ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
405       

                Figure 4. Pupil Wheel Bearing Installation 

The geared motors support wheel rotational speeds of  1.5 RPM. The geared motor sizing was based on  providing sufficient torque margin when taking into account the resistive torques, due to accelerating the  rotational inertia of the wheels and the resistive to rque of the bearings. The appropriate factors were  applied to the resistive torques, in order to calcul ate torque margins. The specified output torque of the  geared motor was based upon previous measured performance of geared motors completed by CDA Intercorp and the estimate of the resistive torques wa s based upon measurements of the BBM filter wheel  bearing at cryogenic temperatures.        The DW mechanism design has to meet the positioni ng and repeatability requirement having considered  the contributing errors. These errors include the variation in stepper motor step angle, geared motor  backlash error, resolver error, ring / pinion gear backlash, tooth pitch variation, thermal distortion and component machining tolerances. Having analyzed the er rors, the granularity of wheel motion (and hence  gear ratios) were sized to be smaller than the required accuracy.   A resolver position sensor provides feedback to confir m that the wheel is in the proper position. An antibacklash gear is used on the resolver output pinion to increase the accuracy of this gear train. Since the resolver is geared up by a factor of 9, the resolv er turns 9 times for every rotation of the wheel. The  resolver provides an absolute read-out, but pending  a loss of memory due to a fault condition, the  resolver would be unable to determine which 1/9 th of a segment the wheel would be in. A reference signal  is therefore provided to indicate a home positio n. The reference signal is provided by a variable  reluctance sensor (VR sensor).   Inner Clamping  Nut  Duplex  Bearing  Static Hub Rotating Pupil Wheel  Bearin g  Outer Clamping  Nut Flexure  Flexure  NASA/CP-2010-216272
406 Geared Motor Testing – Lessons Learned 

The actuation to rotate each wheel is provided by a geared motor. The geared motor is a combination of  a three phase stepper motor (step size of 15 deg.) and a single stage planetary gear. The geared motor  for the PW is shown in Figure 5. The geared motor testing was first completed at r oom temperature. It was important to establish a  baseline of the motor performance at room temperature before testing at cryogenic temperatures.  Any issues with the geared motor would have to be found before introducing the motor drive electronics. The motor drive electronics are a COM DEV design. T he drive electronics cater to room temperature and  cryogenic operation of the motor. This is achieved through a continuous real time measurement of motor winding resistance and a subsequent ad justment of voltage (through time modulation techniques).    The first round of room temperature testing yielded a pull in torque value of 17 N-cm versus a specified  requirement of 23 N-cm. This was thought to be a simple shortfall in motor performance and certainly not a design problem. It was decided to proceed to cryogeni c temperature testing. The testing at cryogenic  temperatures was carried out in a cryogenic chamber t ogether with a feed thru shaft out of the chamber  coupled to a torque measurement machine.     The first test at 35K did not result in full rotation of the motor. The motor exhibited some stalling and an  investigation was started through the material review board process. Considering the observations, the motor performance appeared  normal at room temperature, but the performance at 35K was abnormal.  The cause was immediately thought to be a deficiency in the motor and that the cryogenic environment  must be causing more friction in the motor bearings and planetary gears.  

Figure 5. Geared Motor – Pupil Wheel 

The geared motor vendor (CDA Intercorp) was requested to use their own design of drive electronics with  the geared motor tested again at cryogenic temperature. The material review board chose the motor “no load response rate” as the most appropriate data to  obtain at cryogenic temperature. The “no load  response rate” is the maximum speed that the motor can rotate at, in a stable manner, without any load.  The “no load response rate” provides a good indicati on of the health of the geared motor. Higher friction  in the geared motor would lead to clear c hange in the “no load response rate”.    At 35K with the vendor drive electronics, the gear ed motor was able to rotate at the same “no load  response rate” that was obtained at room temperatur e. This finding proved that  the geared motor was not  suffering from increased friction at cryogenic temper atures. It was therefore concluded that in some way  NASA/CP-2010-216272
408  Table 1: Bearing Design Parameters 

The cryogenic chamber test set up for the wheel / hub assembly is shown in Figure 7. Note that the cryogenic chamber has a cold plate surface, to whic h the device under test (DUT) is directly attached.   Each wheel/hub assembly was run-in for 100 revolutions CW and 100 revolutions CCW at room temperature. The torque of the bearings was then measured. Each bearing torque at room temperature  was in the range of 8 to 12 N-cm. This was certainly acceptable and proved that the bearings had been successfully installed into the wheels. The next set of torque tests would be carried out at cryogenic temperature.    

                   Figure 6. Single Pupil Wheel Bearing & Duplex Pair installed in Pupil Wheel Machining 

The wheel/hub assembly was installed in the cryog enic chamber. The chamber was purged with dry N 2  gas for a period of 2 hours (this was the standard purging used for all cryogenic tests) and then a vacuum  was drawn in the chamber. The DUT was then cooled  down to 32K and a record was maintained of the  wheel and hub temperature. It was noted that the whee ls cooled at a much slower rate than the hub,  leading to a 100K temperature gradient across the bearings.  The PW bearing (cryogenic) torque was measured fi rst. The initial torque measurement was carried out  with a hand held torque meter. The initial torque measurement was abnormally high at 202 N-cm. This result was unexpected and indicated that a problem wi th the bearing was occurring. An investigation of  the problem was started and it was decided to continue rotation of the bearing to further characterize the problem. After 4 revolutions, the torque had dropped from 56 to 46 N-cm. After a further 100 revolutions the torque had reduced to 20 N-cm. After a total of  200 revolutions the torque had reduced to 6 N-cm. 

The FW bearing (cryogenic) exhibited a similar prob lem, with an initial torque measurement of 196 N-cm.  The behavior when further rotated was similar to the PW bearing, with the torque reducing to an acceptable value of 5 N-cm over the first 100 revolutions.    NASA/CP-2010-216272
409  Torque Trace  Machine Feed  Through  Shaft Wheel / Hub  Bearing  Assembly  Cold  Plate           

   Figure 7. Wheel / Hub Assembly in the Cryogenic Chamber 

The bearings were being impeded by something when initially rotated. Over the first four revolutions the  torque reduced but was still very hi gh. Eventually an acceptable value of torque (approx. 5 N-cm) was  achieved on both bearings after 200 revolutions. The fact that the bearing torque at cryogenic  temperature was similar to the ambient torque, after 200 revolutions was encouraging because it implied that the bearings were not permanently distorted by  the wheel and hub at 35K. If they were permanently  distorted, it did not seem credible that they woul d ever run well. It was decided to disassemble the  wheel/hub assembly, to the extent possible, to make  a visual inspection of the bearings. The visual  inspection revealed particles of material in the form  of flakes and small threads in the vicinity of the  retainer and the races. The particle contamination in the bearing is shown in Figure 8.                      Figure 8. Wheel / Hub Assembly in the Cryogenic Chamber   The particles were identified (by chemical analysis) to be MoS 2 and PGM-HT. Therefore, it was concluded  that the retainer was implicated in the problem. The retainer design was reviewed and a dimensional shrinkage analysis of the retainer and race was undertaken.     Flakes  from  the  Retainer  NASA/CP-2010-216272
411                   Figure 9. Pupil Wheel Bearing Test at 35K viewed through Test Fixture Window 

Bearing Test 1  Objective:  To quantify the temperature gradient ac ross the bearing when preload is lost.  Pupil wheel bearing was torque tested with various temperature gradients across the bearing. Result:   Preload was lost when temperature gradient exceeded 5K . This result agreed with analytical prediction.  Preload is lost because outer race is temporarily too large to maintain contact with the balls.   

Bearing Test 2:  Objective:  To measure the bearing initial torque at 35K, having periodically rotated the  bearing from 150K down to the operating temperature. The purpose of the test was to see if a mitigating strategy could be implemented during the cooling down of the bearing. Result:  The rotating of the bearing  during the cool down (temperature gradient > 5K) did not  eliminate the high initial torque when rotated at  35K. Rotating the bearing with no preload on the ball / ra ce system was in-effective.              Bearing Test 3:  Objective:  To measure the bearing initial torque at 35K, having maintained the  temperature gradient across the inner and outer races at  less than 5K. In essence, the bearing would be  cooled in an iso-thermal manner. Providing an iso- thermal cool down of the bearing could be achieved,  the preload between the balls and the races would be maintained during the entire cool down from 295K to 35K. Result:  The temperature gradient across the bearing was maintained during the entire cool down  at 4K or less. Therefore, it wa s assumed that the preload had been maintained on the bearing. However,  a high initial torque of 80 N-cm occurred, but it was observed that the torque reduced down to 10 N-cm in  only 10 degrees of bearing rotation. This rapid dr op in torque had not been observed on any previous  bearing test. It was concluded that the iso-thermal cool down had resulted in better bearing behavior at 35K, but the retainer itself could not be eliminated as  a possible contributor to the high initial torque.   In parallel with the individual bearing tests the flight  wheel / hub assembly (bearings having the enlarged  retainer) was torque tested at 35K. The results of the pupil wheel bearing test was an initial torque of  167 N-cm. Dropping to 31 N-cm after 270 degrees of bearing rotation. The torque trace of the next three and a half revolutions is shown in Figure 10. The torque value drops from 27 N-cm to 13 N-cm in non-linear manner with respect to the rotation angle. Over  the next 50 revolutions the torque reduced to an  acceptable low value of only 7 N-cm.   The wheel / hub assembly still showed a high initial to rque at 35K, despite the use of the larger diameter  retainer. A review of the temperature data during  the cool down revealed that the bearing had  experienced a very high temperature gradient across  the races (at one point the wheel being 100K  warmer than the hub). This large temperature gradient led to a loss of bearing preload and when the  Clearance Retainer with  clearance to  Inner Race Balls between  Races  NASA/CP-2010-216272
412 01234567891011121314151617181920212223242526272829303132 1 146 291 436 581726 871 10161161BEARING   TORQUE (Ncm) ANGLE OF ROTATION  (DEGREES)PUPIL WHEEL BEARING  ‐FIRST  4 REVOULTIONS   CW ‐35 K TORQUE AVERAGE  TORQUE preload is lost the balls and the retainer could “fall” in  the presence of 1G loads into non-optimal locations.  These non-optimal locations would be “locked in” w hen the balls and races re-register, when all bearing  components reach 35K. It is theorized that over the first four revoluti ons the balls are being put back into  optimal locations by the act of rotating the bearing.  

       Figure 10. Wheel / Hub Assembly Torque Test at 35 K 

The test also proved that the bearing high initial torque could not be due to frozen moisture in the MoS 2.  The reason for this conclusion is that a high torque  (if due to this phenomena) should clear itself in a  single rotation of each ball relative to the races. But the bearing took four rotations to recover. It was not  considered plausible that frozen moisture in the MoS 2 could affect the bearing for this many rotations.      

Dual Wheel Mechanism Flight Model Tests   It was decided to proceed with the assembly of the DW  flight model. Initially, both wheels were tested at  room temperature, to establish a clear baseline for performance before testing at 35K. The minimum current for successful operation of each wheel was es tablished at room temperature and then a torque  ratio was calculated, based on the maximum motor drive current available. The torque ratios at room temperature were 4.4 for the PW and 4.1 for the FW . After vibration tests were completed, the torque  ratios were unchanged. Vibration tests were deemed to be successful.                     Figure 11. Dual Wheel Mechanism Flight Model  NASA/CP-2010-216272
414 Item (6)  – The JWST mission cool down is over a seve ral week period. COM DEV expect item (2) to  occur in the mission, but not item (3) to occur. However, it was decided to use the accelerated six day  cool down as the basis for qualification. Future ground  based testing at the inst rument level will also have  to cool down in six days. This is more severe than  the mission itself but now serves as the environment  for qualification.        

The bearing anomalies described could not be eliminat ed from the design. However, a successful test  campaign of the flight model ultimately lead to establishing acceptable torque ratios to overcome the anomalies.       The authors wish to express their gratitude to  COM DEV and Canadian Space Agency management and  to COM DEV technical and test personnel for their considerable support in tackling the complex design,  build and test campaign for the DW mechanism. 

References 

1. National Institute for Standards and Te chnology, 325 Broadway, Boulder, Colorado  

NASA/CP-2010-216272
415 Xatcobeo: Small Mechanisms for CubeSat Satellites –   Antenna and Solar Array Deployment 

Jose Miguel Encinas Plaza *, Jose Antonio Vilán Vilán +, Fernando Aguado Agelet +, Javier Barandiarán  Mancheño *, Miguel López Estévez +, Cesar Martínez Fernández * and Fany Sarmiento Ares * 

Abstract 

The Xatcobeo project, which includes the mechanisms deal t with here, is principally a university project to  design and construct a CubeSat 1U-type satellite. This work describes the design and operational  features of the system for antenna storage and deployment, and the design and simulations of the solar array deployment system. It explains the various pr oblems faced and solutions adopted, with a view to  providing valid data for any other applications that could find them useful, be they of a similar nature or not.   Introduction   Xatcobeo is an educational joint project between Univer sity of Vigo and INTA (Spain National Institute for  Aerospace Technology), which has been selected to be launched on the maiden flight of Vega. The  space segment of the Xatcobeo project is mainly formed by a mechanical structure that complies with the  CubeSat 1U standard. Into this structure, five  subsystems are going to be implemented: a power  subsystem, an on board computer, a Telecommand Tracking and Control system for communications, a solar Panel Deployment Mechanism (PDM), a software reconfigurable radio, and a sensor for measuring  the total amount of ionizing radiation in the space.  Mechanisms

Two different deployment mechanisms are included in the satellite:  • An antenna (four monopoles) deployment  • A solar array deployment. 

Antenna Retention and Deployment Mech anism for a CubeSat 1U Picosatellite  The CubeSat concept makes small-scale satellite launches viable on a low budget. Dimensions are limited to a 100 x 100 x 100 mm cube and mass to 1 kg, which means all built-in components must weigh and measure as little as possible. These limitations have been borne in mind from the outset when designing the mechanism offering storage capacity  and deployment for antennas. The mechanism must  fit a casing measuring 82.6 x 98 mm on the front and with a thickness of 6.5 mm. To fully explain the design needs and solutions chosen, the main problems and solutions are given below.  Problems

The problems can be divided into a series of more or less independent sections:  • General configuration.  The design must leave clear as much of the face  where it is installed as possible, insulate  the antenna connection from the metal of the satellite, and respect the geometry for the antenna position, which will come out at an angle of 45 degrees to the adjacent edge.                                                       * Instituto Nacional de Tecnica Aeroespacial (INTA), Madrid, Spain  + University of Vigo, Lagoas Marcosende, Spain 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  NASA/CP-2010-216272
417 a profile guide for the antenna whilst they are folded. The deployment mechanism operation is analyzed  below and the features of the elements that make it up are described.   Sub-chassis  As explained, the sub-chassis is  the piece upon which all the necessary elements are attached for  deployment mechanism operation.   General requirements and design process

Antenna design basic requirements:  • Each antenna must be located in the center of the edge of the +X face of the CubeSat.  • The exit angle must be 45º ( π/4 rad) with respect to the adjacent edge of each antenna.   A plaque with solar panels to power CubeSat was also expected to be installed on the +X face, so the part used to hold the antenna had to leave as much space as possible for the installation of these panels. Being the largest part in the syst em meant weight had to be kept down to a minimum to give as much  margin as possible to other elements.  In order to save mass, system design began as a set of independent parts, four for antenna attachment and another four as guides to give the folded ant enna curvature and keep them within the available  envelope. However, the large number of attachment elements needed for this set-up made it unviable. The approach was changed and a continuous profile was designed for attaching the antenna and guiding them in their folded path. The profile moved as far out from the shielding as possible so that the greatest  number of solar panels could be installed in the free interior space. The piece to which the antenna was attached was called the sub-chassis. 

Figure 1. Sub-chassis                                             Figure 2. Mechanism built into the Project  Xatcobeo prototype 

Materials selection

The antenna had to be insulated from the metal of the satellite in order to avoid short circuits when touching the shielding faces. There were two options:  • Make the sub-chassis from metal and include a dielectric piece at the antenna contact areas to  insulate them from the rest of the metallic parts.  • Make the sub-chassis entirely from polymeric material.  The first option was seen to have complications with the metal-dielectric interface fitting and there was a risk of contact between the antenna and any other pa rt of the metal sub-chassis, so the sub-chassis  made entirely of polymeric material was therefore chosen as safer in terms of antenna insulation. The material has to fulfill a long list of requirements:  NASA/CP-2010-216272
419   Figure 3. Drill hole and antenna attachment.    Figure 4. Method and location of attachment.  

Antenna Retention  The retention mechanism keeps the antenna folded from the time the satellite is finally assembled and  ready for launch to the moment when the antenna controller indicates they must deploy. It needs to be able to cope with vibrations produced on rocket la unch and the temperature changes from assembly to  deployment, i.e., dilatations produced by heat variati on must not put the system’s retention capability at  risk nor, obviously, cause damage to the material that  would lead to structural breakage or weakening,  which would harm the project’s aims.  Each antenna, or monopole, is located in the center of the edge of the shielding for the +X face of the  CubeSat, coming out at a 45º angle. Each one is 170- mm long, whereas the continuous profile of the subchassis is 325 mm in perimeter. This means that no antenna goes right around the profile or any of the others completely. This led to a problem, as it in volved 4 retention points, one for each antenna. This  meant, furthermore, guiding 4 nylon threads towards the area where the burner element is located, or four burner elements and four burner circuits. The latter was rejected as four circuits and burners would increase overall weight too much, so a way was sou ght to deploy the four ante nnas using a single circuit  and burner element.  The idea to take a thread from the end of each antenna towards the burning area did not seem the most ideal because of the amount of thread needed to co ver the distance and the risk of threads getting  crossed. Furthermore, pivots had to be introduced to redirect the threads to an area where the burner element would not interrupt the solar panel positions.  That is, the most suitabl e configuration was sought  to ensure that the retention thread was as short as  possible and that a single thread was enough to hold  all the antennas at the same time, thus requiring onl y one burning point. This removed the need to place  additional burning points and so kept weight down.  Thus the final retention system co nsisted of an enveloping sheet of t hermoplastic material, fixed at a  single point to the sub-chassis, which was 425-mm long and so allowed four antennas to be enclosed, once they had been folded, and keep them retained agains t the continuous profile of the sub-chassis. A  perforation was made at the same point on all the antenna for the retention thread to pass through from the outside to the inside of the profile, where the burning element was located.   Attachment system

For the sheet to function properly, it needed to be joi ned to the sub-chassis. When enclosing the folded  antenna, the traction tangential to the sub-chassis profile applied to the enveloping sheet would be turned into pressure on the antenna and so achieve a force at every point along them and keep them folded. The  NASA/CP-2010-216272