Impedance control of series elastic actuators: Passivity and acceleration-based control

Series elastic joints allow force and impedance controllers to be implemented on high torque and high power density motors. Several impedance controllers have been proposed whose stability is usually analyzed by means of passivity-based tools such as the $Z$-width characterization. This paper proposes an overview of existing impedance control solutions for series elastic joints and derives the passivity characterizations that are still missing in the literature, thus providing a complete and coherent overview of the existing solutions. Within this overview, we highlight the advantages of impedance control based on positive acceleration feedback showing improved stability robustness and impedance accuracy with respect to existing solutions. These advantages are theoretically motivated (considering ideal conditions) and experimentally validated.