The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

As part of the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. I will be sharing questions from program participants and the answers. This question is from Danaca Jordan

Danaca Jordan is a manufacturing staff engineer working for a specialty chemical company in the U.S. She began her journey to automation in high school as team captain for FIRST Robotics Team #624 and earned a B.S. in chemical engineering from the University of Houston. Jordan is an active member of AIChE and ISA, and is a successful acolyte of the ISA Mentor Program.

Danaca Jordan's Question

How do you determine a measurement is good enough for control?

Great question.

First, the measurement needs to be representative of what you are trying to control and significantly affected by the disturbances you are trying to reject. This is best seen by monitoring changes in the measurement for changes in the manipulated and disturbance streams.

Second, the measurement should have a deadtime less than 1/20 of the cyclic disturbance period; otherwise, the disturbance is too fast and is effectively noise with a risk of amplification by resonance with the natural frequency of the loop.

Third, the measurement threshold sensitivity and resolution and repeatability should be less than one quarter of the allowable control error at setpoint for the entire operating measurement range. The measurement must have enough rangeability to handle low operating points. Often measurement errors are expressed as a percent of full scale and are therefore larger in terms of percent of operating setpoint.

Fourth, the measurement drift should be less than one quarter of the allowable control error between scheduled maintenance.

Fifth, the noise must be small and fast enough to be filtered without appreciably slowing the loop’s ability to see disturbances. The filter time to keep the oscillations in the controller output from noise less than the valve deadband should be less than 1/10 the reset time and not significantly affect the tuning of the controller. For more information on measurement location and selection, see my process control post, How to Succeed – Part 4. 

Additional Mentor Program Resources

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant) and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

About the Author
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.

Connect with Greg