User guide

Watlow EZ-ZONE
®
PM PID Controller • 95 • Chapter 9 Features
On-Off Control (cont.)
Note:
Input Error Failure Mode [faIl] does not function in on-off control mode. The output goes off.
Proportional Control
Some processes need to maintain a temperature or process value closer to the set point than on-off control
can provide. Proportional control provides closer control by adjusting the output when the temperature 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 the process value is to the set point, the lower the output power. 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 simple on-off control. However, when the system settles down, the temperature
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 the proportional band value.
In an application with one
output assigned to heating and
another assigned to cooling, each
will have a separate proportional
parameter. The heating param- e-
ter takes effect when the process
temperature is lower than the set
point, and the cooling parameter
takes effect when the process tem-
perature is higher than the set
point.
Adjust the proportional band
with Heat Proportional Band
[`h;Pb] or Cool Proportional Band
[`C;Pb] (Operations Page, Loop Menu).
Hysteresis
Time
Temperature
The cooling action
switches
on at startup.
Process Temperature
The cooling action switches on when
the process temperature rises above
the set point plus the hysteresis.
Set Point
The cooling action switches off when the process
temperature drops below the set point.
Time
Temperature
Proportional Control
Set Point
Proportional Band
Droop
Overshoot