Bsf08 Spacecraft Attitude Determination And Control V1 0
Transcript of Bsf08 Spacecraft Attitude Determination And Control V1 0
Basics of Spaceflight
S f A i dSpacecraft Attitude D t i ti d C t lDetermination and Control
dachwald@fh‐aachen de
Prof. Dr.‐Ing. Bernd Dachwald
Aerospace Technology DepartmentHohenstaufenallee 6, 52064 Aachen, Germany
dachwald@fh aachen.de
FH Aachen University of Applied Sciences/Winter 2009 / 2010
v1.0
What is Spacecraft Attitude … and Why Do We Have to Control It?Overview and Introduction
• The orientation of the spacecraft in space is called its attitude• Most spacecraft have instruments and/or antennas that must be
d f d l b d hpointed into specific directions. Solar arrays must be pointed into the sun. The thruster must be pointed into the required direction during thrust maneuversthrust maneuvers
• To control the attitude, the spacecraft operators (or the spacecraft's computer, in case of an autonomous system) must have the ability to1. Determine the current attitude2. Determine the error between the current and the desired attitude3. Apply torques to remove the error
• Therefore, the spacecraft needs an attitude determination and control system (ADCS)system (ADCS)
• Attitude determination requires sensors• Attitude control requires actuators
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• Attitude control requires actuators
Attitude Determination & Control ProcessOverview and Introduction
Attitude Requirements:Antenna must point
Thruster must point into thrust direction
ActuatorsActuators
TorqueDemandsTorque
Demands TorquesTorques
Antenna must point into Earth direction
ExternalDi t b T
InternalDisturbance
On‐boardOn‐board
Disturbance Torques‐ Aerodynamic‐ Gravity‐Gradient‐Magnetic‐ Solar Radiation Pressure
stu ba ceTorques
On boardComputerOn boardComputer
MeasuredAttitudeMeasuredAttitude
AttitudeAttitude
Solar arraymust pointinto sund
Sensor mustpoint intotarget direction
Solar Radiation Pressure
„Real“Attitude„Real“Attitude
GroundGround
ControlCommandsControl
Commands
direction
AttitudeSensorsAttitudeSensors
ControlControl
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Measured AttitudeMeasured Attitude
Disturbing Forces and Torques Acting on SpacecraftDisturbing Forces and Torques Overview
• Aerodynamic force / torque from planetary atmospheres, at Earth: altitude / 500 km
• Gravity gradient torque• Gravity gradient torque from planetary gravity fields, ∝ 1/R3, at Earth: altitude ≈ 500‐35000 km
• Magnetic torque g qfrom planetary magnetic fields, ∝ 1/R3, at Earth: altitude ≈ 500‐35000 km
• Solar radiation pressure force / torque i th i l t 1/ 2 t E th ltit d ' 600 kin the inner solar system, ∝ 1/r2, at Earth: altitude ' 600 km
• Force / torque from micrometeorite and debris impactsat all altitudes
• Spacecraft generated forces / torques e.g. from mass movements (mechanisms, propellant, astronauts), at all altitudes
• Their relative importance is a generally a function of spacecraft size, mass,mass distribution, altitude, and design
R: Distance from Earth (center)
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R: Distance from Earth (center)r: Distance from sun
Aerodynamic Torque (simplified)AerodynamicDisturbing Forces and Torques
Aerodynamic drag (simplified):
1 2³ v´
FA =1
2ρCDA⊥v
2³−v
v
´FA: Aerodynamic force
A⊥
v FA
CP
FA: Aerodynamic forceρ: Atmospheric densityCD: Aerodynamic (drag) coefficientA⊥: Area normal to the
vr
FA
CM
A⊥: Area normal to thespacecraft velocity vector
v: Spacecraft velocity vector
TD = r × FA
Aerodynamic torque:
TD r × FA
TD: Aerodynamic torquer: Vector from the center of mass (CM) to the center of pressure (CP)
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r: Vector from the center of mass (CM) to the center of pressure (CP)
Typical Magnitude of Disturbing TorquesDisturbing Forces and Torques Comparison
AltitudeDiagrams like this are strongly dependent on the shape and
100 000design of the spacecraft
R di ti P
10 000
Radiation Pressure
10 000Gravity Gradient
1 000Magnetic Effects
Aerodynamic Effects
100 Torque
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Description of Spacecraft AttitudeAttitude Description
Spacecraft attitude is characterized by the orientation of a spacecraft‐ fixed coordinate system with respect to a reference
Example: orbit‐defined coordinate system Roll, Pitch, Yaw:
coordinate system
p y– Yaw axis is directed toward nadir (i.e. Earth center)– Pitch axis is directed toward the south orbit normal– Roll axis is perpendicular to yaw and pitch axis
Yaw rotation
Roll rotation
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Pitch rotation
Attitude Determination
Attitude Determination Objective and Sensors
• Objective: To determine the attitude, or orientation, or pointing direction of a spacecraft‐fixed reference frame with respect to a known (usually inertial) reference framereference frame.
• Attitude determination requires two or more attitude sensors like
M– Magnetometersmeasure the magnitude and direction of the magnetic field
Sun sensors– Sun sensors measure the position of the sun
– Earth sensorsEarth sensors measure the position of the Earth or the attitude with respect to the horizon
– Star sensors compare some image of the sky with a build‐in library
– Gyroscopes
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measure the rotation of spacecraft without external references
Simple Sun SensorsSun SensorsAttitude Sensors
SunlightS li ht
gSunlight
Sensors
Sensors
ElectronicsSignal
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Sun SensorsSun SensorsAttitude Sensors
• Accuracy limit of a sun sensor is about several arcseconds (0.1 – 0.01 deg) for precise sensors and 0.5 deg for coarse sensorsO t d t i th l t ttit d b t l• One sun sensor measurement does not give the complete attitude but only a direction (only two degrees of freedom of the vector are sensitive to the attitude).
• Two measurements are required to determine the attitude:– by a second independent sensor– by the same sensor but separated significantly in time– by the same sensor but separated significantly in time.
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?
Star SensorsStar SensorsAttitude Sensors
• Starlight strikes the CCD of a camera
• By determining the y gdirection to twodifferent stars in the picture, the
Star 1
Star 2
complete attitude can be determined
• Star sensors are very accurate (typically 1‐3 arcsec, some up to0.001 arcsec) …
• … but they generally do not function well at angular rates above some deg/s (due to their small field of view)⇒ a coarse sensor is
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also required for high angular rates
Typical Attitude Control TasksAttitude Control Tasks
High angular rateArbitrary orientation
Tumbling S/C after separation
Low angular rateArbitrary orientation
Slow‐down angular speed
Low angular rateSun‐pointingLow accuracy
Attain safe attitude (power, thermal)
Low angular rateOperational orientation
Attain operational attitude (payload operations)
Low accuracy
Low angular rateSlew to support orbit operations
pHigh accuracy
o a gu a ateOriented to support orbit maneuverLarge disturbances
S e to suppo t o b t ope at o s
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Reaction JetsReaction JetsAttitude Control
• By expelling a mass (for the spacecraft ) with a velocity c, a thruster exerts a force onto the spacecraft
mF = mc
m < 0
• If the thruster has a moment arm r with respect to the spacecraft‘s center of mass (CM), it exerts a torque about CM:
• A single thruster also changes the spacecraft‘s linear momentum
T = r × FA single thruster also changes the spacecraft s linear momentum p, because(this is typically not desired
F = mcF = p
( yp ybecause it also changesthe orbit of the spacecraft)
r
CM
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Reaction JetsReaction JetsAttitude Control
• Therefore, thrusters are used in pairs, so that
T1 + T2 = 2(r × F)
• Depending on the actual thruster, one can have a very high
F1 + F2 = 0
Depending on the actual thruster, one can have a very high control authority– Cold gas:
F1 = mc1up to 10 N– Hydrazine:
up to 10 kN r1
F1 = mc1
CM
up to 10 kN– Ion thrusters:
10 mN down to < 1 mN
r1
r2
F2 = mc2
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F2 = mc2
Reaction WheelsReaction WheelsAttitude Control
• Rotating the spacecraft does not necessarily require thrusters (conservation of angular momentum!))
• Reaction wheels (RWs) are a common choice for active attitude controlRW id i k d l• RW provide quick and accurate control
• Internal torque only (external torque is still required for de‐saturating the wheels, when they q g , yhave reached their maximum rotation speed)
• Three RWs are necessary for three‐axis control but four wheels are usually used for redundancybut four wheels are usually used for redundancy (tetrahedron mounting)
• Control logic is simple for three independent axes but can get complicated with redundancy
• Static and dynamic imbalances can produce vibrations (attitude jitter)
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vibrations (attitude jitter)• Typical torque is 0.1 − 1 Nm
Basic Concept of Feedback ControlAttitude Control System Design
• Satellite attitude is measured and compared with a desired value ⇒ Attitude error
• Attitude error is used to determine corrective torque to be applied by onboard actuators ⇒ New attitudeC l ti i d fi it l b• Cycle continues indefinitely because– disturbance torques occur– attitude measurement is imperfectattitude measurement is imperfect– attitude correction is imperfect
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ReferencesReferences
[Ro08] Lucy Rogers:It’s ONLY Rocket Science. An Introduction in Plain English.Springer, 2008
[Sw08] Graham Swinerd:How Spacecraft Fly. Spaceflight Without Formulae.Springer 2008Springer, 2008
[Se05] Jerry Jon Sellers:Understanding Space. An Introduction to Astronautics.Third Edition. McGraw‐Hill, 2005
[Gr04] Michael D. Griffin, James D. French: Space Vehicle Design.p gSecond Edition. AIAA Education Series, 2004
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