Page 167 - Advanced Course

KNX ADVANCED COURSE

that it can be used simultaneously as a lux value that can be displayed. The correct
measured brightness value is generally sent cyclically by the sensor. The value differential
and time interval that are used for sending can be set in a wide range in most cases for
the available devices. In addition to the cyclical sending variant, it is also possible to react
quicker with event control if the measured brightness value changes significantly.

4.4.2 Closed-loop Controller types

The example outlined above now requires a closed-loop controller, which uses the
setpoint/actual value comparison to determine how the control value must be modified in
order to return to the desired equilibrium of setpoint = actual value. The details of this
control technology will not be discussed here, only the feasible alternatives.
This so-called closed-loop procedure can contain proportional, integral and differential
feedback components. This means:

proportional: a direct control output ‘y’ is calculated for the dimming actuator from the
setpoint/actual value differential ‘x’ via simple, linear conversion function according to
the function y = a x
.
integral: the control output is integrated with a specific rate i.e. it is zero at the
. .
beginning and reaches the calculated value y = a x t only after a certain period
differential: the control output is determined from the rate of change of the system
deviation y = x/t
Only the P controller can be used directly by these basic functions of control technology.
The use of a P controller however leads to a systematic deviation which cannot be
reduced due to the requirement to avoid oscillations. Only an integrating controller would
function better in our case as it finds a stable state after a certain period in which it
remains while there are no changes to the external light intensity. The D controller reacts
very quickly if there is a substantial change in the parameters. The response to this
change dies down again after a certain period.
Apart from the continuous control of the setpoint, it is now important for an optimum
lighting control system that the change in the brightness level runs almost imperceptibly
for the user. Extreme external variations in the lighting should also not influence the
control output at such a rate that it causes an imbalance. P and D controllers are therefore
rarely used for lighting control systems. Only integral action control is in practice sufficient
to fulfil the requirements of the user.
In most cases, the integral action control has an indirect rather than direct influence on the
control output. We are then using the “integral reset” procedure.
In practice, this means that the closed-loop controller modifies the control output stepwise
and indeed always by the same amount per temporal unit, which is a modification of a true
integral controller, since the steps are always of the same size. In the KNX system, 4 bit
dimming telegrams (DPT 3.007) are predestined for this as they transfer these constant
changes in value that do not contain any different values (apart from the sign) as is the
case with an absolute 8 bit control output. Any considerable variations in the parameters
always cause the same rate of change. Since the change in brightness should take place
gradually, only the step widths 1/64 (1.6%) or 1/32 (3.2%) are recommended for DPT
3.007. 4 bit dimming telegrams are sent continually at a certain rate, provided that the
actual value does not meet the setpoint within the hysteresis. If the hysteresis range is
reached, the controller stops.

Home and Building Management Systems KNX Association
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