Dynamics And Vibration Control Of A Flexible Space Robot For Manipulating A Large Flexible Spacecraft

In the future,the size of some spacecrafts will be larger and larger and the operating environment of space robots will be more harsh.Some flexible appendages,such as solar panels,communication antenna or other large structure will be mounted on the base of spacecrafts.Due to the vibration of large flexible structure,it is very challenging for a space robot with flexible appendages to capture and repair a large flexible spacecraft.Taking the mission about a space robot with flexible appendages capturing a large flexible spacecraft as the research background,this thesis will focus on the modeling,dynamic coupling and vibration control of flexible space robots and compounded systems.An air floating experiment system is established,and experimental research is carried out based on the experimental system.Before capturing the target,space robot with flexible appendages and flexible joints is a typical rigid and flexible coupling system.After capturing,the two spacecrafts with flexible appendages are connected by a flexible space manipulator,and a compounded system is formed.In this thesis,the dynamic equations of the flexible space robot and the compounded system are established by the recursive set method.The dynamic calculation program based on C language is compared with the Adams software.The dynamic coupling characteristics of the flexible space robot system are studied based on the dynamic model.The coupling factor used to describe the size of coupling between elastic motion of flexible appendages and the end-effector motion of the manipulator is defined.By establishing the relationship between joint s' acceleration and the acceleration of elastic variables,the coupling matrix between the joint motion and the elastic motion of appendages is obtained.The coupling map described in joint space is drawn based on the coupling matrix.The developed dynamics calculation program,which lays the foundation for the planning and control of flexible space robot systems,has the advantages of high precision and high efficiency.The coupling factor and coupling map can be applied to design the configuration of space robots and to reduce the vibration of flexible appendages,respectively.The elastic vibration of flexible appendages will be excited by point-topoint motion of space robots.Based on the study of dynamic coupling of flexible space robots,a method of trajectory planning of manipulator joints using the coupling map is proposed to reduce the vibration of flexible appendages.The joint trajectory planning methods of the starting point and the termination point in the same and different minimum coupling curves are discussed,respectively.The proposed trajectory planning method to reduce the vibration is intuitive and feasible,simple and effective.In order to determine the optimal trajectory of the flexible space robot for point-to-point motion with minimum vibration energy or minimum residual vibration,this problem is described by Bolza type optimal control.Then the Radau pseudospectral method is used to solve the optimal solution of the motion trajectory of the flexible space robot.At the same time,the determination of the optimal trajectory by radau pseudospectral method also verifies the correctness of the trajectory planning method based on coupling map.The states of the flexible space robot change in real time during capturing the target and there is a complex dynamic coupling between the manipulator and the base.Hence,it is impossible to use the off-line or open-loop methods to control the vibration of the flexible appendages.Aiming at this problem,this thesis proposes a hybrid control method that simultaneously achieves target tracking and vibration suppression of flexible appendages.Firstly,a closed-loop control system of flexible space robot for autonomous target capturing is established.The methods of relative pose measurement and autonomous trajectory planning are given.Then,a hybrid control law is proposed to satisfy the joint trajectory tracking and the appendage vibration suppression.The stability of the control system is proved by the Liapunov's second method.This method,which has good application value,does not require additional vibration suppression actuator,and the choice of vibration signal is flexible and diverse.After the large flexible target is captured,the structural parameters of large flexible appendages mounted on the target spacecraft are unknown and their vibration can not be measured directly.In this thesis,a compound control method based on vibration wave superposition for vibration suppression is proposed by using the characteristics of vibration wave transmission.Firstly,the principle of vibration wave control is expounded.The vibration wave is divided into transmission wave and return wave,and two kinds of calculation methods of return wave are given.Then a compound control method combining PD control and vibration wave control is designed for the compounded system.The stability of the control system is proved by the Liapunov 's second method.The method transfers the vibration control of the flexible appendages completely to the joint of the manipulator,which has important engineering application value.In order to verify the dynamic characteristics of the system and the vibration control method,a set of ground air flotation experiment system of the flexible space robot is established based on the principle of air flotation simulated microgravity environment.The experimental system fully considers the space microgravity environment and the system flexibility characteristics,which can well simulate the dynamic characteristics of the space robot and the compounded system in the plane.Firstly,the dynamic characteristics of t he compound system are studied based on this experimental system.The influence of planning method and planning time on the vibration of compound system is studied.Then,the vibration control of the compounded system is verified by the compound control method of vibration wave superposition.