Ask Greg McMillan

Greg's Response:

Override control is considerably challenging because the controllers that are not selected tend to windup, the takeover of control is difficult to make bumpless, controller tuning depends upon dynamics that may be quite different for the situation when override occurs, and operator confusion as to what is happening is significant, leading to detrimental intervention. 

The good news is that PID algorithms with filtered positive feedback can make the transition smoother and dynamic first principle simulations can adapt the PID tuning settings to deal with changing process conditions, develop metrics to show time each override control has taken over and consequential process performance for last hour and shift, and provide the testing and training to ensure better understanding and continuous improvement of override control. 

For a deep dive into details and test results, see the Control magazine feature article “The pitfalls and promise of override strategies” that I coauthored with Peter Morgan. For much more on filtered positive feedback see the ISA Technical Report “Proportional-Integral-Derivative (PID) Algorithms and Performance” ISA-TR5.9-2023 that Peter and I led and were recognized for by ISA Excellence in Standards awards.