Generic Attitude and Orbit Control Simulator
development supporting the AOCS software life cycle
E. Mooij and Q.G.J. Wijnands
7th International Workshop on Simulation for European Space Programmes
Noordwijk, November 12, 2002
Introduction
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Extensive experience at Dutch Space has resulted in a generic
approach for development, integration, verification and validation of onboard software for AOCS and Data/Handling systems
Up to phase C/D there is a need for rapid-prototyping, mission and
system definition, and control-law design
Interconnected paths for:
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rapid prototyping
control-algorithm design and verification
on-board software development, and
integration thereof with dedicated (flight) hardware in the control loop
Pre-phase C/D development is characterised by an iterative design
process with many iterations and verification runs, with no direct need
for real-time simulations
The development can be supported by the use of a MATLAB/Simulink
based Generic AOCS facility
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Simulator development
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User Requirements
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GAOCS Simulator elements
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Matlab/Simulink Environment:
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EuroSim Environment
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Physical modelling
Simulator architecture
Functional verification
Example application
Real-Time Workshop
MOSAIC
EuroSim Software in the loop Test Facility
Conclusions and Recommendations
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Matlab/Simulink Environment
Physical modelling (1)
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Equations of (translational and rotational) motion for a rigid
satellite with up to 4 solar panels, with Earth as central
body
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Sun and Moon as perturbing third bodies
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Tabulated atmosphere according to MSIS-86
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Gravitational field according to inverse-square law (+J2, J3
and J4) or GRIM-5C1 (spherical harmonics, n=m=99)
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Geomagnetic field based on IGRF-95 (spherical
harmonics, n=m=10)
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Solar radiation according to inverse-square law
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Matlab/Simulink Environment
Physical modelling (2)
90
9.77
9.78
9.79
9.8
9.81
latitude (deg)
45
0
Gravitational acceleration in
m/s2 at the Earth's surface
(GRIM5-C1, n = m = 8)
9.81
9.8
9.79
9.78
-45
9.77
-90
-180
-120
-60
0
longitude (deg)
60
120
180
90
0
45 -150
latitude (deg)
Difference in gravitational
acceleration in mm/s2
(GRIM5-C1 versus central
field including zonal
harmonics up to J4)
-150
150
0
-150
-300
0
300
-300
0
-300
0
-300
-150
150
500
-600 300
0
150
150 -150
-45
-1
150
300
150
-150
-3
-300
-90
-180
-150
-120
-60
0
longitude (deg)
60
120
180
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Matlab/Simulink Environment
Simulator architecture (1)
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Graphical User Interface, for editing input data and plotting
results
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Generic core for flight dynamics and spacecraft environment
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Additional sub-systems:
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AOCS logic
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Mission manager
Orbit-control logic
Attitude-control algorithm
State estimator
Actuators
Sensors
Dedicated Simulink library with sensors and actuators
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Matlab/Simulink Environment
Simulator architecture (2)
Edit Window: Body Characteristics
Edit Window: Dynamics
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Matlab/Simulink Environment
Simulator architecture (3)
Facility to plot simulation results
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Matlab/Simulink Environment
Simulator architecture (4)
User-defined sub-systems
Generic core
Main Simulink AOCS simulator window
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Matlab/Simulink Environment
Simulator architecture (5)
Simulink AOCS logic sub-system
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Matlab/Simulink Environment
Simulator architecture (6)
Main Library Simulink window
Sensor Library
Actuator Library
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Matlab/Simulink Environment
Simulator architecture (7)
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Performance improvement by replacing Matlab scripts by
C-coded S-functions (so-called CMEX S-functions)
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Simulator porting to EuroSim
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Relatively easy due to use of CMEX functions
Use Real-Time Workshop to autogenerate C-code of Simulink
simulator structure
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Matlab/Simulink Environment
Functional verification (1)
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Time propagation, both in relative and absolute sense, and
frame and co-ordinate transformations
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Environment, consisting of the Earth's gravitational and
magnetic field, the Earth's atmosphere, the motion of Moon
and Sun and the interplanetary environment
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Equations of motion, focusing on both translational and
rotational motion, and the numerical aspects due to the
integration of the differential equations
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Perturbations, of gravitational origin, due to third-bodies
(Sun and Moon), the Earth-magnetic field, the Solar
radiation and the working of the upper atmosphere
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Matlab/Simulink Environment
Functional verification (2)
90
60
60
latitude (deg)
45
50
45
55
35
40
40
0
25
55
60
-45
35
60
-90
-180
30
45
50
55
-120
-60
0
longitude (deg)
60
120
180
Total magnetic field intensity at the Earth's surface (in mT), matches with
results (and extrapolation) in Wertz (1978)
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Matlab/Simulink Environment
Functional verification (3)
Verification date and time, in combination with Earth’s orbit around the Sun
8
2
x 10
1.5
8
3D orbit
x 10
Summer Solstice
relative Sun orbit
1
1
0.5
Z-position [km]
Y-position [km]
Earth
0.5
Autumnal Equinox
0
Vernal Equinox
0
-0.5
Earth
-0.5
-1
2
-1
1
-1.5
-2
-2
8
x 10
Winter Solstice
-1.5
-1
-0.5
0
0.5
X-position [km]
1
1.5
2
2
1
0
0
-1
Y-position [km]
-1
-2
-2
8
x 10
X-position [km]
8
x 10
Relative in-plane and 3D orbit of the Sun “around” the Earth
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Matlab/Simulink Environment
90
Functional verification (4)
Molniya-type orbit
0
-45
-90
-180
-120
-60
0
longitude (deg)
60
120
180
-120
-60
90
45
geostationary
transfer orbit
latitude (deg)
latitude (deg)
45
0
-45
-90
-180
0
longitude (deg)
60
120
180
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Matlab/Simulink Environment
Functional verification (5)
Test #1: Rotational motion due to magnetic perturbations; location above the
equator, qv,0 = 10°, MM,x = 1000 Am2
Conservative rotational motion, mean values of about fV = -2.5°, qV = 17.5° and yV
= -9°
Test #2: Variation in magentic field vector: BV,0 = (1.354,-0.224,-0.430)×10-5 T (t = 1.9113°, d = 0° , R = 8000 km)
Vector product of BB and MM will be zero, only BB,x ¹ 0
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16
12
-2
8
10
12
14
16
18
thetav (deg)
20
22
24
26
5
0
8
6
-10
0
-15
-2
psiv (deg)
2
8
10
12
14
16
18
thetav (deg)
20
22
24
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y-component
z-component
4
-5
-20
x-component
10
-4
-6
-6
14
0
BB (Tesla)
phiv (deg)
2
x 10
-4
0
200
400
600
time (sec)
800
1000
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Matlab/Simulink Environment
Example application (1)
1.4
1.2
Demand
1
0.02s+1
J.s
1
s
Controller
Torque to Rate
Rate to Angle
1
1
Angle
2
pseudo angle (deg)
8s+3
1
0.8
0.6
0.4
Rate
0.2
0
0
5
10
15
20
25
time (sec)
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Implement single-axis PD controller for all three axes
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Add three-axes generic actuator and gyroscope sensor
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Sample (and integrate) sensors at 10 Hz
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Apply three different step functions (command signals)
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Matlab/Simulink Environment
Example application (2)
Control algorithm
State error
400z-397
z-0.006738
+
Step_x
X Controller
1
1
1
z-0.006738
state error
mission profile
Step_y
400z-397
em
commanded
actuator
state
Y Controller
-
400z-397
z-0.006738
Step_z
Z Controller
Mission Manager
0
0
0
1
em
commanded
actuator
state
commanded X-moment
actual X-moment
commanded Y -moment
actual Y -moment
commanded Z-moment
actual Z-moment
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control forces
and
moments
3-axes generic actuator
Actuators
State estimator
Tz
1
em
measure
attitude state
z-1
Tz
1
Tz
z-1
z
z-1
1
Flight
Dynamics
estimated
attitude state
Sensors
Bus selector
Measured angular rate 1 Actual angular rate 1
1
measured state
attitude control
Measured angular rate 2 Actual angular rate 2
Measured angular rate 3 Actual angular rate 3
three-channel gyroscope
em
1
actual_state
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Matlab/Simulink Environment
Example application (3)
X angle (deg)
3
2
1
0
0
5
10
15
time (sec)
20
25
30
0
5
10
15
time (sec)
20
25
30
0
5
10
15
time (sec)
20
25
30
Y angle (deg)
3
2
1
0
-1
Z angle (deg)
3
2
1
0
-1
Step functions for X, Y and Z pseudo angles.
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EuroSim Environment
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Adapt Simulink sub-system models and use RTW to generate C-code
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Use MOSAIC (NLR) to generate shell around RTW output for
implementation in EuroSim
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Re-connect sub-system models and create EuroSim schedule
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Experiences:
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porting is a not straight-forward, cautious process
identified problems (e.g., integration and output order)
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EuroSim Environment
X angle (deg)
3
2
1
0
0
5
10
15
time (sec)
20
25
30
0
5
10
15
time (sec)
20
25
30
0
5
10
15
time (sec)
20
25
30
5
10
15
time (sec)
20
25
30
5
10
15
time (sec)
20
25
30
5
10
15
time (sec)
20
25
30
3
Y angle (deg)
Matlab RTW
Code
2
1
0
-1
Z angle (deg)
3
Code
generated
from
Simulink
2
1
0
-1
-16
x 10
0.5
0
-0.5
-1
0 -16
x 10
1
Y angle (rad)
Code from
C-mex Sfunctions
X angle (rad)
1
0.5
0
-0.5
EuroSim
Schedule
Z angle (rad)
-1
0 -17
x 10
5
0
-5
-10
-15
0
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EuroSim
Software in the Loop Test Facility
bus 1
bus 2
bus 3
actuator
sensor
control
MIL 1553
Data bus
Coupler
LAN
LAN
MIL Front End
VME
Step 2
Control
Software
AOCS Workstation
Mil-1553 interface
EuroSim Mil1553
Mil-1553 interface
Step 1
MIL 1553
Data bus
Coupler
LAN
PCI Bus Bridge
Control
Model
Sensor
Model
1
2
3
VME Bus Bridge
Actuat
Model
PCI Bus Bridge
EuroSim Workstation
LAN
SW Mil-1553
GAOCS Test Facility
Gustavo. J. Ruwiel 06/02/2002
SW Mil-1553
LAN
SW Mil-1553
SW Mil-1553
Mil-1553 interface
VME Bus Bridge
MIL Front End
VME
Testcase 1
1
Sensor
Model
Actuat
Model
EuroSim Workstation
Testcase 2
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Summary and conclusions
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Generic simulator for rigid satellites in Earth environment has
been designed and implemented in Matlab/Simulink
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Each of the individual models as well as the combination of
several models has been evaluated and judged to be correctly
implemented
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Performance improvement by factor of 20, by using C-coded Sfunctions
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Basis for further development of Generic EuroSim AOCS
simulation environment for SIL and HIL testing
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(Potential) customers: HERSCHEL, ConeXpres, EarthCare,
EXPERT re-entry vehicle, EsimSat, ...
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Recommendations
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Modelling:
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EuroSim:
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Additional satellite shapes, i.e., cone and sphere, which includes
the adjustment of the solar-radiation pressure and atmospheric
perturbation models
Addition of tip masses to the appendages
Flexible appendages
Mass variation due to fuel consumption, including sloshing models
Sun as central body for interplanetary missions
Development of GUI, possibly combined with Matlab/Simulink GUI
SILT/HILT:
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GAOCS simulator with MIL-1553 communication
Dedicated (flight) hardware in loop
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