Spacecraft attitude control by 2 wheels with initial angular momentum

The attitude of a spacecraft is usually controlled by 3 reaction wheels. However, to prepare an accident of the wheel, it is important to consider the attitude control by 2 wheels. Especially, when the initial angular momentum of the system is not zero, the system has a drift term and it cannot control the attitude arbitrarily. Therefore, the purpose of this research is to control the direction of a spacecraft´s communication antenna to coincide with the target direction. We separate control laws into two cases by the configuration of wheels; (1) One of the wheel is set along the antenna and (2) Both wheels are not set along the antenna. In the case of (1), by using an idea of equal ratio divergence and a time-state control form, it can control the direction of the antenna and stop rotation of the body. While in the case of (2), it turned out to not be able to stop rotation of the body. So, by using an output zeroing control, it can control the direction of the antenna and keep rotation around the antenna.