Assignment Draft 1 v2

UNIVERSITY OF MAURITIUS FACULTY OF ENGINEERING

Motorised Window Blinds with Light Sensor

(Draft 1)

By

(1) Junaid BAUHADOOR 1310570

(2) Kshema Teynidhi RAMSEWAK 1311167

(3) Kaushaveer GHOORA 1311777

(4) Sajal KISSOON (Leader) 1311789

(5) Akshay HEATHY 1313264

BEng (Hons) Mechanical Engineering (Minor: Energy Systems)

Level 2

ELEC 2012Y – Electrical Machinery and Measurement

October 2014

Introduction

Over the last years, the energy demand of the world for air-conditioning systems has increased

exponentially, due to apparent rise in global temperature. One of the alternatives to using the air-

conditioning is to prevent a room from heating up due to sunlight. However, it would be difficult to

manually control the amount of light needed to maintain the adequate light exposure and also avoid

unnecessarily warming the room up. This is where Smart Windows come into play. The emerging

concept of the window will be more as a multi-functional “appliance-in-the-wall” rather than simply

a static piece of coated glass. By actively managing lighting and heat due to the sun, the Smart

Windows could reduce peak electric loads significantly in many commercial as well as domestic

buildings and increase daylighting benefits.

Smart Windows model

This concept consists of motorised blinds which will be found on the outside of windows. It is done

to prevent harmful waves from attaining the building itself. This avoids unnecessary heating of the

building, as in the more conventional case (where the blinds are found on the inside of the room)

the rays travel from the windows to the inside of the house then reaching the shades.

The system comprises of horizontal blades with string ladders. The system may be activated at a

frequency of 1-5 minutes.

The system should perform the following functions:

1. Rotate rapidly and smoothly to a specified angle over the full tilt angle range.

2. Raise and lower the shade rapidly and smoothly to a specified height above the floor.

3. Respond relatively quickly and accordingly to the information from the sensors or the user.

4. Perform the above processes with minimal noise and disturbance to the surroundings.

The system should be able to rotate about its axes to let only a specified amount of light in the

building unless required otherwise.

The target market for this product is for personal and domestic utilisation; hence it needs to be

relatively low cost, easily installed and practical.

Figure 1: The blinds section of the Smart Windows. [1]

Development

Figure 2: Schematic diagram of the design and operation of the Smart Windows.

1. Choice of Materials for slats.

a) Requirements (Materials)

They should be light, strong and not easily degradable by the environment as the blinds will be

placed on exterior side of the window.

The slats should be of minimal thickness to be able to allow maximum light inside the house when

fully open; however not too thin so as to reduce their overall strength (as they will have to withstand

powerful gusts of wind in case of cyclone). They should also be of adequate width to precisely

regulate the amount of light in. The blinds should also be able to refract light.

Proposed materials: wood, plastic and aluminium.

Wood

Advantages Disadvantages

It is very light. Wood may degrade due to harsh weather.

It is considered to be very strong. Requires frequent maintenance as wood cannot sustain moisture and has to be cleaned frequently to remove dust which accumulates quickly hence making it unsuitable for outside environment

It is aesthetically appealing. Costly compared to the other material due to the treatment it needs to go through to be suitable for this application.

Plastic


Plastic is very light. For this purpose, the thin slabs of plastic would offer less mechanical strength than the other materials

It is quite cheap. Less rigid than the other materials.

Aluminium


Aluminium is very light. Bending can occur if not properly maintained.

It has a high mechanical strength.

Less constant maintenance is required.

Aluminium is resistant to degradation.

Low price: Although the material is not so cheap, its manufacturing cost is much less than that for the other materials.

Hence the chosen material for use will be aluminium.

b) Requirements (Dimensions)

The dimensions chosen have to be relevant to the average dimensions of windows in households

and should be available in various dimensions.

For this report, we will take a dimension of 120 cm by 180 cm for the blind.

The individual slats have to allow maximum light in when fully open. An approximate dimension for

this case is to use slats 6 cm wide.

Thickness is also important so as not to decrease mechanical strength and to allow maximum light in

when fully open. A thickness of around 1 mm will be used.

Each slat will be around 800 g and there will be 30 blades. The total weight should be around 25kg.

2. Modifying power supply to adapt to requirement of motors.

Figure 3: Transformer-Rectifier Unit

A transformer rectifier unit (TRU) is used to adapt the voltage of the mains supply to the

requirements of the motor. This device takes 240 V AC and converts it to 12 V DC. This is achieved by

a step down transformer which converts the AC voltage to the required voltage and then converts it

to DC via a 4 diode full wave rectifier bridge.

a) Working principle of the Transformer-Rectifier Unit (TRU).

During the first half cycle of an AC, the transformer steps down the voltage and steps up current.

The current flows out from the secondary coil and passes through diode 1. As diode 2 is reversed

biased current cannot flow though it and goes through the battery (output). The current then passes

through diode 3 and return back to the coil.

During the second half cycle, the polarity of the current changes and it flows in the opposite

direction. As diode 3 is reversed biased, the current flows through diode 1 and goes through the

battery (output).The current then flows through diode 4 to go back again in the coil.

As a result, the output signal is of the same polarity throughout.

b) Selection of the rectifier.

A rectifier is an electronic device that changes alternating current into direct current. This process is

called rectification.

The three main types of rectifier are as follows:

Half-wave rectifier

When the voltage of the alternating current is positive, the diode becomes forward-biased and

allows current to flow through it.

When voltage is negative the diode is reversed biased and current cannot flow.


Simple and cheap as this arrangement is made up of only one diode.

One half of the input waveform reaches the output thus decreasing its efficiency.

Full wave rectifier

In a Full Wave Rectifier circuit two half-wave diodes are used, one for each half of the cycle. A

multiple winding transformer is used whose secondary winding is split equally into two halves with a

common centre tapped connection. This configuration results in each diode conducting in turn

producing an output during both half-cycles.


More efficient than half-wave rectifiers. Need a center tapped transformer.

In the absence of a center tapped transformer two more diodes are required thus increasing cost and space.

A larger transformer for a given power output is required with two separate but identical secondary windings.

The diodes used have high peak inverse voltage.

Full wave bridge rectifier

This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop

“bridge” configuration to produce the desired output.


It does not require a centre tapped transformer. The only disadvantage of using Bridge rectifiers is that it requires four diodes.

Reduced cost and size compared to full wave rectifier.

Hence the chosen rectifier for use will be the full wave bridge one.

A capacitor is connected in parallel to the output of the rectifier to increase the average DC output

level and make the DC signal smooth.

c) Selection of the transformer.

A step down transformer is used to step down high voltage to lower voltage.

Autotransformer

Autotransformers are different from traditional transformers because autotransformers share a

common winding. On each end of the transformer core is an end terminal for the winding, but there

is also a second winding that connects at a key intermediary point, forming a third terminal. The first

and second terminals conduct the primary voltage, while the third terminal works alongside either

the first or second terminal to provide a secondary form of voltage. The first and second terminals

have many matching turns in the winding. Voltage is the same for each turn in the first and second

terminal.


Small size and cheaper compare to conventional transformer.

No electrical separation between primary and secondary which is risky in case of high voltage levels.

Better voltage regulation. Low impedance hence high short circuit currents for short circuits on secondary side.

If a section of winding common to primary and secondary is opened, full primary voltage appears across the secondary resulting in higher voltage on secondary and danger of accidents.

Conventional step down transformer

Work using the principle of electromagnetism. Both wires in a transformer are wrapped around an

iron core which is insulated using an oil bath so as to decrease its electrical conductivity. When

current flows through the primary wire, it creates a magnetic field, which then induces a current in

the secondary wire on the other side of the transformer.

A step down transformer has greater number of turns in the primary coil than in the secondary so as

to decrease the output voltage.


High efficiency due to electrical insulation. Copper and hysteresis losses.

Hence selected transformer is conventional transformer.

Transformer ratings

Input voltage: 240 V AC

Output voltage: 12 V AC

Turns ratio : 12/240 = 0.05

3. Selection of the Motor.

a) Types of motor.

This project requires the use of two motors:

Tilt motor-This motor allows the slats to rotate in order to vary light intensity.

Draw motor-This motor is used to draw the entire blind in case of very low light intensity or

as per user’s requirements.

To select the required motor, the torque of the motor and the maximum load it will support has to

be established. From calculations, the following data was obtained:

Desired properties of draw motor.

Specification Tubular motor

Diameter of motor 45 mm

Theoretical torque required 5.6 Nm

Torque required from motor (to account for losses) 10 Nm

Maximum load to be supported 40 kg

Desired properties of tilt motor.[2]

Specification Tubular motor

Diameter of motor 45 mm

Theoretical torque required 0.2 Nm

Torque required from motor (to account for losses) >0.5 Nm

Two types of motors are used predominantly in commercially-available shading systems: AC and DC

tubular motors. The motors are typically mounted in the head rail of the shade and sold as a unit.

An AC motor is typically used in applications where raising and lowering of a heavy load is required.

Tilting requires much less power than raising a blind, but the latter function determines the size of

the motor needed for installation in the blind header.

AC Motor


Can be plugged directly in main supply. Moderating the power and speed of the AC motor is difficult for both tilt and lift functions and the speed control circuitry is expensive and not readily available.

Low maintenance required. Motors require effective starting equipment to operate.

Higher life expectancy. Works best for large upfront power.

Speed cannot be controlled externally except while using a Variable Frequency Drive.

DC Motor


Speed of motor can be controlled externally. The voltage from the main supply has to be stepped down and rectified to suit the requirements of the DC motor.

Motors are self-starting and require no external help.

More frequent maintenance may be required.

Works best for small upfront power.

DC systems provide flat torque over a wide speed range.

Hence, based on our system requirements, it can be concluded that a DC motor would be

definitely more suitable for both the tilt and the draw motor. Moreover, a 12 V DC supply would be

appropriate for both motors.

b) Working principle of a DC motor.

Figure 4: A simple DC motor and its various components.

At any instance, the direction of the current in the armature is perpendicular to the field it is passing through, i.e. θ=90⁰. Let the current flowing through the right armature be I A and the current flowing through the left armature be –I A. From the equation ( )

Forces acting on the different sides of the armature are therefore equal in magnitude but opposite

in direction. Since these forces are separated by a distance w, we have the formation of a torque

whose formula is given by:

where is the angle between the magnetic field and the armature.

Thus

When the armature cuts the field lines perpendicularly, no rotating torque acts on it. However, the

DC motor does not stop operating at this point since the latter has been engineered in such a way

such that the inertia of the previous motion is just adequate to overcome this point of null torque.

As soon as this point has been swamped, α again takes up a certain value and eventually its

corresponding torque acts on the armature anew. Also, to prevent the direction of the torque from

reversing, a commutator is used.

4. Selection of sensor.

For this assignment, a light sensor (commonly known as photoelectric devices or photosensors) will

be used to assess the intensity of exterior light. A light sensor generates an output signal by

measuring the radiant energy that exists in a very narrow range of frequencies basically called light,

and which ranges in frequency from infrared to visible up to ultraviolet light spectrum. The light

sensor is a passive devices that convert this light energy whether visible or in the infrared parts of

the spectrum into an electrical signal output.

Photoelectric devices can be grouped into 2 main groups namely:

Those which generate electricity when illuminated which include photovoltaic and

photoemissive cells.

Those whose electrical properties are modified in some way such as photoconductive cells

and photojunction devices.

Based on our requirements we will be considering those photoelectric devices whose electrical

properties are modified in the presence of light.

a) Photoconductive Cell

A photoconductive light sensor does not produce electricity but simply changes its physical

properties when subjected to light energy. The most common type of photoconductive device is the

photoresistor which changes its electrical resistance in response to changes in the light intensity.

Photoresistors are semiconductor devices that use light energy to control the flow of electrons, and

hence the current flowing through them. The commonly used photoconductive cell is called the Light

Dependent Resistor (LDR).

Light Dependent Resistor (LDR)

Figure 5: A Light Dependent Resistor.

The light dependent resistor is made from a piece of exposed semiconductor material such as

cadmium sulphide that changes its electrical resistance from several thousand ohms in the dark to

only a few hundred ohms when light falls upon it by creating hole-electron pairs in the material.

The net effect is an improvement in its conductivity with a decrease in resistance for an increase in

illumination.


Available in many sizes. LDRs are very inaccurate.

The equipment is cheap.

Robust and does not wear out easily.

b) Photojunction device

Photojunction devices are basically PN-junction light sensors or detectors made from silicon

semiconductor PN-junctions which are sensitive to light and which can detect both visible light and

infra-red light levels. Photo-junction devices are specifically made for sensing light.

The most common type of photo-junction devices is the photodiode.

Photodiode

Figure 6: A photodiode.

A photodiode is a type of photo-detector capable of converting light into either current or voltage,

depending upon the mode of operation. Photodiodes operate by absorption of photons or charged

particles and generate a flow of current in an external circuit, proportional to the incident power. It

can be used in the reverse bias mode, turning off when a certain light intensity has been exceeded.

This means that it has two levels of output. Either it is off when the light intensity is exceeded or on

when the light intensity is adequate.


Fast response to change in light levels. Photodiodes are brittle; therefore use is limited for indoor purposes.

Photodiodes are cheap.

Hence the chosen light sensor is the photodiode as it is more suitable compared to the light

dependent resistor given its accuracy.

References:

[1] Adalons of Ipswich, ‘External Venetian Blinds’, http://www.adalons.co.uk/awnings.html,

(accessed 18 October 2014).

[2] Coulisse, ‘Absolute control – A complete range for the automation of window decoration’,

http://en.coulisse.nl/media/downloads/Brochure_Absolute_Control.pdf, (accessed 18

October 2014).

Documents

Assignment Draft 1 v2