A Novel Design of Extended Coaxial Spherical Joint Module for a New Modular Type-Multiple DOFs Robotic Platform

In this study, we propose an extended coaxial spherical joint module (E-CoSMo) with three to four degrees of freedom (DOFs) for a multi-DOF robot platform. The E-CoSMo consists of a coaxial spherical parallel mechanism (CSPM) with three DOFs and one extended DOF based on a universal joint mechanism (UJM) coaxially connected to the CSPM. This structure enables the application of serial link configuration (such as shoulder-elbow) with wide and universal ROMs while allowing all four actuators to be placed in the base. This makes the inertia of the moving link part to be dramatically reduced and thus contributes to decreasing the mechanical impedance of the multi-DOF robot system. In addition, through the effective design of the coaxial spherical joint module, the output rotational torque in a specific axial direction reaches approximately three times then the torque of a single actuator. To optimally implement this, we applied an optimal design approach that considers the mechanical performance and design constraints. The mechanical impedance reduction effect through the proposed module is discussed. The feasibility of the E-CoSMo is also verified through a dynamic simulation. Finally, the proposed mechanism is verified using a fabricated prototype.