Design and Development of an Assistive Ankle Joint for a Portable Orthotic Device

Abstract: An ankle-foot orthosis (AFO) is commonly used to help subjects with weakness of ankle dorsiflexor muscles due to peripheral or central nervous system disorders. Patients having ankle joint disability often suffers from foot drop. This work presents an active ankle-foot orthosis (AAFO) that was designed to allow two degree-of-freedom motion while serving to maintain proper foot position for patients with lower limb disorder. . In this study, an active ankle-foot orthosis (AAFO) is developed which can control the dorsi/ plantarflexion of the ankle joint to prevent foot drop and toe drag during walking. To prevent slapping foot after heel strike, ankle joint has to be controlled actively to minimize forefoot collision with the ground. In the late stance, ankle joint also has to be controlled to provide the toe clearance and help the push-off. The goal of the present work is to design an exoskeleton structure using the available human modelling data.. The trajectory of the ankle joint is planned using two via points. The foot sole will be provided with a flexible layer at the bottom to help reduce the torque requirements of the actuator, since it is observed that foot returns some of the energy given to it by the actuator itself.

Keywords: Assistive devices, AAFO, Ankle joint assistance, Deformable sole, Orthotic devices

I. INTRODUCTION

Orthotic devices are intended to support the ankle, correct deformities, and prevent further occurrences[1]. A key goal of orthotic treatment is to assist the patient in achieving a measure of normal function. Ferris et. al. [15] proposed an ankle-foot orthosis powered by artificial muscles. The orthosis has two pneumatic muscles to control the dorsiflexion and plantarflexion motion of the ankle. Yamamoto et. al.[16] developed a dorsiflexion assist, controlled by a spring. Dorsiflexion correction is achieved via the compression force of a spring within the assist device. Blaya proposed an active ankle-foot orthosis with one degree-of-freedom. The active ankle foot orthosis comprises a force-controllable series elastic actuator (SEA) capable of controlling orthotic joint stiffness and damping for plantar and dorsiflexion ankle motions. There are a number of commercial ankle-foot orthoses manufactured. All these orthoses are single axis or are elastically deformable. The limitation in normal inversioneversion adds to the discomfort and does not provide a natural motion to the ankle [17].
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