Attitude Determination and Control Dr. Andrew Ketsdever
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Attitude
Determination and Control
Dr. Andrew Ketsdever
MAE 5595
Outline
Introduction
Definitions
Control Loops
Moment of Inertia Tensor
General Design
Control Strategies
Spin (Single, Dual) or 3-Axis
Disturbance Torques
Magnetic
Gravity Gradient
Aerodynamic
Solar Pressure
Sensors
Sun
Earth
Star
Magnetometers
Inertial Measurement Units
Actuators
Dampers
Gravity
Gradient Booms
Magnetic Torque Rods
Wheels
Thrusters
INTRODUCTION
Introduction
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
Introduction
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
Introduction
Control Loops
Mass Moment of Inertia
where
H is the angular momentum
, I is the mass moment of inertia tensor, and is the angular velocity
Mass Moment of Inertia
For a particle
For a rigid body
Mass MOI
Mass MOI
Like any symmetric tensor, the MOI tensor can be reduced to diagonal form through the appropriate choice of axes (XYZ)
Diagonal components are called the
Principle Moments of Inertia
Mass MOI
Parallel-axis theorem:
The moment of inertia around any axis can be calculated from the moment of inertia around parallel axis which passes through the center of mass.
ADCS Design
ADCS Design
ADCS Design
ADCS Design
ADCS Design
Control Strategies
Gravity Gradient
Stabilization
Deploy gravity gradient boom
Coarse roll and pitch control
No yaw control
Nadir pointing surface
Limited
to near Earth satellites
Spin Stabilization
Entire spacecraft rotates about vertical axis
Spinning sensors and payloads
Cylindrical geometry and solar arrays
Spin Stability
Satellite Precession
Spinning
Satellite
Satellite thruster is fired to change its spin axis
During the thruster firing, the satellite rotated by a small angle
Determine the angle
Dual Spin Stabilization
Upper section does not rotate (de-spun)
Lower section rotates to provide gyroscopic stability
Upper section may rotate slightly or intermittently to point payloads
Cylindrical geometry and solar arrays
3-Axis Stabilization
Active stabilization of all three axes
Thrusters
Momentum (Reaction) Wheels
Momentum dumping
Advantages
No de-spin required for payloads
Accurate pointing
Disadvantages
Complex
Added mass
Disturbance Torques
External Disturbance Torques
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
Gravity Gradient Torque
Magnetic Torque
Aerodynamic Torque
Solar Pressure Torque
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
Sensors
Attitude
Determination
Earth Sensor (horizon sensor)
Use IR to detect boundary between deep space & upper atmosphere
Typically scanning (can also be an actuator)
Sun Sensor
Star Sensor
Scanner: for spinning S/C or on a rotating mount
Tracker/Mapper: for 3-axis stabilized S/C
Tracker (one star) / Mapper (multiple stars)
Inertial Measurement Unit (IMU)
Rate Gyros (may also include accelerometers)
Magnetometer
Requires magnetic field model stored in computer
Differential GPS
Attitude Determination
Actuators
Attitude Control
Actuators come in two types
Passive
Gravity Gradient Booms
Dampers
Yo-yos
Spinning
Active
Thrusters
Wheels
Gyros
Torque Rods
Actuators
Attitude Control
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