Suksri, T. , Sritheeravirojana, U. , Numsomran, A. , Kongrattana, V. , Werataweemart, T.. "T-DOF PID Controller Design using Characteristic Ratio Assignment Method for Quadruple Tank Process". World Academy of Science, Engineering and Technology, Open Science Index 12, International Journal of Electrical and Information Engineering, (2007), 1(12), 4154 - 4160.