Human Intention-Oriented Variable Admittance Control with Power Envelope Regulation in Physical Human-Robot Interaction

Abstract

Human intention adaptation is usually realized by changing impedance/admittance parameters according to human interaction in physical human-robot interaction (pHRI). However, determining which admittance parameters need to be updated and how to update depends on the context of pHRI and system stability. This paper aims to provide a systematic study for all three admittance parameters considering human intention adaptation and system passivity for ensuring an intuitive and safe pHRI. For intuitiveness, a general human intention framework based on the robot's roles is summarized as a premise for designing human intention-oriented updating rules for admittance parameters. For the changing trends of these parameters (i.e., increasing or decreasing), only those impacting system passivity are studied. A power envelope regulation (PER) concept is then proposed to impose constraints on variable admittance parameters inferred from human intention for maintaining safe interaction. Our initial results suggest that the passivity condition is not a sufficient condition for system stability in pHRI, whereas preserving the power envelope inferred from the passivity condition can maintain passivity as well as avoid mismatch of admittance parameters arising from unpredicted human interaction. As a result, drastic changes in admittance controller dynamics, which usually result in instability, are restrained. The effectiveness of our approaches not only to maintain intuitive interaction but also to preserve passivity is validated via numerical simulation and experiments.