Siemens Module B3 manual Dac

Automation and Drives - SCE

The diagram below shows the layout of a digital controller:

Comparing	Digital
		DAC
			System
Element	Controller

ADC

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
	T I A Training Document		Page 30 of 64	Module
				B3
Issued: 02/2008				Control Engineering with STEP 7