ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Separation Test Results The first separation test was considered successful even though only two of the three panels fully  deployed as seen in Figure 12. The two panels that did fully deploy included the most highly instrumented  panel (left side of figure) which provided the necessary data for model correlation and risk reduction. Figure 12. Separation Test #1 (Image Courtesy: NASA)   The third panel did not fully deploy due to an unexpected asymmetric interference which had not been  accounted for in the dynamic separation model.  The nearly 300 channels of instrumentation during the separation test provided sufficient data to gain a  better understanding of how the various factors such as gravity, air resistance and friction affect the dynamic behavior of the panels as they deploy. Test results also showed dynamic interaction between the  panels was less than predicted and that the single separation event was similar to three independent tests. This was highly beneficial since only two panels deployed. Figure 13 shows the model predictions of the panel deployment before the test and after correlating the model to the data. Figure 13. Fairing Position versus Time    282
After the first separation test, a panel free-fall test was performed on the panel that failed to separate. This test was developed to gather data to reduce uncertainties in the rigid body motion and losses due to air effects and friction. For this test, a single panel was pulled slowly over its center of gravity using an actuator and pulley system. The aero model of a flat plate worked surprisingly well in predicting aeroeffects during ground testing, however, the aero effects were only a small portion of the total factors that effected the fairing deployment response. It should be noted that while the deployment failure was unexpected, the free-fall test had been previously planned and the un-deployed panel position greatly simplified the logistics and implementation of this test. The second separation test was performed with one panel heated to 93 /g113C (200 /g113F) to simulate flight  environments as shown in Figure 14. Panel heating was predicted to increase loads, induce panel  warping and amplify the dynamic response of the fairing during separation. Only one panel was heated to save cost and schedule associated with a more complex heating system. Multiple checkout tests of the heater system were performed prior to separation and data was collected for each checkout to gather  additional data for model correlation including effects on load share.  Figure 14. Separation Test #2 Heated Panel   All three panels successfully jettisoned during the second test (Figure 15). The geometry modification, margins and clearances were validated and it was found that thermal effects were slightly less than  predicted.   283
Figure 15. Separation Test #2 Jettisoned Panels Lessons Learned As with most test programs general lessons learned such as “test early” and “test like you fly” were  continually being weighed against design finalization, cost and schedule constraints. However, some specific lessons were learned during the Orion fairing tests. The following is a list of those lessons learned along with some further information and recommendations.  /g120Titanium with CANADIZE ®coating with a molydisulfide dry lubricant provided adequate galling  resistance for the PSM cup/cone separation interface; however, other materials may have been more  efficient as loads and resulting contact stresses increased. Minimizing gaps and overall relative motion reduces wear.  /g120Coatings and lubricants are very difficult to predict how they will respond to different environments. Testing is critical to understand them and to demonstrate that the selected system will survive both the load levels and the required life span. Part of the selection process and testing should consider  what the consequence will be if/when the lubricant fails. Lubricant compatibility with surrounding  materials is also a concern. During testing of the aero deflectors, one lubricant tested did not survive; it was discovered later that the lubricant had reactivity issues with both of the interfacing materials. Additionally surface finishes of substrates that receive coatings is critical. /g120Titanium is not an ideal material in random vibration environments; it has very little structural damping  which often results in very high amplification of vibration inputs. This was evident during the testing of  both the spring and the hinge (not discussed in paper). In both cases, the response due to a random  vibe input was significantly higher than expected and in the case of the spring resulted in several issues that had to be resolved. /g120Coil springs can exhibit unanticipated response in vibration environments. During random vibration  testing of the spring, it was found that the response was highly nonlinear (motion was excited in all  directions, not just the input direction). Additionally, the amount of motion seen in the spring was  much greater than anticipated, which resulted in the spring contacting the inner guide tube. The spring motion also resulted in a load amplification effect along the spring axis (surging).  284
Design and Lessons Learned on the Deve lopment of a Cryogenic Pupil Select  Mechanism used in the Testing and Ca libration of the Integrated Science  Instrument Module (ISIM) on the James Webb Space Telescope (JWST) Alissa Mitchell*, Thomas Capon*, Jeffrey Guzek**,C l a e f  H a k u n*, Paul Haney*and Corina Koca* Abstract Calibration and testing of the instruments on the Integrated Science Instrument Module (ISIM) of the  James Webb Space Telescope (JWST) is being performed by the use of a cryogenic, full-field, optical  simulator that was constructed for this purpose. The Pupil Select Mechanism (PSM) assembly is one of  several mechanisms and optical elements that compose the Optical Telescope Element SIMulator, or OSIM. The PSM allows for several optical elements to be inserted into the optical plane of OSIM, introducing a variety of aberrations, distortions, obscurations, and other calibration states into the pupil plane. The following discussion focuses on the details of the design evolution, analysis, build, and test of this mechanism along with the challenges associated with creating a sub arc-minute positioning mechanism operating in an extreme cryogenic environment. In addition, difficult challenges in the control system design will be discussed including the incorporation of closed-loop feedback control into a system that was designed to operate in an open-loop fashion. Figure 1.  PSM in OSIM Bench    Figure 2. Light Path through PSM Introduction The JWST Optical Simulator (OSIM) is designed to simulate the optical performance of the JWST telescope elements. The simulator consists of the primary imaging optical path, which relays fiber optic point sources to the focal surface of the ISIM over its field of view. OSIM is comprised of several  subassemblies, including the Optical Bench Module (OBM), into which the PSM is integrated. The PSM is  located at the pupil plane of the OSIM instrument and is mounted on a 2-DOF Pupil Translation  *NASA Goddard Space Flight Center, Greenbelt, MD **Design Interface, Inc. Greenbelt, MD /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 287
Mechanism. Figures 1 and 2 show the location of the PSM within the OSIM optical bench. For a more  detailed description of the OSIM, see reference 2.  Integration of the various modules and subassemblies into OSIM occurred at GSFC in the Spacecraft  Systems Development and Integration Facility (SSDIF) and environmental performance of OSIM was verified in the Space Environment Simulator (SES), where OSIM was tested with the Beam Image Analyzer (BIA). The BIA is a cryogenic 5 DOF mechanism system with cold focal planes designed by NASA/GSFC Electromechanical Systems Branch and was used to measure the performance of OSIM as the simulator for the JWST optical telescope. Figure 3 shows the OSIM being lowered in to the SES  chamber. Figure 4 shows the OSIM during ambient alignment with the BIA in the SSDIF. Figure 3.  OSIM Bench being lowered into the SES  Figure 4. OSIM during Ambient Alignment The PSM provides OSIM with the capability to configure the OSIM pupil plane with various optical  elements, masks, and alignment reference sources. In this mechanism, two 27-inch (68.6-cm) wheel  assemblies are assembled adjacent to one another with 0.1 inch (2.5 mm) of clearance between them. Fourteen different optical elements, eight in each wheel, are then rotated into the optical plane of OSIM  for testing and calibration purposes. Driving requirements for the PSM are to operate in a cryogenic environment and to provide stable and repeatable positioning of the elements in the optical plane during a powered-off condition. During the testing stages of OSIM, the PSM is required to operate at temperatures as low as 100K and with element position placement of less than 45 arc-seconds. A brief description of  the elements located in the two wheels is shown in Figure 5. 

Figure 5. PSM Wheel Optical Elements All of the elements in Wheel 1 were fabricated from 6061 Aluminum and machined to tolerances to meet  the requirements of each individual element. It should be noted that, in addition to mounting the optical  288
elements, heaters and temperature sensors were also mounted on each wheel. In addition, the ADM  Target incorporated an LED light source. These necessitated the incorporation of a cable wrap into both  wheel assemblies. The optical elements in wheel 2 were more complex, precision opto-mechanical  assemblies. An example is the Course Phase Plate Assembly. Figure 6 shows the course phase plate  which incorporated sophisticated fabrication and assembly techniques to allow for the individual mounting of 18 segments that would not distort when cooled from ambient temperature to 100 Kelvin. Figure 6.  PSM Optical Element Phase Plate Design Evolution of the PSM During the conceptual design phase of the PSM, the most challenging design task was to develop a  system with high pointing stability and repeatability in the powered-off state. Two spare CDA gearhead stepper motors that had been used on a previous JWST mechanism life test were available for the  development. This motor was incorporated into the design to turn the primary wheel via friction from a preloaded drive wheel. The step size of the stepper motor allowed for a position resolution of less than 45 arc-seconds, while a high accuracy absolute encoder provided knowledge information to a resolution of approximately 1 arc-second. Figure 7 shows the initial conceptual design of the traction drive system. A peer review was held on the design and significant resources were dedicated to proof-of-concept.  During the initial stages, motor testing was performed as well as testing on high friction representative materials for the optical element wheel and the drive wheel. Several materials were considered for the drive wheel including Vespel, a polymer/carbon fiber blend composite (WearComp ®), and a  polymer/carbon fiber/graphite composite blend (FibreComp®). Tests were performed to capture the  friction coefficient between these potential materials and the aluminum wheel and to determine a viable pairing that offered a high coefficient of friction with a low risk of debris generation. These tests were  performed in ambient pressure and in vacuum at several temperatures, down to 100K, and displayed an  unacceptable amount of variation in the coefficient of friction.   

Due to concerns with motor life, power dissipation, the variability of the friction data under cold  temperature, and a concern that the motor drive wheel would slip during operation, it was decided that pursuing a different design route for the PSM would be less risky. As the development schedule for the  289
PSM was constrained, the goal was to limit design modifications to the friction-based PSM and to  incorporate the high-accuracy, low-bandwidth sensor from the original concept. In addition, restricted wire  counts in the flex harness that provided power and signal to the encoders precluded the addition of a  high-rate position sensor to the design. Figure 7. Preliminary PSM Friction Drive Design The team began a conceptual redesign incorporating a brushless DC motor to directly drive each PSM  wheel. At a later time, a preloaded follower wheel was added on the outside of the mechanism that would  engage mechanical detents machined into the wheel. In addition to preliminary control system design  modeling, analyses were initiated to determine the optimal detent and follower wheel geometries as well  as the forces on the system. The initial analysis showed that an open-loop control system could be developed to meet the power-off position requirement with minimal changes to the initial friction-based  conceptual design.   As-Built PSM Design In order to rotate the wheels, the stepper motors were replaced by two frameless, 2-phase brushless Aeroflex motors affixed to the center hub assembly, directly driving each wheel. The hub assembly also includes spring-loaded angular contact bearings. In order to meet the 45 arc-second holding and  repeatability positioning requirement, a detent is used on the outside of the wheel with a follower  assembly to roll in and out of the detents and to hold the wheel in place when power is removed. The encoder system was developed to operate in the 100K environment and provided feedback for the mechanism electronics. Ultimately, it was determined that in order to control the mechanism, a high-fidelity dynamic model was needed. The dynamics of the system were modeled along with a PID gain scheduling controller and trajectory planning which utilized 40-Hz feedback from the absolute encoder system. The final design of the PSM is shown in Figure 8. Details of the maturation will be discussedbelow.

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Figure 8. As-Built PSM Design

Motor/ Bearing Hub Assembly To drive the PSM, two frameless, brushless DC motors were selected from Aeroflex and mounted directly  to the frame of the structure. In the sizing of these motors, it was assumed that the primary purpose of the motors was to drive the wheels to the capture region of the follower wheel and, therefore, no control  authority was required. Due to the cryogenic operating temperatures, traditional bearing lubricants were not suitable for the  application, and the bearings needed to be cold-treated to the 100K operating temperatures of the mechanism. Ultimately, a hybrid, dry-running ceramic bearing from Cerobear was selected for this  application. The bearings consisted of a non-magnetic, silicon nitride balls running in Cronidur 30 races  with minimum lubrication from PGM-HT (PTFE/MoS 2/glass fibre composite) retainers. The hardness  properties of the ceramic balls coupled with the material differences between the balls and the steel races reduces the risk of fretting, cold welding, and seizing of the bearing at higher Hertzian stresses. The  expected of number life cycles for the mechanism was 2000, which is an acceptable lifetime to expect for these bearings. To accommodate the large operating temperature range, the PSM bearings were  installed with a soft preload, such that the maximum loaded bearing had a mean Hertzian pressure of  291
101 ksi (696 MPa), which is less than half the mean recommended Hertzian pressure of 217 ksi (1496  MPa) from the vendor. A cross-section of the wheel assemblies is shown in Figure 9. Figure 9. Cross-section of Wheel Assemblies The motors and motor bearings were assembled together in a titanium hub which provided a good  coefficient of thermal expansion (CTE) match to the bearing race material. Figure 10 shows the fixture used to assemble the motor rotor and stator. To accommodate CTE mismatch between the titanium hub and the aluminum wheels and X-frame structure, three pin-in-slot interfaces coupled with controlled fastener preload were used to accommodate sliding during cool-down. For signals transversing the  rotating axis, a flat ribbon cable from 3M was used in a coiled clock spring design to allow for near 360° rotation. Prior to assembly, this ribbon cable was heated on a mandrel to create a coiled shape, which  assisted in assembly. Figure 11 shows the cable wrap assembly. Figure 10. Motor Rotor Manually  Lowered into Stator AssemblyFigure 11. Cabl e Wrap Assembly 292
Follower Wheel Assembly For the follower wheel, Vespel (SP-1) was selected due to the low risk of debris generation with the  aluminum wheel. WearComp was also considered for the follower wheel material, but there were concerns that the machined surface finish would generate debris when in contact with the primary wheel.  The follower wheel assembly consisted of a face-to-face, spring-loaded duplex bearing pair that  supported the follower wheel. The follower wheel was keyed to the rotating shaft while a titanium housing supported the outer races of the bearings. Figure 12 shows the components of the follower wheel assembly.

Figure 12.  Follower Wheel Assembly The follower wheel was preloaded on the rim of the element wheel via beryllium copper beam flexures. Slots in the flexure bolt holes allowed for circumfer ential adjustability of the follower assembly on the xstructure while shims allowed for adjustment of the preload. 

The initial design of the follower wheel assembly consisted of two cantilever beams which radially preloaded the follower against the outside surface of the wheel and mechanical detents. Once the assembly was integrated, it became apparent that the cantilever beam preload system had a few  shortcomings. The cantilever beam spring allowed for the follower to move in and out of the detent, but did not react moments. The arc-shaped tip motion did not adequately provide a purely radial preload to  the follower wheel and did not position the elements in the proper location. The system was replaced with a parallel blade arrangement which allows for near perfect radial motion of the follower as it moves in and  out of the mechanical detents while resisting internal moments. This allowed for the optical elements to  be positioned as originally envisioned. Figure 13 shows the follower wheel assembly integrated onto the  X-frame structure of the PSM.

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Figure 11. As-Built Mechanical Follower Wheel Assembly Optical Encoder Design and Development In order to verify the tight positioning and knowledge requirements, cryogenic absolute encoders were  developed (see Reference 1 for more details). The PSM absolute encoders were able to achieve less  than one arc-second resolution over a nearly 360-degree operational range. The ultra-high resolution wasachieved through a unique process of pattern recognition. A PSM Leviton absolute encoder is shown in Figures 14 and 15, and is composed of an LED and LED pinhole mount, a titanium encoder housing with  an internal baffle, a mirror and flexure, a detachable titanium CCD mount with accompanying CCD camera and heater and a photolithographic soda lime glass scale. The rotary scale disc was comprised of a pattern containing a periodic series of lines; each line is etched with unique and identifiable features.     Figure 14. PSM Encoder Readhead    Figure 15. PSM Encoder Cross Section The encoder functions in the following manner. The LED source emits light and the pinhole collimates the  light source to a point source. The light then enters the housing, which is baffled to eliminate undesirable  stray light. Once the light exits the housing the scale casts a shadow and the image sensor captures the 294
unique pattern at that location. In addition to capturing the pattern, the image sensor compares its fixed  pixel columns with the centroid of each line on the scale. With the combination of unique and identifiable line features and relative line centroid locations, an absolute position measurement with arcsecondresolution is achieved. Figure 16 and 17 show the assembled read head and the code disk kinematically mounted on the wheel hub assembly. Figure 16.  PSM Encoder Readhead    Figure 17.  PSM Encoder Disk Optical Encoder Alignment Due to the high accuracy requirement of the PSM encoders, alignment was critical; this includes initial  alignment as well as alignment repeatability. Omitting the translation of the object being measured, if the encoder read head and scale move relative to each other, the position measurement is void. Two readheads were included to increase accuracy by removing run-out of the bearing configuration. Achallenge when operating in a cryogenic environment is designing and accounting for the differing CTEsof the various materials. To control the position of the CCD relative to its mount, a bonding fixture was used. With the CCD position controlled, bonded and fixed in place on the CCD mount, any additional  alignment adjustments were made with the CCD mount, specifically minimal decenter, angular position  and image sensor to scale defocus adjustments. To maintain alignment, the CCD mount was pinned to  the encoder housing and the encoder housing was pinned to the stationary PSM reference.  To ensure the PSM scale was aligned to the rotor and stator over the temperature range, the scale  utilized a titanium 3-point kinematic mount with a flat, a cone, and v-groove features. Titanium flexures and features machined into the glass allowed captured steel balls to remain nestled and preloaded between the glass and the mount. In this configuration, all changes in motion of the scale due to thermal  contraction and/or expansion were relative to the kinematic mount cone feature. However, due to CTE differences between the glass scale and the titanium hub of the PSM, scale recalibration at varying  operational temperatures was performed as part of the PSM testing. In addition, changes to the bias voltage of the CCD were required to allow for good encoder images to be obtained.

PSM Controller Design and Development Due to the tight requirements and the torque disturbances from the mechanical detent, controlling the  Pupil Select Mechanism proved to be much more challenging than originally anticipated. After the baseline of the friction wheel design with geared stepper motor drives was abandoned, and the direct-drive brushless system was implemented, representative testing was performed with an unpopulated 295