Pseudospectral Based Trajectory Planning And Sliding Mode Control Of Space Robots

With the increasing frequency and depth of human exploration for space,the space mission has changed from simple to increasingly complex,and reliable on-orbit service technology becomes more and more important.The development of space robotics provides new possibilities for the traditional human-based mission execution,and has played an important role in the past few decades.It is indispensable in space station construction and maintenance,on-orbit fueling,fault satellite repairing and capturing,astronaut extravehicular activities and space garbage cleaning.As basic technologies in various space missions,the trajectory planning and tracking control of space robots have received extensive attention.However,the task-space control of space robots in the existing works is still limited.Besides,many issues during on-orbit servicing such as uncertainties due to variation of system parameters,the complexity and variability of the external space environment,and input saturation have not been fully considered.Based on existing results,this thesis investigates the trajectory planning,joint space control and task-space control of space robots working in the free-floating mode and free-flying mode.The main contents are as follows:Aiming at the kinematics and dynamics modeling of space robots,the general motion equation,which describes the mapping relationship between the inertial coordinates of an arbitrary point on the system,the pose of the spacecraft and the joint variables of the manipulator,is firstly given by introducing the concept of vector mechanics and augmented body.Then,for the free-flying space robot with fully actuated base spacecraft,the differential kinematics describing the relationship between the velocities of the endeffector and the states of the base and joints are formulated.On this foundation,for the free-floating space robots whose position and attitude of the base spacecraft are both uncontrollable,the motion of the spacecraft and the manipulator is decoupled by using its momentum conservation property,and the differential kinematics is further derived which is independent of the base states.Finally,dynamics of space robots working in the two different modes are established by using the Lagrange equation,providing a basis for the subsequent trajectory planning and tracking control.The two working modes of space robots,free-floating and free-flying modes,play corresponding roles in different stages of on-orbit services.When approaching a longdistance target,the space robot needs to be in free-flying mode.When it is too close to other spacecraft,it is necessary to shut down the thrusts of the base to avoid unexpected collisions.Since spacecraft maneuvering is usually required in the free-flying mode,its fuel consumption affects the on-orbit servicing life of a space robot.As a result,fuel optimization is a priority consideration in trajectory planning for this kind of space robot.On the other hand,pose of the spacecraft is uncontrollable in the free-floating mode,the system exhibits non-integrity properties and the angular momentum equation is nonintegrable.The system configuration is not only related to the manipulator’s current joint states,but also affected by its historical path.Therefore,this thesis studies its minimum spacecraft variation trajectory planning problem,trying to reduce the variation of the base spacecraft by purposely design the manipulator’s joint space trajectory.To deal with the contradiction between the computational accuracy and efficiency when solving the trajectory planning of space robots,an adaptive Radau pseudospectral method is introduced.The Radau pseudospectral method transforms the continuous optimal problem into a nonlinear programming problem at collocation points,and the adaptive law adaptively segments the time interval and allocates the number of points in each segment according to the current calculation error and the smoothness of the solution.Simulation and experiments verify the effectiveness of the proposed method.For the joint space control of free-floating space robots,considering the system uncertainty,a high-order sliding mode controller based on non-singular terminal sliding mode and super-twisting algorithm is proposed.By designing the piecewise function,the singularity esisting in the tradition terminal sliding mode surface at the equilibrium point is avoided,and the continuity and derivability at the segmental points are also ensured.In the reaching phase of the sliding variables,the super-twisting algorithm guarantees finitetime converge property.Compared to existing methods,the proposed controller is more robust and improves the transient performance of joint space traking even in the presence of system uncertainty.Subsequently,the fractional-order resolved acceleration control is proposed to achieve the task-space trajectory tracking of free-floating space robots.By introducing the fractional calculus,the differential or integral order can be adjusted more finely,thereby improving the deficiencies of traditional resolved acceleration control.In order to more accurately compensate for the system uncertainty and alleviate the problem of chattering and excessive control gians caused by the existence of uncertainty,a disturbance observer is designed.The observer ensures that the estimation error for the uncertainty converges in finite-time,and reduces the requirement for the control gains as well.Simulation results validate effectiveness of the proposed controller.Considering the influence of input saturation on the task-space tracking control of space robots,the anti-saturation controllers for free-flying mode and free-floating mode are designed respectively.Since the base spacecraft is fully controllable for free-flying space robots,its task-space control can be diretly mapped to the joint space.Therefore,an adaptive sliding mode controller in the joint space is proposed.By introducing the adaptive parameters,the nonlinear term due to input saturation,external disturbances and the system uncertainty are compensated,ensuring the stability and transient performance of the system.For the free-floating space robot,an anti-saturation slding mode controller based on adaptive disturbance observer and auxiliary system is proposed,with the joint space and the task-space being mapped by the resolved acceleration control framework.The adaptive parameter in the observer is used to reduce the requirement for prior knowledge of bounds of the uncertainty.The auxiliary system generates additional outputs according to the difference between the calculated control inputs and the saturation bounds of the actuators,thereby compensates for the effect of actuator saturation and ensures finite-time converge property of the closed-loop system.Simulation verifies the effectiveness of the proposed methods.