ME176: (Space!) Machine Design
ADCS Design & Hardware
February 20th, 2003
Aaron Rogers
aaron.rogers@aeroastro.com
ADCS Design & Hardware
ME176: Lecture 5
Chart 1
Introductions and Overview
ADCS Design & Hardware
ME176: Lecture 5
Chart 2
Review of Last Section: Orbits
ZECI
i
Earth's
equatorial
plane
 = right ascension of
the ascending node
i = inclination
orbit
normal

YECI
XECI
line of
nodes
orbit
ADCS Design & Hardware
GNC-078
ME176: Lecture 5
Chart 3
Review of Last Section: Orbits Cont.
descending
node
w = argument of perigee
 = true anomaly at epoch
rp = perigee radius
ra = apogee radius
satellite's
position at
epoch

ro
a = semi-major axis = (ra + rp)/2
e = eccentricity = (ra - rp)/(ra + rp)
rp
ra
w
Earth
ascending
node
ADCS Design & Hardware
ME176: Lecture 5
GNC-079
Chart 4
Review of Last Section: Hohmann Transfer
At T0=0 min, R' = RD + DR
where RD=1.5km (nom. baseline),
DR > 300km (20% baseline)
DV1 @ T0: Initiate Transfer Orbit
DV2 @ TF: Circularize Into Target Orbit
At TF~95 min, R' = RD
T ~ 24 min
T ~ 71 min
Transfer Orbit:
Apogee Alt=500km – dAlt
ADCS Design & Hardware
ME176: Lecture 5
Chart 5
Commercial Satellites at GEO
ADCS Design & Hardware
ME176: Lecture 5
Chart 6
(Re) Orientation
•
•
•
•
•
1 - Introduction
2 - Propulsion & ∆V
3 - Launch Vehicles
4 - Orbits & Orbit Determination
5 – Attitude Determination and
Control Sys. Design & Hardware
–
–
–
–
–
–
–
–
Coordinate Frames
Attitude Determination
Environmental Disturbances
Vehicle Stabilization Methods
Attitude Control
Control System Design
Assembly, Integration & Test
Simplifying ACS
• 6 - Power & Mechanisms
ADCS Design & Hardware
ME176: Lecture 5
• 7 - Radio & Comms
• 8 - Thermal / Mechanical
Design. FEA
• 9 - Reliability
• 10 - Digital & Software
• 11 - Project Management
Cost / Schedule
• 12 - Getting Designs Done
• 13 - Design Presentations
Sporadic Events:
•Mixers
•Guest Speakers
•Working on Designs
•Teleconferencing
Chart 7
Homework Questions for 2/27
For your selected Mission:
Comments:
•
Pick two attitude control
approaches that might work
•
•
List the sensors and actuators
necessary to implement each of
them. How accurate / sensitive
would each have to be? Any other
special requirements ($, mass,
volume, power, bandwidth etc.)
Eg. Gravity Gradient, Thompson Spin
& TS with momentum storage are
options for an earth pointer.
•
Also search Web to locate actual
components that might be candidates
- not selections, but possibilities.
•
ACS thrusters want to be in pairs and
far from the CG. Torque coils don’t
care. Sensors have to have a clear
view out etc.
•
Locate them on a “generic”
spacecraft (e.g. a cube or faceted
sphere
•
Make a block diagram of the
feedback control system you
envision
•
Pick your favorite of the two, and
tell me why it’s your favorite
(compare $, mass, complexity,
performance…)
What is the plant, what are the
sensors, what is the actuator suite?
What are the “set points?”
•
Make a “trade table” listing specs /
attributes of each to justify your
selection.
•
ADCS Design & Hardware
ME176: Lecture 5
Chart 8
Homework Questions Cont. for 2/27
For your Favorite of the two:
Comments:
•
List the ACS modes and the
triggers to proceed from one to
another. Diagram with a flow
chart.
•
Modes might include sleep,
initial rate killer, sun or earth
finder, rough point, tight point,
and safe/hold.
•
Suggest a simple algorithm for
your mission mode. Model it on
Excel and show that it has a prayer
of working
•
For instance, measure an error
angle or rate and actuate
something to reduce that error.
•
For example: mass distribution,
symmetry, power, siting,
computation, magnetic /
electromagnetic cleanliness
•
Assume an engineering year
costs $200,000 including the
tools and toys necessary to
play with ACS in the lab.
•
List requirements your selected
ACS imposes on the spacecraft
•
List a candidate component suite
and estimate the cost and labor to
design, build & test.
ADCS Design & Hardware
ME176: Lecture 5
Chart 9
Reading Suggestions for 2/27
–Power:
• SMAD Chapter 11.4
• TLOM Chapter 13, 14
–Mechanisms (SMAD):
• Chapter 11.6
–Extra ADCS Review (SMAD):
• Chapt. 6.2: Orbit Perturbations
• Chapt. 6.2: Orbit Maintenance
• Chapt. 11.1: ADCS
–Extra ADCS Review (TLOM):
• Chapters 6, 11
ADCS Design & Hardware
ME176: Lecture 5
Chart 10
Spacecraft Bus Block Diagram
Electrical Power
Arcjets
Solar
Arrays
Deployment
Mechanisms
70 VDC
– Tanks
– Valves
– Lines
Batteries
Thermal
Control
- Heat Pipes
- Heaters
- OSRs
AJTs
LAE
REAs
Power Regulation
Unit (PRU)
Structure
Mechanisms
Propulsion
Fuse Box
Attitude
Control
Power Bus
Pyro Power
Pyro Fire
Pyro
Relays
Pyro
Control
Bus
RIUs
P/L
RIUs
TT&C
Antennas
OBCs
RF, CMD, TLM
Ranging
Accepted
Commands
Wheel
Control
ESA
MIL-STD 1553 Data Bus
To
Payload
Wheels
TLM Words
UDU
TT&C
Baseband
SSA
IMU
Baseband
CMD/TLM
RF
Couplers
Receiver/
Transmitter
TT&C RF
ADCS Design & Hardware
ME176: Lecture 5
Chart 11
GN&C Coordinate Frames
• Body frame
– Fixed in & rotates with the spacecraft
– Reference for sensor & actuator alignments
– Reference for control torque calculations
• Earth-Centered Inertial (ECI) frame
– Constant orientation in inertial space
– Used to define spacecraft & sun ephemeris for attitude determination
• Orbital frame
– Earth-oriented coordinate frame defines nominal attitude
– Orientation in space depends on spacecraft’s orbital location
• Target frame
– The frame control system aligns the body frame with
– Defined with commanded offsets relative to orbital frame
ADCS Design & Hardware
ME176: Lecture 5
Chart 12
Body Frame
• Coordinate system origin
– Geometric center of separation plane
North
Panel
• Xb-axis (yaw)
– Perpendicular to the separation plane
– Points away from the center of the
spacecraft
(Pitch) ZB
East
Panel
• Yb-axis (roll)
YB (Roll)
– Perpendicular to the E & W panels
– Points toward the east panel
XB
(Yaw)
• Zb-axis (pitch)
– Perpendicular to the north & south
panels
– Points toward the north panel
ADCS Design & Hardware
Separation
Plane
ME176: Lecture 5
GNC-056
Chart 13
Earth-Centered Inertial (ECI) Frame
• XECI
– Parallel to the intersection of Earth’s equatorial plane
and the ecliptic plane
– Positive axis points toward the sun at the vernal
equinox
• ZECI
– Parallel to Earth’s polar axis
– Positive axis points north
• YECI
– Completes the right-handed triad
ADCS Design & Hardware
ME176: Lecture 5
Chart 14
ECI Frame (Continued)
ZECI
ZECI
first day
of summer
XECI
XECI
first day
of spring
YECI
YECI
sun
ZECI
ZECI
first day
of winter
first day
of autumn
XECI
XECI
ADCS Design & Hardware
YECI
YECI
(Seasons are for the Northern Hemisphere)
GNC-050
ME176: Lecture 5
Chart 15
ECI Frame & Orbital Frame
• Yaw is toward zenith
(straight up from Earth)
• Pitch is perpendicular to
the orbit plane
• Roll is perpendicular to
yaw and pitch and points
in the direction that the
satellite is moving
Orbit Normal
+Zo (pitch)
ZECI
(north)
+Yo (roll)
+Xo
(yaw)
YECI
Equator
Direction of Orbit
XECI
ADCS Design & Hardware
GNC-020
ME176: Lecture 5
Chart 16
Target Frame
–During normal operations, the
spacecraft body axes are controlled to
the target coordinate frame
–The orientation of this frame relative to
the orbital frame is defined by the
enabled pointing offsets
• Constant
• Earth-target
• Fourier
ADCS Design & Hardware
ME176: Lecture 5
Chart 17
Attitude Determination: The Problem
Where am I looking in space?!
ADCS Design & Hardware
ME176: Lecture 5
Chart 18
Attitude Determination: Magnetometers
• Magnetometer measures
applied magnetic field,
outputs two or three
magnitudes: B= [X, Y, Z].
• With known orbit model
(IGRF2000) and
ephemeris, can calculate
attitude by comparing
measured vs. expected
field direction.
• Low cost and low power,
though does require
some EMI isolation.
ADCS Design & Hardware
ME176: Lecture 5
Chart 19
Attitude Determination: Sun Sensors
Sun line
projected on the
YSD–ZSD plane
Sun line
of sight


-
XSD
ZSD
Sun line
projected on the
XSD–ZSD plane
 is the true detector elevation angle
 is the measured angle
YSD
ADCS Design & Hardware
ME176: Lecture 5
GNC-075
Chart 20
SSA Mounting & Field of View
+Pitch
49.55°
SSA
boresight
35°
49.55°
Earth Face
East Face
Azimuth
Angle
West Face
Elevation
Angle
Projection of sunline
on yaw-roll plane
+Y (Roll Axis)
+X (Yaw Axis)
GNC-027
ADCS Design & Hardware
ME176: Lecture 5
Chart 21
SSA Problems: Earth Albedo
• Problem: Earth Albedo at Low Altitude
– The SSA sun detection threshold is 20% of the nominal solar
intensity
– At low altitude, Earth albedo can be 40% as bright as the sun
– Albedo can trigger a false sun-presence indication and cause
erroneous sun azimuth and elevation readings
• Conditions that can cause the problem:
– Transfer orbit perigee altitude below 5000 km
– Perigee on Earth’s sunlight side
• Solution (GEO spacecraft)
– Suspend use of sun sensor data when the spacecraft altitude is
below 5000 km.
ADCS Design & Hardware
ME176: Lecture 5
Chart 22
SSA Problems: Moon Interference
• Problem: Moon Interference (Partial
Solar Eclipse)
– The SSA’s detection threshold is 20% of
the nominal solar intensity
• The SSA will detect the sun during a
partial solar eclipse
– During a partial eclipse, the centroid of the
visible solar crescent is offset from the
sun’s true centroid
SUN
MOON
• This produces a 0.1° to 0.2 ° error in the
measured sun angle
• Note
GNC-077
– The sun's visible surface has an angular
diameter of 0.53 deg. as seen from Earth
ADCS Design & Hardware
ME176: Lecture 5
Chart 23
Attitude Determination: Earth Sensors
• Earth sensor assembly (ESA)
provides Roll & Pitch attitude data
• Used to update inertial attitude
reference
North
Positive Pitch
– Data used indirectly in a highly
filtered form during normal
operations
– Data used directly with little
filtering during Earth acquisition
East
Positive
Roll
Scan Mirror
3.82±0.1°
so
n
Se
ADCS Design & Hardware
xis
A
ll
u
rN
ME176: Lecture 5
North Scan
3.82±0.1°
South Scan
Chart 24
GNC-003
ESA Scans With a Pitch Offset
North
±15°
Sensor
Scan
Center
Reference
Pulse
North
East
East
Earth
Earth
No Roll or Pitch Offset
Positive Pitch Offset
GNC-001
Pitch is determined from the offset between the
center reference pulse and the center of the Earth.
ADCS Design & Hardware
ME176: Lecture 5
Chart 25
ESA Scans With a Roll Offset
North
±15°
Sensor
Scan
North
East
East
Earth
Earth
No Roll or Pitch Offset
Positive Roll Offset
GNC-002
Roll is determined from the difference
between the lengths of the north and south
scans across Earth.
ADCS Design & Hardware
ME176: Lecture 5
Chart 26
ESA Problems: Multiple Targets
• ESA response to this
condition:
ESA
scans
Earth
Sun
(3° effective
diameter)
ADCS Design & Hardware
GNC-031
ME176: Lecture 5
– Detects two targets in
the south scan
– Automatically inhibits
the south scan
– Uses north scan for
pitch angle calculations
– Uses north scan and
standard chord for roll
angle calculations
– Outputs sun presence
bit = 1 (multiple targets
detected)
Chart 27
ESA Problems: Non-Distinct Targets
• ESA response to
this condition:
ESA
scans
Earth
Sun
(3° effective
diameter)
ADCS Design & Hardware
GNC-032
ME176: Lecture 5
– Detects only one
target in the south
scan
– Continues to use the
south scan (scan is
not inhibited)
– Outputs erroneous
pitch and roll angles
– Outputs sun
presence bit = 0
(only one target
detected)
Chart 28
Attitude Determination: Star Trackers
Pinhole
Lens
Image
30°
Field of
View
Active Pixel
CMOS
Imager
Pattern
Recognition
Software
Processor
Star
Catalog
Roll, Pitch,
and Yaw
• Utilizes a light sensitive
Attitude
(x, y, z)
medium (CMOS, CCD)
• Pattern recognition of detected
images against internal star
catalog
• Acquisition, track modes
• Extremely high precision
(typically high cost)
• Sensitive to stray light (baffles)
ADCS Design & Hardware
ME176: Lecture 5
Chart 29
Attitude Determination: Propagation
• Data output from the IMU CPU:
– The sampled angular outputs developed
by each gyro
– The sampled acceleration outputs
developed by each accelerometer
• Data is typically only available during
significant orbit adjust maneuvers!
– Integrates linear and angular rates in order
to propagate state vectors
– Typically operates much faster than sensor
measurements are taken
– Important when attitude update is not
available (e.g. no sun).
ADCS Design & Hardware
ME176: Lecture 5
Chart 30
IMU Functional Block Diagram
Gyro
A
Interface
Electronics
Accelerometer
1
Gyro
B
Interface
Electronics
Sensor outputs, rate
mode commands, and
status (1553B data bus)
Sensor
Processor
and I/O
Electronics
1
Sensors
Power
Supply
1
Input Power
Relay Commands
Gyro
C
Interface
Electronics
Accelerometer
2
Gyro
D
Interface
Electronics
ADCS Design & Hardware
Sensor
Processor
and I/O
Electronics
2
Sensor outputs, rate
mode commands, and
status (1553B data bus)
ME176: Lecture 5
Power
Supply
2
Input Power
Sensors
GNC-029
Chart 31
Gyro & Accelerometer Alignments
• Gyro A
+Z
(pitch)
– Positive sense axis is in the Y-Z
plane, offset 125.3 from the +Z
axis
Gyro
D
Gyro
A
• Gyros B, and C
– Positive sense axes are offset
125.3 from the +Z axis
– Projections of the sense axes on
the X-Y plane are 60 from the +Y
axis (the positive roll axis)
Gyro
C
• Gyro D
– Senses rotation about the +Z axis
(the positive pitch axis)
• Accelerometers
+X
(yaw)
Gyro
B
ADCS Design & Hardware
+Y
(roll)
– Positive sense axes point in the +X
direction
GNC-033
ME176: Lecture 5
Chart 32
Attitude Determination Options
Operational Recurring
Flexibility
Cost
ADCS
Complexity
Design
Impact
Development
Risk
Low
Low
Low
Medium
Medium
Low
Medium
Medium
Medium
(Software)
< 0.5 deg
Medium
Medium
Medium
Low
Medium
(Software)
Star Tracker
< 0.1 deg
High
High
Low
High
Low
AeroAstro Mini
Star Tracker
< 0.5 deg
High
Low
Low
Low
Medium
(Software)
Option
Accuracy
GPS + INS
< 1 deg
Low
Sun Sensor +
Magnetometer
< 5 deg
Sun Sensor +
Horizon Sensor
ADCS Design & Hardware
ME176: Lecture 5
Chart 33
Environmental Disturbances
• Atmospheric Drag (LEO)
– Function of Ballistic Coefficient, Altitude
• Solar radiation
– Function of Surface Area, |CG – CSP|
– Produces torque about all three axes
– Varies with season and time of day
• Payload transmissions (recoil effect)
– Primarily a pitch torque
Total Environmental
Disturbance Torque
ADCS Design & Hardware
ME176: Lecture 5
Chart 34
Environmental Disturbances Cont.
• Geomagnetic field (compass
needle effect)
Geomagnetic
Torque
– Due to residual uncompensated
dipole, varies with R-3
– Primarily a yaw and roll
disturbance
– Pitch torque produced only when
solar storms temporarily distort the
geomagnetic field
• Gravity Gradient (LEO)
– Due to asymmetric mass
distribution
– Torques about pitch and roll axes
– Function off-nadir angle (theta), R-3
Gravity
Gradient
Torque
• Thermal radiation (recoil effect)
– A function of the heat radiated
from various spacecraft surfaces
ADCS Design & Hardware
ME176: Lecture 5
Chart 35
Environmental Disturbances Cont.
• Solar wind (flow of charged particles)
– Very small effect
– Earth’s magnetosphere deflects the solar wind before it
strikes the spacecraft
• Force can increase temporarily (for a few hours) during strong solar
storms that distort the magnetosphere
• Worst-case is still a small effect
• Micrometeoroid impact
– Occasional small events (several times a year)
– Most impacting particles are so small that the effects are
barely noticeable
– Angular impulse almost always <0.5 in.lb.sec (<0.06 Nms)
ADCS Design & Hardware
ME176: Lecture 5
Chart 36
Anarchy Happens: Unstabilized
Pros
•
•
ADCS Design & Hardware
ME176: Lecture 5
Cheap,
Simple,
Reliable
Can still
determine
attitude
Cons
•
Complicates
Radio Antennas
•
Many missions
impossible (eg
imaging)
•
How to ensure
thermal
balance?
•
How to guard
against spin-up?
Chart 37
Passive Stabilization: Gravity Gradient
Pros
•
Cheap,
Simple,
Reliable
•
Complicates
Radio
Antennas
•
Can still
determine
attitude
•
•
Typical
pointing
performance:
±5°
Many
missions
impossible (eg
imaging)
•
How to ensure
thermal
balance?
•
Major
deployable
•
How simple &
cheap is it?
•
Very weak
GG Torque = 3w2∆I = 3 x (2π/6000s) 2 x 1 kg-m2
= 3x10-6 N-m
= 2 millionths of a foot pound
ADCS Design & Hardware
Cons
ME176: Lecture 5
Chart 38
Passive Stabilization: Permanent Magnet
Pros
•
•
•
ADCS Design & Hardware
Cheap,
Simple,
Reliable
Pointing
sideways
often handy
Passive yet
strong
ME176: Lecture 5
Cons
•
No yaw
control
•
Flip 2x per
orbit at poles
•
Damping?
•
Pointing
typically
±5°
Chart 39
Passive Stabilization: Aerodynamic
Pros
•
Only simple
way to detect
& point
straight ahead
•
Pointing
sideways often
handy
•
Passive yet
strong
•
Pointing
typically ±3°
Cons
•
Yaw damped
but not
controlled
•
Narrow
altitude range
=> short
lifetime
•
May require
deployables
•
Damping may
be necessary
Aero Torque = 1/2rAV2(cp-cg)
= 1/2 x 10-10kg/m3 x 1m2 x 70002 m2/s2 x 1 m
(values @ 300 km)
= 2.5 x 10-3 N-m = 1.9 thousandths of a foot- pound
ADCS Design & Hardware
ME176: Lecture 5
Chart 40
Spin-Stabilization
Pros
A non-spinning body subjected to a •
torque impulse will begin to tumble
and continue doing so.
Disturbances
cancel / avg out
•
Easy attitude
determination
•
Thermal
rotisserie
•
Typical pointing
performance ±2°
Cons
•
Sensor
deconvolution
•
Only one
locale nadir
pointing
•
CG Control
A spinning body subjected to a torque
impulse will precess its spin axis and
otherwise will be unaffected
ADCS Design & Hardware
ME176: Lecture 5
Chart 41
Spin Stabilization and Mom. of Inertia
Neutrally Stable
Minimum Axis
IX < IY, IZ
Unstable
Intermediate Axis
IY < I X < I Z
For stable, minimum ACS design, SS
prefers rotation about the principle
axis aligned along the thrust vector,
PX, where in general: IX > 2*(IY, IZ).
Absolutely
Stable
Principal Axis
IX > IY, IZ
ADCS Design & Hardware
ME176: Lecture 5
Rotation is possible, with nutation
damping, about minimum axis, where:
PX, where in general: IX < IY, IZ.
Chart 42
Thompson (non) Spinner
Momentum Wheel
(non-spinner)
Pros
•
Non-spinner can stare and track
subsatellite and lateral to
subsatellite points
ADCS Design & Hardware
Disturbances &
thermal loads
cancel / avg out
•
Inherently
stabile
•
Antennas
broadside to
earth (+ 3 dB)
•
No moving parts
•
Scan pattern for
sensors
•
Whole earth
nadir pointing
spinner
ME176: Lecture 5
Cons
•
Solar panel
usage (1/π)
•
Non-spinner
requires single
mo. wheel
•
CG Control
Chart 43
Sun (non) Spinner
Pros
Coarse Sun
Sensor (12)
•
Huge electric power
gen.
•
Roll angle
hard to
determine
•
Stabile thermal /
illumination
environment
•
Attitude
solution in
umbra
requires filter
•
High performance
at low cost
•
•
Pointing accuracy
0.2°
CG critical difficult with
deployables
•
Pointing knowledge
0.05°
Magnetometer
X-coil
Coil
Driver
Flight
Computer
Horizon
Z-coil
Fine Sun
Sensors (2)
Y-coil
Crossing
Indicator
(HCI)
Cons
Non-Spinner: add just one wheel.
Q: On which axis?
ADCS Design & Hardware
ME176: Lecture 5
Chart 44
Three-Axis Stabilized
Pros
•
Arbitrary
pointing &
staring
•
Simple
propulsion for
station keeping
•
Mass
distribution not
critical
Cons
•
Difficult
thermal
control &
power
generation
•
High power
required
•
Cost, mass &
complexity
•
•
•
•
Spin-up
Wheel
control
Lost wheel
Torque noise
4 wheels divide three axes
ADCS Design & Hardware
ME176: Lecture 5
Chart 45
Vehicle Stabilization Methods
Accuracy
Operational
Flexibility
Design
Cost
Complexity
Development
Risk
Low
Low
Low
Low
Low
Spin-Stabilized
Medium
Low
Medium
Low
Low
3-Axis Control
High
High
Medium
Medium
Medium
Option
Passive Stabilization
• Magnetic
• Atmospheric
ADCS Design & Hardware
ME176: Lecture 5
Chart 46
Spin vs. 3-Axis Stabilization
Rank
Parameter
Spin Stabilization
3-Axis Stabilization (Thrusters)
Disturbance
Rejection
Directly proportional to stack MOI and spinrate. High rpm might constrain AD, OD, and
ConOps. Will require propellant for spin/despin.
Requires appropriate sizing of
thrusters and propellant.
2
Sensitivity to Stack
Moments of Inertia
Large dependency on payload mass properties
(MP). Will likely require trim mass, frequent
measurements of stack MP, and update of spin
rate.
ACS can accommodate spacecraft
plant through modification of
software.
3
Thrust
Misalignment
See (1). Commanded thrust will precess and
nutate spacecraft attitude.
See (1).
4
Slewing
Requires “turning” stiff rotation vector.
Easily accomplished; see (1).
5
Pointing
Inertial pointing only; accuracy highly
dependent upon MOI and spin rate (2).
Capable of high accuracy, tracking,
offset commanding.
6
C.G. Management
CG migration measurement must be extracted
from off-axis rotation rates and may require
analysis/update of spin rate (2).
CG migration can be determined
from system performance and is
easily accommodated; see (1).
7
Propellant
Management
Inherently settles propellant for primary orbit
adjust, but may inhibit fuel flow near end of
life.
May require a short settling burn
before start of primary ignition.
8
Slosh
High spin rates can reduce slosh effects.
Slosh effects handled through
software and filter design.
1
ADCS Design & Hardware
ME176: Lecture 5
Chart 47
Attitude Control H/W: Torque Rods/Coils
• A torquer consists of a coil (or
two redundant coils) around a
soft iron core.
• Coil magnetizes the iron core
– Long, slender core magnetizes
easier and more uniformly than a
short, “fat” bar.
– Longer bar uses less power and
coil mass.
• Subject to hysteresis saturation
effects.
• Typically with wheels and/or
gravity gradient booms.
ADCS Design & Hardware
ME176: Lecture 5
Chart 48
Attitude Control H/W: Reaction Wheels
Pitch
RWA 3
RWA 2
RWA 4
RWA 1
Roll
Yaw
GNC-036
ADCS Design & Hardware
ME176: Lecture 5
– The positive
momentum/torque axes are
all 45 from the pitch axis
– The projections of the
momentum/torque axes onto
the yaw/roll plane are all 45
from the yaw and roll axes
Chart 49
Reaction Wheel vs. Momentum Wheel
Reaction Wheel
• Bi-directional
• Operates over a
large speed range
(positive & negative)
• Generates torque by
controlling motor
current
Momentum Wheel
• Unidirectional
• Operates in a narrow
range about a high
nominal speed
• Torque depends on
difference between
commanded speed &
current speed
Mechanically, there is no difference between a
reaction wheel and a momentum wheel.
ADCS Design & Hardware
ME176: Lecture 5
Chart 50
Typical Pitch Momentum Profile
Pitch Momentum
– Daily oscillation is due mainly to
solar radiation pressure
– Long-term slope is due mainly to
payload transmissions
0
1
ADCS Design & Hardware
2
3
4
Elapsed Time (Days)
ME176: Lecture 5
5
6
Chart
51
GNC-041
Yaw/Roll Momentum Interchange
• If there are no disturbances
– The angular momentum vector has a constant magnitude
and a constant direction in space
• The spacecraft rotates once per orbit
– Necessary for the payload to continuously face Earth
• As viewed from the spacecraft coordinate system
– Angular momentum is exchanged between the yaw and
roll axes
– The wheel speeds vary once per orbit, with the yaw and
roll momentum 90 out of phase
ADCS Design & Hardware
ME176: Lecture 5
Chart 52
Momentum Interchange (Continued)
yaw
roll
angular
momentum
roll
yaw
roll
angular
momentum
Earth
angular
momentum
yaw
roll
angular
momentum
yaw
ADCS Design & Hardware
• If there are no
disturbances, the
angular momentum
vector has a constant
magnitude and a
constant direction in
space.
GNC-042
ME176: Lecture 5
Chart 53
Typical Roll & Yaw Momentum Profiles
Yaw & Roll Momentum
Roll
Yaw
0
0
1
ADCS Design & Hardware
2
3
4
Elapsed Time (days)
ME176: Lecture 5
5
6
GNC-043
Chart 54
.
Attitude Control H/W: Thrusters
Want 6-DOF
Control
ADCS Design & Hardware
ME176: Lecture 5
Chart 55
Attitude Control Options
Option
Torque
Pointing
Authority Accuracy
Recurring
Cost
Disturbance Development
Rejection
Risk
Gravity Gradient
Low
Low
Low
Low
Low
Momentum Bias, 1
Reaction Wheel
Assembly (RWA)
High,
Single-Axis
Medium
Medium
Medium
Medium
RWA (3 or 4)
High, AllAxes
High
High
Low
Medium
Magnetic Torquers
Low, AllAxes
Low
Low
Low
Low
Thrusters
High, AllAxes
Medium
Medium
High
Medium
Thrusters + RWAs
High, AllAxes
High
High
High
High
ADCS Design & Hardware
ME176: Lecture 5
Chart 56
GN&C Block Diagram
ACS Algorithms (resides in OBC)
Sensors
IMU
(1)
B
u
s
SSA
(2)
ESA
(2)
RWA
Tach
(4)
R
I
U
S
c
h
e
d
u
l
e
r
ODDS LOGIC
Sensor
Processing
Logic
Redundancy
Mgt. Logic
Control
Mode
Specific
Logic
Timed Pulse
Attitude
Command
Processing
Actuators
0.2 lb
REA
(12)
B
u
s
5 lb
REA
(6)
Thruster
PID
Controller
Automatic
Switching Logic
RWA
Momentum
Mgt. Logic
S
c
h
e
d
u
l
e
r
R
I
U
LAE
1
AJT
(4)
Attitude Determination and Ephemeris Propagation Logic
RWAs
(4)
1553 Data Bus
ADCS Design & Hardware
1553 Data Bus
ME176: Lecture 5
Chart 57
Spacecraft Feedback Controller
“point at sun”
Q = 0 => V=0
V = Volts
Set
point
Error Angle =
Sun Sense - 0
Error
T = -k(Q)
Control
Algorithm
+/- Thruster
Torque  time
Actuator
d2Q/dt2 = Torque/I
Plant
(satellite)
Disturbances
Sensor
Sun Sensor
VQ
ADCS Design & Hardware
ME176: Lecture 5
Chart 58
0th Order Angle Controller
ADCS Design & Hardware
ME176: Lecture 5
Chart 59
1st Order Angle Controller
ADCS Design & Hardware
ME176: Lecture 5
Chart 60
Assembly, Integration & Test
ADCS Design & Hardware
ME176: Lecture 5
Chart 61
ACS Test Setups
• Level 1 (sometimes all in 1 computer)
Control
Algorithm --> Controller
• Level 2
Real Control software
Real sensors
Spacecraft
Real output to
actuators --> Controller
ADCS Design & Hardware
Sensor Signals
Plant
Actuator Commands
Simulated
Environment
Plant
Actuator
Commands
ME176: Lecture 5
• Actuator model
• Actuator commands
• Dynamic model
• Orbit / Universe model
• Sensor Model
=> Sensor Signals
• Actuator model
• Dynamic model
• Orbit / Universe model
• Sensor simulation
=> simulated environment
Chart 62
More ACS Test Setups
• Level 3
(Big $)
Spacecraft in vacuum on
zero-mass, zero-friction
simulation mount
• Level 4
(on-orbit tweaking)
ADCS Design & Hardware
ME176: Lecture 5
Chart 63
Simplifying ACS
•
–
Use simple ones: sun, magnetometers
–
Use payload instruments as sensors
•
–
•
System Design
–
–
–
–
but beware accuracy limits and loop time
Design using low-cost vendors, flight spares etc.
Development and Test
–
–
–
–
•
•
Sensors
Build testability into design
Use Matlab or equivalent
Simulate dynamics and sensors with external PC
•
Use safe modes and assume final tweeks on orbit
Managing the Payload
–
–
–
–
–
–
Can it search a little bit?
Scanning vs. staring
Larger apertures = shorter
integration duration
Duty cycling to avoid interference
Self registration and non-real-time attitude
reconstruction
–
–
Actuators
–
–
–
•
ME176: Lecture 5
Air core torque coils where possible
single wheel vs. 4-wheel momentum
storage
avoid propulsion (toxic, leaks, fluid
handling, safety, lifetime limits)
Alternative Approaches
–
–
–
ADCS Design & Hardware
Choose simple modes - spinners, gg
Avoid deployables
Relax pointing / determination accuracy
Use switching antennas and other
techniques to eliminate some pointing
requirements
Basic autonomy / safing on board Handle anomalies on the ground
Attitude Determination vs. Control
Wide FOV instruments / multiple
instruments
Unstabilized and passive stabilization
Chart 64