Background study and literature review
The
overall system can be divided in to three main categories such as,
·
Identifying the light level
·
Controlling
·
Dimming
Identifying the light level
When
identifying the available light level at a specific location of a given area it
should be considered some factors such as,
·
Effect of the shadows
·
Effect of the reflection of the light from the walls and the
floor
·
Available daylight amount in the given area at the given
moment
When
identifying the available light level there are three methods that can be followed.
Those are,
·
By a simulation software such as DIALux or MATLAB.
·
By an open loop sensor setup.
·
By a closed loop sensor setup.
Using simulation software
DIALux
is a software which can be used to design, calculate and visualise the light
variation throughout a single room, whole floors, buildings and outdoor scenes.
And this is software can be downloaded for free and this is used as a planning
tool by over 680 000 lighting designers worldwide. From this software it can be
designed the lighting design as well as can calculate the amount of light level
at a specific location of a given structure with the availability of daylight.
Therefor this software can be used for the detection of the light level
purposes.
But
there are limitations when using this software for our project. Those are,
o When the structure of the
building changes, the lighting distribution throughout the building will also
change. So if we use this software for detection purposes of the light level in
our project, then it will have to apply the design of every building which we
install the system. So it will not be user friendly and will be difficult to
use.
o When calculating the light
level inside the building with the presence of daylight, the effect of the
clouds, shadows seasonal change will not be considered. Therefor the values we
acquire from this software will not valid with our projects
Due
to the limitations mentioned above, this software will not be used in our
project.
Using open loop sensor setup
In
this method the sensor doesn’t look directly into the area that it controls.
Most of the time the lighting sensors placed outside the area which’s light
level should be calculated so it can measure the amount of day light only. In
other words the sensors used in this setup only measures daylight not the total
amount of light level which receives from both daylight and artificial light
bulbs. So in this method it does not take in to account that the measured light
level from the sensor when the sensor is placed outside the area, is different
from the actual amount which flows through the windows to the specified area.
Therefor
this method will not be compatible with our needs in the project.
Using closed loop sensor setup
In
this method, the sensor will look directly to the area which’s light level
should be controlled. So in here the main objective is to maintain the lux
level which sense by the sensor at a constant value.
In
order to get the measurements from the sensor, it will be mounted directly
above the work plane in most of the time. But the position may vary according
to the type of sensor been used. The number of sensors that been used in this
method will be higher than both of the methods stated above.
Light detectors
As
described in both open loop and closed loop sensor setup, unlikely as when
using simulation software, the light sensors will be needed to measure the
amount of light presence. There are various types light sensors (light
detectors) exist in the world at this moment which are used to measure the
amount of light in a given area.
The
light sensor is a device which generates an electrical signal output which is
proportional to the strength of the light amount which falls on to the surface
of the device. In other words, basically, the light sensor converts the light
energy in to electrical signal output. Due to that conversion light sensors
known as photo sensors of photo electric devices, since they converts photons
(light energy) in to electrons (electrical signal).
These
photoelectric devices can be divided in to two main categories based on which characteristic
in the device changes in presence of light.
·
The devices which generate electricity when it is illuminated.
- Photovoltaic cells – solar
cell
Photovoltaic
cells are made from single crystal silicon PN junctions. These are somewhat
similar to photodiodes but the light sensitive region is larger in this device.
When the device is illuminated the light energy causes electrons to flow
through the PN junction.
The
most common type of photovoltaic light sensor is the Solar Cell. Solar cells
convert light energy directly into DC electrical energy in the form of a
voltage or current. However a limitation in the solar cell, when using it as a
light sensor is that the solar cell works best using the suns radiant energy
rather than with artificial light.
The
light level can be measured using this device by measuring the current. And the
amount of available current from a solar cell depends on the light intensity,
the size of the cell, and the cell’s efficiency.
The
advantages of photovoltaic cells are ne need of an external power supply,
robust in construction.
These
type of devices are been made of light sensitive material such as caesium. When
this device struck by a photon with sufficient energy, the light sensitive
material will release free electrons. The amount of the energy of a photon
depends on the frequency of the light. Therefor when the frequency is high the
energy of the photons will be high resulting the number of electron released by
the device higher.
Photo
emissive cell is a device which contains cathode and an anode mounted in a
vacuum tube which is made of glass. The cathode is the photosensitive material
while the anode is made out of nickel or platinum. When the photons with
sufficient energy falls on the cathode, electrons are emitted and will flow to
the anode causing an electric current.
The
disadvantages in these devices are a direct power supply will be required, expensive
and generates extremely small current. And the advantages are the emission is
instantaneous, the maximum current is proportional to the intensity of the
light and the sensitivity is somewhat high.
Photo
multiplier tube is a device which used to detect light level when the intensity
is very weak. In this tube there are number of dynodes and each time an
electron strikes a dynode, it gains enough momentum to create a larger number
of secondary electrons. Due to this multiplication process, there will be a
sufficiently large current although the light intensity is very low.
By
measuring the generated current, the intensity of the light can be measured by
these type of devices.
- Photo junction devices –
photodiodes, phototransistors
Photo
junction devices are made from silicon semiconductor PN-junctions. And it can
detect both visible light and infra-red light levels. There are many types of
photo junction devices. And those are,
Photodiode
The
construction of the photodiode light sensor is somewhat similar to that of a
conventional PN-junction diode. But the difference is the outer case of the photodiode
is either transparent or has a clear lens to focus the light onto the PN
junction. In this way the sensitivity of the device is increased.
When
used as a light sensor, the current of a photodiodes when the lux level is
zero, is about 10uA for geranium and 1uA for silicon type diodes. When the
illumination of the junction increases the leakage current also increases. Thus,
the photodiodes current is directly proportional to light intensity falling
onto the PN-junction. Therefor by measuring that current the light intensity
can be measured. The advantage in photodiodes when used as a light detector is
the response to the changes in the light levels is faster. While the
disadvantage is the current is relatively small although the device is fully
lit.
Phototransistors
The
difference of phototransistor from photodiode is that there is a built gain in
the phototransistor. Therefor it is basically a photodiode with amplification.
The operation of the phototransistor is same as in photodiode except that they
can provide current gain from 100 to several thousands and are much more
sensitive than the photodiode.
Most
phototransistors are NPN types and the outer casing is similar to photodiode
for the same reason. By measuring the current, the amount of light level can be
measured.
Figure
2- VI Characteristics of a Phototransistor
Photodarlington transistors
The
sensitivity of this device is higher than both photodiode and phototransistor
due to the reason that it uses a second bipolar NPN transistor to provide
additional amplification. But its response is slower than that of an ordinary
NPN phototransistor. And the net gain of the photodarlington can be greater
than 100,000 as photodiode. By measuring the current, the amount of light level
can be measured.
Photo thyristor
This
is a light activated thyristor or Silicon Controlled Rectifier and it can be
used as a light activated switch in AC applications. However the sensitivity is
usually very low when compared to equivalent photodiodes or phototransistors.
But this device cannot be used to measure the light level due to its low
sensitivity.
·
The devices which changes their electrical property when
light presence.
A
photoconductive light sensor does not produce electricity but simply changes
its physical properties when subjected to light energy, so it becomes more
electrically conductive when it is exposed to light due to the absorption of
electromagnetic radiation such as visible light, ultraviolet light, infrared
light, or gamma radiation. Photoresistor is the most common type of
photoconductive device. This type of semiconductor devices use light energy to
control the flow of electrons in the device.
Light Dependent Resistor (LDR)
Light
Dependent Resistor (LDR) is the most commonly used photoconductive cell. LDR is
made from a piece of exposed semiconductor material such as cadmium sulphide
which changes its electrical resistance from several thousand Ohms to only a
few hundred Ohms when the device exposed to the light from no light incident. So
the net effect is the conductivity is improved with a decrease in resistance
for an increase in illumination.
The
LDR function within the same general spectral range as the human eye and it is
an advantage when using these devices for applications which require such
requirements. One disadvantage is that the photoresistive cells have a long
response time requiring many seconds to respond to a change in the light
intensity. Therefor the sensitivity is low.
Therefor
according to the variation of the electrical resistance, the light intensity
can be measured by using this devices.
|
Characteristics
|
Light
sensors
|
|
Photo
multiplier tubes
|
Photodiodes
|
Phototransistors
|
LDR
|
|
Sensitivity
|
Excellent
|
Very
Good
|
Very
Good
|
Very
Good
|
|
Linearity
|
Good
|
Excellent
|
Good
|
Good
|
|
Stability
|
Very
Good
|
Very
Good
|
Good
|
Poor
|
|
Performance
to cost ratio
|
Fair
|
Good
|
Excellent
|
Excellent
|
|
Cost
|
High
|
Low
|
Very
low
|
Very
low
|
|
Ruggedness
|
Poor
|
Excellent
|
Excellent
|
Excellent
|
|
Physical
Size
|
Large
|
Small
|
Small
|
Small
|
Table
1-Comparison of some light sensor characteristics
Drawbacks of the existing sensors
There
are some drawbacks that needed to be faced when using the sensors described
above for measuring light level in our project.
·
Large number of sensors that needed to be used
In
our project, we design the system mainly for covering larger areas such as office
areas, corridors and halls etc. . . . . That large area will be divided in to
number of zones in order to smoothly control the light level throughout the
complete area. Therefor there is a need of measuring the lux level of each zone
in order to control the light level of that zone individually. Therefor to
measure the lux level of each zone. Throughout the whole area, a large number
of sensors will be needed. And various types of sensors will be needed to use
in order to fulfil various types of tasks other than measuring light level,
such as occupancy control. Therefor using a large number of sensors will make
the system,
o Complicated to control
o Increase the cost of the
final system
o Decrease the efficiency of
the system
Due
to the disadvantages stated above, using the existing light level measuring
sensors will not be compatible with the needs of our project.
·
Positioning a sensor
Usually
a sensor which use to measure the light level will only measure the intensity
of the light which falls on to the surface of that sensor. This fact will be a
disadvantage of these types of sensors when we use them for our project.
Because in our project when the sensors are been used to measure the light
level which falls on to a table in an office area, there will be a huge problem
when selecting a place to fix the sensor. If the sensors described above been
used, then it should be fixed on the table. If that position is been used then
the sensor should always been exposed to the light. But fixing that type of
sensor on a table is not practical since it might get covered from the
equipment such as books, papers, etc., and also from shadows. Therefor using the above described sensors for
our project will cause problems by the measured light level by the sensors been
incorrect.
Using a camera as a lux meter
As
discussed above there are various types of sensors which used in application in
today’s world in order to measure the light level of an interested area. The
drawbacks which needed to be overcome when using those type of sensors as we
are using them to measure light intensity, is also briefly discussed above.
Therefor
for our project in order to achieve the objectives, we had to use a sensor
which can overcome those obstacles.
The
best solution is for the problems discussed earlier is to use a camera as a lux
meter to measure the light level. A single camera can cover a more area than a
sensor can cover. As an example if the coverage area is an office area, and if
a single camera cannot capture the complete area, then more than one camera can
be used and those camera can be integrated together to the controller.
The
pixel values in an image from a camera are proportional to the luminance in the
original scene. Therefor a camera can act as a luminance meter. From the
relationship between the luminance value and illuminance value, the illuminance
of the selected area can be calculated.
According
to Hiscocks, illuminance is defined as a measure of the light falling
on a surface, measured in lux while luminance is the measure of light radiating
from a source, measured in candela per square meter. Illuminance is a linear function
of the luminance, but must take into account the reflectance of a surface.
A
still digital camera can perform static brightness measurement at multiple
points of interest. A video camera, on the other hand, is capable of performing
dynamic and real-time measurement at any point of interest in the entire scene.
Using a digital camera
The
advantages in using a camera as an illuminance meter are,
·
A camera is able to capture a larger scene. This speeds up
the measuring process and allows multiple measurements at the same instant.
·
For luminance measurement, the field of view (FOV) of the
sensor must be smaller than the source. The FOV of a luminance meter is about
1◦. The FOV of a digital camera pixel is on the order of 150 times smaller, so
it can measure small area light sources.
Communication methods
Communication protocols designed for lighting controlling systems
BACnet
This
is a communication protocol which is specially designed for fulfil
communication purposes in building automation and controlling system. Therefor
this also satisfies the need in lighting control systems.
DALI
(Digital Addressable Lighting Interface)
This
is a non-proprietary lighting control protocol. In this protocol, it contains
with two wires which is been installed connecting DALI ballasts to DALI
controllers. Digital commands will be send through these two communication
wires to the ballasts to control the light bulb. Although the wires are
bidirectional, the signal will be sent only in one direction at a given time.
The bidirectional function is used when the controller send a signal to the
ballast about the dimming level, and when the ballast replies with the request
information. Each DALI ballast has a non-volatile memory which contains its own
settings, such as address, group assignments, scene levels, and fade rate. A
DALI device can be controlled individually via its short address. Additionally,
DALI devices can be arranged into groups in which all devices of the same Group
can interact with each other.
The
DALI specification defines a constant-current bus that operates at a maximum of
250 mA and a nominal 16 V. The data is transferred using a Manchester format at
1,200 bits/s, which is fast enough for lighting controlling purposes. The basic
protocol definition includes a single master device (controller) and up to 64
controlled devices (ballasts). The master sends out a 16-bit command or request.
The ballast device can optionally reply with an 8-bit response. A ballast
device cannot send data on the bus unless it is requested by a controller
device.
KNX
KNX
is designed to be independent of any particular hardware platform. A KNX Device
Network can be controlled by anything from an 8-bit microcontroller to a PC,
according to the needs of a particular implementation. The most common form of
installation is over twisted pair medium. When using this type of medium, the signalling
speed is 9600 bit/s. Devices within same physical segment are addressed with
8-bits. Maximum segment length is 1000m.
LonWorks
Unlike
other protocols LonWorks is a completely peer-peer network. Instead of moving
data through a “Master” device, any device can exchange data with any other
LonWorks device on the network. LonWorks can use twisted pair, Ethernet or even
a power line as its communication channel. The two-wire layer operates at 78
kbit/s using differential Manchester encoding.
ZigBee
ZigBee
is one of the wireless high-level communication protocol, which has designed as
a low-power, low data rate, and close proximity wireless ad hoc network. ZigBee
has a defined rate of 250 kbit/s, which is compatible with lighting controlling
applications.
The
maximum communication distance is dependent on the physical environment, but
distances up to 250 ft. are possible. The ZigBee protocol provides the ability
to create a self-organizing low-data-rate mesh network with up to 65,536 nodes.
Disadvantages
with the ZigBee protocol is when compared to the DALI protocol, the ZigBee
stack requires a greater electronics cost at each fixture. Additional software
will be required to process lighting commands and requests for status
information. But this is less expensive than other wireless personal area
networks (WPANs), such as Bluetooth or more general wireless networking such as
Wi-Fi.
The
main advantage is easy installation. When installing this protocol, it is not
necessary to physically lay wires. Therefor this method is beneficial with the
existing buildings since there will be no need of reconstructing.
Other communication protocols
Inter-IC Bus (I2C)
I2C
is a multi-master, multi-slave, packet switched, single-ended, and serial
computer bus which was introduced by Philips Semiconductor as a standard for
connecting networked integrated circuits. I2C is intended for application in systems
which connect microcontrollers and other microcontroller-based peripheral
devices. It is a two wire serial bus, which is divided as serial data and
serial clock. The serial data wire is bi-directional but data may flow in only
one direction at a given time.
Since
this is a multi-master multi-slave bus, each device which is connected to the
bus is identified by a unique address. And the devices on the bus are defined
as masters or slaves. The slave device is a device which is controlled by the
master while master devices are usually microcontrollers. A master initiates a
data transfer on the bus and generates the clock. And it also generates the
control signals which are placed on the data wire. A slave can either receive
or send data depending on the master. In I2C bus, it can be installed more than
one microcontroller, therefor it is known as a multi-master bus.
The
bus can operate in three modes with different data rates. Data on the bus can
be transferred at rates of up to 100kbit/s in the standard-mode, up to 400
kbit/ in the fast-mode, or up to 3.4 Mbit/s in the high-speed mode. The speed
is preferable with the need of our project.
I2C
bus supports two addressing schemes: 7-bit address and 10-bit address. Up to
1024 devices are allowed to be connected to the bus. The 7-bit address scheme
has shorter message length and requires less complex hardware. Devices with 7
and 10 bit addresses can be mixed in the same system.
Due
to the reasons discussed above, this protocol can be recommended to use in
lighting controlling systems.
