Burkert Type 1110 - Handbuch (Seite 25)

1110 - 23
DIGITAL INDUSTRIAL CONTROLLER
english
The controlled system is split into the two subsystems F
S1
and F
S2
. The controlled
variable PV1 is measured on the subsystem F
S1
and the auxiliary controlled variable
PV2 is measured on the subsystem F
S2
.
The auxiliary control loop consists of the auxiliary controller F
R2
and the subsystem
F
S2
. The setpoint for the auxiliary control loop is given by the output variable CO1 of
the main controller F
R1
, which constitutes the main control loop together with the
auxiliary control loop and the subsystem F
S1
. The setpoint of the main control loop is
specified as SP1.
A prerequisite for interaction between the two control loops is that the auxiliary
control loop must have a faster time response than the main control loop, i.e. the
essential delays are encountered in the sub-loop F
S1
. Disturbances Z2 influencing
the subsystem F
S2
are balanced out by the faster auxiliary control loop and
disturbances Z1 influencing the subsystem F
S1
are balanced out by the main control
loop.
Example:
Control of the temperature in a tank heated with hot steam can be mentioned as an
example of a cascade control. A fast auxiliary control loop for control of the
hot steam flow rate is superimposed on the slow temperature control loop with the
main controller F
R1
.
5.6 Controller with additional functions for cascade control
5.6.1 Cascade control
In a cascade control, two control loops are interlinked so that one control loop (the
main control loop) is superimposed on the other (the auxiliary control loop). We
therefore speak of a double control loop (Figure 13).
Figure 13: Cascade control
SP1 PVd 1
Setpoint
generator Main controller Auxiliary
controller Subsystem 2 Subsystem 1
Main control loop
Auxiliary control loop
FR1 FR1 FS2 FR1
PV1 PV2
Z1 Z2
CO2 PVd2 CO1