This paper presents the design and development of a novel mechanically overdamped actuator with adjustable stiffness (MOD-AwAS). The novelty of MOD-AwAS compared to other variable stiffness actuators relates to its mechanical design, which prevents oscillations at the output link. Almost all variable stiffness actuators have an overshooting problem that require a sophisticated control algorithm to be able to perform accurate positioning. MOD-AwAS can regulate the stiffness from zero to its maximum (theoretically infinite) in less than 0.2 s by changing the position of the pivot point of its lever mechanisms. MOD-AwAS employs only one rotational spring with no pre-deflection, which gives it full accessibility to its energy storage capacity. Experimental results are presented to show the ability of MOD-AwAS to control its position accurately with a wide range of stiffness adjustment.

This document explores the modeling and performance of the power train for a lower extremity exoskeleton during stair ascent and descent gait cycles. While the exoskeleton was optimized for walking gait cycles, the power train has not been evaluated under stair gaits. The power train for this exoskeleton is a novel design concept that locates torque motors at the back of the exoskeleton. After taking joint position data during stair gait ascent and descent cycles, joint velocities and accelerations of an unencumbered subject were found. Joint torque and power requirements of the unencumbered subject were scaled to include the effects of the extra mass and the position of the mass. After building a mathematical model to represent the power train, the output power of the motor for each joint was compared to the required joint power to actuate each joint across stair ascent and descent gait cycles. Overall it was found that the selected power train could provide assistance positive assistance to the operator under these loading conditions.