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How Do You Measure Control System Performance?


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.


For the automation profession to gain the recognition it deserves, we need to show the impact of our achievements on improving plant performance. This question is from Sridhar Dasani, with my answer and additional input from co-founder of the ISA Mentor Program Hunter Vegas and mentor Brian Hrankowsky.

The question helps us seek a suite of practical metrics in terms of key performance indicators (KPIs) that can have an impact on plant performance. At the end is a link to a Control Talk column that offers metrics more directly related to plant efficiency and plant capacity.


Sridhar Dasani, MS, P.Eng., is chief technology officer and founder of PYGO, Inc. In Canada.


Sridhar Dasani’s Question

As a new engineer supporting plant operations, I regularly attend monthly meetings where the operations team presents its key performance indicators in terms of the production rates, savings, and uptime, and maintenance personnel present their SAP-scheduled maintenance numbers. I always wonder how an automation engineer supporting DCS system administration or process control can present his/her performance? Based on your experience, would you be able to suggest what KPIs can be presented during these meetings, anything from system health to loop performance to start with?


Hunter Vegas’ Answer

If the plant is continuous you can look at:

(1) Percent of time the control loops are in their highest mode (e.g., auto, cascade, remote cascade).

(2) Alarm rates (per week, per month). Obviously this is somewhat dependent on the plant operations but if you are doing an alarm rationalization project it gives you a feel for how well you are doing.

(3) If you have made a control system change to the process you can document before/after performance (within range of setpoint, production rate, quality, etc.).

(4) Non-routine instrument work orders generated per month gives you a clue on instrumentation reliability. Those are some that immediately come to mind. No doubt Greg will have a longer list.


ISA Mentor Program

The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.


Greg McMillan’s Answer

(5) Percent of time and number of times PID output hits output limit.

(6) Standard deviation of the process variable.

(7) Number of oscillations where the process variable has crossed the setpoint by more than a deadband or noise band (may or may not want to detect noise) forgetting results older than an analysis period.

(8) Peak error for a constant setpoint forgetting results older than an analysis period.

(9) Integrated absolute error (IAE) forgetting results older than an analysis period.

(10) Rise time: time to reach setpoint (within deadband around setpoint) for setpoint change.

(11) Overshoot and undershoot for setpoint change.

(12) Settling time: time to settle out at setpoint (stay for more than four dead times within deadband around setpoint) after setpoint change. Even better as noted by Hunter is if you could tie what you do to operations KPIs on process capacity (production rate and onstream time) and efficiency (energy and raw material use and waste). This could be done by noting if the KPIs are better because of changes you made or from better loop metrics you achieved. For a view on how to use a virtual plant to develop and demonstrate online KPIs, see the Control Talk column Getting innovation back into process control.

Brian Hrankowsky’s Answer

(13) How many operator actions are logged in the audit trail/event history?

(14) How long are the "waits" in the process? (How long is the heat up or cool down before the next value added step can take place and what is the duration of prompts lingering for an operator?)

(15) How many batch “hold” events are there and what has been the duration?

(16) How often are interlocks tripping?

(17) How often are estop or shutdown events occurring?

(18) How much paper work does it take to run the process? Any change that is to be made to a control system must be thoroughly functionally tested by realistic simulations of the process’s dynamic response. The ability of the control system improvement to deal with abnormal besides normal operating conditions must be verified. The commissioning and performance of improvements should be closely monitored to ensure they meet plant requirements.


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), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).


Greg McMillan
Greg McMillan
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 digital twin for exploring new opportunities.

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