Controller parameter self-tuning when control loop mode switching for multi-loop PID control system of chemical process

Chemical process is generally a multivariable system, but its main control scheme is decentralized multi-loop PID conventional control. Since there are different degrees of coupling in the multivariable system, there are mutual influences between the control loops. When the other control loops switch between the manual/automatic modes, the equivalent controlled object of this loop will mutate so that the original control parameters of this loop will be inappropriate and the control performance will become worse even the closed-loop system is unstable. In order to avoid this situation, the stability of the control loop mode switching should be studied from the perspective of the whole system, so the multivariable frequency domain Nyquist array design method is adopted. Based on the Nyquist stability criterion under diagonal dominance, the stability changing of each control loop before and after mode switching is quantitatively analyzed from the Gershgorin circle boundary points, so as to determine the adjustment direction and size of the controller gain for each loop. The controller parameter self-tuning of each loop at the moment of control loop mode switching is realized to compensate the disturbance caused by the control loop mode switching and ensure the closed-loop stability of the whole system. The multi-loop PID control system of Shell heavy oil fractionator is used as an example, when the three PID control loops are put into use in turn, the control parameter self-tuning according to the boundary points of Gershgorin circle makes the closed-loop system still maintain certain control performance, otherwise the closed-loop system will be unstable. © 2022, Editorial Board of CIESC Journal. All right reserved.