Abstract
This paper presents the PID controller design for coupled-tank process using characteristic ratio assignment (CRA). The coupled-tank process is a challenge plant for testing the performance of PID controller because it is a high uncertainty nonlinear model. For this paper, the characteristic ratio assignment (CRA) which is satisfied specification of performance of control system. Furthermore, it is very convenient as a fast adjustment damping ratio as well as a high speed response. Finally, the simulation results can be illustrated the validity of our approach by MATLAB.