New tuning formulas for a nonlinear PID control scheme

Many processes operated in chemical process industries show time-varying and highly nonlinear characteristics. This paper proposes an enhanced nonlinear PID (NPID) controller for the improvement of setpoint tracking or disturbance rejection responses and new tuning formulas for a FOPTD process model. The NPID controller has a structure with a first-order filter in the derivative term to avoid possible Derivative Kick. The parameters of the NPID controller are expressed in terms of the ratio L/& tau; of the time delay L to the time constant & tau; in the process by using the dimensionless approach. Repeated optimizations are performed for each value over the ranges of 0.01 to 1 and 1 to 3 of L/& tau; and over the ranges of 5 to 30 of the filter parameter N to obtain the average of optimal parameter values that minimize the integral of absolute error performance criterion. By using the least-squares method with together the calculated optimal values and the rule formulas, the tuning rules are obtained. A set of simulation works on the five processes are carried out to demonstrate tracking and disturbance performance and robustness against the noise of this approach.