Watlow PM PLUS 6, Руководство пользователя

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Watlow PM PLUS™ 6 • 90 • Chapter 8: Features
Time
Temperature
The heating action switches off when the process
temperature rises above the set point .
The heating action
switches on at startup .
On/Off Hysteresis
The heating action switches on when the process temperature
drops below the set point minus the hysteresis .
On/Off System Cycles
Set Point
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 .
The cooling action switches off when the process
temperature drops below the set point .
Set Point
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 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 swinging 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 the set point minus the process value divided by
proportional band times 100. 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 parameter takes effect when the process
temperature is higher than the set point.
Adjust the proportional band with Heat Proportional Band or Cool Proportional Band (
Operations — Setup -
Control Loop
).
Time
Temperature
Proportional Control
Set Point
Proportional Band
Droop
Overshoot
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. Integral 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. Adjust the integral
with Time Integral (
Operations — Setup - Control Loop
).