vacon • 170
Parameters
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7.2.17
FeedForward
F
EEDFORWARD
FUNCTION
Feedforward usually needs accurate process models, but in some simple cases a gain + offset type
of feedforward is enough. The feedforward part does not use any feedback measurements of the
actual controlled process value (water level in the example on page 170). VACON
®
feedforward
control uses other measurements which are indirectly affecting the controlled process value.
Example 1:
Controlling the water level of a tank by means of flow control. The desired water level has been de-
fined as a setpoint and the actual level as feedback. The control signal acts on the incoming flow.
The outflow could be thought of as a disturbance that can be measured. Based on the measure-
ments of the disturbance, we can try to compensate for this disturbance by simple feedforward con-
trol (gain and offset) which is added to the PID output.
This way the controller would react much faster to changes in the outflow than if you just had mea-
sured the level.
Figure 63. Feedforward control
PID
+
+
LT
FT
9099.emf
FFW
Level ref
Level control
Outflow
control
