Watlow PPC-2000 - Manual (Page 253)

PPC-2000 User’s Guide Chapter 6: Tuning and Control
Doc.# 30002-00 Rev 2.3 Watlow Anafaze 225
Distributed Zero Crossing (DZC)
With DZC outputs, the PID algorithm calculates an output
between 0 and 100%, but the output is switched on a variable
time base. For each AC line cycle the controller decides whether
the power should be on or off. There is no fixed cycle time since
the decision is made for each line cycle. When used in
conjunction with a zero crossing device, such as an SSR,
switching is done only at the zero crossing of the AC line, which
helps reduce electrical noise.
Using a DZC output should extend the life of heaters. Since the
time period for 60Hz power is 16.6 ms, the switching interval is
very short and the power is applied uniformly. DZC should be
used with SSRs. Do not use DZC output for electromechanical
relays.
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 or voltage applied to the heater as phase-angle firing
does.
Analog Outputs
For analog outputs, the PID algorithm calculates an output
between 0 and 100%. This percentage of the analog output
range can be applied to an output device via an analog module
or a digital module in conjunction with an SDAC.
Output Filter
The output filter smooths PID control output signals. It has a
range of 0-255 scans, which gives a time constant of 0-170
seconds for the PPC. 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 processes
are very sensitive and would otherwise require a large PB,
making normal control methods ineffective. Using the output
filter allows a smaller PB to be used, achieving better control.
Also, use the digital 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.