Design of a Robotic Spherical Wrist with Variable Stiffness

This paper discusses the design of an adjustable force compensator for a spherical wrist dedicated to robot milling and incremental sheet metal forming applications. The design of the compensator is modular and can be introduced to any existing manipulator design as a single multi-body auxiliary system connected with simple mechanical transmission mechanisms to the actuators. The paper considers the design of the compensator as an arrangement of elastic springs mounted on moving pivots. The moving pivots are responsible for adjusting the stiffness of the wrist-compensator coupling. Special attention is given to two compensation schemes in which the value of the external force can be known or unknown, respectively. The simulation results show that the analytical derivation of the compensator leads the main actuators to spend zero effort to support the external force.