Parametric design of an active ankle foot orthosis with passive compliance

Abstract

A two degree of freedom active ankle foot orthosis (AFO) for post stroke rehabilitation purposes was designed and manufactured. The AFO was cable actuated, with the force supplied by custom-built air muscles due to their low weight and compliance. A novel actuator linkage was designed in order to allow the entire actuation system to be situated behind the leg, meaning the AFO was lightweight and discrete. The novel two degree of freedom joint allowed a significantly more natural gait and comfortable user experience than conventional one degree of freedom AFOs. The effect of wearing the AFO in a passive state was analysed using a low cost inertial measurement unit (IMU), which showed that it had minimal impact on key gait parameters such as stride length and velocity. It was also trialled in a passive condition by multiple users, including 3 stroke rehabilitation physiotherapists, and was found to be comfortable to wear and easy to be put on. A parametric design process was developed, which produces a custom design based on an individual's anthropometric data. This kind of custom design would give much better conformity, decreasing skin friction and increasing comfort. It has been concluded that a custom two-degree of freedom active AFO could be of benefit after a stroke as it allows more complete rehabilitation of the ankle joint.