Adding robustness to nominal output-feedback controllers for uncertain nonlinear systems: A nonlinear version of disturbance observer

• Published in: Automatica (Oxford) Vol. 44; no. 10; pp. 2528 - 2537
• Main Authors: Back, Juhoon, Shim, Hyungbo
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.10.2008; Elsevier
• Subjects: Applied sciences; Computer science; control theory; systems; Control theory. Systems; Disturbance observer; Exact sciences and technology; Modelling and identification; Nonlinear output feedback; Performance recovery; Robust control; Singular perturbation; Robust control; Performance recovery; Disturbance observer; Nonlinear output feedback; Singular perturbation; Transient response; Disturbance rejection; Non linear control; Feedback regulation; Steady state; Transients; Closed feedback; Uncertain system; Observer; Global stabilization; Robustness; System identification; Output feedback
• ISSN: 0005-1098; 1873-2836
• DOI: 10.1016/j.automatica.2008.02.024

The linear disturbance observer (DOB) approach has been successfully employed as a tool for robust control and disturbance rejection in practice. In this paper, we present a nonlinear DOB which inherits all the benefits of the classical linear DOB approach. In particular, we prove that the proposed nonlinear DOB recovers not only the steady-state performance but also the transient performance of the nominal closed-loop system under plant uncertainties and input disturbances. One novel feature of the proposed controller is that it is an add-on type inner-loop output-feedback controller; thus any type of outer-loop controller can be combined to enhance the closed-loop system performance. Our result is a semi-global one and the stability proof is given when the nonlinear system has a well-defined relative degree with a stable zero dynamics.