Control Of Close-proximity Tethered Satellite System In Deep Space

Multiple spacecraft formation flying for deep space exploration is a new development trend of space science and technology in 21 st century. As a special form of multiple spacecraft formation flying, the tethered satellite system has certain benefits ranging from fuel consumption to high precision compared with the free formation flying. So it is especially suitable for space-borne infrared telescopes and enhances the capability of human in exploring deep space. However, the tethered satellite system in deep space is challenging with its complexity and diversity, the dynamics and control problem is one of the key issues for the application of tethered satellite system. On the base of the existing research achievements, this paper focus on the control problem of tethered platform attitude, relative position and attitude-orbit coupling for tethered satellite system with support of the research subject “Research on dynamics and control of tethered satellite system in deep space”. The main contents of this dissertation are as follows:For the platform attitude control problem of multi-body rotating tethered satellite system, the design and analysis of robust optimal control for actuated and underactuated system has been researched in depth. The objective of the control is to change the array angular rate while stabilizing the compound pendulum motion. The nonlinear attitude motion equation of multi-body rotating tethered satellite system is given first, the dynamics is reduced based on the oscillation synchronization then. Considering dynamics uncertainty and bounded disturbances, a robust optimal control scheme is proposed, which combines optimal control theory, adaptive method and robust integral of error method. The optimization and robustness properties of the nonlinear control are taken into account concurrently. In addition, the feasibility of controlling the array spin rate and relative attitude without thrusters is investigated. A change of coordinates by nonlinear diffeomorphism is used for the complex dynamics of non-complete. Then, by applying the robust optimal control scheme, the underactuated tethered satellites system tracking problem is solved. Numerical simulation results of two-body and three-inline tethered satellite system demonstrate the effective of the proposed control strategies.For the relative position control problem of a spinning multi-body Coulomb satellite system in deep space, the relative motion equations of two-body and three-body satellite system are derived and analyzed first. In the presence of the Coulomb control input saturation and external disturbance, a robust control algorithm of two-body Coulomb system is developed by introducing a well-defined smooth function and using a Nussbaum function. The Nussbaum function is introduced to compensate for the nonlinear term arising from the input saturation. On this basis, considering the state constraints caused by the control range of Coulomb force and the potential collision among satellites, a nonlinear controller is proposed combining with an auxiliary function and backstepping philosophy. The auxiliary function is designed for the state constraints. With the proposed method, the Coulomb satellite system can achieve the reconfiguration even if the system contains constraints in both state and control. Considering the situation without angular velocity feedback, an output-feedback reconfiguration controller is designed based on the filter method. For the three-body Coulomb satellite formation flying freely in deep space, actively controlled Coulomb forces are used to stabilize the formation shape to a desired triangular configuration. The control problem of is challenging because the system is nonlinear and nonaffine, and the system constraints need to be taken into account. The proposed solution to the control problem is a nonlinear model predictive controller ensuring stability and optimal control performance while satisfying system constraints. Numerical simulations illustrate the effectiveness of the proposed control strategies.The last part of the thesis investigates the attitude and orbit stability control for a two-body tethered satellite system in a long term. It is assumed that the center of mass of the system is located near Earth-Moon collinear libration point for a detailed analysis. The nonlinear dynamic model in the circular restricted three-body problem is derived by utilizing analytical mechanics theory, and the design of SDRE optimal controller for the stability is put forward. Considering the velocity information is not easy to obtain in the orbit motion, an SDRE state observer is developed to observe the velocity information. Based on this, the SDRE output-feedback is designed. Then, taking the effects of environment disturbances to the system in the long term into account, the analysis on environment disturbances related to the solar radiation pressure effects, orbitial eccentricity and solar perturbations are carried out. For the robust control problem of two-body tethered satellite system, a closed-loop control strategy based on the indirect robust control scheme and theq-D method is proposed with precise and good robust performance. Theoretical analysis and numerical simulation results are presented to validate the feasibility of the proposed dynamics model and proposed control strategy for the robust reconfiguration and station-keeping mission.