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Modeling Gravity Drained Tanks Data Using Software
In previous posts, we investigated a graphical analysis method for fitting a first order plus dead time (FOPDT) dynamic model to step test data for both the heat exchanger and the gravity drained tanks processes.
Describing process behavior with an approximating FOPDT dynamic model is the third step of our controller design and tuning recipe. Thus, it is a critical step for quickly achieving desired controller performance while avoiding time consuming and expensive trial and error methods.
The reason we studied graphical modeling is because it is a useful way to isolate the role of each FOPDT model parameter, and in particular, appreciate what each says about a controller output (CO) to process variable (PV) relationship.
As we learned in these investigations, for a change in CO:
- process gain, Kp, describes the direction and how far the PV moves,
- time constant, Tp, describes how fast the PV responds,
- dead time, 0p, describes how much delay occurs before the PV first begins to move.
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