Determination of Bandwidth of Q-filter in Disturbance Observers to Guarantee Transient and Steady State Performance under Measurement Noise

TL;DR

This paper analyzes how to select the bandwidth of the Q-filter in disturbance observers to ensure both transient and steady-state performance under measurement noise, using Lyapunov methods and singular perturbation theory.

Contribution

It provides a systematic approach to determine the Q-filter time constant that guarantees performance in noisy environments, addressing limitations of previous methods.

Findings

Derived an open interval for the Q-filter time constant under measurement noise.

Showed that reducing the time constant improves performance only in noise-free cases.

Provided theoretical conditions for balancing robustness and performance in disturbance observers.

Abstract

Q-filter-based disturbance observer (DOB) is one of the most widely used robust controller due to its design simplicity. Such simplicity arises from that reducing the time constant of low pass filters, not only ensures robust stability but also enhances nominal performance recovery -- ability to recover the trajectory of nominal closed-loop system. However, in contrast to noise-free environment, excessively small time constant can rather damage the nominal performance recovery under measurement noise. That is, minimizing time constant is no longer immediately guaranteeing nominal performance recovery. Motivated by this observation, this paper concentrates on determination of time constant to ensure transient and steady state performance. This analysis uses Lyapunov method based on the coordinate change inspired by the singular perturbation theory. As a result, we present an affordable noise level and open interval for the time constant that guarantees both the required performances. The analysis can also lead to theoretical demonstration on that excessively reducing time constant is assured to achieve target performance only for noise-free case.