Two-step IMC-PID method for multiloop control system design

Proportional-integral-derivative (PID) controller tuning method based on the internal model control is one of the simplest tuning rules and provides excellent performances for various processes. It has one design parameter which determines the speed of closed-loop response. As the design parameter changes, the controller gain changes while integral and derivative times usually remain constant. Hence, the method can be extended easily to the design of multiloop control systems because there are simple methods to find stable multiloop proportional controller gains. However, integral and derivative times independent of the design parameter can cause poor closed-loop responses for some processes such as those with small dead times. For such processes, 2 degree of freedom control systems are usually required, and a two-step method Where a proportional controller is designed first and then a PID controller is designed for the compensated system can be used to design the 2 degree of freedom control systems. This two-step method is applied to the design of multiloop control systems for interacting multivariable processes.