This ISA Mentor program question is from Danaca Jordan (USA):
“How should the need for manual operation be handled?”
If at all possible, manual actions should be eliminated to remove the discontinuity, inconsistency, and delay inherent in any human correction. The fastest and most abrupt and unpredictable disturbance is an operator action. The best of the operator actions can be automated and from increased pattern recognition from a consistent response be continuously improved. The magnitude of the opportunity was exemplified in the Control Talk column Show Me the Money – Part 1. Operator manual activity is most intense during startup, transitions, and abnormal operations. The more an operator says a manual action cannot be automated the greater the opportunity as discussed in my modeling and control blog from yesteryear Exceptional Opportunities in Process Control – Startup and Abnormal Operation.”
Given that there is a manual action required, consider slowing down transitions in manual operations by the use of velocity limits on the setpoints of PID and analog output (AO) blocks. Be sure to use the PID dynamic reset limit option in any controller trying to manipulate a velocity limited valve or flow setpoint to prevent the burst of oscillations when a PID output tries to change faster than the final control element or secondary loop can respond. For an example of the importance of the dynamic reset limit see Shinskey’s article in Control magazine, The Power of External-Reset Feedback.
Please refer to the article PID Controller Tuning Rules and click this link to download the accompanying PID Tuning appendices for information on how to get the most out of your PID controllers.
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly “Control Talk” columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.
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