User's Manual

Automation and Drives - SCE

T I A Training Document Page 20 of 64 Module

2.7.2 Three Position Controllers

The three position controllers represent the second important class of discrete controllers.

The difference regarding the two position controllers consists in the following: The controller output

can handle three different values: positive influence, no influence, and negative influence of the

controlled variable.

An example is control by means of a valve that can be adjusted electrically but that itself can only be

completely open or completely closed. Let’s take, for example, water level control. As soon as the

water level exceeds a maximum value, the valve motor is triggered with a positive direction of

rotation, and the valve is opened. The control system remains inactive -that is, the motor is idle-

until the water level drops below a minimum value. When this is the case, the motor is triggered into

the negative direction of rotation, and the valve is closed. Thus the actuator knows three states:

rotating valve motor with positive direction of rotation, idle motor, and rotating motor in negative

direction of rotation.

Preface Fundamentals Discontinuous Action Controller Controller Block (S)FB41 Setting the System Appendix

Issued: 02/2008 Control Engineering with STEP 7