The virtual screw: Concept, design and applications

The research work reported here was motivated by a class of two-limb parallel Schönflies-motion1 generators, which offer simplicity, isostaticity, and symmetry. The crucial components required to construct a parallel two-limb robot of this class are a cylindrical drive, namely, a cylindrical motion generator, and a load-carrying link playing the role of the moving platform in parallel robots, with four degrees-of-freedom, i.e. 3D translation and rotation about a vertical axis, which operates based on the same cylindrical motion generator. However, the design requirements of such components call upon screw joints with an unusually large pitch, which are not available off-the-shelf; the authors thus propose a cable-driven virtual screw with an arbitrarily large pitch. This concept is elaborated on with regard to its various alternatives, each suitable for different circumstances and applications. Furthermore, the concept of variable-pitch virtual screw is introduced, which enables the designer to adapt the specification of the robot to any given task. In addition, the novel application of the virtual screw in the context of the architecture of Schönflies-motion generators is studied. Finally, the authors report the design and fabrication of two prototypes to conduct a comparison between the screw joint and the virtual screw.