Control Loop Improvement

McMillan and Weiner Talk With Jacques Smuts About Improving Control Loops

By Greg McMillan, Stan Weiner

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Greg: I had the pleasure of interviewing Jacques Smuts on improving control loops,. a subject near and dear to my heart. Jacques is the president of OptiControls Inc and the author of the book Process Control for Practitioners that provides a very understandable and practical approach to control loop optimization.

Stan: What do you do for plants?

Jacques: I do process control consulting and training to improve the performance of control loops. You would think after 20 years it would become old hat, but I am constantly surprised at how key fundamentals are not understood by people responsible for tuning.

Greg: In the midst of teaching a course on opportunities in process control at a plant I used to work at outside New Orleans, I realized the veterans were hanging on each word, but the young engineers were pretty much blowing me off. I found out most of their job was paper work in a Six Sigma program, and none had spent any time trying to tune or improve loops. As I lead the ISA Mentor Program, I am astonished how little time my protégés have to work on control loops.

Jacques: I am lucky that I can focus on tuning loops. This has unfortunately been reduced to a small fraction of what practitioners at plants are responsible for today. Tuning is a skill that requires practice. Many don't appreciate how much you really have to know beyond the basic tuning relationships. You need to know if the valve works properly. You need understand if a process needs to be tuned fast or slow and whether you need to schedule tuning. I enjoy the depth of the field. We lose sight of what the user needs to know and what the user does know. Increasingly, the user has more responsibility and less time for tuning and learning.

Greg: I was impressed with your chapter on valve problems where you describe tests for the various sources of limit cycling. Most other books don't make a distinction between dead band and stiction or the importance of valve problems in general. In university textbooks, the valve is perfect, and the whole emphasis is on a process time constant and gain and a setpoint response, reducing the loop to a servo problem. This may work for aerospace, but not for industrial processes with sloppy valves, slow measurements, analyzer sample and cycle times, large and variable dead time, interactions, unmeasured disturbances and non-self-regulating responses.

Stan: What has been the impact of digital positioners?

Jacques: Nowadays I find a lot less dead band, but also some erratic behavior from weird positioner tuning. I am amazed at how many tuning settings are on a modern positioner. Each manufacturer has a proprietary algorithm and tuning parameters. Tuning guidelines are slim to none. It helps to know the positioner is a closed loop controller that could have overshoot and limit cycles. Also, be aware that the response time depends on step size. I have seen a response time increase from 2 sec for a 2% change to 30 sec for 0.5% change. I look at past history to see the size of increments in the PID output to determine what the controller will have to deal with. I have also seen a positioner develop a limit cycle with a flow loop in manual.

Greg: The problem may be caused by the combination of stiction in the valve and integral action in the positioner. The solution would be to turn off integral action in the positioner or turn on integral dead band in the positioner. In a way, the old analog positioners were smarter than today's digital positioners by being proportional-only controllers. Controller offset is inversely proportional to controller gain and is very small because the positioner gain is so high (e.g., gain > 50). Offsets may be a cosmetic concern when manually stroking a valve, but in practice are inconsequential because the process controller will manipulate the signal as needed, eliminating the effect of the offset.

Stan: Are auto tuners the complete solution?

Jacques: People expect the software to be an engineer rather than a tool. Most people try, fail, get frustrated and stop using tuning software. Many try to tune from bad data, not realizing that more than minimal knowledge is needed. Loop tuning and performance monitoring software requires human skills to get the most out of them.

Greg: I found a production unit where all the gain settings were 1, and the reset settings were 1 repeat per minute. For the loops that were oscillating, the operator narrowed the output limits to achieve a sort of on-off control. When instructed the loops needed to be tuned, the operators were turned loose on tuning. Do you see operators tuning loops?

Jacques: Some plants have operators tuning loops. Some operators have had a class on tuning, but when I ask, they tell me the math was too much. If the Zeigler-Nichols tuning relationships are over their heads, you have problems. We do need to take advantage of their first-hand knowledge. Operators know important aspects of equipment performance, but sometimes have their own interpretations of the causes for poor control. You need to get operator observations rather than conclusions and explore the cause-and-effect relationships. In a liquid-gas separator, operators said you needed to close the outlet valve first and then open it to lower the level; otherwise the level would keep going up. I was tempted to discount this requirement, but found out under certain conditions a vortex formed. Closing the valve broke the vortex.

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