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

Abstract Hydraulic swing cylinders have tremendous potential in legged robots for their direct heavy torque output to the joints. However, the practical usage faces the obstacles such as internal leakage, oversized shape, and redundant weight, which limit the dynamic performance of legged robots. To solve the above problems, this paper proposes a novel swing hydraulic actuator with a compact size and high torque-to-mass ratio by bending the hydraulic linear actuator, named a circular swing actuator. First, the basic principle of the circular swing cylinder is introduced, then the mechanical structure considering machinability is given, and the ultimate dimension is φ153×70 mm3. The mechanical properties of the circular swing cylinder are verified by numerical simulation and experiments. Its torque-to-weight ratio reaches 471.7 Nm/kg, and the lower internal leakage and zero external leakage are satisfied with the customized sealing system. The step response and position control experiments are conducted, and the frequency and tracking accuracy meet the demands of hydraulically actuated legged robot joints. The proposed hydraulic circular swing actuator offers a good joint actuator choice to design a legged robot with agile motion ability.