Problems Encountered During the Recertification of the GLORY Solar Array Dual Axis Gimbal Drive Actuators Marc Saltzman (1), Joseph P. Schepis (2) , Michael J. Bruckner(3) X11 Sterling Dynamics LLC, 20475 Quarterpath Trace Circle, Sterling, VA, 20165, USA, Email: [email protected](2) NASA Goddard Space Flight Center, Code 544, 8800 Greenbelt Road, Greenbelt, MD, 20771, USA, Email: [email protected](3) Stinger Ghaffarian Technologies, Inc., 7701 Greenbelt Road, Greenbelt, MD, 20770, USA, Email: [email protected]ABSTRACT The Glory observatory is the current incarnation of the Vegetation Canopy Lidar (VCL) mission spacecraft bus. The VCL spacecraft bus, having been cancelled for programmatic reasons in 2000, was nearly integrated when it was put into storage for possible future use. The Glory mission was a suitable candidate for using this spacecraft and in 2006 an effort to recertify the two axis solar array gimbal drive after its extended storage was begun. What was expected to be a simple performance validation of the two dual axis gimbal stepper motors became a serious test, diagnosis and repair task once questions arose on the flight worthiness of the hardware. A significant test program logic flow was developed which identified decisions that could be made based on the results of individual recertification tests. Without disassembling the bi-axial gimbals, beginning with stepper motor threshold voltage measurements and relating these to powered drive torque measurements, both performed at the spacecraft integrator's facility, a confusing picture of the health of the actuators came to light. Tests at the gimbal assembly level and tests of the disassembled actuators were performed by the manufacturer to validate our results and torque discrepancies were noted. Further disassembly to the component level of the actuator revealed the source of the torque loss. 1. INTRODUCTION The Glory observatory will fly a three instrument suite to advance the understanding of aerosols and solar irradiance. The Advanced Polarimeter Sensor (APS) in combination with two cloud cameras will determine the global distribution of natural and anthropogenic aerosols and clouds allowing quantification of direct and indirect effects on global warming. This capability will significantly reduce the current 40% uncertainty of the effect of aerosols in the radiative forcing function. The second primary instrument, the Total Irradiance Monitor (TIM) will provide for continued measurement of solar irradiance to determine the Sun's direct and indirect effect on the Earth's climate. Fig. 1 shows the Glory observatory and a two-axis Solar Array Drive Assemblie (SADA). The SADAs are configured in a pan — tilt (alpha under beta) orientation. lcll^ 1411W ,gieo Figure I Glory Observatory and the Solar Array Drive Assembly In 2003, the Glory pre-program study phase identified the nearly completed spacecraft bus from the VCL observatory program as a viable option for accommodating the Glory instrument suite. Among many components that weathered the three year storage period between VCL cancellation and Glory program startup were the SADAs. As the SADAs were originally delivered in 2000 they will be approximately 9 years old once they are finally on-orbit in late 2009. While in storage the VCL bus was properly housed in a sturdy container with an inert Nitrogen cover gas. Nevertheless, the SADAs were thus identified as having a level of risk due to their age particularly with the uncertainty associated with how the Pennzane lubricant may have migrated or degraded during the long dormant period. In addition, the revised spacecraft configuration requires driving the solar arrays in a higher inertia configuration at higher speeds, raising additional questions about the output torque capabilities in terms of torque margin. https://ntrs.nasa.gov/search.jsp?R=20090033100 2020-04-15T14:40:29+00:00Z
Transcript
Problems Encountered During the Recertification of the GLORY Solar Array Dual AxisGimbal Drive Actuators
Marc Saltzman (1), Joseph P. Schepis (2) , Michael J. Bruckner(3)
The Glory observatory is the current incarnation of theVegetation Canopy Lidar (VCL) mission spacecraft bus.The VCL spacecraft bus, having been cancelled forprogrammatic reasons in 2000, was nearly integratedwhen it was put into storage for possible future use. TheGlory mission was a suitable candidate for using thisspacecraft and in 2006 an effort to recertify the two axissolar array gimbal drive after its extended storage wasbegun. What was expected to be a simple performancevalidation of the two dual axis gimbal stepper motorsbecame a serious test, diagnosis and repair task oncequestions arose on the flight worthiness of the hardware.
A significant test program logic flow was developedwhich identified decisions that could be made based onthe results of individual recertification tests. Withoutdisassembling the bi-axial gimbals, beginning withstepper motor threshold voltage measurements andrelating these to powered drive torque measurements,both performed at the spacecraft integrator's facility, aconfusing picture of the health of the actuators came tolight. Tests at the gimbal assembly level and tests of thedisassembled actuators were performed by themanufacturer to validate our results and torquediscrepancies were noted. Further disassembly to thecomponent level of the actuator revealed the source ofthe torque loss.
1. INTRODUCTION
The Glory observatory will fly a three instrument suiteto advance the understanding of aerosols and solarirradiance. The Advanced Polarimeter Sensor (APS) incombination with two cloud cameras will determine theglobal distribution of natural and anthropogenic aerosolsand clouds allowing quantification of direct and indirecteffects on global warming. This capability willsignificantly reduce the current 40% uncertainty of theeffect of aerosols in the radiative forcing function. Thesecond primary instrument, the Total Irradiance Monitor(TIM) will provide for continued measurement of solarirradiance to determine the Sun's direct and indirect
effect on the Earth's climate. Fig. 1 shows the Gloryobservatory and a two-axis Solar Array DriveAssemblie (SADA). The SADAs are configured in apan — tilt (alpha under beta) orientation.
lcll^
1411W
,gieo
Figure I Glory Observatory and the Solar Array DriveAssembly
In 2003, the Glory pre-program study phase identifiedthe nearly completed spacecraft bus from the VCLobservatory program as a viable option foraccommodating the Glory instrument suite. Amongmany components that weathered the three year storageperiod between VCL cancellation and Glory programstartup were the SADAs. As the SADAs wereoriginally delivered in 2000 they will be approximately9 years old once they are finally on-orbit in late 2009.
While in storage the VCL bus was properly housed in asturdy container with an inert Nitrogen cover gas.Nevertheless, the SADAs were thus identified as havinga level of risk due to their age particularly with theuncertainty associated with how the Pennzane lubricantmay have migrated or degraded during the long dormantperiod. In addition, the revised spacecraftconfiguration requires driving the solar arrays in ahigher inertia configuration at higher speeds, raisingadditional questions about the output torque capabilitiesin terms of torque margin.
The test approach evolved during the fall of 2006 withthe intent of measuring basic performancecharacteristics of the two stepper motors in each of thetwo SADAs for comparison against their baseline datarecorded in year 2000. The original approach was toship the SADAs back to Moog Chatsworth Operations,the manufacturer of the SADAs, for test. However,several programmatic factors eventually led to adecision to perform SADA retesting and verification atOrbital Sciences Corp.
The original performance characterizations at Moog'sfacility were measured at the individual actuator levelbefore assembling the two actuators (alpha and beta)into the completed biaxial gimbals. Except for directoutput torque measurements, all the actuatorperformance measurements could be performed byOrbital at the biaxial gimbal level. With specialfixturing, beta actuator output torque could be measureddirectly. Assessment of alpha actuator torqueperformance would have required separation of thebiaxial gimbals into separate actuators which wasbeyond the scope and risk Orbital was willing to bear.Accordingly, the test program was designed to use thepast (year 2000) and present (2006) alpha thresholdvoltage measurements as an assumed bridge forvalidating torque capability.
Threshold voltage is a measure of the design margin themotor has to start without losing synchronization. Thethreshold voltage measurements were understood to bea measure of whether or not actuator internal losseswere normal/in-family. The logic, thought now to beflawed, was that if the threshold voltage measurementsof the alpha and beta actuators were both within a smallfraction of the baseline values and the torque capabilityof the beta actuator was similarly consistent then thetorque capability of the alpha actuator could be assumedto be satisfactory as well.
3. TEST SETUPS AND INITIAL TEST RESULTS
Fig. 2 shows the electronics interface test schematic,and the predicted oscilloscope plots. The test dependedon being able to use the Glory flight Electronic DriveUnit (EDU) which has a normal input voltage range of22 to 35 volts. Voltage can be reduced below 22 voltsbut internal power supply and control circuits requireabout 16 volts to function properly. This is muchgreater than the baseline 7 to 8 volts across the windingsrequired to initiate and sustain movement of theactuators (i.e. threshold movement). For this reason thenormal voltage range had to be furnished to the EDUand the external circuit dropping resistors shown in the
schematic were used to drop the output voltage acrossthe windings. The fixturing for threshold voltagetesting was such that both actuators were free to turn.Tab. 1 shows the threshold voltage test results.Inspection of these results shows that the new thresholdvoltage results were in-family with those recorded byMoog in 1999/2000. At this point it appeared testingwould soon result in SADA re-certification for flight.
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Figure 2 Threshold Voltage Testing
Fig. 3 shows the setup for ambient torque testing at fullflight driver voltages. The test arrangement included aHimmelstein torque transducer and a Planetrol gearreducer to amplify the effect of the Placid Industrieshysteresis brake. Flexible couplings and carefulalignment ensured there were no misalignment inducedtorque errors. The torque versus time plot of Fig. 4
shows a typical loss of synchronization (torquedropout). The simultaneously generated position versustime plot shows the same dropout. The objective was tofind a brake induced torque resistance which the
Figure 3 Torque Test Setup
•0
1
a
Table I Threshold Voltage Measurements in 199912000 (Moog) and 2007 (Orbital)
actuator could drive through the entire range of motion.This was defined as the actuator's torque capability atthe particular operating speed utilized.
Torque capability in the 150 in-lb to 190 in-lb(16.9 N-m to 21.5 N-m) range was produced prior toshipment but, as demonstrated by the plots in Fig. 4,there was either 30% degradation in torque or a seriousflaw existed in the test apparatus. At least two dayswere spent checking the calibration of the torque testsetup. The final test results for both SADAs are shownin Tab. 2. In this table, "N/A" indicates theconfiguration could not be tested without disassembly.
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Figure 4 Torque Test Data
According to our calculations, the threshold voltage testshould have had sufficient sensitivity to detect internal
loss changes on the order of 30%. This mysterioustorque loss was very perplexing. Moog was contactedto validate our torque testing approach, and they didvalidate it. Despite test setup confirmation indicatingour torque measurement approach was not flawed, thesentiment at the time was that something was amisswith the setup and that the SADAs were, most likely,acceptable. A large contributor to this sentiment wasthe fact that the threshold voltage measurements werevery reassuring. Nevertheless, NASA and Orbitalagreed to follow the logic flow of our recertificationdecision tree and send the units back to Moog for a setof confirmation tests. At the time, it was assumed thatsome detail of the test setup or apparatus was flawedand that the inconsistency between the threshold testdata and the torque test data would be resolved whenMoog retested the actuators using the original testequipment.
Figure S Moog Test Actuator Mounted in Altered GlorySADA Test Apparatus
NASA, Orbital and Moog worked together closely andan approach was quickly derived to carefully measurethe torque of a non-flight test actuator at Moog's facility
OqtFsolar Array -0Inertia
Simulator 150%
Table 2 Torque Measurements in 199912000 (Moog) and 2007 (Orbital)
Moog @ 32 VDC 176 176 184 188Glory @ 28 VDC NA 113 NA 145
Percentage lowerthan Moog min.
-33% -23%
using their test equipment but testing across the entirerange of operation in the way Orbital had tested theSADAs. While this was accomplished at Moog, Orbitaladapted their torque test setup to accept the Moog testactuator shown in Fig. 5. The Orbital torque results forthe test actuator were in close agreement with thosefrom Moog indicating approximately 30% torque lossand thus adding even more concern that the SADAshad, in fact, degraded over the past several years ofstorage and occasional use with Glory integration andtest activities. The original test program wascompleted and an executive test summary written todocument the results.
4. INERTIA TESTING
While not part of the original test plan, inertia testingwas added to the test program because there wasconcern that the speed of 2°/sec available for resettingthe solar array positions during eclipse might be toofast. As such, the actuators could lose synchronizationand lose steps or possibly stall. The test setup used forinertia testing was similar to that shown in Fig. 3except that the brake was replaced with a solar arrayinertia simulator as shown in Fig. 6. A typical plot ofangular position from the actuator potentiometer (thesecondary was usually used) versus time is shown inFig. 7.
spokes to adjust inertia. While the test increasedconfidence that the inertia can be started at 2° /secondthe amount of backlash in the test system precludedcomplete certainty of that conclusion. The majority ofbacklash was attributed to the test setup gear reducerand not in the SADA itself. The presence of backlashis non-conservative with respect to starting the inertiabecause more time is available for the momentum toincrease which means the torque required from theactuator can be lower.
Figure 6Inertia Testing Setup
The inertia used in this test setup was 150% of the solar To achieve a more conservative, test the actuator speed
array inertia that the Alpha actuator will be required to was increased to 40/second and then 6°/second. In all
drive during each orbit. The inertia wheel consisted of cases the inertia was started without any lost steps.
6 masses which could be positioned using threaded Additional test were conducted with the backlash
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Figure 9 Harmonic Drive Wave Generator Bearingwith Defect Areas
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Figure 8Initial Steps in a Detailed Test Strategy
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manually removed by pre-rotating the inertia load priorto starting the actuator at 2°/second.
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Figure 7 SADA 2 Inertia Test Record
5. FAILURE INVESTIGATION
The test services agreement with Moog was expandedto include return of the worst case SADA S/N 1 toMoog for testing followed by SADA S/N 2 if thetorque findings continued to evidence degradation.This was a small part of a detailed test strategy, whichis shown in Fig. 8.
CCW SARA 2lnertia ambient
—Run 2 CCW SARA 2 Inertia ambient
—Run 1—Run 1 CW SADA2lnertia ambient
—Run 2 CW SADA 2 Inertia ambient
80 1
smooth. An attempt was made tomechanically remove the discoloration using apointed plastic stick but this was notsuccessful.
SADA 1 was returned to Moog in January 2008 andsubsequently tested with the following activities andobservations:
a. The alpha drive showed no degradation from1999.
b. The beta drive showed the 30% to 40%degradation measured by Orbital.
C. The SADA 1 motor stator housing wasremoved allowing the rotor/harmonic wavegenerator to remain installed. Moog observedthat rotor rotation was `lumpy' with repetitivecyclic torque peaks of approximately 6.5 inch-ounces (45.9 mN-m) versus the Moog
threshold voltage and torque testing for compari sonwith the results recorded earlier by Orbital. Theseresults also placed in question the validity of using
that all is well with an actuator.
requirement for new hardware of 2 inch-ounces (14 mN-m).
d. There remained a good quantity of grease/oilslurry present in the wave generator bearing.
e. The amber colored lubricant inside theharmonic drive wave generator bearing wassomewhat darker in places than the shade fornew lubricant (50% Pennzane oil & 50%Pennzane grease).
f. The cleaned wave generator bearing races hadsmall patches of discoloration with a grainyappearance from either embedded material ora divot. These are shown in Fig. 9. Threesuch patches on each side were located on theinner race at the major diameter areas andcoincided with the spacing of the balls. Theremaining portions of the races were clean and
Sim
SARA 2 was returned to Moog and retested on1/30/2008. The beta drive showed 20% to 30% outputtorque degradation. The beta actuator wasdisassembled and showed the same discolored areasshown in Fig. 9, but not quite as prominently. Prior todisassembly the actuators were subjected to both
threshold voltage testing as a conclusive verification
At this point the flow path led NASA, Orbital andMoog to the decision to disassemble the wavegenerator bearings from the beta actuators and subjectthe races and balls to laboratory testing. After twoseparate investigative sessions at Seal Laboratories,Moog's assessment was that the discolored areas were,in fact, pitted areas most likely due to stress corrosion.Fig. 10 shows a visual summary of the typical fmdings.Except for a very minor amount of discolorationobserved on the alpha actuator of S/N 1, the alpha
engineering team. The individual actuators werereassembled and subjected to a complete series ofacceptance tests including two axes of vibration andtwo thermal vacuum cycles. The actuators werereceived at Orbital in early June 2008 and integratedonto the Glory observatory in early July. Fig. 11shows the SADAs successfully integrated onto theGlory observatory during solar array deploymenttesting.
8. LESSONS LEARNED
• A little electrolyte, a little oxygen, and lots ofstorage time can lead to a problem.
• Retesting mechanisms after a long storageperiod (several years) is highly recommended.
Figure 11 SADA Integrated onto Glory during SolarArray Deployment Testing
Use careful controlled storage with an N2cover gas and periodic sampling when storingsensitive mechanisms for extended periods.The mechanisms community needs to take amore careful look at the use of thresholdvoltage testing — it didn't appear to bepredictive for the Glory SADAs."Engineering curiosity" and vigilance arecritical attributes to cultivate in the act ofproblem solving.
bearings looked good — but the fact that there was evena small amount of discoloration made them suspect.
6. ROOT CAUSE DETERMINATION
Moog determined that the harmonic drive supplier hadused a chlorinated solvent as part of the cleaningprocess. This practice has long since been prohibitedbut at the time the VCL SADA components weremanufactured the process was still in use. It is thoughtthat some small amount of this solvent had not beenfully washed away. This in combination with thepresence of sodium in the grease thickener providedthe constituents for a salt solution only awaiting thearrival of small amounts of water vapor, the presenceof oxygen, and, lastly, the availability of lots of timefor the corrosion process to attack the bearing surfacesunder most stress. The elliptical wave generator plugprovided the stress. The evidence for the presence ofchlorine and sodium is shown in the electron dispersionspectrographic (EDS) analysis portion of Fig. 10. Until2004, the SADAs were stored with the VCL busbagged and with an N2 cover gas but it was neverconfirmed that the cover gas had not dissipated, thatthe bag was entirely leak tight, or that trace amounts ofmoisture did not exist in the purge gas. In addition,after 2004, the SADAs were exposed to the atmosphereas they were required for integration and test purposes.There remains the mystery as to why the alphaactuators were not nearly as degraded as the betaactuators. Nevertheless, the root cause was concludedto be stress corrosion due to the incomplete cleaning ofthe harmonic drive components leading ^presenceof small amounts of chlorine, sodium, water vapor,oxygen, and lots and lots of time.
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Figure 10 Laboratory Investigation Findings
9. CONCLUSIONS7. REASSEMBLY AND TEST
Based on the aforementioned findings, Orbital revisedthe Moog service agreement to include completereplacement of all harmonic drive bearings in bothSADAs 1 & 2. While not contributing to thedegradation the cotton phenolic wave generator bearingretainers were vacuum impregnated using an extendedtime of 3 days as recommended by the NASA
As of June 2009, the Glory SADAs continue to operatenominally as the Glory Observatory continues forwardtowards launch. The SADA test program lasted a totalof 14 months from the first day of retest at Orbital(April 2007) until receipt of the refurbished SADAsfrom Moog (June 2008). From a pragmatic standpointit would have been better to have sent the SADAs back
to Moog as soon as it was decided that re-verificationwas required. However, the overall experience gainedworking more intimately with these interestingcomponents and later working with the professionals atMoog was as stated in a popular advertisement,"Priceless". It is felt that our organization's (NASA)ability to specify, procure, and operate thesemechanisms have been enhanced. The authors are alsoreminded of the importance of vigilance with respect toprocessing and contamination control at the componentand Observatory levels as well as the importance ofmaintaining good documentation for potential usewhen problems arise.
ACKNOWLEDGEMENTS
The authors would like to express their gratitude toDan Owens and Dan Hogan at Moog ChatsworthOperations who contributed so much to the loss oftorque investigation and timely re-assembly/return ofthe Glory Solar Array Drive Assemblies. Many thanksare also extended to Nasim Khawaja, Greg Yinger, andthe other Orbital manufacturing personnel for the rapidturn-around on the various test fixtures. And manythanks to Art Pokorny for his electrical engineeringhelp and Senior Electrical Technician, Rich Protzmanfor his expertise and resourcefulness in rapidly fieldingsolutions to electrical test setup challenges.
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