Watlow Series 982 - Manual (Page 129)

WATLOW Series 982 User’s Manual 9.15 Software Features, Chapter 9
Software
V a lv e
Ac t ua t or
G a s - f ire d Furna c e
input 1
input 2
output 1 (close)
output 2 (open)
slidewire input
limit
gas
flow
gas
valve cut-off
valve
limit
sensor
temperature
sensor
TL
TL
W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT
DISPLAY
SERIES 982
MODE
HOLD
RUN
Figure 9.15 -
Gas-fired furnace
with slidewire feed-
back.
NOTE:
See Chapter 4 for
more infromation
about Slidewire
Hysteresis and
Hunt.
Sample Application
A Series 982 controls the gas valve for a gas-fired furnace to heat treat
large metal parts. First the controller must be “married” to the slidewire
feedback from the valve actuator. To do this, first set the Input 2 prompt
[`In2] to slidewire [SLid]. Advance to the Learn Low Resistance prompt
[LrnL]. Close the valve manually to the minimum resistance reading from
the slidewire. Select [`YES] in the upper display and press the Mode key
µ to advance to the Learn High Resistance prompt [LrnH]. Manually
open the valve (maximum slidewire resistance). Select [`YES] in the upper
display and press the Mode key µ. At this point both the high and low
resistance values have been learned and stored in the range low 2 and
range high 2 parameters.
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 [`rL2] and Range High [`rH2] prompts.
[`rH2] must be greater than [`rL2].
Once the control is operating, adjust the hunt [Hunt] parameter, to mini-
mize valve oscillations. The hunt parameter sets up a dead band on both
sides of the current valve position. The desired valve position is then com-
pared to the actual position. If the difference is greater than one-half of the
hunt value, the Series 982 repositions the valve to achieve the tempera-
ture set point. Once repositioning is complete, the dead band is recalculat-
ed for the new valve position. If your valve opens and closes quickly, try
increasing the [Hunt] value. If the valve’s momentum causes it to “coast”
after the contact opens, try increasing the {SHyS} value.