Recently the Time Delay Control(TDC) method has been proposed as a promising technique in the robust control area, where the plants have nonlinear dynamics with parameter variations and substantial disturbances are present. TDC method, however, requires the measurement of all the state variables,
together with their derivatives. This requirement imposes a severe limitation on the applications to most real systems. In order to solve this measurement proble, we proposed an observer design method that can stably reconstruct the state variables and their derivatives. the stability of the overall system has analyzed and proved. Then for a simulation study, the controller/observer based on our design method has been applied to a nonlinear plant, the result of which confirmed that the controller/observer performs
satisfactorily as predicted. Finally we made experimentations on a DC servo motor that is subject to substantial amount of inertia variations and external disturbances. the results showed that the controller/observer
performs quite robustly under thos variations and disturbances, and is much less sensitive to sensor noise than the controller using numerical differentiations.