Siemens Module B3 用户手册

 

 

 

 

 

 

 

   

 

 

 

 

T I A  Training Document 

Page 20 of 64 

  Module 

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 

B3 

Issued: 02/2008                                                                                                                                     Control Engineering with STEP 7