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A team of researchers from KTH Royal Institute of Technology (KTH) and Karolinska Institutet, both located in Stockholm, Sweden, have developed a soft, robotic ankle exoskeleton consisting of actuation and transmission systems to assist 2 degrees of freedom simultaneously: dorsiflexion and eversion, then performed several proof-of-concept experiments on non-disabled persons.
Overview of the soft ankle exoskeleton designed to counteract drop foot and excessive inversion.
According to doctoral student Xiaochen Zhang at the research group KTH Moveability, Department of Engineering, who presented the work for his thesis, the design of the ankle exoskeleton can assist both drop foot and excessive inversion. “It also introduces a novel multi-objective Human-in-the-Loop optimization protocol aiming to maximize the different assistive effects simultaneously,” he said.
The actuation system consists of 2 motors worn on a waist belt. The transmission system provides assistive force to the medial and lateral sides of the forefoot via Bowden cables. The coupling design enables variable assistance of dorsiflexion and inversion at the same time, and a force-free controller is proposed to compensate for device resistance. The team first evaluated the performance of the exoskeleton in 3 seated movement tests: assisting dorsiflexion and eversion, controlling plantarflexion, and compensating for device resistance, then during walking tests. In all proof-of-concept experiments, drop foot tendency was simulated by fastening a weight to the shoe over the lateral forefoot.
In the first 2 seated tests, errors between the target and the achieved ankle joint angles in 2 planes were low; errors of <1.5° were achieved in assisting dorsiflexion and/or controlling plantarflexion and of <1.4° in assisting ankle eversion. The force-free controller in test 3 significantly compensated for the device resistance during ankle joint plantarflexion. In the gait tests, the exoskeleton was able to normalize ankle joint and foot segment kinematics, specifically foot inclination angle and ankle inversion angle at initial contact and ankle angle and clearance height during swing.
“Our exoskeleton is designed to assist people with drop foot caused by stroke or other neurological injuries,” said Zhang. “We are hopeful that it could significantly improve their safety while walking by reducing the risk of tripping and falling and then enhance their mobility and quality of life.”
