Archive for the ‘ADACS’ Category

Reaction Wheel Assemblies

Reaction Wheel Assemblies are used on LEO and GEO satellites to control errors in Pitch, Roll and Yaw axises. The assembly consists of the electronics to monitor and control the wheel, a motor and the wheel its self. A Reaction wheel works by applying current to a motor that spins a wheel with a given mass and is based on the common principal that every action has an equal and opposite reaction. By applying current to increase or decrease the wheel speed control torques can be generated in the given axis. As the wheel is spun faster it gains stored momentum in the axis and therefore in systems that employ reaction wheels, periodic momentum management is required to prevent the wheel from exceeding its recommended speed range. Momentum unload procedures typically use opposing thruster firings to drive the wheel speed down or up and reduce the stored momentum.

In a zero momentum biased control system when the unload procedure is activated, it drives each axises momentum to zero momentum. In advanced control systems this function is enabled during the standard station keeping maneuvers and can conserve fuel.

When a fourth wheel is included for redundancy, the wheel’s orientation is set such that each wheel has an influence on 2 axis and the ACS or ADACS system uses the alignment of each wheel and axis to calculate the required torque and distribute them over the wheels to control errors in an axis. In this configuration a minimum of three wheels are required to operate the satellite and the fourth wheel can be turned on when needed to replace a failed or failing wheel. The system can also distribute the torques over all four wheels if desired, providing it has been designed to do so.

Magnetic Torque Coils

LEO satellites can utilize the Earths magnetic fields of flux to maintain pointing in 2 of the 3 axis by employing Magnetic Torque Coils, also called Torque Rods.

Since these coils are electromagnets one end of the coil will be positive (North) and other end will be negative (South). These coils are aligned with the axis to be controlled and energized to produce a magnetic field that will attract it to or repel it from the Earth’s lines of magnetic flux. To actively control the torque from these coils the ADACS system regulates the amount of current, it can be increased or decreased and the direction of current flow can be reversed to change the polarity of the magnetic field being generated. ADACS calculates the pointing errors in each axis, converts it to the required current and applies it to the coil to generate torque to correct the errors.  The ADACS system will de-energize them as the satellite passes over the North or South pole, reverse the polarization and re-energize them as required.  During the brief time the coils are de-energized the momentum of the satellite will allow it to maintain it’s pointing.

Very precise pointing can be achieved by the use of a combination of Reaction Wheels and Magnetic Torque Coils.

Earth Detector

At the altitude of a LEO orbit the satellite is close to the Earth and would require sensors with an extremely wide field of view to sense the leading and trailing edges of the Earth. For instance at GEO the field of view would be on the order of 30 degrees where at LEO it could be more than 160 degrees.  Therefore ADACS systems typically use Earth Detectors to sense the presents of the Earth as a course indication of pointing and are located on the face of the satellite that will be Earth pointed. One exception to this is when they are used on spin stabilized satellites and are mounted so they sweep across the Earth.

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Shining light on satellites and how they operate. Drawing from over 30 years of knowledge and experience in all phases of the life of a satellite from concept, to operations, and through end of life. You will find short topics intended to give you an understanding of how they work, the general concepts, and principals used along with information on ground systems. There is also a section dedicated to topics that can be used as basic concept training along with links to animations and 3D models I have created.