Impact of Imperfect Exoskeleton Algorithms on Step Characteristics, Task Performance, and Perception of Exoskeleton Performance

Abstract

Lower-limb exoskeletons may experience errors in operational settings, where an expected assistive torque is missing. These errors may affect user's gait strategies and perception of the exoskeleton's performance, leading to impacted human-exoskeleton fluency and user trust in the system. In this study, we introduced five different exoskeleton control algorithms with fixed error rates up to 10% error (90% accuracy). Two groups of participants (N=22, 11 per group) walked with a bilateral ankle exoskeleton while completing a targeted stepping task and experienced each controller twice, but in different orders. The impact of exoskeleton error rates was assessed on step characteristics (step length and width), task performance (absolute task error), and perception of exoskeleton performance (survey responses). Step character-istics were not impacted by exoskeleton errors, but multiple participants were not able to achieve acceptable task accuracy and increased task error over time across all error rates. Increasing error rates negatively impacted users' perception of algorithm predictability, exoskeleton supportiveness, and probability of future usage. Perceived predictability and future usage probability transitioned from positive to negative between 2% and 5% error. Understanding the effect of increasing exoskeleton error rates informs minimum algorithm accuracy to support human-exoskeleton fluency and performance for gait-assist exoskeletons.