CONTROL LOOP PERFORMANCE ASSESSMENT FOR TWO DEGREE OF FREEDOM CONTROLLER: ANALYSIS OF MODEL-PLANT MISMATCH

Abstract

Controllers are vehicles to implement decisions designed for improved productivity. Performance assessment of controllers is necessary to estimate the health of such controllers so as to maximize their ability to realize optimal decisions/ targets. Traditionally, such performance assessment algorithms have focused largely on the single degree of freedom control structure, and generate performance indices either for the servo or the regulatory case. However, in many applications such as grade transitions or batch processes, it is important to achieve good tracking in the presence of unknown disturbances. A two degree of freedom controller is eminently suited to realize these objectives. This paper therefore extends performance assessment strategies for control loops with single degree of freedom controller, to control loops with two degree of freedom controller (2DOFC) structure. The proposed methodologycalculates two performance indices (viz. for the servo and regulatory case each for set point change and disturbance change) that are based on the minimum variance benchmark. We also examine effect of model plant mismatch on the servo and regulator performance of the control loop system.