ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
191 Lessons Learned  For EMA testing, it was necessary to first command the actuator away from null and then to displace the  duty cycle accordingly. This was done because the type of LVDT used in this particular actuator created a control discontinuity as the actuator passed through null. The preferred solution would have been to change out the LVDT for one more optimally suited. However, schedule constraints did not allow for the exchange of this LVDT, so it was decided to simply avoid duty cycles that commanded the actuator through the null position.  Figure 11. Discrete Frequency Duty Cycle    The discrete frequency response duty cycle shown in Figure 11 was the other programmed duty cycle used. It was generated to assess the frequency response of the integrated EMA TVC system. This duty cycle consisted of a set of 17 sinusoidal waveforms covering the frequencies 0.5 Hz, 1.0 Hz, 2.0 Hz, …, 16 Hz. Unfortunately, a fixed number of 30 cycles was programmed for each frequency. The preferred method would have programmed a fixed duration for each frequency so that the number of data samples acquired would be equal. The duty cycle used resulted in a broadening of the individual spectral response curves with increasing frequency. Although this error was not identified at the time of testing, sufficient spectral data was present in the recorded command and response signals to perform the needed assessment.  To each of the 17 individual sinusoidal waveforms of the discrete frequency duty cycle a Hanning window was applied, creating a smooth, gradual rise in amplitude to a maximum followed by an equally gradual reduction of amplitude. A wait time of 1 second was introduced between each waveform. This technique was used as a result of lessons learned during a previous modal assessment in which it was observed that whenever abruptly starting and stopping sinusoidal waveforms an impulse component was introduced with its associated broadband excitation. This technique was verified by Fourier analysis to be a very effective means of applying monochromatic (single discrete frequency) excitation.  The recorded command signal and the actuator response, acquired via LVDT, were analyzed by applying a discrete Fourier transformation individually over time intervals corresponding to each successive frequency in the duty cycle. The highest four frequencies (13 Hz, 14 Hz, 15 Hz and 16 Hz) were discarded because the transform of the response did not meet the necessary criteria for inclusion. The  coefficient of variation (⁄ߤߪ=ܸܥ representing the ratio of the standard deviation to the mean of a  computed discrete Fourier transform, was used as this criterion. Only those responses for which the  calculated CV was greater than 1 were selected.
192 Results and Conclusions 

All test objectives were accomplished without any unexpected outcome. Functional integration of all developmental ETVC components was performed. Controller parameters were adjusted for smoother operation and to account for differences due to the larger inertial load at the MSFC laboratory facility compared to that at Moog facilities. This need provided an opportunity to demonstrate the ability to tune the controller “in the field”. Peak power draw and voltage droop was demonstrated to be controlled to acceptable levels as was repeated operation on a single battery charge.  The dynamic behavior of the EMA system was determined for step and frequency response. Figure 12 shows the response of the EMA actuator to a large commanded step in the extend direction under the test stand inertial load. This step occurred at the start of the aggressive ETVC Demonstration Duty Cycle. Response as recorded by the LVDT and the optical angular encoder, which provided a much cleaner signal, are plotted along with the recorded command signal.  

Figure 12. Step response of EMA actuator  Gain and phase response spectra of the underpowered EMA are shown in Figure 13. Poor performance above 4 Hz was expected under the inertial load and power limitations imposed by the test. Frequencies above 12 Hz were discarded because the response did not meet the acceptance criteria for inclusion. This is equivalent to the magnitude of the response being below the effective noise floor of the data acquisition system.  The highest battery cell temperature was recorded on 2 September 2011. The temperature log for that day, shown in Figure 14, captures a total of three EMA tests. A single thermocouple measured a single out of family temperature event at 2,010 seconds, elapsed from the start of the log, which corresponds to the end of the 2 nd EMA test performed that day. At the same time, the temperature nearest that value,  recorded by another thermocouple, was compared. The difference is not significantly greater than the standard deviation measured across all temperature data at this same time. This means that the amount by which this single value is out of family is insignificant. It should be noted that even for this worst case example, the rise in temperature was not appreciable, the temperature returned to an in family value post test and, in no case, did any cell temperature ever approach a level of concern.
193   Figure 13. Gain and Phase Spectra of Underpowered EMA 

Figure 14. Lithium Ion Battery Temperature Log (worst case)  

References 

1.  Garrison, Michael and Scott Steffan.  "Two Fault Tolerant Electric Actuation Systems for Space  Applications."  Association Aeronautique et Astronautique de France (AAAF) Changes in Aeronautical  and Space Systems , Challenges for On-Board Energy Conference, Avignone, France (26 - 28 June  2006).  2.  Garrison, Michael and Scott Steffan.  "Two Fault Tolerant Electric Actuation Systems for Space  Applications." AIAA-2006-4939, 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference,  Sacramento, CA.  (9 - 12 July 2006).  3.  McMahon, William A. "Apollo Experience Report – Guidance And Control Systems: CSM Service  Propulsion System Gimbal Actuators." NASA TN D-1969 (July 1975).  4.  Wayne, Duke. "OMS/RCS Subsystem Presentation." NASA GNC Presentation at NASA Kennedy  Space Center, (4 June 1996).
194 5. White, Robert A. “High Voltage Design Criteria.” Marshall Standard MSFC-STD-531, George C.  Marshall Space Flight Center (September 1978).  6. Dunbar, William G. “Corona Onset Voltage of Insulated and Bare Electrodes in Rarefied Air and  Other Gases.” Air Force Aero-Propulsion Laboratory Technical Report AFAPL-TR-65-122, Air Force  Systems Command, Wright-Patterson Air Force Base (June 1966).  7.  United States Environmental Protection Agency. Hydrazine Hazard Summary-Created in April 1992;  Revised in January 2000.
198 Moreover, the margin of safety with respect to shear force is significant, as in the real application, the  composite brush experiences a shear force approximately 1000x smaller than the minimum ultimate shear force resistance when the slipring’s motion is initiated.  Table 1. Shear test results summary  Series 1 Shear Force Std deviation  Thermocycled Samples (8 samples) 442 N 14 N  Reference Samples (2 samples) 448 N 4 N  Series 2    Thermocycled Samples (8 samples) 427 N 20 N  Reference Samples (2 samples) 448 N 20 N   Figure 2 provides pictures of shear test samples after shear testing to illustrate the fracture mode of the composite brush glued assembly. The same fracture mode has been observed on all test samples.  

Figure 2. Shear test samples   Figure 3 illustrates the shear fracture propagation mode. A crack initiates on the side of the composite brush where it is held in the shear test fixture, and then propagates up to the glue interface, underneath the blade. The glue interface surface then breaks very quickly as the contact surface between the blade and composite brushes reduces rapidly. The initiation of the crack in the composite brush suggests that this component has a shear-stress limit lower than the glued interface.  Glue layer microsections  Examples of transverse micro-sections views of the composite-brush/flexible blade assembly made after life tests are presented in Figure 4. The Copper Beryllium blade section is shown on the top-half of the micro-section views (uniform color) and the composite brush sections on the lower half (marbled section). All micro-section views have shown that the integrity of the gluing system of the composite brushes is not compromised by both the thermal cycling and life tests performed on the breadboard models.
200 To assess the potential impact of thermal vacuum cycling on the mechanical fixation of the sintered  braids, composite brushes from both breadboard models have been tested and the results compared with the ones obtained on the QM/FM composite brushes fabrication batch. The flexible braid pull test results are summarized in    Table 2. 

Table 2. Pull test results summary   Pull Force Std deviation  composite brushes from BBMs (thermal vacuum cycled) 58.3 N 4.8 N  composite brushes from QM/FM production lot (not cycled) 52.5 N 2.8 N   No major impact of the thermal cycling on the resistance of the sintered connection can be observed from the pull test results. In fact, in most cases the rupture mainly occurred on the flexible braid rather than on the sintered joint. This is illustrated in Figure 5 on a composite brush assembly that encountered both fracture modes during the flexible braid pull tests.  

Figure 5. Pull test results on composite brushes sintered cables with both rupture modes    The maximum design loads on the flexible braids are generated by the vibration and shock environment and principally due to the inertial loads on the flexible braids. These loads are marginal with respect to the ultimate traction load.  These pull tests have proven the suitability of the sintered connection between the flexible braids and the composite brush for the BepiColombo slipring assembly design.  Electrical grounding between composite brush and blade  In this new composite brush Assembly the spring blades are not part of the electrical line anymore. Since the flexible blades are connected to the composite brush using electrically insulating glue at one end, and supported by a brush holder made of insulating material at the other end, the electrical grounding of the blades needed to be insured by tests or by the use of an additional grounding lug.  Continuity tests have been performed on the composite brushes assembly of both breadboard models after the validation tests. The tests were fully successful and proved that the electrical continuity was guaranteed between all blades and composite brushes, despite the use of electrically insulating glue.   Using a purely theoretical approach on the insulating properties of the thin layer of glue used, the spring blades should have been electrically insulated. However, the electrical grounding tests have enlightened
201 that the assumption of a uniform barrier of glue between the composite brush and the flexible blade is not  realistic. In fact, a good electrical conduction between the blade and the composite brush is ensured by discrete electrical contact points arising from the high surface roughness of the composite brush in combination with the gluing process performed under preload. In order to guarantee the electrical grounding of the flexible blades during QM/FM production, an electrical continuity check is performed on every composite brush assembly.  Anthony-Brown tests on silver plated flexible braid  The applicable requirements from the ECSS call for a 2- μm silver plating thickness on copper conductors.  While standard space grade cables commonly have a silver thickness of 2 μm, it was not possible to  procure ultra-flexible braids in standard Space grade with a silver coating thicker than 1 μm for this  special type of braid.  “Red Plague” is a well-known phenomenon that causes corrosion of copper on silver plated copper cables. The risk of occurrence is increased in the presence of humidity, particularly in condensed form. With the selected composite brush assembly, this is typically the case considering the thermally insulating sleeving of the flexible braids in humid atmosphere and submitted to temperature variations.  Samples of the flexible braid production batch have been equipped with the PTFE shrink-sleeves and provided to ESA’s Test Center (ESTEC). Anthony-Brown tests are currently being performed for the assessment on red plague contamination control.   Breadboard Model Validation Tests   The new composite brush sliding contact system has been validated at system level on two breadboard models (MTM and MPO slipring assemblies). Figure 6 provides a picture of one of these breadboard models for illustration purposes.  Both models shared the same composite brush contact system and were fully representative of the flight hardware in terms of materials, contact configuration and dimensions, with the exception of the number of tracks that was reduced in the breadboard models. The main implic ation is that the breadboard models  have a reduced overall length with respect to the QM/FM design.
202   Figure 6. BepiColombo BBM 1 (configuration with contacts not fully populated)   The validation test sequence for both breadboard models is schematized in Figure 7. Breadboard model  validation tests mainly consisted of mechanical vibration and shock tests, thermal cycling and life tests with numerous electrical functional tests to demonstrate the adequacy of the contact technology to this high temperature application.   

Figure 7. Breadboard models validation sequence   In addition, the preload force drop due to the combination of the wear of the composite brushes and flexible blade material creep has been measured and assessed. These measurements were then used to compute the friction coefficient of the contact system from torque measurements performed both in air and in vacuum as well as with and without brush holders in order to remove the contribution of the bearings on the overall measured torque. The main results of these BBM validation tests program are described in the next subparagraphs.  Thermal vacuum cycling, mechanical and accelerated lifetime tests  BBMs have been tested under representative mechanical, thermal and accelerated lifetime (number of cycles including qualification margin) of the BepiColombo mission. During the lifetime testing, monitoring of the dynamic resistance and friction torque was performed, in order to measure the evolution of these critical parameters.  Figure 8 and Figure 9 present the static and dynamic RMS contact resistance evolution during lifetests (measured with a current of 1A). Both graphs show a slight increase with the number of revolutions, which reflects the impact of the generated wear particles on the sliding contact electrical characteristics. However, these resistance values remain very small and well within the specification.
203   Figure 8. Static contact resistance evolution – RMS value 

Figure 9. Dynamic resistance – RMS value 

The design of the composite brush preloading system is inherently critical with respect to the mechanical environment, mainly due to the configuration of this system that has a concentrated mass at the end of a spring blade. The weight of the composite brush design has been minimized and the preloading system designed to avoid eigenfrequencies in the vibration frequency spectrum. Moreover, the mechanical dimensioning of the preloading system is such that lift-off of the composite brush is prevented as well. Vibration and shock tests have been performed on both breadboard models to validate the design of the composite brush suspension system and demonstrate that the composite brushes and tracks were not affected by these mechanical environments.  An illustration of the contact system after vibration and shock tests on a breadboard model is shown in Figure 10. Slight traces were visible on all tracks near the composite brushes. After close inspection, it 05101520 Before vibrationsAfter vibrations0 revolution 14000  revolutions28000  revolutions42000  revolutions56000  revolutions70000  revolutionsContact static resistance [mOhm]in AIR in VACUUM 012345 Before vibrationsAfter vibrations0 revolution (1rpm)14000 revolutions   (0.007rpm)28000 revolutions  (1rpm)42000 revolutions (1rpm)56000 revolutions (1rpm)70000 revolutions (0.007rpm)Dynamic resistance - RMS measured  with 1A [mV/A] in AIR in VACUUM
205   Figure 11. Friction coefficient measurement results 

Dynamic contact resistance  Examples of the dynamic electrical contact resistances measured over a full revolution and after run-in of the composite brushes are presented in Figure 12. These measurements were performed at the nominal current of 12.5 A per track on four different tracks in air and vacuum. All results have shown a low electrical contact resistance in both vacuum and air conditions with great consistency between the different measured tracks. Moreover, the performances of the composite brushes are significantly better in vacuum environment, which, again, confirms the adequate selection of the composite brush material for space applications.  The RMS values of the dynamic electrical contact resistances are of particular interest, as contact resistance is a key driver for the thermal dissipation at the contact interfaces due to the electrical power transfer. This is particularly critical considering the thermal environment of the BepiColombo mission.   Figure 13 summarizes the RMS values of the dynamic contact resistance curves presented in Figure 12, with the addition of measurements performed before run-in of the composite brushes. These results show that with the selected composite brushes technology, RMS values of the contact resistance are very low, especially under vacuum. Composite brushes run-in also shows its benefits, with an overall reduction of the dynamic contact resistances. These results show that composite brush technology becomes optimal under vacuum and is furthermore improved by the effect of the run-in, which correspond to the flight configuration.  in vacuum in air0.30 0.25 0.20 0.15 0.10 0.050.00friction coefficient
206   Figure 12. Dynamic contact resistance curves over 360°– VACUUM & AMBIENT (after run-in) 

Figure 13. RMS dynamic contact resistance – VACUUM & AMBIENT (after run-in)  Lessons learned and improvements 

Validation test programs performed at component and breadboard model levels have shown that the selected composite brushes contact technology is fully compliant with the BepiColombo requirements. In view of QM and FM slipring assembly production, some improvements are being carried out, namely:    For the breadboard models, the composite brushes were manufactured as closely as possible to  the cylindrical shape of the track, with the objective of performing a run-in directly on the assembled slipring assemblies. However, breadboard models tests have showed that wear is extremely low, and that a significant number of revolutions were required to obtain a perfect fit of the composite brushes to the tracks. Figure 14 illustrates the composite brushes contact surface before and after run-in tests performed on the breadboard models. Track A 0 mΩ5 mΩ10 mΩ15 mΩ20 mΩ 0 ° 30 ° 60 ° 90 ° 120 ° 150 ° 180 ° 210 ° 240 ° 270 ° 300 ° 330 ° 360 ° angular positioncontact resistancevacuum ambientTrack B 0 mΩ5 mΩ10 mΩ15 mΩ20 mΩ 0 ° 30 ° 60 ° 90 ° 120 ° 150 ° 180 ° 210 ° 240 ° 270 ° 300 ° 330 ° 360 ° angular positioncontact resistancevac uum ambient Track C 0 mΩ5 mΩ10 mΩ15 mΩ20 mΩ 0 ° 30 ° 60 ° 90 ° 120 ° 150 ° 180 ° 210 ° 240 ° 270 ° 300 ° 330 ° 360 ° angular positioncontact resistancevac uum ambientTrack D 0 mΩ5 mΩ10 mΩ15 mΩ20 mΩ 0 ° 30 ° 60 ° 90 ° 120 ° 150 ° 180 ° 210 ° 240 ° 270 ° 300 ° 330 ° 360 ° angular positioncontact resistancevacuum ambient BepiColombo - Carbon brush technology RMS Contact resistance - AMBIENT 0 mΩ10 mΩ20 mΩ 12.5 A 12.5 A 12.5 A 12.5 A track A track B track C track Drms valueambient before run-in ambient after run-in BepiColombo - Carbon brush technology RMS Contact resistance - VACUUM 0 mΩ10 mΩ20 mΩ 12.5 A 12.5 A 12.5 A 12.5 A track A track B track C track Drms valuevacuum before run-in vacuum after run-in
207   In order to accelerate the run-in process for the QM and FM production lines, the brush holder  assemblies are run-in on a dedicated abrasive tooling representative of the final track dimension. In order to further accelerate the run-in process, wear particles are removed by a continuous flow of demineralized water during the run-in procedure.   

Figure 14 – Overview of composite brushes before and after run-in 

 After thermal tests, a slight brownish coloration has been observed on the Kynar sleeves covering  the ultra-flexible braids of the composite brushes. Initially translucent, with a pale yellowish coloration, they have slightly darken most probably due to the harsh thermal environment of the experienced thermal test (see images on the left of Figure 15), which were close to the Kynar temperature limits. Additional tests performed on the Kynar sleeves of the breadboard models did not reveal any degradation of the mechanical and electrical insulation properties. 

However, for the QM/FM design, it has been decided to switch to a PTFE-type heat shrink sleeve, which could sustain higher temperatures and avoid further shrinkage during the operational life of the slipring assemblies. Additional thermal tests have shown that this shrink sleeve was not affected by the exposition to the thermal environment defined for the BepiColombo mission (see images on the right of Figure 15).    In the initial design of the QM/FM slipring assemblies, the flexible blades were grounded using an  additional cable fixed to the opposite end of the composite brush using a terminal lug. However, continuity tests performed between the composite brushes and the flexible blades have all shown that the grounding of the flexible blades is ensured through the glued interface between the composite brushes and the suspension blades. The grounding cables have been removed in the QM/FM contact system design and replaced by a continuity test performed on each composite brush assembly during their production.
208   Figure 15. Change of heat shrink sleeve type (left: old, right: new) 

 The gluing process carried out on the composite brushes flexible blade assembly during  breadboard model production has shown that it requires careful attention. In order to enhance the  QM/FM gluing process, the amount of glue and the location of its application have been defined and documented. Additionally, acceptance criteria have also been defined to guarantee a proper distribution of the glue, a precise alignment of the composite brush and in general a full repeatability of this process. Figure 16 illustrates the final configuration of the QM/FM composite brush as part of the acceptance criteria. 

Figure 16. Close-up view of glued interfaces