Experimental Validation of a Variable Stiffness Joint with Antagonistic Springs

Abstract

The variable stiffness joints (VSJ) provide nonlinear stiffness behavior. The VSJ can be used in articulated systems to improve the safety of human-robot interaction. A new VSJ is proposed in this paper based on the principle of antagonistic springs. The stiffness behavior is tuned when the lengths of the springs are adjusted. A motor is used to control the movement of a set of racks and gears. This latter set changes the springs’ lengths and thus the joint stiffness. Based on the mathematical model and in the case of the revolute joint, it is demonstrated that it’s possible to obtain a constant force due to the stiffness. The joint is attached to a link; the simulation results show that the joint provides the desired force at the end of the link. Physical experiments on the prototype were performed to verify the validity of the analysis results. The experimental results show that the control of the springs’ length allows, to vary the stiffness and therefore maintains a constant force at the end of the link regardless of the angle joint rotation.