Monthly Archive: July 2007

Jul
30

Limit Cycle Periods – Part 1

After watching a PI controller take an incredibly long time to cycle through a very small resolution limit, I figured it would be helpful to develop an equation that would provide an estimate of the period of the limit cycle (sustained equal amplitude oscillation) from a valve resolution limit (e.g. stick-slip). The attached equation shows …

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Jul
20

Freedom to Optimize PID Controller Structure – Part 2

The “Two Degrees of Freedom” structure functionally can give you the smooth transition online between 4 choices of PID structure. The user can adjust the set point weights for proportional action and derivative action between zero and one. If the set point weights are zero. there is no step or bump from a set point …

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Jul
14

Freedom to Optimize PID Controller Structure – Part 1

There are 8 different choices for controller structures in a modern DCS. Which one is the best for your application and does it take a download to change it? One choice is for integral only controller (no proportional or derivative action). The most common I-only control is a valve position controller (VPC) which is trying …

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Jul
08

Gainfully Tuned

If a control loop is oscillating, would it be best to increase or decrease the controller gain? The standard answer of decreasing the controller gain is right for a decaying or growing oscillation in a relatively fast self-regulating loop (loop whose PV quickly goes to a steady value when in manual and disturbances have dissipated). …

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