Home appliances

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UNIT-V
HOME APPLIANCES
 Basic principle and block diagram of microwave oven;
 Washing machine hardware and software,
 Components of air conditioning and
 Refrigeration systems,
 Proximity Sensors and
 accelerometer sensors in home appliances.
1. BASIC PRINCIPLE AND BLOCK DIAGRAM OF MICROWAVE
OVEN:
Microwave oven definition
A microwave oven (commonly referred to as a microwave) is a kitchen appliance that
heats and cooks food by exposing it to microwave radiation in the electromagnetic spectrum.
This induces polar molecules in the food to rotate and produce thermal energy in a process
known as dielectric heating. Microwave ovens heat foods quickly and efficiently because
excitation is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a homogeneous, high
water content food item; food is more evenly heated throughout (except in heterogeneous,
dense objects) than generally occurs in other cooking techniques.
Microwaves
A microwave is a signal that has a wavelength of one foot (30.5 cm) or less. This
converts to a frequency of 984 MHz, so all frequencies above 1000 MHz (1 GHz) are
considered microwaves. The frequencies immediately below this border are considered ultra-
high frequencies. The microwave oven uses microwaves of frequency 2.4 GHz (12.5 cm
wavelength) to cook food. Microwave ovens have a magnetron usually concealed in the roof
of the oven, specifically designed to make use of the energy in the microwaves. Electricity
applied to the magnetron tube is used to create microwave energy.
Microwaves enter the cooking area through openings inside the oven. A turntable or
tray is located at the bottom of the oven. Microwaves cannot pass through the metal walls of
the oven, but they can penetrate such materials as glass, porcelain and paper, the materials
out of which microwave-safe cookware is constructed. Microwaves do not heat cookware,
though cooking vessels will eventually get hot from the heat generated by the food. 4

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concave reflectors are located on the left and back sides to concentrate the microwave
energy on the food. In state-of-art microwave ovens, the wave reflector system (WRS) and
dual-wave emission system (DES), Fig., ensure that the food is always uniformly cooked.
Transit time
Every electronic product with two or more terminals will have inter-electrodes
capacitance across the terminals and inductance in series with the terminals. Both effects
limit the usefulness of the device at high frequencies. Values of 2 pF and 0.02 μ are typical
inter-electrode reactance‟s that erode the high-frequency signals.
Fig. (a) The wave reflector system (WRS) (b) The dual-wave emission system (DES)
A second limitation is the time it takes for the electron to travel from one electrode to
another electrode called the transit time. At microwave frequencies the time for one cycle of
RF energy is often shorter than the transit time of the device. The measures required to
improve one effect are the opposite of what is required to improve the other, and are
therefore counterproductive. For this reason the principle of using the transit time (as in
magnetrons) instead of fighting it, has become basic to many of today's microwave devices.
Magnetrons
The word magnetron is a conjunction of the words magnet and electrons and
identifies one of the major components, a very powerful magnet. The second major
component is a cylindrical copper block, drilled and channeled as shown in Fig. The centre
opening is called the interaction chamber. The holes drilled around the outer edge have a
diameter equal to one-half wavelength at the operating frequency and are called resonant
chambers. There will always be an even number of resonant chambers, usually not less than
6 and not more than 16. With the magnetron used as a diode the copper block becomes the
anode and a directly heated cathode is placed at the centre of the interaction space.

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The chamber is sealed with top and bottom cover plates and the air is drawn out to
form a vacuum. The output connection is a wire loop in one of the chambers that feeds to a
coaxial cable fitting on the side wall of the block anode. Because the anode is exposed to the
user it is placed at ground potential and the cathode is at a high negative potential. The
magnetron will only operate as an oscillator (never as an amplifier) and finds its greatest use
as a power oscillator.
Fig. The anode cylinder block of a multi-cavity magnetron
The frequency of the magnetron will remain most stable when any one channel differs
in phase from its immediate neighboring channels by an exact multiple of π/4 radians. Best
results are obtained at 4π/4 radians (π radians = 180°). This is called the mode of operation.
To ensure this phase shift of 180° alternate channels are strapped together as shown in
below Fig.
Fig. Strapping of alternate anode channel pole pieces to ensure 180° phase shift,
mode operation A and C are strapping contacts, B and D are not

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Wave guides
The part of a microwave system that established the theory of operation for all of the
other devices is the interconnecting hardware called wave guide. The conductors of
microwave energy constitute a departure from conventional cables in that they resemble a
coaxial cable with the centre conductor removed, Fig. Microwave energy is carried through
the waveguide by reflection along its inside walls.
Fig. The evolution of waveguides (a) Two quarter-wave shorted sections of
transmission line. (b) Multiples of a (c) Standard waveguide with designations (no
flange). The narrow side dimension b is 0.4 to 0.5 times the wide side dimension a.
This is possible only if the guide is larger than one-half the length of the applied voltage
wave. Thus the wave guide size is directly related to frequency. The waveguide factors that
deserve attention are :
1. attenuation losses per unit length
2. size selection
3. coupling methods
4. guide impedance and
5. power-handling ability
MICROWAVE OVEN BLOCK DIAGRAM:
The block diagram of a microwave oven is given in below Fig. The mains plug and socket
are three-pin earthing type. The fast blow ceramic fuse is of 15 A, 250 V. Interlock switches

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are linked with the oven door. Power will be applied to the mains transformer only when the
oven door is closed. At least one interlock switch is in series with the transformer primary,
hence even a spot of dirt in the relay or trial, cannot turn the oven on when the door is
open.There is yet another interlock across the power supply line. It normally remains open. If
the door alignment is not correct it will be activated, putting a short circuit (crowbar) across
the line and making the fuse to melt. Thus, the microwave oven is a failsafe device.
The voltage induced in the secondary winding is about 2000 V (rms) at 250 mA for normal
domestic ovens. The transformer also has a tertiary winding for the magnetron filament. The
high voltage return circuit is fastened directly to the chassis through the transformer frame. A
half-wave doublers configuration is used for the rectifier, with a peak inverse voltage of about
12000 V. One end of the diode is connected to the chassis.
Fig. Functional block diagram of a microwave oven
The bleeder capacitor (1 μF) should always be discharged before touching anything inside
when the cover is removed. The high value bleeder resistor is slow to discharge; further it
may be open. The thermal protector is a PTC thermistor. The primary current decreases
when the temperature rises abnormally. It senses the temperature of the magnetron as it is
bolted to the magnetron case and is so connected electrically that its resistance comes in
series with the primary circuit. The controller is a microprocessor chip with a clock. It is
activated by key-pad switches and sets the cooking time. It senses the temperature and
moisture, sets the power levels and runs the display. There are three power levels. For HIGH
the microwave generator remains on continuously; for MEDIUM it remains on for 10 seconds

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and off for 10 seconds; for LOW it remains on for 5 seconds and off for 15 seconds. The
controller activates the microwave generator using either a relay or a triac.
LCD Timer with alarm
Most microwave ovens feature at least one timer with an alarm. Older appliances
used mechanical timers, but modern microwave ovens and cooking ranges feature electronic
timers using digital circuitry. The concept of a timer is sketched in Fig. In this system, the
keypad is the input and both the digital display and alarm buzzer are the output devices. The
processing and storage of data occur within the digital circuits block in Fig. (a).
Fig. Digital timer system
A somewhat more detailed block diagram of a digital timer is shown in Fig. 50.7 (b).
The digital circuits block has been subdivided into four blocks. They are the time-base clock,
the self stopping down counter, the latch/decoder/driver, and the magnitude comparator. The
input controls block presets the time held in the down counter. The time base is a stable
multivibrator which generates a known frequency. In this case, the signal is a 1 Hz square
wave. The accuracy of the entire timer depends on the accuracy of the time-base clock.
Activating the start input control causes the down counter to decrement. Each lower number
is latched and decoded by the latch/decoder/driver. This block also drives the display.
SINGLE-CHIP CONTROLLERS
Most of us are familiar with general-purpose microcomputers such as the IBM PC and
its clones and the Apple Macintosh, which are used in more than half of our homes and in
almost all of our businesses. These microcomputers can perform a wide variety of tasks in a

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wide range of applications depending on the software (programs) they are running. There is
a more specialized type of microcomputer call a microcontroller which is not a general-
purpose computer. Rather, it is designed to be used as a dedicated or embedded controller
which helps monitor and control the operation of a machine, a piece of equipment, or a
process. Microcontrollers are microcomputers because they use a microprocessor chip as
the CPU, but they are much smaller than general-purpose microcomputers because the
input/output devices they normally use are much smaller. In fact, some of the input/output
devices as well as memory are usually right on the same chip as the microprocessor. These
single-chip microcontrollers are employed in a wide variety of control applications such as:
appliance control, metal-working machines. VCRs, automated teller machines, photocopiers,
automobile ignition systems, antilock brakes, medical instrumentation, and much more
Fig. Single-chip microcontroller block diagram
Types of microwave ovens
There are two main types of microwave ovens in the market. The first type cooks by
microwaves only while the second is a microwave convection oven, which is in fact a
combination of ovens. While microwave ovens remain popular, there is now a great demand
for combination ovens also.
Heating through microwaves has the following advantages over conventional electrical
heating systems
1. Quicker heating saves time.
2. System is very clean.
3. Preserves natural flavours.

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4. Retains most of the nutrients.
5. Enhances the colour and texture of food.
6. System can be combined with a conventional heating process.
2. WASHING MACHINE :
A washing machine (laundry machine, clothes washer, or washer) is a machine used
to wash laundry, such as clothing and sheets. The term is mostly applied to machines that
use water as opposed to dry cleaning (which uses alternative cleaning fluids, and is
performed by specialist businesses) or ultrasonic cleaners. Laundry detergent is frequently
used to clean clothes, and is sold in either powdered or liquid form.
Electronic controller for washing machines
The task here is simply to identify the input and output devices used in electronic washing
machines and to construct a block diagram showing their connections to the controller.
Detailed information about the characteristics of sensors and actuators can be added at a
later stage.
Fig. Inputs and outputs in an electronic washing machine
There are many acceptable ways of representing the system. It would, for example, be
possible to consider the display to be internal to the controller and therefore not show it
separately. Similarly clock circuitry used to time the operation of the machine is considered
here to be contained within the controller. It could equally well be considered as an external
component. The block diagram is a good starting point for the generation of the specification

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since it shows very clearly the structure of the complete system. The block diagram makes
no assumptions of the form of the controller.
It could be implemented using an electromechanical timer, or a microcomputer, or a
range of other technologies. Many modern washing machines now use microcomputer to
control their various functions, replacing the electromechanical controllers used in earlier
models. Clearly it is not practical to consider all aspects of such a system, but it is instructive
to look at some elements of the design. At various stages of the washing cycle the drum is
required to rotate at different speeds. These include: a low speed of about 30 revolutions per
minute (rpm) while clothes are washed: an intermediate speed of about 90 rpm while the
water is pumped out and a high speed of either 500 or 1000 rpm to spin dry the clothes. Let's
consider how the microcomputer should control the speed of the motor. Since a domestic
washing machine is a very high-volume product, the design should attempt to minimize the
amount of hardware required. This necessitates a close look at the choice of sensors and
actuators to select low-cost items. Our first decision must be whether the system will be open
loop or closed loop. Since although an open-loop system is theoretically possible using a
synchronous motor the cost of such a system for high-power variable-speed applications is
prohibitive. The system will therefore be closed loop using a motor to drive the drum and
some form of sensor to measure its speed.
One of the simplest methods of speed measurement is to use a counting technique
illustrated in Fig. It uses a fixed inductive sensor to produce a pulse each time it is passed by
a magnet which rotates with the drum. This produces one pulse per revolution of the drum
which can be used to determine its speed. The speed of the motor will be controlled by the
power dissipated in it. The simplest way of speed control is to use a triac. The power could
be controlled by some form of electronic circuitry, but the hardware requirement can be
reduced if the microcomputer controls the power directly by firing the triac at an appropriate
time during its cycle. To do this the controller must detect the zero crossing of the ac supply.
This will require circuitry to detect the crossing point while protecting the processor from high
voltages. A block diagram of the system is shown in Fig. At any time in the washing cycle the
program determines at what speed the drum should rotate. From knowledge of the required
speed and the actual speed as obtained above, the controller can determine whether to
increase or decrease the power dissipated in the motor.
The motor power is determined by the timing of the triac firing pulse. If the triac is
fired at the beginning of each half of mains cycle it will remain on for the remainder of the half
cycle and the motor will operate at full power. The longer the processor waits before firing the

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triac, the less will be the motor power. The processor thus varies the delay time with respect
to the zero crossing point of the mains by an appropriate amount to increase or decrease the
power in the motor as determined by the difference between the actual and required speeds.
Fig. Washing machine control
This method of controlling the motor speed is very processor intensive. It consumes a large
amount of processor time and will require a considerable amount of effort in writing and
developing the software. However, this approach uses very little hardware and is thus very
attractive for such a high-volume application.
Fig. Displacement sensors using counting
Washing machine hardware
A system is an assembly of components united by some form of regulated interaction
to form an organized whole. We will examine a microcomputer system, using a washing
machine control as an example. The input peripherals consist of

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Fig. Washing machine—hardware
1. Temperature sensor which senses the washing water temperature. (The
analog/digital converter changes the analog values to binary numbers).
2. Safety cut-out switch.
3. Keyboard for program selection.
4. Water level gauge.
5. Motor for washing drum.
6. Power switches for motor, heater, etc.
7. Heater for washing water.
8. Water inlet valve.
9. Water suction pump.
10. Control lamps and indicators.
The units listed above i.e. the washing machine as well as its mechanical components,
electrical units and electronic components are known as hardware.
Hardware and software development:
We will now examine how a system is developed. The example used for this is, of course, a
simple washing machine control. The development will follow the broad pattern shown in Fig.

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Fig. Developing the system for washing machine control
The problem definition is based on the requirements of the specification. It is also
necessary for the redesign of the existing unit. It is a means of determining what a system's
performance is capable of and what is required from it. Data flow charts are used to identify
all the hardware elements of a system at this stage for a general broad picture of the
structure of the installation. Program flow charts permit the costs of the necessary software
to be established in the development stage and represent useful aids for the designer. The
decision to go ahead with the developments of a system is governed by economic appraisal
and technical feasibility of the plan. To establish these criteria the required operating speed,
memory storage capacity and costs of the component parts of the system must be
determined. Subsequently the structure of the problem is analyses and the final production
costs deduced. There are two alternative approaches for hardware development. On one
hand, a universal system may be considered which has not been designed to cope with any
one specific problem. On the other hand a specially designed system may be decided upon
in which the components used are specially selected for their suitability to deal with the
problem under consideration. Such optimization is generally not possible when standard
systems are employed. For software development a detailed program sequence plan must
first be established. This is then written in the appropriate code and fed into a computer or
into a development system. The program is then translated into the language required by the
machine and a simulation of the operation sequence is carried out. Any errors found in the
program are corrected (this is known as debugging) and the software is then available for
use.

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Fig. 51.6 Data flow chart for a washing machine control
After the hardware and software has been developed the system is tested. An examination is
carried out to determine whether the system can satisfy all the demands which may be put
upon it, i.e. “Can the machine perform every function which may be required from it. It is not
now a question of testing the program (this has already been done during the program
development) but the system is now under scrutiny.

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3. AIR CONDITIONING:
Air conditioning is the process of treating air in an internal environment to establish
and maintain required standards of temperature, humidity, cleanliness, and motion. This is
how each of these conditions is controlled:
1. Temperature: Air temperature is controlled by heating or cooling the air. Cooling
technically means the removal of heat, in contrast to heating, the addition of heat.
2. Humidity: Air humidity, the water vapour content of the air, is controlled by adding
(humidification) or removing (dehumidification) water vapour from the air.
3. Cleanliness: Air cleanliness or air quality is controlled by either filtration, the removal
of undesirable contaminants using filters or other devices or by ventilation, the
introduction of outside air into the space which dilutes the concentration of
contaminants. Often both filtration and ventilation are used in an installation.
4. Motion: Air motion refers to air velocity and to where the air is distributed. It is
controlled by appropriate air distributing equipment.
Sound control can be considered an auxiliary function of an air conditioning system even
though the system itself may be the cause of the problem. The air conditioning equipment
may produce excessive noise requiring additional sound attenuating (reducing) devices as
part of the equipment. The above description does not imply that every HVAC (heating,
ventilation and air conditioning) system regulates all of the conditions described. A hot water
or steam heating system consisting of a boiler, piping, and radiation devices (and perhaps a
pump) only controls air temperature and only during the heating season. These types of
systems are common in many individual homes (residences), apartment houses, and
industrial buildings.
A warm air system, consisting of a furnace, ducts, and air outlet registers, also
controls air temperature in winter only. However, by the addition of a humidifier in the ducts,
it may also control humidity in winter. Warm air systems are popular in residences. Some
residences have combination of air heating and air cooling equipment that provides control of
temperature and humidity in both winter and summer. Some degree of control of air quality
and motion is provided in air-type heating and cooling systems.

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Air conditioning systems used for newer commercial and institutional buildings and
luxury apartment houses usually provide year round control of most or all of the air conditions
described. For this reason, it is becoming increasingly popular to call complete HVAC
systems environmental control systems. Most air conditioning systems are used for either
human comfort or for process control. Air conditioning enhances our comfort. Certain ranges
of air temperature, humidity, cleanliness, and motion are comfortable; others are not. Air
conditioning is also used to provide conditions that some processes require. For example,
textile printing, and photographic processing facilities as well as computer rooms and
medical facilities, require certain air temperature and humidity for successful operation.
Components of air conditioning systems
Heat always travels from a warmer to a cooler area. In winter, there is a continual
heat loss from within a building to the outdoors. If the air in the building is to be maintained at
a comfortable temperature, heat must be continually supplied to the air in the rooms. The
equipment that furnishes the heat required is called a heating system. In summer heat
continually enters the building from the outside. In order to maintain the room air at a
comfortable temperature, this excess heat must be continually removed from the room. The
equipment that removes the excess heat is called a cooling system.
An air conditioning system may provide heating, cooling, or both. Its size and complexity
may range from a single space heater or window unit for a small room to a huge system for a
building complex. Most heating and cooling systems must have the following basic
components:
1. A heating source that adds heat to a fluid (air, water, or steam).
2. A cooling source that removes heat from a fluid (air or water).
3. A distribution system (a network of ducts or piping) to carry the fluid to the rooms to
be heated or cooled.
4. Equipment (fans or pumps) for moving the air or water.
5. Devices (e.g., radiation) for transferring heat between the fluid and the room.
All-water air conditioning systems:
A typical hydronic (all water) heating system is shown in below Fig. Water is heated
at the heat source (1) usually a hot water boiler. The heated water is circulated by a pump (2)
and travels to each room through piping (3) and enters a terminal unit (4). The room air is
heated by bringing it into contact with the terminal unit. Since the water loses some of its
heat to the rooms, it must return to the heat source to be reheated.

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Fig. Arrangement of basic components of a (hydronic) hot water heating system
If steam is used in a heating system, the components still work in the same manner, with the
exception that a pump is not necessary to move the steam; the pressure of steam
accomplishes this. However, when the steam cools at the terminal unit, it condenses into
water and may require a condensate pump to return the water to the boiler. A hydronic
cooling system Fig functions in a similar manner to the hydronic heating system. Water is
cooled in refrigeration equipment called a water chiller (1). The chilled water is circulated by
a pump (2) and travels to each room through piping (3) and enters a terminal unit (4).
Fig. 52.2 Arrangement of basic components of (hydronic) chilled water cooling system
Hydronic systems are popular for HVAC systems that require both heating and
cooling. This is because it is possible to use the same piping system for both by connecting a
hot water boiler and water chiller in parallel, Fig. 52.3, using each when needed.

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Fig. Arrangement of basic components of a hydronic heating and cooling system
All-air air conditioning systems
All-air systems use air to heat or cool rooms. They may also have the added
capability of controlling humidity and furnishing outdoor ventilation, which hydronic systems
cannot do. A typical all-air heating and cooling system is shown in Fig. Air is heated at the
heat source (1), such as a furnace. It may also be a coil circulating hot water, or steam,
heated by a boiler. The heated air is circulated by a fan (2) and travels to each room through
supply air ducts (3). The supply air enters the room through outlets called air diffusers or
registers (4) that are designed to provide proper air distribution in the room. When the
warmed supply air enters the room, the room is heated. A humidifier (10) may also be
included to maintain comfortable room humidity in winter.
Fig. Arrangement of basic components of an all-air heating and cooling system
In summer, air is cooled when it flows over a cooling source (5), usually a coil of tubing
containing a fluid cooled by refrigeration equipments. When the cooled supply air enters the

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room, the room is cooled. Because a room‟s size is fixed, the same volume of air that enters
the room must also exit. This is usually accomplished by return air ducts (6). The air is then
heated or cooled again and re-circulated. An outdoor air intake duct (7) may be provided for
introducing fresh outdoor air for increased air quality. Similarly, the same volume of air must
be exhausted (8). Provision may be made for cleaning the air with air filters (9) and for
humidifying the air (10).
4. REFRIGERATION
An environmental control system that includes cooling and dehumidification will
require a means of removing heat from the conditioned spaces. Because heat flows from a
higher to a lower temperature, a fluid with a temperature lower than the room design
temperature must be made available, to which the excess room heat can be transferred.
A natural heat sink that is used occasionally for cooling water is atmospheric air. In climates
where the humidity is extremely low, evaporative cooling of air may reduce both the air and
water temperature low enough so that either can be used for cooling. A refrigeration system
extracts heat from a substance at a temperature lower than the ambient and transfers the
extracted heat to the atmosphere at a temperature higher than the ambient. A refrigeration
system is termed as :
1. A heat recovery system when its refrigeration effect is utilized for cooling an area or a
fluid and the heat rejection is put to some beneficial use.
2. A heat pump when it is used for cooling during summer and heating in winter by
incorporating suitable accessories for the change over from the cooling to heating
modes and vice versa.
Food preservation, both for processing and storage, is one of the significant applications of
refrigeration. Food processes calls for chilling, freezing, quick freezing, or freeze drying.
Typical applications are the domestic refrigerators and home freezers, ice cream
manufacturing and storage, drinking water coolers, beverage cooling, cold storages, process
cooling of meat, fish, dairy products, fruits, vegetables, transport refrigeration etc.
Refrigeration is very vital to the chilled and frozen-foods industry for maintaining the cold
chain i.e. a supply of such foods from the farm to the consumer. Transport refrigeration is an
important link in this chain. The conditions of temperature and relative humidity are dictated
by the application for which the refrigeration system is intended.

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Refrigerants:
Refrigerants are heat carrying mediums which during their cycle in the refrigeration system
absorb heat at a low temperature level and discard the heat so absorbed at a higher level.
These refrigerants have been used since the 1930s because of their excellent
characteristics. They have good physical properties for performance temperatures, pressure,
oil mixing feature, heat transfer, specific, etc. They are non-toxic, stable, and inexpensive.
Chlorofluorocarbons (CFCs) are composed of chlorine, fluorine, and carbon atoms. Some in
this group are R11, R12, and R114. Hydro chlorofluoro carbons (HCFCs) are composed of
hydrogen, chlorine, fluorine, and carbon atoms. Some in this group are R122 and R123.
Hydro fluorocarbons (HFCs) are composed of hydrogen, fluorine and carbon atoms. Some of
these are R134a and R125
REFRIGERATION SYSTEMS
A schematic flow diagram showing the basic components of the vapour compression
refrigeration system is shown in Fig. To aid in understanding; some typical temperatures for
air conditioning applications are also indicated.
Process 1–2: At point (1), the refrigerant is in the liquid state at a relatively high
pressure and high temperature. It flows to (2) through a restriction, called the flow control
device or expansion device. The refrigerant loses pressure going through the restriction. The
pressure at (2) is so low that a small portion of the refrigerant flashes (vapourises) into a gas.
But in order to vapourise, it must gain heat (which it takes from the portion of the refrigerant
that did not vapourise, thus cooling the mixture and resulting in low temperature at (2).
Process 2–3: The refrigerant flows through a heat exchanger called the evaporator.
This heat exchanger has two circuits. The refrigerant circulates in one, and in the other, the
fluid to be cooled (usually air or water) flows. The fluid to be cooled is at a slightly higher
temperature than the refrigerant, therefore heat is transferred from it to the refrigerant,
producing the cooling effect desired. The refrigerant boils because of the heat it receives in
the evaporator. By the time it leaves the evaporator (4), it is completely vapourised.
Process 3–4: Leaving the evaporator, the refrigerant is a gas at a low temperature
and low pressure. In order to be able to use it again to achieve the refrigerating effect
continuously, it must be brought back to the conditions at (1)–a liquid at a high pressure. The

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first step in this process is to increase the pressure of the refrigerant gas by using a
compressor. Compressing the gas also results in increasing its temperature.
Fig. The vapour compression refrigeration system
Process 4–1: The refrigerant leaves the compressor as a gas at high temperature and
high pressure. In order to change it to a liquid, heat must be removed from it. This is
accomplished in a heat exchanger called the condenser. The refrigerant flows through one
circuit in the condenser. In the other circuit, a cooling fluid flows (air or water) at a
temperature lower than that of the refrigerant. Heat therefore transfers from the refrigerant to
the cooling fluid, and as a result, the refrigerant condenses to a liquid (1).
The refrigerant has returned to its initial state and is now ready to repeat the refrigeration
cycle. Of course the processes are actually continuous as the refrigerant circulates through
the system. The absorption system uses the principle that some gases will be absorbed by
certain other substances. There are many pairs of substances that have this affinity for one
another. We are all aware of how table salt absorbs water vapour from the air, thus making it
difficult to pour. Yet another combination is lithium bromide (LiBr) and water, lithium bromide
will absorb large quantities of water vapour. This pair is used in many refrigeration systems.
Domestic refrigerators
The refrigerator is an essential part of almost every household for preserving food
and thereby reducing wastage. The primary function of a refrigerator or freezer is to provide

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food storage space maintained at a low temperature for the preservation of food. Its essential
secondary function is the formation of ice cubes for domestic consumption. A storage
temperature of 0° to 4°C (32° to 39°F) is satisfactory for the preservation of most of the fresh
foods. For the short term storage of frozen foods, however, temperatures much below the
freezing point are required.
The evaporator in the domestic refrigerator, formed as a box, serves as a freezer for
the storage of frozen food as well as for making ice cubes. It is mounted above the food
storage space. The evaporator is held at a temperature of about –18°C (0°F) and the general
storage space is cooled by natural convection. Mechanical vapour-compression cycle as well
as the absorption cycle is adopted for domestic refrigerators and freezers. The mechanical
vapour-compression system has an edge over the absorption system because of its
compactness and more efficient use of electrical energy. Hence the mechanical vapour-
compression system is almost universally adopted. The rear view of a single door vapour-
compression refrigerator is given in Fig.
Fig. Evaporative condenser

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Fig. Refrigeration by absorption
Fig. A single-door domestic vapour-compression refrigerator (rear view)

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5. SENSORS DEFINITION:
A sensor is a device that detects and responds to some type of input from the physical
environment. The specific input could be light, heat, motion, moisture, pressure, or any one
of a great number of other environmental phenomena. The output is generally a signal that is
converted to human-readable display at the sensor location or transmitted electronically over
a network for reading or further processing.
Classification of sensors based on property is as given below:
1. Temperature - Thermistors, thermocouples, RTD‟s, IC and many more.
2. Pressure - Fibre optic, vacuum, elastic liquid based manometers, LVDT, electronic.
3. Flow - Electromagnetic, differential pressure, positional displacement, thermal mass,
etc.
4. Level Sensors - Differential pressure, ultrasonic radio frequency, radar, thermal
displacement, etc.
5. Proximity and displacement - LVDT, photoelectric, capacitive, magnetic, ultrasonic.
6. Biosensors - Resonant mirror, electrochemical, surface Plasmon resonance, Light
addressable potentio-metric.
7. Image - Charge coupled devices, CMOS
8. Gas and chemical - Semiconductor, Infrared, Conductance, Electrochemical.
9. Acceleration - Gyroscopes, Accelerometers.
10. Others - Moisture, humidity sensor, Speed sensor, mass, Tilt sensor, force, viscosity.
PROXIMITY SENSOR DEFINITION:
A proximity sensor detects the presence of objects that are nearly placed without any
point of contact. Since there is no contact between the sensors and sensed object and lack
of mechanical parts, these sensors have long functional life and high reliability. The different
types of proximity sensors are Inductive Proximity sensors, Capacitive Proximity sensors,
Ultrasonic proximity sensors, photoelectric sensors, Hall-effect sensors, etc.”
[i] Inductive Proximity sensors – They have an oscillator as input to change the loss
resistance by the proximity of an electrically conductive medium. These sensors are
preferred for metal targets.
WORKING:
 Inductive sensors are thee non-contact proximity sensors detect ferrous targets,
ideally mild steel thicker than one millimeter.

24
 They consist of four major components: a ferrite core with coils, an oscillator, a
Schmitt trigger, and an output amplifier.
 The oscillator creates a symmetrical, oscillating magnetic field that radiates from the
ferrite core and coil array at the sensing face.
 When a ferrous target enters this magnetic field, small independent electrical
currents called eddy currents are induced on the metal‟s surface. This changes the
reluctance (natural frequency) of the magnetic circuit, which in turn reduces the
oscillation amplitude.
 As more metal enters the sensing field the oscillation amplitude shrinks, and
eventually collapses. (This is the “Eddy Current Killed Oscillator” or ECKO principle.)
 The Schmitt trigger responds to these amplitude changes, and adjusts sensor output.
When the target finally moves from the sensor‟s range, the circuit begins to oscillate
again, and the Schmitt trigger returns the sensor to its previous output.
If the sensor has a normally open configuration, its output is an on signal when the target
enters the sensing zone. With normally closed, its output is an off signal with the target
present. Output is then read by an external control unit (e.g. PLC, motion controller, smart
drive) that converts the sensor on and off states into useable information.
It should be noted that metallic contaminants (e.g. filings from cutting applications)
sometimes affect the sensor‟s performance. Inductive sensor housing is typically nickel-
plated brass, stainless steel, or PBT plastic.

25
[ii] Capacitive Proximity sensors – They convert the electrostatic capacitance variation
flanked by the detecting electrode and the ground electrode. This occurs by
approaching the nearby object with a variation in an oscillation frequency. To detect
the nearby object, the oscillation frequency is transformed into a direct current voltage
which is compared with a predetermined threshold value. These sensors are
preferred for plastic targets.
WORKING:
Capacitive proximity sensors can detect both metallic and non-metallic targets in powder,
granulate, liquid, and solid form. This, along with their ability to sense through nonferrous
materials, makes them ideal for sight glass monitoring, tank liquid level detection, and hopper
powder level recognition.
 In capacitive sensors, the two conduction plates (at different potentials) are housed in
the sensing head and positioned to operate like an open capacitor.
 Air acts as an insulator; at rest there is little capacitance between the two plates.
 Like inductive sensors, these plates are linked to an oscillator, a Schmitt trigger, and
an output amplifier.
 As a target enters the sensing zone the capacitance of the two plates increases,
causing oscillator amplitude change, in turn changing the Schmitt trigger state, and
creating an output signal.
 Note the difference between the inductive and capacitive sensors: inductive sensors
oscillate until the target is present and capacitive sensors oscillate when the target is
present.

26
Because capacitive sensing involves charging plates, it is somewhat slower than inductive
sensing ranging from 10 to 50 Hz, with a sensing scope from 3 to 60 mm. Many housing
styles are available; common diameters range from 12 to 60 mm in shielded and unshielded
mounting versions. Housing (usually metal or PBT plastic) is rugged to allow mounting very
close to the monitored process.
If the sensor has normally-open and normally-closed options, it is said to have a
complimentary output. Due to their ability to detect most types of materials, capacitive
sensors must be kept away from non-target materials to avoid false triggering. For this
reason, if the intended target contains a ferrous material, an inductive sensor is a more
reliable option.
Applications
 Used in automation engineering to define operating states in process engineering
plants, production systems and automating plants
 Used in windows, and the alarm is activated when the window opens
 Used in machine vibration monitoring to calculate the difference in distance
between a shaft and its support bearing
PHOTOELECTRIC SENSORS
Photoelectric sensors are so versatile that they solve the bulk of problems put to
industrial sensing. Because photoelectric technology has so rapidly advanced, they now
commonly detect targets less than 1 mm in diameter, or from 60 m away. Classified by the
method in which light is emitted and delivered to the receiver, many photoelectric
configurations are available. However, all photoelectric sensors consist of a few of basic
components: each has an emitter light source (Light Emitting Diode, laser diode), a
photodiode or phototransistor receiver to detect emitted light, and supporting electronics
designed to amplify the receiver signal. The emitter, sometimes called the sender, transmits
a beam of either visible or infrared light to the detecting receiver.
All photoelectric sensors operate under similar principles. Identifying their output is thus
made easy; darkon and light-on classifications refer to light reception and sensor output
activity. If output is produced when no light is received, the sensor is dark-on. Output from
light received, and it‟s light-on. Either way, deciding on light-on or dark-on prior to purchasing

27
is required unless the sensor is user adjustable. (In that case, output style can be specified
during installation by flipping a switch or wiring the sensor accordingly.
PRINCIPLE
Different definitions are approved to distinguish sensors and transducers. Sensors
can be defined as an element that senses in one form of energy to produce a variant in same
or another form of energy. Transducer converts the measure and into the desired output
using the transduction principle.
Based on the signals that are obtained and created, the principle can be categorized into
following groups namely, Electrical, Mechanical, Thermal, Chemical, Radiant, and Magnetic.
Let‟s take the example of an ultrasonic sensor.
An ultrasonic sensor is used to detect the presence of an object. It achieves this by emitting
ultrasonic waves from the device head and then receiving the reflected ultrasonic signal from
the concerned object. This helps in detecting the position, presence and movement of
objects.
Since ultrasonic sensors rely on sound rather than light for detection, it is widely used to
measure water-levels, medical scanning procedures and in the automobile industry.

28
Ultrasonic waves can detect transparent objects such as transparent films, glass bottles,
plastic bottles, and plate glass, using its Reflective Sensors.
WORKING
The movement of ultrasonic waves differs due to shape and type of media. For example,
ultrasonic waves move straight in a uniform medium, and are reflected and transmitted back
at the boundary between differing media. A human body in air causes considerable reflection
and can be easily detected.
The travelling of ultrasonic waves can be best explained by understanding the following:
1. Multi-reflection
 Multi-reflection takes place when waves are reflected more than once between the
sensor and the detection object.
2. Limit zone
 The minimum sensing distance and maximum sensing distance can be adjusted. This
is called the limit zone.
3. Undetection zone
 The undetected zone is the interval between the surface of the sensor head and the
minimum detection distance resulting from detection distance adjustment. The figure
is shown below.
The Undetection zone is the area close to the sensor where detection is not possible due to
the sensor head configuration and reverberations. Detection may occur in the uncertainty
zone due to multi-reflection between the sensor and the object.

29
Applications
Sensors are used in many kinds of applications such as:
 Shock Detection
 Machine monitoring applications
 Vehicle dynamics
 Low power applications
 Structural Dynamics
 Medical Aerospace
 Nuclear Instrumentation
 As pressure sensor in Mobiles „touch key pad‟
 Lamps which brighten or dim on touching its base
 Touch sensitive buttons in elevators
Accelerometer sensor working principle:

30
One of the most common inertial sensors is the accelerometer, a dynamic sensor capable of
a vast range of sensing. Accelerometers are available that can measure acceleration in one,
two, or three orthogonal axes. They are typically used in one of three modes:
 As an inertial measurement of velocity and position;
 As a sensor of inclination, tilt, or orientation in 2 or 3 dimensions, as referenced from
the acceleration of gravity (1 g = 9.8m/s2);
 As a vibration or impact (shock) sensor.
There are considerable advantages to using an analog accelerometer as opposed to an
inclinometer such as a liquid tilt sensor – inclinometers tend to output binary information
(indicating a state of on or off), thus it is only possible to detect when the tilt has exceeded
some thresholding angle.
PRINCIPLES OF OPERATION
Most accelerometers are Micro-Electro-Mechanical Sensors (MEMS). The basic principle of
operation behind the MEMS accelerometer is the displacement of a small proof mass etched
into the silicon surface of the integrated circuit and suspended by small beams.
Consistent with Newton's second law of motion (F = ma), as an acceleration is applied to the
device, a force develops which displaces the mass. The support beams act as a spring, and
the fluid (usually air) trapped inside the IC acts as a damper, resulting in a second order
lumped physical system. This is the source of the limited operational bandwidth and non-
uniform frequency response of accelerometers. For more information, see reference to
Elwenspoek, 1993.
Types of Accelerometer
There are several different principles upon which an analog accelerometer can be built. Two
very common types utilize capacitive sensing and the piezoelectric effect to sense the
displacement of the proof mass proportional to the applied acceleration.
Capacitive
Accelerometers that implement capacitive sensing output a voltage dependent on the
distance between two planar surfaces. One or both of these “plates” are charged with an
electrical current. Changing the gap between the plates changes the electrical capacity of the
system, which can be measured as a voltage output. This method of sensing is known for its
high accuracy and stability. Capacitive accelerometers are also less prone to noise and
variation with temperature, typically dissipates less power, and can have larger bandwidths
due to internal feedback circuitry. (Elwenspoek 1993)

31
Piezoelectric
Piezoelectric sensing of acceleration is natural, as acceleration is directly proportional to
force. When certain types of crystal are compressed, charges of opposite polarity
accumulate on opposite sides of the crystal. This is known as the piezoelectric effect. In a
piezoelectric accelerometer, charge accumulates on the crystal and is translated and
amplified into either an output current or voltage.
Piezoelectric accelerometers only respond to AC phenomenon such as vibration or shock.
They have a wide dynamic range, but can be expensive depending on their quality (Doscher
2005) Piezo-film based accelerometers are best used to measure AC phenomenon such as
vibration or shock, rather than DC phenomenon such as the acceleration of gravity. They are
inexpensive, and respond to other phenomenon such as temperature, sound, and pressure
(Doscher 2005)
6. OVERVIEW OF OTHER TYPES THAT ARE LESS USED IN
AUDIO APPLICATIONS
Piezo resistive
Piezo resistive accelerometers (also known as Strain gauge accelerometers) work by
measuring the electrical resistance of a material when mechanical stress is applied. They are
preferred in high shock applications and they can measure acceleration down to 0Hz.
However, they have a limited high frequency response.
Hall Effect
Hall Effect accelerometers work by measuring the voltage variations caused by the change
in magnetic field around them.
Heat transfer
Heat transfer accelerometers consist in a single heat source centered in a substrate and
suspended across cavity. They include equally spaced thermo resistors on the four side of
the heat source. They measure the internal changes in heat due to acceleration. When there
is zero acceleration, the heat gradient will be symmetrical. Otherwise, under acceleration, the
heat gradient will become asymmetrical due to convection heat transfer
Specifications
A typical accelerometer has the following basic specifications:
 Analog/digital

32
 Number of axes
 Output range (maximum swing)
 Sensitivity (voltage output per g)
 Dynamic range
 Bandwidth
 Amplitude stability
 Mass
Analog vs. digital:
The most important specification of an accelerometer for a given application is its type of
output. Analog accelerometers output a constant variable voltage depending on the amount
of acceleration applied. Older digital accelerometers output a variable frequency square
wave, a method known as pulse-width modulation. A pulse width modulated accelerometer
takes readings at a fixed rate, typically 1000 Hz (though this may be user-configurable based
on the IC selected). The value of the acceleration is proportional to the pulse width (or duty
cycle) of the PWM signal. Newer digital accelerometers are more likely to output their value
using multi-wire digital protocols such as I2C or SPI. For use with ADCs commonly used for
music interaction systems, analog accelerometers are usually preferred.
Number of axes:
Accelerometers are available that measure in one, two, or three dimensions. The most
familiar type of accelerometer measures across two axes. However, three-axis
accelerometers are increasingly common and inexpensive.
Output range:
To measure the acceleration of gravity for use as a tilt sensor, an output range of ±1.5 g is
sufficient. For use as an impact sensor, one of the most common musical applications, ±5 g
or more is desired.
Sensitivity:
An indicator of the amount of change in output signal for a given change in acceleration. A
sensitive accelerometer will be more precise and probably more accurate.
Dynamic range:
The range between the smallest acceleration detectable by the accelerometer to the largest
before distorting or clipping the output signal.

33
Bandwidth:
The bandwidth of a sensor is usually measured in Hertz and indicates the limit of the
near-unity frequency response of the sensor, or how often a reliable reading can be taken.
Humans cannot create body motion much beyond the range of 10-12 Hz. For this reason, a
bandwidth of 40-60 Hz is adequate for tilt or human motion sensing. For vibration
measurement or accurate reading of impact forces, bandwidth should be in the range of
hundreds of Hertz. It should also be noted that for some older microcontrollers.
The bandwidth of an accelerometer may extend beyond the Nyquist frequency of the
A/D converters on the MCU, so for higher bandwidth sensing, the digital signal may be
aliased. This can be remedied with simple passive low-pass filtering prior to sampling, or by
simply choosing a better microcontroller. It is worth noting that the bandwidth may change by
the way the accelerometer is mounted. A stiffer mounting (ex: using studs) will help to keep a
higher usable frequency range and the opposite (ex: using a magnet) will reduce it.
Amplitude stability:
This is not a specification in itself, but a description of several. Amplitude stability
describes a sensor's change in sensitivity depending on its application, for instance over
varying temperature or time (see below).
Mass:
The mass of the accelerometer should be significantly smaller than the mass of the
system to be monitored so that it does not change the characteristic of the object being
tested.
OTHER SPECIFICATIONS INCLUDE:
 Zero g offset (voltage output at 0 g)
 Noise (sensor minimum resolution)
 Temperature range
 Bias drift with temperature (effect of temperature on voltage output at 0 g)
 Sensitivity drift with temperature (effect of temperature on voltage output per g)
 Power consumption
OUTPUT
An accelerometer output value is a scalar corresponding to the magnitude of the
acceleration vector. The most common acceleration, and one that we are constantly exposed
to, is the acceleration that is a result of the earth's gravitational pull. This is a common

34
reference value from which all other accelerations are measured (known as g, which is
~9.8m/s^2).
Digital output
Accelerometers with PWM output can be used in two different ways. For most
accurate results, the PWM signal can be input directly to a microcontroller where the duty
cycle is read in firmware and translated into a scaled acceleration value. (Check with the
datasheet to obtain the scaling factor and required output impedance.) When a
microcontroller with PWM input is not available, or when other means of digitizing the signal
are being used, a simple RC reconstruction filter can be used to obtain an analog voltage
proportional to the acceleration. At rest (50% duty-cycle) the output voltage will represent no
acceleration, higher voltage values (resulting from a higher duty cycle) will represent positive
acceleration, and lower values (<50% duty cycle) indicate negative acceleration. These
voltages can then be scaled and used as one might the output voltage of an analog output
accelerometer. One disadvantage of a digital output is that it takes a little more timing
resources of the microcontroller to measure the duty cycle of the PWM signal.
Communication protocols could use I2C or SPI.
Analog output
When compared to most other industrial sensors, analog accelerometers require little
conditioning and the communication is simple by only using an Analog to Digital Converter
(ADC) on the microcontroller. Typically, an accelerometer output signal will need an offset,
amplification, and filtration. For analog voltage output accelerometers, the signal can be a
positive or negative voltage, depending on the direction of the acceleration. Also, the signal
is continuous and proportional to the acceleration force. As with any sensor destined for an
analog to digital converter, the value must be scaled and/or amplified to maximally span the
range of acquisition. Most analog to digital converters used in musical applications acquire
signals in the 0-5 V range. The image at right depicts an amplification and offset circuit,
including the on-board operational amplifier in the adxl 105, minimizing the need for
additional IC components. The gain applied to the output is set by the ratio R2/R1. The offset
is controlled by biasing the voltage with variable resistor R4. Accelerometers output bias will
drift according to ambient temperature. The sensors are calibrated for operation at a specific
temperature, typically room temperature. However, in most short duration indoor applications
the offset is relatively constant and stable, and thus does not need adjustment.
USES
A system consisting of two orthogonal sensors is capable of sensing pitch and roll. This is
useful in capturing head movements. A third orthogonal sensor can be added to the network

35
to obtain orientation in three dimensional spaces. This is appropriate for the detection of pen
angles, etc. Verplaetse has outlined the bandwidths associated with various implementations
of accelerometers as an input device. These are:
Location Usage Frequency Acceleration
Head Tilt 0-8 Hz xx
Hand , Wrist, Finger Cont. 8-12 Hz 0.04-1.0 g
Hand, Arm, Upper Body Cont. 0-12 Hz 0.5-9.0 g
Foot, Leg Cont. 0-12 Hz 0.2-6.6 g
Depending on the sensitivity and dynamic range required, the cost of an
accelerometer can grow to thousands of dollars. Nonetheless, highly accurate inexpensive
sensors are available.

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