Self-Regulating Pre-Tensioning Parameter Strategy for Control of Cable-Driven Knee Exoskeleton

Knee exoskeletons can offer additional strength and stability for individuals with mobility impairments by providing proper assistance. However, the precise timing of assistance torque for the knee joint poses a challenge to the control of cable-driven exoskeleton systems. Conventional torque-position switching control with a pre-tensioning strategy often fails to meet the precision requirements. In this paper, we developed a self-regulating pre-tensioning parameter strategy to enhance the control performance of cable-driven knee exoskeletons. A specific knee exoskeleton capable of providing both extension and flexion assistance was selected for study. Through geometric analysis of the human-exoskeleton interaction system, a more accurate kinematic relationship has been established, and a state variable named “tightness” has been introduced to describe the cable state. During exoskeleton-assisted walking, the pre-tensioning parameter is iteratively regulated based on tightness, aiming to bring the cable to a critically-tensioned state at the moment of heel strike. Preliminary experimental results demonstrate the strategy significantly improves control precision, the root-mean-square-error of torque tracking with this strategy are reduced by 88.7% and 77.4% for extension and flexion assistance, respectively, compared to scenarios without pre-tensioning. This enhanced control strategy shows promise for further advancing the capabilities of exoskeletons in facilitating human locomotion.