Reconfigurability Analysis of a Class of Parallel Kinematics Machines

A novel parallel kinematics machine (PKM) stemming from the 3-SRU (spherical-revolute-universal) under-actuated joints topology is presented in this paper. The concept here proposed takes advantage of a reconfigurable universal joint obtained by locking, one at a time, different rotations of a spherical pair. Such local reconfiguration causes a slight, yet crucial, modification of the robot legs mobility which is enough to provide the end-effector with different kinds of motion. In particular, the kinematic chain is converted to two different 3-URU architectures (universal-spherical-universal) able to provide the moving platform with essentially different mobilities. The paper is dedicated at formally demonstrating the motion capabilities offered by such parallel architectures. To this aim, the first part of the paper describes the mechanical structures and formalizes the kinematic problem through appropriate sets of polynomial equations. Then, an analysis of the equations is proposed to uniquely identify the mobilities of the moving platform. At last, a concept design is proposed for the reconfigurable spherical platform.