Ergodic Shared Control: Closing the loop on pHRI based on information encoded in motion.

Kathleen Fitzsimons, Todd Murphey

Abstract:

Advances in exoskeletons and robot arms have given us increasing opportunities for providing physical support and meaningful feedback
in training and rehabilitation settings. However, the chosen control strategies must support motor learning and provide mathematical task
definitions that are actionable for the actuation. Typical robot control architectures rely on measuring error from a reference trajectory. In
physical human-robot interaction, this leads to low engagement, invariant practice, and few errors, which are not conducive to motor
learning. A reliance on reference trajectories means that the task definition is both over-specified—requiring specific timings not critical
to task success—and lacking information about normal variability. In this article, we examine a way to define tasks and close the loop
using an ergodic measure that quantifies how much information about a task is encoded in the human-robot motion. This measure can
capture the natural variability that exists in typical human motion—enabling therapy based on scientific principles of motor learning. We
implement an ergodic hybrid shared controller(HSC) on a robotic arm as well as an error-based controller—virtual fixtures—in a timed
drawing task. In a study of 24 participants, we compare ergodic HSC with virtual fixtures and find that ergodic HSC leads to improved
training outcomes.