Robot Impedance Control and Passivity Analysis with Inner Torque and Velocity Feedback Loops

TL;DR

This paper analyzes how inner control loops and filtering affect the stability and passivity of impedance control in robotics, and proposes a design methodology using torque and velocity feedback loops to optimize performance.

Contribution

It provides an extensive analysis of stability regions considering inner loop bandwidth, filtering, and sampling, and introduces a new impedance controller design method with positive velocity feedback.

Findings

Inner loop bandwidth significantly influences impedance stability.

Filtering and sampling affect passivity and stability regions.

Proposed velocity feedback enhances torque loop bandwidth without complex controllers.

Abstract

Impedance control is a well-established technique to control interaction forces in robotics. However, real implementations of impedance control with an inner loop may suffer from several limitations. Although common practice in designing nested control systems is to maximize the bandwidth of the inner loop to improve tracking performance, it may not be the most suitable approach when a certain range of impedance parameters has to be rendered. In particular, it turns out that the viable range of stable stiffness and damping values can be strongly affected by the bandwidth of the inner control loops (e.g. a torque loop) as well as by the filtering and sampling frequency. This paper provides an extensive analysis on how these aspects influence the stability region of impedance parameters as well as the passivity of the system. This will be supported by both simulations and experimental data. Moreover, a methodology for designing joint impedance controllers based on an inner torque loop and a positive velocity feedback loop will be presented. The goal of the velocity feedback is to increase (given the constraints to preserve stability) the bandwidth of the torque loop without the need of a complex controller.