IN THE NAME OF
ALLAH
THE MOST BENEFICENT
THE MOST MERCIFUL
SPACERAFT
DYNAMICS AND CONTROL
[email protected] ; 0321-9595510
DR. QASIM ZEESHANBE, MECHANICAL ENGINEERING
NATIONAL UNIVERSITY OF SCIENCE AND TECHNOLOGY, NUST, PAKISTAN, 2000
MS, FLIGHT VEHICLE DESIGNBEIJING UNIVERSITY OF AERONAUTICS AND ASTRONAUTICS, BUAA, P.R.CHINA, 2006
PhD, FLIGHT VEHICLE DESIGNBEIJING UNIVERSITY OF AERONAUTICS AND ASTRONAUTICS, BUAA, P.R.CHINA, 2009
SPACECRAFT
ATTITUDE
DYNAMICS
AND CONTROL
SPACECRAFT
DYNAMICS AND
CONTROL
DISTURBANCE
TORQUESLECTURE #
SPACECRAFT
DYNAMICS AND
CONTROL
EXTERNAL
DISTURBANCE
TORQUESLECTURE #
External Disturbance Torques
Orbital Altitude
To
rqu
e
Solar
Press.
Drag
Gravity
Magnetic
LEO GEO
NOTE: The magnitudes of the torques is
dependent on the spacecraft design.
SPACECRAFT
DYNAMICS AND
CONTROL
INTERNAL
DISTURBANCE
TORQUESLECTURE #
Internal Disturbing Torques
Examples
Uncertainty in S/C Center of Gravity (typically 1-3 cm)
Thruster Misalignment (typically 0.1 – 0.5 )
Thruster Mismatch (typically ~5%)
Rotating Machinery
Liquid Sloshing (e.g. propellant)
Flexible structures
Crew Movement
Disturbing Torques
FrT
IHT
DISTURBANCE
TORQUES
GRAVITY GRADIENTLECTURE #
Gravity Gradient Torque
Earth Sensor uses the
principle of gravity gradient
sensing, which consists in
the very slight change in
gravity as you move away
from the center of the earth.
When an elongated mass is
placed in this environment, it
will tend to align itself with
this field, since one part of
the mass is pulled more
strongly than the other. By
measuring this
displacement, of the mass
trying to align itself, it is
possible to know where the
earth is !
Gravity Gradient Torque
Diagram showing effect of
gravity gradient on a proof
mass. (
a) the proof mass inside the
satellite is perpendicular to
the earth. Both sides of the
mass being the same
distance from the earth,
there is no detectable
effect.
b) if the satellite rotates by a
given amount, the mass
inside it is of course
rotated in the same way.
c) Since now one side is
closer to the earth and
sees more pull, an
additional rotation (torque)
will occur as shown in (c).
Gravity Gradient Torque
2sin2
33 yzg II
RT
where:
verticalfromaway deviation maximum
inertia of moments mass S/C ,
radiusorbit
parameter nalgravitatio sEarth'
gradientgravity maximum
zy
g
II
R
T
z
y
DISTURBANCE
TORQUES
MAGNETIC TORQUELECTURE #
Magnetic Torque
where:BxmTm
meters radiusorbit
m tesla10 7.96moment magnetic sEarth'
poles theabove pointsfor 2
equator theabove pointsfor
field magnetic sEarth' ofstrength
mAmp dipole magnetic residual S/C
torqueedisturbanc magnetic
315
3
3
2
R
M
R
M
R
M
B
m
Tm
*Note value of m depends on S/C size and whether on-board compensation is used
- values can range from 0.1 to 20 Amp-m2
- m = 1 for typical small, uncompensated S/C
DISTURBANCE TORQUES
AERODYNAMIC TORQUELECTURE #
Aerodynamic Torque
where:
gpaa ccFT
2
2
1AvCF D
gravity ofcenter C
pressure catmospheri ofcenter C
velocity
area sectional-cross
2.5 - 2 are valuesS/C typical drag oft coefficien
density catmospheri
torqueedisturbanc caerodynami
g
pa
v
A
C
T
D
a
DISTURBANCE
TORQUES
SOLAR PRESSURELECTURE #
Solar Pressure Torque
where:
gpssrp ccFT
iAc
FF s
s cos1
angle incidencesun
S/Cfor 0.6 value typical1,0factor ereflectanc
surface dilluminate of area
light of speed
m
Wdensity flux solar
gravity ofcenter c
pressureradiation solar ofcenter
torqueedisturbanc presureradiation solar
2
g
i
A
c
F
c
T
s
s
ps
srp
FireSat
Example
Disturbing Torques
All of these disturbing torques can
also be used to control the satellite
Gravity Gradient Boom
Aero-fins
Magnetic Torque Rods
Solar Sails
Spin Stabilized Satellites
Gravity Gradient Stabilization
Gravitational attraction:
f = μm/r2
Top: f1 > f2 ⇒ torque is out of the page
Bottom: f1 > f2 ⇒ torque is into the page
In both cases, the torque is a restoring torque,
tending to make the satellite swing like a
pendulum
Gravity Gradient Stabilization
In the 60s was viewed as “free”
attitude control
• In general, “G2” is not accurate
enough, spacecraft can even flip over
• Not really free, because of boom mass
• However, OrbComm and TechSat 21
use gravity gradient with flexible solar
panels on an extensible wrapper
around the boom
• The Moon is gravity-gradient
stabilized; Lagrange (1736-1813)
showed this
• In the 60s was viewed as “free”
attitude control
• In general, “G2” is not accurate
enough, spacecraft Tech SAT 21
SPACECRAFT DYNAMICS
AND CONTROLATTITUDE DETERMINATION AND CONTROL SYSTEM
ADCS
LECTURE #
Attitude Determination and
Control System
Attitude Determination and Control Subsystem (ADCS)
Attitude control is the exercise of control over the orientation of an object with respect to an inertial frame of reference or another entity (the celestial sphere, certain fields, nearby objects, etc.).
Controlling vehicle attitude requires sensors to measure vehicle attitude, actuators to apply the torques needed to re-orient the vehicle to a desired attitude, and algorithms to command the actuators based on
(1) sensor measurements of the current attitude
(2) specification of a desired attitude.
The integrated field that studies the combination of sensors, actuators
and algorithms is called "Guidance, Navigation and Control" (GNC)
Attitude Determination and Control
System
Torques
Control
actuator
On-board
computer
Ground
control
Attitude
sensor
Attitude
Controller Actuator Spacecraft
Sensors
Commands
Attitude
Torques
Measured
attitude
Error signal
Attitude Determination and
Control System
Attitude Determination and Control Subsystem (ADCS)
Stabilizes the vehicle
Orients vehicle in desired directions
Senses the orientation of the vehicle relative to reference (e.g. inertial) points
Determination: Sensors
Control: Actuators
Controls attitude despite external disturbance torques acting on spacecraft
Attitude Determination and Control
System
Attitude Determination and
Control System
ADCS Design Requirements and Constraints
Pointing Accuracy (Knowledge vs. Control)
Drives Sensor Accuracy Required
Drives Actuator Accuracy Required
Rate Requirements (e.g. Slew)
Stationkeeping Requirements
Disturbing Environment
Mass and Volume
Power
Reliability
Cost and Schedule
ADCS Design
ADCS Design
ADCS Design
ADCS Design
ADCS Design
THANK YOU FOR YOUR INTEREST
Recommended
