Optimal Joint Configuration for Ankle Exoskeleton: Force-Based Evaluation of Three Different Designs

The development of wearable robots is inherently challenging due to their interaction with the complex structure of the human body. Full kinematic compatibility (FKC) between the exoskeleton and the user is rarely achieved, resulting in misalignments between robot joints and anatomical ones, hindering the movement of the user and compromising the effectiveness of the device. This study presents three distinct ankle exoskeleton joint designs and evaluates their effectiveness through physical human-robot interaction (pHRI) by measuring force exchanges at the interfaces. Each joint design differs in terms of kinematic configuration and Degrees of Freedom (DoF). The upper and lower braces of the exoskeleton have been equipped with three force-sensitive resistors (FSRs) each to monitor the pressure in the front, lateral, and rear regions of the braces. The three different joints were tested during a walking trial performed on a treadmill at 3.5 km/h. Ten healthy subjects participated in the experiment. The analysis of the experimental results, using a repeated measure ANOVA, revealed significant differences in the distribution of forces. The study's findings underscore the critical role of joint design in enhancing pHRI, indicating that the slider joint, among those tested in this study, provides an optimal balance between mechanical functionality and user comfort.