Research On Passive Variable Structure Cable-driven Parallel Robot Oriented To Orientation Workspace Expansion

Cable-driven parallel robots(CDPRs)are a subclass of parallel manipulators wherein cables are used instead of rods used as the robot legs.CDPRs exhibit a large workspace and a high carrying capacity.However,the orientation workspace of the CDPRs is limited.Adaptive CDPRs are a kind of robot whose position of attachment point or actuated reel point can change.Due to the change in the position of the attachment point or actuated reel point,the orientation workspace and motion performance index of adaptive CDPRs will be improved,but at the cost of extra drive and complex control strategy.In this paper,the modeling theory,design method,and control strategy of the passive variable structure chain cable-driven parallel robot(PVSCDPRs)are studied to expand the orientation workspace of the CDPRs without increasing the drive and improve the performance of CDPRs.This paper proposes a kind of PVSCDPR,and the modeling theory of PVSCDPR is studied.Inspired by the cable wrapping phenomenon and to avoid the cable slipping phenomenon,this study proposes a spatial continuous constrained cable chain with low pairs.The PVSCDPR is a kind of adaptive CDPR,which can change the structure of the cable branch chain without additional driving.According to the change continuity of the cable branch chain structure,the PVSCDPR can be divided into spatial continuous constrained PVSCDPR and planar discontinuous constrained PVSCDPR.The kinematic model and dynamic model of PVSCDPR were studied.Based on analyzing the equilibrium point of the spatial continuous constrainted cable branch chain,we established the kinematic and dynamic mapping relationship between the workspace and joint space of PVSCDPR.This research provides a theoretical basis for the design of the PVSCDPRs for large orientation workspace.Aiming at the problem that the orientation workspace of CDPRs is limited,the kinematics performance evaluation index related to the orientation workspace of CDPRs is analyzed.With this index,the orientation workspace characteristics of CDPRs are analyzed,and the kinematics description of the orientation workspace characteristics of the CDPRs and orientation workspace expansion method are obtained.Based on the modeling theory of PVSCDPRs,the design and research of planar discontinuous constrained PVSCDPR and spatial continuous constrained PVSCDPR oriented to large orientation workspace are carried out.A planar discontinuous constrained PVSCDPR with two variable structure cable chains is proposed.According to the workspace and dexterity index,the adaptive point position and size of the variable structure cable chains are optimized based on a genetic algorithm.The results show that the planar discontinuous constrained PVSCDPR has a more fabulous orientation workspace and dexterity.A kind of spatial continuous constrained PVSCDPR is proposed,in which all the cable chains are rotational pairs,and the cable chains are continuously constrained.The analysis shows that the robot has the ability of a significant rotation around a single axis.Aiming at the complex problem of motion control of CDPR caused by cable flexibility,the control strategy of PVSCDPR was studied.Considering the unidirectional tension driving characteristics of the cable,the variable structure CDPR belongs to the redundant CDPR,and the solution of redundant cable tension is also an essential part of the motion control of PVSCDPR.The cable tension calculation needs to satisfy the real-time requirements of motion control.The optimization method of cable tension for redundant PVSCDPR is proposed.The optimization algorithm is based on the search algorithm of redundant cable tension convex space and has real-time performance.Considering how to track the joint cable length and cable tension at the same time,a cable length and cable tension control strategy based on admittance control for the cable drive module is proposed.And the trajectory tracking control strategy of PVSCDPR is designed.This control strategy is applicable to PVSCDPR.This research provides a control method for the system prototype and orientation workspace verification experiment.The principle prototype of PVSCDPR is built,which applies to the experimental verification of the planar discontinuous constrained PVSCDPR and the spatial continuous constrained PVSCDPR.The verification experiments of the large orientation workspace of the planar discontinuous constrained PVSCDPR and the spatial continuous constrained PVSCDPR were carried out.In the trajectory tracking experiment of the planar discontinuous constrained PVSCDPR,the ability of smooth switching between different configurations of the planar discontinuous constrained PVSCDPR is verified.In the trajectory tracking experiment of the spatial continuous constrained PVSCDPR,it is verified that the adaptive point of the spatial continuous constrained PVSCDPR is always at the stable equilibrium point,and the configuration of the spatial continuous constrained PVSCDPR is continuously changing.In the course of the trajectory tracking experiment of PVSCDPR,the positive cable tension verified the feasibility of the trajectory tracking control strategy of the PVSCDPR.The small trajectory-tracking error also proves the trajectory-tracking ability of PVSCDPR.