Assessment of Proportional Integral Derivative Control Loops for Large Dominant Time Constant Processes

This manuscript deals with the assessment of parallel form of proportional integral derivative (PID) control structure for tracking the reference input designed for large dominant time constant processes whose dynamics are slow (integrating processes). The theoretical bound of integral absolute error (IAE) which is established for unstable first order process is extended to pure integrating process without using any approximations. This relies on direct synthesis tuning (DS) and the theoretical bound is obtained from the transfer function of closed loop system subjected to ramp input changes. An error based performance index is formulated on the basis of this IAE theoretical bound and actual IAE, to measure the behaviour of the controller employed for non self regulating (integrating) processes. This error based index evaluates the performance of closed loop controller and specifies whether the controller requires retuning or not. A sequence of simulated examples is used to illustrate the benefit and effectiveness of this new performance assessment method.