Watlow Controller Series D8, Руководство пользователя

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D8 Series User’s Guide
Chapter 4: Operation and Setup
79
Setting Up Cascade Control
Cascade control is used to control thermal systems with long lag times, which cannot be as
accurately controlled with a single control loop. The output of the first (primary) loop is used to adjust
the set point of the second (secondary) loop. The secondary 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 primary 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 Cascade 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 below.
• 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 below.
• 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.
Set Point of Secondary Loop
(Engineering Units)
High Set Point
Low Set Point
-100%
Output of Primary Loop (Percent of Full Scale)
100%
Figure 4.9 – Secondary Set Point When Primary Loop Has Heat and Cool Outputs