ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Lessons Learned From the Windsat BAPTA Design and On-Orbit Anomalies  Steve Koss’ and Scott Woolawaf  Abstract  The Windsat spin drive assembly is a high precision constant speed drive assembly with challenging  design requirements. Many difficulties were encountered during the design and build of the spin drive  assembly. Additionally, after 2 years of nominal on-orbit operation the Windsat spin drive assembly spun  out-of-control, resulting in a temporary cessation of the W indsat/Coriolis mission. After a lengthy  investigation, telemetry analysis revealed that the resolver based tachometer data was likely being  corrupted by two distinct kinds of slip ring noise. Workarounds were devised to successfully bring W indsat  back on-line, and another similar spin anomaly was recovered from 5 months later. This paper will  concentrate on the many lessons learned from the Windsat spin drive development and subsequent on-  orbit anomaly. Additionally, the goal of the paper is to convey the “key things that were done well” as well  as the “things that should be done different next time” (good vs. bad heritage).  introduction  Windsat is the primary payload on the Coriolis spacecraft. The Windsat payload size is roughly 2-meters  square by 3-meters tall with a mass of 450 kg. Windsat is a demonstration program to evaluate the ability  to exploit passive microwave polarimetry to measure the full ocean surface wind field (wind speed and  wind direction) from space. This payload is very similar to the NPOESS CMlS payload currently under  design. The Windsat payload instrument spins continuously at 31.6 revolutions per minute via a spin drive  mechanism called the Bearing And Power Transfer Assembly (BAPTA). For accurate geolocation, the  BAPTA is a precision device with a spin rate accuracy of 4.05% and a position feedback accuracy of  i  Figure 1. WindsatICoriolis (left) and Wind Speed/Direction Data (right)  f. U.S. Naval Research Laboratory, Washington, DC  Ball Aerospace Corp, Boulder CO  Proceedings of the 3@ Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  209
depending on how the brush might "lift off" due to debris). Additionally, unlike the typical gold plated rings,  the Windsat rings are solid soft gold with a hard gold flash coating on top (Figure 4).  post Lire I (  Figure 3. Windsat / DSCS Slip Rings  Figure 4. Windsat / DSCS Slip RinglBrush Contact Cross Section  Design & Manufacture Lessons Learned  There were numerous lessons learned during the design, build, and test phase of the Windsat BAPTA  and slip rings. What will be discussed are the "key things that were done well" as well as the "things that  should be done different next time"  Test Validated Toraue Marains  The performance test program for the Windsat BAPTA required it to meet its rate and position accuracy  requirements when operating under a 4X worst-case drag torque. This test was performed by measuring  the cold drag torque (worst-case) and driving the BAPTA against an open loop torque motor (acting as a  brake) to produce the 4X drag torque. The BAPTA was found to function well under this load but the  increased BAPTA motor current caused the resolver based tachometer feedback signal to get noisy. The  noise was due to secondary power supply feedback into the resolver-to-digital chip under high load.  Adding an RIC filter and a Zener Diode circuit between the secondary power supply and the resolver-to-  digital chip eliminated this noise. By performing performance testing at worst-case torque, this problem
Motors - Not Challenaina But Acts Like a “Maanet” For “Mundane” Problems  For various reasons spaceflight motors seem to have an unusual amount of minor manufacturing  problems, which at best impact schedule and at worst impact reliability. As such, special attention should  be paid to the motor contract and it is strongly recommended that an extra thorough pre-ship  test/inspection/documentation review be conducted at the motor vendor prior to accepting motor delivery.  On Windsat, motor manufacturing defects drove us to procure motors from 2 different vendors and  neither vendor produced defect-free motors - in the end we flew the motor that had the least defects. A  listing of problems that have been encountered with both the Windsat BAPTA motors as well as a few on  other programs follows:  1) Windings potted in Stycast thermally conductive epoxy. Stycast cracked (Figure 6) after thermal  cycling as a result of CTE mismatch between Stycast & copper windings. Resulted in X-rays of  motor to look for crack induced strain on windings. Also resulted in “painting” the Stycast with  Urelane in an effort to contain any pieces of Stycast that might pop off and jam the air gap or  bearings. On the Windsat BAPTA, we ended up flying a motor that had the least cracks in the  Stycast. While Stycast winding encapsulant may not cause a problem on small motors, on large  motors like the 147-mm-diameter Windsat BAPTA motor it is probably best to avoid potting  windings in Stycast.  4) Figure 6. Fine-Focus X-Rays, Cracked Stycast (left), Nicked Hookup Wires (right)  Lamination stack painted with “fluidize”. Problems with fluidize adhesiodflaking off. Riskkoncern  that fluidize flakes could get into bearings.  Magnets or non-corrosion resistant backiron nickel plated. Problems with nickel plating  adhesiodflaking off. Problems with epoxy washcoat bubbling/peeling off. Riskkoncern that  platingkoating flakes could get into bearings. Use corrosion-resistant materials to the greatest  extent possible to minimize need for plating & epoxy coating. When coating is required perform  tape test & detailed inspections.  Hookup wire strain relief problems. Various instances of insufficiently designed strain relief in  multiple vendorddesigns wherein wire insulation damaged (Figure 6) or wires broken. Strain relief  is something that rarely gets enough attention.  Problems with air gap tolerances. Tolerance stack analysis required both at room temperature  and worst-case temperatures. Analysis should be verified by test/measurement.  Problems with motor cleanliness. Particulate contamination, which could get into bearings &  reduce lubricant life. In extreme cases debris in tapped/helicoiled holes, which could jam, air gap  or bearings. (Watch out for helicoil tangs, small tap pieces, etc)  21 3
down to -2OC. Even with a larger slip ring heater care would need to be taken with thermal gradients and  brushhing alignment.  BAPTA Speed Relative to Bus in SCS Frame  I I I  I I I I I I I I  4000 4100 4200 4300 4400 4500 4600 4700 4800 4900  Time (t), [sec]  Figure 11. Original On-Orbit “A-Side” Spin Anomaly  BAPTA Relative Speed Estimates in SCS Frame  .....  .....  .....  P ........ , ............  ...............  .........  .lJI i.. ....  I ..I: .....  .....  ......  ...... ResolverBasedSpeed  Tw” Speed (Fm IMU, MA, R  WI II m  4  \I V yi  3 8.04 8.05 8.06 8.07 8.08 8.09  x104 Time (t), [sec]  Figure 12. Original Anomaly “B-Side” Troubleshooting Low-Speed Spin  21 9
Additionally, it was postulated that a “blobs” of slip ring debris could have been the cause of the original  A-side instability which occurred at the operational temperature of 25-3OOC. However, with limited on-orbit  telemetry there is no easy way to confirm this. One item of note is that while we had full electrical  redundancy and multiple redundant (parallel) slip ring contacts, all of the rings have the samekommon  wear life (i.e., wear life is not redundant). With these anomaly theories in hand a workaround was devised  to power on the payload on for several days to warm up the slip rings prior to BAPTA / momentum wheel  spin up. (The payload had to be powered off during spin up to minimize the risk of blowing a fuse on the  spacecraft power bus from the “peak” current draw during spin up.) On June 13, 2005 the workaround  was successfully implemented and the BAPTA operated nominally on the B-side.  On October 27, 2005, however, the BAPTA went unstable again, this time on the B-side. During this  second “6-side BAPTA anomaly”, the BAPTA went unstable in a manner similar to, but not quite the  same as the original A-side anomaly (Figure 13).  B-Side Anomaly 10127105  40  35  30  25  E  15  10  5  0  5.95 IO‘ 6 IO‘ 6.05 IO’ 6.1 IO’ 6.15 IO’ 6.2 IO’ 6.25 IO’  Seconds Past Midnight  Figure 13. Second On-Orbit “B-Side” Spin Anomaly  Noteworthy during the B-side anomaly, the speed had several unstable “blips” followed by recoveries until  it finally went fully unstable and triggered a spacecraft momentum induced safe hold and spin down  (Figure 14).  The second anomaly on the B-side presented the opportunity to confirm/refute the theory that slip ring  debris “ingestion” was the cause of the original A-side anomaly. -If slip ring debris induced noise were  indeed the cause of the A-side anomaly then there was a good chance (based on our DSCSll life test unit  experience) that subsequent running had cleared the debris and “cleaned up” the noisy A-side resolver  rings. Therefore, troubleshooting was conducted at low speed (0.3 rpm) on both the A and 6 sides of the  BAPTA after the slip rings were warmed (to 15°C) to avoid the cold temperature slip ring noise  phenomenon. Indeed the 6-side remained noisyhnstabie but the A-side was now cleanktable after the  4.5 months of additional run-time since the original A-side anomaly (Figure 15).  220
8-Side Anomaly 10127105  I I  I I  2  1.5  I  K n  1  0.5  0 E-Side 0.3 rpm lest 11/1/05  I I I  j. i ........................ C .......____, ...........  ;r j :a :  j. j  i. i  i. j  ;a j ................................................  ...... I I  ..................................  j. .j  mi ;r  ?I .................... :.I ....... fl ;  ....& ,I 1 10 jm  .................  i ='-""'  7.78 IO4 7.79 IO' 7.8 10' 7.81 10' 7.82 IO' 7.83 IO' 7 84 IO' 6 63 10'6.635 IO* 6 64 10'6;645 IO6 6 65 10'6.655 IO' 6.66 10'6 665 IO'  Seconds Past Midnight Seconds Past Midnight  Figure 15. Second On-Orbit Anomaly "A & B-Side" Troubleshooting  Windsat was then successfully restored to operational status again on the A-side on November 6, 2005.  Procedures have been developed to provide a speedy recovery should future anomalies of this nature  arise. Additionally, if the condition eventually arises wherein both A&B sides are noisy at the same time it  may be possible to run the motor open loop (forwards or backwards) in a troubleshooting mode to clean  221
Door  r-O.64 mm  ( 2.4mm  0.25 mm 4 t  Imm I  Bulkhead  Figure 1. Light- Path Labyrinth Cross-section View  Fabrication of parts was completed on the Micron 600U 5-axis high-speed milling machine as shown in  Figure 2. The small sizes and angles of the light path protrusions make it necessary to use this machine.  An additional advantage of the high-speed machine is the extremely low amount of stress that is added to  the part by the machining as opposed to the level normally added by typical machines. Stress is a  concern due to the fact that cryogenic temperatures can cause the part’s stress to increase and warp the  movable door. Therefore, as well as using the high-speed machine, Aluminum 6061-T651 is used to  minimize stress. Aluminum 6061 -T651 is heat treated to stress relieve the material.  Figure 2. Picture of door being cut on Micron machine  After rough-cut fabrication, the door is taken and soaked in liquid nitrogen to determine if part warping at  cryogenic temperatures is an issue. Key measurements taken before and after this process are compared  and it is found that the door’s critical dimensions do not permanently move any significant amount.  Therefore, it is determined that the cryogenic environment will not warp the door and affect its operations.  224
Next, the dewar bulkhead plate is fabricated. Aluminum 6061- T651 is again used to minimize stress.  Figure 3 shows the exploded isometric view of the mechanism.  End of Travel  End  C  """ILVII Dewar Bulkhead  0279 mm (1 1 .O in)  Figure 3. Exploded Isometric View of Light Shield Mechanism  Motor Selection  One cryogenic compatible stepper motor is used to drive the light shield door mechanism. The motor was  procured from CDA lntercorp with a right angle gear head, a planetary gear train, dry film Molybdenum Di-  Sulfide (MoS2) coated parts, a motor shaft mounted resolver for position feedback, and a keyed motor  shaft interface to the door. Motor design parameters are discussed below.  Toraue  Analysis indicates the light shield mechanism requires, with margin, a maximum driving torque of  0.19 Nom (1.67 inolb) in the worst-case gravity orientation. In this orientation, the door is required to be  held open or closed against gravity by the motor detent alone as power must be disabled during detector  tests. This motor detent torque with margin turns out to be the driving design parameter and calculates to  0.1 Nom (0.88 inolb). A healthy factor of safety of 6 is used to bring the motor output detent torque to a  0.6 Nom (5.3 inolb) value. The procured motor provides a measured detent torque of 0.75 Nom (6.67 in-lb).  Gearinq  A right angle gear-head is essential for this mechanism design as the volume space available to mount  the motor and mechanism to the dewar bulkhead is limited. The total motor gear ratio is 187:l implying  one motor step equals the door motion of 0.16 degree. This small step size is required so that if during  door motion the motor bounces back or misses a step near the fully closed position, the gap left "open"  does not allow a significant amount of light to enter the light path. Analysis confirms the gap is not large  enough to degrade the light shield capability of the mechanism.  225
Crvoaenic Lubrication  Dry lubrication on the motor components is required for proper cryogenic temperature operation. A  sputtered Molybdenum Disulfide (MoS2) coating was used on the motor gears, bearings, and moving  parts. MoS2 was chosen for this mechanism based on the ability to provide any thickness needed and  also its heritage on numerous previous flight programs such as COBE, Cassini, SIRTF, and others.  Drawbacks in using many dry film lubricants, including MoS2 are that the coating should be operated in a  dry oxygen and water vapor free environment. Operation should not occur at humidity levels above about  40% due to the reactivity of the coating with oxygen and water vapor.  The MoS2 dry film lubricant sputtering process was provided by Hohman Plating Inc. In a vacuum, at  149°C (3OO0F), an electron emitter breaks up the lubricant material into atomic size particles that  bombard the parts. The coating particles adhere to the parts' surfaces and build up thickness over time.  Coatings can be co-sputtered with different elements to improve crystalline structures and lower the  friction coefficient. The majority of coatings are co-sputtered with Nickel giving a 0.02 friction coefficient.  More advanced co-sputter options are Antimony O3 (p = 0.01) or Antimony O3 with Gold (p c 0.01). This  project co-sputtered with nickel as the coefficient of friction meets the required amount and is less  expensive than the other co-sputter options. Figure 4 shows the dry film thickness along the tooth of a  motor gear part as seen in a cross-section view.  Dpto  tlme ,  :I1 .Zr):bl  Figure 4. Mo~ coating thickness measured along width of gear tooth near tooth tip of a  representative motor gear part. Thickness measured from mounting adhesive to gap.  Nominally, pohman plates parts to a thickness of 3000-1 0000 Angstroms (0.3 to 1 pm or 1.1 8 x 1 O5 to  3.93 x 10 in) over 50 minutes. Considering the low life cycle requirements of the mechanism, the  associated mechanical loads it would undergo, and margin, a MoS2 coating thickness of 2.5 pm (0.0001  in) was considered. Initially, the relationship of lubricant coating buildup over time with the sputtering  process was thought to be linear. However, through the coating of several sample gears, analysis shows  226
it to not be so. At first the thickness and time were extrapolated linearly to that for 2.5 Nm (0.0001 in)  which resulted in a 3-hour long application. Destructive analysis was performed on a sample gear by the  materials group at NASA Goddard, and the average thickness of the coating was measured to be  17.8 pm (0.0007 in). Another gear sample was then coated at a lesser time of 70 minutes and the  measured thickness of the coating ranged from 0.5 to 3.8 pm (ZX~O-~ to 1.5 x loe4 in). Based on the  gathered data shown in Figure 5, it was decided that an application duration time of 80 minutes would be  sufficient for the light shield mechanism motor parts. All parts are coated for the 80-minute duration  resulting in a thickness of 1.2 um (0.00005 in). The final dry lube coating applied to the motor parts is  based on the sample gear data and the minimum motor assembly tolerances for nominal fit. After the  coating process was complete, all motor parts were successfully reassembled and tested.  9.00E-06 -  8.OOE-06 -  7.OOE-06 -  6.00E-06 - E  u)  5.00E-06 - Y u c c .-  4.00E-06 -  3.OOE-06 -  2.00E-06 -  1.OOE-06 -  O.OOE+OO 7 MoS2 Coating Thickness vs. Time  t  t  v  **  A 8 -  + * 1 .OOE-05  I  The bearings in the motor are also dry lubricated with MoS2. Lubrication thickness is the same as that for  all other motor parts, 1.27 pm (0.00005 in). The support bearing on the opposite side from the motor on  the bulkhead is a radial bearing and is not sputter coated. Instead, the support bearing is a “BarTemp”  bearing from The Barden Corporation. “BarTemp” bearings have a cage made of a Teflon-coated, highly  compressed material with very fine glass fibers and molybdenum disulfide impregnation. During  operation, the rotation of the bearing causes the balls to rub off small amounts of the cage which coats  the raceways with a thin lubrication layer.  Mechanism Control  The light shield mechanism motor is a two phase DC stepper motor with 30 degree steps and a tots1 right  angle gear ratio of 187:l. Nominal operation occurs at 24 VDC. Since the mechanism must operate at  room temperature and at 20 Kelvin, a current controlled electrical drive system is used for maintaining  proper performance margins under all operating conditions. A Newport 300 Series current control  amplifier drives the light shield mechanism motor. Step pulse rate and phase current levels are settable  227
quantities through a front panel or macro driver software which controls the mechanism to open or close  the light shield door in approximately one minute. Software time out and TTL limit switch interfaces allow  electronically disabling motor power redundantly with this controller. Motor resolver analog signal is read  and converted to a TTL quadrature signal that the Newport controller can read. Motor power is disabled  when contact is made with one of the end of travel switches in either the open or closed position of the  light shield door. The motor shaft resolver is used as a method of determining the door position.  Over Travel Analysis  As the light shield reaches its end of travel in either the open or close direction, it comes into contact with  an end-of-travel switch. This switch then cuts off power to the motor and the door stops its motion. In the  close direction there is a hard stop that does not allow the door to over travel. Travel in the open direction,  however, does not have a hard stop. Therefore, stress analysis is performed on the tab that comes into  contact with the switch to determine the maximum amount of force that can be handled. These  calculations determine that the load felt on the tab is 20 times smaller than the yield load of the door  material, including margin. Therefore, at the slow speed that the motor rotates, the tab contacting the  switch will not yield if the motor attempts to continue to rotate, and the shield door will not over travel.  Cryogenic Testing  Before delivery to the project, the light shield mechanism is tested in a dewar at 20K in the operational  configuration. The dewar bulkhead is attached to an interface plate to mate the mechanism to the  chamber. Once assembly is complete, the chamber is closed and the then rotated 180" to orient the  mechanism to its operational configuration. This places the mechanism so that gravity is acting to open  the door from its closed position.  Test results are forthcoming and will be presented at the conference.  Special thanks goes to Mike Dube and Mark Mclendon of the Materials Group at Goddard for their  destructive analysis and insight in evaluating the dry film coating of motor parts.  228
Development Tests of a Cryogenic Filter Wheel Assembly  for the NlRCam Instrument  Sean McCully’, Charles Clark*, Michael Schermerhorn*, Filip Trojanek*, Mark O’Hara*,  Jeff Williams* and John Thatcher*  Abstract  The James Webb Space Telescope is an infrared-optimized space telescope scheduled for launch in  201 3. Its 6.5-m diameter primary mirror will collect light from some of the first galaxies formed after the big  bang. The Near Infrared camera (NIRCam) will detect the first light from these galaxies, provide the  necessary tools for studying the formation of stars, aid in discovering planets around other stars, and  adjust the wave front error on the primary mirror (Fig. 1). The instrument and its complement of  mechanisms and optics will operate at a cryogenic temperature of 35 K. This paper describes tests and  test results of the NlRCam Filter Wheel assembly prototype.  ..-  Figure 1. The NlRCam instrument  Introduction  The Filter Wheel assembly is one of three types of mechanisms on NIRCam. There are a total of four  Filter Wheel assemblies on NIRCam, and these assemblies are situated at the pupils of both the  longwave and the shortwave beams for both of the NlRCam optical benches. The Filter Wheel assembly  mission is to position optical filters and pupil lenses as well as other wavefront sensing elements into  respective optic beams.  The Filter Wheel assembly prototype was built to retire certain risks and concerns. The end-of-life bearing  drag torque is a critical parameter necessary in determining the drive current. The motor torque constant  and phase resistance at temperature is critical to meeting operating margins for torque and power.  Finally, the position control and stability using an inductive feedback position sensor and cogless DC  motor needed to be characterized.  * Lockheed Martin Space Systems Company, Palo Alto, CA  Proceedings of the 38 Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  229