Watlow PPC-2000 - Manual (Page 129)

PPC-2000 User’s Guide Chapter 3: Operating with AnaWin 3
Doc.# 30002-00 Rev 2.3 Watlow Anafaze 105
Cascade Control
Cascade control is used with thermal systems with long lag
times, which cannot be as accurately controlled with a single
control loop. To accomplish this, the output of the primary
channel is used to adjust the set point of the secondary channel.
The secondary channel executes the actual control.
In some applications, two zone cascade control systems are
used. In such systems, the primary channel’s output is used for
heat control and the secondary channel’s output is used for a
heat boost. Such arrangements are used in metal applications
such aluminum casting. The PPC’s cascade control feature
allows the primary channel’s output to be used both to
determine the secondary channel’s set point and as an actual
control output.
The secondary channel’s set point is determined according to
the heat and cool output values from the primary loop and the
user supplied Max Set Point and Min Set Point parameters. Figure
3.6 and Figure 3.7 on page 106 illustrate the relationship
between the primary channel’s output and the set point of the
secondary channel.
When the primary channel has both heat and cool outputs
enabled, the secondary channel’s set point is equal to the Min
Set Point setting when the cool output is at 100%. (See Figure
3.6) When the primary loop has only its heat output enabled,
the secondary channel’s set point is equal to the Min Set Point
setting when the heat output is at 0%. (See Figure 3.7 on page
106) In either case, the secondary channel’s set point is equal
to the Max Set Point setting when the heat output is at 100%.
For a cool only primary channel, the secondary channel’s set
point is equal to the Min Set Point when the primary channel’s
cool output is at 100% and equal to the Max Set Point when the
primary channel’s cool output is at 0%.
Figure 3.6 How the Secondary Channel’s
Set Point is Determined When the
Primary Channel Has Heat and Cool
Outputs
S
et Point of the
S
econdary Channel
(Engineering Units)
Max Set Point
Min Set Point
-100%
Primary Channel’s Output (% of Full Scale)
100%