Watlow Series 96 - Manual (Page 25)

Wa t l o w S e r i e s 9 6 F e a t u r e s ■ 5 . 6
Proportional plus Integral (PI) Control
The droop caused by proportional control can be cor-
rected by adding integral (reset) control to the sys-
tem. When the system has settled down the integral
(reset) value is tuned to bring the temperature or
process value closer to the set point. Integral (reset)
determines the speed of the correction. However,
this may increase the overshoot that occurs at start-
up or when the set point is changed. Too much inte-
gral (reset) action will make the system
unstable.
Integral (reset) is cleared when the process value is
outside of the proportional band.
Reset is measured in repeats per minute. A low
reset value causes a slow integrating action.
Integral is measured in minutes per repeat (the
inverse of reset). A low integral value causes a fast
integrating action.
View or change the Output 1 integral or reset value
with Integral 1 [It1`] or Reset 1 [rE1`] (PID 1
Menu). Integral appears if Units Type [Unit]
(Global Menu) is set to [``S1]. Reset appears if
[Unit] is set to [``US].
Integral Value Equivalent Reset Value
1 minute 1 repeat per minute
2 minutes 0.5 repeats per minute
3 minutes 0.33 repeats per minute
4 minutes 0.25 repeats per minute
Table 5.6 – Converting between integral and reset values.
Figure 5.6b Proportional plus integral control.
Time
Temperature
Set Point
Proportional Band Droop corrected
Droop
Proportional Control
Some processes need to maintain a temperature or
process value closer to the set point than an on/off
control can provide. Proportional control provides
closer 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 swing-
ing as widely as it would with a simple on/off con-
trol. However, when a system settles down, the tem-
perature 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.
The PID 1 proportional band can be viewed or
changed with Propband 1 [Pb1`] (PID 1 Menu).
Figure 5.6a Proportional control.
Time
Temperature
Set Point
Proportional Band Droop
Overshoot