Research On Spacecraft Nonlinear Attitude Control Methods Considering Time-delay Characteristic

Time delays are inevitably introduced into spacecraft attitude control system,components such as actuator and attitude sensors are origins when switches on or during attitude determination process.The performance of control system is therefore deteriorated,leading at worst to instability.The strong coupling nonlinearity of spacecraft attitude model,in addition to time delay,makes the problem more complex,noticing that the character equation involves exponentials and the system model changes from finite dimensional ordinary equation to infinite dimensional delay differential equation.For this reason,traditional frequency domain methods or Lyapunov function based time-domain method are not sufficient any more.In recent years,Lyapunov-Krasovskii stability theory,established by Krasovskii for linear time-delay system analysis,has become the main time-domain method for researching nonlinear time-delay system and is extended to spacecraft attitude control system.This development leads to researches on attitude control strategies for spacecraft under the impact of time-delay.Under this background,this dissertation addresses the spacecraft attitude stabilization problem in the presence of disturbances and unknown time-delay.This dissertation proceeds as follow:Spacecraft attitude kinematic model and dynamic model are established,in which Modified Rodrigues Parameters(MRPs)are employed for attitude description from the perspective of Lipschitz-based schemes to tackle nonlinear term in the spacecraft model.The MRP original set and MRP shadow set are combined to achieve globally nonsingularity,Spacecraft dynamic model usded for attitude control is established,and the property of spacecraft attitude model is given.Then time-delay system stability theory and mostly used inequalities are given as preliminaries for attitude controllers design in the following chapters.Considering rigid spacecraft attitude stabilization problem in the presence of unknown time-delay induced by the actuator,a linear state-feedback controller is designed,and then a Lyapunov-Krasovskii functional is constructed for stability analysis.Based on the Lipschitz assumption and free weight matrix method,the local stability conclusion is obtained in the form of a linear matrix inequality.After that,the problem of spacecraft model parameter uncertainty is considered,and the controller is accordingly designed.It is proved that this close-loop system is uniformly asymptotically stable.Finally,numerical simulations are performed to illustrate the effectiveness of the proposed control strategies.The stabilization problem of rigid spacecraft in the presence of time-delay in attitude measurement is addressed in chapter 4.A stability conclusion of linear time-delay system is employed to backstepping the spacecraft attitude model,and a nonlinear controller is designed during this process.Considering the impact of unknown measurement delay and external disturbances on spacecraft,a nonlinear controller is designed combining Extended State Observer with backstepping method.To solve the stabilization problem in the presence of both the measurement delay and actuator delay,a model reduction method is employed as an improvement of the aforementioned backstepping method,and the obstacle imposed by the nonlinear term in spacecraft attitude motion mathmatical model is therefore handled during model transformation,at the meanwhile,a memory state feedback controller is designed.The stability is proved and then illustrated through simulation,the required control energy for attitude stabilization is effectively reduced.Considering the attitude stabilization problem in the presence of long time-delay induced by actuator,time delay compensation idea is adopted.Firstly,first-order hyperbolic partial differential equation is used to model actuator state,which consequently leads to a Partial Differential Equation-Ordinary Differential Equation(PDE-ODE)cascade model of the system,in this way the stability of time delay attitude control system can be analyzed in a strict way.The whole system undergoes then a two-dimensional backstepping transformation,that is,one dimensional on the spacecraft attitude ordinary differential equation,and the other on the actuator state partial differential equation.A boundary controller is designed on this basis.Finally,the target system is proved exponentially stable,and the stability of the original system is proved through the transform relationship.Noticing that though the time-delay is assumed to be exactly known,however,simulation results show the proposed controller is robust to time-delay deviation and gravity gradient torque.Considering spacecraft attitude control problem in the presence of unknown time delay,a time-delay adaptive estimator is designed to improve the aforementioned boundary controller,simulation results show this improved nonlinear boundary controller is effective to achieve attitude stabilization.