Design and Validation of a Torque-Controllable Knee Exoskeleton for Sit-to-Stand Assistance

Individuals with poststroke hemiparesis often exhibit mobility deficits, particularly during tasks requiring high lower limb torques. The sit-to-stand transition is consistently marked by asymmetrical weight-bearing between the paretic and unaffected legs. One way to improve characteristics of stroke sit-to-stand may be to provide assistive knee extension torque with a powered exoskeleton. To perform research on the biomechanical effects of assisting sit-to-stand, a unilateral powered knee exoskeleton is required, which can accurately control torque. This paper introduces a novel series elastic actuator capable of producing the full torques and speeds required for sit-to-stand (80 Nm, 3 rad/s). It utilizes a unique transmission configuration with a series fiberglass beam spring that improves torque-control and reduces output impedance. The actuator is incorporated into a unilateral orthosis and a high-level sit-to-stand controller is implemented. A small validation study with three able-bodied subjects performing sit-to-stand is presented, demonstrating the ability to appropriately provide assistance.