Watlow CPC400 - Manual (Page 85)

CPC400 Series User’s Guide Chapter 3: Operation and Setup
Doc. 0600-2900-2000 Watlow Anafaze 71
Proportional-Only Control on the Primary Loop
The PID parameters of the primary loop must be tuned to
produce the desired effect on the set point of the secondary
loop. The primary loop typically uses proportional-only con-
trol. Disabling the integral and derivative components of
PID makes the secondary set point a predictable function
of the primary loop’s process variable.
The proportional band is selected so that the set point of
the secondary loop has the desired relationship to the pro-
cess variable of the primary loop. For an example, see Cas-
cade Control Example: Water Tank on page 71.
How To Set Up Cascade Control
1. For the primary cascade loop:
• Configure proportional-only control. For an ex-
ample, see Cascade Control Example: Water Tank
on page 71.
• Enter the desired set point. See Changing the Set
Point on page 54.
2. For the secondary cascade loop:
• Set up PID control as you would for a standard
closed-loop application.
• Set up the parameters in the Cascade menu. See
Cascade Menu on page 127.
NOTE! Cascade control cannot be used on the same con-
trol loop as ratio control.
Cascade Control Example: Water Tank
A tank of water has an inner and outer thermocouple. The
outer thermocouple is located in the center of the water.
The inner thermocouple is located near the heating ele-
ment. The desired temperature of the water is 150°F,
which is measured at the outer thermocouple.
Using cascade control, the outer thermocouple is used on
the primary loop (in this example, loop 1), and the inner
thermocouple is used on the secondary loop (loop 2). The
heater is controlled by loop 2.
As the temperature of the inner thermocouple drops from
150 to 140°F, the set point of the secondary loop should rise
from 150 to 190°F.
Table 3.10 and Table 3.11 show the setup for this applica-
tion.