Research On System Design And Variable Stiffness Control Of Cable-Driven Space Robots

Space robots play an increasingly important role in on-orbit services such as capturing,detecting,and maintaining satellites.The key to carry out these services effectively is achieving successful satellite capture.Aiming at the problem of on-orbit satellite acquisition,this paper intends to design a cabledriven space robot system with the characteristics of ’mechanical-electrical’ separation.The paper establishes a kinematic model of a series-parallel coupled cable-driven space robot,proposes a dynamic acceleration planning algorithm based on convolution,and develops an analytical stiffness model,along with a variable stiffness control method for the cable-driven space robot to avoid rigid collision impact during satellite capture.The extreme environment with large temperature difference,strong radiation and interference in space is very easy to damage the space robot.However,electrical equipment such as motors installed at joints and requiring special protection will increase the quality and size of moving parts.Therefore,this paper designs a cable-driven space robot system with the characteristics of ’mechanical-electrical’separation.In the ’mechanical-electrical’ separation characteristics,’machine’ refers to the mechanical components such as the mechanical arm including the structure of the arm and the joint,and the electrical components such as the ’electric’ command control box include components such as motors,drives,and controllers.The control box based on the rotary quick-change mechanism is designed,which can not only realize the rapid replacement and maintenance function,but also can be placed in the highly protected star body to improve the reliability in the harsh space environment.The control system of cable-driven space robot is built to provide the basis for motion planning and variable stiffness control.The cable-driven space robot has many degrees of freedom,and the coupling relationship between the cable and the joint is complex.It is very difficult to achieve flexible and stable motion control.Therefore,based on the structural characteristics of its series-parallel coupling,this paper establishes the kinematic model of the cable-driven space robot,and establishes the mutual mapping relationship between the cable space,the joint space and the task space.Furthermore,considering that the expected displacement of the cable belongs to four cases: super long displacement,long displacement,medium displacement and short displacement,a dynamic acceleration algorithm based on convolution is proposed.The proposed algorithm reduces the impact of excessive acceleration on the cable-driven space robot during start-up and variable speed by dynamically setting the jerk to ensure the smooth movement of the cable-driven space robot.In the capture process of free-floating satellites with non-cooperative characteristics,the on-orbit 6or 7 degree-of-freedom space manipulators often need to withstand huge rigid impact.Rigid and flexible variable stiffness capture can greatly reduce or avoid rigid collision impact.In order to complete the variable stiffness satellite capture task,this paper first establishes the analytical stiffness model of the cable-driven space robot,and analyzes the main factors affecting the stiffness of the cable-driven space robot.Then,the stiffness mesh of the cable-driven space robot is drawn to characterize the relationship between the overall configuration,cable tension and stiffness.Based on the main factors affecting the stiffness and the stiffness mesh,a variable stiffness control method considering the overall configuration of the cable-driven space robot and the cable tension is proposed.The proposed variable stiffness control method is mainly to adjust the active and passive control points to optimize the overall configuration,supplemented by the cascade PID control cable tension.This method simultaneously controls the configuration and tension of the rope-driven space robot to achieve the desired stiffness and realize the variable stiffness control of the rope-driven space robot.This paper completes the development of the cable-driven space robot prototype and builds a cabledriven space robot experimental system.The kinematics model,motion planning method,stiffness model and variable stiffness control method are verified by experiments.The experimental results show that the design of the cable-driven space robot system is reasonable and the variable stiffness control is effective.