PLATINUM
TM
Series User’s Guide
47
Reference Section: Programming Mode (PRoG)
5.6.1
Cascade Control using Remote Setpoint
The remote Setpoint feature of the PLATINUM
TM
Series controllers can be used in a variety of
applications where Setpoints can be sent to the controllers from remote devices such as a manual pots,
transmitters, computers, etc. This feature can also be used to set up a “cascade control” system, where
the remote Setpoint input is generated by another controller. Figure 5.3 shows a generic diagram of a
cascade control system and Figure 5.4 shows a typical example, in this case a heat exchanger
application.
Figure 5.3 Generic Cascade Control Diagram
Figure 5.4 Heat Exchanger with Cascade Control
Cascade control schemes can provide tighter control of a process when you have two linked variables,
one of which has a much slower (typically 4X or more) response than the other. The slower responding
variable is used as the input to the primary or master controller, and the faster responding variable is
used as the input to the secondary or slave controller. The output of the primary controller is scaled to
be used as the Setpoint for the secondary controller.
In the heat exchanger application in Figure 2, the primary goal of the application is to control the
temperature of the effluent. Therefore, the desired effluent temperature becomes the Setpoint for the
primary controller, which is a temperature controller (TC). The process input for the temperature
controller is the measured temperature of the effluent (TT). The output of the temperature controller is
the flow Setpoint for the secondary controller, which is a flow controller (FC). The process input for the
secondary (flow) controller is the flow rate of the steam that is used to heat the process flow through
the heat exchanger (FT). The output of the secondary (flow) controller is a control signal for the
proportional valve controlling the flow of the steam.
By isolating the slowly changing effluent temperature control loop from the rapidly changing flow
control loop, a more predictable, robust, and tighter control scheme results.
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