Control of unstable processes with two RHP poles, a zero and time delay

A lead or lag compensator in series with a proportional-integral-derivative (PID) controller is designed for unstable processes with two unstable poles and a zero with time delay. Such transfer functions arise in modeling an isothermal continuous stirred tank reactor (CSTR) carrying out autocatalytic reactions, in crystallizers and in nonisothermal CSTRs. The direct synthesis method is used for the design of the controller. The desired closed-loop transfer function is specified to get the required PID controller with lead/lag compensator. Simulation studies are given to show the effectiveness of the proposed method. For systems with and without negative zero, significant improvement is obtained when compared to the method proposed by Lee et al., (Chem. Eng. Sci. 2000; 55: 3481-3493). (C) 2006 Curtin University of Technology and John Wiley & Sons, Ltd.