Simulink 3D Animation 5.1

Design of Satellite Flight Software

This demo illustrates how to use Aerospace Blockset to perform some of the steps of flight software design for a satellite.

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Contents

Modeling Satellite Dynamics

The satellite travels in a circular orbit with 0 degrees inclination at 1500 kilometers. Satellite rigid body dynamics can be quickly modeled using several blocks from Aerospace Blockset to model a two-body six-degrees-of-freedom system. Aerospace Blockset blocks are highlighted in red and subsystems containing Aerospace Blockset blocks are highlighted in orange. This model takes into account the effects from the earth's oblateness (bulging at the equator due to spinning)using the 1984 World Geodetic System (WGS84) gravity model.

Open Simulink Web view of a satellite dynamic model

Modeling Control Logic for Flight Software

There are three main components to any satellite flight software: failure detection, navigation, and attitude control. When examining the flight software for this model, you can see that it uses Aerospace Blockset in its modeling. Aerospace Blockset blocks are highlighted in red and subsystems containing Aerospace Blockset blocks are highlighted in orange.

Open Simulink Web view of the control logic

The three main components work together as follows: The Failure Detection subsystem determines if any of the thrust actuators or sensors has failed. Because the design task for this demo includes only the attitude control design, the failure detection system in this model is simplified. The simplified failure detection model serves as a working placeholder to provide signals to indicate that no failure has occurred.

The Navigation subsystem manages the course of the satellite with respect to three translational degrees-of-freedom. Because the design task includes only the attitude control design, the navigation system in this model is simplified. This simplified navigation model serves as a working placeholder to provide pointing commands until the full navigation model is supplied.

The Attitude Control subsystem manages the desired orientation of the satellite with respect to three rotational degrees-of-freedom. Attitude is important for making sure that the satellite antenna and instruments will be able to function correctly by pointing at the desired location.

Open Simulink Web view of attitude control model

Modeling Hydrazine Thrust Actuators Control and Failure Management

The Thruster Control and Failure Management subsystem contains logic to turn axis torque commands into individual thruster commands for the hydrazine thrust actuators. The thrust actuators use Aerospace Blockset in the model. Aerospace Blockset blocks are highlighted in red and subsystems containing Aerospace Blockset blocks are highlighted in orange.

There are two steps to convert axis torque commands into individual thruster commands: failure detection and thruster actuation. The model completes failure detection first. The model checks all thruster packs for failures. If there are no failures, the model executes the torque commands for all the axes simultaneously. If a thruster pack fails, only one axis at a time can be actuated. During a failure, the axis that executes is the one determined by thrust control to have the axis with the largest torque command. When the failure detection completes, the model determines which thrusters to fire for a specific torque command. The satellite has four packs of three thrusters each, for a total of twelve thrusters. To produce the appropriate moments, only two must be fired. The model uses the axis, the sign of the command, and the existence of a failure to determine which two thrusters to fire. The control and failure management logic for the thruster actuator system is modeled using Stateflow.

Open Simulink Web view of thruster control and failure management logic model

Detailed Satellite System

A detailed satellite model contains detailed models of components needed for the design of the attitude control and the actuator failure management. Aerospace Blockset blocks are highlighted in red and subsystems containing Aerospace Blockset blocks are highlighted in orange.

Open Simulink Web view of detailed model

You can use the Pointing Mode subsystem to set the pointing mode manually during simulation. In the Pointing Mode subsystem, switches can be set to alternate between the different pointing modes: nadir, Earth-fixed, and no pointing. Nadir points the satellite towards the Earth and turns on the sensor. In this mode, the sensor is used only for visualization purposes. Earth-fixed pointing mode maintains a constant orientation of the satellite with respect to the Earth. Ideally, this mode should be inertial so the mode can track the sun. The satellite drifts and conserves fuel in no pointing mode.

Watch the video of satellite motion simulation. (2 minutes, 56 seconds)