Performance improvements of cascade and feed-forward control schemes for industrial processes

In this paper a comparison among different control schemes, frequently implemented in industrial processes is presented. These control structures range from simple configurations to complex structures, in order to reproduce problems to be faced in the choice of the more suitable control system of a real industrial process, a spray dryer, which presents significant problem of quality control in the presence of external disturbance. In particular, feedback control, feedback plus feedforward control, cascade control, cascade plus feedforward, and double-feedback plus feedforward are analyzed and compared. The control structures are evaluated on the basis of different criteria, including structural simplicity and ease of realization, performance indices (as maximum allowable deviations on controlled variable, integral of the absolute value of control error, total variation of control action) and constraints on manipulated variables. As final result, a cascade plus feedforward configuration is indicated as more suitable for the assigned specifications and its robustness to errors in the model and variations in the operating conditions is accounted for.