High-speed Fiber Bragg Grating 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.

FPGA-Based High-Speed Optical Fiber Sensor Based on Multitone-Mixing Interferometry

We report a real-time high-speed fiber Bragg grating (FBG) interrogator based on a fiber-optic interferometer. The signal processing is performed by using a low-cost field-programmable gate array (FPGA) system, which is programed to implement a phase-generated carrier (PGC) demodulation algorithm with multitone mixing (MTM) to provide distortion-free signals with high tolerance to modulation depth variations and light intensity fluctuations. The system can stream data at rates up to 1 MS/s and allows multiplexed processing up to two channels. Experimental results show simultaneous measurements of two FBGs, one of which was actuated at frequencies up to 100 kHz. The system features a 3-dB bandwidth of 280 kHz, and a dynamic wavelength resolution of 4.7 fm/Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\mathrm {1/2}}$ </tex-math></inline-formula> . We also demonstrate a strong reduction of distortion using the MTM approach with respect to the standard technique. Finally, we study the origin of the noise, demonstrating a reduction in common noise sources by using one of the FBGs as a reference. The system can measure FBGs centered at any position within the spectral band of the source, is polarization-independent, and is easily scalable to more than two measurement channels from the same interferometer.

High-speed interrogation of multiplexed fiber Bragg grating sensors for monitoring dynamic strain responses during a flexible plate impact on a water surface

The investigation of fluid-structure interaction during the impact of a flexible plate on a water surface has received much attention. Measurement of highly transient, distributed strain of the plate during the slamming event is of great interest. Multiplexed fiber Bragg grating (FBG) strain sensors provide a promising solution for such measurement since these sensors are inherently waterproof and are immune to electromagnetic interference. However, in order to monitor the highly transient, distributed strain responses (up to 20 kHz), high-speed simultaneous interrogation of multiplexed FBG sensors is required, which is challenging by using commercial optical interrogators. We present a tunable-wavelength-filter-based optical interrogation system for high-speed simultaneous interrogation of multiplexed FBG strain sensors and demonstrate its application for structural monitoring of a flexible plate during the slamming event. The interrogation system employs a piezoelectric-transducer-controlled Fabry–Perot tunable filter. By operating the tunable filter at its resonant frequency and demodulating the sensor signal based on a peak tracing method, we demonstrated an interrogation speed of 100 kHz, an interrogation range of 98 nm, and an interrogation resolution of 5 pm. For proof-of-performance, the interrogation system was used to monitor the vibrational responses of a cantilever plate under impact loading and the measurement of vibration modes up to 6.785 kHz was demonstrated. Finally, the slamming experiments were carried out with six multiplexed FBG strain sensors mounted on a flexible plate. The dynamic strain measurement of the plate during the slamming event was successfully demonstrated by using the high-speed FBG interrogation system.

An edge-filter FBG interrogation approach based on tunable Fabry-Perot filter for strain measurement of planetary gearbox

• An improved TFP-based edge-filter FBG interrogation approach is proposed. • A feedback channel is adopted to control the creep of the piezoelectric actuator. • The TFP is driven by both the coarse voltage and fine voltage. • The FBG interrogation approach is applied to the strain measurement of ring gear. Fiber Bragg gratings (FBGs), with smaller size, are suitable to measure the tooth root strain of ring gear which is an ideal signal for fault diagnosis of planetary gearboxes. In order to identify the fault features in the strain signal, a high-speed and high-resolution FBG interrogator is needed. Compared with wavelength-scanning interrogation approach, the edge-filter approach can achieve such a performance more easily and economically. However, conventional edge-filter approaches based on fixed edge filters have low applicability. The adoption of a tunable Fabry-Perot filter (TFP) increases the flexibility of the edge-filter system, but the creep characteristics of the piezoelectric actuator in TFP may cause interrogation errors. This paper proposes an improved TFP-based edge-filter FBG interrogation approach with creep control ability. First, the interrogation principle of the proposed approach is analyzed by comparing with the conventional edge-filter approaches. The creep control method is presented by employing a feedback channel and two kinds of TFP driving voltage. Next, a FBG interrogator based on the proposed approach is established. The creep control performance, static interrogation performance and dynamic interrogation performance are all tested. The results show that the interrogator has the wavelength repeatability of ~ 0.1 pm at 1 kHz and ~ 0.2 pm at 5 kHz respectively. Finally, the proposed edge-filter approach is applied to measure the tooth root strain of ring gear in a planetary gearbox. The measured strain signal has a high quality, and the tooth fault feature can be identified obviously.

High-Speed FBG Interrogation With Electro-Optically Tunable Sagnac Loops

We report high-speed Fiber Bragg Grating (FBG) interrogation based on fiber interferometry. An electro-optical modulator, used as a tunable retarder, is implemented in a fiber Sagnac loop, enabling fast tuning of the interference fringes. Applying an active modulation scheme, the wavelength of the FBG can be retrieved without any requirement of the spectral position of the FBG with respect to the fringes, with no need of tunable sources or spectral analysis. We report experimental results of wavelength measurements with a bandwidth of 100 kHz (only limited by our data acquisition rate) and a dynamic wavelength resolution of σ = 3.7 fm/√Hz (corresponding to a strain resolution of 3.1 nϵ/√Hz). We also show that the system can track a continuous tunable laser sweep of 40 nm along the C-band. The technique is polarization-independent and allows wavelength division multiplexing schemes using only one interferometer for all the sensing points. The system is fully based on low-cost off-the-shelf fiber-optical components, and the free-spectral-range of the interferometer can be easily modified by just changing the length of the polarization-maintaining fiber segment.

Method of damage location determination based on a neural network using a single fiber Bragg grating sensor.

In this paper, a structural damage identification algorithm based on a single fiber Bragg grating (FBG) sensor isproposed. The signal detected by the FBG can be analyzed by the wavelet packet decomposition and back propagation neural network to obtain the damage location information. A high-speed FBG demodulation system based on a tunable Fabry-Perot filter and unbalanced Mach-Zehnder (M-Z) interferometer is designed to respond to a signal with a frequency range from 0 to 4kHz, which will increase the sensing accuracy. This algorithm is verified by the aluminum plate model, which can simulate the generation of damage in reality. The experimental results show that a single FBG sensor is enough to realize accurate damage location identification according to this algorithm, and identification accuracy can reach 90.0%.

Distortion-tolerated high-speed FBG demodulation method using temporal response of high-gain photodetector

Abstract A distortion-tolerated fiber Bragg grating (FBG) demodulation method for high-gain or high-speed sensing is proposed. The temporal response of photodetector (PD) on FBG reflection spectrum for different electric gains and wavelength scanning frequencies is investigated theoretically and experimentally. Simulation results show that high electric gains and high wavelength scanning frequencies will induce a tailed distortion on the right side of the FBG reflection peak output by the PD. A temporal response function of PD is presented to fit the peak with distortion and determine the peak position on FBG reflection spectrum. Experiments with varying degrees of FBG acquisition distortion are carried out. Compared with the traditional Gaussian fitting method, the measurement error of the temporal-response based method is limited within ±1.777 pm and the standard deviation is less than 0.940 pm, which are respectively improved by 6.87 times and 4.38 times at least.

Multichannel High-Speed Fiber Bragg Grating Interrogation System Utilizing a Field Programmable Gate Array

We demonstrate a multichannel high-speed fiber Bragg grating (FBG) interrogation system utilizing a field programmable gate array (FPGA). A temperature-controlled Fourier domain mode locking laser was operated with a sweep rate of 50.7kHz, sweep bands of 35nm, and an output power of 0.91mW. This laser enabled an FBG measurement speed of up to 9.9 s. Real-time processing could be achieved by incorporating a high-speed data acquisition system (DAQ) mounted on an analog to digital converter and an FPGA. The DAQ performed in-line processing with four 14-bit analog channels and a sampling frequency of 250MHz. The internals of the FPGA were implemented parallel processing for high-speed FBG detection to allow for the simultaneous measurement of the multiplexed FBG in three channels. We demonstrated that this system is capable of high-speed vibration measurements above several tens ofkHz with a multichannel.

Performance Optimization Design for a High-Speed Weak FBG Interrogation System Based on DFB Laser.

A performance optimization design for a high-speed fiber Bragg grating (FBG) interrogation system based on a high-speed distributed feedback (DFB) swept laser is proposed. A time-division-multiplexing sensor network with identical weak FBGs is constituted to realize high-capacity sensing. In order to further improve the multiplexing capacity, a waveform repairing algorithm is designed to extend the dynamic demodulation range of FBG sensors. It is based on the fact that the spectrum of an FBG keeps stable over a long period of time. Compared with the pre-collected spectra, the distorted spectra waveform are identified and repaired. Experimental results show that all the identical weak FBGs are distinguished and demodulated at the speed of 100 kHz with a linearity of above 0.99, and the range of dynamic demodulation is extended by 40%.

Real-time detection of low-velocity impact-induced delamination onset in composite laminates for efficient management of structural health

Abstract Carbon fiber reinforced polymer (CFRP) laminates have been becoming primary structures in the aerospace industry because of their high specific strength and stiffness. However, CFRP laminates have susceptibility to low-velocity impact events which can easily induce internal or hidden damages such as delamination. Such impacts frequently arise during maintenance, flight operation or in-service events. Thus, composite structures have to be irregularly inspected in addition to the periodic maintenance for ensuring the structural health. However, such irregular inspections can inherently incur the overall maintenance cost because it has to be performed in all suspicious cases of damages. For this reason, the methodology for accurately realizing the onset of delamination induced by low-velocity impact events is required for reducing the operating cost of composite structures. In this paper, the potential of using high speed fiber Bragg grating (FBG) sensing system for detecting the delamination onset was studied for thick CFRP laminates. Because FBG sensors can be simply multiplexed to capture the structural responses, the proposed method in this study can be quite attractive for an efficient impact monitoring system. To obtain the impact response signals and contact force histories, several low-velocity impact experiments were performed in a range of 1–30 J. From the signal processing of these experimental data, the meaningful damage index was introduced using the detail components of wavelet transformed sensor signals. Although this result is in the preliminary step, such damage index can be useful for applying an in-situ impact damage assessment system to the real composite structures in the near future.

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.

The measurement of dynamic strain and resonant frequency for three-dimensional solids partially immersed in water using free-edge bonded fiber Bragg grating sensors

In this work, the feasibility of liquid level detection based on structural vibration modes is investigated. We specifically consider impact-induced vibration modes of an aluminum solid partially immersed in a water tank. A free-edge bonded fiber Bragg grating (FBG) sensor and a corresponding high-speed FBG filter-based demodulation system are employed to measure the transient impact responses. To perform more accurate in vacuo finite element analysis (i.e., for the mode shapes, resonant frequencies, and the impact responses), elastic constants of the aluminum solid and impact loading history of a steel ball are obtained beforehand respectively according to the elastic wave theory and with a polyvinylidene fluoride (PVDF)-based technique. Our experimental results show good correspondences of intensities and shifts of the impact-induced resonant modes with liquid levels. The modes of the same type (e.g., double modes in out-of-plane or in-plane dominated modes) are shown to possess similar behaviors in relationships between shifts of the resonant modes and the liquid level. This work also provides related experimental evidence for future researchers interesting in simulation of complicated fluid-structure interaction (FSI) problems for three-dimensional structures.

FBG sensor array-based-low speed impact localization system on composite plate

A fiber Bragg grating (FBG) sensors-based impact localization system on composite structure and a novel localization algorithm independent of wave velocity were proposed. Six FBG sensors constitute two isosceles right triangle FBG arrays. Impact signals were detected by a high-speed FBG interrogation system. Morlet wavelet transform was employed to extract time differences of impact signals. The straight lines equations, which are through impact source and FBG sensors of right-angled vertices of FBG arrays, can be obtained by the time differences. The coordinate of impact source is the intersection of straight lines. Testing experiments were carried out on composite plate within 400 mm × 400 mm monitor area. The experimental results showed that the maximum and average errors were 20.92 and 8.67 mm, respectively. This article provides a simple and stable impact source localization system independent of wave velocity.

Delay Compensated FBG Demodulation System Based on Fourier Domain Mode-Locked Lasers

Fourier domain mode-locked lasers are excellent wavelength-swept sources for high-speed fiber Bragg grating (FBG) demodulation. When the lasers are applied for remote FBG sensing, the transmission delay could make the sensing signal deviating significantly from its real position due to the short sweep period of the lasers, which can cause large wavelength demodulation error. An autocalibration method for compensating the delay is proposed by inserting a Fabry–Perot etalon with a wavelength marker into the reference optical path. Besides, the wavelength sweeping nonlinearity can be real-time monitored via the multiple combs of the Fabry–Perot etalon and absolute wavelength of FBG can be derived using interpolation algorithm. Experimental results show that this demodulation method is efficient and accurate.

Impact localization on composite wing using 1D array FBG sensor and RMS/correlation based reference database algorithm

In this paper 1-dimensional (1D) array fiber Bragg grating (FBG) sensor configuration is proposed for impact localization of random impact points on composite wing. Firstly, 2D and 1D array FBG sensor localization results are compared and then 1D array FBG sensor configuration is implemented on a full scale composite wing of Jabiru UL-D aircraft. A high speed FBG interrogator is used to sample the impact signals at 100kHz and reference database impact localization algorithm is used to estimate the impact location. The sensor configuration and impact localization techniques presented in this paper demonstrates an excellent means of decreasing the number of required sensor for localizing impact points. Moreover, through a parametric study it is demonstrated that using 1D array sensor configuration and reference database algorithm, two FBG sensors covering 4.2m×0.6m wing area can approximate the impact location with maximum localization error limited to about 35.0mm, which is as effective as using 1D array configuration with six FBG sensors.

An integrated approach for structural health monitoring using an in-house built fiber optic system and non-parametric data analysis

Multivariate statistics based damage detection algorithms employed in conjunction with novel sensing technologies are attracting more attention for long term Structural Health Monitoring of civil infrastructure. In this study, two practical data driven methods are investigated utilizing strain data captured from a 4-span bridge model by Fiber Bragg Grating (FBG) sensors as part of a bridge health monitoring study. The most common and critical bridge damage scenarios were simulated on the representative bridge model equipped with FBG sensors. A high speed FBG interrogator system is developed by the authors to collect the strain responses under moving vehicle loads using FBG sensors. Two data driven methods, Moving Principal Component Analysis (MPCA) and Moving Cross Correlation Analysis (MCCA), are coded and implemented to handle and process the large amount of data. The efficiency of the SHM system with FBG sensors, MPCA and MCCA methods for detecting and localizing damage is explored with several experiments. Based on the findings presented in this paper, the MPCA and MCCA coupled with FBG sensors can be deemed to deliver promising results to detect both local and global damage implemented on the bridge structure.

Low velocity impact localization system of CFRP using fiber Bragg grating sensors

Carbon fiber reinforced plastics (CFRP) structures are very susceptible to barely visible impact damage induced by low velocity impact. Thus knowledge of the position of an impact event on a CFRP structures can be useful information for detecting this type of damage. In this paper, low velocity impact localization system using fiber Bragg grating (FBG) sensors and localization algorithm for CFRP structures were investigated. The FBG wavelength shifts, caused by impact, were captured by a high speed FBG interrogation system. Wavelet transform was used to weaken signal noise. After that, the arrival time differences of each leading wave were calculated to produce the input data sets for least squares support vector machines (LS-SVM) model training. Finally, FBG impact localization system was constructed and validated on a CFRP plate with 300mm*300mm*2mm experiment area. The experimental results showed that the impact position abscissa and ordinate localization errors were all less than 5mm, and the maximum and average impact localization errors were 5.8mm and 3.7mm, respectively. This paper provided a reliable method for CFRP structures impact localization.

Detection of Impact Damage in Composite Structures Using High Speed FBG Interrogator

In the case of composite structures, low-velocity impact events can cause various types of damage that are mostly hidden inside the laminates or occur on the side opposite to the impact. Thus, damage cannot be easily detected by visual inspection or conventional NDT techniques. Moreover, if the damage occurs between the scheduled inspection periods, the possibilities of extensive damage or catast rophic failure can be increased. For these reasons, built-in NDT systems, such as the real-time impact monitoring system, are required in most composite structures for high reliability and ease of maintenance. In this paper, such an impact monitoring system, including impact location detection and damage assessment techniques, wasstudied for composite flat and stiffened panels. In order to acquire the impact-induced acoustic signals, four multiplexed fiber Bragg gratings (FBGs) and commercial high-speed FBG interrogator were used. In signal processing for the detection of impact locations and damage occurrences, neural networks and wavelet transforms were adopted. Finally, these algorithms were embodied using MATLAB and LabVIEW software to obtain a user-friendly interface.

Application of high-speed fiber bragg grating sensing system for safety monitoring of modern ships

Structural health monitoring (SHM) can be understood as the integration of sensing intelligence and possibly also actuation devices to allow the structure loading and damaging conditions to be recorded, analyzed, localized and predicated. SHM sensing requirements are very well suited for fiber optic technology. The fiber Bragg grating (FBG) sensor is a promising sensor of SHM for many applications because of its good optical properties, small size, low insertion loss, antiRF, anti-electromagnetic interference, breakdown voltage, radiation-resistant, light weight, small and large signal bandwidth. There also are reports of the application of FBG sensors for SHM of large industrial and civil works such as dams, bridges [1−2] , aero plane [3] , and tunnels [4,5] . The development of ship’s SHM takes advantage of multidisciplinary work that is being done, and most of the applications reported are about ship hull monitoring systems [6,7] . Previous studies mostly focus on the measurement of static stress [8] . For a modern ship, the thruster is a key part of the propulsion machinery, and the stability of its related equipments has a great influence on the propulsion efficiency of the ship. Electric resistance strain sensors are difficult to be used for the stress measurement for the thruster because of the electromagnetic disturbance and other complicated environment. Low-speed FBG sensors cannot meet the requirements due to the propulsion machinery’s 100 Hz arising vibration frequency. The high-speed FBG sensing system can be used to measure dynamic stress and achieve the goal of safety monitoring in advanced ships. In this letter, the application of a high-speed FBG sensing system for SHM of modern ships is proposed. We apply this sensing system to give on-site tests to an advanced ship. Strain distribution of the bases and supporting frames of its large turbine thrusters are measured both onshore and in the sea when it runs at different work conditions. The maximum natural frequency of this measured structure is 335 Hz. To test all the monitoring points synchronously, an 8-channel senor system is applied, and the data acquisition frequency was 1000 Hz. Most monitoring points are set with single-line sensors for unilateral stress, and the others are set with triple directions sensors to measure shear stress. Temperature compensation is taken with additional FBGs to eliminate the effect of cross-sensitivity between temperature and strain on FBG sensors. As for every single monitoring point, the max and average stress-gas turbine power factor (GTPF): N/Ne curves are analyzed. Then we synthesize all the results from three-time onshore tests and acquire the static stress-GTPF curves. Results from the test at sea are discussed together, thus we can accomplish completely prediction of the dynamic stress-GTPF curve when we did not obtain dynamic data in every work condition because of some objective reason. We examine the vibration amplitude of the wave length of signal light from each specific monitoring point under the same work condition for all tests and find it barely affected by the environment factors, unlike the absolute wave length. Those experimental results are accordant with mechanical theories, and the final reports have obtained clients’ approval. FBGs are periodic alterations in the index of refraction of the fiber core, and only the specified wavelength (Bragg wavelength) related to its grating period is reflected for an input light wave form a broadband source.

Fracture Monitoring of Composite Wind Turbine Blade Using High-Speed Bragg Grating Sensor System

This paper introduces a FBG (fiber Bragg grating) based AE (acoustic emission) sensing system for use in health monitoring of composite wind turbine blades. In this study, a multiplexing high speed FBG sensor system was developed with a spectrometer-type demodulator based on a linear photo detector. Pencil break test was performed using an FBG sensor and the results were compared with the results of piezo-based AE sensor. For the performance test of fracture sensing in composite materials, a down-scaled wind turbine blade was fabricated and drop impact tests were performed. Arrayed 4 FBG sensors were installed in the skin of wind turbine blade and impacts of 15 J energy were applied by a drop weight. The frequency characteristics of impact induced AE signals were examined with short-time Fourier transform focused on the leading waves. Finally, the onset of fractures in composite structure was successfully assessed using arrayed FBG sensors.