1 / 25

Attitude & Orbit Control Subsystem

Attitude & Orbit Control Subsystem. 26 April 2007. Contents. Key Requirements AOCS Design Description Functional block diagram AOCS modes AOCS Hardware Description Hardware Functions/ characterization Interface Summary (Power, Bi-level, Discrete, analog, serial bus)

cleta
Download Presentation

Attitude & Orbit Control Subsystem

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Attitude & Orbit Control Subsystem 26 April 2007

  2. Contents • Key Requirements • AOCS Design Description • Functional block diagram • AOCS modes • AOCS Hardware Description • Hardware Functions/ characterization • Interface Summary (Power, Bi-level, Discrete, analog, serial bus) • AOCS Software Development

  3. Contents (cont’d) • Major Trade-offs • Star camera orientation • Thruster configuration • Jitter analysis (rigid body) • Sun Sensor configuration • Design and Analysis • ASH mode • Navigation filter • Attitude estimator • Off loading • Guidance • Normal mode

  4. AOCS Key Requirements • Orbit Altitude • Orbit Inclination • Equator Crossing Time • Attitude Control Accuracy • Attitude Control Accuracy Goal • Attitude Control Bandwidth • Attitude Knowledge

  5. AOCS Key Requirements (cont’d) • Attitude Maneuvers • Spacecraft Jitter • On-board Orbit Determination • Satellite Autonomous Operations • Over-sampling • Maneuver Agility

  6. Sun sensor GPS MAG Star camera IMU AOCS Design Description: Functional block diagram 1: Attitude acquisition and Safe-hold (ASH) sub-mode: Stabilize (STAB) Sun tracked (STRA) Sun locked (SLO) 2: Normal mode (NM) sub-mode: Geocentric attitude pointing (GAP) Maneuver (MAN) Fine imaging pointing (FIP) Sun pointing (SUP) 3: Orbit control mode (OCM) ASH Mode manager : STAB, STRA, SLO ASH Mode control - Reaction Whl command - Magnetoquer command satellite Telecommand Commanded NM Mode manager : quaternion GAP, MAN, FIP, SUP Attitude estimation Normal Mode control - Reaction Whl command Orbit - Magnetoquer command estimation 3 OCM Mode control - Thruster command other

  7. ASH Mode STRA A A STAB SLO AOCS Design Description: AOCS modes TC TC SUP A TC ARO GAP MAN TC OCM TC A TC FIP TC TC Normal Mode A : Automatic transition TC : Telecommanded transition ARO : Attitude Reconfiguration Order (from any submode)

  8. AOCS Hardware Description: • Sensors: • Sun Sensors • Magnetometers • Inertial Measurement Unit (IMU) • Star Camera with two Camera Heads • Actuators: • Reaction Wheels • 3 Magnetic Torquer • 1 RCS (cold gas) with 4 thrusters

  9. Attitude maneuver performed by a cluster of 4 whls • Wheel capacity • 20 deg/min for each axis based on current whl capacity • Possible to increase agility for specific axis from ( , ) • 25 % torque margin a b Major Trade-offs : Maneuver Agility

  10. Major Trade-offs : Magnetorquer sizing Wheel Control Preliminary analysis shows: • Wheel unloading control in NM mode, Maximum command magnetic command shall be able to retain wheels angular momentum variation induced by the environment disturbing torques • Detumbling control In ASH mode, maximum command magnetic command shall be able to stabilize the spacecraft within 2 orbits • Cross denote wheel control has been absent from the control loop and enforced S/C with nadir attitude in eclipse and sun pointing attitude in sunlight • H was calculated by integrating T off-loading + Tdist instead of feeding from wheel speeds S/C (nadir /Sun pointing) , wheel off-loading control law

  11. Earth Sun direction 7.5 deg -Zsc 39 deg Sun masking Available for roll maneuver: 59.8 deg 23 deg Earth limb masking 28.6 deg CHU los +Ysc Major Trade-offs : Star camera orientation • Sun is a point source, Sun masking angle: 39 deg • Earth is an extended source, Earth masking angle (from Earth limb): 23 deg CHU B los +Y CHU A los Xsc +Z CHU los +X Rr Ysc Rx Zsc

  12. Major Trade-offs : Star camera orientation (cont’d) Conclusion: • Based on the simulation results, at least one of the two CHUs will be always kept out from blinding. • To extend roll maneuver capacity from +/- 25 deg to +/- 35 deg, elimination of 10 deg either in Sun or Earth exclusion angle is needed

  13. COM y x 4 z 2 1 3 Major Trade-offs : Thruster configuration • Four thrusters configuration • Only one of the two thruster branches is used after 1 failure • Propulsion module is centred around centre of mass (COM), the thruster configuration cannot create any torque aligned on Y axis. • Orbit control • On Y axis: • No capacity around Y, Y axis is always controlled by wheels. • On X and Z axes: • In the nominal case, the thruster is performed by firing the 4 thrusters simultaneously. • In a degraded case (one thruster failure), the pair that includes the failure thruster is no longer used and the thruster is performed with the remaining thrusters. The X or Z axis is therefore control by wheels • Off-modulating Control. The pair (1,2) control Z axis, the pair (3,4) control X axis

  14. Argo PDR – AOCS Jitter Analysis

  15. Preliminary Performance Analysis: Jitter analysis (rigid body) Objective: Analyze whether pointing req. for 0.5” ∀ freq > 0.015 Hz is achievable. Method: Frequency domain analysis. Results: Normal Mode (FIP, MAN sub-modes) + time delay

  16. Jitter Conclusion Required specification achievable. Given 0.0061 Hz cl-BW, Relative Accuracy: 47.20” + 2nd order LPF with 4 Hz sampling rate  output: pointing error ~ 0.19”, for freq > 0.015 Hz .

  17. Argo PDR – AOCS Omni-directional Sun Sensor (OSS)

  18. OSS Conclusions • Maximum OSS sun direction error < 12 deg. • Sensitivity analysis will be done after PDR. Those including: variation of mean albedo, unequal cell degrade, mismatch of measurement resistors, head misalignment, and variation of backside radiation.

  19. Preliminary Performance Analysis: ASH mode • Objective: • To reduce the initial rate, after that to track Sun and control the solar array toward Sun while it is in eclipse or daylight. • To keep the satellite in safe state once any contingency or anomaly happened. • Method: Eclipse Sun presence automatic TC Normal Mode STAB STRA automatic SLO B-dot control law B-dot control law B-dot control law (X,Z) Sun acquisition control law (Y) Wheel off-loading control law (Y) ASH Mode

  20. Preliminary Performance Analysis: ASH mode (cont’d) Conclusion:Control law works. • The satellite spins down from the initial rate of 2.5°/s at each axis within 2 orbits, then transits from STAB to STRA. • STRA/SLO cyclic transition demonstrates Sun acquisition function well. • Angular momentum of each wheel is in the designed working range.

  21. Argo PDR – AOCS Navigation Filter Design (NAV)

  22. NAV requirement • Orbit determination (Normal mode) • Position: 25 m (3D-3s) • Velocity: 1.8 m/s (3D-3s),

  23. Argo PDR – AOCS Inertial Attitude Estimation (IAE)

  24. Inertial Attitude Estimation (IAE) • Hardware: • Star camera (ASC) • Gyro (IRU) • Measurements:

  25. IAE Conclusions • LPF is good enough + fast & easy to design/implement. • Angular error < 40 arc-second, rate error < 0.5 deg/hr. • Data fusion – camera head misalignment

More Related