EFFECT OF NONLINEARITIES
Nonlinearity is not just a change in the open loop gain. Any change in dynamics or deviation from a response describable by a fixed open loop gain, dead time, and time constants is considered to make the loop nonlinear.
The dynamics in 99 percent of the control loops in the process industry are not constant. We have seen from previous chapters how equipment, process, valve, variable speed drive (VSD), measurement design, installation, and operating conditions affect the overall loop dynamics. There is always a tradeoff between control loop performance and robustness, which is the ability to tolerate changes in dynamics without causing excessive oscillations. The tuning that gives the minimum ...