Siemens LOGO!, Manual

Page: 154 / 534

LOGO! functions
4.3 Special Functions
LOGO!
154
Manual, 04/2011, A5E03556174-01
4.3.6 Calculating the gain and offset of analog values
A sensor is connected to the analog input and converts a process variable into an electrical
signal. This value of signal lies within the typical range of this sensor.
LOGO! always converts the electrical signals at the analog input into digital values from 0 to
1000.
A voltage of 0 to 10 V at input AI is transformed internally into a range of values from 0 to
1000. An input voltage exceeding 10 V is shown as internal value 1000.
Because you cannot always process the range of values from 0 to 1000 as predetermined by
LOGO!, you can multiply the digital values by a gain factor and then shift the zero of the
range of values (offset). This allows you to output an analog value to the LOGO! onboard
display, which is proportional to the actual process variable.
Parameter Minimum Maximum
Input voltage (in V) 0 ≥ 10
Internal value 0 1000
Gain -10.00 +10.00
Offset -10000 +10000
Mathematical rule
Actual value Ax =
(internal value at input Ax • gain) + offset
Gain and offset calculation
The gain and offset is calculated based on the relevant high and low values of the function.
Example 1:
The available thermocouples have the following technical data: -30 to +70 °C, 0 to 10 V DC
(i.e. 0 to 1000 in LOGO!).
Actual value = (internal value • gain) + offset, thus
-30 = (0 • A) + B, i.e. offset B = -30
+70 = (1000 • A) -30, i.e. gain A = 0.1
Example 2:
A pressure sensor converts a pressure of 1000 mbar into a voltage of 0 V, and a pressure of
5000 mbar into a voltage of 10 V.
Actual value = (internal value • gain) + offset, thus
1000 = (0 • A) + B, i.e. offset B = 1000
5000 = (1000 • A) +1000, i.e. gain A = 4