A Proposal of Fiber Sensing System Using Low Reflectivity FBG

The monitoring of temperature during operation of lithium ion batteries is crucial for a stable and safe performance of a battery system. However, a major challenge is to obtain exact internal temperature progression upon charge and discharge processes. Though commonly used resistance temperature sensors can give considerably accurate temperature values, they are limited to the application outside of the cell due to the distortion of their signal through induction effects of the applied cell currents, if installed inside the pouch cell and therefore, they are not suitable for in situ measurements. This poses serious problems especially for big cell stacks, as core temperatures for those cells differ greatly from the outer parts and particularly the surface of the cell encasement. Fiber Bragg Grating (FBG) sensors on the other hand, provide data that are unaffected by induction effects as their operating principle is solely based on the reflection of light of certain wavelengths within a glass fiber. However, a specific difficulty is the distinction between temperature and pressure effects, as the reflected signal is sensitive to both. Often times additional tubing around the sensor is needed to eliminate pressure effects or an additional sensor is placed in close proximity, inside or outside, to the FBG-sensor as a reference. The former, however, can affect the cell stack physically acting as a defect spot with unwanted side effects.In our work, we show that a careful calibration of the FBG-sensor can be accomplished, so that both effects can be separated and the FBG-sensor can then be used to gather information not only about temperature but also about volume changes in an operating cell. Therefore, a temperature calibration took place, with the cell being held at a constant SoC. Subsequently, a calibration of the SoC - dependent pressure-induced mechanical deformation was performed at very low C-rate to minimize thermal effects. The so gathered calibration parameters can then be applied to all performed cycling tests, regardless of the C - rate. A calibration routine and mathematical solution to separate the influences of temperature and pressure to receive exact in situ temperature values for a 10 Ah lithium ion battery cell is presented. The calibration method is applied to cycling and rate tests at various C-rates.The results show, that wavelength shifts upon charging and discharging processes are mainly resulting from volume changes of the active cell components, meaning that the FBG – sensor has a very high sensitivity towards pressure changes. With the before mentioned calibration method the impact of mechanical deformation on the FBG - sensor can be subtracted from the raw data and allow for a direct temperature reading. The temperature data correlates with data gathered from commonly used sensors placed on the outside of the pouch cell. Tests with varying discharge rates confirm these results and show, that the method produces precise temperature data even at different C-rates, even though only slow C-rates were applied during calibration.In detailed comparison, the FBG - sensors show a faster response towards temperature changes compared to conventional external temperature sensors, which can be related to the insulating effect of the pouch foil. In case of high discharge rates, we were able to show that the measured temperatures inside of the pouch cell are higher in comparison to the outside ones. Furthermore, SoC - induced changes in pressure measured by two FBG-sensors, which were located at different positions on the cell stack, are almost identical. This indicates that pressure build-up within a pouch cell occurs evenly through the cell stack.

Laboratory core flooding experiments in reservoir sandstone under different sequestration pressures using multichannel fiber Bragg grating sensor arrays

Fiber Bragg Grating (FBG) sensor is a new type of optical sensor that is light in quality, small in volume, easy to achieve distributed measurement, and easy to embed. When FBG sensor technology is introduced to the damage detection of the CFRP shaft, the shortcomings of conventional damage detection technologies, including ultrasonic testing and radiographic testing, can be effectively overcome. Considering the working load, operating environment and life requirements of the CFRP shaft , combined with the relevant national and industry standards for composite material testing, Prepare three CFRP shaft with embedded fiber gratings (non-destructive CFRP shaft, single hole CFRP shaft , double hole CFRP shaft), the hole diameter is 1mm, and the layering of CFRP shaft is ([0 / 90 / 45 / 0 / 0 / 45 / -45 / 0 / -45 / 0 / 90 / 90 / 0 / -45 / 0 / -45 / 45 / 0 / 0 / 45 / 90 / 0 ]). FBG is embedded between 18 and 19 layers, pasted on the surface of CFRP shaft with 10 FBG sensors (named FBG1, FBG2... FBG10). The strain information of the CFRP shaft is obtained through fiber grating to obtain the deformation of the CFRP shaft under a three-point bending load. Each CFRP shaft is loaded 12 times (A1, A2, A3, B1, B2, B3, C1, C2, C3, D1, D2, D3). Through the BP neural network method, the mapping relationship between strain distribution and damage model was established. In the model, the FBG sensor measures the strain of the CFRP shaft and forms the input of the neural network. Combined with the damage situation of the composite material, the learning samples of the network formed. After learning and training, the neural network can damage recognition. To better verify the accuracy of the neural network to distinguish and identify the CFRP shaft non-destructive, single-hole, and double-hole damage patterns. The finite element analysis was used to simulate the damage of three CFRP shafts under the same experimental conditions, and extract the eigenvalues at the same position to form the input of the network, combined with the damage situation of the composite material, constitutes a learning sample of the network. The experiment shows that the CFRP shaft experiment based on BP neural network verifies the reliability of the CFRP shaft damage recognition method. The finite element analysis is used to extract the strain information of CFRP shaft measuring points with different damage modes. The damage position information and strain information were input into the BP neural network. The experimental results are in good agreement with the simulation results, which verifies the accuracy of the BP neural network to distinguish the CFRP shaft non-destructive, single-hole, and doublehole damage patterns.

Railway tracks are the direct supporting structures of the trains, which are vulnerable to produce large deformation under the temperature stress or subgrade settlement. The health status of track is critical, and the track should be routinely monitored to improve safety, lower the risk of excess deformation and provide reliable maintenance strategy. In this paper, the distributed optical fiber sensor was proposed to monitor the continuous deformation of the track. In order to validate the feasibility of the monitoring method, two deformation monitoring tests on one steel rail model in laboratory and on one real railway tack in outdoor were conducted respectively. In the model test, the working conditions of simply supported beam and continuous beam in the rail model under several concentrated loads were set to simulate different stress conditions of the real rail, respectively. In order to evaluate the monitoring accuracy, one distributed optical fiber sensor and one fiber Bragg grating (FBG) sensor were installed on the lower surface of the rail model, the strain measured by FBG sensor and the strain calculated from FEA were taken as measurement references. The model test results show that the strain measured by distributed optical fiber sensor has a good agreement with those measured by FBG sensor and FEA. In the outdoor test, the real track suffered from displacement and temperature loads. The distributed optical fiber sensor installed on the rail can monitor the corresponding strain and temperature with a good accuracy.

In this study, measurements from low-impact velocity experiments including embedded and surface mounted optical fiber Bragg grating (FBG) sensors were used to obtain detailed information pertaining to damage progression in two-dimensional laminate woven composites. The woven composites were subjected to multiple strikes at 2m/s until perforation occurred, and the impactor position and acceleration were monitored throughout each event. From these measurements, we obtained dissipated energies and contact forces. The FBG sensors were embedded and surface mounted at different critical locations near penetration-induced damaged regions. These FBG sensors were used to obtain initial residual strains and axial and transverse strains that correspond to matrix cracking and delamination. The transmission and reflection spectra were continuously monitored throughout the loading cycles. They were used, in combination with the peak contact forces, to delineate repeatable sensor responses corresponding to material failure. From the FBG spectra, fiber and matrix damage were separated by an analysis based on signal intensity, the presence of cladding modes, and the behavior of individual Bragg peaks as a function of evolving and repeated impact loads. This provided an independent feedback on the integrity of the Bragg gratings. A comparison by number of impact strikes and dissipated energies corresponding to material perforation indicates that embedding these sensors did not affect the integrity of the woven systems and that these measurements can provide accurate failure strains.

A novel and simple multi-wavelength fiber ring laser has been developed to implement an optical source of a fiber Bragg grating (FBG) sensor array in which respective wavelength component of the oscillation output provides an optical source of one of the wavelength-division multiplexed (WDM) FBG vibration sensors. The laser consists of a semiconductor optical amplifier (SOA) as a gain medium and of FBGs as wavelength selection elements. Since an SOA is an inhomogeneous gain medium, stable multi-wavelength oscillation can be realized to serve an optical source for the FBG vibration sensor array. In the experiment, a dual-wavelength fiber ring laser has been fabricated and tested, and simultaneous two-point vibration measurement has been successfully demonstrated.

Knowledge of the gas flow distributions, their mass velocity and turbulences, in chemical reactors, thermodynamic engines, pipes, and other industrial facilities may help to achieve a more efficient system performance. In our novel approach, optical fibre Bragg grating (FBG) sensors have been used for measuring the temperature of a heated element, adapting the principles of conventional hot-wire-anemometers. Because of the multiplexing capability of FBG sensors, the gas mass flow distribution can be measured along the sensor array. The length of the heated and sensor-equipped element can be easily adapted to the cross section of the gas flow, from <10 cm up to several metres. The number and distances of FBGs distributed over this length defines the spatial resolution and is basically limited by the sensor signal processing. According to FBG sensor lengths < 5 mm, spatial resolutions of gas flow measurements of less than 1 cm can be achieved.

Studying the temperature field of photovoltaic modules is important for improving their power generation efficiency. To solve the problem of traditional sensors being unsuitable for measuring the spatial temperature field, we designed a real-time detection scheme of the photovoltaic module temperature field based on a fiber Bragg grating (FBG) sensor array. In this scheme, wavelength division multiplexing and space division multiplexing technologies were applied. The multi-channel FBG sensor strings were arranged on the surface and in the near field of the photovoltaic module. Different FBG strings were selected through optical switches, and the wavelength of the FBG string was addressed and demodulated using the tunable laser method and a peak-seeking algorithm. A measurement experiment of the photovoltaic module temperature field was carried out in an outdoor environment. The experimental results showed that the fluctuation law of the photovoltaic module surface and near-field temperature is basically consistent with that of solar radiation power. The temperature of the photovoltaic module decayed from the surface to space. Within 6 mm of the photovoltaic module surface, the temperature sharply dropped, and then the downward trend became flat. The lower the solar radiation power and the higher the wind speed, the faster the temperature decay. This method provides technical support for measuring the temperature field of a photovoltaic module and other heat source equipment.

In this paper, one large Fiber Bragg Grating (FBG) sensor sensing network for bridge local health monitoring was studied, designed and implemented to Harbin Sifangtai Bridge, China. According to the hot point principle and finite element analysis results, FBG sensors installation scheme was made to monitor the local strain of key elements, respectively. 60 FBG sensors had been installed in monitoring points along the height of I-shape girders distributed in three main monitoring sections along the bridge. Monitoring results in load testing period was obtained, the static and dynamic results show that FBG sensors can monitoring the local strain precisely, and the local strain response is in the safe range under test vehicle load. Monitoring results in service period was obtained, 9 months after load test. Results show that maximum local strain responses in three main monitoring sections are in the safe range, under the dynamic vehicle load. Comparison of the monitoring results from FBG sensor sensing network are used to unveil the local health condition of Sifangtai Bridge reflected by local strain response.

In this study, a down-scaled wind turbine blade was designed and fabricated using glass and carbon fiber materials for the skin and stiffener, respectively. In the course of its fabrication, an array of FBG (fiber Bragg grating) sensors was embedded in the composite laminates. The embedded FBG sensor array was used to measure the residual strain in the stiffener before and after the curing process, and after fabrication of the blade, the FBG array was used to monitor the structural conditions, including structural dynamic behavior during the structural testing of the blade. The results of the tests showed that the FBG sensor array effectively measured the residual strain distribution of the blade stiffener before and after the curing process. And the measured natural frequencies and mode shapes by the FBG array matched the results obtained from the FE analysis and conventional accelerometers.

One challenging need for inspection capabilities is in adhesively bonded joints between composite components, a common location of premature failure in aerospace structures. In this work we demonstrate that dynamic, full spectral scanning of FBG sensors embedded in the adhesive bond can identify changes in bond quality through the measurement of non-linear dynamics of the joint. Eighteen lap joint specimens were fabricated with varying manufacturing quality. Ten samples also included fiber Bragg grating (FBG) sensors embedded in the adhesive bond for real-time inspection during a simulated flight condition of these single-lap joints. Prior to testing, pulse phase thermography imaging of the pristine specimens revealed defects such as air bubbles, adhesive thickness variations, and weak bonding surface between the laminate and adhesive. The lap joint specimens were then subjected to fatigue loading, with regular interrogation of the FBG sensors at selected load cycle intervals. The FBG data was collected during vibration loading of the lap joint to represent an in-flight environment. Changes in the lap joint dynamic response, including the transition to non-linear responses, were measured from both the full-spectral and peak wavelength FBG data. These changes were correlated to initial manufacturing defects and the progression of fatigue-induced damage independently measured with pulse phase imaging and visual inspections of the failure surfaces.

Optical fiber Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibers. The gap between the fiber end- faces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fiber and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.

Focus on sensors

Embedded fiber Bragg grating (FBG) sensors are attractive for in-situ structural monitoring, especially in fiber reinforced composites. Their implementation in metallic structures is hindered by the thermal limit of the protective coating, typically a polymer material. The purpose of this study is to demonstrate the embedding of FBG sensors into metals with the ultimate objective of using FBG sensors for structural health monitoring of metallic structures. To that end, ultrasonic additive manufacturing (UAM) is utilized. UAM is a solid-state manufacturing process based on ultrasonic metal welding that allows for layered addition of metallic foils without melting. Embedding FBGs through UAM is shown to result in total cross-sectional encapsulation of the sensors within the metal matrix, which encourages uniform strain transfer. Since the UAM process takes place at essentially room temperature, the industry standard acrylate protective coating can be used rather than requiring a new coating applied to the FBGs prior to embedment. Measurements presented in this paper show that UAM-embedded FBG sensors accurately track strain at temperatures higher than 400 °C. The data reveals the conditions under which detrimental wavelength hopping takes place due to non-uniformity of the load transferred to the FBG. Further, optical cross-sectioning of the test specimens shows inhibition of the thermal degradation of the protective coating. It is hypothesized that the lack of an atmosphere around the fully-encapsulated FBGs makes it possible to operate the sensors at temperatures well above what has been possible until now. Embedded FBGs were shown to retain their coatings when subjected to a thermal loading that would result in over 50 percent degradation (by volume and mass) in atmospherically exposed fiber.

Strain response of FBG sensors are investigated at various temperatures from 298 K to 4.2 K. Numerical modelling is carried out for acrylate coated, substrate-free fiber Bragg grating (FBG) sensors at room temperature of 298 K and cryogenic temperatures of 77 K, 10 K and 4.2 K. A 1550 nm Bragg wavelength (λB) FBG sensor is modelled and simulated for applied strain (ε) ranging from 0 to 800 µm/m. The Bragg wavelength shifts (ΔλB) thus obtained are compared with the experimentally investigated values obtained by subjecting the FBG sensor to axial strain, with its sensing part not being bonded to any surface. The MTS25 tensile machine with a cryostat under vacuum conditions (10-4 mbar pressure) is used for the experiments and the required temperatures are maintained using liquid Nitrogen (LN2) and compressed Helium gas (He). The Bragg wavelength shift (ΔλB) versus induced strain (ε) is regressed with a linear polynomial function and the strain sensitivity obtained in both the cases are discussed.