General methodology for type synthesis of symmetrical lower-mobility parallel manipulators and several novel manipulators

In the area of parallel robots, the use of lower-mobility parallel manipulators for many tasks requiring fewer than six degrees of freedom (DoF) has drawn a lot of interest. This paper treats one fundamental of problem in the study of lower-mobility parallel manipulators: type synthesis. Using reciprocal screw theory, we define the mechanism constraint system and limb constraint system. We then investigate the relations between the mechanism constraint system and the limb constraint system under different geometrical conditions. Three tables describing the relations are presented. Based on the three tables, we propose a constraint-synthesis method for type synthesis of symmetrical lower-mobility parallel manipulators. The method used in the paper to construct the kinematic structure of the limb of lower-mobility parallel manipulators with prescribed DoF is simple and systematic. Examples are included to illustrate the application of the method and some novel lower-mobility parallel manipulators with 3-, 4- and 5-DoF are synthesized.