Saturated Adaptive Control of Antagonistic Muscles on an Upper-Limb Hybrid Exoskeleton

Functional electrical stimulation (FES) is a viable rehabilitation method for individuals affected by neurological injuries. When FES is combined with a powered exoskeleton, a hybrid exoskeleton is created. To control hybrid exoskeletons, a user’s muscles must be stimulated while simultaneously activating the exoskeleton’s motors. In this paper, an adaptive position-based controller is developed to stimulate a user’s biceps and triceps muscle groups on an upper-limb hybrid exoskeleton. For user comfort and stability, the stimulation input is saturated and the remainder is redirected into the exoskeleton’s motor. The motor is actuated using a robust feedback controller along with the excess input from the user’s saturated muscle controller. A Lyapunov stability analysis is conducted to prove that the closed-loop position error system is uniformly ultimately bounded using the two controllers. This approach showcases a novel way to safely limit stimulation without sacrificing overall performance via a conditionally offloaded neural network on hybrid exoskeletons. Experiments were conducted on four participants without injuries for validation and to demonstrate the efficacy of the proposed approach.