  This paper presents and experimentally demonstrates a novel framework for
variable assistance on lower body exoskeletons, based upon safety-critical
control methods. Existing work has shown that providing some freedom of
movement around a nominal gait, instead of rigidly following it, accelerates
the spinal learning process of people with a walking impediment when using a
lower body exoskeleton. With this as motivation, we present a method to
accurately control how much a subject is allowed to deviate from a given gait
while ensuring robustness to patient perturbation. This method leverages
control barrier functions to force certain joints to remain inside predefined
trajectory tubes in a minimally invasive way. The effectiveness of the method
is demonstrated experimentally with able-bodied subjects and the Atalante lower
body exoskeleton.
