Watlow EZ-ZONE
®
ST
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70
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Chapter 7 Features
Adjust the proportional band with Heat Propor-
tional Band
[`h;Pb]
or Cool Proportional Band
[`C;Pb]
(Operations Page, Loop Menu).
Time
Temperatur
e
Proportional Control
Set Point
Proportional Band
Droop
Overshoot
Proportional plus Integral (PI) Control
The droop caused by proportional control can be cor-
rected by adding integral (reset) 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 correc-
tion, but this may increase the overshoot at startup or
when the set point is changed. Too much integral ac-
tion will make the system unstable. Integral is cleared
when the process value is outside of the proportional
band.
Adjust the integral with Time Integral
[``ti]
(Op-
erations Page, Loop Menu).
Proportional plus Integral plus Derivative (PID)
Control
Use derivative (rate) control to minimize the overshoot
in a PI-controlled system. Derivative (rate) adjusts the
output based on the rate of change in the temperature
or process value. Too much derivative (rate) will make
the system sluggish.
Derivative action is active only when the process
value is within twice the proportional value from the
set point.
Adjust the derivative with Time Derivative
[``td]
(Operations Page, Loop Menu).
Time
Temperatur
e
PID Control
Set Point
Reduced Overshoot
Proportional Band
Proportional Band x 2
Heating Slows
Dead Band
In a PID application the dead bands above and below
the set point can save an application’s energy and
wear by maintaining process temperature within ac-
ceptable ranges.
Proportional action ceases when the process value
is within the dead band. Integral action continues to
bring the process temperature to the set point.
Using a
positive dead band value
keeps the two
systems from fighting each other.
Time
Temperature
Positive Dead Band
Set Point
Heat Output Active
Cool Output Active
When the
dead band value is zero
, the heating
output activates when the temperature drops below
the set point, and the cooling output switches on when
the temperature exceeds the set point.
Time
Temperature
Zero Dead Band
Set Point
Heat Output Active
Cool Output Active
When the
dead band value is a negative value,
both heating and cooling outputs are active when the
temperature is near the set point.
Time
Temperature
Negative Dead Band
Set Point
Heat Output Active
Cool Output Active
Adjust the dead band with Dead Band
[``db]
(Op-
erations Page, Loop Menu).
