Type Synthesis of Low-dof Parallel Robots Based on Screw Theory

In this paper a new procedure for type synthesis of low-DoF parallel robots based on screw theory is presented. The motivation of these work is that for many industrial applications, in particular in the machine-tool field, less than 6 degree of freedom (6-DoF) may be needed to complete the task. For example, for milling operation in the machine-tool domain, the rotation of the platform around its normal is not needed, as the spindle will manage this DoF, hence only 5-DoF are needed. Type synthesis consists basically in finding all possible types of parallel robots capable of generating a specified motion pattern of the moving platform. The type of motion pattern of the moving platform is specified in agreement with the degrees of freedom of the task to be developed. Previous research on parallel manipulators were concentrated mainly on 6-DoF parallel robots, but today parallel robots with fewer than 6-DoF, also called low-DoF parallel robots, are being increasingly studied. In comparison with 6-DoF parallel robots, a low-DoF parallel robot has the advantages of more simpler mechanical design, lower manufacturing cost, simpler controller and, in general, reducing the number of DoF will increase the maximum motion range of the remaining DoF increasing the workspace. The type synthesis procedure present in this paper is developed in five steps. First, the motion pattern is defined in agreement with the DoF necessary for task execution. Second, using reciprocal screws we define the robot twist and wrench system. Third, the legs of parallel robot are enumerated using linear combinations of screws and the assembling of parallel robot is done. Fourth, as twists and wrenches are instantaneous, the identification of instantaneous parallel robots is necessary. And fifth, a method for validation of actuated joints is present. Parallel robots with 4-DoF and 5-DoF are synthesized and tables are presented using a special notation. A functional representation of each parallel robot with 4-DoF and 5-DoF and actuated joints are presented. The advantage this procedure in relation of others procedures based on screw theory, found in literature, is the completeness of type synthesis process.