Tag Archive: controller+tuning process+dynamics valve+dynamics

Aug
14

Limit Cycle Periods – Part 3

Any process with split ranged valves is prone to limit cycling for many reasons. The periods are unpredictable. Some are process related. For example, the transition between steam and coolant creates a severe discontinuity. There is a change of phase and disruptive change in magnitude and sign of the effect. There is no happy medium …

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Aug
07

Limit Cycle Periods – Part 2

Last week we discussed how a resolution limit (stick-slip) in a control valve can cause a saw tooth oscillation in the controller output for a self-regulating (steady state) process. For a flow or liquid pressure loop where the process time constant is small, the oscillation in the process variable (PV) is a square wave. For …

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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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