Watlow Controller Series D8, Руководство пользователя

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D8 Series User’s Guide
Chapter 4: Operation and Setup
80
Set Point of the Secondary Loop
(Engineering Units)
High Set Point
Low Set Point
0%
Output of Primary Loop (Percent of Full Scale)
100%
Figure 4.10 – Secondary Set Point When Primary Loop Has Heat Output Only
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 control. 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 process variable of the primary loop. See the example below.
How To Set Up Cascade Control
1. For the primary cascade loop:
– Configure proportional-only control. See the example below.
– Enter the desired set point. Changing the Set Point on page 66
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. Cascade Menu on page 128
NOTE! Cascade control cannot be used on the same control 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 element. The desired
temperature of the water is 150°F, which is measured at the outer thermocouple.