Iranian Journal of Chemical Engineering (IJChE)

Abstract Abstract – pH control is crucial in biological and water treatment processes, yet it poses significant challenges due to its nonlinear characteristics. Conventional Proportional-Integral-Derivative (PID) controllers, while operator-friendly, often struggle to maintain satisfactory performance in highly nonlinear systems, especially when subjected to disturbances, measurement delay, and measurement noise. On the other hand, gain scheduling is a technique to cope with the process nonlinearity while maintaining the PID simplicity for the operators. In this paper, a gain-scheduled digital proportional-integral (PI) controller for pH control is presented, where the controller parameters are adapted using fuzzy logic. The control error and its numerical derivative are fed to the fuzzy inference unit with 7 membership functions, fuzzifying the severity of the control deviation. Then, the PI proportional and integral gains are updated in the defuzzification step. A simulation of a benchmark pH process with measurement delay was carried out under various scenarios. The results show that the proposed fuzzy-PI controller significantly outperforms the conventional PI controller. This is especially true in cases of feed disturbance and considerable setpoint changes, where the nonlinear process deviates significantly from the nominal point. In a combined disturbance/setpoint change scenario, the fuzzy-PI controller reduces the integral of absolute error by 54%.