DECOUPLING OF MULTI VARIABLE SYSTEM AND ITS APPLICATION IN MODEL REDUCTION

Abstract

In a conventional decoupling control scheme, process inputs (signals to valves or set points to lower level flow controllers) are produced as a time-weighted combination of process controller outputs. This report presents a comparative study of simplified, ideal and inverted decoupling. The stability, robustness and implementation of the three decoupling methods are studied. Two simulation examples and an experimental process are used to show these advantages in comparison with simplified decoupling. It is demonstrated that robust performance and robust stability of a nominally stable control system are equivalent for the three decoupling methods when the controllers are tuned to obtain identical nominal performance. A relation is derived between the presence of right-half plane (RHP) zeros of a process in series with its simplified decoupler and the instability of the ideal and inverted decouplers for the same process. This report also describes a potential implementation problem related to the particular structure of the inverted decoupling. Finally, a recapitulative table of the main advantages and limitations of each decoupling -method is presented. Because of this, inverted decoupling may be a good and easy way to improve the performance of industrial TITO (two inputs and two outputs) processes with interaction problems (when it can• be applied).