Siemens PCS 7 manual Stabilization of Unstable Control Loops

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Stabilization of Unstable Control Loops

3 Stabilization of Unstable Control Loops

Regarding the stabilization of unstable control loops, integral processes and mono- tone unstable or oscillating unstable processes have to be distinguished.

In general only an analysis in frequency domain is helpful for oscillating control loops. As an example, displacements of unstable poles to the stable domain can be examined using root locus analysis [also see Related Literature /3./]. These oscillating control loops are a common issue in the context of mechanical systems (spring-damper-systems, elas-tical roboter arms) but can rarely be found in process plants. Oscillations in process plants can rather be attributed to malfunctions of slave control loops, e.g. in the valve position controllers.

In the following only the stabilization of integral processes will be discussed, due to the practical relevance in process engineering. A proportional-only controller is suf- ficient to stabilize integrating processes, as confirmed by systems dynamic consid- erations (e.g. root locus). Thus the problematic interaction of the integral part of a PI controller with the integral part of the plant is avoided. However, persistent con- trol deviations caused by disturbances at the input of the process have to be ac- cepted, if no integral action is used in the controller. Example: The proportional- only controller is not able to hold the level exactly at its set point if the feed is vary- ing.

3.1Manual Parameterization of a Proportional-only Controller for Integrating Processes

Figure 3-1 Unit-step response of an integrating process

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MPC Level

V 1.0, Beitrags-ID: 42200753

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Contents Applikationen & Tools Page Online-support.automation@siemens.com Warranty and Liability Table of Contents Preface Objective of the ApplicationIntroduction Basic Principles of Model Predictive ControlStable and Unstable Control Loops Whithout compensation With Ohne Integral Ausgleich Examples of Unstable Control Loops Level ControlPressure Control in Tanks Position Control Stabilization of Unstable Control Loops Unit-step response of an integrating processt 1s PID Tuner Configuration of MPC with Slave Controller Starting PointConnection in CFC MV1 MV2 Connection of MPC and slave controller Commissioning Simulation Example OS picture of the example project Conclusion Internet Link Specifications Related LiteratureBibliography History Version Date Modifications
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Manual 6 pages 24.98 Kb

PCS 7 specifications

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