Just Noticeable Difference of Impedance Parameters while Walking in an Ankle Exoskeleton

Exoskeletons have been used to augment or enhance human performance in a variety of settings. For exoskeletons to be used most effectively, researchers need to know how changing the control parameters affects human perception. This has been investigated for impedance parameters while in static positions, but has not yet been investigated while walking. This study investigated the ability of users to detect small changes in stimuli in exoskeleton impedance control parameters during locomotion. The just noticeable difference (JND) was calculated using a psychometric function by having subjects report when they were able to detect a stimulus change across a range of stiffness and ankle angle control rate magnitudes. The JND was approximately 42% for ankle stiffness and 49% for ankle angle control rate. This is over 3 times larger than the JND for impedance parameters in a static position. It is also possible that the JND is higher than reported. It is unclear if the reference stimulus affected the JND. Any learning that may have occurred over the course of the experiment did not seem to affect the ability to fit the JND. The increased JND may be caused by a variety of factors including increased cognitive load due to dual tasking and tactile gating. This suggests that these exoskeleton control parameters could be varied significantly without users being able to determine a difference, potentially allowing for additional parameters, such as metabolic cost, to be included in an optimization procedure.