A Compact Rotary Series Elastic Actuator for Human Assistive Systems

TLDR

Human assistive robots require precise, high‑torque, back‑drivable, low‑impedance, and compact hardware, yet worm‑gear torque amplification differs from the nominal ratio because of friction, complicating controller design. The paper designs a compact rotary series elastic actuator that incorporates friction effects into its model and develops a robust control algorithm to precisely regulate torque output amid nonlinearities. The actuator uses a worm gear to magnify motor torque within a compact space, and its mechanical design, dynamic model, and robust control algorithm are presented and experimentally validated. Experiments with a human subject demonstrate the cRSEA’s performance during human interaction.

Abstract

Precise and large torque generation, back drivability, low output impedance, and compactness of hardware are important requirements for human assistive robots. In this paper, a compact rotary series elastic actuator (cRSEA) is designed considering these requirements. To magnify the torque generated by an electric motor in the limited space of the compact device, a worm gear is utilized. However, the actual torque amplification ratio provided by the worm gear is different from the nominal speed reduction ratio due to friction, which makes the controller design challenging. In this paper, the friction effect is considered in the model of cRSEA, and a robust control algorithm is designed to precisely control the torque output in the presence of nonlinearities such as the friction. The mechanical design and dynamic model of the proposed device and the design of a robust control algorithm are discussed, and actuation performance is verified by experiments. Experimental results with a human subject are also presented to show the performance of the cRSEA while interacting with humans.