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Micro-Arcsecond X-ray Imaging Mission Pathfinder (MAXIM-PF)

Micro-Arcsecond X-ray Imaging Mission Pathfinder (MAXIM-PF). Eric Stoneking Paul Mason May 17, 2002. ACS. ACS Drivers. Very tight attitude and translation control requirements 1 arcsec is limit of existing state of the art

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Micro-Arcsecond X-ray Imaging Mission Pathfinder (MAXIM-PF)

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  1. Micro-Arcsecond X-ray Imaging Mission Pathfinder (MAXIM-PF) Eric Stoneking Paul Mason May 17, 2002 ACS

  2. ACS Drivers • Very tight attitude and translation control requirements • 1 arcsec is limit of existing state of the art • Subarcsec attitude, sub-millimeter translation control to be achieved through technology under development • “Super star tracker” • Very stable gyros • Micro-thrusters • Swarm sensors • Formation Flying • Requires inter-spacecraft sensors and communication • Requires distributed control laws , fault detection, safing algorithms MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  3. Technologies • Key Hardware Technologies • Sensors • “Super Star Tracker” • Quad cell laser beacon tracker • Very low-drift gyros ( < 1 uas/day) • Swarm Sensor • Low bias Accelerometer • Micro-Newton Thrusters • Formation Flying Algorithms • Formation acquisition and maintenance • Micro-thrust Control • Disturbance estimation and rejection • Parameter estimation and adaptation • CG migration/fuel usage • Bias/drift estimation MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  4. ACSControl Modes • Coarse Formation Acquisition • Omni RF ranging with small programmed maneuvers to solve “Lost in Space” • Maneuver to assigned positions in formation (within meters) • Fine Formation Acquisition • Acquire laser beacons in star trackers • For Phase 2, freeflyers acquire swarm sensors • Maneuver Detector to acquire science target • Science • Hold attitude and relative position • Maneuver • Execute commanded attitude/translation maneuver while maintaining formation • Translation requirements relaxed from Science mode • One day in Phase 1, ~ 1 week in Phase 2, dependent on thrust level • Safehold • Point solar arrays to Sun • Collision avoidance MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  5. ACS Requirements Imposed On Other Sub-Systems • “Super Star Tracker” (Laser beacon tracker + low-drift gyros) needed for detector control (Instrument) • Thruster impulse bit < 20 mN-sec (Propulsion) • Omni RF used for coarse formation acquisition (Comm) • Lowest structure mode should be > 10 Hz, to minimize interaction with attitude control loop (Mechanical) MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  6. ACS Concerns and Comments • Technologies, while not “miracles”, still carry significant development risk • Concerns • Contamination due to thruster firing • Lost in Space problem • Misalignment of Star Trackers, gyros, optics • Due to tolerances of the Phase 1 S/C connections • If impulsive thrusters are used, drive frequencies must be chosen to stay from structural resonant frequencies • Tight control and knowledge requirements • Requires higher control bandwidths • Ensure quiet motion in formation mode • Advanced estimation and control techniques are needed • Trade bandwidth against estimator complexity • Control authority levels should overlap • During retargeting coarse control is utilized • Settling times • Maintaining the formation control during retargeting will help to provide a quiet structure MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  7. Future Studies • Expansion to full MAXIM mission architecture • Several freeflyers will have the capability to lead a subgroup • ACE and C&DH should be developed to handle an increase in the number of S/C • Tighter safehold and collision avoidance constraints • Direct inter-FF ranging? • Higher Formation and individual S/C Bandwidth • Increase the number of reference fiducials on Hub MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  8. Backup Slides • Sensor Configurations • Components • Trade Studies MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  9. Detector S/C Hub S/C Super Star Tracker centers on Laser Beacon Laser Beacon illuminates Detector S/C Super gyros hold inertial attitude Laser Detector measures range by time-of-flight of reflected laser beam Normal Star Tracker places Laser Beacon against fixed stars Reflector Cube reflects laser beam back to Hub for ranging Coarse Ranging by omni RF comm link Hub/Detector Sensor Configuration MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  10. Freeflyer S/C Hub S/C Swarm Sensor measures range by bouncing RF, laser off Hub Reflector Cube Small Laser Beacon Normal Star Tracker places Hub beacon against fixed stars Coarse Ranging by omni RF comm link Hub/Freeflyer Sensor Configuration MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  11. Attitude/Translation Requirements and Sensors: Optics Hub MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  12. Attitude/Translation Requirements and Sensors: Detector MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  13. Attitude/Translation Requirements and Sensors: Freeflyer MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  14. ACS Components Optical Hub MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  15. ACS Components Detector MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  16. ACS Components Free Flyer MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  17. ROM ACS Labor Cost Note: 1) Estimated cost derived from MAP cost in $K MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  18. Attitude and Position Information Thrust commands Thrust Thrust Attitude and position Attitude and position Phase 1 Command structure • ACE/C&DH in charge of the unit sensor/actuators • Receive measurements from freeflyer attitude sensors • Sends thruster commands to freeflyer MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  19. Formation • Configuration: • Expandable • Increase the number of free flyers with several acting as local leaders • Redundancy • For the full version local leaders can take the place of the hub or detector • Communication issues • Reduces communicate traffic • Improves local and global autonomy MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  20. Formation Configuration • Detector: • Communicates with ground and Hub • Has more fuel and thrust authority for retargeting • Additional safehold communication/ranging capabilities can be utilized to provide position of self and hub (full mission) • Optical Hub: • Provides command for formation structure and retargeting • Safehold beacon used to keep free flyers near • In safehold sends detector updates on current estimated location of FF and self (full mission). • Freeflyers: • In Safehold, execute collision avoidance and stay close to Hub • Freeflyers can lead a subgroup as numbers of S/C grows (full mission) • Can replace some of the functionality of the Hub (full mission) MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  21. High Accuracy Formation Control Technologies • External and Internal Disturbance estimation • Estimate fuel usage and CG migration • Sensor bias and drift • Uncertainty bounds • Localized disturbance levels • Other system parameters • Control • Utilize estimated states compensation scheme • Adaptive/Robust schemes can account for variations in parameters (Mass Properties, CP-CG offset, local variations in solar pressure) • Phase 2 may employ distributed control schemes to decentralize control • Reduces risk by S/C-level redundancy • May reduce computational load on Hub MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  22. Trades performed • Reaction Wheels vs. Thrusters for Attitude Control • Reaction wheels would be jitter sources • Continuous micro-thrust needed for translation control • Recommendation: Use thrusters for attitude as well as translation control • Do Freeflyers talk to each other? • Inter-FF comm would simplify “Lost in Space” solution • Direct measurement of FF-FF ranges • Inter-FF comm complicates RF comm system • More channels required • Recommendation: No FF-FF comm • Avoids complicating RF comm system • “Lost in Space” may be solved with Hub-FF ranging, with small programmed maneuvers MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

  23. Sensor/Actuator Resolution • Minimum Impulse Bit = 20 mN-sec achievable by PPTs or FEEPs • Assumes 100-sec limit cycle on 10 mm translation control, and 100-kg S/C • PPTs provide 10 mN-sec or less • FEEPs provide 1 mN thrust resolution • Accelerometer Resolution Required ~= 1.0x10-9 m/s^2 • Acceleration “bit” is thruster resolution divided by S/C mass • FEEP thruster resolution = 1.0E-6 N, S/C mass < 1000 kg • Onera (GRACE) accelerometer resolution = 3.0x10-9 m/s^2 • Right order of magnitude MAXIM-PF, May 13-17, 2002Goddard Space Flight Center

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