Performance assessment of control loops involving unstable systems for set point tracking and disturbance rejection

In this article, a performance assessment method that measures the performances of set point tracking as well as disturbance rejection characteristics of PID controllers designed for unstable processes is proposed. The lower bounds of integral absolute errors (IAE(0)) are derived from the desired closed loop transfer function subjected to set point changes which may be of step or ramp types. The performance index is developed by considering Direct Synthesis (DS) method for PID controller design. Based on the theoretical bound IAE(0), a dimensionless index is elucidated as the ratio between IAE0 and the actual IAE obtained from closed loop response under step set point changes and load variable changes. Considering this IAE based index as the performance measure of PI/PID control loops, the effectiveness of the controller is evaluated and tested on different types of unstable first order or second order plus time delay processes. Simulation studies for both set-point changes and disturbance rejection on various numerical examples are performed to prove the validity of the suggested performance benchmark. (C) 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.