Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.1 Overview
Analog value processing
10
Function Manual, 06/2014, A5E03461439-AC
Transducers
Controllers are only capable of processing analog values in the form of bit patterns. For this
purpose, transducers which can be connected to the analog module measure physical
variables such as pressure or temperature. The analog input module reads this analog value
in the form of current, voltage or resistance. To enable the CPU to process the current or
voltage measured, an integrated analog-to-digital converter in the analog input module
converts the value into a 16-bit integer value. Depending on the type of measurement, you
can use the following transducers:
●
Voltage transducers
● Current transducers
–
2-wire transducers
– 4-wire transducers
● Resistance transducers
–
4-wire connection
– 3-wire connection
– 2-wire connection
● Thermocouples
Additional information on wiring various types of transducers to analog inputs can be found
in Wiring transducers (Page 65).
Example
Speed is acquired using a transducer which converts the speed range of 0 to 1500 rpm to a
voltage range of 1 to 5 V. At a measured speed of 865 rpm, the transducer outputs a value
of 3.3 V.
The resulting voltage value is calculated as follows:
The graph below illustrates voltage acquisition:
Figure 2-1 Speed measurement