Structure-Controlled Variable Stiffness Robotic Joint Based on Multiple Rotary Flexure Hinges

Abstract

Variable stiffness actuator (VSA) plays an important role for achieving safety and robustness for robots to physically interact with environments. This article proposes a structure-controlled approach in developing a new variable stiffness robotic joint with the merits of compactness, high stiffness, and high displacement-to-stiffness ratio. Unlike the common methods of using antagonist springs and lever concept, the structure-controlled approach achieves stiffness variation through the adjustment of mechanical structure configuration inside the actuator. Specifically, the stiffness variation is realized by changing the effective second moment of area through rotating a combination of leaf flexure hinges arranged in a radial configuration. A range of configurations are studied and the configuration we identified has the advantages of minimum unbalanced twists in the mechanism while providing high transmission torque, large stiffness, range and minimum lateral buckling effects. The performance characteristics are accomplished through a novel variable stiffness mechanism and a position-based control strategy for both position and torque control modes. A prototype system was implemented to verify the characteristic and performance of the design concept. It demonstrated that the new VSA system is effective for realising independent joint-stiffness and joint-position control of robot joints.