Brooks A. Childers

Recent experiences with implementing a video-based six-degree-of-freedom measurement system for airplane models in a 20-foot-diameter vertical spin tunnel

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.

Fiber Optic Sensors for Health Monitoring of Morphing Aircraft

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.

High density strain sensing using optical frequency domain reflectometry

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.

Characterization of a fiber Bragg grating (FBG) based palladium tube hydrogen sensor

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 Stackable Bonding-Free Flextensional Piezoelectric Actuator

A flextensional actuator was designed using commercial multilayer stacked actuator so as to produce large displacements at intermediate force levels. The simple design chosen eliminated the need for bonding the actuators into the frame and permitted easy series connection of multiple units. To satisfy the need for a fiber grating tuning device to interrogate an array of Bragg grating fiber optic stress sensors, a tuning device using four series connected units was constructed. The unit performs well, but the actual measured amplification is less than theoretical expectation. The problem was traced to unwanted flexing of the simple original frame and a hinged more robust flexing beam construction was shown to eliminate the problem.

Quantitative Analysis of a Class of Subsurface Cracks Using Shearography and Finite Element Modeling

The application of a full field non-contacting measurement system for nondestructively evaluating (NDE) subsurface flaws in structures has been conducted using Electronic Shearography. Shearography has primarily been used as a qualitative tool for locating areas of stress concentration caused by anomalies in materials[1–4]. NASA has been applying optical techniques such as these to NDE inspection of aircraft lap joint integrity, composite material defects, and pressure vessel quality assurance. This paper examines a special class of defects manufactured in thin metal panels and serves as a testbed for interpreting the displacement gradients produced on a simple well-characterized sample with known defects. Electrode discharge machining (EDM) notches were cut into panels to simulate subsurface cracks. Shearography was used to determine the detectability of subsurface cracks ranging in size from 0.8 mm to 25.4 mm fabricated in both steel and aluminum test panels. Finite element modeling was used to verify and quantify experimental results obtained in these tests. Very good agreement existed between both the experimental and predicted displacement models.

Clouds and the Earth's Radiant Energy System (CERES) instrument level 1 science data validation plan for geolocated radiances

From spacecraft platforms, the clouds and the Earths 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 (ITS90). 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.

Videometric system using VITC for computer-aided image management

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.

Integrated regenerative multicellular optical fiber grating control system

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 tiber optic laser capability.

Mode shape analysis using a commercially available peak-store video frame buffer

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.