Watlow CLS200 - Manual (Page 174)

Chapter 8: Tuning and Control CLS200 Series User’s Guide
158 Watlow Anafaze Doc.# 0600-3050-2000
Distributed Zero Crossing (DZC)
With DZC outputs, the PID algorithm calculates an output
between 0 and 100%, but the output is distributed on a
variable time base. For each ac line cycle, the controller de-
cides whether the power should be on or off. There is no
fixed cycle time since the decision is made for each line cy-
cle. When used in conjunction with a zero crossing device,
such as a solid state relay (SSR), switching is done only at
the zero crossing of the ac line, which helps reduce electri-
cal noise.
Using a DZC output should extend the life of heaters. Since
the time period for 60 Hz 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 and voltage applied to the heater as phase-an-
gle 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%. 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 sig-
nals. It has a range of 0 to 255 scans, which gives a time
constant of 0 to 170 seconds for a CLS216, 0 to 85 seconds
for a CLS208 or 0 to 43 seconds for a CLS204. Use the out-
put filter if you need to filter out erratic output swings due
to extremely sensitive input signals, like a turbine flow sig-
nal or an open air thermocouple in a dry air gas oven.
The output filter can also enhance PID control. Some pro-
cesses are very sensitive and would otherwise require a
large proportional band, making normal control methods
ineffective. Using the output filter allows a smaller propor-
tional 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.