Optimal PI tuning rules for flow loop, based on Modified Relay Feedback Test

Normally PID controller tuning rules are derived using linear models of the plant or process, with nonlinearities neglected. This factor often results in the deterioration of loop performance which is tuned using these rules. In the present paper, optimal tuning rules are derived based on a more accurate nonlinear model of the flow process, in which the nonlinearity of the pneumatic actuator (most widely used in the process industries) is considered. This nonlinearity is dynamic and exists even if the valve static characteristic is linear. The proposed tuning rules are coupled with the Modified Relay Feedback Test (MRFT) that was recently proposed in the literature. It is shown that the use of the presented tuning rules along with MRFT provides an advantageous result for control performance of flow loops. Simulations are provided.