Page 166 - Advanced Course

KNX ADVANCED COURSE

4.3 Usable Bus Devices

Since we are specifically discussing the maintenance of a constant level of illuminance,
only actuators that allow variable illumination are permitted. They must therefore be
dimming actuators. A required characteristic of KNX dimming actuators is that it must be
possible to control them in three different ways: switching, relative dimming via 4 bit
information and absolute dimming via 8 bit information. Sensors for this control system
must first only be able to measure the actual brightness value with sufficient accuracy and
frequency without supplying upper limit values in the required band width. As regards
accuracy, the sensor that offers a logarithmic resolution is the most beneficial. This means
that is must be able to carry out more precise measurements in the lower range than in
the upper range as the human eye reacts less to absolutely identical changes with an
increasing level of brightness. When judging whether a closed-loop control system is
better or worse than another type of system, only a relative estimation of accuracy is
necessary. If an adjustment to 1500 lux is required for example and the maximum system
deviation is +/- 150 lux i.e. 10 %, this is equivalent to an adjustment to 500 lux at an
absolute value of +/- 50 lux.
The third component of this control system is a controller which takes over the actual
control task. It receives the measured brightness value as an input and compares it with
the setpoint which is available e.g. as a parameter (fixed value) or as an object value (can
be modified at any time via the bus). From these two values, it can determine the output
(control value) according to the implemented algorithm (control function) and send it to the
actuator.
In practice, particularly due to cost and space restrictions, the manufacturers of these
types of components decide to integrate the controllers into the sensors or even to build
the entire controller and sensor component into the actuator.

4.4 Characteristics of Sensors and Actuators

4.4.1 Sensors

As stated above, brightness sensors that are used for lighting control should have a
measured-value resolution available that is adapted to the setpoint. A system deviation
below approx. +/- 15% is still not noticed by the user. The accuracy or tolerance of the
measuring sensor as well as the losses caused by A/D conversion in the KNX sensor
must be added together i.e. lie below this figure. Example: The setpoint is 600 lux; the
tolerance is therefore +/- 90 lux. The individual possible measured values must thus be
separated by less than 90 lux. The fact that indirect measurements are always carried out
causes a problem. It is not the luminous flux of the lamp that is measured, but the
reflected light output from the reflected surface. It is easy to imagine that a dark surface is
less reflective than e.g. a white desk. The structure of the surface can also influence the
recording of the measured value. In principle, it can be said that the light is scattered to a
greater or lesser degree and the measured value of the sensor is lower than for direct
measurement. Due to the varying reflectance factors, sensor heads must have a variable
gain available which can be adapted to the respective requirements. This gain must be
carried out in front of the A/D converter as otherwise the resolution is reduced by the gain
factor and the control precision decreases. The measured value can thus be adapted via
this gain factor under the named requirements in the course of a calibration procedure so

Home and Building Management Systems KNX Association
Lighting Control Lighting Control_E0310a.doc 7/34