Research On Drive Unit Of Large Cable-driven Parallel Robot

Large cable-driven parallel robot is a new type of parallel robot,which uses cable instead of rigid link.It has the characteristics of large workspace,large load-mass ratio and high modularity.Its working principle is that the length and tension of the cable are controlled by several identical drive units,so as to adjust the position and posture of the moving platform at the end of the cable.Therefore,the development of a drive unit with good dynamic quality is the first step in designing a cable-driven parallel robot.However,the elasticity of the cable,the unidirectionality of the force,the wrap angle and friction of the pulley have some effects on the cable position control and tension control of the drive unit.Therefore,this paper focuses on the mechanical and electrical system design,system modeling and motion control of the drive unit,including the following parts:Firstly,a drive unit with linearization of cable length and tension is designed.In this process,the overall design index of the cable-driven parallel robot is firstly decomposed into the design parameters of the drive unit,and the design scheme of the winding mechanism and the arranging mechanism is put forward.The mechanical system design of the drive unit is completed by optimizing the basic dimensions of the main components of the drive unit and calculating the important dimensions of the key components based on the energy optimization.And the real-time electrical system is designed and built based on the BECKHOFF platform.Completed the development of interactive interface and test program under TwinCAT software.Secondly,the kinematics and dynamics models of the drive unit were completed.In the process of kinematics model,considering pulley wrap angle,the actual revised cable retraction length model is established.The error compensation rule of pulley radius and motion position of moving platform for kinematics correction model is calculated to verify the correctness of the model.In the process of dynamics model,the cable model is simplified to a mass-free spring model,and the dynamic elastic deformation of cable is considered,and the driving list is established.The dynamic model of the element is validated by experimental identification and compared with the theoretical model.Then,a compensation control algorithm is established for the redundancy of the test platform.On the basis of the identified model,the forward channel and disturbance transfer function of the drive unit are obtained.A cable position controller is designed on the non-redundant drive unit,which consists of kinematic compensation composed of pulley wrap angle compensator and dynamic compensation composed of forward channel series correction,feed-forward compensation and disturbance feed-forward compensation.The cable tension controller is designed on the element.For the redundancy of two drive units in the 1-DOF test platform,the passive mode based on trajectory planning and the active mode based on interaction force are proposed.Finally,the multi-mode dynamic response characteristics of the drive unit are experimentally tested.It is verified that the kinematics compensator reduces the cable length error by about 85%and the dynamic compensator reduces the lag angle by about12.3~o.At the same time,the dynamic response and steady-state accuracy of the drive unit to the constant and sinusoidal forces are tested.The working mode proposed in this paper is adopted in the joint space,and the trajectory of the moving platform can be planned to5Hz in Cartesian space when the position error of the moving platform is less than 1.5mm and the lag angle is within 3~o.The drive unit system with good dynamic quality is obtained.