Watlow CPC400 - Manual (Page 102)

Chapter 4: Tuning and Control CPC400 Series User’s Guide
88 Watlow Anafaze Doc. 0600-2900-2000
Control Outputs
The controller provides open collector outputs for control.
These outputs normally control the process using solid-
state relays.
Open collector outputs can be configured to drive a serial
digital-to-analog converter (Serial DAC) which, in turn,
can provide 0 to 5V
Î (dc), 0 to 10 V Î (dc) or 4 to 20 mA con-
trol signals to operate field output devices.
Output Control Signals
The following sections explain the different control output
signals available.
On/Off
When on/off control is used, the output is on or off depend-
ing on the difference between the set point and the process
variable. PID algorithms are not used with on/off control.
The output variable is always off or on (0 or 100 percent).
Time Proportioning (TP)
With time proportioning outputs, the PID algorithm calcu-
lates an output between 0 and 100 percent, which is repre-
sented by turning on an output for that percent of a fixed,
user-selected time base or cycle time.
The cycle time is the time over which the output is propor-
tioned, and it can be any value from 1 to 255 seconds. For
example, if the output is 30 percent and the cycle time is
ten seconds, then the output will be on for three seconds
and off for seven seconds. Figure 4.5 shows examples of
time proportioning and distributed zero crossing (DZC)
waveforms.
Figure 4.5 Time Proportioning and Distribut-
ed Zero Crossing Waveforms
Time Proportioning (30%)
On
Off
0 3 10
Seconds
(Cycle Time = 10)
Distributed Zero
0 1 3 4 6
AC Cycle
Crossing (33%)