Abstract: Basic principles and characteristics of three structural synthesis methods (based on screw theory, displacement subgroup and position orientation characteristics(POC) respectively) for robot mechanism are described and compared. This comparative study shows that the method based on POC has the following notable advantages：The obtained mechanism is non-instantaneous mechanism and the DOF is full-cycle DOF. Existence condition of the obtained mechanism is more general (relative to screw theory based method). The basic structural equations reveal the explicit mapping relations among topological structure, POC and DOF of mechanisms. Much less operation rules are required and all types of mechanisms are covered (relative to displacement subgroup based method). The conclusion is that the POC based method is an effective new one for creative mechanism topological structure design. It is an important research field to establish a systematic theory integrating topological structure, kinematics and dynamics of mechanisms based on a single open-chain unit.

Key words: Dynamics, Kinematics, Robot mechanism, Structural synthesis