Page 179 - Advanced Course

KNX ADVANCED COURSE

5.3 Usable Bus Devices

5.3.1 General

A separation between the sensor and actuator technology is advisable. The control
system takes over the evaluation of the sensors (including push buttons) and triggers the
actuators.

5.3.2 Sensors

Almost all the devices that can evaluate external inputs can be considered as sensors for
brightness control. This includes the simple two-step control via a binary input that is
coupled with a floating contact of a light sensor. The disadvantage in this case is that the
switching points and hysteresis cannot be influenced via the KNX by telegram or at least
by downloadable device parameters.
Analogue inputs with KNX capability which supply a measured value in DPT 9.004 format
in lux units are better in this case. The sensors that offer an almost logarithmic resolution
of the measured signal are beneficial and of course completely cover the required area. If
this is not the case, the measured-value resolution – as mentioned above – should be
sufficiently precise for light control values approx. >50% so that visible fluctuations in the
brightness level do not occur in the area in which the artificial light is predominant. As we
saw above with closed-loop control, the maximum step width may not be higher than a
3.2% dimming value differential. This characteristic must of course also apply here. This
means that we should have at least 50/3.2 = 16 steps in the range 50%-100% for the
dimming setpoint. You can now easily specify whether a sensor is sufficient in practice or
is not precise enough.
Example: A sensor records 1000 lux of external brightness, the dimming level lies at 50%.
The resolution of the sensor dimming level lies at 50%. The resolution of the sensor must
be at least 1000 / 16 = 6.25 lux. A small query regarding the theory – if this sensor with a
resolution of 62.5 lux already had a dimming level of 50% at an external brightness of 500
lux – is it suitable or not?

5.3.3 Actuators
As stated above under 3.4.3, there is no limit applicable here. All the actuators designed
for normal dimming can be used. All the binary outputs can be used again for two-step
control.

5.3.4 Controllers

The brightness control system covers several different control curves. It is therefore a
good idea to implement the functions ‘measure external light level’ and ‘trigger actuators’
in a special module which doesn’t have to be directly linked with the light sensor via cable.
In this case, it is possible to concentrate on the optimum position for placing the
measuring sensor, as the measuring signal is already formatted as a DPT 9.004 telegram
in the KNX-coupled sensor and then routed through the KNX network as appropriate. A
controller module is then located in a distribution board which derives a range of different
control curves from the incoming measured value. It is of course always possible to
interface another system via a gateway in which DDC functions are also available. This is

Home and Building Management Systems KNX Association
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