Positioning systems for optical metrology applications are continuously challenged to maintain high resolution, accuracy, and repeatability over ever-expanding ranges of motion and payload masses due to the need for larger optical apertures and sensor field of regard. To address these demands, new facility capabilities have been developed for the X-Ray & Cryogenic Facility (XRCF) at the NASA Marshall Space Flight Center. These upgrades included re-purposing the center of curvature optical bench (CoCOB) hexapod, originally designed for James Webb Space Telescope program testing, to provide precision positioning of large test articles. To maximize the potential range of test article sizes, and ultimately the utility of the facility, studies were undertaken to evaluate the maximum payload capacity and simultaneous range of motion combinations of the CoCOB hexapod without requiring significant hardware modifications. The analytical tools developed to assess and display the possible range of motion combinations for various center of rotation locations are discussed along with the results of the study. A hexapod load study was also completed to determine maximum allowable payload mass across a wide array of possible hexapod poses. Modeling was used to assess unmeasured deflections as a function of the displacement away from the hexapod’s null position and the associated impact on accuracy performance. The methodologies and results of each of these studies are presented in detail. Software techniques to prevent collisions between internal hexapod geometry and avoid overtravel of both actuator stroke and end-joint angular travel ranges are also discussed.
The James Webb Space Telescope’s (JWST) center of curvature optical bench (CoCOB) hexapod was repurposed to enhance NASA Marshall Space Flight Center’s X-Ray & Cryogenic Facility (XRCF) optical metrology capabilities. This upgrade unlocked higher test article load capacity and extended the allowable ranges of motion of the CoCOB hexapod. The hexapod was also coupled to a new long-stroke, linear motion axis to form a 7 degrees of freedom system and allow for high precision testing of larger diameter test articles. A 9 degrees of freedom motion system, consisting of a three-axis linear mount in an X-Y-Z configuration and a commercially available hexapod, was also designed to allow high resolution positioning of the focal plane instrumentation over a large range of motion. A modern control architecture and graphical user interface was developed for the CoCOB hexapod and additional motion stages to permit streamlined commanding and operation. This paper discusses the justification for re-using the CoCOB hexapod by highlighting its unique precision motion control capabilities in a high vacuum and optically clean environment. The design, key component selection, and environmental compatibility for each of the additional motion stages is presented along with testing results for achieved range, repeatability, and minimum step size performance for all motion axes. Finally, a summary of the motion control system architecture and its flexibility to address tomorrow’s optical metrology needs are presented.
The X-Ray and Cryogenic Facility (XRCF) at Marshall Space Flight Center is the world’s largest x-ray optic calibration facility and NASA’s premier cryogenic optical test facility. Built specifically to calibrate the Chandra telescope, the facility contributed to several other x-ray missions until 2005 when it became dedicated to normal incidence optical testing at cryogenic temperatures. Recently the facility’s x-ray test capability has been returned to service and updated. New beam monitors, focal plane detectors, and test article and instrument positioning systems have been added. The x-ray data acquisition system has been updated. A real-time position monitoring metrology system is being developed that will enable calibration of large diameter optics via partial illumination in a diverging beam. The newly expanded x-ray test capabilities of the facility will be discussed.
Development and testing of a lightweight-kinematic optical mount with integrated passive vibration-and-shock
mitigation technologies and simple / robust optical alignment functionality is presented. Traditionally, optical mounts
are designed for use in laboratory environments where the thermal-mechanical environments are carefully controlled to
preserve beam path conditions and background disturbances are minimized to facilitate precise optically based
measurements. Today's weapon and surveillance systems, however, have optical sensor suites where static and dynamic
alignment performance in the presence of harsh operating environments is required to nearly the same precision and
where the system cannot afford the mass of laboratory-grade stabilized mounting systems. Jitter and alignment stability
is particularly challenging for larger optics operating within moving vehicles and aircraft where high shock and
significant temperature excursions occur. The design intent is to have the mount be suitable for integration into existing
defense and security optical systems while also targeting new commercial and military components for improved
structural dynamic and thermal distortion performance. A mount suitable for moderate-sized optics and an integrated
disturbance-optical metrology system are described. The mount design has performance enhancements derived from the
integration of proven aerospace mechanical vibration and shock mitigation technologies (i.e. multi-axis passive isolation
and integral damping), precision alignment adjustment and lock-out functionality, high dimensional stability materials
and design practices which provide benign optical surface figure errors under harsh thermal-mechanical loading. Optical
jitter, alignment, and wave-front performance testing of an eight-inch-aperture optical mount based on this design
approach are presented to validate predicted performance improvements over an existing commercial off-the-shelf
(COTS) design.
Delivery of Orbital Replacement Units (ORUs) to the International Space Station (ISS) and other on-orbit destinations is an important component of the space program. ORUs are integrated on orbit with space assets to maintain and upgrade functionality. For ORUs comprised of sensitive equipment, the dynamic launch environment drives design and testing requirements, and high frequency random vibrations are generally the cause for failure. Vibration isolation can mitigate the structure-borne vibration environment during launch, and hardware has been developed that can provide a reduced environment for current and future launch environments.
Random vibration testing of one ORU to equivalent Space Shuttle launch levels revealed that its qualification and acceptance requirements were exceeded. An isolation system was designed to mitigate the structure-borne launch vibration environment. To protect this ORU, the random vibration levels at 50 Hz must be attenuated by a factor of two and those at higher frequencies even more. Design load factors for Shuttle launch are high, so a metallic load path is needed to maintain strength margins. Isolation system design was performed using a finite element model of the ORU on its carrier with representative disturbance inputs. Iterations on the model led to an optimized design based on flight-proven SoftRide MultiFlex isolators. Component testing has been performed on prototype isolators to validate analytical predictions.
The Herschel Space Observatory (formerly known as FIRST) consists of a 3.5 m space telescope. As part of a JPL- funded effort to develop lightweight telescope technology suitable for this mission, COI designed and fabricated a spherical, F/1, 2 m aperture prototype primary mirror using solely carbon fiber reinforced polymer (CFRP) materials. To assess the performance of this technology, optical metrology of the mirror surface was performed from ambient to an intended operational temperature for IR-telescopes of 70K. Testing was performed horizontally in a cryogenic vacuum chamber at Arnold Engineering Development Center (AEDC), Tennessee. The test incorporated a custom thermal shroud, a characterization and monitoring of the dynamic environment, and a stress free mirror mount. An IR-wavelength phase shifting interferometer (IR PSI) was the primary instrument used to measure the mirror surface. From an initial surface figure of 2.1 microns RMS at ambient, a modest 3.9 microns of additional RMS surface error was induced at 70K. The thermally induced error was dominated by low-order deformations, of the type that could easily be corrected with secondary or tertiary optics. In addition to exceptional thermal stability, the mirror exhibited no significant change in the figure upon returning to room temperature.
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