A novel hydraulic swing actuator with high torque density for legged robots

Hydraulic swing actuators hold significant potential for legged robots due to their ability to deliver high torque directly to joints. However, their practical implementation is hindered by issues such as internal leakage, excessive size, and redundant weight, which restrict the dynamic performance of legged robots. To address these challenges, this paper proposes a novel circular swing hydraulic actuator that offers a compact design and a high torque-to-mass ratio inspired by the snail’s structures. First, the basic motion principle of the circular swing actuator is introduced, drawing from the snail’s spiral motion. Then, the split-shell mechanical structure with enhanced machinability and better assembly precision is presented, with a final dimension of ϕ153 × 70 mm³. Its mechanical properties are validated through numerical simulations and experiments, achieving a torque-to-weight ratio of 471.7 Nm kg⁻¹. Additionally, a customized sealing system ensures minimal internal leakage and zero external leakage. Step response and position control experiments demonstrate that the actuator’s frequency response and tracking accuracy meet the requirements for legged robot joints driven by hydraulic systems. This circular swing actuator provides a promising solution for designing agile, high-performance legged robots.