Effects of Knee Joint Misalignments on Human-Exoskeleton Interaction Dynamics

One aspect that characterizes exoskeletons is their close physical contact with the user. Physical human-exoskeleton interaction (pHEI) is strongly affected by kinematic incompatibilities generated in the human-exoskeleton kinematic chain, often resulting in undesired joint misalignments and interaction forces. However, a systematic analysis to highlight this relation is difficult to execute. In this work we use a methodology for the study of pHEI based on an active dummy leg named Leg Replica. Thanks to this system, our aim is to highlight the relations between key pHEI metrics such as joint misalignment, interaction forces and relative displacements, underlining the role and effects of joint misalignment in controlled conditions. We first validated a kinematic model available in the literature relating human-exoskeleton knee misalignment with the resulting device's motion relative to the limb. The model was further used to investigate which is the relation between the kinematic of the leg-device system and the forces produced between the two bodies. We discovered that the model predictions were consistent with the experimental kinematic observations and strongly related with the peaks reached by shear forces during the motion. Shear forces were found to play a key role in the overall interaction and related with the lost of energy during the motion. These results underscore the strong influence that misalignments can exert on the generation of force and motion, even under simplified and controlled conditions.