Abstract: Based on basic structure theory of parallel robotic mechanisms，a novel type synthesis methodology for kinematic structures of 6-DOF weakly-coupled parallel mechanisms is presented, which considerates the hybrid chain as the “limps” connecting the moving platform with the base. The 27 promising 6-DOF parallel kinematic structures with lower coupling degree between independent loops, actuated by 6 rotation joints (or 6 prismatic joints) amounted at the base, are presented originally , the platform of which can possesses general spatial motions, I.e. three translations and three Rotations . The much more alternatives to these kinematic structures also could be obtained by altering the location and the type of the driving joints or by changing some partial sub-structures relevant to the branch loops. The classification of these parallel kinematic structures is identified according to the four structure evaluation criteria suggested here, I.e.(a)structure topological symmetry; (b)coupling degree value between independent loops; ? structure simplicity (with fewer links and fewer joints); (d ) manufacturability, from which optimal 6-DOF parallel kinematic structures are recommended in terms of different design requirements.

Key words: Hybrid chains, Kinematic structures, Parallel mechanisms, Parallel robotics, Single-opened chains, Type synthesis