Towards a series elastic actuator with electrically modulated stiffness for Powered Ankle-Foot Orthoses

Series elastic actuators offer several benefits for powered ankle foot orthoses. One major benefit they offer for this application is the reduction of motor power requirements, which enables the reduction of motor weight. However, series elastic actuators commonly have a fixed stiffness value, which only yields optimal power reduction for one set of gait parameters such as gait type, user weight, and gait speed. These parameters vary during the normal use of orthotic devices. This paper presents a new variable stiffness series elastic actuator that can compensate for these variations. Our actuator uses a dielectric elastomer as the series elastic element so that the stiffness of the actuator can be electrically modulated, unlike current variable stiffness actuators that modulate their stiffness with a second motor. Experimental results indicate the viability of this approach for modulating stiffness and verify that the actuator generates forces meaningful for gait assistance.