Unit III
Permanent magnet moving coil instruments (PMMC)
Construction
The permanent magnet moving coil instruments are most accurate type for direct current
measurements. This consists of a coil on a metal or insulated former which is suspended in a
permanent magnetic field. When the current flows through the coil a torque will be set up.
The coil then gets displaced against a torque provided by a pair of hair spring which provides
the restraining torque.
Generally the shape of the coil is rectangular & is placed in a radial magnetic field. The
springs are generally of phosphors bronze & are provided at each end of the spindle. The
springs which are wound in the opposite directions provide torque which is proportional to the
deflection.
The hair spring further act as carrier of a current, supplying the coil.
Working principle
Instrument coil current I ampere & deflection of it produces Ɵ due to current I ampere, the torque
produced by the spring that restrain the coil movement= KƟ Nm the spring factor K is constant at
steady deflection of the instrument.
Torque due to the coil current= restraining torque of the spring= KƟ.
Deflection is proportional to current. Hence the scale is uniform.
For indicating instruments the former is made of a metallic aluminum former.
In this former eddy current develops a damping torque which opposes the movement of the disk
about the final position & prevents oscillations of the pointer by critical damping action.
The PMMC instrument is shown in the below images
.
Advantages of PMMC
 It has uniform scale.
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
With a powerful magnet, its torque to weight ratio is very high. So operating current of
PMMC is small.
 The sensitivity is high.
 The eddy currents induced in the metallic former over which coil is wound, provide
effective damping.
 It consumes low power, of the order of 25 W to 200 mW.
 It has high accuracy.
 Instrument is free from hysteresis error.
 Extension of instrument range is possible.
 Not affected by external magnetic fields called stray magnetic fields.
Disadvantages of PMMC

PMMC is Suitable for direct current measurement only.

Ageing of permanent magnet and the control springs introduces the errors.

The cost is high due to delicate construction and accurate machining.

The friction is due to jewel-pivot suspension.
Moving Iron or MI Instrument
Definition: The instrument in which the moving iron is used for measuring the flow of current
or voltage is known as the moving iron instrument. It works on the principle that the iron
place near the magnet attracts towards it. The force of attraction depends on the strength of
the magnet field.
Construction and working of Moving Iron Instrument
The plate or vane of soft iron is used as the moving element of the instrument. The vane is so
placed that it can freely move in the magnetic field of the stationary coil. The conductor makes
the stationary coil, and it is excited by the voltage or current whose magnitude is used to be
measured.
The moving iron instrument uses the stationary coil as an electromagnet. The electromagnet is
the temporary magnet whose magnetic field strength increases or decreases with the magnitude
of the current passes through it.
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Working of the Moving Iron Instrument
The moving iron instruments use the stationary coil of copper or aluminium wire which acts as
an electromagnet when an electric current passes through it. The strength of the magnetic field
induces by the electromagnet is directly proportional to the current passes through it.
The plates or vane of the iron pass through the coil increases the inductance of the stationary coil
(the inductance is the property of the conductor which increases their electromotive force when
the varying current passes through it).
The electromagnet attracts the iron vane. The vane passes through the coil tries to occupy the
minimum reluctance path (the reluctance is the property of the magnet which opposes the flow
of electric current).
The vane passes through the coil experience a force of repulsion caused by the electromagnet. The
repulsion force increases the strength of the coil inductance.
This happens because the inductance and reluctances are inversely proportional to each other.
Advantages of the MI Instruments
The following are the advantages of the moving iron instruments.
Universal use – The MI instrument is independent of the direction of current and hence used for
both AC and DC.
Less Friction Error – The friction error is very less in the moving iron instrument because their
torque weight ratio is high. The torque weight ratio is high because their current carrying part is
stationary and the moving parts are lighter in weight.
Cheapness – The MI instruments require less number of turns as compared to PMMC instrument.
Thus, it is cheaper.
Robustness – The instrument is robust because of their simple construction. And also because
their current carrying part is stationary.
Disadvantages of Moving Iron Instruments.
The following are the disadvantages of Moving Iron Instrument.
1. Accuracy – The scale of the moving iron instruments is not uniform, and hence the
accurate result is not possible.
2. Errors – Some serious error occurs in the instruments because of the
hysteresis, frequency and stray magnetic field.
3. Waveform Error – In MI instrument the deflection torque is not directly proportional to
the square of the current. Because of which the waveforms error occurs in the instrument.
4. Difference between AC and DC calibration – The calibration of the AC and DC are
differed because of the effect of the inductance of meter and the eddy current which is
used on AC. The AC is calibrated on the frequency at which they use.
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Tariff
The cost of electrical generation of electrical energy consists of fixed cost and
running cost. Since the electricity generated is to be supplied to the consumers, the
total cost of generation has to be recovered from the consumers. • Tariffs or energy
rates are the different methods of charging the consumers for the consumption of
energy. It is desirable to charge according to the maximum demand (KW) and the
energy consumed (KWh).
Objectives of Tariff :
(i) Recovery of cost of capital investment in generating equipment, transmission
and distribution system.
(ii) Recovery of the cost of operation, supplies and maintenance.
(iii) Recovery of the cost material, equipment, billing and collection, cost as well as
for miscellaneous services.
(iv) A net return on the total capital investment must be ensured.
(v)
Requirements of Tariff:
1. It should be easier to understand.
2. It should provide low rates for high consumption.
3. It should be uniform over large population.
4. It should encourage the consumers having high load factors.
5. It should provide incentives for using power during off peak hours.
6. It should have a provision of penalty for low PF
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38
4.1 Tariff
Types of Tariff
(i) Simple Tariff: When there is a fixed rate per unit of energy consumed, it is
known as simple tariff (Uniform Rate Tariff). This is the most simplest of all
tariff. In this type, the price charged per unit is constant. It means, the price
will not vary with increase or decrease in number of units used.
Disadvantages: • The cost per unit delivered is high. • There is no discrimination
among various types of consumers. Flat Rate Tariff: When different types of
consumers are charged at different uniform per unit rates, it is said to be
(ii) Flat rate Tariff: In this type, the consumers are grouped into different classes.
Each class is charged at different uniform rate, the different classes of
consumers may be taken into account of their diversity and load factors. Since
this type of tariff varies according to the way of supply used, separate meters
are required for lighting load, power load etc.
(iii) Block rate tariff: When a given block of energy is charged at a specified rate
and the succeeding blocks of energy are charged at progressively reduced rates
is called as block rate tariff. In this type, the energy consumption is divided
into many blocks and price per unit is fixed in each block.
(iv) Two Part tariff: When the rate of electrical energy is charged on the basis of
maxi- mum demand of the consumer and the units consumed it is called twopart tariff. In this type, the total charge to be made from the consumer is split
into two components. i.e., fixed charges and running charges. The fixed charges
depend upon the number of units consumed by the customer. Thus the
consumer is charged at a certain amount per kW of maximum demand + a
certain amount per kWh of energy consumed.
Total charges = Rs(X kW + Y kWh)
• It is easily understood by the consumer. • It recovers fixed charges which
depend upon the maximum demand of the consumer independent of the units
consumed. • This form of tariff is generally used for industrial customers.
Disadvantages: • Consumer has to pay the fixed charges irrespective of the
fact whether he has consumed or not the electrical energy. • There is always
error in assessing the maximum demand of the consumer.
(v) Three part Tariff: When the total charges to be made from the consumer is
split into three parts, fixed charge, semi-fixed charge and running charge, it is
known as three-part tariff. This type of tariff is applied to big consumers. The
principle objection of this type of tariff is the charges are split into three
components (fixed charge, charge per kW of maximum demand, charge per
kWh of energy consumed).
(vi) Maximum demand tariff: It is similar to two-part tariff. The only difference is
the maximum demand of the consumer is calculated by installing a maximum
demand meter at his premises. This type of tariff is mostly applied to the bulk
consumers.
(vii) Time-Of-Day tariffs (TOD, peak-load): Rates vary depending on when the
service is being used. For example, the operator would charge higher prices
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during peak use hours and lower prices during off-peak hours to reflect the cost
of generation. This structure requires sophisticated measurement of customer
usage, such as metering technologies. It encourages consumers to use less
power during peak hours. With decreasing costs of TOD meters, use of the
TOD tariff structure is becoming more common.
(viii) Seasonal tariffs: These rates allow higher charges for electricity in summer
and winter when demand for cooling or heating is higher. Typically they are
used in climates where utilities experience significant seasonal cost
differences. With traditional regulation, sea- sonal rates reduce net revenue
stability for utilities by concentrating revenue into the weather-sensitive
seasons.
(ix) Power factor tariff: The tariff in which the power factor of the consumers is
taken into account is known as power factor tariff.
Domestic Wiring
Introduction A network of wires drawn connecting the meter board to the
various energy consuming loads (lamps, fans, motors etc) through control and
protective devices for efficient distribution of power is known as electrical wiring.
Electrical wiring done in residential and commercial buildings to provide power
for lights, fans, pumps and other domestic appliances is known as domestic
wiring. There are several wiring systems in practice.
They can be classified into:
Tree system: In this system branches are tapped from the main circuit at required
points. This involves many joints making the location of the fault point difficult.
Though the method is economical it is visually unappealing with scattered fuses
and is affected by large voltage drops.
Distribution system: This system is more organized in the sense that the main circuit is
drawn to several distribution centers and connected to the distribution boards.
Branches are tapped from these distribution boards. This system of wiring has an
aesthetic appeal, as they are without joints and also makes the location of the fault
point easier. All the points are maintained almost at the same potential. Each
circuit is provided with an independent fuse. Provides flexibility for repair and
maintenance. This system is widely preferred for indoor wiring though expensive.
Types of Wiring:
(i) Cleat wiring
(ii) CTS wiring or TRS wiring or batten wiring
(iii) Metal sheathed wiring or lead sheathed wiring
(iv) Casing and capping
(v) Conduit wiring
Cleat Wiring:
(i) In this type of wiring, insulated conductors (usually VIR, Vulcanized Indian
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Rubber) are supported on porcelain or wooden cleats.
(ii) The cleats have two halves one base and the other cap.
(iii) The cables are placed in the grooves provided in the base and then the cap is placed.
(iv) Both are fixed securely on the walls by 40mm long screws.
(v) The cleats are easy to erect and are fixed 4.5 –15 cms apart.
(vi) This wiring is suitable for temporary installations where cost is the main
criteria but not the appearance.
Figure: Cleat Wiring
Advantages:
(i) Easy installation
(ii) Materials can be retrieved for reuse
(iii) Flexibility provided for inspection
(iv) Relatively economical
Disadvantages:
(i) Appearance is not good
(ii) Higher risk of mechanical injury.
Casing and Capping
Figure: Casing and capping
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(i) It consists of insulated conductors laid inside rectangular, teakwood or PVC
boxes having grooves inside it.
(ii) A rectangular strip of wood called capping having same width as that of
casing is fixed over it.
(iii) Both the casing and the capping are screwed together at every 15 cms.
(iv) Casing is attached to the wall. Two or more wires of same polarity are drawn
through different grooves.
(v) The system is suitable for indoor and domestic installations.
Advantages:
(i) Cheaper than lead sheathed and conduit wiring
(ii) Provides good isolation as the conductors are placed apart reducing the risk of
short circuit
(iii) Easily accessible for inspection and repairs
(iv) Since, the wires are not exposed to atmosphere; insulation is less affected by
dust, dirt and climatic variations.
Disadvantages:
(i) Highly inflammable
(ii) Usage of unseasoned wood gets damaged by termites
(iii) Skilled workmanship is required.
Conduit wiring
(i) In this system PVC (polyvinyl chloride) or VIR cables are run through
metallic or PVC pipes providing good protection against mechanical injury
and fire due to short circuit.
(ii) They are either embedded inside the walls or supported over the walls, and are
known as concealed wiring or surface conduit wiring (open conduit)
respectively.
(iii) The conduits are buried inside the walls on wooden gutties and the wires are
drawn through them with fish (steel) wires.
(iv) The system is best suited for public buildings, industries and workshops
Figure: Conduit wiring
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Advantages:
i.
ii.
iii.
iv.
v.
vi.
vii.
No risk of fire and has good protection against mechanical injury
The lead and return wires can be carried in the same tube
Earthing and continuity is assured
Waterproof and troubleshooting is easy
Shock proof with proper earthing and bonding
Durable and maintenance free
Aesthetic in appearance
Disadvantages:
(i)
(ii)
(iii)
(iv)
(v)
Disadvantages:
Very expensive system of wiring
Requires good skilled workmanship
Erection is quite complicated and is time consuming
Risk of short circuit under wet conditions (due to condensation of water in tubes).
Factors affecting the choice of Wiring System:
The choice of wiring system for a particular installation depends on technical factors
and economic viability.
1. Durability: Type of wiring selected should conform to standard specifications, so that
it is durable i.e. without being affected by the weather conditions, fumes etc.
2. Safety: The wiring must provide safety against leakage, shock and fire hazards for the
operating personnel.
3. Appearance: Electrical wiring should give an aesthetic appeal to the interiors
4. Cost: It should not be prohibitively expensive.
5. Accessibility: The switches and plug points provided should be easily accessible. There
must be provision for further extension of the wiring system, if necessary.
6. Maintenance Cost: The maintenance cost should be a minimum
7. Mechanical safety: The wiring must be protected against any mechanical damage
PROTECTIVE DEVICES
1. Protection for electrical installation must be provided in the event of faults such
as short circuit, overload and earth faults.
2. The protective circuit or device must be fast acting and isolate the faulty part
of the circuit immediately.
3. It also helps in isolating only required part of the circuit without affecting the
remaining circuit during maintenance.
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4. The following devices are usually used to provide the necessary protection:
Fuses, Relays, Miniature circuit breakers (MCB), Earth leakage circuit
breakers (ELCB)
Fuse
1. The electrical equipment’s are designed to carry a particular rated value of
current under normal circumstances.
2. Under abnormal conditions such as short circuit, overload or any fault the
current raises above this value, damaging the equipment and sometimes
resulting in fire hazard.
3. Fuses are pressed into operation under such situations.
4. Fuse is a safety device used in any electrical installation, which forms the
weakest link between the supply and the load.
5. It is a short length of wire made of lead / tin /alloy of lead and tin/ zinc
having a low melting point and low ohmic losses.
6. Under normal operating conditions it is designed to carry the full load current. If
the current increases beyond this designed value due any of the reasons
mentioned above, the fuse melts (said to be blown) isolating the power supply
from the load.
Characteristics of Fuse Material
The material used for fuse wires must have the following characteristics:
1. Low melting point
2. Low ohmic losses
3. High conductivity
4. Lower rate of deterioration
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Different types of fuses:
1. Re-wireable or Kit -Kat fuses: These fuses are simple in construction, cheap and
available up-to a current rating of 200A. They are erratic in operation and their
performance deteriorates with time.
2. Plug fuse: The fuse carrier is provided with a glass window for visual inspection
of the fuse wire.
3. Cartridge fuse: Fuse wire usually an alloy of lead is enclosed in a strong fibre
casing. The fuse element is fastened to copper caps at the ends of the casing.
They are available up-to a voltage rating of 25kV. They are used for protection
in lighting installations and power lines. Miniature Cartridge fuses: These are
the miniature version of the higher rating cartridge fuses, which are extensively
used in automobiles, TV sets, and other electronic equipments.
4. Transformer fuse blocks: These porcelain housed fuses are placed on secondary
of the distribution transformers for protection against short circuits and
overloads.
5. Expulsion fuses: These consist of fuse wire placed in hollow tube of fibre lined
with asbestos. These are suited only for outdoor use for example, protection of
high voltage circuits.
6. Semi-enclosed re-wireable fuses: These have limited use because of low breaking capacity.
7. Time-delay fuse: These are specially designed to withstand a current overload
for a limited time and find application in motor circuits.
MCB (Miniature Circuit Breaker):
Figure: MCB
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1. MCB’s are electromechanical devices which protect an electrical circuit from an
over current.
2. The over current, in an electrical circuit, may result from short circuit,
overload or faulty design.
3. An MCB is a better alternative to a Fuse since it does not require
replacement once an overload is detected.
4. Unlike fuse, an MCB can be easily reset and thus offers improved operational
safety and greater convenience without incurring large operating cost.
5. The principal of operation is simple.
6. An MCB functions by interrupting the continuity of electrical flow through
the circuit once a fault is detected.
7. In simple terms MCB is a switch which automatically turns off when the
current flowing through it passes the maximum allowable limit.
8. Generally MCB are designed to protect against over current and over
temperature faults (over heating).
9. There are two contacts one is fixed and the other moveable.
10. When the current exceeds the predefined limit a solenoid forces the moveable
contact to open (i.e., disconnect from the fixed contact) and the MCB turns off
thereby stopping the current to flow in the circuit.
11. In order to restart the flow of current the MCB is manually turned on.
12. This mechanism is used to protect from the faults arising due to over current or over load.
13. To protect against fault arising due to over heating or increase in temperature a
bi-metallic strip is used.
14. MCBs are generally designed to trip within 2.5 millisecond when an over
current fault arises.
15. In case of temperature rise or over heating it may take 2 seconds to 2 minutes
for the MCB to trip.
16. The following image shows the different internal parts of an MCB with top casing removed.
EARTHING
1. The potential of the earth is considered to be at zero for all practical
purposes as the generator (supply) neutral is always earthed.
2. The body of any electrical equipment is connected to the earth by means of a
wire of negligible resistance to safely discharge electric energy, which may be
due to failure of the insulation, line coming in contact with the casing etc.
3. Earthing brings the potential of the body of the equipment to ZERO i.e. to the
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earth’s potential, thus protecting the operating personnel against electrical
shock.
4. The body of the electrical equipment is not connected to the supply neutral
because due to long transmission lines and intermediate substations, the same
neutral wire of the generator will not be available at the load end. Even if the
same neutral wire is running it will have a self-resistance, which is higher than
the human body resistance. Hence, the body of the electrical equipment is
connected to earth only.
5. Thus earthing is to connect any electrical equipment to earth with a very low
resistance wire, making it to attain earth’s potential.
6. The wire is usually connected to a copper plate placed at a depth of 2.5 to
3meters from the ground level.
The earth resistance is affected by the following factors:
1.
Material properties of the earth wire and the electrode
2. Temperature and moisture content of the soil
3. Depth of the pit
4. Quantity of the charcoal used
Necessity of Earthing:
1. To protect the operating personnel from danger of shock in case they come in
contact with the charged frame due to defective insulation.
2. To maintain the line voltage constant under unbalanced load condition.
3. Protection of the equipments
4. Protection of large buildings and all machines fed from overhead lines against
lightning.
Methods of Earthing:
1. The important methods of earthing are the plate earthing and the pipe earthing.
2. The earth resistance for copper wire is 1 ohm and that of G I wire less than 3 ohms.
3. The earth resistance should be kept as low as possible so that the neutral of
any electrical system, which is earthed, is maintained almost at the earth
potential.
4. The typical value of the earth resistance at powerhouse is 0. 5 ohm and that at
substation is 1 ohm.
Types of Earthing
1. Plate earthing 2. Pipe earthing
PLATE EARTHING
• In this method a copper plate of 60cm x 60cm x 3.18cm or a GI plate of the size
60cm x 60cm x 6.35cm is used for earthing.
1. The plate is placed vertically down inside the ground at a depth of 3m and is
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embedded in alternate layers of coal and salt for a thickness of 15 cm.
2. In addition, water is poured for keeping the earth electrode resistance value well
below a maximum of 5 ohms.
3. The earth wire is securely bolted to the earth plate.
4. A cement masonry chamber is built with a cast iron cover for easy regular
maintenance.
5. Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X 3.0 M.
6. Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more Contact of
Earth and reduce Earth Resistance.
7. Make a mixture of Wood Coal Powder Salt & Sand all in equal part
8. Wood Coal Powder use as good conductor of electricity, anti corrosive, rust
proves for GI Plate for long life.
9. The purpose of coal and salt is to keep wet the soil permanently.
10. The salt percolates and coal absorbs water keeping the soil wet.
11. Care should always be taken by watering the earth pits in summer so that the
pit soil will be wet.
12. Coal is made of carbon which is good conductor minimizing the earth
resistant. Salt use as electrolyte to form conductivity between GI Plate Coal
and Earth with humidity.
13. Sand has used to form porosity to cycle water & humidity around the mixture.
Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the
mid of mixture.
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PIPE EARTHING
1. Earth electrode made of a GI (galvanized) iron pipe of 38mm in diameter and
length of 2m (depending on the current) with 12mm holes on the surface is
placed upright at a depth of 4.75m in a permanently wet ground.
2. To keep the value of the earth resistance at the desired level, the area (15 cm)
surrounding the GI pipe is filled with a mixture of salt and coal.
3. The efficiency of the earthing system is improved by pouring water through
the funnel periodically.
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4. A cement masonry chamber is built with a cast iron cover for easy regular
maintenance.
5. The GI earth wires of sufficient cross- sectional area are run through a 12.7mm
diameter pipe (at 60cms below) from the 19mm diameter pipe and secured
tightly at the top as shown in the following figure.
6. When compared to the plate earth system the pipe earth system can carry
larger leakage currents as a much larger surface area is in contact with the soil
for a given electrode size.
7. The system also enables easy maintenance as the earth wire connection is
housed at the ground level.
Figure: Pipe Earthing
Sensors
A device which detects or measures a physical property and records, indicates, or
otherwise responds to it.
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Types of Sensors:
(i) Acoustic & sound sensors e.g.: Microphone, Hydrophone.
(ii) Automotive sensors e.g.: Speedometer, Radar gun, Speedometer, fuel ratio meter.
(iii) Chemical Sensors e.g.: Ph sensor, Sensors to detect presences of different gases or liquids.
(iv) Electric & Magnetic Sensors e.g.: Galvanometer, Hall sensor (measures flux
density), Metal detector.
(v) Environmental Sensors e.g.: Rain gauge, snow gauge, moisture sensor.
(vi) Optical Sensors e.g.: Photodiode, Phototransistor, Wave front sensor.
(vii) Mechanical Sensors e.g.: Strain Gauge, Potentiometer (measures displacement).
(viii) Thermal & Temperature sensors. e.g.: Calorimeter, Thermocouple, Thermistor,
Gardon gauge.
(ix) Proximity & Presences sensors: A proximity or presences sensor is the one
which is able to detect the presences of nearby objects without any physical
contact. They usually emit electromagnetic radiations and detect the changes in
reflected signal if any. e.g.: Doppler radar, Motion detector.
Ultrasonic / Level sensor
As the name indicates, ultrasonic / level sensors measure distance by using ultrasonic
waves.The sensor head emits an ultrasonic wave and receives the wave reflected back
from the target. ultrasonic / level sensors measure the distance to the target by
measuring the time between the emission and reception.
Figure: Ultrasonic Waves
An Optical sensor has a transmitter and receiver, whereas an ultrasonic / level
sensor uses a single ultrasonic element for both emission and reception. In a
reflective model ultrasonic
/ level sensor, a single oscillator emits and receives ultrasonic waves alternately.
This enables miniaturisation of the sensor head.
The distance can be calculated with the following formula:
Distance L = 1 × T ×2C
where L is the distance, T is the time between the emission and reception, and C is
the sonic speed. (The value is multiplied by 1/2 because T is the time for go-andREVA University
return distance.)
Other Applications Ultrasonic material testing (to detect cracks, air bubbles,
and other flaws in the products), Object detection, position detection, ultrasonic
mouse, etc.
Figure 4.7: Block Diagram
Figure : Using Aurdino
Displacement sensor
Measurements of size, shape and position utilize displacement sensors.
Ex: diameter of part under stress (direct), Movement of microphone diaphragm to
quantify liq- uid movement through heart (Indirect). A Displacement Sensor
measures and detects changes (displacement) in a physical quantity. The Sensor
can measure the height, width, and thickness of an object by determining the
amount of displacement of that object.
A Measurement Sensor is a device that measures the dimensions of an object by converting changes in amount of light into electrical signals when the object interrupts
a wide laser beam.
A Measurement Sensor measures the position and dimensions of an object.
Figure: Displacement Sensor
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Rotary Encoders
A Rotary encoder, also called a shaft encoder, is an electro-mechanical device that
converts the angular position or motion of a shaft or axle to an analog or digital
signal. It is used for sensing in myriad applications on motors paired with drives and
automated machinery for everything from consumer electronics, elevators, and
conveyor speed monitoring to position control on au- tomated industrial machines
and robotics. They track the turning of motor shafts to generate digital position and
motion information.
There are two main types: Absolute and Incremental (relative). The output of
absolute en- coders indicates the current position of the shaft, making them angle
transducers. The output of incremental encoders provides information about the
motion of the shaft, which is typically further processed elsewhere into
information such as speed, distance and position.
Figure: Rotary Encoder Sensor
Photoelectric sensor
A photoelectric sensor emits a light beam (visible or infrared) from its light-emitting
element.
A reflective-type photoelectric sensor is used to detect the light beam
reflected from the target. A thrubeam type sensor is used to measure the change in
light quantity caused by the target crossing the optical axis.
Figure: Photoelectric sensor
Figure: Through Beam
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The transmitter and receiver are separated. When the target is between the
transmitter and receiver, the light is interrupted.
Both the light emitting and light receiving elements are contained in same
housing. The light from the emitting element hits the reflector and returns to the
light receiving element. When a target is present, the light is interrupted.
Strain Gauge Sensor
The strain gauge has been in use for many years and is the fundamental sensing
element for many types of sensors, including pressure sensors, load cells, torque
sensors, position sensors,
Figure: Retro reflective model
etc. The majority of strain gauges are foil types, available in a wide choice of
shapes and sizes to suit a variety of applications. They consist of a pattern of
resistive foil which is mounted on a backing material. They operate on the
principle that as the foil is subjected to stress, the resistance of the foil changes in
a defined way.
Pressure sensor
Since a long time, pressure sensors have been widely used in fields like Automobile,
Manufacturing, Aviation, Bio medical measurements, Air conditioning, Hydraulic
measurements etc. A few prominent areas where the use of pressure sensors is
inevitable are:
1. Touch Screen Devices: The computer devices and smart phones that have touch
screen displays come with pressure sensors. Whenever slight pressure is applied on
the touch screen through a finger or the stylus, the sensor determines where it has
been applied and accordingly generates an electric signal that informs the processor.
Usually, these sensors are located at the corners of the screen. So when the pressure
is applied, usually two or more such sensors act to give precise location information
of the location.
2. Automotive Industry: In automotive industry, pressure sensors form an
integral part of the engine and its safety. In the engine, these sensors monitor the
oil and coolant pressure and regulate the power that the engine should deliver to
achieve suitable speeds whenever accelerator is pressed or the brakes are applied
to the car.
For the purpose of safety, pressure sensors constitute an important part of AntiLock Braking system (ABS). This system adapts to the road terrain and makes sure
that in case of braking at high speeds, the tires don’t lock and the vehicle doesn’t
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skid. Pressure sensors in the ABS detail the processor with the conditions of the
road as well as the speed with which the vehicle is moving. Air bag systems also
use pressure sensors so that the bags get activated to ensure the safety of the
passengers whenever high amount of pressure is experienced by the vehicle.
3. Bio Medical Instrumentation: In instruments like digital blood pressure
monitors and ventilators, pressure sensors are needed to optimize them according
to patient’s health and his requirements.
4. Industrial Uses: Pressure sensors are used to monitor gases and their partial
pressures in industrial units so that the large chemical reactions take place in
precisely controlled environmental conditions. In oil industry, sensors detail with
the depth that the oil rig has reached while exploring.
Aviation: In the airplanes, these sensors are needed to maintain a balance
between the atmospheric pressure and the control systems of the airplanes. This not
only protects the circuitry and various internal components of the airplane but also
gives exact data to the system about the external environment. Also, particular
levels of air pressure need to be maintained in the cockpit and the passengers lobby
to provide nominal ground like breathing conditions.
5.
6. Marine Industry: For ships and submarines, pressure sensors are needed to
estimate the depth at which they are operating and for detailing the marine
conditions so that the electronic systems can remain safe. Oxygen requirements of
under water projects are also regulated by the pressure sensors. The sensing
element reacts to the force or pressure of the process, creating an output signal that
can be interpreted by a readout device or a data-collection device. The sensing
element, therefore, is the heart of the transducer or load cell.
Figure: Pressure Sensor
REVA University
REVA University