Robot Impedance Control and Passivity Analysis with Inner Torque and Velocity Feedback Loops
This work addresses stability and performance limitations in impedance control for robotics, which is incremental but provides practical design insights for roboticists.
The paper analyzes how inner control loop bandwidth, filtering, and sampling frequency affect the stability region and passivity of impedance parameters in robotics, and proposes a method using inner torque and positive velocity feedback loops to enhance torque loop bandwidth while maintaining stability, supported by simulations and experimental data.
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.