Relaxing the Conservatism of Passivity Condition for Impedance Controlled Series Elastic Actuators This paper proposes a practical and less conservative passivity analysis for series elastic actuators (SEAs) by introducing load port definition and shows that the achievable stiffness by the impedance control of SEA can be set higher than the inherent stiffness of SEA depending on the condition of the load dynamics. Since SEA can inherently measure or estimate a transmitted force thanks to its embedded spring element, impedance control is often exploited to render compliant behaviors related between the motion and the force. Although the stability of the SEA control system is of great importance, the conventional passivity analysis gives conservative criteria, and indeed limits the actual actuator performance. To tackle the conservatism of the conventional passivity in SEAs, we first explore the dynamic characteristics of SEA including load dynamics, which has been ignored for the sake of simplicity of the passivity analysis by excluding uncertain load dynamics. The inclusion of the load dynamics into the passivity analysis allows us to properly derive the less conservative limit of achievable stiffness by impedance control and the factors that determine the limit. The proposed analysis is verified by numerical simulations and applied to a passivity observer design for experimental validation on an actual SEA setup.

Design of a Series Elastic Actuator with Doublelayer Parallel Spring for Lower Limb Exoskeletons This paper presents a novel Series Elastic Actuator (SEA) with doublelayer parallel spring for lower limb exoskeletons (LLE). The SEA can improve the humanrobot interaction comfort and have the ability to absorb shock and vibration. In order to make the assembly process simple and convenient, the series elastic unit adopt split structure, which includes an inner ring structure, an outer ring structure and rectangular springs as elastic element. At the same time, in order to meet the requirements of the stiffness and dimensions of the SEA, a doublelayer parallel spring arrangement is adopted. The stiffness of SEA is 312Nmrad, which is obtained by the stiffness calibration experiment. In the experiments, the SEA is used in the knee joint of LLE and an impedance controller is designed to achieve knee joint tracking motion. Finally, the experiments results show the position tracking performance of the LLE, which illustrates the effectiveness of the SEA.