Watlow Series D8 - Manual (Page 138)

Chapter 5: Tuning and Control Series D8 User’s Guide
120 Watlow Anafaze Doc. 0600-3120-2000
Output Filter
The output filter digitally smooths PID control output signals.
It has a range of 0 to 255 scans, which gives a time constant
of 0 to 85 seconds for a CPC408 or 0 to 43 seconds for a
CPC404. Use the output filter if you need to filter out erratic
output swings due to extremely sensitive input signals, like a
turbine flow signal or an open air thermocouple in a dry air
gas oven.
The output filter can also enhance PID control. Some process-
es are very sensitive and would otherwise require a large pro-
portional band, making normal control methods ineffective.
Using the output filter allows a smaller proportional band to
be used, achieving better control.
Also, use the filter to reduce the process output swings and
output noise when a large derivative is necessary, or to make
badly tuned PID loops and poorly designed processes behave
properly.
Reverse and Direct Action
With reverse action an increase in the process variable causes
a decrease in the output. Conversely, with direct action an in-
crease in the process variable causes an increase in the output.
Heating applications normally use reverse action and cooling
applications usually use direct action.