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Jun
09

Review of Deminar #5 – PID Tuning for Self-Regulating Processes

PID Tuning for Self Regulating Processes - Greg McMillan Deminar

You can click on the above to view and hear the recording of the Deminar.

In Deminar #5 we first show that for a self-regulating process, the process variable will line out (reach a steady state) when the controller is in manual unless there are continual disturbances. The self-regulating response is most commonly encountered response because there are more flow loops than any other type of loop. Liquid pressure loops and temperature control loops in continuous operations have a self-regulating response. Level normally has an integrating response but in the Deminar we show test results for a conical tank level with self-regulating response due to gravity discharge flow. The flow across the discharge valve is proportional to the square root of the liquid head as the level increases, the discharge flow increases and vice versa. The self-regulating or steady state process gain increases with level as a result. The significant increase in cross sectional area with level due to the conical shape causes a dramatic increase in the process time constant that creates a stabilizing effect. The process response at high level is much slower enabling the use of more aggressive tuning settings. However, the test results show these settings at low level cause excessive oscillation. The adaptive level controller is able to keep the set point response smooth and consistent over the level range. For more details you can check out the Control magazine article “Adaptive Level Control”

Most of the Deminar focuses on how an auto tuner, adaptive tuner, and adaptive controller can be used to improve the response of liquid flow and liquid pressure loops. The principle nonlinearities are the control valve characteristic for the flow loop and pump curve for the pressure loop.

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