Event-triggered adaptive terminal sliding mode tracking control for drag-free spacecraft inner-formation with full state constraints

This paper investigates the reconstruction and maintenance of the inner-formation system for an innovative drag-free spacecraft with double spherical test masses in consideration of unknown model uncertainties, external disturbances, and actuator saturation. First, spherical test mass 1 is prescribed as the ‘leader’ of the inner-formation system, which is freely floating in the nearly pure gravitational orbit without any control. The nonlinear relative orbit dynamics of the outer spacecraft and spherical test mass 2 with respect to spherical test mass 1 is deduced and the micro configuration of the inner-formation system is introduced. For the outer spacecraft, the aim is to track the spherical test mass 1 in real-time, such that both the center of masses coincided with each other. For the spherical test mass 2, the goal is tracking the reference trajectory, which is designed as a space circular fly-around spherical test mass 1 based on the Clohessy-Wiltshire equation. The state constraints caused by the limit cavity size are analyzed. Second, to achieve these goals with satisfied performance, an event-triggered adaptive terminal sliding mode tracking control strategy is developed for the inner-formation. The adaptive terminal sliding mode technique is implemented to overcome the model uncertainties and external disturbances and to accomplish the reconstruction process in a finite time. Two kinds of barrier functions are utilized to tackle the state constraints. The relative threshold event-triggered mechanism is embedded to save the computation source onboard, evidently. The simulation results demonstrate the efficacy and superiority of the proposed method.