User preference of applied torque characteristics for bilateral powered ankle exoskeletons

Abstract

For robotic ankle exoskeletons, the choice of controller and the many associated controller parameters (e.g. magnitude or timing of assistance) directly affect the user's performance, comfort, and biomechanics. Some studies have proposed tuning such controllers for individual users using optimization techniques to minimize (or maximize) a physiological objective, such as metabolic energy expenditure. One drawback to this approach is that it necessitates having one predefined, measurable objective function that is relevant in all situations. In reality, people may prioritize many different less-quantifiable metrics, such as stability, comfort, pain, or perceived effort, in any given situation. In this paper, we present a method of exoskeleton controller customization that is based on subject preference, which likely encodes many of these subjective cost functions at once. In this pilot study, two subjects self-tuned their own controller parameters in two dimensions, by directly manipulating the magnitude and timing of bilateral ankle exoskeleton assistance using a touch screen tablet interface. Subjects exhibited high repeatability in identifying their preferred exoskeleton parameters at each of three walking speeds (coefficient of variation between 0.914.5%). Subjects' preferences differed from each other, which highlights the importance of individual customization. The results from this pilot study will inform future experiments that incorporate rigorous measurement of subject preference into the control of robotic ankle exoskeletons. This research has the potential to provide more complete insights into important elements of exoskeleton control that will be useful outside of the steady-state, laboratory environment.