44
Appendix; PI Regulation
For inverse control; modification of the setpoint and
feedback signal is required.
Example:
A transmitter has a 0-15 foot span equal to 0-20mA. The
PID set-point must be set at 12 feet for inverse control.
• Enter 20-0 mA in parameter 413. The normal signal in
figure A is converted to an inverse signal as shown in
figure B.
• Calculate the set point for inverse control.
Setpoint = 100 – (4/5 x 100) = 20%
It can be seen in figure C how the set point is calculated.
• If the internal set-point is used, ensure that parameter
412 (voltage input) is set to no operation.
• The values in parameter 115 & 116 must be reversed.
The display at min. feedback in parameter 115 becomes
display at max. feedback and parameter 116 becomes
display at min. feedback.
• Connections and set-up are identical to normal control.
0 12 15 Feet
Inverted signal from par:413 (current)
where 12 ft. represents 100%-80%=20%
Figure C.
Setpoint = 20%
% Setpoint
100
External Setpoint
55
60
0/4-20 mA transmitter
parameter 413 (0/4-20mA)
+
–
–
+
External setpoint
using potentiometer.
1 kOhm wire wound.
Parameter 412 (0-10V)
Link
VLT control terminals
12
27
16
18
50
53
55
Internal Setpoint
55
60
0/4-20 mA transmitter
parameter 413 (0/4-20mA)
–
+
Internal setpoint
parameter 205
Set parameter 412
to (no operation)
Link
Start/Stop
Parameter 402 (start)
VLT control terminals
12
27
16
18
50
53
55
Start/Stop
Parameter 402 (start)
20
mA
Normal signal from transmitter where
12 ft. represents 16mA (80%)
0 7.5 15 Ft
Figure A.
100
%
0 7.5 15 Ft
Figure B.
Inverted signal from par:413
12 ft. represents 100%-80%=20%
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