Example 8: Motorized Valve Control - Watlow PM PLUS 6 User Manual

Example 8: Motorized Valve Control

A typical scenario where a motorized valve is used is to regulate the flow of fluid which impacts the loop pro-
cess value. A valve is opened or closed by closing contacts to drive the value in the intended direction. Motor-
ized Valves come in a number of configurations. Some valves have a position feedback mechanism that allows
the control to measure the valve's position via an internal potentiometer called slide-wire. Other valves take an
analog signal and have a localized control mechanism that regulates the valve position. The actual valve
position is not critical because it is a part of a closed loop control.
The controller can measure the potentiometer resistance 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 con-
troller to measure valve position.
2. The controller and the valve are more
expensive.
3. Additional wiring is required for the
slide-wire feedback.
The Motorized Valve control algorithm is designed to work with a type of valve that provides two discrete sig-
nals: 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 position. 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:
1. Program the Special Output Function to Motorized
Valve.
2. Set Source Function A to either Heat or Cool Power
and set Source Instance A to match the control loop,
typically 1.
3. Enter the amount of time in seconds that the valve
requires power to go from a closed state to an open
state. 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.
4. 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 programmed in the same manner.
Definitions:
• Current Position is an approximation of the valve's position as it relates to a power level (0 - 100%) where
0% is fully closed and 100% is fully open.
• Dead Time is the minimum on time that the valve will travel once it is turned on in either the closed or
open direction. Dead Time = Valve Dead Band / 100 * Valve Travel Time.
Watlow PM PLUS™ 6
Motorized Valve Control
Analog Input 1 Source A
Control Loop 1
• 110 •
Heat Power
Source Function A
Special Output Function 1
®
Output 1 = Close
Valve
Actuator
Output 2 = Open
Gas Flow
Chapter 9: Applications
Output n
Special Function
Output 1
Close
Open
Special Function
Output 2
Output n
Gas Furnace
Temperature
Sensor