Lighting Control
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Table of Contents
1
2
3
4
General ..................................................................................................................... 4
Conventional Brightness Control: Sun shines – Light switches off............................. 4
Principle .................................................................................................................... 5
Constant Lighting Control .......................................................................................... 5
4.1
4.2
4.3
4.4
Constant Lighting Control: Areas of Application, Objective ................................ 6
Types of Closed-loop Control ............................................................................ 6
Usable Bus Devices .......................................................................................... 7
Characteristics of Sensors and Actuators .......................................................... 7
4.4.1
4.4.2
4.4.3
4.5
4.6
Parameterisation Notes, Flags, Possible Errors, Bus Load etc. ........................10
Parameterisation Example................................................................................12
4.6.1
4.6.2
4.6.3
4.6.4
4.6.5
4.6.6
4.7
Alignment of the Measuring Sensor ..............................................................16
Several Light Strips with a Varying Proportion of External Light: ...................16
Fundamental Mismatch ................................................................................17
Brightness Control....................................................................................................18
5.1
5.2
Areas of Application, Objective .........................................................................18
Types of Open-loop Lighting Control ................................................................18
5.2.1
5.2.2
5.3
5.4
5.5
5.6
Continuous Control .......................................................................................18
Two-step Control ..........................................................................................19
Usable Bus Devices .........................................................................................20
5.3.1
5.3.2
5.3.3
5.3.4
6
Functions ......................................................................................................12
Example .......................................................................................................13
Parameters ...................................................................................................13
Group Addresses..........................................................................................15
Linking of Sensor and Actuator Objects, Operation ......................................15
Additional Notes ...........................................................................................16
Installation Notes ..............................................................................................16
4.7.1
4.7.2
4.7.3
5
Sensors ......................................................................................................... 7
Closed-loop Controller types.......................................................................... 8
Actuators ....................................................................................................... 9
General ........................................................................................................20
Sensors ........................................................................................................20
Actuators ......................................................................................................20
Controllers ....................................................................................................20
Parameterisation Notes, Flags, Bus Load etc. ..................................................21
Parameterisation Example................................................................................22
Installation Notes ..............................................................................................25
Brightness Control, combined with Master/Slave Control .........................................25
6.1
6.2
Objective ..........................................................................................................25
Principle ...........................................................................................................25
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6.3
6.4
6.5
7
Available Devices .............................................................................................26
Parameterisation Example................................................................................26
Installation and Solution Notes .........................................................................27
Appendix Tasks .......................................................................................................29
7.1
7.2
7.3
Task 1: Lighting – Control dependent on External Light ....................................29
Task 2: Lighting – Closed Loop Control Master-Slave with separate Actuator ..33
General hints for the calibration of lighting closed loop control .........................34
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1 General
The lighting in modern buildings is generally no longer switched manually. The users in
principle demand the implementation of intelligent open- and closed-loop lighting control
systems. This should result in the efficient operation of the lighting. Efficiency has three
meanings in this context: firstly, there should be an obvious saving of energy; secondly,
intelligent lighting systems should largely prevent lights from being switched on
unnecessarily which should lead finally to an automatic protection of resources!
2 Conventional Brightness Control: Sun shines – Light switches
off
If dimmed fluorescent lamps are used for example instead of switched lamps, the service
life of the fluorescent material is lengthened considerably even when the maximum power
is reduced by only 10%. This again results in cost savings and protection of the
environment. A well-designed lighting control system also means a more pleasant working
environment for the user e.g. from office workstations. It is also worth mentioning in this
context that closed-loop lighting control should – where possible – use existing (free)
external light which results in a two-fold energy saving, particularly during the warmer
seasons of the year. A reduced level of artificial light means less electrical power, less
waste heat and thus also a lower cooling capacity if the rooms are air-conditioned.
Moreover, scientific studies have proven, that an integrated blinds and illumination
automation will reduce the sun induced heat energy into a building up to 80%,
Thus even a 4-fold saving can be made, perhaps in certain buildings avoid to install air
condition at all.
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Light strip 1
Light strip 2
~ 26 %
Light strip 3
~ 48 %
Receiver
~ 70 %
Receiver
Receiver
500 lux required light intensity
Necessary
artificial light
Existing daylight
Goal: Brightness control optimised for all areas of the room
3 Principle
Closed-loop and open-loop lighting control systems are based on the modulation of the
lighting level inside the room by the measurement of either the level of external light
(independent variable) or the measurement and feedback of the level of internal light
(dependent variable) which contains a variable proportion of external light. In both
variants, the primary goal is to maintain a required level of internal light as constant as
possible. In general, it is a feature of a distributed system such as KNX that the individual
tasks in the closed-loop or open-loop control systems are distributed among different
devices: sensors, actuators and controller modules. This has both benefits and
disadvantages as outlined below in detail.
4 Constant Lighting Control
A closed-loop control circuit is used for constant lighting control. The required level of
brightness in the room or the level of lighting at the desk is measured as a controlled
variable together with the interference from the external light and then fed back to the
actuators in the appropriate manner.
Controller
W
Z
Controlled system
Controlling system
Y
Actuator
Contr. variable
X
+
X+Z
Measuring device
Brightness
sensor
W ... Reference variable (e.g. brightness, setpoint)
Y ... Control value (dimming value 1-100%)
Z ... Interference (level of external light)
X ... Actual value (lux value at workstation)
Figure 1: Constant lighting control
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4.1 Constant Lighting Control: Areas of Application, Objective
This type of control system is mainly used in commercial installations where it is important
to adhere to certain regulations such as those governing the workplace but not to make
more light available than necessary. The aim is to create optimum working conditions
while at the same time saving costs. The user should also be offered an optimum level of
convenience and the internal room controller should operate independently of other
parameters which cannot be detected here.
4.2 Types of Closed-loop Control
A distinction is made between Closed-loop Control and the so-called Integral Reset. In the
first control system, the control value is directly influenced by the setpoint/actual value
differential. For example, an absolute dimming value is sent to the actuators via a
feedback function (dependent on the measured lighting level but with a variable proportion
of external light). The negative feedback of the control value on the comparison point of
the closed-loop control circuit as well as the system deviation with forward gain are
governed by one function. The greater the influence on the control value, the higher the
failure rate of the system deviation.
Integral Reset on the other hand operates according to the two-step principle. The control
value for the brightness level of the actuators is modified indirectly or gradually, namely
via relative dimming telegrams. After this type of dimming process, the lighting sensor
measures the surface again that is to be constantly illuminated, compares it with the
setpoint and then decides again in which direction further dimming should take place. The
respective level of change carried out to the control value remains constant at each new
activation. It is not dependent on the degree of system deviation
Two-step control is actually not a proper closed-loop control system as it rarely achieves
the required setpoint. This type of control only has an opening/closing point instead of
continuous dimming processes which must be provided with a sufficiently large hysteresis
to prevent the triggering of continuous opening/closing operations. This type of lighting
control is described in more detail under the section “Brightness control”.
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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
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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.
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Figure 2: Example of a closed-loop controller with an integrated sensor
In the objects of figure 1, it is possible to detect that further possibilities can be
implemented apart from the actual dimming function (= control value). These options
enable a very convenient use of the control system:
Setpoint adjustment via absolute (direct) value control
Master /slave function for additional lighting circuits
Toggling between manual and automatic mode via all operator functions which the
dimming actuator is aware of (i.e. switching, dimming and value setting)
The previous statements about the procedure used by DPT 3.007 2 do not however mean
that an integral reset can only be carried out with DPT 3.007 only. This procedure can
also be implemented with DPT 5.001 telegrams. The controller thus starts with an initial
value, mainly 0 or 255, and sends further telegrams as with DPT 3..07, which now contain
8 bit values that are reduced or increased by a constant value compared to the previous
value. Some devices also manage to retrieve the present dimming value of the actuator to
be controlled and start from this one instead 0 or 100%. The benefit of this procedure
compared to DPT 3.007 appears to be that the resolution increases until it reaches steps
of 0.4%. This means it is possible to make even finer adjustments in the dimming process.
The bus load should however be taken into account since it rises when smaller changes in
the value are carried out.
The device shown in figure 2 is an example for 8-bit closed loop control.
4.4.3 Actuators
No particular requirements are placed on the dimming actuators initially as only the 4 bit
or 8 bit object is required for integral reset. It is however important to be able to select
whether this type of actuator should switch off or not when dimming down continually. It is
likewise the case with the opposite process. If the closed-loop control is switched on, it
should be possible to switch on using 4 bit dimming commands. The timing/dimming curve
must be active for both 8 bit value dimming and the 4 bit dimming process as the actuator
otherwise dims brighter or darker step by step. The KNX dimmers and dimming actuators
that are currently available on the market fulfil almost all these conditions which is not
always the case with older devices. To optimise the dimming process, the dimming speed
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of the actuator and dimming step width and send interval of the controller should match
precisely:
If e.g. 1/64 dimming telegrams are sent every 2 seconds, the dimmer must be set so that
it makes a full cycle of 0 – 100% in 128 sec and no faster.
If it should also be possible to operate the dimmer manually, a shorter dimming period is
of course required. Differences can therefore be found in this regard between the
available devices as not all of them offer this possibility (see the note in the next section).
4.5 Parameterisation Notes, Flags, Possible Errors, Bus Load etc.
The previously named condition (dimming period of 128 sec) is optimised for the control
system but not for manual operation. A compromise must often be made: the actuator
dims at a slightly slower rate e.g. in 8 sec and the controller transmits at a slightly faster
rate to avoid stepwise brightness processes. This increases the bus load however.
Alternatively 1/32 is used as a step width instead of 1/64 to reduce the bus load again.
This produces a further problem in the case of very bright luminaires (over-dimensioned or
still new). The changes in brightness can already be greater than the set hysteresis after a
single step of 1/32. This results in continuous fluctuations as the luminaire continually
exceeds or falls below the target value.
500 lx
Hysteresis
Interior
lighting
External brightness
Figure 3: Mismatch of dimming step width and hysteresis
The extension of the hysteresis or reduction of the dimming step width can provide some
help:
500 lx
Hysteresis
Interior
lighting
External brightness
Figure 4: Correct ratio of dimming step width and hysteresis
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A better option is the use of dimming actuators with two time bases. This option has
become more common since dimming actuators based on BCU2 have come on to the
market. It is then really possible to implement a manual and automatic function: manual
operation = short dimming time base, automatic = long dimming time base.
Figure 5: Objects of a switch/dim actuator with a dual time base
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4.6 Parameterisation Example
4.6.1 Functions
We use 3 KNX components in this layout for a complete control system: a sensor with an
integrated controller (type = integral reset by 8 bit), a dimming actuator with a dual
dimming time base and a 3-fold push button with text display that enables the following
functions.
Function
Effect
Switches actuator on/off, interrupts the CLL*)
Switch light manually (basic)
control
Dim light manually (basic)
Dims the actuator, interrupts the CLL control
Switch automatic CLL control on/off (basic)
Starts the CLL control with the parameterised set
point or stops it
Set value manually for lighting
Sets the brightness value at the actuator between
(continuously) (optional)
0 and 100%, interrupts the CLL control
CLL control triggered by a movement / presence
Presence (optional)
detector; or simpler, just by switch
Setpoint shift (optional)
Variable LUX value setpoint control
Only for commissioning (or for advanced users to
Calibration
recalibrate later)
*) CLL: closed loop lighting
The usual range in which setpoint values can fluctuate is between 250 lux (lighting level
for less demanding activities) and 1500 lux (level for light-intensive laboratory
workstations where optical equipment is used such as lenses, microscopes etc.). The
sensor must maintain a system deviation of max. +/-15% throughout the range.
Two further functions/group addresses are required to calibrate the arrangement after the
installation:
measured lux value (to compare with the external lux meter)
calibration trigger
The brightness sensor/controller already mentioned above also enables calibration to be
carried out. The address ‘measured lux value’ is not really always necessary but strongly
recommended to be used in order to have an indication about how far the real measured
value and that one of the KNX sensor differ before the calibration is started. The
calibration process will calculate just a (for the user not accessible) gain factor between
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the physically measured light level and the either parameterized or via object adjusted
setpoint.
4.6.2 Example
The sensor would – after the initial full application download – send a value of 200. But a
Luxmeter would see 500 Lux. The setpoint of the sensor is also 500 Lux. (It has to be –
otherwise the calibration would be faulty). So after the calibration trigger telegram has
been sent, the new measured value would be returned with “500”.
Only a connection between the controller and the actuator is now missing:
Automatic value control (Master value)
4.6.3 Parameters
Figure 6: Control parameters of the closed loop controller
Explanations:
Operating mode:
“Constant light level control” is used for continuous dimming
Number of slaves: Only 1 master channel required, so no slaves = 0
Send meas. brightn.: (optional) the sensor value can be displayed and monitored
Min. variation…:
“15 Lux” means, with deviations of equal or more than 15 Lux a new
value will be sent.
Setpoint value:
“Parameter” or “Comm. Object”; the latter one allows flexible setting
of the setpoint
Max. var. from setp.: Half of the double sided hysteresis (varies from 15 to 60 Lux)
Max. step size …:
varies from 0.5 up to 3%; = difference between 2 master value
telegrams;
Transmit next…:
2 sec; i.e., together with “max. step” 0-100% = 120 sec.
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Start and finish…:
The controller will firstly read the value status, then calculate the
next telegram to be sent, and also stop with a 0% value telegram
Figure 7: Parameter (extract) of the switch/dim actuator
This setting enables a manually operated dimming speed of 5 sec per 100%.
The factor for the “dimming time 2” (plus its base timer “seconds”) rules an automatic
dimming time of 120 sec.
This time equals exactly the setting of the master channel of the brightness controller.
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4.6.4 Group Addresses
To summarise, all the group addresses are listed here again:
Figure 8: Group address configuration in a convenient constant lighting control system
4.6.5 Linking of Sensor and Actuator Objects, Operation
In our example a push button with text display (LCD) is used to provide proper labelling of
the control functions and the (optional) display of values.
Very often, however, customers don’t want to spend so much money, so they only use a
local pushbutton for simple switching and dimming control as manual override, and the
automatic activation will be done e.g. by a central scheduler, or by the twilight sensor on a
weather central.
Figure 9: Group address links between the 3 KNX devices
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4.6.6 Additional Notes
In general, the actuator is also addressed via central functions (see above) which switch
the lights on/off directly or also activate/deactivate the control for example via a time
switch. This must also be taken into account in the controller. A direct central switching
operation must also switch off the controller with positive drive. These addresses must
therefore also be linked to the disable objects of the controller; otherwise the controller will
always act against this external input and try to dim up the lights again.
4.7
Installation Notes
Many factors are decisive for the optimum installation of a brightness control system.
Some are listed here:
4.7.1 Alignment of the Measuring Sensor
The surface that is to be measured should be as undisturbed as possible i.e. should never
indicate different surface characteristics. External light should not directly penetrate the
receiving lens if possible, likewise artificial light.
Building ceiling
Correct
Incorrect
Lamp
Receiver
Suspended
ceiling
Mounting height 2.5 - 6 m
Correct
Incorrect
Light cone
0
4
4
w
n
A
_
A
5
9
1
Calibration required
Measuring sensor +/-15%
Daylight
Figure 10: Optimum position of the measuring sensor (graphics do not resemble the sensor
exactly)
4.7.2 Several Light Strips with a Varying Proportion of External Light:
In this case, which rarely occurs, a single brightness sensor is only sufficient if it is
possible to generate different control values for the individual actuator channels. If you do
not have a controller that can do this, a separate sensor must be installed for each light
strip. As mutual influences (mostly unwanted) can arise, a partitioning of the sensors must
take place and the overlapping of the lighting surfaces underneath the individual strips
must be minimised. It can, however, invariably lead to oscillation processes or at least an
unexpected distribution of brightness (although the measured values of the sensors are
correct). The technically better solution nowadays is to only control one strip (whereby it is
not possible to simply select any strip but the strip which is installed at the window side,
which reacts the most sensitively to daylight changes). The other lighting strips are simply
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interfaced via a control function e.g. offset adjustment is a so-called master/slave circuit
(see the explanations in section 5).
Light strip 1
~ 26 %
Light strip 2
~ 48 %
Light strip 3
~ 70 %
Receiver
Slave 2
Slave 1
Master
500 lx required illuminance
Necessary
artificial light
Existing
daylight
Figure 11: Brightness control of a light strip, combined with offset control for the other light
strips
4.7.3 Fundamental Mismatch
A mismatch always occurs in this system. This is based on the one hand on the different
lighting spectrum of natural light and artificial light and on the other hand, on the different
angle of illumination. The more external light comes in, the greater the measuring error,
because the natural light also is picked up by the sensor rod directly instead of only from
the controlled surface by reflection. A curve of the controlled variable is produced
dependent on the external brightness which reaches its minimum in a ratio of 50:50
between the proportion of external and internal light. The system must always be
balanced under these lighting conditions.
Ф internal
[lx]
Optimum adjustment value
Actual
value
Setpoint
500
250
Actual
internal value
Proportion of
artificial light
Proportion of
external light
Ф external
[lx]
Figure 12: Behaviour of the actual value dependent on the level of external light
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5 Brightness Control
5.1 Areas of Application, Objective
In contrast to lighting control in which you wish to achieve individual and optimum levels of
light intensity as mentioned above, brightness control is mostly considered with regard to
minimising installation costs without fully losing sight of the goal of reducing energy costs.
It is however agreed between the planner and user that significant deviations from the
setpoint/actual value can occur in the control system. This is particularly the case when
only switchable lamps are integrated in the control system.
Figure 13: Principle of brightness control
5.2 Types of Open-loop Lighting Control
For brightness control, a distinction is made between continuous control and two-step
control. A sensor measures an external brightness value which is independent of the
internal illuminance that is to be set. Starting from this measured value, the internal control
value is determined at first via any calculation function which the room illuminance uses to
reach the required setpoint. Technically this is an open control loop. The feedback loop is
missing. This has both advantages and disadvantages. It is beneficial that this type of
lighting control can never oscillate since the feedback loop has been omitted as
mentioned. Starting with the measured value of a single sensor, many different control
curves can be differentiated. The disadvantage is that a very extensive adjustment must
be carried out for continuous control which takes at least one day.
5.2.1 Continuous Control
Continuous control – like the closed-loop control above – requires dimmers or dimming
actuators which can be infinitely adjusted. As an automatic adjustment of the control value
cannot take place (due to the missing feedback loop), this must be carried out by selecting
a control function. In the simplest case, this can be a straight line determined by two pairs
of values: a) maximum external brightness above which there should be 100% interior
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lighting and b) minimum brightness level above which the light should be switched off.
Moreover, the control module must be in a position to carry out a hysteresis in order to
ease control in the extreme range between switched on and switched off.
Figure 14: Characteristic curve for brightness control – a hysteresis is advisable
5.2.2 Two-step Control
The characteristic curve described in the previous section is further simplified if only two
states are possible for the controlled lamps: ON or OFF.
Switching value
Switch Off point
1
Hysteresis
Switch On point
0
0
[ lux ]
2000
4000
6000
Measured value of brightness sensor
Figure 15: Principle of two step control
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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
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not a KNX lighting controller but the conversion is carried out externally in the DDC which
then sends new dimming control values in response via the gateway to a new measured
value of the lighting sensor. (E.g. gateways to Profibus).
Figure 16: Conversion of the measured value of the light sensor in different control curves
5.4 Parameterisation Notes, Flags, Bus Load etc.
Having already discussed all the important characteristics of the actuators and sensors
involved in the ‘Lighting control’ section, we can now briefly summarise here: statements
regarding flags (readability of status objects) and inclusion of manual operation in the
automatic system (time-limited interruption of control) also apply here of course. If the
lighting should also be switched/dimmed manually via local push buttons or set directly to
values, the control system must also be aware of the addresses, otherwise manual
operation leads inevitably to an immediate correction by the automatic system.
As we no longer have a control loop, it is also possible under certain conditions to
implement minimum and maximum limit values for the dimming control value within which
the control system operates. Otherwise it is switched off (manual operation). To prevent
frequent switching in the two limit ranges, minimum ON and/or maximum OFF times can
be requested as well as an ON/OFF hysteresis. The module that can fulfil these additional
requirements is of course preferable to those with pure control curve functions.
The bus load also plays a role here: it is not merely a question of value dimming but also
whether and how the cyclical and/or event–controlled sending of the lighting measured
value should be set best. It should first be considered that each new brightness value from
the sensor results in the same number of value dimming telegrams as active control
curves.
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5.5 Parameterisation Example
Figure 17: Links between switch sensors, actuators and the control unit, consisting of the
brightness sensor and control module
In the above diagram, it can be seen that it is not only the external brightness and
dimming control values (‘set value’) from the controller to the dimming actuators that are
important, but also the ON/OFF lighting command, the optional 4 bit dimming and the 8 bit
value setting of the manual push button. The application shown is – as in the closed-loop
control described in paragraph 3 – able to convert increases and decreases in the
dimming value into an adjustment of the control curves. The logic operation of the status
value is required however as this determines the parallel displacement of the curve. What
is still missing in this diagram would be a true automatic ON/OFF function which is
controlled via a time switch. This KNX clock would also use the group addresses
‘Enable/disable control’ and ‘Control ON/OFF’.
Figure 18: Parameter of 2 different brightness sensors: linear vs. logarithmic value
calculation
The setting for “Sending on change” should not be too precise. The 1st sensor allows 5%
changes, always related to the last sent value. Such a value calculation goes along with
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the logarithmic impression of brightness by the human eye and is much better than a
linear output (equidistant values) of sensor 2. Still an option of limiting the amount of bus
telegrams per time unit should be available (not visible in this example here).
Regarding sensor 2 (Figure 18 lower image): If a measured value interval of 64 lux is still
sufficient in the lower range (see 2nd example above), then the factor should be 8 instead
of 4 as shown. During cyclical repetition of the measured value – if parameterised –values
like those shown above are completely unsuitable: assuming that 10 control curves are
implemented, up to 11 telegrams in total can occur every 650 msec. The bus load would
then already be over 30%. The aim should however be to remain below 2%. Assuming
that a maximum of 45 telegrams can be sent on the line on average, then 2% would
correspond to 0.9 telegrams / sec. In the case of the aforementioned 11 telegrams per
measured value, this means that a measured value may be sent approx. every 12
seconds. This timing resolution is probably quick enough, primarily, because the controller
calculates the new control values in proportion and does not always send constant
dimming steps as in integral-action closed-loop control. The correct parameter
combination for sending the cyclical measured values is then:
Base = 130 ms; Factor = 12 sec / 130 ms = 92
Dimming
value
Measured
250
value
250
0
250
500
200
1000
150
2000
125
3000
75
4500
25
8000
25
1000
8000
Figure 19: Value table for lighting control: a monotone falling curve is important
The table shown in the diagram figure 19 can be specified arbitrarily at first. An
approximation of a comparable constant lighting control system can mainly be achieved
over 3 points. The two values of complete darkness or the level of daylight that is
sufficient to light up a room or part of it without artificial light can be quickly determined
using a lux meter. The value pair that starts the proportional range should be verified next:
in our example this is 500 lux / control value 250 (98%). If there are still considerable
deviations in the intermediate lux values, it is possible to ‘recalibrate’ the particularly poor
values a few days later.
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Figure 20: Parameter of a controller module
It is then possible to check whether the curve-specific parameters of the controller (if
available) fulfil the requirements of the customer. In our example, that would be
the behaviour after bus voltage recovery; the open-loop control would be switched on
here
the minimum On time: if the external brightness should vary within the limit range so
that the control value for the dimmer can fluctuate around the switching point, then the
light remains switched on for at least 5 minutes
the hysteresis limit values shown mean in our example that the light is only switched
off at approx. 9000 lux (since there is then an underrange in the control value of 20)
and switched on again at approx. 600 lux because the control value of 30 has been
achieved again.
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5.6
Installation Notes
The installation instructions for the open-loop lighting control system are reduced to notes
about the installation of the sensor head:
It must be directed outside or be installed outside the building.
Its recording of the lighting level may not be influenced by seasonal variations such as
leaves on the trees which stand between it and the sky or snow on the receiving lens.
Its measurement may (for interior installation) also not be invalidated by the shutter.
An installation behind the shutter or roller blind should be avoided where possible.
The presence of shutters require a particular type of control: if proportionally-controlled
shutters are present, the louvres of the shutters can be adjusted in parallel to the
external brightness instead of the light.
For an optimum adaptation of rooms that face different directions, it is advisable to use
at least 2 differently positioned sensors: one in a south-east direction and another in a
north-west direction. Large buildings may require even more sensors which should
then be placed as perpendicular as possible to the respective façade and point
upwards.
6 Brightness Control, combined with Master/Slave Control
6.1 Objective
With combined open-loop/closed-loop lighting control, you are in general pursuing the aim
of saving costs without having to relinquish the benefits of a partially true closed-loop
control system. These can be:
a simple setting procedure
optimum lighting conditions in at least one position in the room
always the correct lighting level in the rooms even when combined with light direction
and sun protection systems.
6.2 Principle
An internal sensor measures the lighting level of a surface that should be regulated – as in
closed-loop control which was described in detail above. The measured values of the
sensor are further processed in a control program resulting in a control value which is
used to trigger the actuator that is responsible for the direct light strip. To eliminate the
requirement for further sensors (and also their deviation), it transfers a full control curve
via offset adjustment to all other curves. The required offset adjustment can be
determined by 2-3 simple measurements: at an artificial lighting level of 25%, 50% and
75%, the necessary offset of the controlled light strips is determined in comparison to the
regulated strip to arrive at the required setpoint in lux. The largest recorded differential
(upwards) per strip is then taken as this guarantees that the minimum lighting level never
falls below the setpoint.
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6.3 Available Devices
Since the offset adjustment is derived from the dimming value of a regulated dimming
actuator, it would be possible for example to use the active status response (8 bit) of this
type of dimming actuator and simply add it to the established offset in a function module
or visualisation program. The newly specified control value would then be sent to the next
controlled light strip via another group address.
This procedure is however only advisable if the function module or visualisation program
mentioned are also available or if there is a cost benefit in using them compared to a
multi-layered lighting control system.
In terms of device implementation, it is much simpler in any case to use multi-channel
dimming actuators with an integrated sensor connection and controller application. It is
also possible here to configure internal offset connections between the actuator channels
so that fewer bus telegrams can be sent. This solution not only saves device addresses
but also costs in general.
6.4 Parameterisation Example
In the following example, 3 light strips in one room should be regulated or controller with a
single dimming actuator and only one light sensor. A time-limited manual operation
(switching/dimming) of all 3 channels should be possible using a 4-fold push button. The
fourth rocker should switch all 3 dimming channels back to automatic mode. A special
‘cleaning light’ function should also be implemented: when a further
1-fold push button is pressed, the light should be set to full brightness for 1 hour, the
closed-loop/open-loop control should be deactivated and then everything should be
switched off.
Figure 21: Example of a complete 3-channel room control system with an automatic
‘cleaning-light’ function: only 3 bus devices are required
The following can also be detected in figure 21: emergency operating mode via local push
buttons if the KNX should fail. In the case of the device shown, these push buttons have
the additional function of triggering an automatic calibration of the respective sensor
channel, if it acts as a master sensor in the closed-loop control.
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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 !
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Figure 23: same as above, but at more external light – worst case situation !
Figure 24: again same configuration, but at complete darkness – rows 2 and 3 “waste”
energy!
What is the conclusion of the notes mentioned above?
If the offset of the slave is positive, then it will always be able to reach 100%, but cannot
dim down to 0%, it will turn off with the master. If it is negative, it can switch off, but not
reach 100% of dimming level.
So there is never a 100% positive solution as with individual light sensors per each row of
luminaries. However, using the configuration with the master at the window, and the
slaves with positive offsets in the inner parts of the room, will provide a good compromise.
The only slight disadvantage is, that the master will either have to turn off late (at more
than 500 lx, which avoids the “gap”, but causes a much higher level at darkness, or there
will be a situation, when the master turns off at 500 lx, where rows 2 and 3 still would need
some light, but will switch of with the master.
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This is an issue, which can be cured when a light controller with a dynamic offset control
is used (a multiplicative offset instead of a constant one). In this case the slave follows the
master via a linear function like this: S = M x ( 1 + O ). O : = Offset [%]; S = Dimvalue [%]
Slave; M = Dimvalue Master [%]. It can easily be seen that at M = 100% and O > 0 the
value of S reaches 100%. At very small values of M, however, the absolute difference
between S and M will come close to 0!
Figure 25: Light controller with multiplicative offset
7 Appendix Tasks
7.1 Task 1: Lighting – Control dependent on External Light
A room that is fitted with two light strips should receive a brightness control system. Up
until now, the light strips have been switched and dimmed individually.
The following are used:
1 x 2-fold switch sensor, switching and dimming objects (4 bit)
2 x switch/dim actuators, objects for switching, dimming, value setting and value status
must be present.
Group addresses for manual control:
L1 Switch
L2 Switch
L1 Dim
L2 Dim
First put this simple series circuit into operation.
The dimmable lighting in the room should now also be controlled by a brightness control
module dependent on the external light.
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Figure 26: Lighting – Control dependent on external light
Required devices:
Brightness sensor e.g. Siemens 5WG1 254 3 AB 02 to record the brightness level
(external light)
Brightness control module e.g. Siemens 5WG1 342 1 AB 01
2-fold push button
Project design
Define the following new group addresses










Brightness value of sensor (control module, sensor)
Calibration request (control module)
Dimming value for calibration (control module)
Brightness value for calibration (control module)
L1 Value set (--> switch/dim actuator + control module)
L2 Value set (--> switch/dim actuator + control module)
L1 Value status (--> switch/dim actuator + control module)
L2 Value status (--> switch/dim actuator + control module)
Control L1 enable / block (2-fold push button, automatic)
Control L2 enable / block (2-fold push button, automatic)
Link the bus devices (Control 1 and 2 for L1 and L2).
Check the read flag in the brightness sensor GE 253.
Use the default parameter setting of the bus devices.
Commissioning
Put the brightness control module and brightness sensor into operation.
You can now determine the control characteristics using the calibration objects.
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Establishing the control characteristics for L1 and L2
Switch off the ambient lighting (artificial sun)
Dim the lighting to the required brightness value. Note that the window luminaire (L1)
requires a lower modulation (see also the sketch of the characteristic curve below).
Mark the group address ‘Calibration request’ and select the item “read/write…”. In the
telegram monitor then check if it is already online, otherwise connect to the bus first
before the next step !
Now be sure the selected group address still is selected; click on the “write” – button
and enter the value ‘1’ for control characteristic 1 (L1), then click on “OK”.
Change to the ‘Read value’ page and read out the status of the group address
‘Dimming value for calibration’. Note the value.
Now read out the status of the group address ‘Brightness value for calibration’. Also
note this value.
Repeat the process for control characteristic 2 (L2).
An interpolation point of the two control characteristics is now known. Determine further
points (at least 3) with different ambient light values (simulated external brightness
values), whereby the last one should be carried out at the maximum brightness level of
the ‘artificial sun’. Your determined values (hex code) could look as follows:
Characteristic 1
Characteristic 2
Measured value of
sensor
Dimming value L1
Measured value
of sensor
Dimming value
L2
02A3
FF
02A3
B8
0651
7A
0651
4F
0F0A
37
0F0A
07
2714
01
2714
00
Converting the determined interpolation points of the characteristic curves
As only decimal values can be entered in the parameters for the interpolation points of
the characteristic curves, the determined hex values must be converted into decimal
values. To do so, use the calculator (scientific) in the ‘Programs’ folder under
‘Accessories’.
Example:
Control curve 1
Control curve 2
Dimming value
Dimming value
255
255
L1
L2
184
122
Measured
value
55
675 1617
3850
Measured
value
79
7
675 1617
10004
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Load control characteristics
Enter the established values in the parameters of the brightness control module and
download the application.
Functional test
Test the function.



Enable/disable automatic control.
Switch lighting on/off manually.
Adjust characteristic curve by manual dimming.
Optimisation
Modify the parameters of the switch/dim actuators to dim brighter gradually.
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7.2 Task 2: Lighting – Closed Loop Control Master-Slave with
separate Actuator
Illumination level at a workplace shall be kept constant at 500 lux. Additionally it should be
possible to change the parametrised luxvalue setpoint (300 – 800 lux), and the light shall
be also manually operable (i.e., automatic / manual changeover shall be possible).
As a device for this task the brightnesscontroller type Siemens UP255 shall be used.
During the commissioning procedure it has to be calibrated, to adopt it to the prevailing
light- and reflexion conditions.
Process description see further below!
A device to control shall be a 4-gang push button which also can set 16 bit values, or a
suitable binary input or a push button interface.
Functions of this 4-gang push button:
Rocker A: lighting manually on/off + dimming
Rocker B: Automatic on/off
Rocker C: Presence on/off
Rocker D: Upper button setpoint 600 lx; lower button setpoint 400 lx
Alternatively (and much better) a display and operation unit would be recommendable,
where beside the normal operating functions you can also see the current brightness level
indication, and which permits a nearly continuous adjustment of the setpoint.
Example: Siemens 5WG1585-2AB11 (UP 585) Display- and operation unit.
Project design
Create the following group addresses
•
•
•
•
•
•
•
•
•
•
Licht E on/off
Light F on/off
Light E dimming
Light F dimming
Sensor setpoint adjust
Calibration trigger
Automatic control on/off
Presence on/off
Dimmvalue set auto Licht E
Dimmvalue set auto Licht f
Parametrise and link the bus devices
Check the necessary read flags in the sensor UP255 and the S/D actuator
Use default settings of the device if not otherwise defined
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Used devices
Brightness controller UP255
25 S1 Brightness control 909601
Push button 4-gang delta profile
12 S4 On/Off/Dim/Blnd/Displ241301
Dimming actuator like in the basic project
Commissioning
Save your work
Download the devices and test
7.3 General hints for the calibration of lighting closed loop control
1) Start ETS
2) Open the required dim actuators, configure their parameters, create the groups
and link them, finally download to the actuator
3) Configure the brightness controller as well: depending on your preference with a
fixed setpoint (parameter) or a variable one by object.
4) Don’t forget to link the object “calibration” to a group address!
5) Now adjustment at 50% operating point follows: Prerequisite: External light level in
the room >= 50% of setpoint.
6) To achieve that, switch on the lights and close the shutters
7) Now place a precision luxmeter underneath the sensor
8) Then adjust the measured luxvalue at the sensor that it shows approx. 50% of the
setpoint. Example: >= 250 lx at setpoint of 500 lx. Best results are made when
direct value control (8 bit) is used to set the required dimming value instead of 4-bit
dimming.
9) It must be ensured that daylight level is constant enough
10) When the luxmeter now shows the correct setpoint, simply send a telegram to let it
calibrate. (this can be achieved either by ETS, or a test push button)
11) The controller should respond with the parameterised setpoint. Now the control is
calibrated and can be used.
12) If there is no controller feedback, then the reason can be: value out of range,
caused by too bright or too dark surface, too much side light
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