There is interest in the rotorcraft community to develop health monitoring technologies. Among these technologies is
the ability to monitor the transmission planetary gear system. The gearbox environment does not lend itself to traditional
sensing technologies due to the harsh environment and crowed space. Traditional vibration-based diagnostics are based
on the output from externally mounted sensors, usually accelerometers fixed to the gearbox exterior. This type of system
relies on the ability of the vibration signal to travel from the gears through the gearbox housing. These sensors are also
susceptible to other interference including electrical magnetic interference (EMI). For these reasons, the development of
a fiber optic-based transmission monitoring system represents an appealing alternative to the accelerometer due to their
resistance to EMI and other signal corrupting influences. Aither Engineering has been working on integrating the fiber
optic sensors into the gearbox environment to measure strain on the ring gear of the planetary gear system. This
application utilizes a serial array of wavelength division multiplexed fiber Bragg grating (FBG) sensors. Work in this
area has been conducted at both the University of Maryland, College Park and more recently at the NASA Glenn
Research Center (NGRC) OH-58 transmission test rig facility. This paper discusses some of the testing results collected
from the fiber optic ring gear sensor array. Based on these results, recommendations for system requirements are
addressed in terms of the capabilities of the FBG instrumentation.
Internal components of the planetary stage of a helicopter transmission have proven difficult to diagnose due to the
complex geometry of the gearbox and the inability to place sensors inside the planetary gear system. The goal of the
research presented in this paper is to use fiber optic sensors to monitor the strain response of planetary gears on the
surface of the ring gear. As the planetary gears traverse the locations of the fiber optic strain sensors, a local strain
profile will be recorded. By synchronous data processing of the strain profiles of each planetary gear, an average
response signal will be generated. Deviations from the average response profile will provide an indication to the
existence of damage within the planetary stage. This paper will present results from experiments conducted on a
transmission test rig at the University of Maryland. Fiber Bragg grating sensors were selected because of their
multiplexing capabilities and localized strain measurement attributes. Initial test results have proven the feasibility of
using FBG sensors to monitor the strain response due to the planetary stage. Research to be conducted includes the
development and application of advanced damage detection algorithms that take advantage of the novel attributes of this
approach.
Systems Planning and Analysis, Inc. (SPA) has recently planned, installed, and tested a fiber Bragg grating (FBG) strain sensor system to validate FEM predictions of a new submarine design undergoing American Bureau of Shipping (ABS) certification testing. Fiber optic triaxial, biaxial, and uniaxial gage locations were selected based on the FEM analysis. FBGs were placed on six optical fibers with two fibers (33 sensors) mounted internally to the hull and four fibers (64 sensors) mounted externally. Testing was performed by lowering the submarine to the design depth and recording strain measurements. The optical sensor signals were transmitted directly to the water's surface and monitored by top-side interrogation instrumentation through over 2000 feet of optical cable. Measured temperature-compensated strain values were compared to the FEM predicted strain values with excellent results. To the author's knowledge, this successful test represents the first time that FBG sensors have been used to certify a submarine design and to validate FEM analysis on a large-scale structure.
The research presented in this paper demonstrates the feasibility of detecting deformations of wing-like structures (cantilever plates) using a distributed, multiplexed fiber Bragg grating (FBG) sensor system. Overall, the work accomplished during the research effort demonstrates the ability of a multiplexed network of FBG sensors to measure strain across a structure and have those strains determine the deformation or shape of the structure. A neural network approach and a structural mechanics approach were examined to determine the deformation of the cantilever plate based on the recorded strain readings from the FBG sensors. This paper presents analytical and experimental results for both approaches. The experimental setup consisted of a cantilever plate with 36 FBG sensors mounted in longitudinal, transverse, and off-axis orientations on the plate’s surface. The neural network method performed best when trained with experimentally measured strain and deflection data. The structural mechanics approach demonstrated more accurate comparisons to the ground truth shape, taken with a laser range finder, than the neural network approach. In all experimental cases using the structural mechanics approach, the root-mean-square error between the ground-truth measurements and the FBG sensor based measurements were less than 0.52 inches (approximately 10% of maximum deflection). In approximately 30% of the test cases, the root-mean-square error was less than 0.2 inches.
Systems Planning and Analysis, Inc. (SPA) has developed a novel statistical approach to estimating the remaining useful life of aircraft components based on known usage monitoring data. The analysis technique is known as the Remaining Useful Life Estimation (RULE) methodology. The basic premise of RULE is to determine conservative predictions for the component loads and fatigue life values from Monte Carlo simulations based on a desired component reliability. Then, as the aircraft's usage is monitored, the component life can be calculated with a known reliability based on the conservative predictions generated by the Monte Carlo simulation. The RULE methodology, which has been successfully tested on small-scale analytical problems, is ideally suited to be integrated into both rotorcraft and fixed-wing aircraft. Furthermore, modifications to the technology may prove to be applicable to wide variety of health and prognostic problems.
Today's society places an extremely high price on the value of human life and injury. Whenever possible, police and paramilitary actions are always directed towards saving as many lives as possible, whether it is the officer, perpetrator, or innocent civilians. Recently, the advent of robotic systems has enable law enforcement agencies to perform many of the most dangerous aspects of their jobs from relative safety. This is especially true to bomb disposal units but it is also gaining acceptance in other areas. An area where small, remotely operated machines may prove effective is in local aerial surveillance. Currently, the only aerial surveillance assets generally available to law enforcement agencies are costly helicopters. Unfortunately, most of the recently developed unmanned air vehicles (UAVs) are directed towards military applications and have limited civilian use. Systems Planning and Analysis, Inc. (SPA) has conceived and performed a preliminary analysis of a low-cost, inflatable, lighter- than-air surveillance system that may be used in a number of military and law enforcement surveillance situations. The preliminary analysis includes the concept definition, a detailed trade study to determine the optimal configuration of the surveillance system, high-pressure inflation tests, and a control analysis. This paper will provide the details in these areas of the design and provide an insight into the feasibility of such a system.
Tomorrow's battlefields will be dominated by the force that is able to obtain the greatest amount of information. Although battlefield situation information can come from a number of source, one particularly important segment of intelligence gathering is aerial surveillance. For the small, lightly armed unit in the field, the location and force structure of local enemy troops is critically important. Systems Planning and Analysis has conceived and performed a preliminary analysis of a low-cost, lighter-than-air surveillance system. The preliminary analysis includes the concept definition, a detailed trade study of determine the optimal configuration of the surveillance system, high-pressure inflation test, and a flight stability analysis. This paper will provide details in these areas of the design and provide insight into the feasibility of such a system.
This paper describes the installation and testing of a large-scale fiber optic sensor network on the British Trimaran Research Vessel (RV) Triton. In this on-going project with the Naval Surface Warfare Center, Carderock Division (NSWCCD) and the British Defence Science and Technology Laboratory (formerly DERA), Systems Planning and Analysis, Inc. (SPA) recently completed the installation and at-sea testing of an integrated hardware/software structural monitoring system for rough sea trial testing of the RV Triton. This paper describes a fiber optic Bragg grating sensor network comprising of 51 sensors, the interrogation system, and the processing algorithms used to simultaneously record strain data at both high- and low-speed frequencies, including triggering of the high-speed channels. This paper also details the sensor layout and installation process with emphasis on lessons learned during this procedure. The test procedure and sample data results are presented. Finally, conclusions are drawn from our experience and recommendations are given for future large-scale installations of optical sensing systems.
The goal of this project is to obtain structural health monitoring data using optical fiber Bragg grating sensors embedded in the filament-wound composite tube of a load carrying pile. This technology has applications for structural health monitoring of bridges, piers, and other post-supported infrastructure. These sensors provide strain data for the life history of the pile including pre-installation, installation, residual strains due to the installation process, and health monitoring for the lifetime of the pile. This paper describes the embedding process for the multiplexed fiber Bragg grating sensors into a 2-ft diameter, 60-ft long composite shell including composite manufacturing lay-up, sensor orientation, and ingress/egress of the optical fiber lead from the composite shell. The paper then discusses in-field tests of the monitoring system. Results from the sensor system will provide civil engineers with a greater understanding of load transfer in the composite pile system. Long term structural monitoring of the composite piles will also be demonstrated with this technology. The purposes of these tests are to: 1) determine the survivability of the sensor arrays during a pile driving process, 2) measure strains on the filament wound composite shell following the pile driving process, and 3) determine if structural integrity issues are observed from the strain data.
The force that is able to obtain the greatest amount of information will dominate tomorrow's battlefields. Although battlefield situation information can come from a number of sources, one particularly important segment of intelligence gathering is aerial surveillance. For the small, lightly armed unit in the field, the location and force structure of local enemy troops is critically important. Systems Planning and Analysis has conceived and performed a preliminary analysis of a low-cost, lighter-than-air surveillance system. The preliminary analysis includes the concept definition, a detailed trade study to determine the optimal configuration of the surveillance system, high-pressure inflation tests, and a control analysis. This paper will provide the details in these areas of the design and provide an insight into the feasibility of such a system.
Recent advances in fiber optic sensors have shown the great potential for Bragg gratings to be used as strain sensors. Optical fiber Bragg gratings offer significant advantages over traditional resistance foil strain gages, including a high degree of multipexibility, compact size, immunity to electromagnetic noise interference, and resistance to most chemicals. However, Bragg grating strain sensors have met only limited success in real-world applications. Two reasons for their limited presence is the inherent temperature sensitivity of the dual-parameter Bragg grating and the lack of experience of engineers with fiber optic sensors. This paper describes the development of a fiber Bragg grating strain sensor that attempts to address both of these issues. The flat-pack strain sensor incorporates a pair of Bragg gratings into a single package. One grating is bonded tightly to the pack and acts as a combined thermal-mechanical strain sensor. The second Bragg grating is packaged loosely within the sensor and is used to measure only temperature, which can then be subtracted from the tight grating, providing a temperature-compensated strain reading. By packaging the two gratings into a configuration that is similar to resistance strain gages, we expect that many of the technical and practical implementation issues of optical sensor technology will be overcome. This paper describes the details of the design and experimental testing of prototype sensor packages to validate the functionality of the fiber optic strain gage.
A CMOS imager-based spectrometer is used to interrogate a network containing a large number of Bragg grating sensors on multiple fibers as part of a proprietary structural health monitoring system. The spectrometer uses a Prism-Grating-Prism (PGP) to spectrally separate serially multiplexed Bragg reflections on a single fiber. As a result, each Bragg grating produces a discrete spot on the CMOS imager that shifts horizontally as the Bragg grating experiences changes in strain or temperature. The reflected wavelength of the Bragg grating can be determined by finding the center of the spot produced. The use of a random addressing CMOS imager enables a flexible sampling rate. Some fibers can be interrogated at a high sampling rate while others can be interrogated at a lower sampling rate. However, the use of a CMOS camera brings several specific problems in terms of signal processing. These include a logarithmic pixel response, a low signal-to-noise ratio, the long pixel time constant, obtaining sufficient process priority for the control program, and proper selection of the window of interest. In this paper we investigate computer algorithms and hardware solutions to address these problems. We also present experimental data to validate these solutions including calibration data and initial field-testing data with 24 sensors on 4 fibers.
This paper discusses structural health monitoring data obtained using an optical fiber Bragg grating (FBG) sensor system consisting of sensors embedded in the filament-wound composite marine pile. A composite marine pile is a tube containing a cement core that is used to support bridges, piers, and other structures. This system has applications for structural health monitoring of these structures. This paper presents the results of tests that retrofit two existing composite piles with 30 Bragg grating sensors. Each pile was retrofitted with three arrays, two arrays consisting of 6 gratings and one consisting of 3 gratings for strain and temperature measurements, respectively. Grooves were cut in the piles to allow for adhesive installation of the sensor arrays, and fiberglass cloth tape was laminated over the arrays to protect the optical fiber during the pile driving process. Data were collected prior to and during the pile driving process using a commercial off-the-shelf FBG interrogation system. The purposes of these tests were to (1) determine the survivability of the sensor arrays during the pile driving process, (2) measure residual strains on the filament wound composite tube following the pile driving process, and (3) determine whether structural integrity issues are observed from the strain data.
Systems Planning and Analysis, Inc., under sponsorship from the Office of Naval Research (ONR), has developed a proprietary structural health monitoring system for large- scale structures based on optical fiber Bragg grating (FBG) sensors. This paper describes the operational capabilities of the health monitoring system and recent rest results on large-scale naval structures performed at Naval Surface Warfare Center, Carderock Division (NSWCCD). DSWDM technology developed under this effort is electro-optics based and has been shown to provide significantly higher sampling rates than comparative FBG interrogation technologies. The baseline system under development interrogates more than 120 sensors along five fiber channels. The prototype DSWDM system also possesses a number of advantages intrinsic to optical fiber sensors as compared to resistance strain gages (RSGs). These advantages include extremely low installed weight and volume, immunity to electromagnetic interference and corrosive environments, and low signal attenuation and drift. The LPD-17 propeller was retrofitted with 24 FBGs and strain and temperature data was recorded for direct comparison with conventional RSGs. Static and dynamic full- scale propeller operating loads were applied to the LPD-17 propeller. Sampling rates of approximately 1.7 kHz were demonstrated using the DSWDM system that provided good agreement in measured strain levels between FBGs and reference RSGs.
As the size, weight, and cost of miniature electronics have continued to decrease, the feasibility of developing portable, low-cost, unmanned surveillance systems has increased tremendously in recent years. Today, manufacturers are developing small, unmanned surveillance aircraft for both military and civilian applications. However, many military, paramilitary, and civilian applications require the surveillance platform to remain stationary over a given area for extended periods. Furthermore, most current unmanned surveillance aircraft are too large to be carried by the operators in the field and require special support equipment. This paper describes the preliminary design and analysis of a lighter-than-air vehicle based surveillance system that is lightweight, portable, and will allow extended station keeping over a desired position. This new system can be applied to military surveillance and reconnaissance, border patrol/law enforcement, search and rescue, surveying, and photographic mapping.
A sensitivity based modal damage detection and characterization approach is applied to a rotating composite beam in vacuum. This damage detection methodology is analytically shown to be capable of detecting changes of both mass and stiffness in the rotating beam. The algorithm developed in this work is based on a finite element model of the rotating beam in the flap and lag directions. Elemental sensitivities are derived including an enhanced stiffness sensitivity arising from centrifugal loading of the composite beam. Damage in the structure is detected by performing a constrained minimization on the modal residuals of the structural response.
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