me176: ( space! ) machine design adcs design & hardware february 20th, 2003
DESCRIPTION
ME176: ( Space! ) Machine Design ADCS Design & Hardware February 20th, 2003. Aaron Rogers [email protected]. Introductions and Overview. Review of Last Section: Orbits. Review of Last Section: Orbits Cont. d. e. s. c. e. n. d. i. n. g. s. a. t. e. l. l. i. t. e. - PowerPoint PPT PresentationTRANSCRIPT
ADCS Design & Hardware ME176: Lecture 5 Chart 1
ME176: (Space!) Machine DesignADCS Design & HardwareADCS Design & Hardware
February 20th, 2003
Aaron [email protected]
ADCS Design & Hardware ME176: Lecture 5 Chart 2
Introductions and Overview
ADCS Design & Hardware ME176: Lecture 5 Chart 3
GNC-078
XECI
YECI
ZECI
line ofnodes
orbitnormalEarth's
equatorialplane
i
orbit
= right ascension of
the ascending node
i = inclination
Review of Last Section: Orbits
ADCS Design & Hardware ME176: Lecture 5 Chart 4
ascendingnode
descendingnode satellite's
position atepoch
rpra
ro
= argument of perigee
= true anomaly at epoch
rp = perigee radius
ra = apogee radius
a = semi-major axis = (ra + rp)/2
e = eccentricity = (ra - rp)/(ra + rp)
GNC-079
Earth
line of nodes
Review of Last Section: Orbits Cont.
ADCS Design & Hardware ME176: Lecture 5 Chart 5
R'
At T0=0 min, R' = RD + Rwhere RD=1.5km (nom. baseline),R > 300km (20% baseline)
V1 @ T0: Initiate Transfer OrbitV2 @ TF: Circularize Into Target Orbit
T ~ 24 min T ~ 71 min
Transfer Orbit:Apogee Alt=500km – Alt
At TF~95 min, R' = RD
Review of Last Section: Hohmann Transfer
ADCS Design & Hardware ME176: Lecture 5 Chart 6
Commercial Satellites at GEO
ADCS Design & Hardware ME176: Lecture 5 Chart 7
• 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
• 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
(Re) Orientation
ADCS Design & Hardware ME176: Lecture 5 Chart 8
Homework Questions for 2/27For your selected Mission:• 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.)
• 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…)
Comments:
• 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.
• 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 9
For your Favorite of the two:
• List the ACS modes and the triggers to proceed from one to another. Diagram with a flow chart.
• Suggest a simple algorithm for your mission mode. Model it on Excel and show that it has a prayer of working
• List requirements your selected ACS imposes on the spacecraft
• List a candidate component suite and estimate the cost and labor to design, build & test.
Comments:
• Modes might include sleep, initial rate killer, sun or earth finder, rough point, tight point, and safe/hold.
• 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.
Homework Questions Cont. for 2/27
ADCS Design & Hardware ME176: Lecture 5 Chart 10
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 11
TT&C RFTT&C RF
TT&CBaseband
AttitudeAttitudeControlControl
AJTs LAE
Pyro Pyro PowerPowerPyro FirePyro Fire
RF, CMD, TLMRanging
Baseband CMD/TLM
MIL-STD 1553 Data BusTLM WordsAccepted
Commands
PropulsionPropulsion
REAs
TT&C Antennas
Pyro Control
Wheel Wheel ControlControl
70 VDC
Electrical PowerElectrical Power
Power BusPower Bus
ArcjetsSolar
Arrays
Deployment Mechanisms
Batteries
Power Regulation Unit (PRU)
Fuse Box
– Tanks– Valves– Lines
Mechanisms
Structure
ThermalControl- Heat Pipes- Heaters- OSRs
Pyro Relays
OBCs UDUP/L
RIUs
Wheels
SSA
IMU
RF Couplers
Receiver/Transmitter
ToToPayloadPayload
ESA
Bus RIUs
Spacecraft Bus Block Diagram
ADCS Design & Hardware ME176: Lecture 5 Chart 12
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 13
Body Frame• Coordinate system origin
– Geometric center of separation plane
• Xb-axis (yaw)– Perpendicular to the separation plane– Points away from the center of the
spacecraft
• Yb-axis (roll)– Perpendicular to the E & W panels– Points toward the east panel
• Zb-axis (pitch)– Perpendicular to the north & south
panels– Points toward the north panel
XB(Yaw)
YB (Roll)
(Pitch) ZB
EastPanel
NorthPanel
SeparationPlane
GNC-056
ADCS Design & Hardware ME176: Lecture 5 Chart 14
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 15
ECI Frame (Continued)
XECI
ZECI
YECI
sun
XECI
ZECI
YECI
XECI
ZECI
YECI
XECI
ZECI
YECIfirst dayof autumn
first dayof spring
first dayof winter
first dayof summer
(Seasons are for the Northern Hemisphere) GNC-050
ADCS Design & Hardware ME176: Lecture 5 Chart 16
ECI Frame & Orbital Frame
ZECI (north)
Orbit Normal
+Zo (pitch)
Direction of Orbit
XECI
+Yo (roll) +Xo(yaw)
YECI
Equator
GNC-020
• 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
ADCS Design & Hardware ME176: Lecture 5 Chart 17
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 18
Attitude Determination: The Problem
Where am I looking in space?!
ADCS Design & Hardware ME176: Lecture 5 Chart 19
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 20
ZSD
-
XSD
YSD
GNC-075
Sun lineof sight
Sun lineprojected on theXSD–ZSD plane
is the true detector elevation angle
is the measured angle
Sun lineprojected on theYSD–ZSD plane
Attitude Determination: Sun Sensors
ADCS Design & Hardware ME176: Lecture 5 Chart 21
SSA Mounting & Field of View
+X (Yaw Axis)
+Y (Roll Axis)
Ea
st F
ace
Earth FaceW
est
Fa
ce
SSAboresight
49.55°
49.55°35°
GNC-027
+Pitch
Sunline
AzimuthAngle
ElevationAngle
Projection of sunlineon yaw-roll plane
SSA
bore
sigh
t
ADCS Design & Hardware ME176: Lecture 5 Chart 22
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 23
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
• This produces a 0.1° to 0.2 ° error in the measured sun angle
• Note– The sun's visible surface has an angular
diameter of 0.53 deg. as seen from Earth
GNC-077
MOON
SUN
ADCS Design & Hardware ME176: Lecture 5 Chart 24
Attitude Determination: Earth Sensors
Scan Mirror
Positive Pitch
PositiveRoll
North
North Scan
South ScanSenso
r Null A
xis
3.82±0.1°
East
3.82±0.1°
GNC-003
• Earth sensor assembly (ESA) provides Roll & Pitch attitude data
• Used to update inertial attitude reference– Data used indirectly in a highly
filtered form during normal operations
– Data used directly with little filtering during Earth acquisition
ADCS Design & Hardware ME176: Lecture 5 Chart 25
North
East
±15°SensorScan
Earth
No Roll or Pitch Offset
North
East
CenterReference
Pulse
Earth
Positive Pitch Offset
GNC-001
ESA Scans With a Pitch Offset
Pitch is determined from the offset between the center reference pulse and the center of the Earth.
ADCS Design & Hardware ME176: Lecture 5 Chart 26
North
East
±15°SensorScan
Earth
No Roll or Pitch Offset
North
East
Earth
Positive Roll Offset
GNC-002
ESA Scans With a Roll Offset
Roll is determined from the difference between the lengths of the north and south
scans across Earth.
ADCS Design & Hardware ME176: Lecture 5 Chart 27
ESA Problems: Multiple Targets• ESA response to this
condition:–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)
Earth
Sun(3° effectivediameter) GNC-031
ESAscans
ADCS Design & Hardware ME176: Lecture 5 Chart 28
ESA Problems: Non-Distinct Targets
• ESA response to this condition:
– 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)
Earth
Sun(3° effectivediameter) GNC-032
ESAscans
ADCS Design & Hardware ME176: Lecture 5 Chart 29
Attitude Determination: Star Trackers
• Utilizes a light sensitive 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)
Roll, Pitch, and YawAttitude(x, y, z)
Processor
Image
StarCatalog
Pattern Recognition
Software
Pinhole Lens
Active Pixel CMOS Imager
30° Field of
View
ADCS Design & Hardware ME176: Lecture 5 Chart 30
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 31
IMU Functional Block Diagram
SensorProcessor
and I/OElectronics
1
PowerSupply
1
Input Power
GyroA
InterfaceElectronics
Accelerometer1
GyroB
InterfaceElectronics
GyroC
InterfaceElectronics
Accelerometer2
GyroD
InterfaceElectronics
SensorProcessor
and I/OElectronics
2
PowerSupply
2
Input Power
Relay Commands
Sensor outputs, ratemode commands, and
status (1553B data bus)
Sensor outputs, ratemode commands, and
status (1553B data bus)
Sensors
Sensors
GNC-029
ADCS Design & Hardware ME176: Lecture 5 Chart 32
Gyro & Accelerometer Alignments• Gyro A
– Positive sense axis is in the Y-Z plane, offset 125.3 from the +Z axis
• 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 D– Senses rotation about the +Z axis
(the positive pitch axis)
• Accelerometers– Positive sense axes point in the +X
direction
GNC-033
+X(yaw)
+Y(roll)
+Z(pitch)
GyroD
GyroA
GyroC
GyroB
ADCS Design & Hardware ME176: Lecture 5 Chart 33
Attitude Determination Options
Option AccuracyOperational Flexibility
RecurringCost
ADCS Complexity
Design Impact
Development Risk
GPS + INS < 1 deg Low Low Low Low Medium
Sun Sensor + Magnetometer
< 5 deg Medium Low Medium MediumMedium
(Software)
Sun Sensor + Horizon Sensor
< 0.5 deg Medium Medium Medium LowMedium
(Software)
Star Tracker < 0.1 deg High High Low High Low
AeroAstro MiniStar Tracker
< 0.5 deg High Low Low LowMedium
(Software)
ADCS Design & Hardware ME176: Lecture 5 Chart 34
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 EnvironmentalDisturbance Torque
ADCS Design & Hardware ME176: Lecture 5 Chart 35
Environmental Disturbances Cont.• Geomagnetic field (compass
needle effect)– 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
• Thermal radiation (recoil effect)– A function of the heat radiated
from various spacecraft surfaces
GravityGradientTorque
GeomagneticTorque
ADCS Design & Hardware ME176: Lecture 5 Chart 36
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 37
Anarchy Happens: Unstabilized
Pros• 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?
ADCS Design & Hardware ME176: Lecture 5 Chart 38
Passive Stabilization: Gravity Gradient
• Cheap, Simple, Reliable
• Can still determine attitude
• Typical pointing performance:±5°
Pros
Cons• Complicates
Radio Antennas
• Many missions impossible (eg imaging)
• How to ensure thermal balance?
• Major deployable
• How simple & cheap is it?
• Very weakGG Torque = 32∆I = 3 x (2π/6000s) 2 x 1 kg-m2
= 3x10-6 N-m = 2 millionths of a foot pound
ADCS Design & Hardware ME176: Lecture 5 Chart 39
Passive Stabilization: Permanent Magnet
Pros• Cheap,
Simple, Reliable
• Pointing sideways often handy
• Passive yet strong
Cons• No yaw
control
• Flip 2x per orbit at poles
• Damping?
• Pointing typically ±5°
ADCS Design & Hardware ME176: Lecture 5 Chart 40
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 41
Spin-Stabilization
Pros• 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
A non-spinning body subjected to a torque impulse will begin to tumble and continue doing so.
ADCS Design & Hardware ME176: Lecture 5 Chart 42
Spin Stabilization and Mom. of Inertia
Neutrally Stable
Unstable
AbsolutelyStable
IX < IY, IZ
IY < IX < IZ
IX > IY, IZ
Principal Axis
Intermediate Axis
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).
Rotation is possible, with nutation damping, about minimum axis, where:PX, where in general: IX < IY, IZ.
Minimum Axis
ADCS Design & Hardware ME176: Lecture 5 Chart 43
Thompson (non) Spinner
Pros• 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
Cons• Solar panel
usage (1/π)
• Non-spinner requires single mo. wheel
• CG Control
Non-spinner can stare and track subsatellite and lateral to subsatellite points
Momentum Wheel (non-spinner)
ADCS Design & Hardware ME176: Lecture 5 Chart 44
Pros• Huge electric power
gen.
• Stabile thermal / illumination environment
• High performance at low cost
• Pointing accuracy 0.2°
• Pointing knowledge 0.05°
Cons• Roll angle
hard to determine
• Attitude solution in umbra requires filter
• CG critical - difficult with deployables
Magnetometer
Fine Sun Sensors (2)
Z-coil
X-coil
Y-coil
Coarse SunSensor (12)
Flight Computer
Coil Driver
Horizon CrossingIndicator (HCI)
Non-Spinner: add just one wheel.
Q: On which axis?
Sun (non) Spinner
ADCS Design & Hardware ME176: Lecture 5 Chart 45
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 46
Vehicle Stabilization Methods
Option AccuracyOperational Flexibility
Design Cost
ComplexityDevelopment
Risk
Passive Stabilization
Low Low Low Low Low• Magnetic• Atmospheric
Spin-Stabilized Medium Low Medium Low Low
3-Axis Control High High Medium Medium Medium
ADCS Design & Hardware ME176: Lecture 5 Chart 47
Spin vs. 3-Axis StabilizationRank Parameter Spin Stabilization 3-Axis Stabilization (Thrusters)
1Disturbance
Rejection
Directly proportional to stack MOI and spin-rate. High rpm might constrain AD, OD, and ConOps. Will require propellant for spin/de-spin.
Requires appropriate sizing of thrusters and propellant.
2Sensitivity 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.
3Thrust
MisalignmentSee (1). Commanded thrust will precess and nutate spacecraft attitude.
See (1).
4 Slewing Requires “turning” stiff rotation vector. Easily accomplished; see (1).
5 PointingInertial pointing only; accuracy highly dependent upon MOI and spin rate (2).
Capable of high accuracy, tracking, offset commanding.
6 C.G. ManagementCG 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).
7Propellant
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.
ADCS Design & Hardware ME176: Lecture 5 Chart 48
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 49
Attitude Control H/W: Reaction Wheels
– 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
GNC-036
Yaw
Roll
Pitch
RWA 1
RWA 2
RWA 3
RWA 4
ADCS Design & Hardware ME176: Lecture 5 Chart 50
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 51
Typical Pitch Momentum Profile.
0 1 2 3 4 5 6Elapsed Time (Days)
Pit
ch M
om
entu
m
GNC-041
– Daily oscillation is due mainly to solar radiation pressure
– Long-term slope is due mainly to payload transmissions
ADCS Design & Hardware ME176: Lecture 5 Chart 52
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 53
Momentum Interchange (Continued)
GNC-042
Earth
yaw
roll
yaw
roll
yawroll
yaw
rollangular
momentum
angularmomentum
angularmomentum
angularmomentum
• If there are no disturbances, the angular momentum vector has a constant magnitude and a constant direction in space.
• If there are no disturbances, the angular momentum vector has a constant magnitude and a constant direction in space.
ADCS Design & Hardware ME176: Lecture 5 Chart 54
Typical Roll & Yaw Momentum Profiles
.
0
0 1 2 3 4 5 6Elapsed Time (days)
Yaw
& R
oll
Mo
men
tum
RollYaw
GNC-043
ADCS Design & Hardware ME176: Lecture 5 Chart 55
Attitude Control H/W: Thrusters
Want 6-DOF Control
ADCS Design & Hardware ME176: Lecture 5 Chart 56
Attitude Control Options
OptionTorque
AuthorityPointingAccuracy
Recurring Cost
Disturbance Rejection
DevelopmentRisk
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, All-
AxesHigh High Low Medium
Magnetic TorquersLow, All-
AxesLow Low Low Low
ThrustersHigh, All-
AxesMedium Medium High Medium
Thrusters + RWAsHigh, All-
AxesHigh High High High
ADCS Design & Hardware ME176: Lecture 5 Chart 57
GN&C Block DiagramGN&C Block Diagram
SensorProcessing
Logic
RedundancyMgt. Logic
ControlMode
SpecificLogic
AutomaticSwitching Logic
Attitude Determination and Ephemeris Propagation Logic
P I DController
ODDS LOGIC
Timed Pulse
MomentumMgt. Logic
AttitudeCommandProcessing
I M U(1)
RIU
Bus
Scheduler
0.2 lbREA(12)
5 lbREA(6)
LAE1
AJT(4)
Thruster
RWA
1553 Data Bus1553 Data Bus
ACS Algorithms (resides in OBC)ACS Algorithms (resides in OBC)
SSA(2)
ESA(2)
RWATach(4)
RWAs(4)
SensorsSensors ActuatorsActuators
RIU
Bus
Scheduler
1553 Data Bus1553 Data Bus
ADCS Design & Hardware ME176: Lecture 5 Chart 58
Spacecraft Feedback Controller
Plant(satellite)
Setpoint Error
ControlAlgorithm
Sensor
“point at sun”= 0 => V=0
V = Volts
Error Angle =Sun Sense - 0
T = -k+/- ThrusterTorque time
tTorque/I
Sun SensorV
Disturbances
Actuator
Actual
ADCS Design & Hardware ME176: Lecture 5 Chart 59
Angle Stabilization with position only feedback
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
Time (seconds)
Angle ErrorError Angle Rate
0th Order Angle Controller
ADCS Design & Hardware ME176: Lecture 5 Chart 60
Angle Stabilization with position + rate feedback
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
1.20
Time (seconds)
Angle Error
Error Angle Rate
1st Order Angle Controller
ADCS Design & Hardware ME176: Lecture 5 Chart 61
Assembly, Integration & Test
ADCS Design & Hardware ME176: Lecture 5 Chart 62
ACS Test Setups
• Level 1 (sometimes all in 1 computer)
Controller Plant
Sensor Signals
Actuator Commands
• Actuator model• Actuator commands• Dynamic model• Orbit / Universe model• Sensor Model
=> Sensor Signals
ControlAlgorithm -->
• Level 2
SpacecraftController
Plant
Simulated
Actuator Commands
Real Control softwareReal sensorsReal output to actuators -->
• Actuator model• Dynamic model
• Orbit / Universe model• Sensor simulation=> simulated environment
Environment
ADCS Design & Hardware ME176: Lecture 5 Chart 63
More ACS Test Setups
• Level 3 (Big $)
• Level 4(on-orbit tweaking)
Spacecraft in vacuum on zero-mass, zero-friction
simulation mount
ADCS Design & Hardware ME176: Lecture 5 Chart 64
Simplifying ACS• Sensors
– Use simple ones: sun, magnetometers
– Use payload instruments as sensors• but beware accuracy limits and loop time
– Design using low-cost vendors, flight spares etc.
• Development and Test– 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
• System Design– 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
• Actuators– Air core torque coils where possible
– single wheel vs. 4-wheel momentum storage
– avoid propulsion (toxic, leaks, fluid handling, safety, lifetime limits)
• Alternative Approaches– Attitude Determination vs. Control
– Wide FOV instruments / multiple instruments
– Unstabilized and passive stabilization