A reaction wheel of similar capability would require megawatts of power. For a few hundred watts and about 100 kg of mass, large CMGs have produced thousands of newton meters of torque. Instead of producing torque by spinning up the reaction wheel or varying the rotor speed, a control moment gyroscope produces torque by tilting the rotor’s spin axis without necessarily changing its spin speed. Video by DSSL TechnionĬontrol moment gyroscopes are very much like reaction wheels but offer an additional degree of freedom by mounting the reaction wheel onto a gimbal. Control Moment GyroscopesĬontrol Moment Gyro Simulator. Drawn from Nano Avionics and New Space Factory. COMAT and Nanoavionics CubeSat sized reaction wheel specifications for comparison and as a sample. In an implementation, reaction wheels are usually operated around some nominal spin rate to avoid stiction effects. Static & dynamic imbalances can induce vibrations so we suggest mounting reaction wheels on isolators. Usually, four wheels are used for redundancy the subsequent controller math needs to account for the wheel speed biasing equation. For three axes of torque, three wheels are necessary, like the image on the left. Reaction wheels create torque on the spacecraft by creating equal but opposite torques on the reaction wheels, which are flywheels on motors. Right: Nanoavionics CubeSat Reaction Wheels Control System SatBus 4RW0: 4 reaction wheels redundant 3-axis control system, enabling precision pointing of the small satellite. Left: COMAT REACTION WHEELS 40 configured in three orthogonal axes. The control logic is simple for reaction wheels that are mounted in independent (orthogonal) axes but can get trickier with reaction wheels that offer redundancy in a configuration. Reaction wheels offer internal torque only, which means that the system still needs an external torque source to dump momentum and/or desaturate the accumulated momentum in the wheels. They’re highly reactive and offer continuous feedback control. Reaction wheels are the most common actuator for active control. Reaction Wheels Reaction wheel rotor underneath its housing. They are useful for initial acquisition maneuvers, like detumbling and nadir-pointing. They have often used in Low Earth Orbit (LEO) satellites. Magnetorquers are commonly used for coarse attitude control and to desaturate angular momentum build-up of the satellite, particularly for dumping momentum in reaction wheels or control moment gyroscopes. Image by Aekjira Kuyyakanont, Suwat Kuntanapreeda, and Nisai H. Example specifications of a magnetorquer. Magnetorquer operating concept Image by Aekjira Kuyyakanont, Suwat Kuntanapreeda, and Nisai H. Where m is the magnetic dipole vector, B the magnetic field vector (for a spacecraft it is the Earth magnetic field vector), and τ is the generated torque vector. The dipole interacts with the magnetic field generating a torque: Where n is the number of turns of the wire, I is the current provided, and A is the vector area of the coil. The magnetic dipole generated by the magnetorquer is expressed by the formula: The electromagnetic coil’s field is controlled by switching current flow through the coils. Magnetic torquers differ from passive magnetic damping in that they can change the magnetic strength of their electromagnetic coils, instead of relying on the constant strength of a permanent magnet. Magnetic torquing is similar to passive magnetic damping in that both types of control use a magnetic field. IIT Bombay Student Satellite Team Satellite 101 wiki about Magnetorquers Magnetorquer used in Pratham Image by Aero IITB.
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