Experimental Evaluation of Knee Exoskeleton Interface Mechanisms

Abstract

Mobility loss or impairment, whether due to injury, disease, or aging, affects quality of life, leads to poorer healthcare outcomes, diminished life satisfaction, and increased healthcare costs. Exoskeletons, mechanical devices worn to assist or augment the wearer’s capabilities, are a method to mitigate these negative outcomes. They can reduce strain and injuries associated with physical activities or restore mobility to those affected by impairments. A major challenge for exoskeletons is ensuring user comfort. Literature indicates that comfort is crucial for their adoption. Discomfort often arises from a mismatch between the user’s natural motion and the mechanical motion of the exoskeleton, causing excessive compression or shear on the user’s limbs. One proposed solution is to add passive degrees of freedom between the user and the exoskeleton to better match their movements. While this has shown promise in simulations, experimental verification is lacking. This research experimentally verified if adding passive degrees of freedom to knee exoskeletons improves user comfort and to determine their optimal stiffness and orientation. To test this, modular connection mechanisms were developed to allow motion between the exoskeleton frame and cuffs in one degree of freedom while restricting others. These mechanisms could be configured to add some stiffness to the active degree of freedom. The connection mechanisms were tested on both a leg phantom and on a human participant. During testing, interface pressures between the limb and the cuff, as well as limb and exoskeleton positions, were measured to compare different configurations. The results support adding passive translation along the axis of the shank as well as in the transverse axis of the shank. However, the leg phantom had limited predictive ability, highlighting the need for human studies to be conducted where possible. Further research is necessary to better characterize the effect of passive degrees of freedom through human trials.