For a PID controller to perform as expected, changes in the controller output must lead to changes in the opening of one or more final control elements, which must affect the measured variable for the PID controller. Driving a final control element to either limit disrupts this sequence. This requires integral tracking, not output tracking. If the final control element has been driven fully open, control actions to increase the opening cannot be implemented, but control actions to decrease the opening can be. The opposite is true if the final control element is driven fully closed.
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Cecil Smith is president of Cecil L. Smith, Inc., Houston, a firm specializing in automation technology that he founded after leaving the faculty of Louisiana State University. Also cited as one of the 50 most influential people in the field, he teaches courses on control topics and has authored or co-authored 14 books, including “Digital Control of Industrial Processes and more than 100 articles. You can email him at firstname.lastname@example.org. He is also one of our experts in our Ask the Experts forum. You can pose your instrumentation and control questions to him here. http://www.chemicalprocessing.com/experts/instrumentation-control/