Operation Using a Programmable Logic Controller (PLC)
Flow Control System
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: These inputs are timing critical. Graco recommends that users wire them discretely to the controller. Alternatively these two inputs can be driven by 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 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 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 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 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 Startup and Defaults
The pressure table is stored in volatile memory, so the table values will be lost 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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