ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Precision Linear Actuators for the Spherical Primary Optical Telescope  Demonstration Mirror  Jason Budinoff* and David Pfenning*  Abstract  The Spherical Primary Optical Telescope (SPOT) is an ongoing research effort at Goddard Space Flight  Center developing wavefront sensing and control architectures for future space telescopes. The 03.5-m  SPOT telescope primary mirror is comprise9 of six 0.86-m hexagonal mirror segments arranged in a  single ring, with the central segment missing . The mirror segments are designed for laboratory use and  are not lightweighted to reduce cost. Each primary mirror segment is actuated and has tip, tilt, and piston  rigid-body motions. Additionally, the radius of curvature of each mirror segment may be varied  mechanically. To provide these degrees of freedom, the SPOT mirror segment assembly requires linear  actuators capable of <lO-nm resolution over a total stroke of 5 mm. These actuators must withstand high  static loads as they must support the mirror segment, which has a mass of -100 kg. A stepper motor  driving a differential satellite roller screw was designed to meet these demanding requirements. Initial  testing showed that the actuator is capable of sub-micron repeatability over the entire 6-mm range, and  was limited by 100-200 nm measurement noise levels present in the facility. Further testing must be  accomplished in an isolated facility with a measurement noise floor of <5 nm. Such a facility should be  ready for use at GSFC in the early summer of 2006, and will be used to better characterize this actuator.  Introduction  Future large (>6 m) space telescopes such as the James Webb Space Telescope, SAFIR, and beyond  require segmented primary mirrors to package into launch vehicle payload fairings of diameters less than  their apertures. Requisite architectures to align or “phase” the individual segments into a single optical  surface after launch and deployment are required. Current techniqyes used on large ground-based  telescop:s such as Keck include precision segment edge sensors and various types of wavefront  sensors . However, phasing a large number of segments requires a significant amount of computing  resources which can reduce observing efficiency. Maintaining a “phased” array of mirror segments in a  challenging environment such as low-earth or L2 orbits remains to be seen. The SPOT research testbed  will explore a new method of phasing segmented mirrors. The SPOT telescope architecture has possible  application to a robotically assembled telescope for ISS, as shown in Figure 1.  2t-  F  Figure 1. A possible application of the SPOT telescope architecture: a telescope mounted on the  “top” (zenith) end of the 21 truss  Wavefront Sensina and Control  A relatively recent technique utilizing image-based wavefront sensing has been pursued by the GSFC  optics branch, Code 551. By placing a point source and camera at the center of curvature of a spherical  * NASA Goddard Space Flight Center, Greenbelt, MD  Proceedings of the 3gh Aerospace Mechanisms Symposium, langley Research Center, May 7 7-79,2006  1
mirror, a direct measurement of its surface wavefront error is possible. Taking various defocused images  at the center of curvature and using an iterative transform solver, the phase error of the reflected  wavefront can be determined3. From the phase error tip, tilt, defocus, and other Zernike terms (currently  truncated to the first 15 terms) can be recovered. This information is used to position the mirror segments.  To further develop this approach to phasing mirror segments, the SPOT internal research & development  project was started in 2004.  u,  B - P  or, - 0"  E$ c)  -  0  (1) Fuewue Caineia hlteiface 8 /  Souice at ROC  (3) WFS Alpritluils (DSP) _---_---___  DSP Processor  Figure 2. SPOT Testbed Schematic  Nanometer-level positioning of -1 00-kg mirror segments was required, as well as a high-load nanometer  displacement actuator to mechanically bend the mirror segments to adjust their radius of curvature. As the  program had limited funding, low-cost actuators were designed to meet these requirements.  SPOT Background  The Spherical Primary Optical Telescope (SPOT) is a GSFC internal research & development program  initiated in 2003. The goal of the SPOT effort is to develop a robust architecture which will reduce the cost  of large-aperture, segmented primary mirror space telescopes. The SPOT telescope architecture is based  upon two key technology developments: 1 ) a high-rate, center of curvature, iterative transform phase-  diversity phasing algorithm, and 2) a low-cost mirror segment. The SPOT demonstration telescope is a  03.5-m segmented spherical primary. The primary consists of 6 identical hexagonal segments measuring  876 mm point-to-point, in a 1-ring configuration, without a central segment. However, only 2 segments are  being fabricated for this effort. Two segments are the minimum amount required to successfully  demonstrate the phasing architecture. Each segment has rigid-body position control in tip, tilt, and piston.  Each segment also has mechanical radius-of-curvature control. Some of the relevant requirements for the  SPOT mirror segments are given in Table 1.  2
Table 1. Pyrexm Mirror Segment Requirements  Tipmilt Range  Tipmilt Resolution  Tipmilt update rate  Position Hold  Actuator Thermal Stability  Static Load Mirror Requirement I Value I Units I Note  Size I 0876 I mm (inch) 1 Point-to-point hex  f 2.0 degree  0.05 arcsecond As allowed by focus resolution  1 HZ  0 Amp Power-off hold  35 kg Mirror mass -1 00 kg, assume 3 Micronddeg As small as practicable I Focus update rate I I  I I I I actuators I  To provide rigid-body positioning in tip, tilt, and defocus of the segments, which will weigh -50 kg each, a  tripod mechanism with custom actuators was designed at GSFC.  Design of the Segment Assembly Tripod  The mirror segment must have 3 rigid-body degrees of freedom: tip and tilt rotations and piston, a vertical  translation. A segment assembly is shown in Figure 3.  Figure 3. The SPOT Mirror Segment Assembly Tripod
Kinematics of the Seqment Assemblv & Grubler’s Mobilitv Criterion  The mirror requires only tip tilt and piston adjustment, 3 degrees of freedom. A hexapod would provide 6  degrees of freedom; but we don’t need 6. Therefore, a tripod was selected to provide the rigid body  motions required. The end joints of each strut must constrain a number of degrees of freedom. For  example, a ball-in-socket joint constrains 3 translations but is free to rotate, allowing 3 rotations. From  kinematics, Grubler’s mobility criterion states that F, the number of degrees of freedom in a system, can  be defined by:  i= 1  Where  h = 6, the degrees of freedom in the space the mechanism will be operating in  n = the number of links in the system  j =the number of joints in the system  f, =the degrees of freedom allowed (unconstrained) at the th joint  For the SPOT tripod, each leg consists of 2 links and 3 joints. Ground is considered a rigid link, and the  mirror is the “end effector” or output link. For this system, the following values are used:  h = 6, we shall consider the system exists in 6 degrees of freedom  n = 8, ground and the mirror are each one link, and each leg has 2 links  j = 9, each leg has 3 joints (base, linear, and upper) x 3 legs  f, = 2, base joint of leg 1, XY flexure allowing 2 rotations  f2 = 2, base joint of leg 2, XY flexure allowing 2 rotations  f3 = 2, base joint of leg 3, XY flexure allowing 2 rotations  f4 = 1, linear joint of leg 1, allowing 1 translation  f5 = 1, linear joint of leg 2, allowing 1 translation  f6 = 1, linear joint of leg 3, allowing 1 translation  f7 = 2, upper joint of leg 1, XY flexure allowing 2 rotations  fa = 2, upper joint of leg 2, XY flexure allowing 2 rotations  f9 = 2, upper joint of leg 3, XY flexure allowing 2 rotations  Using the above values, the system degrees of freedom are calculated as:  i=l  The number of system degrees of freedom is 3, corresponding to tip, tilt and piston. The use of XY  flexures, allowing f, (for I = 1 ..3, 7..9) = 2, is justified. Ball joints could be used, but the additional passive  degree of freedom at each leg (roll) would have to be subtracted out of the Grubler criterion equation to  keep F = 3. The XY flexures will allow hysteresis-free angular motion at the cost of increased force  proportional to displacement.  Actuator Design  The actuator is shown in Figure 4.  4
Figure 4. The SPOT linear actuator, shown with a six-inch ruler for scale.  Figure 5. An exploded view of the SPOT linear actuator  Stetmer Motor/Harmonic Drive Gearhead  The HD14 1OO:l harmonic Drive gearhead  Phytron 500 step per revolution or 0.72 degree step size  3-phase stepper motor, 2.5 Amp winding  Agilent HEDL 5540 500 line (A quad B = 2000 counts) incremental encoder on motor output ZSS 52.500.2.5.Kl -HEDL-HDl4/100  5
Figure 6. The Phytron ZSS 52 / HD14 actuator  Differential Satellite Roller Screw  Several options exist for rotary-to-linear motion: lead screw, ball screw, or roller screw. Generally the most  precise of these is the satellite roller screw. A differential roller screw was selected. After several months  of vendor interaction, a differential roller screw was selected and sized. The smallest, readily available  precision roller screw has a pitch of 0.5 mm. A lead of this size can produce 10 nm steps using the 0.72"  stepper motor and 1OO:l harmonic drive.  Using a differential roller screw, the effective lead can be reduced by 2 orders of magnitude, but at the  cost of a stroke limitation to -6 mm. The theoretical attainable step size drops to 0.4 nm (see below). A  RollvisTM differential satellite roller screw utilizes equal thread pitch on the nut and the shaft, but varies the  nut/shaft thread pitch diameters and the number of starts on the nut and shaft. The effective lead of such  a differential satellite roller screw can be calculated by:  Where  D, = Nut thread pitch diameter, mm  D, = Shaft thread pitch diameter, mm  P =thread pitch in threaddmm  N, = number of thread starts on nut  N, = number of thread starts on shaft, negative for opposite handedness to nut starts  For the SPOT actuator, the values were varied within reasonable limits until a minimum value for effective  lead was found. Using the following values:  D, = 29 mm  Ds=19mm  P = 0.5 threadslmrn  Nn = 4  Ns = -6
The minimum effective lead, Lff, was found to be 0.02 mm per revolution, or -21 microns per revolution. A  bind condition determines the total stroke, which for this differential roller screw is -6 mm. Rollvis Swiss  S.A., a Swiss manufacturer of precision roller screws, fabricated the roller screw as Model  RV160/19,02.R1.604350, custom designed for maximum resolution.  The 5 rollers roll around the shaft and are held in a rotating retainer ring at each end of the nut. A sun gear  at each end of the nut and mating roller gears at the ends of each roller keep them in proper clocking as  they rotate around the shaft. The shaft, nut and rollers are 410 stainless steel. The lubricant for the roller  screw is lsoflex Topas NCA 52, manufactured by Kluber Lubrication. It is a synthetic oil with a calcium  thickener. The differential satellite roller screw is shown in Figure 7 and 8.  Figure 7. Custom Differential Satellite Roller Screw  Figure 8. Nut end details showing the timing gear teeth  Helical Couding  A standard flexible shaft coupling from Helical Products Company, Inc. was used to couple the motor  output shaft to the satellite roller screw shaft. Such couplers allow torque to transmitted despite small axial  misalignments between the shafts. A model HRM-125-12mm-12mm coupling was used. This coupler  uses 2 pairs of cup-point set screws to secure the motor and screw shafts. The coupling is 17-4PH H900  stainless steel. The coupling is shown in Figure 9.  7
Figure 9. Helical Coupling  Bearinas  Barden 1 O6HCDUL back to back duDlex oair. ABEC-9. SAE52100 steel  30-mm bore diameter, 15-degree contaci angle, 14 7.1 4-mm (9/32") diameter 440C balls, 36-kg (80-lb)  heavy preload  Static load capacity 1005 kg (221 6 Ib)  Machined p ca  Winsorlube oil  Figure 10. Barden 106HC Duplex bearing pair  Lea End Flexures  XY flexures  Crossed flexure  Torsional rate  Fat iq u e/c ycl e I if e  8
Where  K = torsional spring rate  r = radius of notch cut  b =thickness of flexure section  t = width of notch E = Modulus  For the SPOT tripod, each leg consists of 2 links and 3 joints. Ground is considered a rigid link, and the  mirror is the “end effector” or output link.  Conclusion  Further testing to fully characterize nanometric step size, repeatibilty, and linearity must be accomplished  in a quiet facility. The actuator performance will also be measured with an actuator built into a mirror  segment. Actuator positioning performance will be indirectly measured by mirror radius of curvature  change per commanded step.  References  1.  2.  3.  4.  5.  6.  7.  8. 9. Budinoff, Jason G. “SPOT Mirror Segment Assembly Requirements  - Revision C, March 2004  Howard, Joseph ”Optical Design Study for NASA’s Spherical Primary Optical Telescope” SPIE  Budinoff, Jason G, Michels, Gregory J. “Design & Optimization of the Spherical Primary Optical  Telescope (SPOT) Primary Mirror Segment” SPIE 5877-42  Dean, B, Smith, S, Budinoff, J. “Image-Based Wavefront Sensing for the Control of Space Optics”  USAF AMOS Maoi Technical Conference, September, 2000  Chanan, G., Troy, M., Ohara, C. “Phasing the Primary Mirror Segments of the Keck Telescopes:  A Comparison of Different Techniques” Proc. SPIE, 4003, 188-201, 2000  Martinez, L.M., Yaitskova, N., Dierickx, P., Dohlen, K. “Mach Zender Wavefront Sensor for  Phasing Segmented Telescopes”  Tsai, Lung-Wen, ‘Robot Analysis: The Mechanics of Serial and Parallel Manipulators” 01 999 John  Wiley & Sons, Inc.  Paros, J.M., Weisbord, I. “How to Design Flexure Hinges” Machine Design vol 37, ppl51-156,  1965  Lobontiu, Nicolae “Compliant Mechanisms: Design of Flexure Hinges” 02002 CRC Press LLC 5524-1  9
The CRlSM Motor/Encoder Assembly and Diaphragm Bearing Assembly Design  Jeffrey Lees" and Ed Schaefe;  Abstract  This paper will describe the thin section angular contact bearings and WS2 dry film lubrication used on the  compact Reconnaissance Imaging Spectrometer for Mars (CRISM) motor/encoder and diaphragm  bearing assemblies.  Introduction  CRISM will use targeted observations to search for evidence of aqueous activity and to characterize the  geology and composition of surface features' on Mars (Figure 1). Global measurements acquired  repeatedly throughout the Martian year will provide information on atmospheric water vapor, CO, and  aerosols complementary to that from other MRO instruments.  The Optical Sensor Unit (OSU) consists of an optical system, a cryogenic system, and focal plane  electronics gimbaled about a single axis to allow scanning over k60" from nadir. Its mechanical design  builds on proven technology from previously successful APL instrument designs. The base housings are  fabricated from titanium that provides high stiff ness and thermal isolation within the same component.  The gimbal bearings are a precision assembly designed to operate in a -60°C environment. The gimbal is  driven directly by a brushless DC motor paired with a BE1 20-bit incremental position encoder. The  encoder disk is co-mounted directly to the bearing shaft beside the motor rotor and the read heads are  mounted to the bearing housing alongside the motor stator. The electrical signals and purge are passed  through a twist capsule in the center of the motor/encoder bearing assembly. A second bearing pair is  mounted in a parallel diaphragm bearing housing that provides high stiffness in the lateral directions to  the gimbal axis and flexibility along the gimbal axis to compensate for differential expansion of the  instrument and spacecraft. The spectrometer housing is passively cooled to -90°C using a flexible link to  the anti-sunward radiator. The anti-sunward radiator passes through the center of the diaphragm bearings  in a thermally isolated mount, and thus rotates with the OSU; its FOV is independent of gimbal position.  4  I  Figure 1. CRISM instrument  Johns Hopkins University Applied Physics Laboratory, Laurel, MD  Proceedings of the 38 Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  11
CRISM Thermal Design  The CRlSM thermal design provides both active cooling of the IR focal plane to cryogenic temperatures  to reduce dark current and passive cooling of the spectrometer housing to -80°C for low background.  Simultaneously it keeps the electronics section near -40°C (Fig. 2). Cryogenic cooling is provided by three  Ricor K508 integral Stirling cryocoolers. The multi-cooler configuration requires “thermal switching”  between coolers. A cryogenic diode heat pipe assembly consisting of heat pipes and a thermally isolating  mounting assembly connects the active cooler with the focal plane while isolating if from the two dormant  coolers2. Each of the three diode heat pipes is connected to the focal plane on one end and to a cooler on  the other. The focal plane electronics are mounted in the bottom of the gimbal housing and are  maintained at -40°C. The housing along with tantalum plates provide EM1 shielding and minimize cable  length to the focal planes. Table 1 lists the CRISM expected flight operating temperatures.  Table 1. CRISM Flight Operating Temperatures  -40”c 7, I Component ~ I I Min. I Max. I  mousina I “C I -1  IR FPA 120  Crvo-Coolers I “C I -25 I 20  c T  / 113K -20°C 4- -7 7 -1 20°C  I  Figure 2. CRISM Thermal Zones  12
CRISM Bearing System  The CRlSM instrument rotates the OSU do" from nadir. The OSU weighs 20.9 kg (45.9 Ibf) and is  supported by the motor/encoder (Figure 3) and the diaphragm (Figure 4) bearing assemblies. The motor-  encoder side used a thin-section angular-contact duplex pair mounted back to back and was designed to  take the non-axis moments, the entire thrust load, and its share of the radial loads associated with launch  event (Figure 5). These bearing were designed for a 266.9-N (60-lbf) axial preload. The diaphragm side  used a thin-section angular-contact duplex set mounted face to face and was designed to take only the  radial loads associated with the launch event. The diaphragm bearings were designed for a 66.7-N (15-  Ibf) axial preload.  7 Spacer/Seal  Bearing  Spacers  Switch Titanium  Spindle  2x Limit  Titanium  Housing Motor Stator  Motor Rotor - Encoder Disk  Purge  Fitting  - Twist  Capsule i  Figure 3. CRISM MotorEncoder Assembly  Preloading  Angular contact bearings should be preloaded as lightly as necessary to achieve the desired results. A  duplex pair is a pair of bearings that have a pre-determined amount of preload built into them. This was  accomplished by grinding the inner or outer ring a sufficient amount to eliminate all internal clearance  within the bearing commonly referred to as the preload offset. There are, however, several disadvantages  to preloading bearings:  Increased running torque  0  0 Sensitivity to misalignment Sensitivity to differential thermal expansion  13
Parallel Corrugated  Diaphragms  m  I  s 7 1  I k- Bearings  Flex link  L Interface  r /- Titanium Housing  Figure 4. CRISM Diaphragm Bearing Assembly Anti-Sunward  Radiator  The CRlSM duplex bearing pairs were separated by titanium spacers so that the preload offset would  remain constant over temperature. However, the difference between the 440C inner and outer rings and  the titanium shaft and housing resulted in a reduction of clearance as temperature decreased (Figure 6).  A reduction of clearance results in a decrease of the contact angle. However, the bearings are only going  to experience substantial axial loads during the launch. The bearings were tested to -196°C and  continued to rotate freely. The few disadvantages of preloading are more than offset by the following  advantages:  Reduces axial and radial runout of the rotating shaft. Required for the encoder disk to read head  alignment  Reduces the shaft deflection under load and improves its assembled stiffness  Removes free play in the bearing set, keeping the bearing set loaded in-order to avoid skidding of  the balls  Minimizes the peak stresses that occur during the maximum loading events by ensuring the load  on the bearings is shared by more balls in each bearing 0  Decreases bearing noise  In addition to the axial preload, the CRlSM bearings employed a light interference fit, 12.7 pm (0.0005 in)  in the bearingkhaft fit and 15.2 pm (0.0006 in) in the bearing/housing fit.  14
Encoder Disk-  Interface  Bearing -  Bearing 7  Spacer  Bearin -  0.80  0.70  h C  ~ 0.60  0 9 .-  v  $ 0.50  C  (d 5 0.40 !!!  -  (d  2 0.30 E  C -  0.20  0.10 Bearing -  Retainer  Figure 5. CRlSM motodencoder spindle  - -  L - Clearance at LMC  I I l~l~l~l~l~l~l~l~l  -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20  Temperature (C) 12.5  12.0  h w  Q) U v  e,  w  11.5 2 -  c 0  (d  C c  6  11.0  10.5  Figure 6. CRlSM motor/encoder bearing clearance  15
CRISM Bearing Tests  Identical sets of sputtered MoS2-coated and WS,-coated thin-section Teflon-toroid angular contact  duplex-pair bearings were purchased simultaneously (Figure 7). Prior to running the tests, an initial set of  pictures using a scanning electron microscope were taken of both types of bearings (Figure 8 through  Figure 10). Figures 8 through 10 show that MoS2 appears to have a much rougher surface than WS2. We  believe this is why the MDlS bearings never “felt” smooth.  Figure 7. WS, and Mo S2 life test bearings  Figure Sb. MoS2 Coating  17
rigure iua. w& boaring Figure IUD. ~os~~oaring  Following the test of the WS2-coated bearings in which 57952 cycles were completed, we were  concerned that the bearings seemed much rougher than they were prior to the test. After a visual  inspection of the bearings, a significant amount of debris in the bearings was found. Post-test pictures  were taken with a standard microscope, Figure 11 through Figure 13. There were several types of  contamination found:  0 White particles  0  0 Brown particles on the toroids  Brown film on the raceways  We were concerned that the contamination found in the bearings following the test would present a real  problem, both in terms of lifespan and smoothness. However, upon review of the data, the rougher feel  was not adversely affecting the motor control system. The bearings were disassembled and further  inspected. We could find no evidence of wear in the raceways or the WS2 coating. The life test was also  significantly longer than the expected lifetime of the instrument at Mars.  Bearinq Test Conclusion  The WS2 coating is more than adequate for a slow moving oscillating gimbal requiring dry film lubrication.  We were satisfied enough with the WS2 performance that we did not test the sputtered MoS2 bearings,  we had found the solution we were looking for. The Teflon toroids would likely continue to break down  and contaminate the bearings with additional particles that the instrument may no longer be able to rotate  as precisely as is required. We believe the Teflon toroids will ultimately be the life limiting factor of these  18