Siemens Module B3 Manual De Usuario
Automation and Drives - SCE
T I A Training Document
Page 29 of 64
Module
2.9 Digital Controllers
So far, mainly analog controllers were discussed; that is, such controllers that derive the controller
output variable -also in an analog way- from the existing system deviation that exists as analog
value. We are already familiar with the diagram of such a control loop:
Analog
Controller
Controller
System
Comparing
Element
Element
Often, however, it has its advantages to evaluate the system deviation digitally. On the one hand,
the relationship between system deviation and controller output variable has to be specified much
more flexibly if it is defined through an algorithm or a formula with which a computer can be
programmed respectively, as when it has to be implemented with an analog circuit. On the other
hand, a clearly higher integration of circuits is possible in digital engineering, so that several
controllers can be accommodated in the smallest space. And finally, by dividing the computing time it
is even possible -if the computing capacity is sufficiently large- to use a single computer as
controlling elements of several control loops.
To make digital processing of the variables possible, the reference variable as well as the feedback
variable have to first be converted in an analog-digital converter (ADC) into digital variables. These
are then subtracted from each other by a digital comparing element, and the difference is transferred
to the digital controlling element. Its controller output variable is then converted again in a digital-
analog converter (DAC) into an analog variable. The unit consisting of converters, comparing
element, and controlling element appears to the outside like an analog controller.
Preface Fundamentals Discontinuous Action Controller Controller Block (S)FB41 Setting the System Appendix
B3
Issued: 02/2008 Control Engineering with STEP 7