Fiber Bragg Grating System Research Articles

Dynamic Strain Measurement of Rotor Blades in Helicopter Flight Using Fiber Bragg Grating Sensor.

Fiber Bragg grating (FBG) sensor has a lot of advantages over the resistance strain gauge and has been used in many applications. However, there are few applications of rotor blade dynamic measurement in helicopter flight. In this paper, a method for blade dynamic strain measurement using an FBG sensor in a helicopter's real flight is presented. The corresponding measurement system is established and can eliminate the effects of the helicopter's electromagnetic environment on the electrical sensing components in the measurement system with the orthogonal frequency-division multiplexing modulation. The measured dynamic strains on the rotor blades of the helicopter in real flight contain six harmonic frequencies with the vibration characteristics of rotor blades, indicating that the established FBG measurement method and system have practical engineering applications.

Feasibility of a lung airway navigation system using fiber-Bragg shape sensing and artificial intelligence for early diagnosis of lung cancer.

Currently early diagnosis of malignant lesions at the periphery of lung parenchyma requires guidance of the biopsy needle catheter from the bronchoscope into the smaller peripheral airways via harmful X-ray radiation. Previously, we developed an image-guided system, iMTECH which uses electromagnetic tracking and although it increases the precision of biopsy collection and minimizes the use of harmful X-ray radiation during the interventional procedures, it only traces the tip of the biopsy catheter leaving the remaining catheter untraceable in real time and therefore increasing image registration error. To address this issue, we developed a shape sensing guidance system containing a fiber-Bragg grating (FBG) catheter and an artificial intelligence (AI) software, AIrShape to track and guide the entire biopsy instrument inside the lung airways, without radiation or electromagnetic navigation. We used a FBG fiber with one central and three peripheral cores positioned at 120° from each other, an array of 25 draw tower gratings with 1cm/3nm spacing, 2 mm grating length, Ormocer-T coating, and a total outer diameter of 0.2 mm. The FBG fiber was placed in the working channel of a custom made three-lumen catheter with a tip bending mechanism (FBG catheter). The AIrShape software determines the position of the FBG catheter by superimposing its position to the lung airway center lines using an AI algorithm. The feasibility of the FBG system was tested in an anatomically accurate lung airway model and validated visually and with the iMTECH platform. The results prove a viable shape-sensing hardware and software navigation solution for flexible medical instruments to reach the peripheral airways. During future studies, the feasibility of FBG catheter will be tested in pre-clinical animal models.

Analysis of Failure Mechanics and Energy Evolution of Sandstone Under Uniaxial Loading Based on DIC Technology

Energy evolution process of rock deformation is conducive to essentially reveal the rock failure mechanism and is of great significance to uncover the breeding of dynamic disasters in rock engineering. To characterize the damage evolution of energy dissipation during rock failure, the digital image correlation (DIC) technique is proposed to describe the rock failure mechanics and its energy evolution process. The uniaxial compression experiment of sandstone specimen was carried out, and the whole field deformation and failure characteristics of the rock had been captured by the DIC system. Measurement accuracy was verified by the fiber Bragg grating (FBG) sensor, the elastic region of the specimen was divided according to the location of strain localization band (SLB), and the evolution process of elastic strain energy of the rock was analyzed. The results show that the time history development of rock strain obtained by the FBG and DIC system matches identically, and the deviation of peak axial strain of both means is less than 5%, which verifies the applicability of DIC system. The uncoordinated evolution of rock deformation displacement field is discussed to reveal the crack development and failure form of the sandstone specimen under uniaxial compression. The energy evolution of the elastic region of the specimen is revealed, and the development of releasable elastic strain energy would be divided into three stages, which correspond to the stress–strain characteristics of rock failure mechanics. This study could provide an alternative analytical method for the experimental rock mechanics research studies.

A Novel FBG-Based Triggering System for Cardiac MR Imaging at 3 Tesla: A Pilot Pre-Clinical Study

This first-ever study demonstrates the applicability of a fiber Bragg grating (FBG) system for MR cardiac triggering of cardiovascular magnetic resonance at 3 Tesla. The unique patented system senses body movements caused by cardiac activity using a non-invasive ballistocardiography (BCG) sensor. The pilot research compares a novel FBG-based system with clinically used triggering systems based on electrocardiography (ECG) and pulse oximetry (POX). The pilot pre-clinical study was conducted on 8 subjects at a Siemens Prisma 3T MR Scanner. The study compares images from two basic cardiac sequences, TRUE FISP (Free Induction Decay Steady-State Precession) and PSIR (Phase Sensitive Inversion Recovery), using objective methods and subjective evaluation by clinical experts. The study presents original results that confirm the applicability of optical sensors in the field of cardiac triggering having a number of advantages in comparison to conventional solutions, such as no eddy current interference, ease of placement of the sensor on the patient’s body, and senor reusability. The proposed FBG-based system achieves comparable results with the most frequently used and most accurate ECG-based and POX-based clinical systems. In terms of subjective evaluation by experts, the FBG system outperformed the POX-based system used in clinical practice.

First Steps into Catheter Guidance Including Shape Sensing for Endovascular Aneurysm Repair Procedures

Introduction: Currently, endovascular aortic repair (EVAR) procedures are guided using fluoroscopy and 2D digital subtraction angiography. The research project Nav EVAR (funded by the German Federal Ministry of Education and Research, grant number 13GW0228C) aims to reduce the x-ray exposure and the administration of contrast agent. This project includes the development of a guidance system based on electromagnetic (EM) tracking for catheter location and Fiber Bragg Gratings (FBG) for catheter shape sensing. The goals of this study were to evaluate the accuracies of the catheter location and the catheter shape sensing separately, and to carry out an initial experiment combining both technologies. Methods: A custom-made 9F catheter was build containing an Aurora EM sensor (Northern Digital Inc.) at its tip and a FBG system (DTG©, FBGS Technologies GmbH). The EM sensor position was obtained by the EM tracking system and then transformed into the computed tomography (CT) planning data by using metallic markers placed on the patient's torso. An optimized shape sensing model was used to reconstruct the first 38 cm of the catheter. The combination of both technologies allowed the localization of the reconstructed shape. The EM tracking system was evaluated with a human torso model including a 3D-printed patient-specific aortic system. The catheter was inserted into the iliac artery and the EM sensor accuracy was assessed at five positions (Figure 1) by obtaining the ground truth from CT scans. The FBG system was evaluated by inserting the catheter into a 3D-printed patient-specific aortic system. The accuracy was assessed by comparing the reconstructed shape with the ground truth from a CT scan. The feasibility of the combined system was tested by visualizing the located shape with MeVisLab software (MeVis Medical Solutions AG). Results: For the EM tracking system, the error was 1.60 ± 0.86 mm (mean ± standard deviation, maximum error 2.56 mm). For the FBG system, the error was 1.13 ± 0.43 mm (maximum error 2.11 mm) over the whole 38 cm. Figure 2 shows the CT scan with the reconstructed shape. Regarding the combined guidance, the located shape moved in the same way as the catheter in the real scenario. Conclusion: Both technologies provided an error of less than 5 mm in realistic endovascular scenarios. This accuracy is required to ensure that the renal arteries are not blocked by the stent placement. The EM sensor accuracy was comparable to other studies [1-2]. Previous studies of FBG systems focused on medical needle shape sensing [3-4]. The shape reconstruction error was higher compared to previous studies, but our endovascular was more complex and realistic [5]. The combined system showed promising results. However, these are initial experiments that do not take into account any anatomical deformations during EVAR procedures. Future work will include the optimization of our catheter guidance system, intraoperative imaging for updating the actual anatomy and further evaluations with patient-specific models. Disclosure: Nothing to disclose

Advantages of fiber Bragg gratings for measuring electric motor loadings in aerospace application.

Electric motors are being investigated in-depth for their application in aerospace. Part of this investigation is the characterization of the loadings from the propulsion devices, in a stationary setup, usually accomplished through the utilization of load cells. The majority of the load cells used in this application are designed around a resistance-based strain gauge. However, electric motors radiate electromagnetic interference (EMI) when in operation, which degrades the signal retrieved through the strain gauge, due to the gauge's metallic construction acting as an antenna for the EMI. To demonstrate the advantage of fiber Bragg gratings (FBGs), with their immunity to EMI, a load cell implementing both sensor technologies was designed and subjected to the same mechanical loading and EMI, with a flywheel coupled to a brushless DC motor. The load cell had a sensitivity of 8.59 ± 0.18 N and 2.49 ± 2.49 N through the strain gauge and FBG system, respectively. The strain gauge signal contained the mechanical loading signal embedded in wideband noise and spikes (that increased linearly with motor angular velocity), while the FBG signal did not, with little noise. The raw strain gauge signal, at a maximum, had a signal power ratio (mechanical signal power divided by the overall signal power mean) of 21.06 at 104.72 rad/s; the FBG signal, at a minimum, had a signal power ratio of 40.09 at 52.36 rad/s. Therefore, on the basis of the mechanical tests performed in this work, the recommended sensor of choice for electric propulsion in aerospace applications is the FBG.

Manufacturing and reliability of a fiber Bragg grating strain sensor designed for uniaxial compression test of rocks

In this work we present an optical Fiber Bragg Grating (FBG) strain sensor appropriate for uniaxial compression test of rocks. The rocks are a complex aggregate of materials and may exhibit in some cases mineral grains of size bigger than the conventional electrical strain gages used. Furthermore, commercial surface mountable optical FBG strain sensors are packaged in planar configuration, which is not appropriate for the cylindrical rock cores in uniaxial compression test. The optical FBG sensors were designed and manufactured manually fitted for cylindrical rock samples. A calibration process was carried out in order to stablish the mechanical performance of the FBG packaging; later, a compression test was conducted on hard rock sample and the strain was recorded by the traditional electrical strain gage (ESG) and FBG system. Under this comparison, the FBG sensors reaches a larger recording area, sensitivity under 100 microstrain and a reliable strain transfer, all of which is appropriate to measure strain for heterogeneous rock composition and minerals grain size.

A self-demodulated fiber Bragg grating for investigating impact-induced transient responses of phononic crystal beams

This work presents a fiber Bragg grating (FBG) displacement sensing system to experimentally investigate the dynamic behaviors of phononic crystal (PC) beams through impact-induced early short time or long time transient responses. Based on the couple-mode theory and the optical transfer matrix (T-matrix) formulation, we first show that it is feasible to achieve linear displacement sensing using a single FBG without extra demodulators such as matching gratings. To validate its effectiveness, the proposed self-demodulated FBG system is applied to measure the point-wise transient displacement responses of a cantilever PC beam subjected to steel ball impacts. To demonstrate the transient sensing performance, the measured early short time transient responses are compared with the corresponding ones predicted by the method of reverberation-ray matrix (MRRM) theoretically and the finite element method (FEM) numerically. The excellent agreements verify together the experimental, theoretical and numerical results. Finally, conducting Fast-Fourier transform (FFT) to the early short time or long time transient responses gives the frequency responses that clearly show the existence of the band gaps. In addition to providing a new displacement sensing method using the self-demodulated FBGs, this work also offers another route to investigate the band structures of PC beams through the frequency responses transformed from the early short time transient responses, long before the structural damping becomes dominant.

Statistical Analysis of Stress Signals from Bridge Monitoring by FBG System.

In this paper, a fiber Bragg grating (FBG)-based stress monitoring system instrumented on an orthotropic steel deck arch bridge is demonstrated. The FBG sensors are installed at two types of critical fatigue-prone welded joints to measure the strain and temperature signals. A total of 64 FBG sensors are deployed around the rib-to-deck and rib-to-diagram areas at the mid-span and quarter-span of the investigated orthotropic steel bridge. The local stress behaviors caused by the highway loading and temperature effect during the construction and operation periods are presented with the aid of a wavelet multi-resolution analysis approach. In addition, the multi-modal characteristic of the rainflow counted stress spectrum is modeled by the method of finite mixture distribution together with a genetic algorithm (GA)-based parameter estimation approach. The optimal probability distribution of the stress spectrum is determined by use of Bayesian information criterion (BIC). Furthermore, the hot spot stress of the welded joint is calculated by an extrapolation method recommended in the specification of International Institute of Welding (IIW). The stochastic characteristic of stress concentration factor (SCF) of the concerned welded joint is addressed. The proposed FBG-based stress monitoring system and probabilistic stress evaluation methods can provide an effective tool for structural monitoring and condition assessment of orthotropic steel bridges.

Real-time estimation of delamination occurrence induced by low-velocity impact in composite plates using optical fiber sensing system

Low-velocity impact is the one of the most critical events to reduce the reliability of composite structures, because it can generate hidden damages such as delamination inside the structures. In order to address this problem, impact monitoring systems have been suggested using various types of built-in sensors. Generally, such impact monitoring systems are consisted of impact localization and damage assessment. In this paper, a methodology of impact-induced delamination assessment was studied using fiber Bragg grating (FBG) sensors. Because of its multiplexing capability, FBG sensors are advantageous of monitoring large areas of target structures. However, it is hard to be applied in impact monitoring systems due to its low sensitivity and narrow bandwidth. In this study, a quantification method of delamination was examined with maintaining the merits of FBG sensors. For the acquisition of impact-generated acoustic emission (AE) signals, the commercial FBG system was adopted with a sampling frequency of 100 kHz. Then, the acquired AE signals during the low-velocity impact fracture tests were analyzed with the wavelet transform (WT) method for measuring the delamination areas due to different energies. Finally, it can be concluded that the quantities of WT detailed portions are linearly correlated with the delamination areas.

Fiber Optic Measurement System for Fresnel Reflection Sensing: Calibration, Uncertainty, and Exemplary Application in Temperature-Modulated Isothermal Polymer Curing

Fiber Bragg grating (FBG) sensors and Fresnel reflectometer sensors have emerged as promising tools for monitoring process state parameters during composite manufacturing. However, multifunctional fiber optic sensor systems exploiting the versatile nature of these sensors are not commercially available. In principle, most FBG measurement systems are equipped with the required hardware to measure the refractive index (RI) by interrogating Fresnel reflectometer sensors. However, the implications of using an FBG system for this purpose have not been previously studied. Therefore, we calibrated a commercial FBG measurement system in the refractive index range between 1.3 and 1.6 at 1550 nm, based on data comparison with an independent RI measurement technique, and determined the measurement uncertainty. Additionally, we investigate the suitability of the proposed Fresnel reflectometer sensor for use in temperature-modulated isothermal epoxy curing experiments for detecting the glass transition by exploiting a concept that resembles temperature modulated optical refractometry. This paper describes in detail the calibration routine, the determination of the measurement uncertainty, and the application of Fresnel reflectometer sensors for glass transition detection during isothermal curing. We observed an offset between the measured RI and the theoretical value in the range of $10^{{{\hbox{--}} 3}}$ . The measurement system uncertainty was found to be of the order of $10^{{{\hbox{--}} 4}}$ . That is comparable to the accuracy of most commercial refractometers. Furthermore, we showed that Fresnel reflectometer sensors are suitable for glass transition detection during isothermal curing. The results show that commercial FBG measurement systems are viable options for multifunctionality sensing in online process monitoring applications.

FBG-Based Monitoring of Geohazards: Current Status and Trends.

In recent years, natural and anthropogenic geohazards have occured frequently all over the world, and field monitoring is becoming an increasingly important task to mitigate these risks. However, conventional geotechnical instrumentations for monitoring geohazards have a number of weaknesses, such as low accuracy, poor durability, and high sensitivity to environmental interferences. In this aspect, fiber Bragg grating (FBG), as a popular fiber optic sensing technology, has gained an explosive amount of attention. Based on this technology, quasi-distributed sensing systems have been established to perform real-time monitoring and early warning of landslides, debris flows, land subsidence, earth fissures and so on. In this paper, the recent research and development activities of applying FBG systems to monitor different types of geohazards, especially those triggered by human activities, are critically reviewed. The working principles of newly developed FBG sensors are briefly introduced, and their features are summarized. This is followed by a discussion of recent case studies and lessons learned, and some critical problems associated with field implementation of FBG-based monitoring systems. Finally the challenges and future trends in this research area are presented.

Acoustic emission localization based on FBG sensing network and SVR algorithm

In practical application, carbon fiber reinforced plastics (CFRP) structures are easy to appear all sorts of invisible damages. So the damages should be timely located and detected for the safety of CFPR structures. In this paper, an acoustic emission (AE) localization system based on fiber Bragg grating (FBG) sensing network and support vector regression (SVR) is proposed for damage localization. AE signals, which are caused by damage, are acquired by high speed FBG interrogation. According to the Shannon wavelet transform, time differences between AE signals are extracted for localization algorithm based on SVR. According to the SVR model, the coordinate of AE source can be accurately predicted without wave velocity. The FBG system and localization algorithm are verified on a 500 mm×500 mm×2 mm CFRP plate. The experimental results show that the average error of localization system is 2.8 mm and the training time is 0.07 s.

Simulation of FBG Wavelength Signal Demodulation Based on Sideband Filter

The wavelength of Fiber Bragg grating (FBG) sensor signal demodulation simulation system based on the quasi-linear relationship between the transmission side band of optical filters and wavelength was established by the simulation software of MABTLAB. The simple system only consists of broadband source, sideband optical filters, couplers, a FBG, data acquisition and processing system. The relationship between the information of FBG central wavelength and the transmitted light intensity of sideband filter were obtained when the alternating and gradually varying signal applied to the FBG sensors, respectively. The FBG wavelength demodulation can be achieved by sideband filtering, and results are consistent with the theory.

Strain and Displacement Controls by Fibre Bragg Grating and Digital Image Correlation

ABSTRACTTest control is traditionally performed by a feedback signal from a displacement transducer or force gauge positioned inside the actuator of a test machine. For highly compliant test rigs, this is a problem since the response of the rig influences the results. It is therefore beneficial to control the test based on measurements performed directly on the test specimen. In this paper, fibre Bragg grating (FBG) and Digital Image Correlation (DIC) are used to control a test. The FBG sensors offer the possibility of measuring strains inside the specimen, while the DIC system measures strains and displacement on the surface of the specimen. In this paper, a three‐point bending test is used to demonstrate the functionality of a control loop, where the FBG and DIC signals are used as control channels. The FBG strain control was capable of controlling the test within an error tolerance of 20 µm m−1. However, the measurement uncertainty offered by the FBG system allowed a tolerance of 8.3 µm m−1. The DIC displacement control proved capable of controlling the displacement within an accuracy of 0.01 mm.

Strain as Damage Indicator for Truss and Frame Structures

Damage detection techniques are becoming more and more important issues for prolonging the lifetime of the old structures. In the paper the strain measurements on offshore oil platform leg model as well as the preliminary measurement system installed on jack-up rig is presented. During experiments performed on frame model of the platform leg different types of damage scenarios were tested. The Fiber Bragg Grating (FBG) strain sensors were capable to detect damage. After laboratory experiments, preliminary FBG system was installed on the real construction (jack-up rig) excited by environmental and operation-induced forces. It is evident that strain measurement based on FBG technology is very promising technique for use in truss/frame structures.

Practical Application of Monitoring System Based on Optical Sensors for Marine Constructions

The paper presents structural health monitoring (SHM) system, dedicated to marine structures. The considered system is based on the fibre optic technique with Fibre Bragg Grating (FBG) sensors. The aim of this research is recognition of possible practical applications of the fibre optic techniques in selected elements of marine structures. SHM and damage detection techniques have a great importance (economical, human safety and environment protection) in the wide range of marine structures, especially for ships and offshore platforms. In the paper monitoring system of the Horyzont II and Dar Młodzieży ships and offshore oil platform is presented. Practical implementation of safety system based on optical sensors meets several difficulties. There has been installed the FBG system and its measurement results have been compared with classical techniques, e.g. piezoelectric accelerometers. The investigations have been performed for undamaged and damaged structure. Different types of failures have been modelled and tested. Damage detection ability has been specifying on the base of static and dynamic structural characteristics.

DYNAMIC MONITORING THE STRAIN OF COMPOSITES DUE TO HIGH SPEED COLLISIONS USING FIBER BRAGG GRATING NETWORK

This paper describes the Fiber Bragg Grating (FBG) sensor system for a dynamic monitoring strain due to high speed collision. The authors investigate whether or not this FBG system can be used for the real time detection of both the impact and the damage generated by high speed collisions. A three-component strain sensor system was preliminary designed for real-time detection the location of collision and the damage extent such as the ranges of the elastic deformation and the plastic deformation. The basic optical and electronic theories about this system, as well as the mechanism and mechanics model of FBG sensors, are expressed in this paper.DOI: http://dx.doi.org/10.5755/j01.mech.18.1.1277

Development of FBG Sensing System for Outdoor Temperature Environment

We have experimentally designed a practical pass-through type FBG temperature sensor. The objective of this study is to design and built a prototype outdoor fibre Bragg grating (FBG) temperature sensor system. Its performance is evaluated at different times of the day. In order to reduce the optical losses of the FBG system, the shortest optical fiber path used to connect the FBG system is 55.5m. It has a total connector loss 4.0dB and fibre loss of 0.3dB thus giving a total loss of the system as 4.3dB. The FBG sensor system is connected to the tunable laser source (TLS) and optical spectrum analyzer (OSA). The TLS is used to provide a broadband light source via a fibre optic cable of wavelength 1550nm. The OSA is used to display the transmission and reflection spectrum to give the Bragg wavelength _B, bandwidth and power dip. The output spectrum can be obtained through direct connection to the FBG. Result of transmission and reflection spectrum show the sensitivity which is calculated from the slope of the graph. The FBG temperature sensor system has an average sensitivity of 9.1 pm (°C)-1 based on the transmission spectrum. It has an average sensitivity of 10.6 pm (°C)-1 based on the reflection spectrum.

Comparison of temperature distribution measurement methods with the use of the Bragg gratingsand Raman scattering of light in optical fibers

Two types of fiber-optical measurement systems are compared: a line with a large number of point sensors based on fiber Bragg gratings (FBGs) interrogated by a tunable continuous fiber laser and a distributed system based on optical time-domain reflectometry (OTDR) of the Raman scattering of radiation of a pulsed fiber laser. Methods for increasing the measurement accuracy with the use of additional calibration of the Bragg wavelength shift over a fiber interferometer in the FBG system and spectral filtration of the Stokes and anti-Stokes components of the Raman scattering with the use of spectral-selective fiber couplers in the OTDR system are proposed and implemented. Physical effects on system parameters are analyzed, compared, and optimized for applications with monitoring of the temperature distribution in turbogenerators and oil wells.