Here's how to improve control loops

McMillan and Weiner talk with Jacques Smuts about improving control loops

By Greg McMillan, Stan Weiner

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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.

Stan: Is there a considerable amount of hand-holding needed?

Jacques: Patience, explanations and time are essential, particularly for systems with significant dead time. Many years ago I worked at a production plant for whiskey. The dry distiller grain solids (DDGS) is a valuable by-product for cattle feed, and was dried in a kiln with a dead time of about 9 minutes. The main disturbance came from the level control of a syrup tank that feeds into the wet grain and was manually set by the operators because level control was deemed not possible. I tuned the level controller and added feed-forward control to the kiln. Operators said it wouldn't work. I had a difficult time getting the operators to leave the moisture controller in automatic because the control was not smooth because of clumps. I sat with the operators and showed them the controller had taken all the action they would take (e.g. 40%), and there was nothing more to do than wait 9 minutes.

Greg: Looks like what the operators needed was a moisture calculation one dead time into the future on a trend chart described in Control Talk Blog "Future PV Values Are the Future."  Dead time is the most difficult thing for humans for deal with. Tuning can take the dead time into account avoiding overreaction and providing a consistent response. Does tuning ever become boring?

Jacques: After working on thousands of loops, you would think you have seen everything, but almost every plant I work at I see something new. I found a controller with a derivative time of 300 minutes. The low output limit was 75%, and the high output limit was 78%, so the loop was essentially in manual. Recently I found loops on an oil rig that were never tuned since they were commissioned in 2004. No attempt was made to rectify severely oscillating loops. They just lived with them unless they caused a shutdown.

Greg: What about configuration problems?

Jacques: Configuration can prevent a loop from ever performing properly. Configuration engineers often haven't had time to learn tuning. I had an override control system that was not working. The initialization was done in the wrong blocks. I had to wait till shutdown to have it fixed. I don't need to know how to do configuration for each of the many types of DCS, but I do need to recognize when the configuration is a problem.

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