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

Step Response

	14
	12
	10
Amplitude	8
									ki
	6
	4
								1
	2
	00	5	10	15	20	25	30	35	40	45	50
		t1				Time (sec)

12	MPC Level
	V 1.0, Beitrags-ID: 42200753