D8 Series User’s Guide
Chapter 5: Tuning and Control
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The combination of DZC output and a solid state relay can inexpensively approach the effect of
analog, phase-angle fired control. Note, however, DZC switching does not limit the current and
voltage applied to the heater as phase-angle firing does.
Three-Phase Distributed Zero Crossing (3P DZC)
This output type performs exactly the same as DZC except that the minimum switching time is three
ac line cycles. This may be advantageous in some applications using three-phase heaters and three-
phase power switching.
Analog Outputs
For analog outputs, the PID algorithm calculates an output between 0 and 100 percent. This
percentage of the analog output range can be applied to an output device via a Dual DAC or a
Serial DAC.
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 an eight-loop controller or 0 to 43 seconds for a four-
loop controller. Use the output filter if you need to filter out erratic output swings due to extremely
sensitive input signals, such as a turbine flow signal or an open air thermocouple in a dry air gas
oven.
The output filter can also enhance PID control. Some processes are very sensitive and would
otherwise require a large proportional 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 increase in the process variable causes an increase in the output. Heating
applications normally use reverse action and cooling applications usually use direct action.
