Comparative study of two methods for type synthesis of robot mechanisms

Basic principles and main characteristics of two methods for structure synthesis of robot mechanisms (the subgroup-based method and the position and orientation characteristics (in short, POC)-based method) are studied and compared in this paper. The comparison deals with mathematical tools, symbolic representation of topological structure, mathematical representation of POC of motion output link (e.g. moving platform of a parallel mechanism, end link of a serial mechanism, etc.), POC equations of serial and parallel mechanisms and the operation rules of POC matrices, and characteristics of synthesized mechanism, etc. The comparative study shows both methods have some common characteristics (e.g. non-instantaneous mechanism with full-cycle DOF can be obtained, mechanism existence criteria are general, not applicable to certain mechanisms such as Bennett mechanism, etc). But the POC-based method is totally different from the subgroup-based method. Its main characteristics include: (1) topological structures of a mechanism can be represented with only R, P and H pairs; (2) POC matrix is used to describe the POC of relative displacement between any two links of a mechanism; (3) fewer rules are needed for POC matrices; (4) only simple mathematical tools (such as vector algebra, set theory, etc.).