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Attitude Determination and Control
Introduction to
Attitude Determination and Control1
https://artes.esa.intESA
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W1 Introduction
W2 Matlab Operations
W3 Frames
W4 Time
W5 Rigid Body Kinematics-1
W6 Rigid Body Kinematics-2
W7 Rigid Body Kinematics-3
W8 Kinematic Differential Equations
W9 Spacecraft Sensors
W10 Spacecraft Actuators
W11 Attitude Dynamics-1
W12 Attitude Dynamics-2
W13 Attitude Control-1
W14 Attitude Control-2
Instructor: Prof.Dr.Nevsan ŞengilOffice:104 Ext:6200Office hours: Open door policyReference Books: Fundamentals of Spacecraft Attitude Determination and ControlF.Landis Markley, John L. Crassidis, Springer 2014Space Vehicle Dynamics and ControlBong Wie, AIAA Education Required S/W: Matlab Student VersionEmail: nsengil(at)thk.edu.trPrerequisities: NoneGrading: 30% Quizzes, 30% Midterm, 40% Final
Introduction
• This course provides the fundamental concepts and mathematical basis for spacecraft attitude determination and control.
• It is intended to serve acourse for undergraduate and graduate students.
• A primary motivation of this course is to develop the theory of attitudedetermination and control from mathematical models to solving algorithms.
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eoportal.org
• Attitude Determination and Control System– Attitude is the three-dimensional orientation of a
spacecraft with respect to a specified reference frame.
– Attitude systems include the• sensors,
• actuators,
• avionics,
• algorithms,
• software, and
• ground support
equipment used to determine and control the attitude of a spacecraft.
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Introduction
Introduction
• We will introduce computer-based (MATLAB examples) problems at the end of each week.
• This promotes solving the real problems using computer programs.
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peasss.eu
• Both Orbit and attitude control is required.• Attitude and Orbit Control System (AOCS) is used
for this purpose.• This course discusses all aspects of spacecraft
attitude determination and control. • This course assumes introductory courses in
physics and calculus through elementary differential equations.
• Some background in control theory would be helpful but is not essential.
• This section will continue to expand upon each of the five areas of the spacecraft control discipline.
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Spacecraft Control
Spacecraft Control
• Spacecraft control is usually synonymous with “Attitude Control,” the engineering discipline of keeping a satellite orspacecraft pointed in the right direction.
• However, in the last 20 years the umbrella of spacecraft control has expanded to include close orbit control including automatic rendezvous and docking, formation flying and close maneuvering.
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www.crystalkiss.com
• In the past orbit control was synonymous with “mission planning”, which can be thought of as low sampling ratefeedback control.
• In modern spacecraft it may be necessary to control relative position as tightly, if not more tightly than, spacecraft attitude.
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Spacecraft Control
https://sci.esa.int/web/cassini-huygens/-/31240-getting-to-saturn
• Mnemonics:
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Mnemonic Meaning
ACS Attitude Control System
AOCS Attitude and Orbit control System
ADS Attitude Determination System
ADACS Attitude Determination and Control System
ADCS Attitude Determination and Control System
Spacecraft Control
• As an engineering discipline, spacecraft attitude determination and control embodies five distinct areas:– Control system design
– Dynamics and modeling of systems
– Software design
– User interface design
– Spacecraft operations
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Spacecraft Control
howthingsfly.si.edu
• The first two involve sensor measurements, often using a combination of different sensors, to get an estimate of thespacecraft pointing.
• The third and fourth involve the design and implementation of the control loops.
• The last, distribution, involves taking control demands and converting them into demands on the actuators.
• For example, a thruster may be pulsewidth modulated so torque or angular acceleration demand must be converted into pulsewidthsfor the thrusters.
• In many cases, control distribution and control are combined into one step.
• Early control systems were analog and the sensors were designed, in some cases, to produce outputs that could be used directly.
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Control System Design
Dynamics and Modeling of Systems• The second area of spacecraft attitude
determination and control can be decomposed into:– Modeling
• Modeling is the creation of computer software to replicate the physical behaviorof the system.
• Only those physical characteristics that are relevant to the problem being studied should be modeled.
• For example, for the purposes of attitude control design, the mechanical and thermal dynamics of an earth sensor are rarely of interest and are usually neglected.
• Simpler models are easier to test and debug and run faster.
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www.nasa.gov
• Simulation
– The numerical models are embedded in a simulation.
– Since it is rarely practical to test control systems on real satellites, the designer must rely on simulation to validate his or her designs.
– Simulations can range from all software running on the same platform, to software models with the control system running on a flight or equivalent board or one in which actuator and sensor hardware are integrated into the simulation.
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Dynamics and Modeling of Systems
• In the first block diagram all of the models and the control software are part of the simulation.
• Everything is software.• This is the easiest to set up and
maintain.• However simulations may involve
hundreds of thousands of lines of code and large databases of parameters.
• Configuration management to allow reproduction of simulation runs is not a trivial task and not something that should be taken lightly.
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Dynamics and Modeling of Systems
• The second diagram uses the actual flight software running on its own processor.
• This allows testing of the flight software with the simulation and requires knowledge of hardware interfaces and networking.
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Dynamics and Modeling of Systems
• The final block diagram uses hardware actuators and sensors as well as the flightsoftware.
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Dynamics and Modeling of Systems
Software Design• Ultimately, spacecraft attitude determination and
control engineers must also be software engineers, whether they write in C++ or use a block diagram language.
• Most of a spacecraft control system has little to do with control theory, but rather is related to how the satellite will operate and interact with spacecraft operations.
• Aside from controlling the satellite, the software must:– Implement the user interface (command and telemetry)
– Switch operational modes
– Provide fault detection
– Provide redundancy management
– Plan maneuvers, operations, etc.17
The Spacecraft Control Engineer’s Job
• How much time does a satellite control system designer spend on these tasks?
• Roughly 5% of a spacecraft controlengineer’s time is in actual control loop design.
• About 10% of his or her time will be spent modeling the hardware.
• The other 85% will be spent writing software.
• This latter number includes writing analysis tools and simulations.
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en.wikipedia.org
Pointing Accuracy• Also referred to as the attitude control accuracy.
• This generally refers to how well the attitude of the vehicle in question can be controlled with respect to a commanded direction.
• When given as a requirement, it is an absolute bound on the error allowed in the spacecraft’s orientation with respect to the commanded orientation.
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www.kleinsatelliten.de
Pointing Knowledge
• Also referred to as the attitude determination accuracy, this refers to how well the orientation of the spacecraft is known with respect to an absolute reference.
• This term can be used for either real-time or after the fact orientation knowledge.
• When given as a requirement, it is an absolute bound on the error allowed in the knowledge of thespacecraft’s orientation with respect to an absolute reference.
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www.ballaerospace.com
• Sensor accuracy is the sum of the ultimate accuracy of the measurement, determined by the object used by the sensors, and all the errors in the measurement such as mounting tolerances and thermal effects.
• Pointing knowledge is the overall accuracy of attitude determination sensor suite.
• Stars are the most accurate source for a measurement followed by the Sun and Earth. 21
Pointing Knowledge
https://artes.esa.int/projects/t3-star-tracker
• Pointing knowledge can sometimes be improved after the fact, such as by monitoring the earth’s magnetic field and later post-processing the output of a magnetometer for better knowledge of a telescope’s pointing direction for a particular photograph.
• The pointing knowledge of a spacecraft is usually better than its pointing accuracy, especially if it is not required to be available in real-time.
• For example, control systems which use thrusters as the sole actuators generally have a pointing accuracy limited to half the thrusters’ pulse width plus the attitude determination accuracy.
• Systems which incorporate wheels into the attitude control system can usually operate close to the attitude determination accuracy so that the pointing accuracy and the pointing knowledge are very close to each other.
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Pointing Knowledge
Jitter• Jitter refers to the errors in attitude of
a frequency too high to be controlled by the attitude control system.
• When given as an attitude control performance requirement, it is a specified angle bound or angular rate limit on short-term, high-frequencymotion of the spacecraft.
• Each spacecraft has an inherent controller period required to sense an attitude error and implement a correction.
• This period determines the controller’s bandwidth by the relationship 1/t.
• Disturbances of a frequency above the bandwidth are not attenuated by the attitude control system.
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What A Spacecraft Attitude Control Engineer Needs To Know
• Dynamics
– Rigid body
– Aerodynamics
– Aeroelasticity (for launch vehicles)
– Multibody dynamics
– Electromechanical systems
– Hydraulic systems (for launch vehicles and large vehicles like the Space Shuttle)
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Mission Programs | NGC Aerospace Ltdngcaerospace.com
Disturbances
• Solar pressure
• Aerodynamic drag
• Radiation pressure
• Albedo pressure
• Magnetic torques
• Outgassing
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What A Spacecraft Attitude Control Engineer Needs To Know
https://www.wired.com/story/spacex-is-launching-a-solar-sail-the-size-of-a-boxing-ring/
• Kinematics
– Coordinate frames
– Transformation matrices
– Quaternions
– Euler angles
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What A Spacecraft Attitude Control Engineer Needs To Know
Euler angles - Wikipedia
• Control
– Single-Input-Single-Output (SISO)
– Multi-Input-Multi-Output (MIMO)
– Nonlinear
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What A Spacecraft Attitude Control Engineer Needs To Know
Control System Tuning in Simulink Made Easy -MATLAB & Simulink
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• Sensors
– Gyros
– Accelerometers
– GPS
– Earth sensors
– Sun sensors
– Star sensors
– Magnetometers
– Potentiometers
– Temperature sensors
– Current sensors
What A Spacecraft Attitude Control Engineer Needs To Know
Coarse Sun Sensor Pyramid - Adcole Corporation
• Actuators– Propulsion systems
– Reaction wheels: • A reaction wheel (RW) is a type of
flywheel used primarily by spacecraft for attitude control without using fuel for rockets or other reaction devices.
• They are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star.
– Magnetic torquers
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What A Spacecraft Attitude Control Engineer Needs To Know
Control moment gyros:
• CMG is a related but different type of attitude actuator, generally consisting of a momentum wheel mounted in a one-axis or two-axis gimbal.
• CMGs are generally able to produce larger sustained torques than RWs with less motor heating, and are preferentially used in larger and/or more-agile spacecraft, such asInternational Space Station.
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What A Spacecraft Attitude Control Engineer Needs To Know
Honeywell Aerospace
• Math
– Differential equations
– Linearization of nonlinear models
– Numerical methods
– Probability and statistics
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What A Spacecraft Attitude Control Engineer Needs To Know
https://www.uiw.edu/smse/academic-programs/math/
Gyroscopic Precession and Gyroscopes
• Video 1:
• Video 2:
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