Closed-Loop Kinematic and Indirect Force Control of a Cable-Driven Knee Exoskeleton: A Lyapunov-Based Switched Systems Approach

Lower-limb exoskeletons can aid restoring mobility in people with movement disorders. Cable-driven offload their actuators away from the human body to reduce weight imposed on user and enable precise control of joints. However, ensuring limb coordination through bidirectional motion joints using cables raise technical challenge preventing occurrence undesired cable slackness or counteracting forces between cables. Thus, motivation exists develop a design framework that integrates both joint loop ensure suitable tracking maintain tension properly. In this paper, two-layer structure consisting high low-level controllers are developed knee-joint exoskeleton system follows desired trajectories adjusts tension, respectively. A repetitive learning controller is designed for high-level knee objective motivated by periodic nature leg swings (i.e., achieve flexion extension). Low-level robust pair cables, each actuated an electric motor, track target motor composed kinematics offset angles mitigate slackness. The computed admittance models exploit measurements tensions as inputs. Each switches its role trajectory acts leader extension) implementing follower slackness). Hence, at any time, one other follower. Lyapunov-based stability analysis global asymptotic guarantee exponential tracking. implemented during swing experiments six able-bodied individuals while wearing cable-driven exoskeleton. comparison results obtained two trials without model exploiting measurements) presented. experimental indicate improved performance, smaller input magnitudes, reduced trial leveraged feedback compared did not feedback.