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Operation Using
a Programmable Logic Controller (PLC)
Flow Control S
ystem
Overview
Flow control is an optional feature that precisely
regulates the flow of material to an automatic spay
device, to help ensure adequate coverage and avoid
sags or runs in the finish coat. The ProMix PD2K
system can control fluid flow by directly controlling
the proportioning pumps. The pumps accurately
dispense a fixed volume of fluid during each stroke.
For this reason, the flow rate of a given pump is
directly proportional to the velocity of the pump. As
long as the gun is open and the system is stable, flow
control is the most effective method for controlling
flow rate.
The flow con
trol system relies on two main inputs for
controll
ing flow rate: Gun Trigger and Control Set
Point.
NO
TE: These inputs are timing critical. Graco
recommen
ds that users wire them discretely to the
controll
er.
Alternatively these two inputs can be
driven b
y the network communications, but latency
could be
an issue for systems requiring precise
timing.
See System Screen 5, page 71, for more details on
configuring these options for ‘Discrete’ or ‘Network’.
NOTE: Flow control cannot be selected with a
manual gun system.
Normal Flow Control
The ProMix PD2K will directly control the speed of
the pump(s) to the programmed flow control set point
to maintain accurate flow rate and ratio. The flow
control set point is set by Network Communications
or the Discrete Input.
The s
ystem is considered to be stable when the
pres
sure readings do not fluctuate and the flow rate
is ma
intained. While the system is considered stable
it wi
ll store (“learn”) the associated pump pressures
to a
table that is used if the gun trigger signal is lost
or r
emoved.
Pressure Control
When the gun trigger signal is removed the system
automatically switches to pressure control mode to
avoidoverpressurizingthefluidlinesandtoallow
smooth transition to flow control if the gun trigger
signal returns. It also works to maintain a consistent
flow rate even though it has transitioned to pressure
control mode if the gun trigger signal is inadvertently
lost.
Gun On/Off Prediction
The pressure table also is used to predict if the gun
has been turned on or off (without a change to the
gun trigger input). The flow control system continually
monitors the desired outlet pressure compared to the
actual outlet pressure. If the actual pressure remains
50% higher than the desired pressure for longer than
10msec, then the system predicts that the gun trigger
has been released. If the actual pressure drops
below the desired pressure longer than 10msec, then
the system predicts that the gun has been triggered.
The gun o
n/off prediction is used in the flow control
algori
thmtopreventthefluidpressurefrombecoming
too hig
h or too low due to a system disturbance. For
exampl
e, if a gun off prediction occurs while the gun
trigge
r input is high, the system will begin to control
to the p
ressure value last stored in the pressure table
for th
e current flow set point.
Syste
m Startup and Defaults
The pr
essuretableisstoredinvolatilememory,so
the t
able values will be lost after a power cycle of the
ProM
ix PD2K controller. This issue is not significant
beca
use the system generally is able to recalculate
new p
ressure table values within a few seconds
(dep
ending on the stability of the fluid system).
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