A fiber optic system for providing permanent downhole casing strain and shape monitoring is described. The optical
sensing technique, deployment strategy, as well as lab and field test results are presented. The system is based on optical
frequency domain reflectometry for reading out densely packed arrays of Bragg grating sensors. The feasibility of the
approach is established and quantitative measurements of casing strain and shape are presented.
We report the use of a fiber-optic distributed sensing system to monitor temperature at a multitude of discrete points on an industrial motor undergoing qualification after a rewinding. This technique involves using optical frequency domain reflectometry to demodulate the reflected signal from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. In this work, high-resolution optical sensing fiber was applied along the stator windings and end-windings of the motor to assess their suitability for long-term temperature monitoring. Performance tests were conducted at different heat loads representing different electrical conditions. Results indicate excellent agreement with collocated Resistance Temperature Devices (RTDs) and demonstrate significant potential for mitigating costly motor failure due to insulation breakdown resulting from highly localized hotspots.
A fiber optic based distributed temperature measurement system was implemented in stator windings (straight copper bars) as well as in the end-windings (curved copper bars) of a motor. Usually, in electrical machines such as motors or generators, only a few conventional temperature sensors are used, whereas the distributed temperature system has the potential of providing very detailed temperature distribution by having hundreds of sensors in a single fiber. The sensors were made of Bragg gratings etched onto the fiber itself. For the present study, the spatial resolution of the sensors is 6 mm (nominally at 1/4” apart). The technique uses Optical Frequency Domain Reflectometry (OFDR) to process the back-reflected light signal indicative of the thermal filed. A prototype fiber optic system was implemented in a motor made by GE industrial systems. The sensing length (length of the stator) for the motor was 0.75 m containing approximately 150 sensors thus providing very detailed temperature data. Performance tests were conducted at different heat loads representing different electrical conditions. Continuous tests for the duration of 19 hours were conducted. The temperature of stator windings varied from ambient (~ 23°C) to approximately 85°C. As reference, Resistance Temperature Devices (RTDs) were installed in adjacent slots to the slot where fiber optic sensors were installed. A total of 8 sensors were installed but data were collected on only 3 fibers. Fiber sensor measurements were found to track the temperature trends very well. The fiber data agreed with RTD data within ± 3°C in the entire duration. The RMS value of difference between the fiber and RTD on one side was 0.3°C, and with the RTD on the other side was 0.5°C. The fiber measurements also showed how hotspots could be missed by using few RTDs, as is done in the industry. The fiber measurements also showed the temperature distribution in the endwindings, an area not normally monitored. The maximum temperature was an acceptable 110°C. The feasibility of this technique for measuring stator-winding temperatures is proved. Still some of the problems faced during the installation and experiments are (a) robustness of fiber and sheathing fiber and (b) fiber survivability during manufacturing process and repair.
We report the use of a fiber-optic distributed sensing system to monitor structural fatigue on an aircraft undergoing a full scale fatigue test. This technique involves using optical frequency domain reflectometry to demodulate the reflected signals from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. The optical fibers, containing a high density of Bragg gratings, were applied along the surface of a Lockheed Martin P-3C Orion fatigue test article to assess the suitability of this technique for long-term structural damage detection and monitoring. Preliminary results indicate good agreement with quasi-collocated foil strain gauges and demonstrate great potential for supplementing or replacing conventional non-destructive evaluation techniques.
We report the use of a fiber-optic distributed sensing system to monitor crack growth on aircraft panels. The system utilizes optical frequency domain reflectometry to demodulate the reflected signals from up to thousands of weakly reflecting gratings photoetched along a single optical fiber. In our experiment, data from a regular array of sensors attached to an aircraft panel were recorded as the panel was subjected to increasing loads. Strain contour maps generated from these data enable clear visualization of the crack growth over time. A similar experiment was also performed using fiber-optic strain sensors embedded in aircraft composite repair patches. The results of these experiments demonstrate the viability of distributed fiber-optic sensing for crack growth monitoring.
This paper presents recent developments in the use of optical frequency domain reflectometry (OFDR) to measure engineering parameters at thousands of locations along optical sensing fibers where weakly reflecting Bragg gratings have been photoetched. Application of the sensing fibers outside of the development lab has revealed several areas for improvement. Some of the problems encountered include polarization fading, non-linear laser tuning, and sensor calibration. This paper considers the use of polarization diversity detection and monitoring of the laser tuning characteristics to provide a more robust OFDR system for both sensor calibration and measurement. Possible modifications to the system are reported along with calibration measurements for quantifying the effects of polarization fading in the sensing fiber.
This paper describes the use of a fiber optic system to measure strain at thousands of locations along optical fibers where weakly reflecting Bragg gratings have been photoetched. The optical fibers were applied to an advanced composite transport wing long with conventional foil strain gages. A comparison of the fiber optic and foil gage systems used for this test will be presented including: a brief description of both strain data systems; a discussion of the process used for installation of the optical fiber; comparative data from the composite wing test; the processes used for the location and display of the high density fiber optic data. Calibration data demonstrating the potential accuracy of the fiber optic system will also be presented. The opportunities for industrial and commercial applications will be discussed. The fiber optic technique is shown to be a valuable augmentation to foil strain gages providing insight to structural behavior previously requiring reliance on modeling.
A system for producing and demultiplexing large numbers of fiber Bragg grating sensors is described. Three thousand gratings were multiplexed in four optical fibers and demonstrated on a large test article. Preliminary results of a strain sensing system in unaltered fiber using OFDR measurements of the Rayleigh scatter is also described.
A method of stretching optical fiber holds interest for measuring strain in smart structures where the physical displacement may be used to tune optical fiber lasers. A small, lightweight, low power tunable fiber laser is ideal for demodulating strain in optical fiber Bragg gratings attached to smart structures such as the re-usable launch vehicle that is begin developed by NASA. A method is presented for stretching optical fibers using the THUNDER piezoelectric actuators invented at NASA Langley Research Center. THUNDER actuators use a piezoelectric layer bonded to a metal backing to enable the actuators to produce displacements larger than the unbonded piezoelectric material. The shift in reflected optical wavelength resulting from stretching the fiber Bragg grating is presented. Means of adapting THUNDER actuators for stretching optical fibers is discussed, including ferrules, ferrule clamp blocks, and plastic hinges made with stereo lithography.
Fiber optic sensors are being developed for health monitoring of future aircraft. Aircraft health monitoring involves the use of strain, temperature, vibration and chemical sensors. These sensors will measure load and vibration signatures that will be used to infer structural integrity. Since the aircraft morphing program assumes that future aircraft will be aerodynamically reconfigurable there is also a requirement for pressure, flow and shape sensors. In some cases a single fiber may be used for measuring several different parameters.
A novel fiber Bragg grating based palladium tube sensor was designed for hydrogen leakage detection in aerospace vehicles. The sensor fabrication method was developed and the sensor response was characterized in terms of total wavelength change, response time and degassing ability. Several factors that influence the sensor performance, including the tube thickness, purging temperature, purging gas, hydrogen concentration, and operation temperature, were studied. The sensor response was improved by reducing the thickness of the palladium tube to around 33 micrometer, optimizing the operation temperature to 95 degrees Celsius, and thoroughly degassing the sensor in nitrogen at 95 degrees Celsius for 4 hours. At these conditions, the total wavelength change was about 0.6 nm, the response time (the time to reach a 0.05 nm wavelength change) was about 2 minutes for the four-hour 4% hydrogen tests.
A new technique is presented for active distributed fiber sensing for interrogating structural integrity and environmental monitoring using an innovation in low power integrated compact tunable fiber optic laser capability.
From spacecraft platforms, the clouds and the Earth's radiant energy system (CERES) scanning thermistor bolometers are designed to measure broadband Earth-reflected solar shortwave (0.3 - 5.0 micrometer) and Earth-emitted long wave (5.0 - greater than 100 micrometer) radiances as well as emitted longwave radiances in the 8 - 12 micrometer water vapor window over geographical footprints as small as 10 kilometers at the nadir. In ground vacuum facilities, the thermistor bolometers and in-flight blackbody and tungsten lamp calibration systems are being calibrated using radiometric sources tied to the international temperature scale of 1990 (ITS'90). Using the in-flight calibration systems, the bolometers will be calibrated periodically before and after spacecraft launch to verify the stability of the bolometers responses and to determine response drifts/shifts if they occur. The in-flight systems calibration analyses along with validation analyses will be used to determine the flight data reduction coefficients (instrument gains and offsets) which will be used to convert the bolometer measurements into calibrated radiances at the top-of-the-atmosphere (approximately 30 km). If a bolometer response shifts or drifts more than 0.5% in the longwave region or more than 1.0% in the shortwave region, and if the validation studies verify the bolometer response change, the flight data reduction coefficients will be corrected. A coastline detection method, using strong contrasting longwave ocean-land scenes, will be used to assess error limits on the geographical locations of the radiances. The detection method was successfully used to assess upper limits (6 km) on the geolocation errors for the Earth radiation budget satellite (ERBS) bolometric measurements of longwave radiances. For CERES, the detection method may be extended to shortwave radiances. In this paper, elements of the CERES instrument level 1 validation plan radiometric strategies are presented as well as the geolocation validation approaches.
Developers of videometric systems must attend to the problems of image storage, retrieval and, for multi-station triangulation, the unambiguous correlation of images with appropriate epochs. For dynamic testing with multiple cameras, this problem is manifest. An `off-the- shelf' component two camera system was recently developed for measuring the six degree-of- freedom time histories of a free flight wind tunnel model. Vertical interval time codes (VITC) were used to correlate fields from each camera station which had been stored onto video cassette recorders (VCR). Subsequent use and development has emphasized the practicality of this approach. This paper discusses the image management technique used along with some details of the particular wind tunnel application. The utility of post-test processing of long sequences of VITC encoded imagery stored to VCR is established.
Time exposure photography, sometimes coupled with strobe illumination, is an accepted method for motion analysis that bypasses frame by frame analysis and re synthesis of data. Garden variety video cameras can now exploit this technique using a unique frame buffer that is a non integrating memory that compares incoming data with that already stored. The device continuously outputs an analog video signal of the stored contents which can then be redigitized and analyzed using conventional equipment. Historically, photographic time exposures have been used to record the displacement envelope of harmonically oscillating structures to show mode shape. Mode shape analysis is crucial, for example, in aeroelastic testing of wind tunnel models. Aerodynamic, inertial, and elastic forces can couple together leading to catastrophic failure of a poorly designed aircraft. This paper will explore the usefulness of the peak store device as a videometric tool and in particular discuss methods for analyzing a targeted vibrating plate using the `peak store' in conjunction with calibration methods familiar to the close-range videometry community. Results for the first three normal modes will be presented.
KEYWORDS: Video, Analog electronics, Calibration, CCD cameras, Image processing, Video processing, Digital video recorders, Cameras, Frame grabbers, Charge-coupled devices
A clear advantage of digital photogrammetric measurement over other, more conventional techniques in the fast sample rate of the data acquisition. CCD cameras and video systems can be used very effectively to analyze dynamic objects or cases of rapid deformation. However, long sequences of images can introduce the penalty of large volumes of digital data, which may not be able or appropriate to be processed in real time. The images are typically stored in analog form, using media such as video tape or video disk, for off line processing subsequent to the image capture. This paper investigates the degradation in accuracy and repeatability caused by the influence of the analog recording. A number of experiments using a Hitachi medium resolution CCD camera, a three dimensional test range and a self calibrating bundle adjustment are described. For cases of near real time monitoring, the ability of frame averaging to reduce the degradation caused by the analog recording is also investigated. The results of the experiments are presented in summary to provide some guidelines as to the degree of degradation which can be expected under similar circumstances.
KEYWORDS: Cameras, Video, Calibration, Systems modeling, Data acquisition, Data modeling, Lamps, Error analysis, Imaging systems, Light sources and illumination
Recent progress in establishing calibration schemes for video cameras promises to significantly enhance the measurement capabilities available to aeronautical engineers studying flight dynamics. Targeting and lighting conditions in working facilities are usually less than ideal from a photogrammetric standpoint. This paper will discuss the implementation of a customized state-of-the-art commercial tracker used to estimate pitch attitude and spin rate of 10 Hz refresh rates for a free spinning model in a 20 foot diameter vertical wind tunnel. Active efforts to augment the measurement accuracy of the real time system by post processing concurrent, time coded, taped data from two independent camera stations will also be described. Independent calibration of interlaced fields, degradations due to storage media, effects of electronic shuttering, limited numbers of target points, estimation of 'optical flow' of data from field to field, vibration, and conflicting requirements for the competing video systems will be discussed.
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