A Unified Passivity-based Control Framework for Position, Torque and Impedance Control of Flexible Joint Robots

TLDR

Flexible joint robot control has seen advances in torque, position, and impedance strategies, with recent work highlighting the relationships among these approaches and emphasizing the role of potential‑energy shaping for impedance control. This paper proposes a unified passivity‑based framework for controlling flexible joint robots. The framework integrates an inner torque‑feedback loop, motor‑inertia shaping, and a potential‑energy function derived solely from motor angles to achieve torque, position, and impedance control. Experimental tests on DLR lightweight robots confirm that the framework delivers robust torque, position, and impedance performance, including gravity compensation, desired Cartesian stiffness, and resilience to model uncertainties, as shown by position controller results and impact experiments.

Abstract

This paper describes a general passivity-based framework for the control of flexible joint robots. Recent results on torque, position, as well as impedance control of flexible joint robots are summarized, and the relations between the individual contributions are highlighted. It is shown that an inner torque feedback loop can be incorporated into a passivity-based analysis by interpreting torque feedback in terms of shaping of the motor inertia. This result, which implicitly was already included in earlier work on torque and position control, can also be used for the design of impedance controllers. For impedance control, furthermore, potential energy shaping is of special interest. It is shown how, based only on the motor angles, a potential function can be designed which simultaneously incorporates gravity compensation and a desired Cartesian stiffness relation for the link angles. All the presented controllers were experimentally evaluated on DLR lightweight robots and their performance and robustness shown with respect to uncertain model parameters. Experimental results with position controllers as well as an impact experiment are presented briefly, and an overview of several applications is given in which the controllers have been applied.