Watlow F4P Series, User Manual, Page: 64 / 124

7 . 6
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F e a t u r e s Wa t l o w S e r i e s F 4 P
On-Off Control
On-off control switches the output either full on or full off,
depending on the input, set point and hysteresis values.
The hysteresis value indicates the amount the process
value must deviate from the set point to turn on the out-
put. Increasing the value decreases the number of times
the output will cycle. Decreasing hysteresis improves con-
trollability. With hysteresis set to 0 the process value
would stay closer to the set point, but the output would
switch on and off more frequently, causing “chattering.”
Set the proportional band to 0 to set the controller to on-
off control mode.
Proportional Band x (A or B) location in software: Opera-
tions Page > Edit PID > PID Set x (1 to 5).
Hysteresis x (A or B) location in software: Operations
Page > Edit PID > PID Set x (1 to 5).
✔
NOTE:
Fail power does not function in on-off control mode.
Figure 7.6a — On-Off Control for Heating and Cooling.
Set Point
Time
Temperature
The heating action switches off when the process
temperature rises above the set point.
The heating action
switches on at startup.
Hysteresis
Process Temperature
Hysteresis
Time
Temperature
The cooling action
switches
on at startup.
Process Temperature
The cooling action switches on when
the process temperature rises above
the set point plus the hysteresis.
Set Point
The heating action switches on when the process temperature
drops below the set point minus the hysteresis.
The cooling action switches off when the process
temperature drops below the set point.
Proportional Control
Some processes need to maintain a temperature or pro-
cess value closer to the set point than on-off control can
provide. Proportional control provides closer control by
adjusting the output when the temperature or process
value is within a proportional band. When the value is in
the band, the controller adjusts the output based on how
close the process value is to the set point; the closer to
set point the lower the output. This is similar to backing
off on the gas pedal of a car as you approach a stop sign.
It keeps the temperature or process value from swinging
as widely as it would with simple on-off control. However,
when a system settles down, the temperature or process
value tends to “droop” short of the set point.
With proportional control the output power level equals
(set point minus process value) divided by propband.
Location in software: Operations Page > Edit PID > PID
Set x (1 to 5).
Figure 7.6b — Proportional Control.
Time
Te
mperature
Set Point
Proportional Band
Droop
Overshoot