A Robust Disturbance Observer for Multivariable Systems

This paper discusses the disturbance estimation problem for continuous-time multivariable dynamical systems with arbitrarily relative degrees. The disturbances, which are assumed bounded, refer to the combination of the external disturbances, the nonlinearities and the model uncertainties of the system. First, the disturbance observer is proposed for multivariable minimum phase systems. Then, the proposed formulation is extended to nonminimum phase systems, where the nonminimum phase systems are approximated by minimum phase systems based on the least square approximation technique. The estimation error of the disturbances is controlled by the design parameters. Further, the accuracy of the estimation also depends on the frequency of the disturbance signal for nonminimum phase systems. Key-Words: Disturbance observer, multivariable systems, relative degree, nonminimum phase dynamical systems, least square approximation.