Design and implementation of a reaction wheel system for CubeSats

This paper describes the dynamics and control of a reaction wheel system for three axis control of a nano-satellite. The dynamics equations governing the reaction wheel system, the controller-reaction wheel interface and the motor control mechanism have been elaborated. The Controller-Reaction wheel interface which includes speed control of the motor of the reaction wheel system from the obtained torques has been described. Due to the presence of continuous disturbance torques in space the reaction wheels will saturate after a certain period, the momentum must be unloaded from the reaction wheels to use them again for attitude control. The results of a momentum dumping strategy using three magnetorquers have been discussed. The speed control system for the motor and the momentum dumping strategy have been tested using a SIL (Software-In-the-Loop) system and all the relevant results have been shown. The BLDC motor used requires a three phase supply. Since the on-board battery provides a DC voltage, power electronic circuits are used to convert this DC to a 3 phase AC using SVPWM method. SVPWM is used to reduce any possible vibrations in the motor due to supply harmonics. The rotor position and speed is determined by both hall effect sensors and the back emf measurements. Since the motor is a slow responding device with a large mechanical time constant, a PID controller is used to speed up any transitions in the rotor speed. The design of this controller has been discussed in detail.