Watlow EZ-ZONE
®
PM Integrated Controller
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133
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Chapter 9 Features
Proportional and (P) Control
(cont.)
than the set point, and the cooling parameter takes effect when the process temperature is higher than
the set point.
Adjust the proportional band with Heat Proportional Band
[`h;Pb]
or Cool Proportional Band
[`C;Pb]
(Op-
erations Page, Loop Menu).
Proportional and Integral (PI) Control
The droop caused by proportional control can be corrected 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. In-
tegral 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. Integral is cleared when the pro-
cess value is outside of the proportional band.
Adjust the integral with Time Integral
[``ti]
(Operations Page, Loop Menu).
Proportional, Integral and 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).
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 acceptable 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
PID Control
Set Point
Reduced Overshoot
Proportional Band
Proportional Band x 2
Heating Slows
Time
Temperature
Positive Dead Band
Set Point
Heat Output Active
Cool Output Active