Transformable parallel-serial manipulator for robotic machining

Robotic manipulators have been conventionally used for position-controlled tasks such as pick-and-place, spray painting and arc welding, where the tools do not experience contact forces from external objects. In recent years, however, there is an increasing interest in using robotic manipulators to perform machining work, for e.g. drilling, polishing, deburring and milling. This is due to the greater dexterity of robots as compared to that of CNC machines (6 axes vs. 3 or 5 axes), as well as the lower cost of industrial manipulators. Nevertheless, in such machining applications, the object exerts external forces onto the tools and thus onto the robots, and robots would generally deflect because of their low joint and link stiffness. This deflection prevents the tools from cutting, drilling or milling the objects into the correct depth, and therefore the dimension of the machined object would not be accurate. In this paper, a novel design of robotic manipulator is proposed, which combines the advantages of both parallel and serial robots, and is suitable for machining applications. The manipulator acts as a stiff parallel manipulator when carrying out machining tasks, but transforms into a serial articulated manipulator when maneuvering through a large area. Experimental results show that the proposed manipulator has a higher stiffness than a serial manipulator of the same size, and that it outperforms conventional serial robots in carrying out machining work.