Fiber Bragg Grating Sensor Array Research Articles

Low-velocity impact localization on a full composite wing of UAV using fiber optic sensors under operating environments

In this paper, low-velocity impact localization algorithms were suggested using fiber optic sensing system. In order to obtain high frequency signals due to low-velocity impact, a commercial high speed fiber Bragg grating (FBG) sensing system (a sampling speed of 100 kHz) was adopted. The test article is a full-scale composite wing box structure as the main wing of an unmanned aerial vehicle. The multiplexed FBG sensor array was attached on the lower surface of the upper skin. Low-velocity impact experiments were performed to obtain the impact signals for the reference data set. The reference data set is composed of the impact signals from the impacts on the reference points. The suggested algorithm in this study is based on the comparisons between impact signals and the reference signals in the database set. The main idea is to determine the reference point which has the most similar data set as the impact location. This idea was validated from the experiments of the non-reference points. As the next step, operational environments were considered to enhance the reliability of the suggested algorithm. The considered environments are the static loads and vibrations. These days, most of UAVs use the electric motors and propellers as the main propulsion system. Thus, harsh vibrations inherently occur and affect the performance of impact localization algorithms. From the experiments under these operating environments, it can be checked that the suggested algorithm has sufficient robustness to such environments.

Research on spacecraft in orbit perception based on artificial neural networks and digital twin technology using grating arrays

In order to improve the safety of spacecraft, the research on artificial neural network and digital twin technology based on, to our best knowledge, a novel fiber Bragg grating (FBG) sensor array is proposed for intelligent sensing monitoring of spacecraft on-orbit collisions. Femtosecond FBG arrays were fabricated on the novel oxide-doped fiber by point-by-point writing technique. The femtosecond FBG is analyzed using the time-dependent perturbation theory of quantum mechanics. The FBG array can achieve high-temperature measurement of 1100 °C and large strain measurement of 15000 µε. The sensing arrays were deployed on the surface of the spacecraft. Constructed the multi-layer perceptron neural network structure and convolutional neural network structure. 1200 samples were trained. Conducted model accuracy testing. The accuracy rate is above 98%, and accuracy verification has been implemented. The digital twin model was designed based on various data such as strain and temperature of the spacecraft structure under impact monitored by FBG sensors. A precise mapping has been formed between the physical entities of spacecraft and digital twins. Empower spacecraft with functions such as self-monitoring, judgment, and response. To ensure the stable and safe operation of spacecraft.

Real-time damage identification in composite structures based on pseudo excitation (PE) approach and fiber Bragg grating (FBG) sensor arrays

At present, most of the damage detection techniques based on global vibration and local guided wave have obvious limitations, which brings difficulties to the safety assessment of structures. To solve this problem, a damage location identification method based on micro-dynamic balance and inverse finite element is proposed. Firstly, the vibration frequency, measuring point density and damage magnitude were determined by using finite simulation composite plate structure. Secondly, a fiber grating sensor is attached to the surface of the laminate to receive the strain response signal of the structure under steady-state vibration, and the global displacement of the structure is constructed by inverse finite element method. Finally, the inverse finite element listing is introduced into the theoretical framework of the dynamic response of the micro-element, and the structural damage identification experiment based on the measured strain data is realized. The experimental results show that this method can effectively identify the structural damage area and has a special sensitivity to structural damage, and the identification accuracy and efficiency are high.

Vibration Suppression of a Flexible Beam Structure Coupled with Liquid Sloshing via ADP Control Based on FBG Strain Measurement

In this study, an adaptive dynamic programming (ADP) control strategy based on the strain measurement of a fiber Bragg grating (FGB) sensor array is proposed for the vibration suppression of a complicated flexible-sloshing coupled system, which usually exists in aerospace engineering, such as launch vehicles with a large amount of liquid propellant as well as a flexible beam structure. To simplify the flexible-sloshing coupled dynamics model, the equivalent spring-mass-damper (SMD) model of liquid sloshing is employed, and a finite-element method (FEM) dynamic model for the beam structure coupled with the liquid sloshing is mathematically established. Then, a strain-based vibration dynamic model is derived by employing a transformation matrix based on the relationship between displacement and strain of the beam structure. To facilitate the design of a strain-based control, a tracking differentiator is designed to provide the strains’ derivative signals as partial states’ estimations. Feeding the system with the strain measurements and their derivatives’ estimations, an ADP controller with an action-dependent heuristic dynamic programming structure is proposed to suppress the vibration of the flexible-sloshing coupled system, and the corresponding Lyapunov stability of the closed-loop system is theoretically guaranteed. Numerical results show the proposed method can effectively suppress coupled vibration depending on limited strain measurements irrespective of external disturbances.

An MR Compatible Vascular Stenosis Method Based on Fiber Bragg Grating

Progressive stenosis of blood vessels is an important cause of cardiovascular diseases. Traditional intervention methods rely on fluoroscopy, which exposes both patients and operators to ionizing radiation. In this paper, we propose a real-time detection method for vascular stenosis intervention based on Fiber Bragg Grating (FBG) sensors that can be used under magnetic resonance imaging (MRI) guidance. An FBG sensor array with 5 sensors is developed and validated using a vascular flow phantom that simulates the morphological features of stenoses. We first verify the sensor for measuring vascular stenosis through flow rate and profile using the FBG sensor array, followed by a detailed assessment of the MRI compatibility of the sensor in terms of imaging artefact and changes in SNR, The error of FBG array is within the range of ±10με, the SNR variation is 3.608%, indicating that the FBG sensor array has excellent MR image compatibility. Experiment results show a good correlation between the degrees of stenoses with strain measurements, thus offering real-time information to aid MR guided vascular intervention.

Strain measurement with multiplexed FBG sensor arrays: An experimental investigation

In conventional rock mechanics testing, radial strain measuring devices are usually attached to the sample's surface at its mid-height. Although this procedure provides a realistic picture of the lateral deformation undergone by homogeneous samples, however, this assumption may not be accurate if the tested rock has significant heterogeneity. Fibre Bragg Grating (FBG) sensors have recently been introduced to various rock testing applications due to their versatility over conventional strain gauges and radial cantilevers. FBG sensors have small size, multiplexing capability, and immunity to magnetic interference. The main objective of this study is to explore and understand the capabilities of FBG sensing for strain measurement during rock mechanics testing, including under confining. To do so, two limestone plugs (Savonnières limestone) and one acrylic Poly Methyl Methacrylate (PMMA) plug, all of 38 mm diameter, were prepared. The acrylic plug and one of the Savonnières samples plugs were subjected to Unconfined Compressive Strength (UCS) tests. The second Savonnières plug was subjected to a hydrostatic test up to 20 MPa confining at room temperature. FBG sensors of 125 μm cladding diameter with ceramics (Ormocer) coating were glued on the surface of each sample, spreading across the entire sample's height. Strain gauges and cantilever-type radial gauges were used on the samples submitted to UCS for comparison. Results show that radial strain measurements and calculated elastic properties derived from the FBG readings for samples are comparable to readings from the conventional strain gauges and cantilever-type devices. Apparent bulk moduli based on volumetric strain computed from FBG radial strain readings during the hydrostatic test on the Savonnières sample was consistent with benchtop measurements conducted on the Savonnières sample and another plug extracted from the same parental block, as well as published literature data. Moreover, variations in the calculated elastic properties are interpreted as evidence that the FBG sensors detected heterogeneities in the samples' inner structure, which can be seen in the density profiles computed from x-ray CT images. Such observation confirms the potential of the presented FBG sensors configuration for 3D strain mapping in rock mechanics tests.

High-Temperature Fiber Bragg Grating Sensor Array Based on Femtosecond Fiber Grating

High-Temperature Fiber Bragg Grating Sensor Array Based on Femtosecond Fiber Grating

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.

A near-wall acoustic wave-based localization method for broken wires in a large diameter PCCP using an FBG sensor array

Wire breakage induced by corrosion is the main cause of pipe bursts for prestressed cylinder concrete pipes (PCCPs). The current Rayleigh-based acoustic fiber optic methods can only locate the broken wires in 1D manner with the accuracy of pipe diameter. This paper proposed a three-dimensional localization method of broken wires in large dimeter PCCP. According to the acoustic emission characteristic of broken wires, the near-wall acoustic wave was extracted as wire-breakage sensitive feature, and the monitoring scheme based on an array of fiber Bragg grating (FBG) sensors was proposed by integrating with genetic algorithms. The feasibility of the method was verified by numerical simulations and experimental investigations. The maximum errors of the circumferential angle and longitudinal distance were 2.23° and 0.01 m, respectively, in the model test. The novel approach pertains to monitoring the broken wires in PCCPs at the high-risk and high-consequence zones for risk-informed operation & maintenance management.

Discriminating Rayleigh backscattering induced false crosstalk in inline interferometric FBG sensor arrays.

The crosstalk between channels is a main factor restricting the performance of interferometric fiber Bragg grating (FBG) time-division and wavelength-division hybrid multiplexing arrays. The time-division crosstalk caused by multiple reflections and the wavelength-division crosstalk due to insufficient isolation are the main crosstalk problems in the Fabry-Perot (F-P) structure, seriously limiting the number of multiplexing devices and the system's applications. Many theoretical research and suppression scheme designs have been done to solve these problems. We have previously found a new crosstalk phenomenon called the false crosstalk in hybrid multiplexing arrays. This paper focuses on this phenomenon and constructs a theoretical model to analyze its causes and influencing factors. The model demonstrates the influences of Rayleigh backscattering (RB) noise and sensor position on the false crosstalk. Both theoretical and experimental results show that the false crosstalk is induced by parasitic interference in the leading fiber and changes with the leading fiber length. This study quantitatively reveals the crosstalk performance degradation with the change of sensor position in the hybrid multiplexing array and provides key support for optimizing the system array design and expanding the large-scale multiplexing capacity.

Deformation and Damage Characteristics of Deep Rock Specimens Based on 3D-DIC and FBG

Abstract Specimen-scale testing of loaded rock parameters is an important research component of rock mechanics testing. In this paper, a method combining 3D-DIC and FBG (fiber Bragg grating) technology is proposed and applied to the study of deformation and damage characteristics of deep limestone and sandstone specimens under uniaxial loading conditions in the Panxie mine area of Huainan coalfield. The selection of optical fiber, the bonding method, and the process of rock specimen fabrication are introduced in detail. Two different fiber Bragg grating sensor arrays were constructed by using both multi-inclination type and orthogonal type deployment of fiber grating to dynamically monitor the strain response of rock specimens throughout the whole process. The results show that both 3D-DIC displacement monitoring and fiber Bragg grating strain captured the rock deformation and failure process well with identical outcomes. Under a continuous load of 0.3 kN/s, the local cracks of the rock specimen and the damage to the rock specimen resulted in different degrees of sudden changes in the fiber strain value. According to the differences in the degrees of brittleness and texture densities of the rock specimens, the local cracks evolution to transfixion of the limestone registered a certain time accumulation and produced more local cracks, while the sandstone at the same stage produced fewer local cracks within a shorter duration. The effective combination of regional noncontact testing and high-precision point contact can dynamically and accurately capture the rock deformation and damage characteristics at the specimen scale. In addition, the combined monitoring method of 3D-DIC and fiber grating can provide assistance in the prediction of rock crack expansion and potential fracture surfaces.

Temperature Field Measurement of Photovoltaic Module Based on Fiber Bragg Grating Sensor Array.

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.

Fiber Bragg grating sensor array for detecting heat flux in vacuum.

In TAE Technologies' current experimental device, C-2W (also called "Norman"), record-breaking, advanced beam-driven field-reversed configuration plasmas are produced and sustained in steady state utilizing variable energy neutral beams, advanced divertors, edge-biasing electrodes, and an active plasma control system [Gota etal., Nucl. Fusion 61, 106039 (2021)]. A novel diagnostic has been developed by TAE Technologies to leverage an industrial fiber Bragg grating (FBG) sensor array to detect heat flux along the wall of the vacuum vessel from a plasma discharge. The system consists of an optical fiber with FBG sensors distributed along its length, housed in a pressurized steel sheath. Each FBG sensor is constructed to reflect a different wavelength, the exact value of which is sensitive to the strain and temperature at the location of the grating in the fiber. The fiber is illuminated with broadband light, and the data acquisition system analyzes the spectrum of reflected light to determine the temperature at the location of each FBG. We have installed four of these vacuum-rated FBG sensor arrays on the C-2W experiment, each with 30 individual FBG sensors spaced at 0.15m intervals along the 5m fiber, with a 100Hz acquisition rate. The measurement of temperature change due to a plasma discharge provides a single data point at each sensor location, creating a 120-point heat map of the vacuum vessel.

Hot Spot Detection of Photovoltaic Module Based on Distributed Fiber Bragg Grating Sensor

The hot spot effect is an important factor that affects the power generation performance and service life in the power generation process. To solve the problems of low detection efficiency, low accuracy, and difficulty of distributed hot spot detection, a hot spot detection method using a photovoltaic module based on the distributed fiber Bragg grating (FBG) sensor is proposed. The FBG sensor array was pasted on the surface of the photovoltaic panel, and the drift of the FBG reflected wavelength was demodulated by the tunable laser method, wavelength division multiplexing technology, and peak seeking algorithm. The experimental results show that the proposed method can detect the temperature of the photovoltaic panel in real time and can identify and locate the hot spot effect of the photovoltaic cell. Under the condition of no wind or light wind, the wave number and variation rule of photovoltaic module temperature value, environmental temperature value, and solar radiation power value were basically consistent. When the solar radiation power fluctuated, the fluctuation of hot spot cell temperature was greater than that of the normal photovoltaic cell. As the solar radiation power decreased to a certain value, the temperatures of all photovoltaic cells tended to be similar.

Crosstalk interference synthesis and polarization switching hybrid interrogating method for inline interferometric fiber Bragg grating sensor arrays

We presented a crosstalk interference synthesis and polarization switching (CIS-PS) hybrid interrogating method designed for inline interferometric fiber Bragg grating (FBG) sensor arrays, targeting the multi-reflection (MR)-induced time division multiplexing (TDM) crosstalk and polarization-induced signal fading simultaneously. The inline interferometric FBG sensor array, which has found wide applications in underwater sensing, generally consists of a series of FBGs written on a commercial single mode fiber (SMF), where the coupling of TDM crosstalk and polarization-induced signal fading often makes signal extraction difficult. In this paper, this coupling was modeled, and the principle of the CIS-PS hybrid method was demonstrated. Experiments showed that even though the FBG reflectivities were all larger than 5%, the measured MR-induced TDM crosstalk reached −50 dB, which was the lowest to the best of our knowledge. The background phase noises were lowered and polarization-independent due to efficient control of polarization-induced signal fading. The demodulation stability of the sensing signal was enhanced from ±2.25 dB to ±0.15 dB.

Multiplexed Active Optical Fiber Bragg Grating Sensor Array Enabled by Femtosecond Laser Induced Nanoscattering Centers

Optical fiber sensors (OFS) such as fiber Bragg gratings (FBGs) are passive and versatile sensor devices that have been used to perform a wide range of physical and chemical measurements. However, the performance and functionality of the OFS are limited by its passivity. This letter demonstrates an active fiber sensor scheme for multiparameter measurements. FBG-based optic fiber sensors are fabricated in an optical fiber core by femtosecond laser direct writing scheme. Guided light scattered out from laser-induced nanograting in fiber core is harnessed by on-fiber coating. Energy conversion films coated on the optical fiber convert scattered optical energy into thermal energy to heat the FBG sensor above ambient temperatures for active measurements. Functionalities of optically heated fiber sensor array are demonstrated for both temperature and liquid level measurements at the room temperatures.

Measurement and Analysis of Soil Temperature Field Based on Fiber Bragg Grating Sensor Array

Studying the soil temperature field and its variation law is of great significance to the real-time service of agricultural production. To solve the problems of low detection efficiency and difficulty of distributed detection, a distributed detection method of soil temperature field using wavelength division multiplexing and space division multiplexing technologies based on fiber Bragg grating (FBG) sensor is proposed. In this scheme, the stainless steel tube encapsulating the FBG string was vertically buried in the measured soil, and the tunable laser method and the peak-seeking algorithm were used to demodulate the wavelength of each sensor of the FBG string. The soil temperature field measurement experiments were carried out in the farmland environment. The experimental results show that the proposed method can detect the soil temperature field in real time. The internal temperature of soil changes periodically with the periodic change of surface environment temperature. With the increase of depth, the influence of surface temperature on soil temperature gradually decreases, and the amplitude of soil temperature oscillation gradually decreases, but the hysteresis effect of temperature peak becomes more and more obvious. The ambient wind speed can reduce the temperature of soil surface and shallow layer. Soil moisture affects the speed of temperature transfer from the surface layer to the deep layers, and the hysteresis effect of temperature peak is more significant in the soil with higher moisture. Solar radiation has a great influence on the temperature of soil surface and shallow layer.

Shape reconstruction based on a multicore optical fiber array with temperature self-compensation.

Temperature variations affect the accuracy of fiber-optic shape sensors; thus, temperature compensation is particularly important. This study developed a temperature self-compensation algorithm and verified the measuring accuracy of shape sensors after temperature compensation. A multicore fiber Bragg grating (FBG) sensor array was calibrated to confirm the consistency of sensor characteristics, and the relationship between the curvature and wavelength shift of FBGs was studied. A variable-temperature experiment revealed the temperature sensitivity of the FBG sensors, and these results were used by the temperature self-compensation algorithm. Further, shape reconstruction before and after temperature compensation was studied. The deformed shapes of the multicore FBG sensor array under different bending conditions were reconstructed. The results obtained after temperature compensation show that the average error between the measured and the theoretical coordinate values as less than 0.33 mm, the maximum error as less than 5.61 mm, and the relative error as less than 3.50%. The proposed temperature self-compensation algorithm has excellent prospects for application to flexible structures.

Analysis on leading-fiber-induced Doppler noise in interferometric FBG sensor arrays using polarization switching and PGC hybrid processing method.

The random disturbance in the leading fiber is considered as a vital noise source in the practical interferometric fiber Bragg grating (FBG) sensor array, which is usually interrogated by periodic laser pulse pair. As the two interrogation laser pluses propagate through the leading fiber in a time-sharing manner, the leading fiber disturbance could cause undesired demodulated phase noises to both the polarization state and the pulse-interval, which are summarized as the polarization fading induced noise and the Doppler noise, respectively. This paper focused on the Doppler noise under the demodulation scheme of polarization switching (PS) and phase generated carrier (PGC) hybrid processing method. A model describing the transformation from arbitrary leading fiber stretching to sensor phase background was presented. The complexity was that the Doppler noise was coupled with the birefringence states, as verified by both simulation and experiment. In response to this issue, a two-stage Doppler noise suppression method was proposed, which is based on the PS and PGC hybrid processing and a reference sensor. A processing procedure was presented where the polarization synthesis must be performed before and the reference sensor was considered. Otherwise, the suppression algorithm will be completely invalid due to the mutual coupling of the Doppler noise and the birefringence. Experimental results showed that only after the first stage of polarization synthesis, identical Doppler noise in the two TDM channels could be obtained, with an amplitude error of 0.02 dB. The second stage involved non-sensitive reference sensor subtraction, which achieved a maximum suppression of about 30 dB, which was the highest to be best of our knowledge. The two-stage Doppler noise suppression method was tested for sinusoidal and wideband leading fiber disturbances, providing a solution for practical interferometric FBG array applications.

Deformation calculation method based on FBG technology and conjugate beam theory and its application in landslide monitoring

Prediction of displacement is an important means for evaluating the safety of civil infrastructures. In recent years, fiber Bragg grating (FBG) sensors have been widely used to measure structural strain and temperature in civil engineering for their obvious advantages of light weight, high precision, strong durability, wide measurement range and long-distance transmission. However, the strain signal directly measured by the FBG sensors should be transformed into the displacement response by mathematical algorithms, thus indirectly predicting the deformation behavior of the instrumented structure. Therefore, based on the FBG sensing technology and conjugate beam theory, this paper presents a displacement algorithm for estimating structural deformation of simply supported beam and cantilever beam. The finite element (FE) numerical simulation was conducted to compare the displacement errors of the proposed algorithm and previous methods in solving for the deflections of the deformed beams with and without an inflection point, which shown that the proposed calculation method has higher accuracy and wider application scope. Additionally, the authors pasted the FBG sensor arrays on the opposite sides of the neutral axis of the inclinometer casing to fabricate an in-place FBG-based inclinometer. The laboratory calibration tests and field applications were also conducted, and the monitoring results demonstrate that the FBG measurement system using this proposed calculation method can effectively determine the deformation movement of a slope.