An approximate inverse system for nonminimum-phase systems and its application to disturbance observer

This paper proposes an approximate inverse system for nonminimum-phase dynamical systems based on the least-square approximation method. The nonminimum-phase systems are approximated by minimum-phase systems. The proposed formulation is applied to the disturbance observation problems for multivariable nonminimum-phase systems with arbitrary relative degrees. The disturbances, which are assumed bounded, are the combination of the external disturbances, the nonlinearities and the model uncertainties of the system. The estimation error of the disturbances is controlled by the design parameters. Furthermore, the accuracy of the estimation also depends on the frequencies of the disturbances. Simulation results show the effectiveness of the proposed method.