Performance enhancement of a class of nonlinear systems by disturbance observers

A large class of single-input single-output (SISO) nonlinear systems is considered. The nonlinearity in a system of this class has the property that its output can be decomposed as the summation of the outputs of a stable SISO linear time-invariant system and a bounded function of time. For the class of systems under consideration, disturbance observers are designed to estimate the effects of nonlinearities and to cancel them subsequently. The designed disturbance observers are thus able to make the nonlinear systems behave linearly and to enhance their performance. Examples are given to show disturbance observers can suppress the effect of dead-zone nonlinearity and limit-cycles caused by backlash