Preliminary design and engineering evaluation of a hydraulic ankle-foot orthosis

Ankle foot orthoses (AFOs) are used to correct motor impairments of the ankle. Whilecurrent AFOs are passive, advances in technology and wearable robotics have openedthe opportunity for a powered AFO. The hydraulic ankle foot orthosis (HAFO) is a devicethat takes advantage of the exceptional power-to-weight and force-to-weight of hydraulicfluid power. The device is untethered, and the power transmission chain isbattery-electric motor-hydraulic pump-hose-cylinder, with the power supply worn atthe waist and the cylinder actuators at the ankle. The fluid power circuit is configured asan 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. Steady-statetorque-velocity performance showed that the prototype can provide 65 N±m of assistancetorque and a no-load velocity of 105 deg/s. Closed-loop position control showed lowsteady-state error but a slow response. The current prototype demonstrates the potentialof hydraulics for providing large torques in a compact, lightweight device. The speed performanceof the prototype is inadequate for normal walking but can be improved byswitching to servo valve control or by developing a custom hydraulic pump.