Alternative Approximation Method for Time Delays in an IMC Scheme

Abstract

Internal Model Control (IMC) algorithms have proven to be quite efficient in designing PID type controllers for time delay systems. However, to compute the equivalent PID controller in a feedback loop, using the IMC tuning approach, implies the approximation of the process time delay. Two of the most widely used approximation methods (first order or series and Pade) are used in this paper and compared to a new approximation method, the Non Rational Transfer Function (NRTF) approximation. In the current paper, the NRTF approximation is presented as an alternative method to the series/Pade approximation techniques for time delay systems in an IMC closed loop design. It is shown that for first order and second order processes, given a certain ratio of the IMC filter time constant and process time delay, the NRTF method could produce better closed loop dynamics, compared to the series/ Pade approximations. Several numerical examples are presented, as well as a case study.