Improving Energy Efficiency of Hopping Locomotion by Using a Variable Stiffness Actuator

In recent years, the development of legged locomotion robots that can achieve both efficiency and versatility has been one of the most important challenges in robotics research. In general, fully actuated systems that can achieve many variations of behaviors show comparatively low energy efficiency, while it is extremely difficult to enrich the behavioral diversity of passivity-based systems that exhibit efficient behaviors. In order to overcome the tradeoff, there has been an increasing interest in the development of actuation technologies, such as variable stiffness actuators (VSAs) that can autonomously adjust mechanical dynamics. However, although many VSAs have been proposed and developed in the past, researchers are yet to clarify how such actuators can improve both energy efficiency and behavioral diversity. From this perspective, the goal of this paper is to investigate a one-legged hopping robot that is equipped with a class of VSA with the intention of explaining how behavioral diversity can be enhanced with modest impact in the energy efficiency. Through a systematic analysis including both simulation and a real-world robot platform, this paper investigates how the natural dynamics of hopping robots can be varied by the actuator resulting in variations in stride frequencies and locomotion speed while maximizing energy efficiency.