Smart Light Controller for Daylight Compensation "Chapter 2"

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.

•   Photo emissive cells

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.

•         Photoconductive cells

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.