A Quasi-Passive Robotic Ankle Foot Orthosis With Speed-Adaptive Stiffness

Ankle foot orthoses (AFOs) are one of the most prescribed mobility devices for individuals with walking disability from conditions like cerebral palsy (CP) and stroke. Current AFO designs offer a fixed stiffness during walking, functioning optimally at only a single speed. The primary goal of this study was to develop and validate a quasi-passive robotic AFO that automatically adjusted stiffness to a user's walking speed. We designed a leaf spring AFO with an adjustable pivot point actuated by a compact linear servo motor. We developed a walking speed estimator using onboard sensors to automatically adjust the pivot point location, and therefore device stiffness. First, we characterized stiffness range, adjustment response time, and battery life during walking. Next, we performed clinical device validation testing in five individuals with CP during stand-to-run acceleration bouts and at constant walking speeds. The AFO exhibited a stiffness range of 60 to 250 Nm/rad during normal walking and up to 300-400 Nm/rad under certain conditions (e.g., running) for the CP participants. The device was able to adjust stiffness by similar to 200 Nm/rad during swing phase in similar to 0.25 seconds. Battery life approached 6000 steps. The on-board controller accurately predicted relative changes in walking speed for the five participants with CP (R-2 = 0.92 +/- 0.04), demonstrating the ability to automatically increase device stiffness with ambulation speed. This study advances the state-of-the-art for quasi-passive AFOs that can function optimally at different ambulation speeds.