A Perturbation-Robust Framework for Admittance Control of Robotic Systems With High-Stiffness Contacts and Heavy Payload

Applications involving serial manipulators, in both co-manipulation with humans and autonomous operation tasks, require the robot to render high admittance so as to minimize contact forces and maintain stable contacts with high-stiffness surfaces. This can be achieved through admittance control, however, inner loop dynamics limit the bandwidth within which the desired admittance can be rendered from the outer loop. Moreover, perturbations affect the admittance control performance whereas other system specific limitations such as "black box" PD position control in typical industrial manipulators hinder the implementation of more advanced control methods. To address these challenges, a perturbation-robust framework, designed for serial manipulators engaged in contact-rich tasks involving heavy payloads, is introduced in this paper. Within this framework, a generalized Perturbation-Robust Observer (PROB), which exploits the joint velocity measurements and inner loop velocity control model, and accommodates the varying stiffness of contacts through contact force measurements is introduced. Three PROBs including a novel Combined Dynamics Observer (CDYOB) are presented. The CDYOB can render wide-range admittance without bandwidth limitations from the inner loop. Theoretical analyses and experiments with an industrial robot validate the effectiveness of the proposed method.