Watlow PPC-2000 - Manual (Page 131)

PPC-2000 User’s Guide Chapter 3: Operating with AnaWin 3
Doc.# 30002-00 Rev 2.3 Watlow Anafaze 107
Figure 3.8 Sample Application Using
Cascade Control
For cascade control the inner TC is selected as the PV Source for
the primary channel, and the outer TC is selected as the PV
Source for the secondary channel. The secondary channel’s
output is used to control the heater.
An engineer familiar with the process has determined that as
the temperature of the inner (primary) TC drops from 150°F to
140°F, the set point of the secondary channel should rise from
150°F to 190°F. Table 3.9 and Table 3.10 on page 108 list the
parameter settings for the primary and secondary channels.
The PID parameters of the primary channel must be tuned to
produce the desired effect on the secondary channel’s set point.
The primary channel typically uses proportional only control.
Disabling the integral and derivative components of PID
makes the secondary set point a predictable function of the
primary channel’s process variable.
The proportional band is selected so the set point of the
secondary channel has the desired relationship to the process
variable of the primary channel. In this application, the
proportional band of the primary channel is set to 10°F.
Table 3.9 Primary Channel Parameter
Settings
Parameter Setting Description
PV Source PPC1:AI 1.1 The inner TC is selected for the primary channel.
Set Point 150°F The desired temperature at the inner TC.
Heat Prop Band 10 As the input drops 10°F, the output increases to 100%.
Heat Integral 0 Only proportional control is used.
Heat Derivative 0 Only proportional control is used.
Heat Output Dest PPC1:Soft Bool
The primary output must be enabled, but is not used to drive
a load.
Power
Control
Tank
Heater
PPC
Channel 2 Heat Output
Inner TC Channel 1 Input
Outer TC Channel 2 Input
Outer
Inner
inner
outer
outer
outer