Shape Memory Alloy Actuated Ankle Foot Orthosis for Reduction of Locomotion Force

Abstract

Humans can be considered inefficient at walking because they are unable to achieve the theoretically ideal "zero energy cost" of steady-state locomotion that is possible for bipeds who have elastic tissues. This inefficiency is mainly due to part of the energy that is generated by the body to complete a single step being dissipated instead of being stored for use in the proceeding step. This suggests that we can improve locomotion efficiency and reduce the metabolic energy cost of walking by manipulating the elasticity of the lower limbs using exoskeletal devices [1]. However, most traditional designs use springs made from regular material that have a constant stiffness. These devices exert a linear force pattern that is not biocompatible because they do not mimic the forces of the muscles or the tendons of the human body. This paper presents an interdisciplinary study of the design of a passive-dynamic ankle foot orthosis mechanism that reduces the biological muscle force requirements during locomotion, thus reducing the metabolic energy cost of walking while maintaining biocompatibility. Shape memory alloy is used as a smart material for an actuator owing to its super-elasticity. This super-elasticity provides a nonlinear stiffness pattern that generates forces comparable to those of healthy muscles.

A Compliant Actuator-Sensor Unit using a Shape Memory Alloy and Capacitive Force Sensing for Active Exoskeleton Interfaces

“…While exoskeleton research often seeks to replace conventional actuation for motion support of the upper [11]- [13] and lower limbs [14], [15], there is limited literature addressing interface and contact to the body challenges. The use of SMAs has been suggested for compact haptic devices and active compression devices for the lower limb [16].…”