1110 - 17
DIGITAL INDUSTRIAL CONTROLLER
english
The control response of a single control loop can be improved substantially in most
cases by feed forward control. The precondition for this is that the disturbance
variable can be measured and recorded.
The disturbance can be fed either to the controller's input or output via a
compensator F
k
(Fig. 7). In the digital controller, the disturbance is fed forward to the
controller's output. The compensator F
k
consists of a PDT-1 element. This element's
P component feeds forward in proportion to the disturbance. The D component feeds
a value that is proportional to changes in the disturbance. Both components can be
chosen freely when configuring or defining the parameters.
Figure 7: Single control loop with feed forward control
a) to the controller’s input
b) to the controller’s output
Example:
Let us take water level control in a steam boiler as an example of fixed setpoint
control with feed forward control. The water level is measured and controlled by way
of the supply of feed water. Here, the outgoing quantity of steam manifests itself in
the form of the principal disturbance. If it is measured and additionally fed forward to
the controller. The controller's response can be improved in this way.
5.3 Controller with additional functions for feed forward control
5.3.1 Single control loop with feed forward control
The
feed forward
controller structure highlighted in Figure 8 is obtained by
configuring the overall structure accordingly. It is based on PID controller 2.
PID controller 1 is not used. Input 1 is used for the controlled variable PV1, while
input 2 serves to feed the disturbance forward to the controller’s output. SP1 is the
setpoint that has to be set.
5.3.2
Feed forward controller
structure
Setpoint
generator
PV
Z
Controller
Partial controlled
system 1
F
R
F
S2
F
S1
Partial controlled
system 2
F
Z
F
K
CO
SP PVd
Compensation element
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