ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Analysis of the jitter testing is still ongoing with spacecraft system models to gain a better understanding  of the effects of wheel micro vibration through the spacecraft with particular interest to imaging missions.  BEARING  FUN DERMENTAL  FREQUENCIES  aao 3.5 1  0  'Shp  Figure 9. Waterfall plot showing coast down micro vibration post life test  QM#2 (Declraded mode)  The wheel operated nominally throughout the life test with no anomalies performing over 130 thousand  zero crossing.  Post life test insDection  The bearings were stripped down, inspected and the cages weighed pre and post life test to ascertain the  amount and characteristics of the wear. Photos from the strip down can be seen in Figure 10, with even  wear around the pocket, clear and even transfer of PTFE material onto the race ways.  Both QM1 (6.1-N preload) and QM2 (2.3 N) exhibited very low cage wear on successful completion of the  life tests carried out compared to that expected. Typically the cage mass loss was of order 1% or less of  the QM1 initial cage mass (with on-loaded bearing cage wear rate being higher than that operated at the  nominal preload). For QM2 the measured cage wear rate was still lower, in fact on the threshold of  measurement for QM2. According to the lifetime model, and previous experimental data, QM1 bearing  cages could have been expected to have lost up to 15% of their cage mass due to wear (0.1g) by the end  of the accelerated life test. The excellent condition of the cages in both QM1 and QM2 suggests that the  improved bearing alignment achieved has a large impact on the wear rate observed comparing to the  EMRW life test.  Extrapolation of wear rates is slightly hazardous because typical wear processes follow a so-called  "bathtub" curve with high initial and final wear rates but a long period of low wear rate equilibrium  performance. Given this a linear extrapolation of mass is likely to under-estimate lifetime, but using such  an approach, it may be concluded that the ultimate lifetime of the bearings in the SMRW wheel may be  considerably in excess of 1 El 0 revs, that is permitting a lifetime with margin of in excess of 16 years at  300 RPM. Clearly however the ultimate lifetime achievable will need to be determined by further  testing/flight experience.  The torque performance of the bearings during the life tests was also very good. After the initial run-in,  the torque noise generated improved with lifetime. For example in QM1, the Standard deviation torque  195
noise during reversals was around 2.5 gcrn initially and slightly improved, to a value of 1.8 gcrn by  completion of testing.  hearing wiih the shield mrnwed Cage shaving wear fines in the centre of pocket  Figure 10. Close up photo of the wear of one of the SMRW QM#1 bearing cage after life test  Conclusion  Both SMRW life tests were successfully concluded with both wheels meeting and exceeding their  requirement; lifetimes substantially in excess of these values may well be achievable.  No SSTL wheel has had a mechanical failure on orbit. All wheels are either still working or were working  nominally when the spacecraft was decommissioned. To date the MRW and the SMRW (through on  ground qualification) have outlived spacecraft life and have still been operating nominally when the  spacecraft has been de-commissioned proving self lubrication is fit for this type of wheel application.  The EMRW, SMRW and SRW covered in the paper are available commercially from SSTL under the  trade names Microwheel and SmallWheel.  References  1. Ball bearing tests to evaluate duroid replacements - ESTL  2. Performance guide; self lubrication bearings, NCT Guide, 1976-ESTL  3. 30 Years of Commercial Components in Space: Selection Techniques without Formal  Qualif ication-SSTL  4. 25 Years Experience With Commercial Components in Space - SSTL, California Paper  196
Design of the ATMS Scan Drive Mechanism  Curtis ~llmon' andpave Putnam  Abstract  The Advanced Technology Microwave Sounder (ATMS) scan drive mechanism is a torque-compensating  single-axis dual-mirror gimbal assembly. The scan drive mechanism will fly as part of ATMS on both  NOAA's NPOESS (National Polar-orbiting Operational Environmental Satellite System) and NASA's NPP  (NPOESS Preparatory Project). The ATMS, a weather monitoring instrument under development by  Goddard Space Flight Center, measures microwave energy emitted by the atmosphere which aids  weather forecasting. The material covered in this paper will focus on the mechanical design of the scan  drive mechanism. The topics covered include the design features of the scan drive mechanism and the  methods used to minimize the transmitted torque disturbances from the scan drive mechanism to the  spacecraft.  Introduction  The ATMS scan drive mechanism is a single-axis continuous-rotation gimbal that feeds microwave  frequency data into the ATMS instrument. The microwave data generated by the scan drive mechanism is  used to develop layered maps of the Earth's atmosphere by measuring the temperature and humidity at  different altitudes. Figure 1 shows the orientation of the scan drive mechanism and ATMS instrument in  flight and a composite plot of the temperature layers showing a developing tropical storm.  Figure 1. ATMS scan drive mechanism flight pattern and data from earlier generation AMSU  showing a tropical storm developing in November 2005. The ATMS Scan Drive  Mechanism is the 3rd generation scanner to provide atmospheric temperature and  humidity data from space for weather prediction.  The ATMS scan drive mechanism is the third generation in a family of scan drives that reaches back to  1978. The ATMS scanner offers a three times improvement over the current generation AMSU scanner in  number of scans per orbit. For comparison, the first generation MSU scanner rotated one scan or one  revolution over 25 seconds. The AMSU second generation scanner rotates one revolution every 8  Lockheed Martin Space Systems Company, Sunnyvale, CA  Proceedings of the 38'" Aerospace Mechanisms Symposium, Langley Research Center, May 17-19,2006
seconds, and the ATMS scan drive mechanism rotates one revolution once every 8/3 seconds. To  achieve the improved scan rate, the ATMS scan drive mechanism inserts three periods of acceleration  and deceleration into each revolution as shown in Figure 2.  1 .  Nadir Earth scene sector I,  AB c I  Figure 2. Scan Drive Mechanism Scanning Pattern: When in scanning mode, the Scan Drive  Mechanism rotates at a constant velocity through the Earth scan and the Cold and Hot  calibration scans. It rotates with constant acceleration or deceleration between these  scans  As with every new design, there were things done very well in the scan drive mechanism and things that  could have been improved upon if time permitted. In the category of things that were done well were the  packaging design, the design for manufacturability, and the disturbance torque minimization features. One  of the things we wished went better was the EMlhadiated susceptibility design and in a middle category  (some good, some bad) was the bearing procurement and testing.  Scan Drive Mechanism Design  Packaaina and Overall Desian.  The overall envelope and packaging of the scan drive is in the category of things that went well. As can be  seen in Figure 3 the scan drive mechanism needed to fit in a very compact space on the ATMS  instrument. The space between the two reflectors had to house two motors, two resolvers, two pairs of  bearings, and a flywheel. The details of packaging the components in this space are illustrated in the  Figure 4 cross section. One unique feature that aided in packaging the gimbal in this small space was the  use of single-string (non-redundant) motors and resolvers. The single-string main and flywheel motors  were driven by redundant wiring and electronics but by making the motors non-redundant we were able to  achieve higher torque margin in a smaller package than possible with a redundant winding design of the  same size. The motors were also an ironless core construction, which used a significantly thinner stator  than a conventional brushless DC motor.  As shown in Figure 4, the scan drive mechanism is made up of main and compensating subassemblies.  The main subassembly rotates the reflectors while the compensating subassembly limits the disturbance  torque into the instrument by driving a flywheel in the opposite direction. The scan drive mechanism can  be driven in both a compensated and uncompensated mode. In the uncompensated mode, the flywheel is  not powered and torque compensation is eliminated.  Weight was another key requirement that went well. The weight and other requirements for the scan drive  mechanism are given in Table 1. To achieve the required weight, the reflectors and the main and  compensating subassembly housings were made of beryllium. The reflector material selection was  important for weight as well as inertia. The spherical shrouds around each reflector were made of 6061 -T6  198
aluminum. Other materials were considered for the shrouds but aluminum was selected for both for weight  and manufacturability.  Figure 3. The Scan Drive Mechanism mounted on the ATMS instrument illustrates the packaging  challenge of this design. Two motors, two resolvers, two pairs of bearings, and a  flywheel must fit in the center section between the two spherical shrouds.  Desian for Manufacturabilitv.  The scan drive mechanism design not only met very tight packaging goals and weight requirement but  was created as a modular assembly that allowed assembly, disassembly (when required), and test as  subassemblies. This approach provided schedule and work load flexibility during assembly and test. The  modular design of the scan drive mechanism can be seen in Figure 5 where the main motor subassembly  and compensation flywheel subassembly are on the work bench together. The subassembly approach  also allowed for the separate balancing of the main assembly with the reflectors and the compensating  assembly with the flywheel.  Fabrication of the spherical shrouds and a repeatable method of assembling and disassembling the  shroud halves with the scan drive mechanism was another important manufacturing challenge. The  shrouds attach to rings that have floating nut plates and locating pins for repeatable reattachment of the  shrouds. The shrouds are split along a horizontal seam to allow removal of the upper shroud half during  reflector alignment verification (see Figure 6). The shrouds are rough machined out of one piece of  aluminum and then split along the horizontal seam. The spherical shape of shrouds provided a maximum  view factor for each reflector and covered the warm calibration source inside the ATMS during earth scan.  199
- 254 mm 7 Beryllium KAV Reflector 155ph Flywheel 397 mm  I Green = Fixed Assy Red = Main Rotating Assy Bke = Cornp Rotating Assy I  Figure 4. Cross Section of the Scan Drive Mechanism illustrates compact packaging and  materials selection used to meet weight and envelope requirements  Reflector Alianment  The reflectors mounted to the main shaft with titanium spiders that allowed axial and rotational alignment.  The spiders attached to the shaft using tapered square holes to ensure repeatability of position during  successive assembly and disassembly operations. In order to meet the required alignment, as shown in  Table 1, there were shims between the spider and the reflector. The shims were pre-machined in  thickness increments of 0.01 mm (0.0005 in). This shim kit allowed for rapid alignment of each reflector to  the spin axis on a coordinate measuring machine (CMM). The CMM was used to define the true angle of  the reflector relative to the spin axis by defining multiple planes whose normal vectors defined a cone. The  upper half of the split shroud was removed during alignment to allow full access across the entire reflector.  Radiated Susceptibilitv.  The cable harnessing and EM1 design on the scan drive mechanism were things that fell into the category  of things-not-done-so-well. Cabling is often one of the last features considered by mechanical designers  and the scan drive mechanism was no different. The scan drive mechanism has two harnesses with  connectors that extend a short distance out of the gimbal. Inside the gimbal the harness travels on the  surface of the gimbal housing inside a cable tray. The harness was originally sheathed with braided  shielding and the surfaces of the cable tray and gimbal housing were coated for conductivity. Despite  these seemingly normal precautions, the scan drive mechanism dramatically failed the first radiated  susceptibility test. Instead of providing a required 60 dB of attenuation in the 1 to 4 GHz frequency range,  the scan drive mechanism harness and shielding were effectively acting like an antenna.  200
Table 1. Requirements and capability for the ATMS Scan Drive Mechanism - to achieve a 3x  increase in the scan rate, the single-axis SDM is accelerated and decelerated between  measurement and calibration scans -  Parameter  Earth View Sector Control Accuracy +f- 35mdeg  ~0.003 N-m between 0.01 and 1 .0 Hz  c0.1 N-m above 33 Hz Disturbance torque into spacecraft  Torque capability Ifunction under 3X worst case drag  Life [continuous service) (11 years  Mass 14 0.9Kg  Average Power 14 3W  There were numerous causes for the radiated susceptibility failure:  1. The beryllium housing halves and cable tray were attached with fasteners spaced too far apart  and there were no provisions for EM1 gaskets (the gaps were radiation sources).  2. There were no provisions for terminating the harness shield to the cable tray or to the entry into  beryllium housing necessary to make a continuous Faraday cage.  3. The beryllium housing mesh air vent was installed with non conductive RTV - an ungrounded air  vent can propagate RF energy.  4. The harness inner shield was tied to the outer shield inside the connector backshells - both  shields terminated at one end can act as an RF antenna.  5. Motor and resolver harnesses were routed together providing coupling paths inside the harness.  Unfortunately these short comings were not uncovered until late in the testing program and schedule did  not permit major design changes to fully correct the problem.  The fixes as shown in Figure 7 consisted of caulking the cable tray and housing halves with silver-filled  epoxy, using silver-filled epoxy to ground the vent screen, and wrapping the harnesses with copper foil  over the braided shielding. These fixes fell short of the required 60-dB attenuation, but fortunately the  requirement was lowered to 40-dB attenuation and the reworked scan drive mechanism passed the  radiated susceptibility test. The painful lesson from this experience is that every wire harness and EM1  task is not necessarily similar to the last design and a quick discussion with a subject area expert can  save weeks of time.  20 1
.-.. . . ' '..?  .h , . 1_, . \r , '., .  f. I  Figure 5. Torque compensation flywheel. The torque into spacecraft due to accelerating and  decelerating the reflectors is compensated for by active control of the counter rotating  flywheel. The flywheel inertia is lower than the inertia of the rotating reflectors so the  flywheel is accelerated at the inertia ratio times the changing rotation rate of the main  rotating assembly.  Figure 6. Upper half of Scan Drive Mechanism shroud is removed to verify alignment of  reflectors and rotation center before and after each environment  202
Gimbal Bearina Handlinq  The manufacturing handling of  bearina toraue characterization the gimbal bearings was good news, bad news story. On the plus side,  tests were implemented as part of the manufacturing plan to detect torque  irregularities. On the down side, the bearings did not have sufficient cleanliness controls. The bearings in  the scan drive mechanism were designed for a life of 130 million revolutions for the reflector shaft  bearings and 230 million revolutions for the flywheel bearings. Cleanliness of the bearings and lubricant  was absolutely essential to achieve bearing life and smooth torque performance. Fortunately, we had a  bearing torque test after the bearings were installed in the gimbal and on the third scan drive mechanism  we found a noisy torque trace on a flywheel bearing duplex pair and on closer inspection we found debris  in the bearing. The most likely sources of debris were:  1. Metallic debris was found and was most likely from tooling used for torque testing at the supplier.  Inspection of the supplier tooling found metallic debris in threaded holes on the tooling.  2. White fibers were found and were most likely from cleanroom wipes or hair bouffants.  Although disassembly and replacement of the bearing was a painful schedule hit, the bearing  contamination was caught before entering acceptance test. None of the other gimbal bearings ever  showed erratic torque performance and a recommended cleanliness program was implemented by our  bearing supplier to eliminate future contamination. Figure 8 shows our bearing torque set-up and samples  of the debris found in the bearing.  m  Air Vent ,  ”.(  Figure 7. During radiated susceptibility testing in the 1 to 4 giga-Hertz frequency range, the scan  drive mechanism harnessing acted as an antenna causing interference with the ATMS  receivers. Various fixes were implemented to reduce the interference but adding the  right EM1 features earlier in the design process would have worked better.  203
A  Figure 8. Contamination was found in the flywheel bearing of flight unit 3. The in-situ bearing  torque test was invaluable at identifying bearing problems during the assembly of the  gimbal. Fortunately disassembly features were built into the design of the scan drive  mechanism.  Disturbance Toraue Reduction.  Torque reduction was an aspect of the scan drive mechanism that went well. The level of torque  disturbance allowed by the scan drive mechanism as shown in the Table 1 is less than 0.027 in-lb (0.003  N-m) across the 0.01 to 1 .O Hertz frequency range. To achieve this level of quiet operation, a number of  disturbance reduction techniques were required. The first was to use a counter-rotating flywheel to  compensate for the torque disturbance caused by accelerating and decelerating the reflectors three times  each scan rotation. Other techniques used to achieve quiet operation included accurate balancing both  the main antenna rotation assembly as well as the flywheel, and careful selection of parts including low  run-out bearings and low cogging motors for both main and flywheel assemblies.  One of the primary reasons for designing the scan drive mechanism as two subassemblies was to allow  access for balancing the main reflector rotation subassembly and flywheel compensation subassembly. In  Figure 5, radial and axial threaded holes for balance weights can be seen on the flywheel sFbassemblr  The balance weight holes were filled with set screws as required to achieve a 80,935 g-mm (4.4 oz-in )  dynamic balance and 216 g-mm (0.3 oz-in) static balance. Figure 9 illustrates the features on each  reflector shroud that allowed balancing of the main shaft and reflector subassembly. The main  subassembly was difficult to balance due to the cutout in the shroud for reflector viewing. The cutout was  balanced by placing a tungsten weight on the hole side of the reflector assembly. Several other smaller  masses were added around the shrouds to account for other inconsistencies in geometry. The main  subassembly was balanced to the same level as the flywheel subassembly.  204
I  Optional  balancing set  screw  locations avsate for  'n top of  IT  (11 10 Note Reflector  rn+ shown  Figure 9. To minimize uncompensated torque disturbances, balancing features inside each shroud  allowed dynamic and static balancing of the rotating main assembly.  One of the unique features of the scan drive mechanism was the counter-rotating flywheel used to  minimize the torque generated by accelerating and decelerating the reflector each scan cycle. The scan  drive mechanism acceleration/deceleration pattern (Figure 2) shortened the scan period and increased  overall gap coverage but generated undesirable torque disturbances. The counter-rotating flywheel  cancelled 92% of the torque from the reflector acceleration profile. The success of this approach is shown  in Figure 10 where the scan drive mechanism was run both with and without the flywheel compensation.  Pointinci Performance  The final link in the scan drive performance was the earth scan pointing. Pointing performance success  was a combination of mechanical alignment and motor and resolver selection. As noted the brushless DC  motor was a low-cogging ironless-core design which resulted in very low torque ripple. The main motor  and reflector assemble position was controlled by a 64-speed and single-speed brushless resolver. The  resulting pointing performance is shown in Figure 11.  205
Compensation Motor Off Compensation Motor On  - -- ~ .._ ~ __-- -*  Jc 1 --  .o; ...... __ ..  Figure 10. Reaction torque cancellation is successful with the compensation flywheel motor on.  The Scan Drive Mechanism can operate in reduced mode (greater torque disturbance)  using only the main motor.  I I I I I I  0 os 1 15 2 2s 3 5.5 4 45 5 -0.8 I  €am %en  I , I I I I 1 I  Figure 11. The Scan Drive Mechanism meets the ultimate test of pointing accuracy for earth scan  as well as during hot and cold calibration. Acceleration/deceleration periods offer the  ability to provide more earth scans per orbit.  206
Acknowledgements  Every successful major project results from the combined efforts of many people. It is with great regret we  don’t have the space to list all of the many people who contributed time, effort, and talent to the ATMS  Scan Drive program. None-the-less, most of those who participated in the Scan Drive program would  agree that the following people were key to the success of the mechanism part of the program. Goddard  Space Flight Center: Sergey Krimchansky, Robert Lambeck, Rick Schnurr; Northrop Grumman Electronic  Systems: Dennis Lord, Terry O’Brien; Lockheed Martin: Ed Boesiger, Caesar Ching, Jeff Fisher, Patrick  Herbert, Stu Loewenthal, A.J. Maher, Larry McGovern, Gordon Smith, Nic Mercer and Julie Price.  References  1) Advanced Technology Microwave Sounder on NPOESS and NPP, Christina Muth, Paul Lee,  Sergey Krimchansky, James Shiue, Allan Webb. NPOES IPO Information center, No. 06125,06-  1 6-2004, http://l40.90.86.6/1POarchive/SCI/sensors/ATMSDraftBriefv4_AlanW ebb. pdf  2) AMSU temperature plots: http://Rm-esiD.msfc.nasa.aov/amsu/  207