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X-Ray Wide Field Imager Mission

X-Ray Wide Field Imager Mission. Attitude Control Scott Starin , Philip Calhoun, Dave Olney Attitude Control Systems Engineering Branch Code 591 16 – 20 April 2012. ACS Overview. Sensors Coarse Sun Sensor (CSS)

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X-Ray Wide Field Imager Mission

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  1. X-Ray Wide Field Imager Mission Attitude Control Scott Starin, Philip Calhoun, Dave Olney Attitude Control Systems Engineering Branch Code 591 16 – 20 April 2012

  2. ACS Overview • Sensors • Coarse Sun Sensor (CSS) • Redundant sets of 8 units aligned to provide 4π-steradian coverage of sky (CSSs cannot be cross-strapped) • Used for Safe mode Sun acquisition and fault detection for instrument protection • Gyro • 1 internally redundant unit • Sense the attitude rate of change • Used in Kalman Filter to propagate and smooth state and to slew at slow rates accurately • Star Tracker / Telescope Alignment Monitor (TAM) (see details on slide 10) • Uses Camera Head Units (CHU) in pairs to provide alignment between science instrument components • One CHU is pointed towards target and used for attitude determination (inertial frame) • Second CHU is pointed back at mirror and used to detect LEDs at focal planes • Any two forward-facing trackers meet requirements and allow for rejection of systematic error • Total of 3 data processing units (DPU) and 6 camera head units (CHU) mounted inside optics • Actuators • Thrusters • 12 – 22 N (5 lbf) thrusters for attitude control and orbit maneuvers • Thrusters full on for orbit maneuvers, off-pulsed for attitude control • Thruster on-pulsed for attitude control during momentum unloading • Reaction Wheels • 4 – 100 Nms, 0.2 Nm reaction wheels • Used for disturbance torque rejection (momentum storage) and for slewing to new targets

  3. ACS Functional Block Diagram Actuators Sensors Reaction Wheels 4 total (pyramid about X) ACS FSW CSS Tot.16 Safe Mode Att. Determination Gyro Attitude Control Mission Attitude Determination Star Tracker/TAM Thrusters 12 Total (See slide #17) Momentum Management Orbit Maintenance

  4. Observatory Coordinate System and Key Terms Observatory Coordinate System Origin is at the Mirror Node +YOBS points from the Mirror Node, forming a right handed orthogonal frame with X and Z Y is the axis for PITCH (side S/A’s are aligned w/ Y) Target +ZOBS points from the Mirror Node to the Target Z is the axis for ROLL (the Boresight is aligned w/ Z) “FORE” is the +Z (FMA) end of IXO “AFT” is the –Z (Instruments) end of IXO +XOBS points from the Mirror Node towards the Sun, X is the axis for YAW Inertia Tensor Used: [7714 0 -790] I=[ 0 4988 0]kg-m^2 [-790 0 5900] Sun

  5. Operations Requirements • Launch • Direct insertion into transfer orbit in full sun with continuous ground contact (have TDRSS capability) • Indefinite duration safe mode available immediately after LV separation • Deployments start right after LV separation • Cruise to L2 • Start with one month commissioning phase for checkouts, calibrations • Continuous DSN contacts during commissioning, then twice daily for 30 minutes for OD during cruise • Correction burns as required • Mirror cover deployed after observatory outgassing • No exposure of aperture to sun light allowed for remainder of mission • Science observations may start during cruise • L2 Insertion • Performed in Operational configuration (10-3 g level forces only) • Observations • Targeted mode: Pointing at a target for ~20 ks, slewing quickly to a new target. Goal of 3-4 targets per day. • Survey mode: Pointing at a single target with small scanning motions for up to 40-60 days. • Momentum unloads may interrupt a given survey • EOL disposal • Passivate observatory, impart 1 m/s towards deep space • Mission Ops • Highly autonomous observatory, 8 x 7 ground staffing • Data latency 2 weeks required, 72 hours goal from completion of observation to product delivery, excludes bright source observations

  6. Requirements and Considerations • Requirements • 3 year (5 year goal) mission lifetime at L2 • Attitude Requirements • Pointing: --/30/30 arcsec, Roll/Pitch/Yaw (3-sigma) • Knowledge: 30/2/2 arcsec, Roll/Pitch/Yaw (3-sigma) • Jitter: 0.8 arcsec, all axes (3-sigma) over 0.33 seconds Slew Requirement • Complete a 60 degree (yaw) slew in 30 minutes (including settling) • Considerations • Two operational regimes: • Survey mode will have approximately the same attitude for 40-60 day spans, broken by minimal momentum dumps • Targeted mode has 3-4 different targets per day, with potentially large slews between them • 30-minute slew is highly desirable but should not drive sizing • Larger wheel size means less power use during slews, perhaps jitter improvement (needs more analysis) • Assumptions • Adequate calibration slews and observation time for sensor alignments • Full survey-mode momentum build-up will be dumped before starting targeted slews

  7. Field of Regard . . . . Target . . . . . . . . . . . . . . Pitch: +/-30 . . Sun Earth H . . . . . . . . . . . Moon . . Yaw: +/-180  . . . . . . . . . . . . . . . . . . . . Roll: +/-30  . . . . . . . . . . . . . . .

  8. ACS Modes, Sensors and Actuators • Orbit Adjust Maneuvers (Delta-V Mode) • Gyro and thrusters • Full-on thrusters for orbit maneuvers • Off-pulse thrusters for attitude control • Momentum Unloading (Delta-H Mode) • Gyro, thrusters and wheels (spin down) • Attitude control on thrusters as wheels torque to commanded momentum level • Science/Mission (Science Mode) • Gyro, star tracker, wheels, Kalmanfilter • Inertial Pointing (point at Sun during coast phase; keep Sun in XZ plane for targets) • Calibration slews and survey scans (science and ACS sensor alignments) • Re-pointing slews (slewing to next science target) • Safe (Safehold Mode) • CSS, gyro and wheels • Point X axis at the Sun; assumes array will return to “home” position

  9. ACS Sensors and Actuators

  10. Telescope Aspect Determination System (TADS) in ACS terms 1 3 2 2 Forward Star Tracker Image Reverse Star Tracker Image TADS Optics Module Components Back to Back Danish Stellar Compass Star Trackers ACS calls forward facing unit “star tracker” and the reverse facing “Telescope Alignment Monitor” TAM. NOTE: “TAM” in ACS section of MEL is the same as “TADS” TADS Instrument Module Components LED Light Path Instrument FOV Movement of FID lights indicates Motion of Spacecraft relative to Optical axis Porthole in FIP Porthole in MIP LED w/ Optics Instrument Light Source is LED/pinhole or Laser Diode/fiber

  11. Star Tracker/TAM Specifications Micro-Advanced Stellar Compass (μASC)

  12. SIRU Instrument Pointing Gyro Specifications

  13. Honeywell Reaction Wheel

  14. Coarse Sun Sensor: Adcole • Adcole analog CSS detector provides close to 2πsteradiancoverage (85 deg half angle) • One set of 8 detectors distributed across theobservatory provides full4πsteradiancoverage • all usual mission attitudes covered by 3 or more eyes • 4 CSS on front of SA panel facing sunward • 4 CSS on back of SA panel facing anti-sunward • Two sets required for redundancy • CSSs cannot be cross-strapped

  15. Four Reaction Wheels – Momentum Storage RWA body alignment (70-deg pyramid around X) RW1 RW2 RW3 RW4 | 0.3420 0.3420 0.3420 0.3420 | | 0.6645 0.6645 -0.6645 -0.6645 | | 0.6645 -0.6645 0.6645 -0.6645 | (bias to Y & Z axes, momentum accumulates mainly along Y, and then may need to be translated into Z) Momentum Capacity with Margin ( 1 wheel = 100 Nms) The center of pressure (Cp) is offset from the center of mass (Cm) by a moderate amount at 0 degrees pitch. Careful alignment might allow smaller wheels to be selected for a modest savings in mass and power, though pitch angles will change the offset. Alternately, the current design is relatively insensitive to pitch angle, with the Cp-Cm offset ranging from 8-12 cm. Maximum buildup will occur mainly about the Y axis during survey periods. All attitudes will target zero roll (Z offset = 0 deg) to maximize power and minimize solar torque. The 21-day buildup will range from 33-49 newton-meter-seconds (Nms). Selecting 100 Nms wheels is necessary to guarantee that Safehold will operate correctly at the end of the 21-day period, with the requisite pre-Phase A 100% margin against maximum wheel momentum storage rating. This margin is required for true redundancy, and also protects against wheels drawing maximum power during Safehold. Elevation (deg) Azimuth (deg)

  16. Slew Capability • Requirement: Slew minimum of 60 deg about X axis and settle in 30 min. • Momentum may need to be dumped to slew quickly in pitch. • Design: • Goal: Slew should provide good trade-off between conservatism and efficiency. Determine slew capability given reasonable demands on system: • Use ~25% of minimum momentum capacity (13.5 Nms) • Use ~25% of torque capability (~0.1 Nm with four wheels) during ramp • Observatory can slew almost 140 degrees in 25 min, allowing 5 min for settling. • Ramp time may be increased to minimize slew transients.

  17. Pitch / Yaw Control Error Budget This analysis is identical to that of the Xray Calorimeter. Note that the WFI mission will have co-aligned CHUs providing onboard knowledge of 2.1 arcsec(3σ), which can be improved through post-processing by reducing starred items below. Uncorrelated items can be RSSed but a more conservative estimate is to Sum items SUM SUM SUM SUM Few things in our favor: Large inertias (torque produces small angles) Motion in star tracker filters out biases Large observation times Note: RWA jitter is allocation (based on experience of similar missions)

  18. Thruster Configuration

  19. Summary Risks: • Failure of one TAM means alignment knowledge for that telescope is not available Potential future work: • Place thrusters to determine moment arms and torque authority • Plenty of latitude for placement locations • No clear drivers yet for small vs large moment arms • Need Jitter Assessment to determine impact to Pointing Budget • Reaction wheel imbalance • Cyrocooler • Passive isolators could be used to reduce jitter if needed • Example: Chandra reaction wheels mounted on isolator Issues/concerns: • Two very different slew regimes: slow scanning vs. fast re-targeting • May require separate modes, adding software expense • Star cameras pointed toward space should be rotated with respect to each other, • Avoids systematic errors, thereby allowing better post-processing • Smaller wheels might be feasible, but the 100-Nms wheels simplify operations, improve responsiveness of the observatory, and increase available time for observations. • Update wheel configuration analysis as design matures.

  20. Acronym list • ACS – attitude control system • CHU – camera head unit • Cm – center of mass • Cp – center of pressure • CSS – coarse Sun sensor • Delta-H – mode that changes angular momentum • Delta-V – mode that changes orbital velocity • DPU – data processing unit • DTU – Danish Technical University (tracker vendor) • Nms – newton-meter-second (unit of angular momentum) • RSS – root-sum-square • RWA – reaction wheel assembly • SA – solar array • SIRU – trademarked name for a particular gyroscope • TAM – telescope alignment monitor

  21. Supplementary Slides

  22. Kalman Filter Performance

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