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



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)
    • Rate Requirements (e.g. Slew)
    • Stationkeeping Requirements
    • Disturbing Environment
    • Mass and Volume
    • Power
    • Reliability
    • Cost and Schedule


Introduction



Control Loops



Mass Moment of Inertia



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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