Sandia National Laboratories currently utilizes two laser tracking systems to provide time-space-position-information
(TSPI) and high speed digital imaging of test units under flight. These laser trackers have been in operation for decades
under the premise of theoretical accuracies based on system design and operator estimates. Advances in optical imaging
and atmospheric tracking technology have enabled opportunities to provide more precise six degree of freedom
measurements from these trackers. Applying these technologies to the laser trackers requires quantified understanding of
their current errors and uncertainty. It was well understood that an assortment of variables contributed to laser tracker
uncertainty but the magnitude of these contributions was not quantified and documented.
A series of experiments was performed at Sandia National Laboratories large centrifuge complex to quantify TSPI
uncertainties of Sandia National Laboratories laser tracker III. The centrifuge was used to provide repeatable and
economical test unit trajectories of a test-unit to use for TSPI comparison and uncertainty analysis. On a centrifuge, testunits
undergo a known trajectory continuously with a known angular velocity. Each revolution may represent an
independent test, which may be repeated many times over for magnitudes of data practical for statistical analysis.
Previously these tests were performed at Sandia's rocket sled track facility but were found to be costly with challenges in
the measurement ground truth TSPI. The centrifuge along with on-board measurement equipment was used to provide
known ground truth position of test units. This paper discusses the experimental design and techniques used to arrive at
measures of laser tracker error and uncertainty.
Conventional tracking systems measure time-space-position data and collect imagery to quantify the flight dynamics of
tracked targets. One of the major obstacles that severely impacts the accuracy and fidelity of the target characterization is
atmospheric turbulence induced distortions of the tracking laser beam at the target surface and imagery degradations.
Tracking occurs in a continuously changing atmosphere resulting in rapid variations in the tracking laser beam and
distorted imagery. These atmospheric effects, in combination with other sources of degradation, such as measurement
system motions (e.g. vibration/jitter), defocus blur, and spatially varying noise, severely limit the useful and accuracy of
many tracking and analysis methods.
This paper discusses the viability of employing stereo image correlation methods for high speed moving target
characterization through atmospheric turbulence. Stereo imaging methods have proven effective in the laboratory for
quantifying temporally and spatially resolved 3D motions across a target surface. This technique acquires stereo views
(two or more) of a test article that has an applied random speckled (dot) pattern painted on the surface to provide
trackable features on the entire target surface. The stereo views are reconciled via coordinate transformations and
correlation of the transformed images. The principle limitations of this method have been the need for clean imagery and
fixed camera positions and orientations. However, recent field tests have demonstrated that these limitations can be
overcome to provide a new method for quantifying flight dynamics with stereo laser tracking and multi-video imagery in
the presence of atmospheric turbulence.
The current and future laser tracking mission requirements of Sandia National Laboratories are discussed. The capabilities of Sandia's existing laser trackers are summarized. The deficiencies of the current laser trackers are identified with respect to future mission requirements. Candidate commercial technologies are addressed to correct the identified deficiencies. Technology gap areas are identified where additional research needs to be conducted prior to developing an effective next generation laser tracking system
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.
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.
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.