ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Filter Wheel Assembly Design  The Filter Wheel assembly contains two independently rotating wheels with 12 optic positions each (Fig.  2). Each optic is centered on a radius that is 112.5 mm from the axis of rotation. The Pupil Wheel  contains a complement of light projectors, wave-front-sensing elements, filters, coronographic wedges,  and weak lenses. The Filter Wheel contains 5-mm filters that are tilted four degrees parallel to the plane  of the optic bench. The NlRCam prescription requires the Pupil Wheel and Filter Wheels to rotate very  close to one another with the final surface of the filters less than 25 mm from the first optic surface in the  Pupil Wheel. The performance of the Filter Wheel assembly is driven by the requirement to locate some  of the elements in the pupil wheel with a repeatability of less than 310 microns. Additionally, the target  location of the prescribed element is to be adjustable on orbit through software. This performance criteria  along with a very modest average thermal dissipation requirement that accompanies the 35 K operating  condition make the Filter Wheel assembly design a challenge.  I"  Figure 2. The NlRCam Filter Wheel Assembly  The team designed each wheel assembly to be mounted directly onto a custom motor rotor. This design  concept is based on heritage designs that have been space qualified and flown before. It is an extremely  efficient design concept for both envelope and mass concerns. Each wheel is driven by a 24-pole, three-  phase, redundantly wound, cogless DC motor. This configuration minimizes the operating power and  space envelope and maximizes the flexibility in commanding positions. A cogless motor has no harmonic  content in the drag term, and it has no position detents. This combination of features allows for a very low  drag actuator with no mechanically favored wheel positions. A 24-pole motor has the added benefit of  simplifying the drive electronics operational scenario since a single 30 degree move from one optic  position to another is accomplished by one complete electrical cycle.  The motor rotor is supported by a back-to-back mounted duplex bearing pair. The choice of bearing type  was driven by heritage and experience. A relatively large bearing was chosen to accommodate a 50-9  launch load and to extend the life expectancy of the dry film lubrication. The lubricant choice was more  difficult. The final decision was based in part on an AMS paper that evaluated various lubricants [l].  Additionally, Lockheed Martin has experience with thin film lubrication on a wide variety of applications.  These experiences lead to a preference for self-lubricating PTFE and MoS2 retainers.  The position feedback was the last aspect to be considered. The team chose a differential inductive  sensor system to measure position. Ramp targets on the face of the optical element wheels converts  linear sensing devices to a rotation. This choice was driven by previous cry0 experience with the  230
technology, envelope, power, mass, and cost concerns. It was made possible by a relatively loose  position requirement.  Filter Wheel Assembly Prototype Design  The prototype design represents a single actuator. The prototype design and test program goals were to  confirm a bearing run-in procedure, quantify bearing, motor drag, and motor capability, characterize the  inductive sensors, and finally verify the controlled repeatability performance. All tests were conducted at  an operating temperature of less than 70 K.  Five 35-mm O.D. duplex bearing pairs with TeflonTM and MoS2 impregnated cage material were obtained  (Fig. 3). The cage material is identical to the proposed flight. The bearing size and preload is identical to  the flight design. However, each prototype bearing is made from 52100 steel rather than 440C as  specified for the flight program.  Figure 3. Filter Wheel Assembly Prototype Bearing  In addition to the bearings, two cogless DC motors were specified and procured and included a  customized rotor with a feature that would accept the bearing pair and mate with the surrogate optic  wheel (Fig. 4). A motor housing was designed to hold the motor stator with a unique flexure design that  accounted for coefficient of thermal expansion differences between the components. A motor shaft and  shaft mount was designed and fabricated. The design allowed for adjustment in the motor air-gap.  Figure 4. Cogless Motor Installed In Prototype  231
A set of inductive sensors were obtained (Fig. 5). The surrogate optic wheel was designed to provide 15-  degree differential ramp targets. The sensor array and sensor track were integrated with the prototype  design.  J  Figure 5. Inductive Sensors  Prototype Test Configuration  In parallel with the design and fabrication of the Filter Wheel assembly prototype, the team designed the  necessary setup to test the assembly at 35 K. After selecting a cryogenic test chamber, the team  specified the required test equipment. In addition, the team designed the appropriate tooling that would  adequately hold the prototype during testing. In order to test the bearing and motor drag, a fixture was  designed to hold the prototype within the cryo-chamber, while an external motor and encoder spun the  rotor and wheel through a ferrofluidic feedthrough. A vacuum rated torque meter was coupled to the  feedthrough inside the chamber to measure torque as close to the prototype as feasible.  Figure 4. Prototype Bearing Cold Test Configuration  232
Bearing Run-in and Cryogenic Drag Test Results  Bearing drag was the first performance characteristic tested. Each bearing was run-in at an ambient  operating environment using a run-in test fixture. A ground support equipment (GSE) motor was used to  rotate the motor at 40 RPMs for 200 revolutions in one direction then 200 revolutions in the opposite  direction. A torque measurement at 1 RPM followed each sequence of 200 revolutions. A total of 10,000  revolutions were accomplished over a period of approximately four hours. Torque telemetry was  continuously obtained by an in-line torque transducer. Post-test inspection and tests indicate that an  average of 40 to 50 angstroms of Teflon lubrication was successfully transferred to the balls and races.  The bearing was then assembled onto the prototype shaft and installed into the cryo-chamber to test  bearing drag at temperature. A bearing resistive torque of 5.4 mN-m (0.8 oz-in) was measured. This  represents an increase of only 3.2 mN-m from the 2.2 mN-m tested at ambient.  While motor and bearing tests continued, additional bearings were run-in using the same 10,000  revolution procedure. These bearings were then inspected and characterized. Periodic inspection and  photography indicates that at 20,000 revolutions, the bearing was in “as-new” condition both in terms of  torque and physical condition (Fig. 5).  I  Figure 5. Prototype Bearing Post-Test inspection (20,110 revolutions)  It was determined that a bearing with a measured torque in excess of 21 mN-m (3.0 oz-in) was no longer  functioning properly. The torque was irregular, and it was accompanied by an audible protest or squeak.  A bearing end-of-life torque limit of 14 mN-m (2 oz-in) was established using the “Knee” of the ambient  torque/life curve and allowing for measurement uncertainty. This is viewed as a very conservative limit.  Motor and Bearing Tests  After the bearing run-in tests were completed, the motor stator was assembled into the prototype housing  for motor and bearing tests at temperature (Fig. 6). The same thermal vacuum test setup was used. A  motor and bearing resistive torque of 9.6 mN-m was measured at temperature. This represents an  increase of 4.2 mN-m over the bearing drag test.  233
Figure 6. Prototype Bearing, Motor, and Sensor Cryo-Test Configuration  The test configuration used to test motor and bearing drag also allowed for the measurement of the motor  torque constant. By spinning the assembly at 50 RPM with the GSE motor and measuring the peak back  emf voltage, the motor torque constant was determined to be 11 50 mN-m/A. Through both ambient and  cry0 test runs, it was also shown that the torque constant was largely insensitive to the temperature and  vacuum change.  Motor phase resistance dropped from 82 ohms to 2.2 ohms at temperature. This measurement was done  with a simple two wire ohm meter in the prototype efforts, and the measurement includes all the GSE  wire-some of it still at ambient temperatures. The flight testing will use a four-wire measurement to  eliminate this measurement error. Using 2.2 ohms in the power calculation is considered conservative.  All mechanical connections to the test shaft were removed, and the assembly was tested cold under its  own power. At each of the twelve positions, breakaway current was determined to be less than 10 mA.  The motor was commanded to take 7 steps at a commanded current level. The motor was reset to pull it  back to the motor zero position, and the commanded current level was increased. The process was  repeated. Motion occurred prior to reaching a 10 mA command (Fig. 7).  A prototype control board was designed and built. Two power amplifiers (with current control) are used to  generate commandable current. Since the net current into the node of a wye-wound motor is zero,-the  third phase is generated as a difference between the first two. The electrical cycle was divided into steps  using a 12-bit controller effectively giving the ability to micro-step. The motor is driven open-loop 30-  degrees (4096 micro-steps) from one position to another. Once in the approximate position, the closed  loop mode is activated, and the position sensors are used to “micro-step” to a final optic position. The  position signal is differentiated and used in a damping circuit while in the closed loop mode. The result is  a nicely damped response. The control loop can be used to drive to any value in the position signal, but  the sensor ramp has been designed (and aligned) such that each optic position is nominally represented  by a zero according to the position signal.  234
Figure 7. Breakaway Current Tests  Position Control Tests  A 17-bit encoder was connected to the test shaft of the prototype in order to obtain position data (Fig. 8).  The sensor track design results in 12 zero-voltage position signals and two higher voltage reference  signals in a single revolution of the wheel. The sensor track was aligned to the motor zero during  assembly. These data were used to obtain a voltage level of 0.09 V that corresponds to the 0.310 mm  position requirement.  Figure 8. Typical Voltage Response of Position Signal  Control scripts were written to command the motor from position to position switching back and forth from  open loop to closed loop modes of operation just as proposed for flight (Fig. 9). A reset was used to pull  the motor to the closest motor zero position. Open loop moves were accomplished by commanding a  speed, a current, and a number of steps (4096). The scripts then closed the position loop and  235
commanded the error to zero. Finally, the motor current was ramped to zero. The whole process was  repeated to accomplish 360 degree rotations. Closed-loop error was found to be acceptable at  approximately 0.09 mm. However, the results indicate an unacceptable position error “spring-back effect  when the holding current is removed. This error is illustrated by the nearly 0.5 V motion near the end of  each position hold.  Figure 9. Nominal Rotation of Wheel to Twelve Positions  Dither and Check Routine  Through a series of build-up and breakdown tests of the prototype, it was determined that the powered-off  position error is the result of both the bearing and the motor. The bearings are large (for a precision  instrument bearing), and they have an outer-land riding cage. It is believed that this cage tends to store  some windup energy when in motion. In addition, the DC motor appears to have a restoring energy.  Additional tests are planned that will help verify the root cause of the motor restoring forces.  It has been shown that increased friction levels reduce the powered-off position error. This is good news  as this means that this particular position error will not get worse at the end of bearing life. It also provides  a possible solution to the problem. After much deliberation, it was determined that introducing another  source of debris into our optic system was not advisable.  A common method for improving position results is to dither back and forth about the final position. This  tends to relieve friction terms, and it has been shown to dramatically improve final positions of the  prototype (Fig. 10). The motor is controlled past the intended final position by 1.4 degrees. This excursion  is based on the ball-to-cage pocket clearance and is intended to specifically address bearing cage  windup. The wheel is then commanded by half that excursion to the opposite side of the final position.  The distance is halved again. The final “dither” is not always possible to see in the position signal.  However, this approach does not guarantee a final position.  236
Figure 10. Results Showing Dither Improvements  The proposed final solution is to dither and check three times before applying a powered-on hold. If the  powered off position error is unacceptable, then a dither is commanded. If the position error is still  unacceptable after dithering 3 times, then the system leaves power on (8 mA) (Fig. 11).  . . .. i  dither  Figure 11 : Typical Dither Details  237
Conclusions  A calculated torque margin of 1.8 is now supported by engineering test results. A motor torque constant  of 1150 mN-m/amp was measured at ambient and operating temperature and meets the specified  minimum of 1100 mN-mlA. A motor drag torque of 4.7 mN-m was measured and is less than the specified  maximum of 5.3 mN-m. Cold bearing testing of the WA prototype successfully characterized the  bearings at various cold temperatures and established an end-of-life bearing torque limit of 14 mN-m (2  oz-in). Further life tests are planned.  A measured motor phase resistance of 2.2 ohms results in an estimated peak power consumption of 89  mW and an average power consumption of 0.44 mW when operated at its maximum duty cycle.  Motor control and current margin have been demonstrated at a temperature of 20 K. A powered-on  closed loop control error of 0.09 mm was measured at operational temperatures. A breakaway current of  less than 10 mA was measured.  Acknowledgements  Development of the NlRCam instrument at the Lockheed Martin Advanced Technology Center is  performed under contract to and teamed with the University of Arizona’s Steward Observatory. The  University of Arizona in turn is under contract to the JWST Project at the NASA Goddard Space Flight  Center.  A great deal of team work was necessary to accomplish these tests. Tom Welsh, Bud Swihart, and  Richard Bnrner played critical roles in getting the hardware to the proper test temperature. Their efforts  and dedication are greatly appreciated.  References  [l] Gould S.G., E.W. Roberts, “THE IN-VACUO TORQUE PERFORMANCE OF DRY-LUBRICATED  BALL BEARINGS AT CRYOGENIC TEMPERATURES,” 23rd Aerospace Mechanism  Symposium, 1989.  238
Cryogenic Nano-Actuator for JWST  Robert M. Warden'  Abstract  An extremely precise positioning mechanism has been developed for use in space for optical positioning  of large mirrors. The design incorporates traditional mechanical components such as gears, bearings and  flexures in a unique configuration covered by two patents. This linear actuator is capable of 10 nanometer  position resolution over a range of 20 mm and can operate under cryogenic conditions. The design,  assembly, construction and testing of this mechanism are presented.  Introduction  The James Webb Space Telescope (JWST) is configured to be a large deployable spacecraft as shown in  Figure 1. A key component of JWST is the Optical Telescope Element (OTE), which consists of all the  components along the optical path including the Primary and Secondary mirrors. The Primary mirror is  about 6.5 meters in diameter and is made up of 18 segments. These mirrors are folded up during launch  and deployed in space. Once unfolded, the mirrors must be deployed away from the launch restraints and  then adjusted very precisely.  Figure 2 shows how each primary mirror segment and the secondary mirror is supported and controlled by  six linear actuators to obtain six degrees of positioning control. Each mirror can be positioned in tip, tilt,  piston, horizontal & vertical decentering and clocking. Two actuators are assembled into a bipod assembly  as shown in Figure 3. The final hexapod configuration is made up of three bipods. In addition, each  primary mirror segment features a central actuator for adjusting the radius of curvature of the segment.  The positioning and focusing of the primary and secondary mirrors require a total of 144 actuators.  7 Actuators  segmen\ A /  Figure 1. James Webb Space Telescope (JWST)  * Ball Aerospace & Technologies, Boulder, Colorado  Proceedings of the 3dh Aerospace Mechanisms Symposium, Langley Research Center, May 17-79,2006  239
Back side of mirror spacecraft framework and the mirror segments. For  the purpose of this paper, the Delta Frame is fixed in  space and the mirror moves relative to it.  Delta Frame (fixed)  Radius of Curvature  strut (6 places) Strut to mirror  attachment  (6 places)  Delta Frame to  Backplane attachment Wiff le Plate  (3 places) I Radius of Curvature  Actuator  mirror, an additional actuator  is used to control the Radius  of Curvature of the mirror.  Actuator Pdr (3 places)  See also Figure 3.  Figure 2. Hexapod Mounting  240
Actuator Requirements  Coarse step size  Axial load  Axial stiffness The Actuator for JWST has two top level requirements:  1. Accurately position the mirror segments.  II Support the mirror segments during ground test and launch  These requirements apply to the actuators supporting the 18 segments of the primary mirror as well as to  the actuators that support the secondary mirror.  e1 .O microns 0.058 microns Yes  1890 N (425 Ib) 2650 N (595 Ib) Yes  24,500 N/mm (140,000 Ib/in) 25,200 N/mm (144,000 Ib/in) Yes Positioninu requirements  The first requirement of positioning was used to generate the following derived requirements:  Position feedback  Cryogenic operation  Nominal length 1. Move the mirror from the stowed position to the nominal deployed position.  2. Move the mirror from the nominal deployed position with 6 degrees of motion.  3. Position each segment to nanometer resolution.  4. Support the segments in a hexapod configuration.  5. Operate at cryogenic & ambient conditions  6. Operate over the life of the mission.  20 microns 12 microns Yes  30 K 20 K Yes  138.8 mm (5.5 in) 138.8 mm (5.5 in) Yes Load Reau irements  The second requirement of support was used to generate the following derived requirements:  1. Support the mirror segments during launch.  2. Support the mirror segments during ground optical testing.  3. Support the mirror segments during ground transportation.  4. Hold the mirror segments in place with power off.  5. Support the segments in a hexapod configuration.  6. Operate at cryogenic & ambient conditions  Mass The derived requirements are summarized in Table 1.  700 gram (1.55 Ib) 665 gram (1.47 Ib) Yes Table 1. Requirements Summary  I Coarse range I >20 mm I 21 mm I yes I  I Axial holding I 380 N (85 Ib) I 890 N (200 Ib) I yes I  242
Early Development  The success of the actuator is primarily due to two important inventions: the fine stage flexure and the  coarse drive coupling. It is important to understand the operation of these two elements before going on to  the overall actuator description. The actuator is comprised of numerous individual design elements that all  work together in order to satisfy the demanding requirements of cryogenic nanometer-level positioning.  However, these two inventions enable the fine adjustment capability over the long range of motion  required for the actuator.  Fine Staae Flexure  The development of the actuator began at Ball Aerospace in 1997 with the invention of the fine stage  flexure. A need was established for amotion reduction'device that could convert a relatively coarse input  motion into a well controlled, optical-level, fine  motion. It was well known that flexures work well  for motion control because they enable motion  without backlash or hysteresis. However, simple  flexures were not able to achieve the large ratio  required for this application.  The solution, partially shown in Figure 4, was the  "Motion Reducing Flexure Structure", developed by  Ball Aerospace and covered by United States  Patent number 5,969,892 dated October 19, 1999.  This compound flexure operates in two stages. As  the middle of the cross bar is moved up and down,  the sides are moved out and in, thereby causing a  small but controlled change to the overall height.  Motion reduction of up to 1OO:l can be achieved  using this design. 1042a  1042%  Figure 4. Flexure Patent Description  Coarse Drive Couplinq  The next development was the use of a single motor to operate both coarse and fine motion. Although the  fine stage flexure provides the nanometer-range positioning accuracy needed for aligning the mirror  segments, it does not accommodate the large range needed for moving the mirrors from the stowed  position to the deployed position. For this reason, a coarse motion feature was needed for the actuator. It  was desired to have one motor drive both the coarse motion and the fine motion.  Figure 5. Coupler To move both the fine stage and coarse stage with a single  motor required the use of a coupling that could switch  between coarse and fine modes or at least disengage the  coarse motion. This was achieved by the invention of a  tumbler type coupling, which connects the fine drive to the  coarse drive as shown in Figure 5. This coupling consists of  two rotating disks, each with a protruding pin. This results in  a deadband or backlash of approximately 90% of the  rotational input. When the motor is reversed, the drive pin  backs away from the driven pin so that the coarse motion is  decoupled from the drive train. In this deadband zone only  the fine stage is engaged, which enables precise mirror  positioning. At the end of this travel, the coarse shaft is  again engaged to enable coarse motion. This mechanism  was developed Ball Aerospace and is covered by United  States Patent 6,478,434 dated November 12,2002.  243