Attitude Control of Two-Wheel Spacecraft Based on Dynamics Model via Hierarchical Linearization

In this paper, we propose an attitude control law for underactuated two-wheel spacecraft under non-zero total angular momentum. Attitude control with non-zero total angular momentum is complicated in the case that the number of reaction wheels equipped on a spacecraft is two. For a spacecraft in this situation, an attitude control law has been proposed based on a kinematics model by Katsuyama et al. [Y. Katsuyama, SICE Annual Conference, pp. 3421-3426, 2013]. However, a dynamics controller is more desirable for a practical system. Thus, in this paper, we expand the controller to a dynamics model. Nevertheless, in the case of dynamics model, the expansion is not straightforward because of the singularity of input transformation. Therefore, we propose to apply the hierarchical linearization technique which separates a system into several subsystems and linearizes the subsystems step by step. Using this method, the input transformation becomes well-defined, and the system is linearized partially. Additionally, the dimension of linearizable state increases compared with the ordinary input-output linearization. Numerical simulation is conducted to show the validity of the proposed controller.