Siemens Module B3 manual Objectives for Controller Adjustment

Automation and Drives - SCE

2.8 Objectives for Controller Adjustment

For a satisfactory control result, selecting a suitable controller is an important aspect. However, even more important is the setting of the suitable controller parameters Kp, Tn and Tv that have to be adjusted to the controlled system behavior. Usually, a compromise has to be made between a very stable but also slow controller, or a very dynamic, more unstable controlled system performance, which, under certain circumstances, tends to oscillate and can become unstable.

In the case of non-linear systems that are always to work at the same operational point -for example, fixed setpoint control- the controller parameters have to be adjusted to the controlled system behavior at this working point. If, as in the case of cascaded controls ñ, no fixed working point can be defined, a controller adjustment has to be found which provides a sufficiently fast and stable control result over the entire work area.

In practice, controllers are usually set based on empirical values. If none are available, the controlled system behavior has to be analyzed exactly, in order to subsequently specify suitable controller parameters, with the aid of different theoretical or practical layout procedures.

One possibility of a definition is the oscillation test according to the

Ziegler-Nichols method. It offers a simple layout suitable for many cases. However, this setting procedure can only be used for controlled systems that permit getting the controlled variable to oscillate autonomously. The following has to be done:

-Set Kp and Tv at the controller to the lowest value, and Tn to the highest value (the lowest possible controller effect).

-Take the controlled system manually to the desired operating point (start the controller).

-Set the manipulated variable of the controller manually to the specified value, and switch to the automatic mode.

-Increase Kp (decrease Xp) until harmonic oscillations can be recognized in the controlled variable. If possible, the control loop should be stimulated to oscillate during the Kp setting by using small, abrupt setpoint changes.

	Preface	Fundamentals	Discontinuous Action Controller Controller Block (S)FB41	Setting the System Appendix
	T I A Training Document		Page 27 of 64	Module
				B3
Issued: 02/2008				Control Engineering with STEP 7