Theben KNX Series Handbook page 93

6.5.2 Response of the P controller
21l
17l
15l
If the filling quantity is 15 l, there is a control deviation of 21 l – 15 l = 6 l
As our actual value lies outside the bandwidth, the control will operate the feed at 100%,
i.e. with 4 l/minute.
The feed quantity (= actuating value) is calculated from the control deviation
(set point – actual value) and the bandwidth.
Actuating value = (control deviation / bandwidth) x 100
The following table illustrates the behaviour and also the limits of the P controller.
Table 1
Filling level Actuating value
15 l
19 l
20 l
The last line shows that the filling level cannot be increased any more, because the inlet feeds
as much water as can be discharged by the losses.
The result is a permanent control deviation of 1 l. The set point can never be achieved.
If the losses were increased by 1 l, the permanent control deviation would be increased by the
same amount, and the filling level would never exceed the 19 l mark.
In case of a room, this would mean that the control deviation increases with decreasing outdoor
temperature.
RAMSES 718 P / RAMSES 718 S
Max. 4l/Min.
Bandwidth
Feed
100% 4 l/min
50% 2 l/min
25% 1 l/min
Set point value
Actual value
Losses
1l/Min
Losses Increase of filling level
3 l/min
1 l/min 1 l/min
0 l/min
Appendix
93