WEG CFW-11 Programming Manual page 244

PID Regulator [46]
3) To define the feedback input: the feedback (process variable measurement) is always done via one of
the analog inputs (selected at P0524). In order to make this guide easier, the AI2 input will be selected
(P0524=1).
4) To adjust the process variable scale: the transducer (sensor) to be used for the process variable feedback
must have a full scale of at least 1.1 times the highest value to be controlled.
Example: If a 20 bar pressure has to be controlled, a sensor with a full scale of at least 22 bar (1.1 x 20) must
be chosen.
Once the sensor has been defined, the type of signal to be read at the input (if current or voltage) must be
selected and the corresponding switch (S1 or S2) be set for that selection.
In this guide it will be assumed that the sensor signal will be 4 to 20 mA (configuring P0238 = 1 and
S1.3=ON).
Afterwards, the gain (P0237) and the offset (P0239) of the feedback signal can be adjusted so that the process
variable be read at the analog input with the highest possible resolution without saturation. In this case, adjust
the parameters P0237 and P0239, according to the following example.
Note!
In order to avoid the saturation of the feedback analog input during regulation overshoots, the signal
must range from 0 to 90 % (0 to 9 V / 4 to 18 mA). This adaptation can be done by changing the
gain of the analog input selected as feedback.
Example:
- Full scale of the transducer (maximum output value)=25 bar (FS = 25);
- Operation range (range of interest) = 0 to 15 bar (OR = 15).
Opting to maintain P0237 = 1.000 and P0239 = 0 (factory setting), which is the most common for most
of the applications:
- P0525 = 50 % (Keypad PID setpoint) will be equivalent to the used sensor full scale value, i.e., 0.5 x FS =
12.5 bar. Thus, the operation range (0 to 15 bar) represents 60 % of the setpoint.
If it is necessary to adjust P0237:
- Considering a gap of 10 % for the process variable measurement range (MR = 1.1 x OR = 16.5), it must
be adjusted at 0 to 16.5 bar. Therefore, the parameter P0237 must be set:
Thus, a setpoint of 100 % represents 16.5 bar, i.e., the operation range in percentage stays from 0 to 90.9 %
(OR=15/16.5).
20
If offset adjustment is necessary, the parameter P0239 must be configured according to the detailed description
of the section 13.1.1.
20-4
FS 25
P0237 = = = 1.52
MR 16.5