Research On Co-Axis Actuated Parallel Robots For Large Workspace And High-Speed Pick-and-Place Applications

Pick-and-place operation is a typical process in industrial production to be utilized for loading/unloading,sorting,packing and so on.Due to their high velocity and high acceleration,parallel robots have been widely recognized as prospective candidates for industrial high-speed pick-and-place operations to improve production efficiency.However,the traditional parallel robots exhibit fundamental drawbacks in large footprint and limited workspace,which greatly restrict their applications,particularly to pick-and-place operations.In this dissertation,for planar 3 degree-of-freedom and 4 degree-of-freedom pick-and-place operations,a family of parallel robots whose actuated joints have a common axis of rotation have been introduced.Not only do they retain the high dynamic performance of parallel robots,but also their workspaces are greatly expanded,which are similar to serial robots'.Thus,a series of researches about structure,kinematics,dynamics,optimal design,motion control,etc.of the co-axis actuated parallel robots have been carried out in this dissertation.The following contents are mainly investigated.In this dissertation,a 3 degree-of-freedom planar parallel robot with common axis actuated joints,named the V3 robot,is the main subject.Firstly,its geometric model,kinematics starting with the loop-closure equations,dextrous workspace based on the inverse kinematics,singularity from the velocity equations and rigid-body dynamic model based on the virtual work principle are derived and analyzed.Then,based on the structure of the V3 parallel robot,this dissertation presents a novel 4 degree-of-freedom parallel robot with common axis actuated joints,named the T4 robot,which is an upgraded design of the V3 parallel robot and capable of three translations and one rotation.It retains large workspace and unlimited rotation capability of the end-effector.Due to the modified type synthesis of the subchains and the introduction of a lifting subchain,this robot overcomes the disadvantages of the V3 parallel robot,which is short of a vertical translation and weak in load bearing capacity.The inverse and forward kinematics are derived and analyzed starting with the loop-closure equations.An algebraic derivation of the dextrous workspace is presented from the inverse kinematic equations.The singularity analysis is discussed based on the relationship between the velocities of the actuated joints and the end-effector.For high-speed pick-and-place operations,three new optimal design methods are proposed for the V3 robot mechanism to obtain optimal geometric parameters.Combining some of the alternative performance indices,which consist of dexterity,velocity,accuracy,acceleration,motor output power and bending stiffness,these optimal design problems are formulated to maximize the workspace having specified desired performances.Simulation results show these methods are effective and that the optimized structures' performances are greatly improved.Considering the flexible-link robot system,the integrated structure and control design method for the V3 parallel robot system is studied.Efficiency is critical requirement for high-speed pick-and-place operations.In order to achieve high acceleration,low moving mass/inertia is inevitable,which leads to increasing flexible effects due to the applied thin and light links.In view of the coupled effects of structure and control parameters in the whole system performance,the integrated method is utilized to improve the performance of the V3 parallel robot system.The dynamic model is derived by the finite element method.The proportional-derivative control strategy is applied in the closed-loop system.Efficiency is critical requirement for high-speed pick-and-place operations.In this dissertation,an efficiency based the integrated structure and control design method is presented for high-speed pick-and-place operations.The structural and control parameters are optimized simultaneously by solving the integrated design problem with accuracy constraints.Simulation shows that the integrated design method presents improved system performance on efficiency,at the same time the accuracy is also guaranteed.Considering the optimized structures and the singularities,singularity-free path planning strategy is studied.This strategy can be used to decide whether the singularity-free path can be achieved and choose the path type by the positions and the orientations of the end-effector at the starting and ending points.At last,the prototype of the V3 parallel robot system is designed and established,which is made of common components and parts in industrial automation equipments.The results of the experiments based on this prototype confirm it can be applied to high-speed pick-and-place operations and the optimal design is effective.