Cascade Control Using Remote Setpoint - Newport Pt32 Operator's Manual

Reference Section: Programming Mode (PRoG) 56
PLATINUM Series Controllers User's Guide

7.6.1 Cascade Control using Remote Setpoint

The remote Setpoint feature of the PLATINUM 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 12 shows a generic diagram of a cascade
control system and Figure 13 shows a typical example, in this case a heat exchanger application.
Figure 12 Generic Cascade Control Diagram Figure 13 Heat Exchanger with Cascade Control
Cascade control schemes provide tighter control of a process when there are 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.