ABB Ability 800xA Series, Operation

Page: 205 / 332

Section 8 Operation Examples Cascade Operation
3BUR002418-600 A 205
Cascade Operation
A cascade control system is a multiple-loop system where the primary variable (in
the master controller) is controlled by adjusting the setpoint of a related secondary
variable (in the slave controller). The secondary variable then effects the primary
variable through the process. A block diagram of this type of cascade system is
shown below.
The main objective in cascade control is to divide an otherwise difficult to control
process into two portions, whereby a secondary control loop is formed around a
major disturbance, thus leaving only minor disturbances to be controlled by the
primary controller.
Cascade Operation in Manual
While in manual, the slave setpoint tracks (automatically remains equal to) the slave
process variable. This characteristic is optional by enabling setpoint tracking on the
slave. It will eliminate the need for the controller to respond immediately to any
deviation which exists when the mode is changed from manual to automatic.
Without setpoint tracking on the slave, the controller responds to the deviation
which exists by ramping the output at the reset rate established when the loop was
tuned.
Figure 89. Block Diagram of a Cascade system