Knee stiffness adjustment for energy efficient locomotion of a legged robot on surfaces with different stiffness

In recent years, there has been an increasing interest in the development of variable stiffness actuators (VSAs) for legged robots. In this paper, we explore how VSAs can be used in legged robots to achieve energy efficient locomotion on compliant surfaces at various stride frequencies. Our legged robot is consisted of an actuated hip joint and a passive knee joint equipped with a VSA, named L-MESTRAN. This VSA is capable of varying stiffness over a large range, maintaining stiffness without consuming energy, and offering a linear joint stiffness. The compliant surface was constructed with stiffness variability. Through simulation and preliminarily experimental results, we show that adjustment of the knee stiffness in the relationship with stride frequency and surface stiffness is beneficial for increasing the energy efficiency of hopping at various stride frequencies.