ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
PSM wheel. This initial testing was promising as the control system was able to command the wheel  position using the high-accuracy, low-bandwidth encoders. However, when inertial simulators for the  optical elements were installed, an open-loop control was not possible with the torque available from the  direct drive motors. Therefore, a closed-loop system was developed within the pre-existing design  constraints. The only sensor on the wheels was the high-precisio n CCD encoder. The readout software was modified  to transmit encoder readings to the motor control rack using UDP packets as TCP packets were not  delivered fast enough. There was still some variability, so timestamps were added to each measurement and the controller was designed to account for this in speed calculations. In its normal configuration, this  encoder only produced 10 measurements per second, and could not produce readings at all above a  certain wheel velocity because of image blurring. Such low sensor bandwidth meant that the optical positioning requirements could not be met through  continuous closed-loop control, so mechanical spring-loaded detents were added to the wheel for this purpose. Thus, in addition to a relatively low amount of motor torque for the given inertia, the control  system had to deal with the detent as a torque disturbance. As the roller navigated the sharp V-notch in  the wheel, it produced very large torques during entry and exit. It seemed impossible to control wheel-exit  trajectories, since the low bandwidth and high disturbance produced high exit velocities, and the resulting blurred images disabled feedback control during recovery. Two critical modifications were made to the  system to correct this problem. First, the sharp V-notch was replaced with a rounded notch, making the  torque disturbance more gradual (see  Figure 18 ). Second, the encoder readout system was modified to  use a shorter exposure time, increasing both the measurement rate and the blurring speed limit. The new  detent profile reduced the exit speed to well below the new blurring speed limit. Together, these  modifications made the system controllable using a gain-scheduled PID controller.

Figure 18. Simulated Detent Disturbance Torque Pr ofile Before and After Mechanical Modification The gain-scheduled PID controller was developed first in a Simulink model of the wheel, motor, detent,  and encoder. While it was very helpful in developing the concept of the controller, the simulation proved too unfaithful for the controller to be directly ported to the physical system. Instead, a similar controller was developed in LabVIEW and the gains tuned during testing. The final version of the controller includes  four states that are executed to move from one detent to the next (see  Figure 19 ). Longer moves are  composed by repeating single moves, since passing through a detent at speed imparts unpredictable and  harmful mechanical shocks to the wheels and optical elements. In all the closed-loop states, the motor is  commutated in software based on the encoder readings. The states are as follows: 296
1.Exit detent. A simple integrator is used to ramp up the torque until the wheel leaves the detent. This step is effectively open-loop, because the feedback system is too slow to react to the detent  torques. Closed-loop control cannot be used while in the “region of influence” of the detent. 2.Null velocity. Once the wheel exits the detent, true closed-loop control can begin. A damping  controller brings the wheel to a controller stop just outside the detent, so the remaining states are  not affected by variations in exit velocity. These variations are clearly visible in circle 2 of Figure  14.  3.Trajectory travel. A linear trajectory with acceleration and deceleration curves is generated to  guide the wheel from its present location to the edge of the next detent. A carefully tuned PID  controller follows this trajectory and ensures the velocity is close to zero when the detent is reached. 4.Enter detent. The closed-loop controller is disabled just inside the “region of influence” of the  detent. Instead, a high-current open-loop commutation waveform is fed to the motor, lowering the  wheel into the detent.

Figure 19. Pupil Select Mechanism Wheel Position Trajectories During Several Moves. Black  Outlines Illustrate the Operation of the Multi-St ate Controller

Several types of fault detection and handling are built into the LabVIEW controller. The main purpose is to  shut off the wheel if encoder data becomes unusable, but also watch for mechanical blockage of the wheel or unstable behavior. Every error condition sends a telemetry code and records data in the log file for analysis. Triggers include: /g120Encoder data rate. If not enough packets are received in one time period, the connection is  assumed to be set to the wrong frame rate or dropped completely, and the wheel is powered off. /g120Poor image decoding. When the encoder CCD image cannot be parsed, the encoder system  transmits an invalid code. If valid frames are received too infrequently, encoder calibration is the  likely cause, and the wheel is powered off. 297
/g120Trajectory tracking error. If the wheel deviates too far from the command trajectory, it is  assumed to be stuck or otherwise out of control, and the wheel is powered off. The master ground system controls the PSM wheels with a special script that configures the encoder data  transmission and performs error detection and automatic retries to move the wheel in a mostly autonomous fashion. This script was developed over the course of several years of use to properly handle edge cases such as powering up the system with the wheel in between two detents. Infinite Microstepping Driver The brushless motor was selected with a 60-V, 2.5-A peak winding configuration. This was to keep the  current in the previously designed cryogenic harnesses within acceptable limits. However, it made most commercial motor drivers unusable, as they typically provide a maximum output around 28 V. When  open-loop control was still intended, an “infinite microstepping” driver was designed for each motor using a pair of 16-bit Texas Instrument digital-to-analog converters and a pair of APEX high-power operational  amplifiers configured as voltage-to-current amplifiers. The linear drives provide more stable fine control than pulse-width modulation drivers, and the current-mode output ensures that variations in harness and motor resistance over cryogenic temperatures does not affect the motor current/torque. Each driver has a dsPIC microcontroller on board that can automatically produce high-fidelity sine/cosine  commutation output or produce a desired phase angle and amplitude. The latter feature was used to achieve closed-loop control, where the LabVIEW program reads the encoder position and determines the optimal phase angle and amplitude required to produce the desired control torque. The drivers performed well in this application as well as their originally intended purpose—additional units were built to drive  separate stepper motor mechanisms with extreme precision. Assembly of the PSM, Functional and Performa nce Testing, and Integration into OSIM The PSM was assembled at NASA Goddard Space Flight Center in Greenbelt, MD and initial  performance testing of the PSM occurred in Spring 2011. The PSM was thermally cycled and taken to  survival temperature limits with functional tests occurring at ambient temperature and pressure before testing, during the cold and hot operational limits, and again at ambient after thermal testing. During these functional tests, the PSM was able to show repeatable and stable element placement of less than 20 arc-seconds and was able to show compliance with all other functional and performance requirements. Figure  20 shows the PSM in the “Big Red” test dewar. Figure 20. PSM During Initial Thermal Vacuum Performance Testing 298
To perform this testing, retroreflectors were used as alignment targets on the PSM X-Structure as a  reference for cathetometer measurements taken of several fiducials and center markings engraved on the optical elements. The known machining error in the fiducial locations was then removed from the final data. For data collection, the wheels were rotated through the range of motion several times, approaching the detent position from different directions, stopping and powering off at each optical element location for measurement. Figure 21 shows the data collected for vacuum optical elements in the wheels.

Figure 21. Error in the Repeatable Average Position (X-Direction) of Central Fiducial on Optical Elements The PSM was integrated into OSIM with the other optical elements and mechanisms in the fall of 2011 in  the Spacecraft Systems Development and Integration Facility (SSDIF) at GSFC. Figure 22 shows the  PSM during integration in the SSDIF. With all of the components installed into OSIM, end-to-end ambient  testing was performed and concluded in April 2012. The OSIM structure was then moved to the Space Environment Simulator (SES) chamber for calibration and verification of OSIM performance in a thermal  vacuum environment. After 4½ months of testing at 100K, OSIM completed its first cryogenic vacuum test on August 2012. A second OSIM cryogenic test was performed at 100K on March 2013 and lasted 4 months. Figure 23 shows the PSM in the OSIM optical bench at the pupil plane between the two fold  mirrors of the optical system. Figure 22. PSM in SSDIF during assembly onto the OSIM bench 299
Figure 23. PSM as oriented in OSIM bench adjacent to fold mirror

Conclusion The PSM is currently located in the SES chamber where it being used successfully in concert with other  OSIM components for the test and calibration of the Integrated Science Instrument Module for JWST. The first three ISIM tests concluded in November 2013 with the next test projected to begin in May 2014. Due to project schedule constraints, the extreme cryogenic operating temperature of the mechanism, and the tight holding and repeatability requirements, several design features were incorporated into the PSM mechanism during different phases of the development. Included was the incorporation of a bearing and  encoder system to survive the extreme operating temperature as well as the implementation of a challenging control system into the design late in the development. A special thank you to the entire OSIM team, especially the integration and test engineers as well as Dr. William Jones at Glenn Research  Center for the use of facilities and expertise for friction characterization.

Lessons Learned /g120Start early on proof-of-concept testing (especially those related to motor, bearing, and sensor  selections) to flush out issues in the design with enough time to affect long lead purchases. Motors  and encoder/sensor selection are key decision points in an electromechanical system design  development and the suitability of these components should be revisited after any subsequentsystem level changes.  /g120Cold treated hybrid bearings consisting of Si 3N4balls, Cronidur 30 races, and a PGM-HT retainer  have been shown to be suitable for cryogenic operations (down to 100K) in mechanisms with a low number of cycles.  /g120Though WearComp may be suitable for minimal debris in friction-based applications, care must be  made that the mating surface is machined appropriately. 300
/g120When modeling a plant, ensure the plant is of a high enough fidelity to represent the dynamic  response of the system accurately. When testing is used in conjunction with simulation, ensure the test item includes a realistic mass or inertia. /g120Closed-loop control always needs a sensor with sufficient bandwidth and speed range. Sensor  capabilities limit control possibilities, so selecting a good sensor is critical to anticipating future  performance needs. /g120In a two-pole system, where position is controlled via torque input, a first-order PID controller  cannot track a quadratic position command (i.e. fixed acceleration curve) and it is better to use a higher-order transfer function as a compensator in these cases. /g120Automatic event logging is critical in systems with many operators to capture intermittent issues. Producing clear and verbose log files from the very beginning of testing saves a lot of time  detecting, diagnosing, and replicating bugs later on without having to re-run tests to duplicate  anomalies. 

References Leviton, D.B. et al. "Cryogenic Optical Position Encoders for Mechanisms in the JWST Optical Telescope  Element Simulator (OSIM)" /g54/g51/g44/g40/g3/g27/g27/g25/g22 (2013) Sullivan, J. et al. "Manufacturing and Integration Status of the JWST OSIM Optical Simulator" NTRS  (2010) 301
requirements. Since the balance requirements were tight and the spin balance to be measured was so  small, the structural resonances of the GMI instrument, as well as the spin balance test fixture and support structure, also required detailed modeling and design to meet stiffness requirements dictated by  the imbalance accuracies to be measured. Figure 2 shows the finite element model of the GMI instrument, the model of the Thermal Vacuum (TVAC) support structure with the dynamometer assembly used for the spin balance, and locations of balance masses available on the GMI instrument. Figure 1. Dynamometer Assembly Setup in Calibration and GMI without RDA and  MR Assemblies Integrated Figure 2. (a) Structural Model of the GM I Instrument Assembly, (b) Dynamometer 304
with the Spin Balance Test Fixture, and (c) Balance mass locations on GMI  Multiple test setups, shown in part in Figures 2 and 3, were developed for balancing GMI at room  temperature and in a vacuum chamber to eliminate error terms associated with forces imparted to the dynamometer due to air drag of the GMI instrument spinning in air. A MATLAB computer program was  developed to process the resultant forces imparted on the dynamometer, together with position encoder data to derive the magnitude and direction of static and dynamic imbalances, and subsequently perform a linear optimization to predict the optimal balance mass solution. This optimal solution, which meets the  static and dynamic imbalance requirements, consists of the values of 21 ‘balance’ masses that can be added and accounts for design constraints specific to each of the balance mass locations, and at the  same time minimizing the overall total mass of balance masses to be added simultaneously. The  optimization can account for order of preference in the choice of masses by assigning higher weights for particular ones to reflect certain particularities such as ease of access, volume or other situational constraints or preferences. Finally, it has been possible to achieve preliminary balancing of GMI before achieving full integration. The software is capable of balancing a partially integrated GMI while the  instrument is being assembled (e.g., before the integration of the RDA – Reflector Deployment Assembly and MR – Main Reflector) by assigning negative inertial properties to the missing subassemblies thatwere subcontracted elsewhere and would not be integrated for the spin balance testing until late in the environmental test program.  Spin Balance Verification  The GMI instrument underwent an initial spin balance in the clean room in July 2011 and initial balance  masses were added prior to GMI instrument environmental testing. Final spin balance and requirement verification and validation was completed in TVAC in February 2012 after the GMI instrument had successfully completed all environmental testing (Figure 3). The methods and steps successfully accomplishing this spin balance of the GMI instrument and showing the requirement was verified via test measurements will be discussed in this paper. Figure 3. GMI Instrument in Spin Balance Test Facility on Dynamometer 305
Evaluation of Imbalance Forces Bringing GMI into balance started from early on in the design phase. As the design progressed, there has  been a continuous effort to keep the rotating hardware as close as possible to static and dynamic balancein order to minimize overall total mass of the instrument, with a plan for three separate axial and radial locations to place balancing masses as shown in Figure 2-c. While a minimum of two axial planes for  balance masses are required to achieve dynamic balancing, provision to use the RDA/MR for a third  location was necessary to maintain lower added masses, because the RDA does not have axial  symmetry, is distant from the mounting plane, and due to other production and schedule constraints.  The basic idea is to determine the static and dynamic imbalances from the measurements of the moving imbalance forces imparted at the mounting plane while spinning GMI at some angular velocity. Load cells are ideal for the task as they only pick variations in load magnitudes. In the absence of other  disturbances, such as structural resonances, air drag, etc., it can be easily shown that the loads imparted  by a non-perfect rotating device on a mounting plane are the result of the centrifugal forces and moments of the imperfection. Assuming an imbalance mass /g80/g3centered at a position /g11/g91/g15/g92/g15/g93 ) in the GMI reference  frame of Figure 4, the reactions are given by the following radial force f /g85and the radial moment Mr   Figure 4. GMI and Dynamome ter Coordinate systems /g2188/g2200=/g3420/g1858/g3051 /g1858/g3052/g3424=/g1865 /g1857 /g2033/g2870/g4676cos /g2033/g1872 sin /g2033/g1872/g4677=/g1865 /g2033/g2870/g4676/g1876 /g1877/g4677, (1) /g2169/g2200=/g3420/g1839/g3045/g3051 /g1839/g3045/g3052/g3424=/g1878 /g3420/g3398/g1858/g3051 +/g1858/g3052/g3424+/g1865 /g1859 /g4676/g3398/g1877 +/g1876/g4677=/g2033/g2870/g4676/g3398/g1865 /g1877/g1878 +/g1865 /g1876/g1878/g4677+/g1865 /g1859 /g4676/g3398/g1877 +/g1876/g4677=/g2033/g2870/g3420/g3398/g1861/g3052/g3053 +/g1861/g3051/g3053/g3424+/g1859 /g2033/g2870/g3420/g3398/g1858/g3051 +/g1858/g3052/g3424, (2a)  where /g1857= /g3493/g1876/g2870+/g1877/g2870is the radial distance from the spin axis/g15/g3/g550/g3/g76/g86/g3/g87/g75/g72/g3/g68/g81/g74/g88/g79/g68/g85/g3/g89/g72/g79/g82/g70/g76/g87/g92/g15/g3 /g74is the magnitude  of gravitational acceleration and /g87is the time in seconds. Note that we replaced the products /g80/g3/g92/g93 and /g80/g3 306
Again, de-rating this level by 50% to account for unknowns in the setup, we need to bring the radial  moment to less than 0.6 N-m (5 in-lb) if we want to show that GMI is statically balanced. Thus, we need to  have a noise level less than 1/10th of 0.056 N-m (0.496 in-lb). Considering the forces in the sensors from  which we compute the moments and the geometry shown in Figure 5, the noise floor needs to be less than 0.5 in-lb (0.06 N-m) /(26.035/2.54) or 0.05 lb (0.22 N). Accounting for three sensors, we have to  have a noise floor better than 0.016 lb (0.071 N). In summary, the requirement imposed on the noise floor of the radial force is 0.004 lb (0.018 N) and the one imposed on the moments is 0.5 in-lb (0.06 N-m) (that is 0.016 lb (0.071 N) in force sensors). The smaller value of the two is 0.004 lb (0.018 N) and is kept for  comparison with the performance of the dynamometer. Figure 5. Noise floor showing measurement s filtered using low bandpass of 10 Hz To measure the noise floor, the dynamometer was placed on four blocks at the four corners of the lower  plate. This configuration was chosen instead of the fully clamped one to simulate the actual balancing configuration in which the dynamometer’s own mode is in the vicinity of 30 Hz versus the 120 Hz+ in the fully clamped configuration. Three measurements were taken. In the first measurement, the data from the load cells is channeled only to an independent secondary data acquisition system (VXI system). In the second test, the data from the load cells is channeled to the BDMS data acquisition (processor to be used  for balancing) and simultaneously to the VXI through a set of “T” connections. The third test has both data  acquisition systems in place except that the data acquisition card of the BDMS was turned off. A digital filter of 10 Hz was applied to the data sets. An example of dynamometer noise floor measurement for the  Y-axis is shown in Figure 5. The maximum noise of 0.00175 lb (0.00778 N )is measured and is found to  be less than the 0.004 lb (0.018 N) needed to carry out accurate measurements to meet GMI spin  balance requirements. Harmonic stinger test This test is intended to calibrate the measurements performed by the dynamometer as a system. The  dynamometer is excited at a frequency of 10 Hz using an MTB 50-lb (220-N) stinger. A calibrated load cell is placed at the interface between the stinger and the dynamometer to measure the input force. The  310
resultant radial load of 0.015 lb (0.067 N) is below the requirement of 0.08 lb (0.36 N), and the measured  mean resultant moment of 0.51 in-lb (0.06 N-m) is below the 10 in-lb (1.1 N-m) equivalent requirement.  This last test provided the necessary argument that the system is capable of predicting a balanced state. Table 1. Comparison of measured im balance to correction magnitudes Dynamometer MATLAB Percent Error  Static  (lb-in) Dynamic  (lb-in2) Static  (lb-in) Dynamic  (lb-in2) Static Dynamic  13.491 139.077 13.371 138.868 0.89% 0.15%   (N-m)   (kg-m2)  (N-m)   (kg-m2)    1.5243 0.040699 1.5107 0.040638    Figure 6. Frequency Response Function of  clamped dynamometer (without GMI) 312
Figure 7. Frequency Response Function in configuration with GMI installed  Figure 8.  Spin Balance Risk Mitigation Test Setup Spin Balancing Verification of GMI   It is shown that GMI was adequately balanced to meet both static and dynamic balancing requirements. The measured imbalances in BRUTUS vacuum testing where found to be 0.017 kg-m (1.47 lbm-in) and0.022 kg-m 2(75.1 lb-in2) meeting the requirements of 0.032 kg-m (2.78 lbm-in) static and 0.055 kg-m2 (187 lb-in2) dynamic imbalances, respectively. The results of Table 2 show the average of three separate measurements of forces, moments and computation of static and dynamic imbalances. Three tests are averaged to give imbalances in vacuum and three other tests carried in open air are averaged to  determine final balancing values. All tests are run at 32 rpm and results show all tests met the  313