Watlow Series F4S/D - Manual (Page 69)

Wa t l o w S e r i e s F 4 S / D F e a t u r e s ■ 6 . 5
Proportional plus Integral (PI) Control
The droop caused by proportional control (reset)
can be corrected by adding integral control. When
the system settles down the integral value is tuned
to bring the temperature or process value closer to
the set point. Integral determines the speed of the
correction, but this may increase the overshoot at
startup or when the set point is changed. Too much
integral action will make the system unstable. Inte-
gral is cleared when the process value is outside of
the proportional band.
Integral (if units are set to SI) is measured in min-
utes per repeat. A low integral value causes a fast
integrating action.
Reset rate (if units are set to U.S.) is measured in
repeats per minute. A high reset value causes a fast
integrating action.
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 6.5a — Proportional Plus Integral Control.
Time
Temperature
Set Point
Proportional Band Droop corrected
Droop
Proportional Integral Derivative (PID)
Control
Use derivative rate control to minimize overshoot
in a PI-controlled system. Derivative adjusts the
output based on the rate of change in the tempera-
ture or process value. Too much derivative will
make the system sluggish.
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 6.5b — PID Control.
Time
Temperature
Set Point
Reduced Overshoot
Proportional Band
Proportional Band x 2
Heating slows
Dead Band
In a multiple PID application the dead bands above
and below the set point can save an application's
energy and wear by maintaining process tempera-
ture within acceptable ranges. Shifting the effective
cooling set point and heating set point keeps the
two systems from fighting each other.
Proportional action ceases when the process value
is within the dead band. Integral action continues
to bring the process temperature to the set point.
When the dead band value is zero, the heating ele-
ment activates when the temperature drops below
the set point, and the cooling element switches on
when the temperature exceeds the set point.
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 6.5c — Cooling Dead Band.
Time
Temperature
Heating Set Point
Heating Proportional Band
Cooling Dead Band
Cooling Set Point
Cooling Proportional Band