In this thesis , we present digital control system by using digital controller especially tri-branch R-S-T. The subject matter handled relates to the design of digital PID controller method to control many kind of plant (such as plant with time delay , disturbance ). The following design method are then presented.1. Digital PID controller2. Poles placement 3. Tracking and Regulation with Independent objective4. Tracking and Regulation with weighted inputThese methods permit the control of system of any order with or without time delay