Yufeng Zhang , Student Member, IEEE, Roger J. Kleinmann, Karen J. Nolan, and Damiano Zanotto , Member, IEEE

Abstract: Motor weakness at the ankle joint is a major contributor to locomotor deficits in stroke, as the ankle provides the largest propulsive moment during walking, compared to the knee and the hip joints. In this paper, a novel lightweight powered ankle–foot orthosis (AFO), named Stevens ankle–foot electromechanical (SAFE) orthosis, is presented and evaluated. The prominent feature of the SAFE orthosis relies on its two alternative actuation modes (AMs): 1) fully active and 2) semiactive. The former mode utilizes two BLDC motors to provide ankle plantar- and dorsi-flexion torques, whereas the latter replaces the motor on the dorsiflexion side with a set of linear springs. This design allows direct investigation of how the introduction of a passive antagonistic element may affect the performance of a powered AFOs. Experimental results from both bench-top tests and human walking tests indicate that the performance of the two AMs are comparable in terms of device transparency and torque tracking. Implications of these results for the design of future powered AFOs are discussed in this paper.