The Application of Series Elastic Actuators in the Hydraulic Ankle-Foot Orthosis

Abstract

Advances in wearable technology have been trending toward more powerful and lightweight devices. A type of wearable device that fits into this trend is the Ankle-Foot Orthosis (AFO), which is a device that restrains or strengthens the movement of an ankle to assist the user during gait. A powered AFO uses an actuator and power supply to add positive power to the ankle but becomes heavier as the power output increases. To address the high power requirement and the need for a lightweight design, a hydraulic series elastic actuator (HSEA) was explored to determine whether it could be used to design a lightweight powered AFO that meets the high peak power demand of gait. Hydraulic power has excellent power density and the ability to lower the weight of the AFO at the ankle by separating the power supply from the actuator by a hose. In addition, a series elastic actuator can take advantages of the high-peak and low-average power profile of ankle gait to store energy and release it during the push off stage of gait. The parameters required for the series elastic actuator were calculated and validated using simulation. The velocity and torque of a gait pattern that requires 250W of peak power were able to be tracked using a 95W power supply. The actuator and power supply overall weight was reduced by 20% with the weight of the actuator at the ankle less than 0.5 kg. A novel design of a HSEA with a clutch capability is proposed for future AFO applications.