A Robot Joint with Variable Stiffness using Leaf Spring

Abstract

Interaction with humans is inevitable for service robots, which results in safety being one of the most important factors in designing the robots. Compliant component is an answer to the safety issue at the cost of performance degradation. In order to reduce the performance degradation, manipulators equipped with variable stiffness have been studied by many researchers. This paper presents a variable stiffness joint (VSJ) designed for a robot manipulator, as well as a control scheme to control the stiffness and position of the VSJ. Compliance is generated by leaf springs and two actuators are used to control the position and stiffness of the joint using four-bar linkages. Two actuators in parallel configuration are connected to the spring. Changing the effective length of the spring results in a change in stiffness. The position of the joint is controlled via two actuators rotating at the same speed in the same direction. A nonlinear controller is used to control the VSJ, and a singular perturbation model is adopted to prove the stability of the closed-loop system. Experiments are conducted to show that the position and stiffness are controlled independent of each other, and having less stiffness at the joint helps in making an unexpected collision with an object safer.