Performance Indices and Tuning in Process Control

Industrial practice in process control shows a very wide gap with respect to the theory, as usually taught at the university. Standard controller tuning is often a compromise between performance and robustness. In this paper we examine some common PI(D) controller tuning rules and the achievable performances for a PI(D) controlled plant modeled as a first order plus time delay. The performance indices considered are IAE and ISE in a particular non-dimensional form including also controller output variations. The output performance indices are weakly dependent on loop gain. Therefore loop gain is essentially constrained by robustness, while the indices are dominated by the frequency response. Conversely the controller output variations are strongly dependent on the loop gain