Output Feedback Control For Spacecraft Subject To Actuator Saturation

With the rapid development of the aerospace,as a carrier of the space mission,spacecraft has gained much attention.Spacecraft is a dynamical system with high couplings and strong nonlinearity,the high precision attitude control of spacecraft plays an important role in the fast and efficient completion of missions.Most of the literatures on attitude control of spacecraft assume that the actuator can supply any requested torque.However,this is unrealistic due to there exists a maximum allowable torque for each actuator.When the requested torque exceeds the maximum value the actuator can supply,the output torque will be limited,and the stability and performance of the control system cannot be guaranteed.On the other hand,attitude sensor is inexpensive and has high measurement accuracy,thus the attitude measurements provide reliable access for the the feedback control.In contrast,angular velocity sensor costs so much and the measurements are sensitive to noise.So,many real available spacecraft are not equipped with velocity sensors,or when the velocity sensors are broken down,if there is no alternative output feedback control,the spacecraft cannot be controlled effectively.Moreover,most of the literatures on attitude control of spacecraft can only achieve asymptotic stability,finite-time control can ensure the system states converge to the equilibrium in finite time,thus it features faster transient and better robustness and attracts many researchers to work on it.Furthermore,due to the topological obstruction,when the Euler angle approach ±? and ±2? respectively,Rodrigues parameters and modified Rodrigues parameters will go to infinity;For unit-quaternion description,there exists two distinct quaternions for each physical attitude.When the continuous control is applied,the unwinding phenomenon exists.So,there is no continuous control law that can achieve attitude control objective globally.This dissertation proposes three output feedback controls for high precision attitude control of spacecraft subject to actuator saturation and attitude measurements only.The main works can be described as follows.First,in order to deal with the singularity of the Rodrigues parameters,we first design an auxiliary variable,and the problem that the control torque will approaches infinity when the Euler angle approaches ±? is completely removed.Then a saturated output feedback finite-time attitude stabilization(SOFT)control is proposed.Lyapunov stability theory and homogeneous technique are employed to show the finite-time stability of the closed-loop system.Numerical simulation results verify the effectiveness of the proposed control.The presented SOFT control fully considers the actuator constraints and by selecting the control gains a priori,the input saturation is removed completely;With the designed auxiliary variable as the input,a nonlinear filter is constructed,and the output feedback control with only attitude measurements is achieved.Thus the cost is deduces and the system reliability is improved;By utilizing of a function featuring small error amplification and large error saturation and involving the homogeneity technique,a finite-time control for spacecraft is designed.Compared to the available asymptotic stable controls,the proposed SOFT control has the advantages of faster transient and higher steady state accuracy;Moreover,the proposed control is independent of the inertia matrix,thus is robust to model uncertainty.So it is more suitable for practical application.Second,by utilizing the hybrid control technique,and fully considering the actuator constraints,a saturated hybrid output feedback global asymptotic attitude tracking ProportionalDerivative plus spacecraft dynamics(SHOPD+)control is proposed.Lyapunov stability theory and La Salle's invariance principle for hybrid system are employed to prove the global asymptotic stability of the closed-loop system,numerical simulation results verify the improved performance of the proposed SHOPD+ control.The proposed SHOPD+ control can ensure the spacecraft move to the desired by a shorter path and achieves the global control,thus the control efficiency is improved and the energy consumption is decreased;Moreover,the proposed control takes into account the influence of the actuator constraints and by selecting the control gains a priori,the proposed control can ensure that the actuator constraints will not be violated;Furthermore,the proposed SHOPD+ control can not only ensure the global control,but also has the advantages of intuitive structure and easy to implement with only attitude measurements.Third,the above proposed global control can only achieve asymptotic stability,which implies that the tracking is achieved as time goes to infinity.In order to improve the convergence of the system states,a saturated hybrid output feedback finite-time PD+(SHOFPD+)control is proposed.Lyapunov stablity theory and homogeneous method are utilized to prove the global finite-time stability of the closed-loop system.Lot of numerical simulation results verify the effectiveness and the improved performance of the proposed control.By selecting the control gains a priori,the input saturation is removed completely;By a kind of nonlinear filter,an output feedback control with only attitude measurements is presented,thus the cost is deduced and the system reliability is improved;Hybrid control technique is utilized to achieve the global control,thus the unwinding phenomenon with unit-quaternion is removed completely and the control efficiency is improved and the energy consumption is decreased;Furthermore,with the utilizing of a function featuring small error amplification,the proposed SHOPD+ control can achieve finite-time stability,thus it features faster transient and better robustness.The advantages of the proposed SHOPD+ control include the simple and intuitive structure,thus it is more suitable for practical applications.