Transcript ASH Mode

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
ASH Mode manager :
STAB, STRA, SLO
MAG
Sun sensor
Telecommand
ASH Mode control
- Reaction Whl command
- Magnetoquer command
Commanded
quaternion
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)
NM Mode manager :
GAP, MAN, FIP, SUP
IMU
Star camera
GPS
Attitude
estimation
Normal Mode control
- Reaction Whl command
- Magnetoquer command
Orbit
estimation
3
other
OCM Mode control
- Thruster command
satellite
AOCS Design Description:
AOCS modes
TC
A
ASH Mode
ARO
A
TC
SUP
TC
STRA
A
STAB
SLO
GA
P TC
TC
TC
MA
A N TC
FIP
Normal Mode
A : Automatic transition
TC : Telecommanded transition
ARO : Attitude Reconfiguration Order (from any submode)
TC
OC
M
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
Angle
(deg)
Duration
(s)
Acceleration
(deg/s2)
Max Rate
(deg/s)
Inertia
(kgm2)
Uncertainty Used Inertia
(%)
(kgm2)
Max H
(Nms)
Avail. Torque
(Nm)
Avail. H
(Nms)
roll
20
60 Spec
0.022
0.667
100
20
120
0.047
1.164
0.052
4.607
pitch
20
60 Spec
0.022
0.667
80
20
96
0.037
0.931
0.046
4.114
yaw
20
120 Spec
0.006
0.333
100
20
120
0.012
0.582
0.019
1.677
Analysis of bang-bang profile
+Y
+Y
cc 
alpha (deg)
beta (deg)
T
RW3
RW3
X
X
Wheel Torque (Nm)
Wheel H (Nms)
RW2
RW2
Time
RW4
RW4
ZZ
RW1
RW1
-X
-X
•
•
•
•
•
Torque
(Nm)
a
a
b
b
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 ( a, b )
25 % torque margin
Major Trade-offs : Magnetorquer
sizing

Tdist
Wheel
Control

H



Toff loading  M  B
 

 
ΔH  B 
M  K  2 


B



Toff  loading
wheel off-loading control

law
H

S/C
(nadir /Sun
pointing)
,


Toff loading  Tdist
 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
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
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
Earth
Sun direction
CHU A los
7.5 deg
39 deg
Sun masking
Xsc
+Z
CHU los
+X
Available for roll
maneuver: 59.8 deg
23 deg Earth
limb masking
Rr
28.6 deg
CHU los
Ysc
Rx
+Ysc
Zsc
-Zsc
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
• 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
COM
y
– 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
x
2
4
z
1
3
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:
Eclipse
STAB
automatic
B-dot control law
ASH
Mode
STRA
Sun presence
automatic
B-dot control law
SLO
TC
B-dot control law (X,Z)
Sun acquisition control law (Y)
Wheel off-loading control law (Y)
Normal
Mode
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-3s)
– Velocity: 1.8 m/s (3D-3s),
Argo PDR – AOCS
Inertial Attitude Estimation (IAE)
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
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