Modeling and Loop Shaping of Single-Joint Amplification Exoskeleton with Contact Sensing and Series Elastic Actuation

TL;DR

This paper presents a robust control strategy for a single-joint exoskeleton that amplifies human strength while maintaining stability across various human and load conditions, using contact sensing and series elastic actuation.

Contribution

It introduces a linear feedback compensator design for exoskeletons that accounts for human impedance variations and load changes, enhancing amplification and stability.

Findings

Controller effectively amplifies human interaction forces within bandwidth.

System stability depends on human impedance and load conditions.

Robust control achieves desired amplification with acceptable stability margins.

Abstract

In this paper we consider a class of exoskeletons designed to amplify the strength of humans through feedback of sensed human-robot interactions and actuator forces. We define an amplification error signal based on a reference amplification rate, and design a linear feedback compensator to attenuate this error. Since the human operator is an integral part of the system, we design the compensator to be robust to both a realistic variation in human impedance and a large variation in load impedance. We demonstrate our strategy on a one-degree of freedom amplification exoskeleton connected to a human arm, following a three dimensional matrix of experimentation: slow or fast human motion; light or extreme exoskeleton load; and soft or clenched human arm impedances. We demonstrate that a slightly aggressive controller results in a borderline stable system---but only for soft human musculoeskeletal behavior and a heavy load. This class of exoskeleton systems is interesting because it can both amplify a human's interaction forces --- so long as the human contacts the environment through the exoskeleton --- and attenuate the operator's perception of the exoskeleton's reflected dynamics at frequencies within the bandwidth of the control.