Clutchable Series Elastic Actuator for a Gait-Assistive Active Orthosis for Subjects with SCI

A spinal cord injury (SCI) disrupts the motor and sensory functions of the nervous system, limitingmotion capabilities and reducing the quality of life of affected people. An active stance-control knee-ankle-foot orthosis was developed and tested to aid incomplete SCI subjects by increasing theirmobility and independence [1]. A further improvement of the orthosis is conducted by the incorporationof elastic actuation to utilize advantages of the compliant system regarding efficiency and human-robotinteraction, as well as the reproduction of the physiological compliance of the knee joint. The optimalstructure and parameters is determined via optimization using elastic actuator models whileconsidering the efficiencies of various components. This leads to the concept of a series elasticactuator with a locked actuator position during the first half of the gait cycle by an additionalmechanism. The series compliance is selected to mimic the physiological stiffness of the knee. Duringthe second half of the gait cycle, a desired motion of the knee is realized with an EC-motor, controlledby means of impedance control. The locking mechanism avoids operation of the motor in a period ofthe gait cycle, where its efficiency is low due to the respective torque-velocity characteristic. Inaddition, the selection of an optimal gear ratio for the second half of the gait cycle maximizesrecoverable energy. Simulations of this clutchable series elastic actuator (CSEA) yield a theoreticalgeneration of 1.52 J per gait cycle in contrast to a consumption of 6.3 J of the directly-actuatedsystem. Control strategy and actuation system are implemented in a test bench, modeling the foot andshank as a pendulum. The conducted experiments provide a proof-of-concept while revealing gearfriction as the main limitation of the system. Future work could improve the prototypic CSEA togenerate a light and aesthetic design for the implementation at the active knee-ankle-foot orthosis. References[1] García-Vallejo, D., Font-Llagunes, J.M., Schiehlen, W. (2016). Dynamical analysis and design of activeorthoses for spinal cord injured subjects by aesthetic and energetic optimization. Nonlinear Dynamics, 84(2):559—581. Short BiographyFlorian Stuhlenmiller studies Mechanical and Process Engineering at Darmstadt University ofTechnology. He works on elastic actuation for assistive robotics since 2012 and (co-) authored severalpapers in international conferences. This work is conducted as part of his final degree project incooperation with the Biomedical Engineering Research Centre at Universitat Politècnica de Catalunyain Barcelona.