Online Adaptive Impedance Control with Gravity Compensation for an Interactive Lower-Limb Exoskeleton

While lower-limb exoskeletons have been increasingly used for gait assistance and rehabilitation, most of them continue to function as assistive devices in the exoskeleton-user relationship as a leader and follower. This limits the user’s ability to interactively contribute to gait control. Therefore, this study proposes an interactive user-exoskeleton control strategy to translate the exoskeletons into interactive compliant companion devices with the exoskeleton-user relationship as the collaborator. This strategy is implemented through online adaptive impedance control with gravity compensation (OAIC-GC). It relies solely on internal pose feedback (joint position) rather than external sensors such as electromyography, torque, or force, as utilized in other assist-as-needed (AAN) control methods. The OAIC-GC can automatically capture the mechanical impedance dynamics of the user’s lower limbs during walking and thus facilitate adaptive, versatile, and personalized gait assistance. It is evaluated using a real lower-limb exoskeleton system with six degrees of freedom (DOFs) across different users engaging in various activities. These activities include symmetrical and asymmetrical walking on a split-belt treadmill at different speeds, as well as walking up stairs. The results indicate a significant improvement in the exoskeleton’s performance in terms of adaptability and movement smoothness under all activities when compared to traditional control. The proposed control reduces joint assistance torque across all exoskeleton joints, enhancing user interaction and comfort. This enables users to actively control their gait patterns, enabling the exoskeleton to operate in an interactive assist-as-needed (IAAN) mode.