Siemens Module B3 manual Three Position Controllers

Automation and Drives - SCE

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
	T I A Training Document		Page 20 of 64	Module
				B3
Issued: 02/2008				Control Engineering with STEP 7