A Compact Rotary Series Elastic Actuator with Wide Deflection Range and Linear Torque Response for pHRI Applications
Mohamed Eraky, Andy Li, Biruk Gebre, Kishore Pochiraju, Damiano Zanotto
AI summary
Problem
Conventional series elastic actuators for physical human-robot interaction struggle to balance compactness, high torque capacity, and wide passive deflection range, often suffering from constrained deflection and high hysteresis due to antagonistic spring preloads.
Approach
The authors developed a compact rotary actuator using ten compression springs arranged on coaxial output rotors that rotate within input chambers, allowing all springs to deflect equally in the same direction with minimal preload to eliminate sliding friction and enable bidirectional loading.
Key results
- Peak torque capacity of 18 Nm and maximum stiffness of 43.8 Nm/rad
- Linear bidirectional torque-deflection response over ±24° with only 2.87% hysteresis
- Closed-loop torque tracking bandwidths ranging from 8.5 to 15 Hz under varying loads
- Accurate assistive torque rendering in an ankle exoskeleton with 1.48 Nm RMSE during treadmill walking
Why it matters
This design advances wearable rehabilitation and assistive robotics by providing a compact, high-fidelity elastic element that improves torque control and user comfort during physical human-robot interaction.
Abstract
This paper presents the design and character- ization 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 fric- tional 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.