Monthly Archive: August 2007

Aug
27

From the Beach

I write wherever I get the inspiration. Headphones are a given. Right now I am listening to Crosby, Stills and Nash’s “Déjà Vu”, which seems especially appropriate since this is a flashback to my career in instrumentation. When I am in a groove, I enjoy the detail of every note and lyric and the thoughts …

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Aug
20

Free Control Technique Buffet

A buffet of techniques that is free to boot (Texas Talk) is available as an E-Book at http://www.modelingandcontrol.com/controltechniques/ While the title is Continuous Control Techniques for Distributed Control Systems, the techniques are applicable to batch processes and in fact were based in some cases on batch loops. It follows in the foot steps of my …

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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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