Expansion modules
55
7.1.1 Conversion of the analogue values to an input range
of 4 to 20 mA
Many analogue sensors use a range from 4 to 20 mA. The main
advantage of these sensors is the easy detection of cable
breaks, because in case of a cable break the current is < 4 mA.
The conversion into real values is done via process variables.
Example:
Pressure sensor, range from 0 to
6000
mbar at channel 1 of the internal analogue
input.
Dmin = 0 mbar => 4 mA;
Dmax
=
6000
mbar => 20 mA
Maximum value of the analogue input: 1994 (raw value) or 2000 (scaled value, see 7.1)
Scaled values from 0 to
2000
thus correspond to 0 to 20 mA
Scaled values from 0.2*
2000
to 2000 then correspond to 4 to 20 mA
AI =
400
(at 4 mA)
AI =
2000
(at 20 mA)
Calculation of the scaling factor
AIcorr = AI-400 AIcorr_max =
2000
-
400
= 1600
Factor
=
Dmax
/AIcorr_max =>
Factor
=
6000
/1600 =
3.75
Calculation of the actual pressure value
Pressure
= (
AI
- 400) *
Factor
Содержание 50.0070.0001.00
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