Control practice: Why are PID loops so difficult to master?

-- Control Engineering, 5/7/2008

While there is some variation on the precise extent of the problem, industrial studies suggest that most people don’t know what to do with PID loops. Perhaps one-third of such applications work reasonably well, which suggests that the other two-thirds are only adequate or worse. Must this be the case?

Getting PID loops to operate well is a major concern in most process industries. Loop tuning related topics continue to be the most heavily researched subject on the Control Engineering Website, year after year. Tuning articles are always heavily read. One would think that by now, loop tuning techniques would be mastered by middle school, but such is not the case. To expand on the statement made earlier, conventional wisdom suggests that one-third of loops work reasonably well, one-third are running fully in manual, and one-third limp along with periodic fiddling. Other sources suggest perhaps half of loops perform adequately.

Why are loops so hard to tune? While at the ABB user group meeting, I tried to find out some answers to this question by attending a relevant technical session and asking around. Here are some suggested reasons:

• Process units frequently operate at production levels that are simply not suited to the equipment. In the real world, process hardware is designed and built for specific production levels and feedstock characteristics. However, production usually deviates from these, which makes a plant harder to control. The greater the deviation, the more control suffers.
• Some processes, due to the dynamics and interactions of the process itself, are simply more difficult than others.
• Process units frequently have multiple loops that interact, and one or two that misbehave can throw others off. The tricky part is identifying which are the most strategic to fix first. It can be difficult to differentiate cause from effect in the real world.
• One size does not fit all. Parameters can be sensitive to operating levels. When a plant is running at full capacity, it may behave. When running at lower capacity, loop parameters may need to change to remain stable.
• Hardware problems will frustrate you every time. If instrumentation, valves, and so forth do not perform reliably, the process unit will never perform reliably. The data may tell you what's happening, such as having valves that stick or hysteresis problems, if you can spot the signs.

Software is available that may provide a solution or at least mitigate the problem. You can buy it and do-it-yourself, or hire a consultant or service to do the work. In the May issue of Control Engineering, our loop expert, Vance VanDoren, does an in-depth analysis of control loop management software. While it may not answer all your questions, it will certainly point you in the right direction.

Also read “Adaptive Controllers Work Smarter not Harder” from Control Engineering and:
The Three Faces of PID
Past PID: Adaptive control is versatile, fast, accurate, report says

—Peter Welander, process industries editor, peter.welander@reedbusiness.com,
Process & Advanced Control Monthly
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