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.
Highlights
When we use the control objectives in [10], [14], [16]–[19], [22], [28], [49] and a quadratic program (QP)-enhanced input-output linearizing controller from [10], [19], we find that a 0.02 m increase in chair height results in foot contact violations of over 100 N
We propose to design the Jacobian matrix Jh arising from the virtual constraints so that Jh is orthogonal to Jand the rows of Jh Bare linearly independent
CRITERIA FOR SUCCESS AND SUMMARY OF THE RESULTS The criteria that we use to determine the success of the standing motions under perturbations are: (1) tracking and steady state error of the SU and support polygon (SP) controllers respectively, (2) torso pitch acceleration, (3) user force expected by the controller, (4) Zero Moment Point (ZMP) constraint violation, (5) friction constraint violations, (6) joint angle limit violation, and (7) motor limit violation
Summary
Lower-limb exoskeletons are assisting patients with mobility impairments, such as the elderly or people with paraplegia. C. CONTRIBUTIONS The objective of the present work is to design user-assisted feedback-stabilized dynamic sit-to-stand trajectories for the exoskeleton, Atalante, shown, using its full dynamic model. CONTRIBUTIONS The objective of the present work is to design user-assisted feedback-stabilized dynamic sit-to-stand trajectories for the exoskeleton, Atalante, shown, using its full dynamic model This is a challenging design problem due to the complexity of the dynamical system and considerations such as user comfort and safety-critical constraints. Since the resulting closed-loop system is underdetermined (aka, over actuated) and must satisfy real-time constraints on joint limits, torque bounds, and ground reaction forces, we combine quadratic programming with input-output linearization (QP I/O) to (robustly) achieve the sit-to-stand motion and to safely come to a stop. After using the exoskeleton several times, the user will learn to provide the nominal force predicted by optimization
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