User Manual

LOGO! functions
4.4 Special functions list - SF
LOGO!
190 Manual, 06/2014, A5E33039675
Functional description
The function fetches the analog values from the inputs Ax and Ay.
Ax and Ay are each multiplied by the value of the A (gain) parameter, and the value at
parameter B (offset) is then added to the relevant product, i.e.
(Ax • gain) + offset = actual value Ax or
(Ay • gain) + offset = actual value Ay.
The function forms the difference ("Δ") between the actual values Ax - Ay.
Output Q is set or reset, depending on difference of the actual values Ax - Ay and the set
thresholds. See the calculation rule below.
Calculation rule
● If the On threshold ≥ Off threshold, then Q = if (actual value Ax - actual value Ay) > On or
Q = 0 if (actual value Ax - actual value Ay) ≤ Off.
● If the On threshold < Off threshold, then Q = 1, if On ≤ (actual value Ax - actual value Ay)
< Off.
Setting the Par parameter
The gain and offset parameters are used to adapt the sensors to the relevant application.
View in programming mode:
Example
In a heating control system, the supply T
v
and return line temperatures T
r
are to be
compared, for example with a sensor at AI2.
A control signal is to be triggered (for example "heater On") when the difference between the
supply and return line temperatures is greater than 15 °C. The control signal is reset when
the difference is less than 5 °C.
The process variable of the temperature is to be shown in parameter assignment mode.
The thermocouples available have the following technical data: -30 °C to +70 °C, 0 VDC to
10 VDC.
Application
Internal mapping
-30 °C to 70 °C = 0 VDC to 10 VDC
0 to 1000
0 °C 300
→ Offset = -30