Recently the Time Delay Contrl(TDC) method has been proposed as a promising technique in the robust control area. TDC method, however, requires the measurement of all the state variables, together with their derivatives. This requirement imposes a severe limitation on the application to most real systems. in order to solve this measurement problem, we proposed an observer design method that can stably reconstrunct the state variables and their derivatives. The stability of the overall system has been analyzed for a class of linear time
invarient(LTI) systems. In addition, it has been estabilished that the closed-loop system error converges to zero for certain classes of uncertainties. then, the controller/observer based on our design method has been applied to a DC servo motor, the result of simulation confirmed that the controller/observer performs satisfactorily as predicted and is much less sensitive to sensor noise than the controller using
numerical differentiations. The results of experimentation shows that the ontroller/observer performs quite robustly under inertia variation and external disturbance.