ABB i-bus KNX Product Manual page 40

5.5.2 B ehaviour of the P controller
4 8 B
Max. 4l/Min.
21l
Strip width
17l
15l
If the filling quantity is 15l, a control deviation is produced of 21l – 15l = 6l
As our actual value is outside the strip width, the controller will actuate the supply
with 100%
i.e. at 4l / minute.
The supply quantity (= control value) is calculated using the control deviation
(set value – actual value) and the strip width.
Control value = (control deviation / strip width) x 100
The table below clearly explains the behaviour and therefore also the limits of the P
controller.
Filling level Control value
15l 100%
19l 50%
20l 25%
In the last line, it is clear that the filling level cannot increase any more because the
supply is allowing exactly the amount of water that can flow out again via losses.
The result is a consistent control deviation of 1l, the set value can never be reached.
If the losses were 1l higher, the remaining control deviation would increase by the
same amount and the filling level would never exceed the 19l mark.
In a room, this would mean that the control deviation increases with a falling outside
temperature.
Set value
Actual value
Losses
1l/Min
Supply Losses
4 l/min
2 l/min
1 l/min
Supply filling level
3 l/min
1 l/min 1 l/min
0 l/min
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