Watlow Electric Integrated Controller User Manual Rev C EZ-ZONE PM - Manual (Page 63)

Wa t l o w E Z - Z O N E ® P M I n t e g r a t e d C o n t r o l l e r • 6 1 • C h a p t e r 9 F e a t u r e s
In an application with one output assigned to
heating and another assigned to cooling, each will
have a separate proportional parameter. The heating
parameter takes effect when the process temperature
is lower than the set point, and the cooling param-
eter takes effect when the process temperature is
higher than the set point.
Adjust the proportional band with Heat Pro-
portional Band [`h;Pb] or Cool Proportional Band
[`C;Pb] (Operations Page, Loop Menu).
Time
Temperature
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 cor-
rection, but this may increase the overshoot at start-
up or when the set point is changed. Too much inte-
gral action 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]
(Operations Page, Loop Menu).
Proportional plus Integral plus Derivative (PID)
Control
Use derivative (rate) control to minimize the over-
shoot in a PI-controlled system. Derivative (rate) ad-
justs 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
Temperature
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
Te
mperature
Positive Dead Band
Set Point
Heat Output Active
Cool Output Active
When the dead band value is zero, the heat-
ing output activates when the temperature drops be-
low the set point, and the cooling output switches on
when the temperature exceeds the set point.
Time
Te
mperature
Zero Dead Band
Set Point
Heat Output Active
Cool Output Active