Setting up cascade control, How the secondary set point is determined, Setting up cascade control 100 – Watlow Series D8 User Manual
Page 118: How the secondary set point is determined 100
Chapter 4: Operation and Setup
Series D8 User’s Guide
100
Watlow Anafaze
Doc. 0600-3120-2000
To complete this configuration, the output for loop 2 must be
configured to provide the 4 to 20 mA analog signal (via the
Serial DAC) that is required by the data logger.
When setup is completed, the controller will produce an out-
put on loop 2 which is linear and proportional to the loop 1
process variable.
Setting Up Cascade Control
Cascade control is used to control thermal systems with long
lag times, which cannot be as accurately controlled with a sin-
gle control loop. The output of the first (primary) loop is used
to adjust the set point of the second (secondary) loop. The sec-
ondary loop normally executes the actual control.
Some applications, such as aluminum casting, use two-zone
cascade control where the primary output is used for the pri-
mary heat control and the cascaded output is used for boost
heat. You can use the primary heat output for both control and
for determining the set point of the secondary loop.
How the Secondary Set Point is Determined
The set point of the secondary loop is determined according to
the heat and cool output values from the primary loop and
user-specified cascade parameters:
•
If the primary loop has both heat and cool outputs, then
the set point of the secondary loop is equal to the Cas-
cade low SP
parameter when the cool output is at 100
percent, and is equal to the Cascade high SP when the
heat output is at 100 percent. See Figure 4.9.
•
If the primary loop has only a heat output, then the set
point of the secondary loop is equal to the Cascade low
SP
parameter when the heat output is at 0 percent, and is
equal to the Cascade high SP parameter when the heat
output is at 100 percent. See Figure 4.10.
•
If the primary loop has only a cool output, then the set
point of the secondary loop is equal to the Cascade low
SP
parameter when the cool output is at 100 percent, and
is equal to the Cascade high SP parameter when the cool
output is at 0 percent.