The development of an innovative two-DOF cylindrical drive: Design, analysis and preliminary tests

The quest for ever faster pick-and-place robots has led to ingenious parallel robots with reduced mobility, e.g., capable of producing motions proper of SCARA systems: three independent translations and one rotation about an axis of fixed direction. These robots are also known as Schönflies-motion generators (SMG). Some parallel versions are commercially available, as are serial-parallel designs. The former are provided with four limbs, the latter with three, the fourth degree of freedom being appended in series with a Delta robot. Parallel robots are more attractive than their hybrid counterparts, but the presence of four legs poses serious challenges to their designers, as limb-interference limits the rotatability of the moving plate. A solution to this problem includes a gear train for rotation-range amplification, but this increases the inertial load on the motors and complicates the design - too many parts - and the control - because of inherent gear backlash and Coulomb friction. Recently, a SMG system was proposed that is supplied with two limbs, arrayed in an isostatic structure, which provides high rotatability of its gripper. This robot is driven by one C (cylindrical)-joint at each limb. As this joint allows for two degrees of freedom, it calls for two motors, that might as well be fixed to the base, which poses interesting design challenges. Reported in this paper is a design solution for the drive of a C joint, which is termed the C (cylindrical)-drive, based on a cylindrical differential mechanism of the RHHR type, with R standing for revolute, H for helical (or screw) joint. The design, kinematics and dynamics of the drive are discussed, along with a realization, and preliminary tests.