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Processing Analog Values
7
Processing Analog Values
7.1
Analog Modules
When you automate processes you will frequently need to acquire or control analog values
such as temperatures, pressures and filling levels. With the exception of the FX
additional modules the base units of the MELSEC FX family can only process digital input and
output signals (i.e. ON/OFF data). Additional analog modules are thus required for inputting
and outputting analog signals.
Basically, there are two different kinds of analog modules:
b Analog input modules and
b Analog output modules.
Analog input modules can acquire current, voltage and temperature values. Analog output
modules send current or voltage signals to the module's outputs. In addition to this there are
also combination modules, which can both acquire and output analog signals.
*
A FX
3GE
without additional modules.
Analog input modules
Analog input modules convert a measured analog value (e.g. 10 V) into a digital value (e.g.
4000) that can be processed by the PLC. This conversion process is known as analog/digital
conversion or A/D conversion for short.
Temperatures can be acquired directly by the analog modules of the MELSEC FX family but
other physical values like pressures or flow rates must first be converted into current or voltage
values before they can be converted into digital values for processing by the PLC. This conver-
sion is performed by sensors that output signals in standardised ranges (for example 0 to 10 V
or 4 to 20 mA). The measurement of a current signal has the advantage that the value is not fal-
sified by the length of the cables or contact resistances.
The following example of analog value acquisition shows a flow measurement solution with a
PLC of the MELSEC FX
Flow measurement device with
voltage or current output
e. g. 50 l/s
FX Beginners Manual
base unit can input two analog signals and output one analog signal (0–10 V or 4–20 mA in each case)
series.
3U
Voltage or
current
e. g. 5 V
or 12 mA
Analog input
module
Digital value
Analog/Digital
conversion
e. g. 2000
Analog Modules
*, without
3GE
Base unit of
FX
series
3U
7 – 1
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