Variable Structure Control For Close Range Rendezvous Phase Of Spacecraft

Space rendezvous is an essential technology, which is widely applied to on-orbit servicing, space assembly, formation flying and other complicated space mission. As the autonomous control scheme is a key factor that determines the success of rendezvous, it has been and continues to be an absorbing area for researchers. When investigating the control scheme, many practical constraints exist, such as external disturbance, faults and time-varying properties of thrusters. Sliding mode control is a kind of nonlinear control method, which is more robust to uncertainties and external disturbance compared with other control methods. Therefore, this dissertation will mainly focus on applying the sliding mode control method to space rendezvous with consideration of some necessary engineering constraints.The shape of most reference orbit in space rendezvous is near-circular, thus this dissertation will mainly focus on the control problem of space rendezvous in near-circular orbits. In the dynamical part, starting from a perturbed two body model, we obtain a dynamical model for the rendezvous in near-circular by employing Taylor and Lagrange expansions, and truncating the expansions at their first order; the non-circularity of the reference orbit is described by time-varying terms, while the difference of the perturbations on two spacecraft is represented by an external disturbance term.As for the space rendezvous with external disturbance and actuator saturation, a fixed-gain and anti-windup sliding mode controller, which attenuates the external disturbance with sliding mode control technique and avoids actuator saturation by constant controller gain, is raised in this dissertation. For the situation when the upper-limit of actuator is time-varying, an adaptive-gain and anti-windup sliding mode controller is raised in order to exploit the ability of actuator, which may be wasted by fixed-gain controllers.As for the space rendezvous with external disturbance and actuator fault, a complex control scheme is raised. This complex control scheme is synthesized with an auxiliary integral sliding mode controller and a guaranteed cost robust controller. The external disturbance is attenuated by the auxiliary controller, while the fault-tolerant controller is used to stabilize the fault rendezvous system with lowest cost.All the simulation result is derived from perturbed two body model, and the simulation part compared the control performance of the fixed-gain and adaptive-gain controllers, and the control performance of the solo and complex control scheme. The results show that adaptive-gain and complex controller perform better than the other controllers in stability, convergence rate, and cost consumption.