Watlow ez-zone pm - Example 9: Motorized Valve Control

Watlow EZ-ZONE
®
PM Integrated Controller • 223 • Chapter 10 Applications
Example 9: Motorized Valve Control
A typical scenario where a motorized valve is used is to regulate the ow of uid which in turn
impacts the loop process value. A valve is opened or closed by closing contacts to drive the value
in the intended direction. Motorized Valves come in a number of congurations. Some valves have
a position feedback mechanism that allows the control to measure the valve’s position via an
internal potentiometer called slide-wire. The controller can measure the potentiometer resis-
tance to determine the initial valve position on power up.
This method may not be desirable
for three reasons:
1) It requires a second input on the
controller to measure valve posi-
tion.
2) The controller and the valve are
more expensive.
3) Additional wiring is required for
the slide-wire feedback.
Other valves take an analog signal
and have a localized control mechanism that regulates the valve position. These are typically
more expensive valves because of the control mechanism built-in plus it requires an analog sig-
nal which is not always available. The actual valve position is not critical because it is a part of a
closed loop control.
The Motorized Valve control algorithm is also designed to work with a type of valve that provides
two discrete signals: one to open the valve and another to close the valve. The algorithm turns
on/off the appropriate signal for an appropriate amount of time to approximate the valve posi-
tion. This works when the valve is inside a closed control loop because when the valve is not in
the correct position, the PID algorithm will adjust the valve further open or close as needed.
These valves have travel limit switches which deactivates the motor once the valve is fully open
or fully closed so the controller can not cause the valve to over travel and burn out the motor, or
the motor is built so it can not overheat at max locked rotor amperes.
To use the motorized feature, the user programs the Special Output Function to Motorized Valve.
Then the Source Function A is selected to either
Heat or Cool Power and Source Instance A is set to
match the control loop, typically 1.
Next the user enters the amount of time in seconds
that the valve requires power to go from a closed
state to an open state. The user enters the dead
band in percent PID power to prevent the valve
from excessive cycling. Larger numbers reduce
activity on the valve and smaller numbers improve
controllability. Select a value that compromises on
these two competing goals.
Lastly, assign an output to Special Output Function 1 that is wired to close the valve. Assign an
output to Special Output Function 2 that is wired to open the valve. Typically, these two outputs
are normally open mechanical relays but solid state relays or switch DC outputs may be pro-
grammed in the same manner.
Control Loop 1
Analog Input 1
Source A
Close
Open
Output n
Output n
Heat Power
Source Function A
Special Output Function 1
Special Function
Output 1
Special Function
Output 2
Motorized Valve Control
Heat Algorithim
Set Point 1
Cool Algorithim
Valve Travel Time
Dead Band
Output 1 = Close
Temperature
Sensor
Gas Furnace
Output 2 = Open
Gas Flow
Valve
Actuator
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