Design And Virtual Simulation Analysis Of A Five-degree-of-freedom Over-constrained Hybrid Manipulator With Extremely Few Kinematic Joints

Among the various types of lower-mobility parallel mechanisms(PMs),the two rotational and one translational degree-of-freedom(2R1T)PMs is one of the most important ones,which have been obtained successful application especially in the field of five degree-of-freedom(DOF)hybrid robot.It has obvious advantages of large workspace and flexible operation in machining complex structural parts in the fields of aerospace,ship and other high-end manufacturing.The fewer the number of kinematic joints of parallel mechanism,the simpler the structure and the easier to realize the design of high structural stiffness.Thus,this paper proposes a method of type synthesis of 2R1 T PMs with extremely few kinematic joints.Besides,based on the four-branch 2R1 T PM 2RPU-UPR-RPR containing extremely few kinematic joints that has been obtained,a swing head with a single DOF is added onto the moving platform(MP)of the PM to adjust the orientation in the other horizontal direction,and the base of the PM is installed on a single DOF mobile worktable to adjust the position in the other horizontal direction.And a five-DOF hybrid robot is then constructed,which can satisfy both the requirements of large workspace and high structural stiffness.Then its scale optimization design and virtual simulation analysis are carried out.The main works are listed as follows:Firstly,based on the theory of reciprocal screw,the ultimate constraint wrenches exerted on the moving platform of 2R1 T PMs with three-branch and four-branch are analyzed and demonstrated.Based on the ultimate constraint wrenches,a series of2R1 T PMs with three and four branches are synthesized,including 2RPU-UPR-RPR and RPU-2UPR-RPR mechanisms.Secondly,two kinds of four-branch mechanisms,RPU-UPR-RPR and RPU-2UPR-RPR,with better structural symmetry,are compared and analyzed by taking workspace and stiffness as evaluation indexes.Then a five-axis hybrid robot is constructed based on the 2 RPU-UPR-RPR mechanism with better performance,and the forward and inverse position solutions of the five-axis hybrid robot mechanism are analyzed by using the equivalent method of serial mechanism,which provides analgorithm basis for the following scale optimization and cutting simulation.Then,based on the characteristics and requirements of a kind of practical aerospace parts,the size of a cylindrical workpiece is defined.Under the condition of satisfying the designed workspace,key dimensions of the parallel mechanism within the five-axis hybrid robot are optimized based on the force/motion transfer performance index,and a set of size combinations making the mechanism have excellent performance are obtained.Finally,based on the optimized key sizes of the hybrid robot,the simulation model of five-axis hybrid robot is established.The motion simulation and surface cutting simulation of the designed robot are carried out by using the open source robot simulation software V_Rep,and the correctness of the kinematics model is verified by an example of machining spherical trajectory.By comparing with the theoretical analysis results,the correctness of the previous type synthesis and inverse kinematics solution model is verified,which lays a foundation for the future practical application of such robots.