Type Synthesis of Three-Degree-of-Freedom Spherical Parallel Manipulators

A spherical parallel manipulator (also called an orientational parallel manipulator or rotational parallel manipulator) refers to a threedegree-of-freedom (3-DoF) parallel manipulator in which the moving platform undergoes a 3-DoF spherical motion or rotates about a fixed point. In this paper, we propose a new method for the type synthesis of non-overconstrained (isostatic) spherical parallel manipulators. In a non-overconstrained three-legged spherical parallel manipulator, each leg is composed of five revolute, prismatic and/or helical joints. Using the proposed approach, non-overconstrained three-legged spherical parallel manipulators can be obtained by the combination of a 3-RRR overconstrained spherical parallel manipulator and three four-bar Delassus linkages with at least one revolute joint. In a 3-RRR spherical parallel manipulator, each leg is composed of three revolute joints whose axes pass through the center of spherical motion. A four-bar Delassus linkage is a single-DoF four-bar linkage composed of four revolute, prismatic and/or helical joints. A number of new non-overconstrained spherical parallel manipulators are also identified, including one obtained by the combination of a 3-RRR spherical parallel manipulator and three Bennett linkages. KEY WORDS—spherical parallel manipulator, type synthesis, Bennett linkage, Delassus linkage, non-overconstrained