A Robotic Ankle-Foot Orthosis for Daily-Life Assistance and Rehabilitation

The ankle plays an important role in human movement as it supplies the majority of energy to support an individual’s walking. In this paper, the authors present a robotic ankle-foot orthosis (RAFO), which is essentially a wearable robot that acts in parallel to the user’s biological ankle for motion assistance. Unlike most of the existing robotic ankle-foot ortheses, the RAFO in this paper is a compact and portable assistive device with full energy autonomy, which enables its use in a user’s daily life without the typical limitation associated with tethered operation. The primary performance goal in the design of the RAFO is to provide a torque capacity equivalent to 35% of a 75 kg healthy person’s maximum ankle torque in slow walking, while keeping the weight of the device less than 2 kg. To reach such goal, the orthotic joint is actuated with a compact flat motor coupled with a two-stage transmission that provides a total 200:1 gear ratio. Additionally, a novel two-degree-of-freedom (2-DOF) joint design is incorporated. In addition to the powered dorsiflexion – plantarflexion, the 2-DOF joint also allows passive inversion – eversion of the joint, which greatly improves the comfort in the prolonged wearing of the device. For the control of the powered joint, a finite-state, friction-compensated impedance controller is developed to provide natural interaction with the user and reliable triggering of the powered push-off in walking. A prototype of the RAFO has been fabricated and assembled, and preliminary results demonstrated its effectiveness in assisting the user’s locomotion in treadmill walking experiments.