Emerson Vission 20/20 Operation And Service Manual page 212

Appendix B • Vission 20/20 Application Procedures
Step 6: Setup of Analog Output for Vessel Level
Control – PID Level Control
Navigate to Auxiliary I/O page 6 and setup analog output
control for vessel level.
CHILLER LEVEL CONTROL
Suppose we are trying to maintain a level of liquid in a
chiller. As the level decreases, we want to stroke a posi-
tioning valve "more open" to allow more liquid to feed
into the chiller.
In the example in Figure B-34, examine the setup of
Aux1: Chiller Level 4,20 Output - on the left side of the
screen. The "Run Always" selection box is not checked,
so the control of the positioning valve will only occur
while the compressor is running.
For the setup in Figure B-34, the Auxiliary #1 analog out-
put signal (which is the 4-20 ma signal to the chiller Level
positioning valve ) is controlled by the Auxiliary Input
"Chiller Level 4,20 input" - which was confi gured in Steps
B - 56
Figure B-34. PID Level Control (Auxiliary I/O – Page 6)
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0
3, 4 and 5 above. PID Control has been selected, with a
50% setpoint.
The "Trigger Input" is enabled and the trigger setpoint
is set at 50% (same as the setpoint). When the trigger
input conditions go to a "true" state ( in this case, the
trigger goes to a true state when the chiller level drops
BELOW the setpoint), only then will the auxiliary analog
output control be enabled. In the above example, when
the chiller level is above 50%, the positioning valve will
be fully closed. As it drops below 50%, then the position-
ing valve will begin to open.
The PID setpoints are selected so that only the P term
(gain) is being used. With these settings, the positioning
valve will be adjusted in response to the "error" from the
desired setpoint.