The fixed-parameter PID controller is ubiquitous in the process industries. The ideal PID controller has the form (see Chapter 5)
where it is understood that any bias associated with steady-state operation is absorbed into the integral term. Other forms for PID control are presented in Chapter 5.
Often it is useful to have a nonlinear controller even if the process is linear. A common example is level control of a surge drum where the primary objective is to maintain a reasonably consistent outlet flow under disturbances in the inlet flow rate, subject to high and low drum level constraints. In this situation, ...