Post on 12-Aug-2015
description
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)• AOCS Software Development
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
AOCS Key Requirements
• Orbit Altitude
• Orbit Inclination
• Equator Crossing Time
• Attitude Control Accuracy
• Attitude Control Accuracy Goal
• Attitude Control Bandwidth
• Attitude Knowledge
AOCS Key Requirements (cont’d)
• Attitude Maneuvers
• Spacecraft Jitter
• On-board Orbit Determination
• Satellite Autonomous Operations
• Over-sampling
• Maneuver Agility
AOCS Design Description: Functional block diagram
Telecommand
estimation Attitude
estimation Orbit
Normal Mode control
- Reaction Whl command- Magnetoquer command
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)
Commandedquaternion
NM Mode manager : GAP, MAN, FIP, SUP
satellite
ASH Mode manager : STAB, STRA, SLO
MAG
Star camera
Sun sensor
GPS
IMU
OCM Mode control- Thruster command
3
other
ASH Mode control- Reaction Whl command- Magnetoquer command
AOCS Design Description: AOCS modes
FIP
MAN
SUP
GAP OCM
ARO
TC TC
TC
TCA TC
TC
A
TC
TC
A : Automatic transitionTC : Telecommanded transitionARO : Attitude Reconfiguration Order (from any submode)
STRA
SLO
STAB
A
A
ASH Mode
Normal Mode
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
Major Trade-offs : Maneuver Agility
• 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
Angle Duration Acceleration Max Rate Inertia Uncertainty Used Inertia(deg) (s) (deg/s2) (deg/s) (kgm2) (%) (kgm2)
roll 20 60 Spec 0.022 0.667 100 20 120
pitch 20 60 Spec 0.022 0.667 80 20 96
yaw 20 120 Spec 0.006 0.333 100 20 120
Analysis of bang-bang profile +Y
-X
Z
RW2
RW1
RW4
RW3 X
cc
Time
T
Torque Max H Avail. Torque Avail. H(Nm) (Nms) (Nm) (Nms)
0.047 1.164 0.052 4.607
0.037 0.931 0.046 4.114
0.012 0.582 0.019 1.677
alpha (deg)beta (deg)
Wheel Torque (Nm)Wheel H (Nms)
Major Trade-offs : Magnetorquer sizing
,
S/C
(nadir /Sun pointing)
Wheel
Control
2B
BHΔKM
BMT loadingoff
H
distloadingoff TT
distT
wheel off-loading control law
loadingoffT
H
Preliminary analysis shows:• Wheel unloading control in NM mode, Maximum command magnetic comm
and 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
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
Earth Sun direction
7.5 deg
39 degSun masking
23 deg Earth limb masking
CHU los28.6 deg
+Ysc
-Zsc
Available for roll maneuver: 59.8 deg
Xsc
YscZsc
CHU los
Rx
Rr
CHU A los
CHU B los
+Y
+X
+Z
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
Major Trade-offs : Thruster configuration
4
312 z
y
x
COM
• 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
Argo PDR – AOCS
Jitter Analysis
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
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 .
Argo PDR – AOCS
Omni-directional Sun Sensor (OSS)
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.
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:
STRASTAB 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)
automaticautomatic
Sun presence
Normal Mode
TC
ASH Mode
Eclipse
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.
Argo PDR – AOCS
Navigation Filter Design (NAV)
NAV requirement
• Orbit determination (Normal mode)– Position: 25 m (3D-3)– Velocity: 1.8 m/s (3D-3),
Argo PDR – AOCS
Inertial Attitude Estimation (IAE)
• Hardware: – Star camera (ASC)– Gyro (IRU)
• Measurements:
q Pros Cons
ASC direct output
deduced from q
accurate blinding, expensive
IRU deduced from
direct output
cheap, robust
drift
Inertial Attitude Estimation (IAE)
• 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
IAE Conclusions