Micromod MOD: 30ML Operation and Template Setup - Manual (Page 49)

MOD 30ML Multiloop Controller
OPERATION
3-17
3.12 CASCADE OPERATION
A cascade control system is a multiple-loop system where the primary variable (in the master
controller) is controlled by adjusting the set-point 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.
3.12.1 Cascade Operation in Manual
While in manual [MAN], the slave set-point tracks (automatically remains equal to) the slave
process variable. This characteristic is optional by enabling set-point 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 set-point tracking on the slave,
the controller responds to the deviation which exists by ramping the output at the reset rate
established when the instrument was tuned.
Master Controller Slave Controller
Slave set-point tracks
slave process
variable.
Master output tracks
slave set-point.
Slave output can be
adjusted manually.
Master set-point tracks
master process variable.
Meanwhile, the output of the master controller tracks a signal from the slave controller which
represents the set-point of the slave automatically adjusted for ratio or bias. The requirement
for balancing the local and remote set-point signals prior to placing the slave controller in
cascade mode is eliminated.