Emerson Vilter Vission 20/20 Operation And Service Manual page 52

even further.
Liquid Injection Control using a 4-20ma motorized valve.
• When a motorized valve is used to control the amount
of liquid being injected into the compressor the pre-
viously mentioned setpoints have a slightly different
function. The Oil Sep. Temp. Override is still used in
controlling the injection solenoid, however the Liquid
Inj. Setpoint 1 is now used as the target temperature
for the PID Algorithm that controls the position of
the motorized valve. The algorithm compares the
actual discharge temperature against the Liquid Inj.
Setpoint 1. The difference between these is the error.
The PID algorithm tries to drive the error to "zero" by
moving the positioning valve to allow more or less
liquid refrigerant to be injected into the compressor.
• PID algorithm can be notoriously hard to tune. As
a result the Vission 20/20 offers a couple of addi-
tional features to help control wild fl uctuations in
oil temperatures that could result in the compres-
sor tripping off. The operator can choose to enable
the minimum value position that automatically sets
the liquid injection motorized value to the specifi ed
value whenever the discharge temperature has fallen
below the Liquid inj. Setpoint 1. This feature nearly
eliminates the overshoot of the PID in the downward
direction and reduces the chance of the compressor
tripping off due to low oil temperature. The operator
can also choose to use an average of the discharge
temperature and the oil manifold temperature as the
control variable. The discharge temperature can vary
quite drastically forcing the PID algorithm to drasti-
cally adjust the motorized value. By averaging the
more stable oil manifold temperature and discharge
temperature, the control variable stabilizes and the
PID is more easily tuned.
• Please note that as stated above, PID algorithms can
be diffi cult to tune and there is no one set of PID val-
ues that will work. The work required for a compres-
sor to meet the requirement of its installation vary
greatly and therefore the amount of heat transfered
to the oil varies just as greatly. We recommend the
operator consult PID tuning guides available from
many different source before attempting to tune this
PID.
Liquid Inj. Setpoint 1:
• Setpoint at which the liquid solenoid will activate if in
solenoid control or if the setting for the control vari-
able for the PID is in liquid motorized value control.
Liquid Inj. Setpoint 2:
• Not yet available
6 – 12
Oil Sep. Temp. Override:
• Defi nes the temperature the oil must reach before the
liquid injection solenoid is allowed to be activated.
P = Proportional (Gain):
• Used to adjust the positioning valve in direct propor-
tion to the difference between the control setpoint
and the discharge temperature (SetPt - DT = error).
The proportional term is a unit-less quantity and is
used for coarse adjustment. This setpoint should be
set to the lowest value that gives adequate response
to the control system. Increasing the proportional
setting increases the control system's sensitivity to
small discharge temperature fl uctuations and the
tendency to hunt.
I = Integral (reset):
• This parameter integrates the error over time, to ac-
count for a small error that has persisted for a long
time. This quantity is used for fi ne adjustment. This
setpoint is used to smooth out discharge tempera-
ture variations. This setpoint should be set high
enough to prevent hunting but not too high or it will
cause control system overshoot.
D = Derivative (rate):
• This parameter accounts for how fast the error is
changing, positively or negatively.
Minimum Valve Open %:
• When enabled, this is the valve position used when-
ever the control variable drops below Liquid inj.
Setpoint 1. Use only if the compressor is tripping off
for low oil temperature due to large overshoots and
all other tuning methods have failed.
Avg. with Oil Manifold Temperature:
• When enabled, averages the Oil manifold tempera-
ture and the discharge temperature. This creates
a more stable control variable and should result in
more stable control.
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 1.9