ON THE EFFICIENCY AND ROBUSTNESS OF DISCRETE PROPORTIONAL-INTEGRAL CONTROL SCHEMES

Feedback control schemes have been widely used in process industries f or many years. They are also increasingly being used in the discrete-p arts manufacturing-industry in recent years. Proportional-integral (PI ) schemes are especially popular, primarily because of their simple st ructure and ease of implementation. This article studies the efficienc y and robustness properties of discrete PI schemes under some commonly encountered situations. For process disturbance, we consider the stat ionary ARMA (1, 1) model and the nonstationary ARIMA (1, 1, 1) model. Process dynamics is studied under a first-order dynamic model, includi ng the special case of pure gain. The efficiency of PI schemes is comp ared with that of minimum mean squared error (MMSE) schemes under thes e models. The PI schemes are seen to be quite efficient over a broad r ange of the parameter space. Furthermore, the PI schemes are much more robust than MMSE schemes to model misspecifications, especially the p resence of first-order nonstationarity. The results here provide addit ional justification for the use of discrete PI schemes.