Feedback Control Design for Robust Comfortable Sit-to-Stand Motions of 3D Lower-Limb Exoskeletons

Margaret Eva Mungai, Jessy W. Grizzle

Abstract: Lower-limb exoskeletons provide people who suffer from lower limb impairments with an opportunity to stand up and ambulate. Standing up is a crucial task for lower-limb exoskeletons as it allows the user to transfer to the exoskeleton from a wheelchair, with no assistance, and can be a precursor to walking. Achieving a safe sit-to-stand motion for the exoskeleton + user system can be challenging because of the need to balance user comfort while respecting hardware bounds and being robust to changes in the user characteristics and the user’s environment. We successfully achieve safe sit-to-stand motions by using constrained optimization to generate two types of dynamic sit-to-stand motions based on two hybrid system descriptions for the exoskeleton, Atalante. Due to the highly constrained nature of the equations of motions, we introduce a method to systematically design virtual constraints for highly constrained systems. We also design two quadratic program-based computed-torque controllers to achieve the sit-to-stand motion and to safely come to a stop in a standing position. We then analyze the closed-loop behaviors of the two sit-to-stand motions under the two controllers using physically motivated robustness tests. The criteria used to determine a successful sit-to-stand motion are: tracking error, the pitch acceleration of the torso, the amount of user force needed to perform the motion, and the adherence to the Zero Moment Point (ZMP), friction, and joint constraints.