Implementation of variable joint stiffness through antagonistic actuation using rolamite springs

Abstract

The addition of variable joint stiffness to an upper limb prosthesis can restore an important function of the natural arm. Design goals for the stiffness and force performance of prosthetic elbow joints are presented. This paper develops configurations of antagonistic actuators, for use in prosthetic arms, with improved energy efficiency, controllability, interaction properties, and size. Implementation of the required quadratic stiffness elements is investigated using rolamites. The rolamite band geometry is designed to generate the required force function. Rolamite design equations are extended to include effects of high force generation and stress concentrations. The effect of using rolamites to generate the spring forces is examined with respect to restrictions placed on the performance of the joint. The performance of this approach to the implementation of a variable stiffness joint is found to be marginally useful in a prosthetic application. Further design improvements are suggested.