Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 33)

General Software Features
3.13
Optimizing Your Process System with the WATLOW Series 988
Slidewire Feedback
Overview
The 988 can control the position of a valve with
a slidewire feedback position indicator. The 988
senses the resistance of the slidewire and
compares it to the range low and range high
settings to determine the valve position. The
controller compares this to the percent output
and takes action to match the two by opening
or closing the valve.
Set the hunt parameter to limit valve
hunting. The value is set for the percent of
output (0.0 to 100.0). When the valve is within
Sample Application
The 988 controls the gas valve for a gas-fired furnace to heat treat large metal
parts. First the 988 must be “married” to the slidewire feedback from the
valve actuator. To do this, first set the input 2 prompt
to slidewire .
Advance to the learn low resistance value prompt . Close the valve
manually to the minimum resistance reading from the slidewire. Select
in the upper display and press the MODE key to advance to the learn high
resistance value prompt
. Manually open the valve (maximum slidewire
resistance). Select in the upper display and press the MODE key. At this
point both the high and low resistance values have been learned and stored in
the range low 2
and range high 2 prompts.
You can also manually set the range low and range high values. From the
slidewire specifications, determine the low and high resistance values and
enter these at the range low
and range high prompts.
Once the control is operating, adjust the hunt parameter, to minimize
valve oscillations. The hunt parameter sets up a dead band on both sides of
the current valve position. The desired valve position is then compared to the
actual position. If the difference is greater than the one-half of the hunt value,
the 988 repositions the valve to achieve the temperature set point. Once repo-
sitioning is complete, the dead band is recalculated for the new valve position.
this dead band, a change in output greater
than half the hunt parameter is required to
trigger action. Output 1 responds to “close”
commands and output 2 responds to “open”
commands.
Requirements
A slidewire configuration uses at least two
inputs and two control outputs. Input 2 must
be a slidewire input. Outputs must be compat-
ible with the slidewire valve actuators.
TL TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
temperature
sensor
limit
sensor
input 2
slidewire input
gas
flow
Gas-fired Furnace
limit
input 1 output 1 (close)
output 2 (open)
Valve Actuator
cut-off
valve gas
valve