Design and Evaluation of a Torque-Controlled Ankle Exoskeleton Using the Small-Scale Hydrostatic Actuator: miniHydrA

A small-scale electro-hydrostatic actuator, termed miniHydrA, was developed based on biomechanical requirements for gait and integrated into an ankle exoskeleton. The key advantage of this actuator concept lies in its compact size and the low mass of its output stage, combined with the ability to deliver high support torques, sufficient for full human assistance. During development, hydraulic cylinder leakage and friction were identified as key challenges. To address control requirements, a dedicated control strategy was proposed and implemented. The prototype exoskeleton was evaluated for joint torque tracking performance across a range of torques (0–120 Nm), both in benchtop tests and during treadmill walking trials. In benchtop experiments, zero-torque tracking was achieved with a mean absolute error ranging from 0.03 to 2.26 Nm across frequencies from 0 to 5 Hz. During treadmill walking, torque tracking errors ranged from 0.70 to 0.95 Nm, with no observable deviations in ankle joint kinematics among the three test subjects. These results show the feasibility of the miniHydrA for remote actuation. Compared to Bowden cables, commonly used in exoskeletons and exosuits, the proposed actuator concept offers two key advantages: it is better suited for high-torque applications, and its friction characteristics can be more accurately predicted and modeled, enabling more effective feedforward control.