Operation Using a Programmable Logic Controller (PLC)
Flow
Flow
Flow Control
Control
Control System
System
System
Overview
Overview
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 control system relies on two main inputs for
controlling flow rate: Gun Trigger and Control Set
Point.
NOTE:
NOTE:
NOTE: These
These
These inputs
inputs
inputs are
are
are timing
timing
timing critical.
critical.
critical. Graco
Graco
Graco
recommends
recommends
recommends that
that
that users
users
users wire
wire
wire them
them
them discretely
discretely
discretely to
to
to the
the
the
controller.
controller.
controller.
Alternatively these two inputs can be
driven by the network communications, but latency
could be an issue for systems requiring precise
timing.
See
, for more details on
configuring these options for ‘Discrete’ or ‘Network’.
NOTE:
NOTE:
NOTE: Flow control cannot be selected with a
manual gun system.
Normal
Normal
Normal Flow
Flow
Flow Control
Control
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 system is considered to be stable when the
pressure readings do not fluctuate and the flow rate
is maintained. While the system is considered stable
it will store (“learn”) the associated pump pressures
to a table that is used if the gun trigger signal is lost
or removed.
Pressure
Pressure
Pressure Control
Control
Control
When the gun trigger signal is removed the system
automatically switches to pressure control mode to
avoid over pressurizing the fluid lines and to allow
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
Gun
Gun On/Off
On/Off
On/Off Prediction
Prediction
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% or 50 psi higher than the desired pressure,
whichever is larger, for longer than 10 msec, then
the system predicts that the gun trigger has been
released. If the actual pressure drops below the
desired pressure longer than 10 msec, then the
system predicts that the gun has been triggered.
The gun on/off prediction is used in the flow control
algorithm to prevent the fluid pressure from becoming
too high or too low due to a system disturbance. For
example, if a gun off prediction occurs while the gun
trigger input is high, the system will begin to control
to the pressure value last stored in the pressure table
for the current flow set point.
System
System
System Startup
Startup
Startup and
and
and Defaults
Defaults
Defaults
The pressure table is reset for every mix cycle, or
after a power cycle of the ProMix PD2K controller.
This issue is not significant because the system
generally is able to recalculate new pressure table
values within a few seconds (depending on the
stability of the fluid system).
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