Siemens Module B3 manual Digital Controllers

Automation and Drives - SCE

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:

Comparing Analog

ElementControllerSystem

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
	T I A Training Document		Page 29 of 64	Module
				B3
Issued: 02/2008				Control Engineering with STEP 7