Hovering Position Analysis and Controller Design for Underactuated Spacecraft

In this paper, a disturbance observer-based underactuated control scheme is proposed for spacecraft formation hovering in the absence of the radial thrust. We derive the nonlinear feasible sets of initial position and desired hovering one, and discuss the relationship between both sets. Then, a new decomposition form of underactuated dynamics is given, and an underactuated controller is designed via using the non-singular fast terminal sliding mode technique. Considering that the loss of radial thrust will lead to unmatched disturbances, a finite time convergent extended state observer is introduced to improve the control accuracy. The Lyapunov-based method gives the stable hovering accuracy of the state errors under the action of the disturbance observer. Numerical simulations also demonstrate the effectiveness of the presented controller.