Research On Attitude Stabilization And Control Method For Capturing Spacecraft

The term on-orbit servicing(OOS)refers to the maintenance of space systems in orbit,including refueling,on-orbit maintenance or upgrade of spacecraft,on-orbit assembly,scrapped satellite push away,and active debris removal,after their deployment.After the mission spacecraft combine with the target,it will form a new assembly that can be called a combined sapcecraft.For the non-cooperative targets,because of its mass,geometry,attitude and other information can not be known in advance,therefore,after capture non-cooperation target,the attitude stability and control of the combined spacecraft must solve the problem of control robustness caused by unknown uncertainty of mass,centroid and moment of inertia.Base on the background of spacecraft capture the unknown target,this dissertation investigates the attitude control algorithm of the combined spacecraft under the constraint of the inertia uncertainties,external disturbances,bounded angular velocity and control torque saturation.The main content of this thesis can be summarized as follows:Before investigating the attitude control method of the combined spacecraft,it is necessary to obtain the accurate attitude dynamics model.For the cahracteristics of the combined spacecraft,it can be abstracted into amulti-boldy system which consisted by spacecraft bodies and multiple sub-bodies.The general attitude dynamics model of the combined spacecraft is deduced based on the momentum moment theorem.The principle of disturbances of parameters and input are obtained by comparing the Pre-combined and post-combined attitude dynamics equations.The simplified attitude dynamics of the combined spacecraft are obtained by assuming that the spacecraft and the target,the thrust are locked and the mass of the all of them are constant.And the attitude control problem of the combined spacecraft is described in the presence of bound angular velocity and control input constrained.First of all,a constrained adaptive backstepping method,which combining constrained backstepping method and radial basis function neural networks,is proposed for the attitude control control of the combined spacecraft with inertia uncertainties,external disturbances and control constraints.Command filters can be used to eliminate the analytic computation of the time derivatives of the virtual controls,and keep the control signal and the state variables with their mechanical constraints and operating limits,respectively.In order to suppress the combined disturbance formed by the inertia uncertainty and external environment,the radial basis neural networks are used to estimate and compensate for the disturbance.The simulation results show that for the continuous smooth attitude reference input,the constrained adaptive backstepping method can accurately track the attitude control input under the control saturation constraint.Then,a cascade control scheme based on the input-to-state stable theory,which combining the command filters and extend state observer,is proposed to deal with the problem that the angular velocity constrained cannot be guaranteed for the muation attitude reference input.In addition,the neural network cannot estimate the disturbance accurately.The interconnected nonlinear system of spacecraft is transform into individually subsystems,the robust controller ensure that each subsystem is input-to-state stable.The cascade control scheme is useful to eliminate the adverse effect of the outer ring of the attitude control system on the inner ring,and keep the angular velocity whithin the limitation.In order to improve the estimation accuracy of the constant interference,the extended state observer is used to estimated the composite disturbance,and the estimation error of the time-varying interference can also be significantly reduced by adjusting the observer gain.The design process of the extended state observer and attitude control law can be independent of each other and led to a cascade structure,so that the stability of the closed loop can also be guaranteed based on the input-to-state stability theory.The simulation results show that the proposed method can effectively eliminate the problem of the angular velocity constraint cannot be satisfied due to mutation reference input,and the accurate estimation of the disturbance term can be realized.In order to improve the robustness of the closed-loop system to the actuator bandwidth,a novel double loop control scheme,which combining command filter and nonlinear disturbance observer technique,is proposed for the attitude control control of the combined spacecraft with inertia uncertainties,external disturbances,bounded angualr velocity and control constraints.Whether constrained adaptive backstepping attitude control or constraint input-to-state stable attitude control,it will cause fluctuations in the states and the control torque when the bandwidth of the actuator below a certain value.Moreover,certain conditions should be satisfied for the estimator's gain to ensure the stability of the estimator when the time-varying gain of the extended state observer is adopted.After analyzing the control structure of the both method,it is found that the double-loop design can be used to replace the recursive design,so as to reduce the influence of correlation between the rings and improve the robustness of the system to the actuator bandwidth.In order to solve the time-varying gain problem of the extended state observer,the disturbance estimator is derived based on the immersion and invariance theory.The I&I disturbance estimator has the advantages of simple structure,less design parameters and accurate estimation of disturbance.The comparative simulation results show that the constraint double loop attitude control method can not only guarantee the angular velocity and control torque constraint in the attitude control process,but also improve the robustness of the system to the actuator bandwidth.