Overview

Proper controller tuning is the largest, quickest, and least expensive improvement one can make in the basic control system to decrease process variability. The detrimental effects of disturbances, interactions, and control valve dead band are minimized by an appropriate selection of tuning settings ^[6.30]. However, the expertise, patience, and time required to compute these settings is significant and most of the methods don't take into account real-world problems like process and measurement noise, periodic upsets, load shifts, and valve stick-slip. In addition, many processes are nonlinear: the tuning settings change with the operating point, and with time. Also, the tuning methods and coefficients change with the ratio of time delay to time lag and the performance objective ^[6.3]. For these and other reasons, the accuracy of tuning settings usually doesn't exceed 20% for industrial processes and a reproducibility of better than 10% should not be expected for settings computed by manual or automated methods.

It is important for the user to realize that there is always a tradeoff between performance and robustness. Tuning settings that provide the smallest errors for load upsets and the fastest set point response are the settings most likely to cause instability for an increase in the process gain or time delay or a decrease in the process time constant. The quarter amplitude response from Ziegler-Nichols is a benchmark for minimum peak error for a load upset, but is too oscillatory and close to instability for most industrial processes. A smooth over-damped...