Preliminary Design and Engineering Evaluation of a Hydraulic Ankle–Foot Orthosis

Abstract

Ankle foot orthoses (AFOs) are used to correct motor impairments of the ankle. While current AFOs are passive, advances in technology and wearable robotics have opened the opportunity for a powered AFO. The hydraulic ankle foot orthosis (HAFO) is a device that takes advantage of the exceptional powertoweight and forcetoweight of hydraulic fluid power. The device is untethered, and the power transmission chain is battery–electric motor–hydraulic pump–hose–cylinder, with the power supply worn at the waist and the cylinder actuators at the ankle. The fluid power circuit is configured as an electrohydraulic actuator (EHA) that is controlled by controlling the electric motor. The first prototype weighs 3.3 kg of which 0.97 kg is worn at the ankle. Steadystate torque–velocity performance showed that the prototype can provide 65 Nآ·m of assistance torque and a noload velocity of 105 deg/s. Closedloop position control showed low steadystate error but a slow response. The current prototype demonstrates the potential of hydraulics for providing large torques in a compact, lightweight device. The speed performance of the prototype is inadequate for normal walking but can be improved by switching to servo valve control or by developing a custom hydraulic pump.