Page 186 - Advanced Course

KNX ADVANCED COURSE

6.5 Installation and Solution Notes

The most important factor in this option of open-loop/closed-loop lighting control is again
the optimum selection of the installation place and the alignment of the sensors. The
essential points have already been mentioned above but there is one additional factor: in
combined closed-loop/open-loop control systems, it must be at least ensured, that in the
situation of complete outdoor darkness all lights must be equally turned on to the same
value (which should be ruled by the specification of the lighting system).

A pure additive / subtractive offset control however means on the other hand, that the
difference between the master and the slave will always be equal or less than the given
value. The master dimming value can range between 0 and 100%, but the slave is limited
to less than this due to the offset. That means: If e.g. slave 1 = master + 20% (master at
the window side), and slave 2 = master + 40%, then both dependant rows can only be
controlled in the range 20 – 100 resp. 40 – 100 %. If the master has reached 0%, again
we have 2 options: the slaves shut off together with the master, then it is too dark there, or
they stay on, then it is too bright. But at least at full darkness all rows are at or above the
required lux value setpoint.

If you look at the same situation, but now with the master in row 3 (darkest area), with a
negative offset to the slaves, this means, at high outdoor light levels the dimming works
fine, but the closer you come to full darkness, the more unequal the distribution of light
would be. In the end row 1 reaches only 60%, row 2 goes to 80%. That this is not the
optimum, is self explanatory.

So the only conclusion can be: The sensor must be mounted in the window side row
which then also must be controlled via closed loop control!

Underneath a couple of illustrations to this problem:

Figure 22: optimized situation, but only at one point !

Home and Building Management Systems KNX Association
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