Siemens Module B3 ユーザーズマニュアル

 

 

 

 

 

 

 

   

 

 

 

 

T I A  Training Document 

Page 18 of 64 

  Module 

 

2.7.1 Two Position Controllers 
 

  The essential feature of two position controllers consists of their knowing only two modes: “On“ and 

“Off“  -which makes them the simplest type of controller. Two-position controllers are used primarily 

when adhering to a setpoint exactly is less important than to keep the control system as simple as 

possible; or, when the actuator or the final control element does not allow for a continuous control 

system.  The heating system mentioned several times above is -with a control loop having a room 

thermometer and a mixer- a continuous control system.  To keep the water temperature in the boiler 

loop constant, typically a two position controller is used since it can, on the one hand, fluctuate by a 

few degrees, and on the other hand it is clearly simpler to switch the burner on and off than to do an 

exact dosing of fuel to be added.  

 

Since theoretically -to adhere to the setpoint exactly- it would be necessary to switch a system on 

and off infinitely fast, the two position controller has a so-called “hysteresis“.  It represents a kind of 

“environment“ around the setpoint within which the actual value may fluctuate. That means, we 

specify a minimum value that is lower than the setpoint, and a maximum value that is a little higher 

than the setpoint.  Only if the actual value exceeds the maximum value or drops below the minimum 

value does the control system react. In most cases, the minimum and the maximum value are 

distanced from the setpoint equally; that is, the hysteresis generates a symmetrical environment 

around the set point.   

In the case of the boiler water temperature, the burner would be switched on, for example, when the 

water temperature drops below the specified setpoint by more than a certain value. The burner 

continues to run until a certain value that is above the setpoint is exceeded. Only then will the burner 

be switched off.  Another typical example is cooling. Usually, a cooler also does not support a 

continuous control system, but only knows the states “On“ and “Off“. It is switched on when the 

actual temperature exceeds the setpoint temperature by a few degrees, and is switched off when the 

actual temperature is a few degrees too low.   

It is therefore typical for the two position controller to periodically fluctuate around the setpoint whose 

amplitude is roughly that of the hysteresis. The selection of the hysteresis depends on how exactly 

the setpoint has to be adhered to.  If we select a large hysteresis, the actual value can deviate more 

considerably from the setpoint.  If we select a smaller one, the setpoint is adhered to more exactly, 

but the system would have to switch more frequently. This again has its disadvantages, such as a 

higher wear of the switching devices, and the actuator or the final control element. 

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

B3 

Issued: 02/2008                                                                                                                                     Control Engineering with STEP 7