On the design of mechanically safe robots based on spatial isotropic force modules and torque limiters

This paper presents a synthesis approach for the design of mechanically safe robots based on the concept of spatial isotropic force module (SIFM). In this concept, a number of torque limiters are included in the structure of the robot in order to limit the feasible forces at the end-effector. SIFMs are proposed in order to ensure that the feasible force space at the end-effector of the robot remains well conditioned for any configuration of the robot, thereby alleviating the configuration dependent transformation between articular torques and Cartesian forces. In this paper, possible architectures of SIFMs are proposed and the conditions required to ensure isotropy of the forces at the end-effector are derived. Then, a three-degree-of-freedom (3-dof) spatial robot including a SIFM is designed to demonstrate the effectiveness of the concept, and the forces that can be applied by the robot along its links are also analyzed as well as the power of potential collisions. Finally, a prototype of a SIFM is built, which is rather compact since the designed torque limiters do not have to be co-located with an actuated joint.