ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
544 need for even more improvements, both to the hardware and test design. Nine months prior, the   Engineering Development Unit  (EDU ) system demonstrated successful, long- term operation between - 50  and 60°C. Needless to say, failures in s uccessive flight TVAC tests at 2 5 and - 35°C were terrible surprises.  

During the two failure investigations —the first  lasting about 13 weeks and the second about 9 weeks —the  design and test team met with s ubject matter experts from both MSFC and from the NESC MSDT to identify  possible causes of the motor overtemperature failures, realistic remediations , and new methods to capture  test data . The major events between December 2018 and September 2019 are illustrated in Figure 1. Each  of these events  will be discussed at greater length.  

Figure 1. NEA Scout AMT TVAC Test Phase Events  

AMT TVAC Test 1, Failure at 25 °C 

The flight NEA Scout AMT , shown in Figure 2 left, was assembled in December of 2018 to the exact  specificatio n represented during the EDU TVAC tesing earlier that year . Notable differences  include :  1. Stepper motor manufacturing lot.   2. Use of flight -like mechanical interfaces , shown in Figure 2 right   3. Use of flight spare Motor Controller Board (MCB)  instead of a protob oard  4. Different power supply   5. Test chamber setup, including new LED lights, thermocouples and col d plate interface 

Figure 2. NEA Scout AMT and Test Configuration  

Following a functional test verifying the electrical, mechanical and test chamber set up, the TVAC chamber  was controlled to 2 5°C, given time to dwell and depressurized to <1.0E -5 Torr . The AMT motors were  powered to begin the first functional test at ambi ent temperature (25° C). About a minute later , the team  noted temperature spikes from TCs mounted near the motor housings . After pausing the operation and  reading the motor coil impedances, it was clear that the motors had begun the process of a cascading s hort  failure. Given the previous TVAC tests for the EDU, the team had a natural inclination to suspect  2
545 overheating of the motor coils, but the root cause was unsure . Previously, it had been a poor thermal  conductive path, which had been addressed for this mechanism. The next 13 weeks would revisit and  uncover a few new contributers that were not discovered during the EDU tests.  

Investigation Method, Failure Mode and Destructive Investigation   The team chose to maintain the test configuration for a time until a detailed plan was developed in order to  preserve any contributions from the test facility and set up. The plan included  (1) collection of all data,  compiled and reviewed to hint at a “smoking gun”, (2) a controlled return to ambient, (3) detailed  documentation of test configuration tear down, (4) return hardware to cleanroom for non- destructive  evaluation, (5) removal of stepper motors from hardware, (6) precision x -ray of steppers, (7) destructive  evaluation of stepper motor internals.  Data logs were compiled and compared to previous temperature and  chamber pressure data. A compiled temperature and pressure data set is shown in Figure 3. The data did  show a temperature spike and a pressure rise in the test chamber  correlating perfectly with a current spike  from the power supply . The small pressure rise is indicative that the lacquers on the motor windings reached  an overtemped state and began to offgas.  Hundreds of pictures were taken throughout the process, the  most valuable of which would come during the destructive evaluation of the failed stepper motors.  

Figure 3. NEA Scout AMT Ambien t Failure Temperature, Current  and Pressure Data  

NASA MSFC has a team and facillity devoted to failure investigation and these resources were vital to the  stepper motor destructive evaluation process . Figure 4 shows some of the sample images of the failed   stepper motor . These motors, which are about 6 mm in diameter, showed clear signs of overheating at first  glance. The discolored (reference the coil at 10 o’clock in the right image of Figure 4) and deformed (same  coil shown in the right image at the top)  exhibited tell -tale signs of the coils overheating. The overheating  was a two- fold failure . First, the lacquer break down emitted material causing the noticable pressure spikes .  Second, the rapid temperature rise causes the copper wire to lengthen, creati ng the wavy, deformed coil .  Y Motor Failure   X Motor Degrades   Chamber Pressure  and  Current  spike simulaneously  as temperature rises  accelerate.
547 into the motors’ sensitivity to thermal environments and conductive paths . Prior to the second test, the team  determined a duty cycle for the motors at the ambient and hot extreme.  

Failure Mode, Preliminary Findings and Destructive Investigation   Since the first failure, the test plan was updated to use a data logging power supply to monitor current and  voltage levels across the motor driver circuits . Analyzing these log files, the team found some peculiar  current behavior which resembled a motor failure on the Tethered Satel lite mission. (The motor on the TSS  mission was determined to have failed due to a phenonmenon called Paschen Discharge.) This current  behavior is shown in Figure 5. Since the faliure occurred at such a low temperature, the team—believing  the causes of fai lure may have been different than before—spent weeks investigating Paschen Discharge.  This effort was never fully realized due to schedule constraints, but the team could never rule out the  possibility completely . The concluded hypothesis was that the heat ed coils offgassed small amounts of  lacquer which filled the small cavity inside the motor . The motor vent paths, sealed by kapton tape and/or  indium, disallowed any free molecules to escape and possibly raised the pressure into the critical 10-4 to  10-3 Torr range. This pressure, coupled with the local magnetic field may have provided a “sweet spot” for  the discharge to occur across particular lengths of winding. This hypothetical event could have caused the  currrent to rise to 0.7 amp, hold for a moment, and once winding temperatures exceeded a failure  temperature, create a short.  

Figure 5. Potential current rise and pause could be Paschen Discharge  

The failed motors from the second TVAC test w ere also taken to the MSFC failure analysis team. This  second investigation yielded some helpful data in regards to the manufacuring variations in the motors,  coils, magnet wire and potting . The motors underwent a typical round of X -ray imaging, which plainly   showed deformed coils, as shown in Figure 6. The destructive evaluation showed a few new irregularities .  One of which showed that the ball bearings inside the motor were single shielded and were installed with  no regard to the shield direction. The next i rregularity was with the laquer . A special instrument allowing  metals and polymers to be clearly distinguished under microscope showed that the laquer was producing
548 very large bubbles/voids in random areas . Figure 7 shows an image of a few bubbles or voids  formed in  the lacquer, either due to testing or remnant of manufacturing processes . This image, amongst others  collected, also gave insight that the coil/stator  design  was not suitable for sustained use in vacuum. The  most prominent detail noticed at this  investigation w as with the potting and mounting of the coils to the  stator . The team assumed the coils were potted to the stator, but a coil was accidentally damaged and  became detached from the stator . Upon further analysis, it became clear that each coi l was potted to the  stator in a single location, with a single dab of adhesive which was not especially thermally conductive . An  example of this potting location is shown in Figure 8.  

Figure 6. Deformed Stepper Mot or Coil under X -ray 

Figure 7. Coils under high magnification showing some evidence of voids in insulation
549   Figure 8. Potting location for coil to stator  

The potting underdesign and presence of voids in wire insulation were clear signs that these motors were  not well suited for a vacuum environment  operation. The conductive paths from the windings to the motor  casing were undersized, and the magne t wire had thin insulation. These findings paired with the  manufacturing variance of the potting and insulation thickness made it clear that the motors would have to  complete a more enhanced thermal acceptance testing to choose the more robust motors from a particul ar  lot. Only after completi ng these tests could the motors be integrated into the flight hardware for an AMT  subsystem TVAC test . The details of the acceptance and burn- in testing will be covered in the next section.  

Lessons Learned  from AMT TVAC Test Failure 2   1. Motors intended for vacuum use should use coil wire with thicker vacuum compatible insualtion.  Some vendors will twice or thrice dip their products for vacuum application.   2. Magnet wire defects such as thin insulation and voids/bubbles can be susceptible to first timewide temperature swings (such as a first time run in vacuum).   3. Motor coils should be potted on all available surfaces to maximize thermal  conduction . These  motors, though advertised as vacuum rated, were contacting th e stator in a single, potted  location. This potting was analyzed and had little to no thermal conductive properties . The team’s  thermal analyst would have assumed these coils to be relying on radiative cooling alone.   4. When using small (<15 mm) DC motors, or  any motor from a non- flight rated vend or, a detailed  motor thermal model should be developed to estimate a steady state temperature or a first cut  duty cycle . Buy some motors, break them open, and figure out how the heat is conducting to the  surface.   5. Acceptance testing in vacuum at the motor level is a must. Again, buy extra motors in case of  failures.   6. Motors may be susceptible to discharge- related failures if the insulation offgasses due to rising  temperatures.  

Motor Conditioning and Burn -in Test Met hod 

An initial effort was made to determine a method to recreate a Paschen Discharge. Exploring the variables  that factor into achieving this effect led to the realization that the conditions could not be duplicated. Focus  turned instead to preventive tac tics. Ultimately every motor failure traced back to coil temperature, either  by causing off -gassing that created an atmosphere to carry a discharge, or by outright melting the wire  coatings, resulting in a short. This points to the wire coatings as the weakest link, and coating inconsistency  between batches as one of the hurdles. A two- part approach was planned. First, develop a conditioning  and vetting test series . Second, determine duty cycles to prevent coil overheating.
550 The conditioning tests were designed to gradually “burn in” the motors by incrementally heating and  allowing to cool in vacuum. Coil temperatures would gradually approach but be kept below manufacturer’s  rated value. This would theoretically allow “bubbles” in the wire coatings to work their way out gradually  through softened coating material without leaving permanent damage.  

A duty cycle is determined as the amount of time the motor is allowed to run in a specific environment  before forcing a rest/recovery period. The run and rest times  were, in this case, selected to limit the coil  maximum temperature, and cool to a minimum temperature selected by the team before running again.  Determining duty cycles were actually a by -product of the conditioning test series, and were established  and r ecorded along the way. This testing would also demonstrate each motor’s individual capability to  survive operation in a vacuum environment. Overall performance during the test series would be used as  a basis for ranking flight motor candidates.  A sample of  the motors final duty cycle is shown in Figure 9.  

Figure 9. Duty cycles for AMT flight motor candidates  at 25 °C and 35° C. 

The four motor candidates shown above were from the exact same vendor lot and were run with the same  max allowable coil temps and cool down targets, but the duty cycles ranged from 36% to 57% at 25 °C and  30% to 48% at 35° C. 

The previous use of thermocouples to monitor coil temperature was insufficient for closely monitoring the  coil temperature due to the thermal lag throu gh the motor casing, indium, and clamshell bracket to the  thermocouple. Instead, the team turned to resistance measurements. Coil temperature correlates to motor  impedance. Resistance of the coils was measured while the motor was off, and immediately being  energized. Resistance- based temperature measurement was calculated for each coil, and verified against  the chamber temperature prior to start of testing. More on this method is discussed in “Testing and Maturing  a Mass Translating Mechanism for a Deep Space CubeSat” from the 44th Aerospace Mechanisms  Symposium.  

For all motors not previously used in a vacuum, a standard vacuum bake- out was performed to reduce the  amount of material that would offgas during the duty cycle and burn in testing.  

Test plan   Three vacuum chamber temperatures were selected for duty cycle determination: room temperature   (25°C), max operational tempature (45° C), and a mid value  (35°C). Motors were mounted in a flight -like  manner , wrapped in indium and installed in EDU chassis; no  drive train hardware was included, allowing  the motors to freely rotate. Maximum coil temperature and minimum cool -down temperatures were  selected, and resistance was determined corresponding to each temperature. Two voltages were selected,  the first being at the low -end of controller capability, the second at expected mission level. In this mission  case, it was 9.8- 12.0 V. Multiple voltages were selected to test the sensitivity of the coil maximum
552   Figure 10. Duty Cycle  Data for two Stepper Motors  

Motor Burn- in   Following a duty cycle discovery run in a vacuum environment, the motors were run per the discovered  duty cycle until 1 5 minutes of motor run time had occurred. Resistances and times for run and cool -down  were recorded to verify that a long duration cycling would not overtemp the motor coils.  This burn in time  was used to help coil insulation material offgas at a slower, more controlled rate, and reduce any  opportunity  for an arcing event to occur . Further, the burn in time would reduce the thermal shock to the insualtion and  reduce the risk of voids forming in the insulation.  

AMT TVAC Test 3, Passed  

The final flight TVAC test plan included even more data capturing requirements: (1) Local temperatures on  AMT logged at 1 Hz, (2) Chamber control temperature taken logged at 1 Hz, (3) Chamber pressure logged  at 1 Hz, (4) Motor circuit current and voltage data logged at 5 Hz, (5) Control Board Memory and Co mmands  logged at 1 Hz and (6) Motor Winding impedances taken immediately after operation termination. These  data sets were combined into a master data set where temperature, current, pressure, and operation  speeds could be compared at each thermal cycle. An example of the combined sets is shown in Figure 11.
553   Figure 11. Combined data set . Note orange line documents X axis change in position. Duty cycle = 40%.  

The extensive measures taken to understand the details of the motors’ internal design paired wit h the  meticulous process of duty cycle determination and burn in testing yielded a long- awaited succesful test .  The third iteration of the NEA Scout AMT test passed with flying colors . The motors, though unable to  perform in vacuum under typical operation,  showed an ability to operate in vacuum for thousands of duty  cycles and an accululative run time of over 10 hours .  

All data points were reviewed in detail following the conclusion of test 3. A single engineer took a few weeks  to align time signatures ac ross each data source and compile the data to create tables and charts similar  to Figure 10 and 11. The data was reviewed and compared with the thermal environments in the test  chamber to inform the final control parameters for the flight motor controller boards . These data sets will  also be valuable to compare with flight data in the event AMT operation telemetry is reviewed or modified .  

Conclusion  

In conclusion, the AMT system TVAC test failures caused the team to develop new processes to determine  a motor’s duty cycle prior to environmental test with limited risk to flight hardware, instrument and collect  data for post -test analysis, and perf orm destructive analysis on failed micro gear motors. The motors used  on the NEA Scout AMT were determined not to be suitable for 100% duty cycle in vacuum, but a  conservative duty cycle enabled the AMT to operate and meet long- term mission requirements . The AMT  motors were instrumented to determine motor health in near real time. Previous failures during NEA Scout   AMT TVAC test always pointed back to an  unfit thermal conduc tive path, most likely due to under -insulated  magnet wire and poor thermal conduction from motor coils to a larger heat sink . It was hypothesized that  Paschen discharges could occur in a small stepper motor such as these if the coil temperatures rises and  offgasses enough material, though this hypothesis could not be recreated or confirmed through further  testing.
554 Major Lessons Learned   1. A Commercial off the Shelf (COTS) non space- qualified motor could  be used for flight purposes if  proper motor conditioning and duty cycling is determined in a comparable space environment   2. Any COTS non space- qualified motor should  be destructively analyzed prior to use to accurately  create a thermal model.   3. TVAC tests including DC motors should log motor temperature, power supply current, power supply  voltage and chamber pressure at 1 second resolution, at minimum.   4. Motor internal pressure could increase to a pressure range suitable for arcing, discharging or  ionizing if magnet wire insualtion breaks down due to overtemperature conditions, leading to failure.  

References  

1. McNutt, L.; Johnson, L.; Clardy, D.; Castillo -Rogez, J.; Frick, A.; and L. Jones. “Near -Earth  Asteroid Scout.” AIAA Space 2014 Conference; 4- 7 Aug. 2014; San Diego, CA; United States.   2. Few, A.; “Development of a High Performance, Low Profile Translation Table with Wire  Feedthrough for a Deep Space CubeSat .” 43rd Aerospace Mechanisms Symposia; 4- 6 May 2016;  San Jose, CA.; United States.   3. Few, A.; Lockett, T.; Wilson, R.; Boling, D.; and Loper, E.; “ Testing and Maturing a Mass  Translating Mechanism for a Deep Space CubeSat.” 44rd Aerospace Mechanisms Symposia; 1618 May 2018; Cleveland, OH.; United States.
555 Design and Development of the GPM Solar Array Drive  Assembly,   Orbital P erformance , and Lessons Learned 

Alejandro Rivera*, Glenn Bock*, Alphonso Stewart**, Jon Lawrence**, Daniel Powers**,   Gary Brown** and Rodger Farley+ 

Abstract  

The Global Precipitation Measurement (GPM) core spacecraft is a NASA/JAXA joint mission launched in  2014 . Developed in- house at NASA GSFC, it s deployable appendages include two large solar arrays each  driven by a single axis solar array drive assembly  and a gimbal equipped high gain antenna. Lessons  learned from the Tropical Rainfall Measuring Mission (TRMM) – Y Solar Array Drive Assembly ( SADA)   anomaly  and Lunar Reconnaissance Orbiter’s (LRO’s)  thermal testing,  influenced the design of the GPM  Solar Array Drive Assemblies . This paper describes the TRMM anomaly, design and development aspects  of the GPM SADA, its  on-orbit health and performance, and finally techniques and orbital maintenance  maneuvers followed by the Flight Operations Team to minimize drag, actuator cycles , and extend the life  of the actuators and the mission itself.  

Introduction  

The GPM  core spacecraft  is a joint mission between the National Aeronautics and Space Administration  (NASA) of the United States and the Japan Aerospace Exploration Agency (JAXA). It is a Low Earth Orbit  (LEO ) spacecraft launched on Febru ary 27, 2014 with a circular 407 km altitude and 65- degree inclination  orbit selected to provide full global precipitation coverage updated ev ery 24 hours . GPM succeeds the  TRMM spacecraft  launched in November 1997 that measured precipitation over tropical and subtropical  regions, from the Mediterranean Sea (35° north latitude) to the southern tip of South Africa (35° south  latitude) . Measurements from GPM core, however,  provide even greater coverage—between the Arctic  Circle (65° north latitude) and the Antarctic Circle (65° south latitude). These measurements, combined  with those from other polar -orbiting satel lites in the GPM constellation, currently  provide global precipitation  datasets every three hours. This integrated approach and unified dataset helps advance scientists '  understanding of Earth's water and energy cycle,  produces enhanced forecasts of hurricanes, floods, and  droughts , and helps enhance Earth’s climate models .  

Figure 1. The GPM Core (left) and TRMM (right) Satellites   ______________________ ___  * KBR Inc  – Space Engineering Division,  Greenbelt, MD   ** NASA Goddard Space Flight Center, Greenbelt, MD   + NASA Goddard Space Flight Center, Greenbelt, MD  (Ret.)  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center , 2020
556 The TRMM SADA Anomaly  

The TRMM satellite mission ended on April 8, 2015 after more than 17 years of very successful operation.  While the mission clearly exceeded its original three- year mission  goal, it experienced a minor  anomaly   involving  one of the So lar Array Drive Actuators . TRMM had two deployable solar arrays  designe d to track  the sun with help of a single axis SADA . A Schaeffer Magnetics rotary actuator identical to that used  on  NASA GSFC’s  X-Ray Timing Explorer (XTE)  mission drove the TRMM SADA, and High Gain Antenna  (HGA ) gimbal . The r otary actuator was a Type 5 ½ (T ype 5 modified  with an output bearing from a T ype 6  drive) . It had a 3- phase, 6- state stepper motor . The harmonic drive was a “silk hat” type with a pitch diameter  of 2.5 in (5 cm)  and a 200:1 reduction. The flexspline was made out of 304L S tainless Steel (SS) , the  circular spline and wave generator used 17- 4 PH SS and 440C SS respectively . Penn zane 2000 synthetic  hydrocarbon oil with 5% lead naphthenate additive was used as lubricant on the gear teeth and bearings .  An internal r otary incremental encoder with three absolute positions provided position, velocity, direction of  travel, and was capable of being used in open or closed loop mode [1 ].  

Figure 2. The TRMM SADA      The SADA also included a cable wrap used to transfer signal and power across the rotary joint. Seventy - six 20 gage wires were sewn together to form two belts that spiral around a central reel . The belts were  36 in (91 cm)  long by 2.5 in (5 cm)  wide and were separated using 0.005- in (27 µm)  Kapton to reduce  sliding frict ion. The cable wrap had a maximum travel of ±300 degrees but the designed use was  ±175 degrees where it could operate in the region of low friction. Finally,  the TRMM SADA included a main  deployment / shoulder hinge bolted onto the output face of the rotary actuator, which rotated the w ing  90 degrees from the spacecraft body with the help of constant torque springs and a rotary viscous damper  kept warm with st rip heaters and thermostats  [1].