Optical Fiber Bragg Grating Sensors Research Articles

Modelling the effect of hot gas torch heating on adjacent tows during automated fibre placement consolidation of thermoplastic composites

Many numerical models of the Automated Fibre Placement (AFP) manufacturing process have been developed to assist in the design process of composite components and structures. Although the tape placement process, in general is a 3-D transient process, limited models considering the 3-D nature of the process exist. This paper describes the development of a 3-D Finite Element (FE) model incorporating the effect of non-concentrated input heat flux distribution on adjacent paths. The model can predict an accurate thermal history of a thermoplastic composite laminate for a complete AFP deposition process. Composite laminates having plies made up of both a single tow and multiple tows have been simulated to study their thermal behaviour during the manufacturing process. Simulations have been conducted both with and without considering the input heat effect on adjacent tows for comparison. Temperatures were measured by embedding two different configurations of optical fibre Bragg grating (FBGs) sensors within the laminate during the lay-up process. This served a dual purpose of validating the model as well as the experimental methodology.

Miniaturized Strain-Free Fiber Bragg Grating Temperature Sensors

This work reports the design, fabrication, and characterization of a miniaturized metallic package for optical fiber Bragg grating (FBG) sensors. The aim of the package is to enhance the temperature sensitivity of the grating and eliminate the cross-sensitivities to mechanical effects, i.e., strain or impulsive forces. The package is simple in its design and developed using off-the shelf components: the FBG is first encapsulated into a steel tube, which is in turn placed inside another larger steel tube, where the fiber is kept loose to prevent that any mechanical effect is transferred to the grating. The package has a length of 25 mm and a diameter of about 2 mm, hence it is very compact. Concerning the performance, the packaged FBG has a linear temperature response in the range 5–50 °C with a sensitivity of 28.9 pm/°C, that is about three times that of a standard FBG. Whereas, concerning the longitudinal force sensitivity, it is decreased by two orders of magnitude in comparison to an unpackaged FBG. The temporal evolution of the device is also evaluated when subjected to a temperature step of about 20 °C, demonstrating a fast response time of 5 s. Finally, the effect of an impulsive force is evaluated by means of a falling mass attached to the fiber, demonstrating a negligible response and a strong resistance of the packaged FBG in comparison to unprotected one, which broke after the same test.

Hybrid of Free Space Optics Communication and Sensor System Using IWDM Technique

In this work, we designed a novel hybrid of free-space optics (FSO) communication and intensity and wavelength division multiplexing (IWDM)-fiber Bragg grating (FBG) sensor system over a single shared FSO transmission channel for both optical and sensor signal transmission. In the proposed system, we use one broadband light source and one FSO link for both optical and FBG sensor signal transmission, which significantly reduces the cost of system installation and complexity. Besides, the FSO transmission channel is used to solve the problem of geographical constraints when the environment is difficult to install the fiber. In addition, IWDM is one of the best multiplexing technique to increase the sensor number by more than two times the conventional wavelength division multiplexing (WDM) method. Besides, to solve the overlap spectra problem of IWDM based FBG sensor system, we proposed the stacked gated recurrent unit (SGRU) algorithm with the neural network (NN) profile method. The results prove that the proposed SGRU with the NN profile method improves the accuracy of wavelength detection techniques with the Gaussian profile method. Moreover, the FSO transmission channel achieves a clear eye diagram even the system is a hybrid of optical communication and FBG sensors system.

Experimental Development of Composite Bicycle Frame

This article focuses on the development of a carbon composite bicycle frame using various experimental methods of structural analysis. Two types of frame specimen were used. The complete frame specimen was tested in accordance with ISO test load cases with the addition of an ergometer test in order to refine the operational strain envelope of such a frame. Resistive strain gauges and optical Fiber Bragg Grating (FBG) sensors were used for this analysis. The FBG sensors were embedded inside the head tube joints during the manufacturing process. The head connection was designed as a geometrically precise form–connection of wound composite tubes, reinforced with a wrap of high-strength unidirectional carbon tapes and carbon fabrics. Additional structural strength laboratory tests were conducted using simplified frame specimens, in order to evaluate the range of the limit case strain ranges. The digital image correlation method was used for the evaluation of the strain distribution in the head tube area. Resistive strain gauges were used for local strain analysis in critical areas. The acoustic emission method was used to detect structural defects before they could influence the stiffness response of the frame. It was found that the joints of the frame tubes are crucial for the strength and safety of the frame. Therefore, attention was also focused on the strengthening of the head tube joint, and on its experimental verification. A positive effect on the strength of the reinforced frame was found by doubling the thickness of the carbon fabric in the head tube joint area.

Monitoring Junction Temperature of RF MOSFET under Its Working Condition Using Fiber Bragg Grating.

When a high-power radio frequency (RF) metal oxide semiconductor field effect transistor (MOSFET) works in low-efficiency situations, considerable power is dissipated into heat, resulting in an excessive junction temperature and a likely failure. In this study, an optical fiber Bragg grating (FBG) sensor is installed on the die of a high-power RF MOSFET. The temperature change of RF MOSFET with the change of input signal is obtained by using the temperature frequency shift characteristic of the FBG reflected signal. Furthermore, the fast and repetitive capture of junction temperature by FBG reveals details of the temperature variation within each RF pulse, which is correctly correlated with input signals. The results show that besides monitoring the temperature accumulation of the chip for a long time, the FBG can also capture junction temperature details of the chip within each pulse period. Finally, a Cauer-type thermal model of the RF MOSFET was constructed based on the temperature information captured by the FBG.

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.

Method for shield tunnel cross-section curve reconstruction based on FBG sensors and neural network

To improve the accuracy of reconstructing morphology in shield tunnel monitoring by fiber-optic Bragg grating (FBG) sensors, a Transformer network-based optimized tunnel cross-sectional curve reconstruction method is proposed to solve the problem of low reconstruction accuracy due to the cumulative error of discrete curvature points. In this research, the optimization method uses all interpolation points as initial points to minimize the cumulative error in fitting the curve with discrete curvature points. Simulation model experiments of the shield tunnel were carried out to verify the accuracy of the optimization method in reconstructing the tunnel section curve. The results show that the shield tunnel simulation model experiments, more accurate tunnel section reconstruction curves were obtained using this optimization method, increasing reconstruction accuracy by 27.3424%.

Application of Optical Fiber Bragg Grating Sensor in Minimally Invasive Medical Treatment

Application of Optical Fiber Bragg Grating Sensor in Minimally Invasive Medical Treatment

In Situ Contact Pressure Monitoring of Press Pack Power Module Using FBG Sensors

For press pack power semiconductor modules, the contact pressure is a key parameter to monitor their operating condition and health status. Direct internal measurement using conventional sensors has been difficult due to the restricted space and strong electromagnetic interference (EMI). This paper proposes a method by using optical fiber Bragg grating (FBG) sensors. To facilitate the FBG measurement, the contact force needs to be converted to an appropriate tensile strain in a tailored spring stack structure within the modified power module, and the in-situ contact pressure is then measured using integrated FBG sensors to detect this tensile strain. The thermal influence on the strain measurement is eliminated by temperature compensation. The sensitivity and accuracy of the proposed method are verified in experiments.

Analysis of Long-Term Prestress Loss in Prestressed Concrete (PC) Structures Using Fiber Bragg Grating (FBG) Sensor-Embedded PC Strands

This study aims to develop a prestressed concrete steel (PC) strand with an embedded optical Fiber Bragg Grating (FBG) sensor, which has been developed by the Korea Institute of Civil Engineering and Building Technology since 2013. This new strand is manufactured by replacing the steel core of the normal PC strand with a carbon-fiber-reinforced polymer (CFRP) rod with excellent tensile strength and durability. Because this new strand is manufactured using the pultrusion method, which is a composite material manufacturing process, with an optical fiber sensor embedded in the inner center of the CFRP Rod, it ensures full composite action as well as proper function of the sensor. In this study, a creep test for maintaining a constant load and a relaxation test for maintaining a constant displacement were performed on the proposed sensor-type PC strand. Each of the two tests was conducted for more than 1000 h, and the long-term performance verification of the sensor-type PC strand was only completed by comparing the performance with that of a normal PC strand. The test specimens were fabricated by applying an optical fiber sensor-embedded PC strand, which had undergone long-term performance verification tests, to a reinforced concrete beam. Depending on whether grout was injected in the duct, the specimens were classified into composite and non-composite specimens. A hydraulic jack was used to prestress the fabricated beam specimens, and the long-term change in the prestress force was observed for more than 1600 days using the embedded optical fiber sensor. The experimental results were compared with the analytical results to determine the long-term prestress loss obtained through finite-element analysis based on various international standards.

Fiber-Optic Bragg Grating Sensors for Biomechanical Analysis of Fracture Healing

Despite the complexity of bone callus formation, the evaluation of fracture healing is usually performed based on x-ray visual inspection, a subjective and imprecise method. The main objective of this study is to access the evolution of bone callus. Tests were performed using a synthetic femur and an in vivo subject (preliminary results). Fiber Bragg Grating (FBG) sensors were glued to the Schanz pins of a circular external fixator for the analysis of static and dynamic strain. Three materials of different densities were inserted in the fracture site (synthetic femur) to simulate the formation of bone callus. The results from the static tests with the synthetic femur show that the strain measured on the Schanz pins presented an expected deformation pattern, according to a previous simulation. It was possible to differentiate bone callus stiffness through strain detected in the Schanz pins. The results from the dynamic tests allowed to find the first resonant frequency of each bone callus, with a difference of 4 Hz between them, measured by the FBG sensor. In the preliminary in vivo tests, the FBG sensor results demonstrated its technical viability to detect the bone callus healing stage. Nonetheless, a comprehensive study using a higher number of subjects is necessary. Being a noninvasive technology, it has the potential to become an important tool to assist medical staff in bone healing analysis.

Pultruded FRP Beams with Embedded Fibre Bragg Grating Optical Sensors for Strain Measurement and Failure Detection.

Composites have added new dimensions to the design and construction of buildings and structures. One of the wider spread composite applications in the construction industry is composite beams or pillars, which can be manufactured through pultrusion processes. These types of construction elements are usually used to withstand the weight of large loads, so their integrity must be guaranteed. Due to optical sensors’ advantages over their electrical counterparts—small sizes, low weight, non-conductive, and immunity to electromagnetic interference—and FBGs having an outstanding position among optical fibre sensors—due to their multiplexation capability and relatively easy monitoring—in this study, we propose the integration of FBG sensors for the observation and analysis of the integrity of structures made with composite beams over time. The validation test results showed the successful embedding integration of FBG-based fibre optical sensors in an FRP pultrusion beam and strain transmission up to 7500 µɛ from the composite test piece to the sensor. Additionally, we were able to anticipate the piece failure by the FBG spectrum deformation.

On the mechanics of MQXFB—the low-beta quadrupole for the HL-LHC

The High Luminosity Large Hadron Collider (LHC) Project target is to reach an integrated luminosity of the LHC of 3000 fb−1, corresponding to a factor 10 increase in collisions with respect to the current accelerator. One of the main components is the superconducting quadrupole called MQXF. It is based on Nb3Sn technology and has a 150 mm single aperture with a field gradient of 132.6 T m−1. The MQXF magnets are currently in preseries production in a joint collaboration between CERN and the US-LHC Accelerator Upgrade Project. The first prototype magnet based on 7 m-long coils (MQXFBP1) was assembled and preloaded in 2019. The testing and disassembly was done in 2020. The coils were equipped with optical fiber Bragg grating (FBG) sensors. Some of the FBG values exhibited strain jumps during preload and their signals were lost during cool-down. The magnet did not reach the nominal current, quenching at 15.15 kA. At the time of writing three MQXFB magnets have been preloaded from which the second prototype is soon to be tested. The objective of this paper is to analyze the mechanical behavior of MQXFB magnets in the light of current knowledge, thanks to earlier short model experiments, as well as describe in detail the process of assembly and preload. We synthesize the mechanical theory of preload and present new considerations on symmetric/asymmetric bladder-key operations and their effects. An extensive comparison between long and short model magnets is presented. In our analysis and FE modeling we take into account the measured coil sizes that vary over position. We introduce a novel technique that allows preload homogenization over the length of the magnet after preload has been done. We analyze the role of the stainless steel shell and show its mechanical behavior. Finally, the relevant data at cold is presented and analyzed.

Performances of PMMA-Based Optical Fiber Bragg Grating Sensor in Extended Temperature Range

PMMA based optical fiber Bragg grating (POFBG) sensors are investigated in an environmental chamber with controlled temperature and relative humidity at temperature extended to 70 °C. At below a critical temperature of 50 °C the POFBG sensor exhibits good linearity and sensitivity for both temperature and humidity sensing. Nonlinear responses are observed at higher temperature, giving rise to varying, reduced magnitudes of sensitivities. An important feature of POFBG humidity sensing is observed at above critical temperature where the POFBG humidity sensitivity turns from positive to negative. A theoretical model based on Lorentz–Lorenz equation is presented to estimate the dependence of POFBG refractive index on temperature and relative humidity. The experimental results qualitatively agree with the theoretical analyses.

FBG Strain Monitoring of a Road Structure Reinforced with a Geosynthetic Mattress in Cases of Subsoil Deformation in Mining Activity Areas.

This paper presents implementation of purpose-designed optical fibre Bragg grating (FBG) sensors intended for the monitoring of real values of strain in reinforced road structures in areas of mining activity. Two field test stations are described. The first enables analysis of the geogrid on concrete and ground subgrades. The second models the situation of subsoil deformation due to mining activity at different external loads. The paper presents a system of optical fibre sensors of strain and temperature dedicated for the investigated mattress. Laboratory tests were performed to determine the strain characteristic of the FBG sensor-geogrid system with respect to standard load. As a result, it was possible to establish the dependence of the geogrid strain on the forces occurring in it. This may be the basis for the analysis of the mining activity effect on right-of-way structures during precise strain measurements of a geogrid using FBG sensors embedded in it. The analysis of the results of measurements in the aspect of forecasted and actual static and dynamic effects of mining on the stability of a reinforced road structure is of key importance for detailed management of the road investment and for appropriate repair and modernization management of the road structure.

Fibre Optic FBG Sensors for Monitoring of the Temperature of the Building Envelope.

Standard sensors for the measurement and monitoring of temperature in civil structures are liable to mechanical damage and electromagnetic interference. A system of purpose-designed fibre optic FBG sensors offers a more suitable and reliable solution—the sensors can be directly integrated with the load-bearing structure during construction, it is possible to create a network of fibre optic sensors to ensure not only temperature measurements but also measurements of strain and of the moisture content in the building envelope. The paper describes the results of temperature measurements of a building 2-layer wall using optical fibre Bragg grating (FBG) sensors and of a three-layer wall using equivalent classical temperature sensors. The testing results can be transmitted remotely. In the first stage, the sensors were tested in a climatic test chamber to determine their characteristics. The paper describes test results of temperature measurements carried out in the winter season for two multilayer external walls of a building in relation to the environmental conditions recorded at that time, i.e., outdoor temperature, relative humidity, and wind speed. Cases are considered with the biggest difference in the level of the relative humidity of air recorded in the observation period. It is found that there is greater convergence between the theoretical and the real temperature distribution in the wall for high levels (~84%) of the outdoor air relative humidity, whereas at the humidity level of ~49%, the difference between theoretical and real temperature histories is substantial and totals up to 20%. A correction factor is proposed for the theoretical temperature distribution.

Partial Discharge Detection in an Oil-Filled Power Transformer Using Fiber Bragg Grating Sensors: A Review

Power transformer is one of the most important high voltage apparatus in any power system network. The life span of such apparatus commonly relies upon its insulation condition. The primary reason for an insulation breakdown is partial discharge. Partial discharges (PDs) are electrical discharges that happen in the insulation system of the transformers. Occurrence of PDs results in deterioration of the transformer insulation system. Hence, measuring and detecting the PD at an initial stage is necessary to avoid complete breakdown of the transformer. Currently, various PD detection and measurement methods, such as, electrical, chemical, acoustic, UHF and optical methods are being used for PD monitoring in power transformers. The conventional PD monitoring techniques like electrical or chemical have some limitations. The optical Fiber Bragg Grating (FBG) Sensor has been used as a PD sensor for PD monitoring in the power transformer to overcome these issues. This paper presents a review of FBG sensors’ application as a sensor for PD monitoring in an oil-filled power transformer. The challenges and future prospects in this area are also discussed.

Smart Sensing of PSC Girders Using a PC Strand with a Built-in Optical Fiber Sensor

This paper presents a multi-functional strand capable of introducing prestressing force in prestressed concrete (PSC) girders and sensing their static and dynamic behavior as well. This innovative strand is developed by replacing the core steel wire of the strand used in PSC structures with a carbon fiber-reinforced polymer (CFRP) wire with a built-in optical Fiber Bragg Grating (FBG) sensor. A full-scale girder specimen was fabricated by applying this multi-function strand to check the possibility of tracking the change of prestressing force at each construction stage. Moreover, dynamic data could be secured during dynamic loading tests without installing accelerometers and made it possible to obtain the natural frequencies of the structure. The results verified the capability to effectively manage the prestressing force in the PSC bridge structure by applying the PC strand with a built-in optical sensor known for its outstanding practicability and durability.

FBG‐Based Sensing for Structural Health Monitoring of Road Infrastructure

Public road infrastructure is developed all around the world. To save resources, ensure public safety, and provide longer‐lasting road infrastructure, structural health monitoring (SHM) applications for roads have to be researched and developed. Asphalt is one of the largest used surface materials for the road building industry. This material also provides relatively easy fiber optical sensor technology installment, which can be effectively used for SHM applications—road infrastructure monitoring as well as for resource optimization when road building or their repairs are planned. This article focuses on the research of the fiber Bragg grating (FBG) optical temperature and strain sensor applications in road SHM, which is part of the greater interdisciplinary research project started at the Riga Technical University in the year 2017. Experimental work described in this article was realized in one of the largest Latvian road sites where the FBG strain and temperature sensors were installed into asphalt pavement, and experiments were carried out in two main scenarios. Firstly, in a controlled environment with a calibrated falling weight deflectometer (FWD) to test the installed FBG sensors. Secondly, by evaluating the real‐time traffic impact on the measured strain and temperature, where different types of vehicles passed the asphalt span in which the sensors were embedded. The findings in this research illustrate that by gathering and combining data from calibrated FWD measurements, measurements from embedded FBG optical sensors which were providing the essential information of how the pavement structure could sustain the load and information about the traffic intensity on the specific road section, and the structural life of the pavement can be evaluated and predicted. Thus, it enables the optimal pavement future design for necessary requirements and constraints as well as efficient use, maintenance, and timely repairs of the public roads, directly contributing to the overall safety of our transportation system.

Applications and research progress of optical fiber grating sensing in thermoplastic composites molding and structure health monitoring

Thermoplastic composites have the characteristics of high strength, fatigue resistance and easy recycling. They have received wide attention and are used in construction, automobile and other fields. An accurate monitoring technique is necessary for monitoring their internal state in the molding process and structure health in the service process. Traditional mechanical and electrical sensors have many disadvantages, such as large size and slow response. Therefore, they are not appropriate for embedded measuring and real-time monitoring. Optical fiber Bragg grating (FBG) sensors present many advantages over traditional and other advanced sensors, such as resistance to electromagnetic interference, light weight and less influence on materials. Generally, FBGs are embedded in composites and/or structures to monitor temperature and strain. The principle of FBG sensor and three key demodulation techniques are introduced firstly, including tunable Fabry-Perot (F-P) filter demodulation, especially the high-speed demodulation technique based on swept wavelength tunable laser, linear filter demodulation and matched grating. Secondly, the progress of monitoring technology based on FBGs for three molding scheme is reviewed, including hot compression molding, fused deposition molding and automated fiber placement molding. Thirdly, the progress of structural health monitoring for thermoplastic composites is also reviewed. Finally, some intelligent thermoplastic composite structures based on FBGs are introduced.