Effect of process dynamics on traditional controller tuning methods

Abstract

Several tuning methods, including Ziegler-Nichols, have been used for decades in the process industries. These methods have the important advantage of ease of use and provide relatively good tuning for the majority of loops. The methods all use a “first-order plus dead time” (FOPDT) model of the process. However, processes do not always fit the FOPDT model; most have multiple lags. The degree to which the process fits the FOPDT model determines how well the traditional tuning methods will work with the process. This paper examines several tuning methods and presents some comparisons and evaluations based on experiments using computer simulations of loops with various combinations of multiple-lag and dead time dynamics. Guidelines for the applicability of the tuning methods are presented, and several aspects of the methods, including the training requirements, are discussed.