Recent blogs appearing on http://www.EmersonProcessXperts.com and http://www.controlguru.com discussing the practical value of process models and various controller rules motivated me to write the following for my “Control Talk” column scheduled for the November issue of Control magazine.
I think most astute control people can devise a case where the literal use of their favorite tuning method is better than another method. In the book titled Handbook of PI and PID Controller Tuning Rules 2nd ed by Aidan O’Dwyer there are over 400 pages of tables of tuning rules. My rules are cited 6 times (incorrectly for dead time dominant processes). Obviously the authors of these rules all thought they had something better to offer.
To help put it all in perspective, I offer the following Top Ten List.
Top Ten Reasons to Devise Your Own Tuning Rule and Simulation Test
10. Opportunity to present papers at your favorite conference (whoops – not possible this year at ISA since there are no sessions on traditional process control)
9. Material to start a blog site
8. Competitive edge to start a consulting business
7. Listing in a book on tuning rules
6. Method named after you (sorry Ignatius Michael Coolman, the IMC acronym is taken)
5. Simulations tailored to prove your point
4. Linear processes without control valves
3. Speed since the time to steady state is just a matter of seconds in your simulation
2. Simplicity by ignoring the prevalence, size, speed, and entry point of unmeasured disturbances in real processes and non-stationary behavior
1. Chance to discount industrial online software for controller tuning as just hearsay
What prompted this impromptu column was the realization that diverse tuning rules have a common basis. For example, the equation for controller gain from the Ziegler Nichols ultimate oscillation, Lambda self-regulating and integrating process, and Internal Model Control tuning rules when set for maximum disturbance rejection reduce to the Ziegler Nichols reaction curve rule. For this discussion, we are focusing on loops dominated by a single time constant so that the dead time to time constant ratio is less than 0.5. It is important to remember that maximum disturbance rejection corresponds to maximum transfer of variability from the process control variable to the controller output, which may not be the entire objective. The following files show the basis and importance of this unification and simplification of tuning rules.