Siemens Module B3 Manuale Utente

 

 

 

 

 

 

 

   

 

 

 

 

T I A  Training Document 

Page 30 of 64 

  Module 

 

The diagram below shows the layout of a digital controller: 

 

 

 

 

 

 

 

 

 

 

  ADC

 System 

Digital 
Controller

 

 DAC 

Comparing

 

Element 

 

 

However, the digital conversion of the controller has not only advantages; this conversion also 

entails various problems.  For that reason, some variables have to be selected sufficiently large in 

reference to the digital controller so that the accuracy of the control does not suffer too much on 

account of digitalization. Quality criteria for digital computers are: 

•  The quantization resolution of the digital-analog converters. 

It indicates how fine the continuous value range is digitally rasterized. The resolution has to be 

selected of a size that no resolutions important to the controller are lost. . 

•  The scanning frequency of the analog-digital converters. 

This is the frequency with which the analog values pending at the converter are measured and 

digitalized. This has to be high enough that the controller can still respond in time if the controlled 

variable suddenly changes. 

•  The cycle time 

In clock cycles, every digital computer processes differently than the analog computer. The speed of 

the computer used has to be high enough that during a clock cycle (during which the output value is 

calculated, and no input value is scanned), the controlled variable can not change significantly. 

The quality of the digital controller has to be high enough so that toward the outside, it responds 

comparably prompt and precise, like an analog controller does. 

 

 

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

B3 

Issued: 02/2008                                                                                                                                     Control Engineering with STEP 7