A Compact Rotary Series Elastic Actuator with Wide Deflection Range and Linear Torque Response for pHRI Applications

Abstract

This paper presents the design and characterization of a new series elastic actuator (SEA) for physical human-robot interaction (pHRI) featuring a compact spring mechanism. The spring mechanism consists of ten compression springs, fitted on output rotors and arranged in a curved formation. The compression springs are enclosed in spring chambers featured in input rotors. This design reduces frictional losses and enables all springs to bear load bidirectionally with minimal preload relative to conventional designs that rely on antagonistic spring arrangements, thereby enhancing deflection range and torque capacity. We introduce the SEA design and experimentally characterize the passive torque-deflection curve and the closed-loop torque tracking bandwidth. Bench testing demonstrates a torque capacity of 18 Nm and a maximum stiffness of 43.8 Nm/rad. As a representative application, the SEA is integrated into an ankle exoskeleton, with the spring mechanism co-located at the ankle joint. Treadmill walking tests with the exoskeleton indicate good torque tracking performance, with a root-mean-square error of 1.48 Nm when applying 12% assistance, corresponding to a peak torque of 17.6 Nm.