Type synthesis and axodes analysis of a class of special 2R1T parallel mechanisms

In this paper, type synthesis of a class of 3-degree-of-freedom (3-DOF) parallel manipulators (PM) is firstly implemented by using a graphical approach. The synthesized PM has two rotational DOFs and one translational DOF, and axes of the two rotational DOFs are always coplanar and perpendicular to the direction of the translational DOF. The motion pattern of the synthesized PMs is defined as 3-RSR type which is different from 3-PPS type, UP type, PU type, or RPR type. Some symmetrical configurations with the desired motion pattern are obtained. In order to know about attractive kinematic characteristic of this class of PMs, the axode characteristic is investigated by taking a typical case i.e. a 3-RSR PM for instance. Base on the analysis of the axodes, it is disclosed that the 3-RSR PM can realize four special motion patterns, i.e. pure conical rolling, pure cylindrical rolling pure spherical rolling and pure elliptical rolling. Finally, a conclusion is drawn that this kind of PMs could not only undergo all motion patterns of the Omni-Wrist III, but also achieve the motion of pure rolling of two curves with equal curvature1.