April 2006

Process Control

Detecting interacting controllers using fast Fourier transforms and correlation coefficients

Using both techniques provides the best solution

Chen, D. H., Lamar University; Lo, P., Chevron Phillips Chemical; Stout, J., Stout, S., Wetuski, P., Nexus Solutions; Swan, W. P., Sasol

Often one poorly tuned loop is the reason for another loop's poor performance. In the case of interacting controllers, the changes in one loop's output have an effect on performance of a second control loop. Problems such as poor tuning with one loop can cause a variation in the process value (PV) of a second loop in which the second loop moves its own output in an attempt to correct the deviation. In a plant with many loops, all controllers with cycling process values cannot be grouped as being caused by the same poorly tuned loop. The difficulty lies in determining which ones are related and to focus attention on those for troubleshooting. Analyzing a group of controllers using fast Fourie

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