Watlow Series F4S/D - Manual (Page 68)

6 . 4 ■ F e a t u r e s Wa t l o w S e r i e s F 4 S / D
Control Methods
On-Off Control
On-off control switches the output either full on or
full off, depending on the input, set point and hys-
teresis values. The hysteresis value indicates the
amount the process value must deviate from the set
point to turn on the output. Increasing the value de-
creases the number of times the output will cycle.
Decreasing hysteresis improves controllability. With
hysteresis set to 0 the process value would stay clos-
er 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:
Operations Page > Edit PID > PID Channel x (1 or
2) > PID Set x (1 to 5) or (6 or 10).
Hysteresis x (A or B) location in software: Opera-
tions Page > Edit PID > PID Set Channel x (1 or 2)
> PID Set x (1 to 5) or (6 or 10).
✔ NOTE:
Fail power does not function in on/off control mode.
Figure 6.4a — 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
process value closer to the set point than on-off con-
trol can provide. Proportional control provides clos-
er control by adjusting the output when the tem-
perature 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 Channel x (1 or 2) > PID Set x (1 to 5) or (6
to 10).
Figure 6.4b — Proportional Control.
Time
Temperature
Set Point
Proportional Band Droop
Overshoot