EFFECT OF PROCESS DYNAMICS ON TRADITIONAL CONTROLLER TUNING METHODS

Several tuning methods, including Ziegler-Nichols, have been used for decades in the process industries. These methods have the important ad vantage of ease of use and provide relatively good tuning for the majo rity of loops. The methods all use a ''first-order plus dead time'' (F OPDT) model of the process. However, processes do not always fit the F OPDT model; most have multiple lags. The degree to which the process f its the FOPDT model determines how well the traditional tuning methods will work with the process. This paper examines several tuning method s and presents some comparisons and evaluations based on experiments u sing computer simulations of loops with Various combinations of multip le-lag and dead time dynamics. Guidelines for the applicability of the tuning methods are presented, and severer aspects of the methods, inc luding the training requirements, are discussed.