ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
352   Figure 8. Oscilloscope trace from noise sensitivity test using 1Ω r esistor (schematic on left) 

Figure 9.  Test interface screen capture of resistor sensitivity test.  Baseline shift in resistance is  evident at (A), and peaks associated with transitions in (B) with all variously filtered noise traces  responding. In the resistance trace (A) the baseline resistance shift is 1 ohm, in the noise trace   (B) the peaks are sev eral ohms.  

Current Level Effects   We found no effect of current on noise in our testing. At the low current levels tested here,  it was not possible  to fully characterize noise at our mid and low range currents. We did show that we were able to resolve  noise levels over threshold  levels that are a function of the applied current itself ( Table 3). We did not find  any instance of noise levels above these thresholds for any lubricant, environment, or rotational speed tested. While we do see an apparent increase in noise l evels for lower currents, as in, we cannot confirm  these are real as the background noise levels are of the same magnitude as th e measured noise.  

   B  A
353 Role of Environment  Pressure  While the neat oil (2001A) and formulated oil (2001) results did not indicate differences between operation  in air and vacuum , the 2001- 3PbNp formulation did. Figure 12 is a graph of the  extended 1- kHz filtered  noise measurement for the UUT in both air and vacuum with 100- mA DC current applied. We found that  the lead naphthenate produced higher noise lev els in vacuum at our highest speed levels but performed  better than the other formulations at lower speeds ( Figure 13). The cause of these differences in noise  performance is not investigated as part of this study.     

Figure 10.  Peak -to-peak noise as a function of time for various surface speeds (rotation rates) and  ring lubricant. These graphs show the instantaneous peak to peak noise (over 0.2  s, solid lines)  and the extended peak to peak noise (over 10  s, bars). Data are presented filtered using a low  pass 10- kHz filter, 1 -kHz filter, and 100- Hz filter.
355   Figure 13.  Box and whisker plot of air and vacuum extended noise results for NYE 2001 -3PbNp  100-mA tests  at 22, 2.2, and 0.22 mm/s . In the 22 and 2.2 -mm/s cases the vacuum noise was higher  than the noise in air, in the 0.22 -mm/s case the opposite is true, the vacuum result has lower noise  than the air result. All statistical tests to a 95% confidence level via analysis using Student’s  t-test. 

Figure 14.  Peak -to-peak RMS noise through  1-kHz low pass filter for both 100  mA and 10 mA
356 Table 3.  Peak to peak RMS noise result for each lub ricant /current/rate configuration. It should be  noted that at current levels below approximately 1- mA background (environmental) noise  dominates the slip ring noise signal. The values presented in this table for the lower currents  tested are below the unfiltered threshold noise levels in Table 2, this was achievable by  incorporating low pass filters, in the case of the results in this table the filter used was 1  kHz. 

Figure 15.  Peak -to-peak RMS noise (from Table 3) for the three lubes studied. Noise was  measured in hard vacuum with 100- mA nominal current applied.  

  Lube Formulation Nominal Current 630/6.0/22 63/0.6/2.2 6.3/0.06/0.22 0.63/0.006/0.022 NYE 2001A 100mA DC 0.00178 0.00143 0.00080 0.00026 NYE 2001A 10mA DC 0.00198 0.00176 0.00154 0.00139 NYE 2001A 1mA DC 0.0127 0.0127 0.0126 0.0126 NYE 2001A 100µA DC 0.120 0.122 0.123 0.120 NYE 2001A 10µA DC 1.25 1.29 1.28 1.26 NYE 2001 100mA DC 0.00131 0.00084 0.00043 0.00020 NYE 2001 10mA DC 0.00195 0.00168 0.00147 0.00136 NYE 2001 1mA DC 0.0122 0.0124 0.0123 0.0122 NYE 2001 100µA DC 0.118 0.120 0.119 0.118 NYE 2001 10µA DC 1.22 1.24 1.24 1.23 NYE 2001-3PbNp 100mA DC 0.00334 0.00106 0.00048 0.00020 NYE 2001-3PbNp 10mA DC 0.00453 0.00171 0.00149 0.00136 NYE 2001-3PbNp 1mA DC 0.0132 0.0124 0.0123 0.0122 NYE 2001-3PbNp 100µA DC 0.119 0.119 0.119 0.119 NYE 2001-3PbNp 10µA DC 1.24 1.25 1.25 1.24Rot.Rate (mrad/s)/rpm/Surf.Speed (mm/s)Measured Slip Ring RMS noise (1kHz low pass filter)
357 Conclusions  

We found that of the three lubricant formulations tested, NYE 2001 perform ed the best in terms of noise.  NYE 2001A (neat Pennzane) was a close second at our highest surface speed (22 mm/s) , while the  formulation of 2001 containing lead napthalate was the worst in terms of noise at that speed. At our  intermediate surface speeds , the 2001- 3PbNp was better or the same as 2001A and at our slowest test  speeds it was nearly identical ( Figure 15). We also found that while 2001A and 2001 noise performance  did not change to a measureable degree between ambient conditions and hard vacuum, the 3PbNp  formulat ion did, with noise increasing in vacuum by  a small but significant amount at our higher two test  speeds (Figure 12) but better at speeds 0.22 mm/s and lower.    No measurable slip ring noise was found at currents less than 1 mA for any configuration tested. Put another  way, RMS 1 -kHz low-pass slip ring noise was less than 0.125 ohm  at 100- µA levels and less than 1.3 ohms  at 10- µA levels.    All tests conducted in this study followed the trend of noise levels reducing with decreasing surface speed (rotation rate) . In most cases tested the peak -to-peak noise levels at our lowest surface speed   (0.0022 mm/s) were indistiguishable from a static slip ring.  

References  

1. Koss, Steve and Woolaway, Scott, “Lessons Learned From the Windsat BAPTA Design and On- Orbit  Anomalies”, 38 th Aerospace Mechanisms Symposium, (2006), pp 209- 222.  2. Courtois, C., Miller, M. et al. “Advanced Slip Ring Solutions (ASR)”, 14th European Space Mechanisms  & Tribology Symposium –  ESMATS, (2011), pp. 313- 317.  3. Mondier, J.B., Sirou, F., and Mä usli, P.A., “Life  Test of the Scarab Instrument Slipring Units”, 9th  European Space Mechanisms & Tribology Symposium –  ESMATS, (2001), pp. 99 -106.  4. Feusier, G., Mä usli, P.A., and Gass, V., “Improved Characteristics of Slipring Assemblies Making use  of Gold on Gold Metallic Contacts”, 10th European Space Mechanisms & Tribology Symposium –   ESMATS, (2003), pp. 169- 175.  5. Pitney, Kenneth E., Ney Contact Manual: Electrical Contacts for Low Energy Uses, J.M. Ney Company, 1973.   6. Holm, Ragnar, Electric Contacts: Theory and Applications, Springer, Berlin, 3 rd printing, 2000.
© 2016 Sierra Nevada Corporation   359 Slip Ring Electrical Anomalies Found After Protoflight Vibration Testing  

Troy Nilson *, Scott Christia nsen*  and Chad Hebert* 

Abstract  

Following protoflight component testing of a Solar Array Drive Assembly (SADA) , post-test electrical  measurements revealed shorted circuits caused by slip ring brush displacement . Initial investigation,  disassembly and analysis suggested the root case was related to insufficient preload in an axial bias  spring constraining the slip ri ng rotor.  Successful rework and re- test supported this conclusion. However ,  later tests of similar hardware for a subsequent program resulted in a  comparable  electrical anomaly .  Original investigation results were review ed and found to still be valid. H owever, additional contributing  factors were identified that led to the reoccurrence of the anomaly.  This paper explores  the discovery,  investigation, analytical approach, and ultimate resolution of a SADA slip ring anomaly .  

Introduction  

Over the past ten years, Sierra Nevada Corporation’s  (SNC) Space System s group has designed,  built  and successfully flown a highly reliable and robust SADA (Figure 1)  capable of transferring several   kilowatts across a structurally integrated rotating interface connecting large solar arrays to spacecraft .  

Figure 1.  SADA System w/ Electronic Control Unit   Commonly experienced with high -reliability , low-volume production, the path to accomplishing this  success has been achieved through desire for quality and focused efforts while working through design  and test challenges. These challenges guided SNC through a series of events that allowed the  engineering team to develop a highly refined SADA design and design process that optimizes the use of  analysis tools such as high -fidelity Finite Element Modeling ( FEM ) to ensure design success. This paper  explores  the discovery, investigation, analytical approach and ultimate resolution of a SADA slip ring  anomaly .                                                  * Sierra Nevada Corporation, Louisville, CO  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016   OUTPUT  ASSEMBLY MAIN HOUSING  ASSEMBLY   w/ SLIP RING ASSEMBLY  MOTOR   ASSEMBLY   Electronic  Control  Unit
© 2016 Sierra Nevada Corporation   360 Environments and Hardware Description  

To help understand the nature and subtle nuances of the SADA slip ring anomaly, it is necessary  to  understand required operat ional and test environments as well as discuss the various SADA  subcomponents  and related design goals . SNC was contracted to develop an electromechanical rotary  device (and related Electronic Control Unit ) designed to structurally support and rotate a greater than   20 kg-m2 inertia payload ( i.e., large  solar array panel) . The design was required to survive a 7-year  mission in a high- vacuum space environment  with thermal extremes  ranging from  -30ºC to + 70ºC. Of  particular interest to this discussion, the SADA was also required to withstand structural loading  associated with random vibration testing  and endure a thermal vacuum life cycle test of more than eighty   thousand rotational cycles without slip ring electrical degradation or other electro mechanical f ailure.   Examples of the test setup, configuration , and levels  are shown in Figure 2 and Figure 3,  

Figure 2.  Photos of SADA Hardware Setup for Vibration Testing  

Figure 3.  Specified  Vibration Test Spectrum   Lateral Axis Test  Vertical Axis Test  Cable Bracket  SADA Mounting Interface (slip  ring and motor are suspended,   unsupported , inside blue fixture
© 2016 Sierra Nevada Corporation   361 At first glance, t he noted g-levels do not appear to be unusual or exceedingly difficult design  requirements . Standard design practices pre-anomaly were to ensure design integrity with positive  margins  up to 3- sigma peak accelerati on loads . However, later evidence demonstrated  this protoflight test  profile , when applied to the SADA mounted in a very stiff test fixture,  combined to produce SADA  resonant  modes  that resulted in unexpectedly high ( i.e., statistically low  probability)  input  loads . In fact,  resonant modes were later discovered to excite SADA components to as high as 2 to 3 times  the control  Grms input level.  Additionally , these high res ponses contained greater than 6- sigma peaks during  component vibration tests.  

The SADA, illustrated in Figure 1, consists of four main subassemblies including the output, main  housing, slip ring, and motor . The output assembly has a payload attachment interface that incorporates   a set of duplex bearings designed to carry  radial, axial and moment loads of the attached payload . The  slip ring assembly shown in Figure 4 and Figure 5  consists of four brush block assemblies each with a  series of gold alloy  wires  (i.e., brushes)  suppor ted and attached to printed circuit boards. Each brush  rides in individual v -grooves machined into the slip ring rotor and each circuit/set of ring grooves (1 to 5  grooves per group) is physically and electrically insulated from each other with a raised insulation barrier  as shown in Figure 5. This slip ring assembly is supported between the output assembly  and another set  of trailing bearings within the main housing assembly. At the opposite end of the SADA , away from  the  output assembly, the geared stepper motor is mechanically grounded to the main housing assembly and  coupled to the output assembly torque transfer shaft passing through a clearance hole in the slip ring  rotor. The com pleted asse mbly weighs approximately 12 kg and measures approximately 20-cm outer  diameter by  23-cm long.  

Figure 4.  Slip Ring Assembly w/ Exposed Rotor and Brushes  

This configuration incorporates several competing design objectives  including  meeting requirements to  support very high moment and axial loads between the SADA mounting flange and the solar array  attachment point while output ting very high stiffness  as well as  minimiz ing output drag to address a low  disturbance t orque performance requirement. The solution was to incorporate a high- precision,   moderately -preloaded duplex bearing mounted as close to the payload as possible. These trades lead to  a cantilevered design optimized for  launch and on- orbit mission  requirements  but less desirable for  component -level vibration tests  and related SADA test boundary conditions .  Slip Ring Assembly   (w/ SADA Output)  Brush Block  Assembly   Exposed Slip  Ring and Brushes   Slip Ring Brushes   Left: Power Groups   Right:  Individual Signal
© 2016 Sierra Nevada Corporation   362 

                       Figure 5.  Illustration Slip Ring Rotor Ring Grooves and Brush Orientation  

Other competing design goals  central to the SADA slip ring design are related to brush preload . The  relatively high cycle life requirement was best achieved through lightly preloaded brushe s where lower   forces would result in less drag and less wear at the contact point between the slip ring brushes and the  rotor.  Reduced brush drag and wear increases  drive torque margins and decreases  debris generation  within the mechanism . Efforts to limit debris generation over time minimized  contamination risk to other  mechanical components within the SADA (such as bearings ) and helped lower  brush electrical stick/slip  noise by allowing  the brushes to remain solidly in contact with and electrically bonded to the slip ring  rotor.  

In contrast, the requirement to sustain random vibration loads was better addressed through higher  brush  preload. Higher  preload provided increased margin against brush/rotor gapping during vibration testing  and increased chances that the brushes would rem ain in assigned groves , keeping  them isolated from  neighboring rings  to prevent unacceptable short circuits. Increased brush preload also minimized brush  contact resistance and sliding contact  noise.  

While these competing design goals were challenging, SNC optimized the design, balanced these  competing requirements and ultimately  developed and manufactured  a highly reliable and viable  mechanism. The design has been integrated into several spacecraft and has several years of  on-orbit  mission success despite various challenges related to a  short circuit anomaly  that occurred during  component -level testing.  

Anomaly Discovery    During a standard post -test electrical check  after a typical random vibration protoflight  test of a newly  developed and built SADA, SNC encountered the first of two key events  associated with the slip ring short  circuit  anomaly . An electrical circuit isolation test confirmed s everal slip ring circuits had  shorted together .  Following this discovery, various other post -test functional checks  were performed. The successful  completion of these performance tests  confirmed the SADA had no physical damage to any other  components and, aside from the electrical anomaly,  was operating as expected.  

Slip Ring  Brushes  Slip Ring  Rotor   Grouped Power Brushes  Individual Signal  Brushes   ← Rotor Inner Diameter  / SADA Rotational Axis  → Slip Ring Brush  Blocks (4X)  Slip Ring Brush   Barriers
© 2016 Sierra Nevada Corporation   363 Through ext ensive design and data review, analysis, and disassembly  of the SADA, SNC confirmed a  random selection of  slip ring brushes had ‘jumped’ across the rotor electrical insulation barrier and moved  from assigned rotor grooves to neighboring grooves . The result  was an anomalous ring- ring short circuit   that caused the  isolation test fault . Figure 6 shows a 3D radiographic image of displaced and shorted  signal  brushes.   

Figure 6.  Radiographic Image of SADA Slip Ring with 4 Shorted Signal Brushes    This image was derived from 3D X -ray imaging processes and techniques not commonly available at the  time of the initial investigation . However, the image is presented here because it clearly illustrates the  condition of the brushes in a similar short circuit condition. S ignal brushes are shown misaligned with the  slip ring rotor grooves as they appear axially along the slip ring rotor. The right side bottom view of Figure  5 is a sketch of t he desired alternating pattern for signal brush position. Four signal brushes shown in  Figure 6 do not follow this pattern , (RED arrows) indicat ing movement away from their assigned grooves   and caus ing a short circuit between adj acent rings .   Due to the enclosed  configuration of the SADA , visual confirmation of the slip ring rotor and brush  condition was not feasible. Prior to the application of this 3D X -ray imaging technology, other methods  such as standard electrical isolation tests and resistance measurements were used to confirm the initial  slip ring  fault.  Electrical isolation  test results allowed the team to adequately identi fy the precise circuits  involved  but they did not provide enough data to confirm  which brush moved to which ring. To collect the  additional data required, the team utilized  high-fidelity resistance measurements of each identified  anomalous slip ring circuit pair . By comparing measured parallel resistance to historical values, the circuit  with the displaced brush( es) could be identified by higher than expected resistance readings.    After testing was conducted to  identify the presence  of the displaced brus hes, it was found  through further  data review and analysis that the slip ring rotor preload spring force was  not providing enough holding  force to axially stabilize the rotor during random vibration testing. The investigation team believed this  insufficient rotor preload allowed the rotor to  “gap” and axially oscillate during test. These oscillations  resulted in very high undamped impact loads each time the rotor re- seated aga inst internal axial  hardstops. This rotor impact imparted forces into brushes beyond established preload that, when  combined with resulting complex nonlinear br ush dynamics , caused the brushes to break contact with the Brush Tips Positioned Correctly   Desired Position of Brush Tips   Out-of-Position Brushes   Noted brushes (RED  arrows) shifted down  away from assigned  grooves   Motor End   Output End  Signal Brushes  Rotor Grooves
© 2016 Sierra Nevada Corporation   367 Root C ause:  Required protoflight  random vibration test levels  and associated test boundary conditions  uncharacteristic of flight environments created response loads that exceeded the design capability of the  SADA brush configuration.  

Figure 8.  Data plot of t est correlated FEM  for lateral Z -axis response  

      Figure 9.  Data Illustrating SADA Primary Structural Modes Aligned with Rotor Shaft Modes  

Anomaly Resolution    The engineering team initially considered various options to physically modify the SADA design and  address the likely  recurrence of shorted brushes . Analysis efforts began to assess whether -or-not  modifications to stiffen primary structural parts of the SADA (i.e., housings, supports, etc.)  would help shift  and separate the SADA primary structural mode from the peak brush mode. Without significant impacts to  the SADA envelop e and mass , FEM analysis results confirmed this was not a viable option. The team
© 2016 Sierra Nevada Corporation   370  

Figure 11.  Cumulative Brush Force ( RMS ) – Force Limiting FEM Predictions  (Lateral Axis)   The engineering team now had a nicely correlated FEM, a strong understanding of slip ring brush  behavior, and recent experience with force- limited vibration test challenges. The desire for a new follow - on program was to utilize the FEM to analytically just ify modification to existing vibration test requirements  and ensure input levels remained well below brush gapping threshold of the exiting design. Several scenarios were evaluated during prior investigations while others were run to help assess new option s  and find a preferable compromise. Table 2 summarizes results from a few scenarios along with brush  preload predictions. While analytical values do not quite achieve the gapping factor design goal of 6 sigma or greater, significant improvement can be seen in the gapping factor column.    This improvement was  possible b ecause brush gapping problems were well understood from prior  investigations. This knowl edge led to a slight but crucial change to the as -built brush preload that simply  involved  narrowing the preload tolerance window . Confidence that the  as-built SADA included these  higher preloads  was gained by  measuring brush preload at both the slip ring subassembly level and after  integration with the SADA. This tolerance change resulted in a 25%  increase over the minimum allowable  brush preload previously specified, minimized the extent of  random vibration profile tailoring  required,  and  preserved the validity of the qualified SADA design. Implementation of this change left the remaining task  of defining and justifying an acceptable random vibration test profile modification.   Table 2.  Examples of  FEM Brush Gapping  Resul ts with  Margin Estimates  

 Random Vibe Environment  FEM Brush Preload Prediction   (grams)  Minimum  Allowable  Brush  Preload  (grams)  Gapping  Factor  (σ)  Z Axis   (lateral)  X Axis  (lateral)  Y Axis  (axial)  Z Axis   (lateral)   550 Hz roll -off, Protoflight   (brushes shorted during test)  1.6 1.1 1.2 4.0  5.1* 2.5  3.2  550 Hz roll -off, Force Limit  30G  (not tested)  1.2 1.0 0.9 4.0   3.3 

350 Hz roll -off, Protoflight/GEVS   (passed tests)  1.0 1.1 1.0 5.0  5.5* 5.0  5.5  *Measured sample average brush force  

The prevailing opinion of design reviewers  was that any modified profile needed to closely match the  required protoflight level  and deviations from these levels had to remain above component minimum  workmanship standards to comply with accepted industry standards defined in NASA technical standard  PSD (g2/Hz) 

Brush Force RMS 

Frequency  (Hz)  First energy peak in  brushes aligns with SADA  first mode near 650Hz   Peak energy in brushes  aligns with SADA second  mode near 1600Hz