaanxi Province, China to monitor its water content distribution in real time. The effectiveness of the monitoring system is verified and the water content distribution of the monitored loess slope are analysed. The in-situ measurement results indicate that in the spatial dimension, the water content in the top and bottom of the slope is low, and that near the scarp is high; in the time dimension, the water content in winter is low while that in autumn and spring is high. The monitored results also suggest that soil water evaporation in winter can significantly enhance the spatial distribution difference of soil water content.

Datacenter Thermal Monitoring Without Blind Spots: FBG-Based Quasi-Distributed Sensing

Computing performance and hardware integrity are tightly linked to adequate thermal management in datacenters. In addition, a large share of its electricity bills comes from cooling systems, so there is a pressing need for enhanced thermal management systems. This paper proposes a fiber Bragg grating (FBG) quasi-distributed sensing device as an alternative to electronic sensors. The main focus is to demonstrate that a FBG-based solution allows thermal measurement at critical non-monitored regions (blind spots) within a server in an acquisition rate of 1 Hz, with thermal sensitivity of 8.757 pm/°C, a thermal resolution smaller than 1 °C and spatial resolution and multiplexing needed to monitoring diverse blind spots where there is virtually no room for adapting conventional sensors. In addition, we also monitor spots where there are already internal sensors next to them (referential spots), such as central processing unit (CPU) and graphics processing unit (GPU). As a result, a simple FBG sensor calibration technique is proposed using those referential spots. The technique proposed can be scaled out to monitor a large number of elements across large areas in a datacenter environment, since quasi-distributed FBG sensing signal can be transmitted over longer optical fibers than those employed in optical distributed temperature systems (DTS).

Monitoring of prestress and bond stress of self-sensing FBG steel strand

In order to investigate the stress distribution in a prestressed steel strand and the development of bond stress between the steel strand and concrete, a new technology of encapsulating quasi-distributed fiber Bragg grating (FBG) sensor in a longitudinal groove of the center wire of a steel strand is proposed. Based on the theoretical analysis of the strain transfer between the FBG sensor and the substrate, specific parameters for measuring the stability of the FBG sensor are discussed. Tension and bending tests on unbonded prestressed reactive powder concrete beams are carried out to explore the self-sensing steel strand characteristics. The test results indicate that the quasi-distributed FBG sensor encapsulated in the center wire of the steel strand can monitor the stress distribution in the steel strand and the damage of the beam efficiently, and the maximum tensile monitoring error is only 3.42%. To analyze the distribution of the prestress and bond stress of steel strand along the anchorage length, the tensile tests are carried out on the self-sensing steel strand. The results show that the quasi-distributed FBG sensor can accurately measure the variations in steel strand stress and bond stress along the anchorage length during the tensile process.

Dilated convolutional neural networks for fiber Bragg grating signal demodulation.

In quasi-distributed fiber Bragg grating (FBG) sensor networks, challenges are known to arise when signals are highly overlapped and thus hard to separate, giving rise to substantial error in signal demodulation. We propose a multi-peak detection deep learning model based on a dilated convolutional neural network (CNN) that overcomes this problem, achieving extremely low error in signal demodulation even for highly overlapped signals. We show that our FBG demodulation scheme enhances the network multiplexing capability, detection accuracy and detection time of the FBG sensor network, achieving a root-mean-square (RMS) error in peak wavelength determination of < 0.05 pm, with a demodulation time of 15 ms for two signals. Our demodulation scheme is also robust against noise, achieving an RMS error of < 0.47 pm even with a signal-to-noise ratio as low as 15 dB. A comparison on our high-performance computer with existing signal demodulation methods shows the superiority in RMS error of our dilated CNN implementation. Our findings pave the way to faster and more accurate signal demodulation methods, and testify to the substantial promise of neural network algorithms in signal demodulation problems.

System and Method of Quasi-Distributed Fiber Bragg Gratings Monitoring Brittle Fracture Process of Composite Insulators

Brittle fracture of the composite insulator is a malignant accident. There is a lack of means to monitor the formation and expansion of stress corrosion cracking in rods, resulting in little understanding of the process of brittle fracture development. First, this article proposes a mechanical model for identifying cracks in rods by the wavelength shift trends of fiber Bragg gratings (FBGs). Second, several FBGs and FBG temperature compensation sensors are pasted on the 10-kV composite insulator rod's surface. Then an optical fiber monitoring system for the brittle fracture process of the rod is established. Finally, the model and system were validated by conducting stress corrosion testing with different tensions. The results show that the surface quasi-distributed FBGs can monitor the temperature and stress in the complete brittle fracture process of a glass-fiber-reinforced polymer (GFRP) rod, which verified the correctness of the mechanical model for wavelength shift identification of cracks in axial cross sections. A temperature difference of 1 °C and the wavelength shift of the FBG on two or more fibers reach 1.1 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">smin</sub> and 0.9 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">smin</sub> can be used as warning thresholds for corrosive heat release, crack initiation, and rapid development of cracks to fracture, respectively. This article provides a quasi-distributed FBGs' monitoring system and method to study brittle fracture of composite insulators in stress corrosion testing. It provides a technical means to evaluate and warn the mechanical properties of composite insulators.

Real-Time Optical Fiber-Based Distributed Temperature Monitoring of Insulation Oil-Immersed Commercial Distribution Power Transformer

Timely detection of the incipient fault of a power transformer is of utmost importance to prevent potential malfunctioning. Monitoring the condition of the transformer insulation oil is the essential step as it contains most of the transformer's health diagnostic information. Thus, the temperature of the oil is considered the most crucial parameter that has to be monitored continuously in real-time. Distributed optical fiber sensors for temperature monitoring has various advantages over traditional methods. In this study, an optical fiber-based distributed temperature measurement of insulation oil of a fully energized 100 kVA commercial distribution transformer is demonstrated. Two types of techniques were deployed to monitor the temperature of the transformer inside and outside of the tank using a distributed Optical Frequency Domain Reflectometry (OFDR) and a quasi-distributed Fiber Bragg Grating (FBG) sensor array respectively. The insulation oil temperature was monitored with different load conditions in a distributed fashion and compared with the conventional method of single point thermocouple and infrared thermal imaging. The test results show very good agreement between the conventional methods and proposed distributed fiber temperature sensors.

Spatially resolved thermometry during laser ablation in tissues: Distributed and quasi-distributed fiber optic-based sensing

Abstract This work assesses quasi-distributed Fiber Bragg Gratings (FBGs) and Rayleigh scattering-based distributed sensing for accurate and millimeter resolved thermometry in media undergoing laser ablation. Mono-, two- and three-dimensional temperature measurements were performed with a network of 1 mm-long FBGs in 10-25-40-FBGs-arrays and distributed-sensing fiber with gauge lengths (GL) of 5.2 and 1.3 mm. Results confirm that the lower the spatial resolution of the sensors, the highest the root mean square error (RMSE) and temperature underestimation with respect to reference sensors (25-FBGs array). Compared to 1 mm-long FBGs, distributed sensing with 5.2 mm GL provided RMSE >7 °C and underestimation >16 °C in agar-gel phantom. Signal-to-noise ratio over time and temperature standard deviation over temperature show better performances of FBGs over distributed sensors. Indeed, any improvement of spatial resolution for distributed sensors determines an increase of noise in the signal. The three-dimensional measurement performed with 420 FBGs with 1 mm-sensing length demonstrated the spatial and thermal gradient occurring in the pancreas undergoing LA, reaching 28 °C over 5 mm. Millimeter-resolved FBGs proved to be effective for accurate thermometry in laser irradiated tissues, and potentially usable in clinical applications.

Experimental study on seepage characteristics of microfracture with different aperture

Exploring the flow mechanism of fluid in rock mass is important in solving the water inrush problems during tunnel excavation. However, it is difficult to obtain an undisturbed rock mass from the actual site conditions and study the flow mechanism of fluid through a fracture network composed of multiple single fractures. Therefore, a solution to simulate rock seepage using rock-like samples with single microfracture was presented in this paper. Water flow through microfracture was tested and the deformation of microfracture was recorded by quasi-distributed fiber Bragg grating (FBG) technology. Experimental data showed that Forchheimer’s law and Izbash’s law could well describe the nonlinear relationship between flow velocity and hydraulic gradient. The coefficient b in Forchheimer’s equation decreased with the increase of microfracture aperture. A critical value of E = 0.8 was proposed to classify the nonlinear flow regime: weak turbulence (E < 0.8) and fully developed turbulence (E > 0.8).

The temperature field modeling for metal plate surface based on PSO-B-spline interpolation and FBG sensors

Abstract The construction of temperature field is of great significance to ensure the normal work of machines. Therefore, it is necessary to consider appropriate methods for temperature measurement and modeling. In this paper, quasi-distributed measurement based on fiber Bragg grating (FBG) sensors is established, due to high accuracy and stability of FBG in sensing temperature on the surface of machines. Additionally, the finite element method and computer-aided geometric design (CAGD) are applied to temperature field modeling, in which three different interpolation methods belonging to CAGD are used to construct temperature field, and a B-spline interpolation improved by particle swarm optimization is presented. According to experiments on a metal plate, measured by a quasi-distributed FBGs network, results indicate that the combination of FBG sensors and PSO-B-spline interpolation provides a better performance in temperature field construction in comparison with many other interpolation methods.

Laboratory Simulation of Flow through Rough-Walled Microfractures under High Hydraulic Gradient

Laboratory experiments on fluid flow through fracture are important in solving the fluid-in-rush problems that happen during the tunnel excavation. In order to study the mechanism of fluid flow through a rough-walled microfracture, fluid flow experiments were carried out and the fiber Bragg grating (FBG) strain sensors were applied to monitor the deformation of the microfracture surface during the seepage process. Considering the difficulty of collection of undisturbed rock samples from the deep locations, a methodology to simulate fluid flow through a fractured rock mass using analog materials containing a single fracture was developed. This method is easy to simulate the fluid flow through a fracture of certain aperture. Experimental data showed that Forchheimer equation could provide an excellent description of the nonlinear relationship between hydraulic gradient and flow velocity, and the variations of Forchheimer coefficients with joint roughness coefficient (JRC) were studied. It was found that the deformation of the microfracture surface subjected to seepage could be accurately captured by the quasi-distributed FBG strain sensors. The test results also demonstrated that the surface strain is significantly affected by hydraulic pressure.

Feasibility Study on Temperature Distribution Measurement Method of Thrust Sliding Bearing Bush Based on FBG Quasi-Distributed Sensing.

According to the characteristics of the temperature distribution of the thrust sliding bearing bush, the principle and method of quasi-distributed fiber Bragg grating (FBG) sensing is used to measure it. The key problems such as calibration, arrangement and lying of optical FBG sensors are studied by using the simulated thrust sliding bearing bush, which was customized in the laboratory. Combined with the thrust sliding bearing bush, the measurement experiments were carried out, which were divided into two groups: Steady-state experiments and transient experiment. The steady-state experiments obtain the temperature data measured by the FBG temperature sensors at each setting temperature, and the transient experiment obtains the relationship between the measured temperature by each temperature sensor and time in the heating and cooling process. The experimental results showed that the FBG temperature sensors had good accuracy, stability and consistency when measuring the temperature distribution of bearing bush.