The Continuously Rotating Performance Analysis Of Mobile Platforms For Three Kinds Of Parallel Manipulators Based On Screw Theory

Compared to serial mechanism，workspace of parallel mechanism is smaller. It results inlower translational ability and rotational ability. This paper analyses three kinds of parallelmechanism whose mobile platform can rotate continuously. Because of these mobile platforms,parallel mechanisms possess larger dexterous workspace, which enlarges the application fieldsof parallel mechanism.Based on constraint screw theory, this paper carries out the detailed continuously rotatingperformance analysis of the three kinds of parallel mechanisms. The kinematic screw systemsand constraint screws system of the limbs and the mobile platform of the three kinds ofparallel mechanisms are written. Then the singular configurations are analysed and thenecessary conditions that the mobile platform can continuously rotate are summarized, whichcan provide some bases for type synthesis of other parallel mechanisms whose mobileplatform can rotate continuously. The main content of this article is as follows:1. The background of the research, especially the present research situation of parallelmechanism whose mobile platform can rotate continuously is described. Then we put forwardthe main research content of the subject and its significance.2. Kinematic screw systems of each branch of three mechanisms are written. By using theprinciple that reciprocal product of screw and reverse screw is zero we determine correspondingconstraint screw systems of the branchs, and respectively obtain kinematic screw systems of theplatforms. It shows that the mobile platforms of the three kinds of mechanism all include thethree motions of1R2P. Among the motions the rotation is a continuously rotating motion.3. The singular configurations of the three mechanisms are analysed, which shows thatsimilar-SCARA robot and the parallel mechanism with redundant actuator both have kinematicsingularity and constraint singularity. But the mechanism with a UPU limb doesn’t have anysingularity. The analysis shows that only constraint singularities affect the continuously rotating performance of the mobile platform, so they must to be eliminated.4. Based on the previous analysis, we sum up some necessary conditions that the mobileplatform of parallel mechanism can continuously rotate in one direction. Some solutions tosolve some problems are put forward. Finally a redundant actuator is used to eliminate theconstraint singularity of the second kind of3-DOF parallel mechanism.