Operating Instructions

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Feedforward function (P3.12.4.1)
Feedforward usually needs accurate process models, but in some cases, a gain + offset type of feedfor-
ward is enough. The feedforward part does not use any feedback measurements of the actual controlled
process value (water level in the example on page 87). Siemens feedforward control uses other mea-
surements 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 defined
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 measurements
of the disturbance, we can try to compensate for this disturbance with a simple feedforward control (gain
and offset) which is added to the PID output.
This way, the controller will react much faster to changes in the outflow than if you had just measured
the level.
Figure 31. Feedforward control.
PID
++
LT
FT
11107.emf
FFW
Level ref
Level control
Ou t f l ow
control