Switched propulsion force libration control for the low-thrust space tug system

The number of micro-satellites launched by commercial rockets and designed by universities and institutions is experiencing rapid growth, inevitably increasing the amount of the space debris of the same size. This paper adopts a low-thrust tethered space tug system to achieve debris deorbit. A switched low-propulsion force control method using only two constant-thrust modes to achieve both deorbit and libration control is presented. This kind of control method benefits from its robustness and low demand on the output accuracy of the thruster. The harmonic-like libration dynamics of the tethered space tug system around the local horizontal configuration is discussed and the stability of the control method is proved. Moreover, the sufficient condition for the switching sustainability is presented. Afterwards, the control effects of such a system are illustrated using numerical simulations. A modified control law to adapt practical demands shows the flexibility of the switching control methodology.