Attitude Control And Control Allocation Of Combined Spacecraft

From 1957,when the first man-made satellite was launched into space,nearly 10,000 effective satellites are now in orbit.Some satellites lose their attitude or orbital mobility due to fuel depletion or damage to power units,causing huge economic losses.In order to extend the operational time of these failed satellites and to remove space debris,this paper investigates the control problem of combined spacecraft after the robot arm captures the failed satellite or space debris.In view of the dramatic change struck by the mass,mass characteristics and actuator configuration matrix after capturing the target spacecraft,this paper studies the attitude control and control allocation problem of the combined spacecraft and main contributions are stated as follows:First,two different kinds of attitude dynamics of combined spacecraft by different actuators are established.For the combined spacecraft actuated by thrusters,the attitude dynamics model is established in the combined spacecraft body frame considering the change of the mass center and thruster configuration matrix after capture.For the combined spacecraft actuated by reaction wheels,the attitude dynamics equation is established based on the law of momentum conservation considering the change of mass center and reaction wheel configuration matrix after capture.Secondly,considering that the mass center and mass characteristics are known and external disturbance is ignored,the attitude and attitude velocity tracking control,and control allocation problem of combined spacecraft are investigated.The control torque is designed by the direct parameter method based on proportional differential feedback.Due to the configuration matrix of the actuator is changed after capture,the control torque needs to be redistributed.For the situation where the configuration matrix is fully known,the improved null-space-based control allocation method is developed.Considering measurement error of the configuration matrix,robust control allocations with perturbation in regular polyhedral,polytopic and linear form are proposed.Finally,numerical simulations demonstrate the effectiveness and superiority of the controller,developed control allocation and robust control allocation method.Then,taking into account of the inertia matrix uncertainty of the combined spacecraft,the attitude tracking control of combined spacecraft with constrained angular velocity and attitude control of combined spacecraft with measurement uncertainty are investigated.On the one hand,a constrained back-stepping control scheme with dynamic ?norm constrained control allocation is developed.Firstly,a disturbance-observer-based constrained backstepping control law is developed to generate the control command signals and can guarantee that the attitude and angular velocity tracking error converge to small neighborhood of zero.Then,with the consideration of physical amplitude and rate constraints on actuators,the dynamic ? norm constrained control allocation problem is solved by linear programming technique.Finally,numerical examples demonstrate the effectiveness of the proposed constrained backstepping control algorithm.On the other hand,a dynamic surface control scheme with robust control allocation is proposed.Firstly,considering measurement uncertainty,attitude dynamics of combined spacecraft is established.Then,a virtual controller is designed and all the states in the closed-loop system converge to a small neighborhood of zero,where the lumped disturbance is compensated by two stable nonlinear disturbance observers.In consideration of uncertain location of mass center in the reconfiguration matrix,a LMI-based robust control allocation is employed to deal with the problem of distributing the three axis torques over the reaction wheels.Finally,numerical simulations are presented to illustrate the proposed method can realize the attitude stabilization control of combined spacecraft.Furthermore,considering the influence of the orbital control torque,the problem of the attitude control of the combined spacecraft with prescribed performance and the robust dynamic control allocation is studied.Firstly,the attitude dynamics of the combined spacecraft considering orbit control torque is established.Then,based on nonlinear disturbance observer,a dynamic surface virtual controller with prescribed performance is designed.The control scheme can guarantee that the attitude converges to small neighborhoods of zero within prescribed bound and the other closed-loop states are uniformly ultimately bounded.Furthermore,considering the uncertainty and dynamic performance in configuration matrix,a robust dynamic control allocation(Rob DCA)strategy is formulated as a min–max optimization problem.Finally,numerical examples demonstrate the effectiveness of the proposed prescribed dynamic surface control scheme and the Rob DCA scheme for combined spacecraft.Finally,considering the actuator fault in the reaction wheels,the attitude faulttolerant tracking control of the combined spacecraft with prescribed performance is studied.Firstly,the attitude tracking dynamics model of the combined spacecraft with reaction wheels considering inertia uncertainty,actuator saturation and reaction wheel fault is established.Then,the ”constrained” state is transformed into ”unstrained” state through prescribed function,and a fault-tolerant controller based on dynamic surfacemethod and nonlinear extended state observer stabilizes the ”unconstrained” state by tuning controller's parameters,which leads to the attitude tracking error convergence to zero with prescribed trajectory and the other states of the closed-loop system is uniformly ultimately stable.Finally,comparative simulations in both actuator faults and actuator fault-free cases are conducted to show that the developed attitude tracking control method can realize control object.