Watlow EZ-ZONE PM Integrated Controller - Manual (Page 135)

Watlow EZ-ZONE ® PM Integrated Controller • 132 • Chapter 9 Features
Auto (closed loop) and Manual (open loop) Control (cont.)
You can easily switch between modes if the Control Mode [`C;M] parameter is selected to appear in the
Home Page.
To transfer to manual mode from auto mode, press the Advance Key ‰ until [`C;M] appears in the
lower display. The upper display will display [AUto] for auto mode. Use the Up ¿ or Down ¯ keys to select
[Man]
. The manual set point value will be recalled from the last manual operation.
To transfer to auto mode from manual mode, press the Advance Key ‰ until [`C;M] appears in the lower
display. The upper display will display [MAn] for manual mode. Use the Up ¿ or Down ¯ keys to select
[AUto]
. The automatic set point value will be recalled from the last automatic operation.
Changes take effect after three seconds or immediately upon pressing either the Advance Key ‰ or the
Infinity Key ˆ.
On-Off Control
On-off control switches the output either full on or full off, depending on the input, set point and hysteresis
values. The hysteresis value indicates the amount the process value must deviate from the set point to turn
on the output. Increasing the value decreases the number of times the output will cycle. Decreasing hyster-
esis improves controllability. With hysteresis set to 0, the process value would stay closer to the set point, but
the output would switch on and off more frequently, and may result in the output “chattering.” On-off control
can be selected with Heat Algorithm [`h;Ag] or Cool Algorithm [`C;Ag] (Setup Page, Loop Menu). On-off hys-
teresis can be set with Heat Hysteresis [`h;hY] or Cool Hysteresis [`C;hY] (Operations Page, Loop Menu).
Note:
Input Error Failure Mode
[faIl]
does not function in on-off control mode. The output goes off.
Proportional and (P) 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 tem-
perature or process value is within a proportional
band. When the value is in the band, the control-
ler adjusts the output based on how close the pro-
cess 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 ap-
proach a stop sign. It keeps the temperature or
process value from swinging as widely as it would
with simple on-off control. However, when the sys-
tem settles down, the temperature or process val-
ue tends to “droop” short of the set point.
With proportional control the output power lev-
el 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 parameter takes effect when the process temperature is lower
Set Point
Time
Temperature
The heating action switches off when the process
temperature rises above the set point.
The heating action
switches on at startup.
Hysteresis
Process Temperature
The heating action switches on when the process temperature
drops below the set point minus the hysteresis.
On/Off System Cycles
Hysteresis
Time
Temperature
On/Off System Cycles
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