Abstract: Type synthesis is an important stage of innovative robot design. It corresponds with the design requirements from scratch according to design tasks using logical reasoning and imaginary thinking to establish logical operation principles considering mechanism characteristics and construct a type design method of parallel robots from the view point of motion combination. Considering that the limb configuration determines the movement capability of rotation characteristics of the distal link in the parallel mechanisms, the real axis movement theorem and virtual axis movement theorem are proposed. The real axis movement theorem refers to that the rotation characteristics of the limb distal link is determined by the position of one revolute joint in the limb and has movement capability along the direction of some translation characteristics of the revolute joint, while the virtual axis movement theorem means that the rotation characteristics of the limb distal link has movement capability along the plane perpendicular to the rotation axis spanned by any two translation characteristics directions. The virtual axis movement theorem is proofed in all six cases by mobility analysis of the virtual closed-loop linkage formed by adding a fixed revolute joint at distal link. Two novel parallel mechanisms are analyzed by applying the proposed theorems.

Key words: motion characteristics of limb distal link, parallel mechanism, real axis movement, virtual axis movement