Unit 5

Introduction Of Transducers
5/9/2018kongunadu college of engineering and technology1
““ENERGY CANENERGY CAN
NEITHER BE CREATEDNEITHER BE CREATED
NOR BE DESTROYED,NOR BE DESTROYED,
IT CAN ONLY BE TRANSFORMEDIT CAN ONLY BE TRANSFORMED
FROMFROM
ONE STATE TO ANOTHER”ONE STATE TO ANOTHER”

Transducer is a device, usually
electrical, electronic, or
electromechanical, that converts one
type of energy to another for various
purposes including measurement or
information transfer.
It must produce the output which
depends on some way on the input
quantity/measurand.

Its output will depends on its input if it
processes upon the actual input.
So transducer extracts some energy
from the measured medium.
Some energy is required to convert the
in input to output.
This energy can be supplied from
outside or by the measurand only.

Transducers are divided into two types:
Active Transducer
Passive Transducer

Active Transducers
An active transducer does not require
an external power supply to provide an
output signal.
These transducers usually rely upon
magnetic inductance or piezoelectric
effects to produce their output signals.
Ex. Piezoelectric crystal used for
acceleration measurement.

Active Transducers
Measurand is converted into output
without any other form of energy
requirements.
They are also called self generating
type transducers.

Passive Transducers
Passive transducers need an external
power supply in order to amplifies the
input and generate an output signal.
All passive transducers change one of,
or a combination of, the three general
circuit parameters; resistance,
inductance, or capacitance.
Ex. Potentiometer is used for
measurement of pressure, displacement
and position.

Basic Requirements Of Transducer
Linearity: Linearity of any transducer is the
prime requirement. A transducer having
linear input output characteristics is a big
plus.
Repeatability: A transducer having this
quality produces the same result again and
again when the same input signal is applied
repeatedly under same environmental
conditions ex. Temperature, pressure,
humidity etc.

Ruggedness: A transducer should be
robust in construction. It should be
mechanically rugged, so that is capable
of withstanding overload.
High Signal To Noise Ratio: The
quality of output signal of transducer
should be good; it should be free from
the internal and external noise.

Highly Reliable: Output of the transducer
should be highly reliable and stable it should
be precise. It should give minimum error in
measurement for temperature variations,
humidity vibrations.
Good Dynamic Response: A transducer
may be called upon to respond to either
slowly varying or dynamic signals. Its output
should be faithful to input when taken as a
function of time.

No Hysteresis: A good transducer is
free from hysteresis. It should not
introduce any hysteresis during
measurement while input signal is varied
from its low value to high value and vice
versa.
Residual Deformation: There should be
no deformation of testing material after
the removal of any pressure after long
period of application.

Variable Resistance Type
Transducer
The variable resistance transducers are
one of the most commonly used types
of transducers. The variable resistance
transducers are also called as resistive
transducers or resistive sensors.
They can be used for measuring
various physical quantities like
temperature, pressure, displacement,
force, vibrations etc.

Transducer
These transducers are usually used as
the secondary transducers, where the
output from the primary mechanical
transducer acts as the input for the
variable resistance transducer.
The output obtained from it is calibrated
against the input quantity and it directly
gives the value of the input.

Principle Of Working Of Variable
Resistance Type Transducer
The variable resistance transducer
elements work on the principle that the
resistance of the conductor is directly
proportional to the length of the
conductor and inversely proportional to
the area of the conductor. Thus if L is
the length of the conductor (in m) and A
is its area (in m square), its resistance
(in ohms) is given by:
R = ρ L/A.

L is the length ,A is cross sectional area and
p is resistivity of the resistance material.
So resistance can be changed if any of
these value p ,L or A is changed.
Measurand is connected to the resistance in
such a way that it varies any one of its
parameters.
A change in the value or R is proportional to
the measurand.
Thus the measurand can be measured by
measuring the change in resistance.

Where ρ is called as resistivity of the
material and it is constant for he
materials and is measured in ohm-m.
The resistance of some materials also
changes with the change in their
temperature. This principle is primarily
used for the measurement of
temperature.

Transducer
 Some of the variable resistive
transducers are:
Sliding wire(potentiometer)
Strain gauge
Load cell
RTD
Thermistor
Hot wire anemometer
Carbon microphone
Humidity sensor

Potentiometer
A potentiometer informally, a pot, is a
three-terminal resistor with a sliding
contact that forms an adjustable voltage
divider. If only two terminals are used
(one side and the wiper), it acts as a
variable resistor or rheostat.
Potentiometers are commonly used to
control electrical devices such as
volume controls on audio equipment.

Potentiometer
The potentiometer is a displacement
transducer.
This is a passive transducer.
It consists of resistive material whose
resistance is proportional to its length.
Slider displacement x is proportional to
the output voltage.
The most common problem is dirt under
the slider.

Potentiometer
One end of conductor is fixed.
The position of the other end is decided
by the slider/brush.
This contact can move along the whole
length of the conductor.
When the body moves the slider also
moves along the conductor so its
effective length changes.
Due to which its resistance also
changes.

Potentiometer
The effective resistance value is
measured between the fixed position of
the conductor and the position of the
slider.
The resistance is calibrated against the
input quantity, whose value can be
measured directly.

Potentiometer

Thermistor
Thermistors are thermally sensitive
resistors, generally composed of
semiconductor materials. All resistors
vary with temperature, but thermistors
are constructed of semiconductor
material with a resistivity that is
especially sensitive to temperature. This
resistance changes predictably with
temperature.

Thermistor
Although Positive Temperature
Coefficient(PTC) units are available,
most thermistors have a Negative
Temperature Coefficient(NTC); that is,
their resistance decreases with
increasing temperature.
The thermistor is an extremely non-
linear device.

Thermistor
The price we pay for this increased
sensitivity is loss of linearity.
These are usually connected to an
electronic circuit that reads out
temprature digitally.

Thermistor
The resistance temperature relation is
generally of the form:
R = R0 exp[β(1/T – 1/T0)]
 R = Resistance at temp. T, Ω
 R0= Resistance at temp. T0, Ω
 β = Constant, Characterstics of
material
T, T0 Absolute tempratures, K

Thermistor
NTC thermistors operates over a range of
-200ºC to +1000ºC.NTCs should be
chosen when a continuous change of
resistance is required over a wide
temperature range. They offer mechanical,
thermal and electrical stability, together
with a high degree of sensitivity.
 PTC thermistors are temp dependent
resistors manufactured from Barium
Titanate and should be chosen when a
drastic change in resistance is required at
a specific temperature or current level.

Thermistor

Advantages Of Thermistor
Inexpensive
Rugged
Reliable
Respond Quickly
Highly Sensitive
Manufactured in a wide range of
shapes, sizes and values.

Humidity Measurement
Humidity is the amount of water vapour
in the air and Humidity Measurement is
a measure of relative amount of water
vapour present in the air or a gas.
The humidity can be expressed in
different ways:
Absolute Humidity
Relative Humidity
Dew Point

Devices that indirectly measure
humidity by sensing changes in
physical or electrical properties in
materials due to their moisture content
are called hygrometers.
The three major instruments used for
measuring humidity in industry are:
The Electrical Hygrometer
The Psychrometer
The Dew Point Meter

Resistance Hygrometer
This is an electrical hygrometer.
It is an active transducer.
These instruments are suitable for
measuring moisture levels between 15%
and 95%.
It has typical measurement uncertainty of
3%.
Atmospheric contaminates and operation
in saturation conditions both cause
characteristics drift.

Principle Of Resistance Hygrometer
Some Hygroscopic Salts exhibit a
change in resistivity with humidity.
Resistive hygrometer humidity sensors
use the change in resistance of a
hygroscopic material between two
electrodes on an insulating substrate.
The hygroscopic salt is deposited
between two electrodes. The resistance
of the element changes when it is
exposed to variations in humidity.

Resistance Hygrometer
The Resistance Hygrometer should
not be exposed to conditions of 100%
humidity as the resulting condensation
may damage the device.
These are accurate to within ± 2.5 %
or ± 1.5 % in some cases.
Response times are typically of the
order of a few seconds.

APPLICATIONSAPPLICATIONS
Humidity sensors can be used not only
to measure the humidity in an
atmosphere but also to automatically
control:
-> Humidifiers
-> Dehumidifiers
-> Air conditioners for adjustment.
5/9/201838 kongunadu college of engineering and technology

Strain Gauge Pressure Transducer
When the tension is applied to the
electrical conductor, its length increases
while the cross section area decreases.
So its resistance changes.
This change can be measured to
measured.
Used for the measurement of force,
stress and strain.

A strain gauge is a passive type
resistance pressure transducer whose
electrical resistance changes when it is
stretched or compressed
A pressure transducer contains a
diaphragm which is deformed by the
pressure which can cause a strain
gauge to stretch or compress. This
deformation of the strain gauge causes
the variation in length and cross
sectional area due to which its
resistance changes.

The wire filament is attached to a
structure under strain and the resistance
in the strained wire is measured.

Construction and Working

Strain gauge pressure transducers are
used for narrow pressure span and for
differential pressure measurments
 Available for pressure ranges as low
as 3 inches of water to as high as
200,000 psig
Inaccuracy ranges from 0.1 % of span
to 0.25 % of full scale

RESISTANCE TEMPERATURE
DETECTOR (RTD)
Resistance temperature detector (RTD)
devices are conductors used for
temperature sensing.
They can be used in bridge method as
well as ohmmeter method to take the
output.
The change in resistance of material
per unit change in temperature should
be as large as possible.

RESISTANCE TEMPERATURE
DETECTOR (RTD)
The material should have high value of
resistivity to get required value in less
space.
Resistance and temperature relation
should be continuous and stable.
Platinum, nickel and copper are the
most commonly used.
Tungsten and nickel alloy are also
used.

APPLICATIONS OF RTDAPPLICATIONS OF RTD
They can be used in average and
differential temp. measurement.
Differential temp. sensing to an
accuracy of 0.05º have been
accomplished in a nuclear reactor
coolant heat rise application.
5/9/201847 kongunadu college of engineering and technology

Variable Capacitance Type
Transducer
A capacitor is a pair of parallel plates
with some insulating dielectric material
between them, which can be used to
store an electrostatic charge.
Capacitive transducer is a passive
transducer used for sensing
displacement. It is a primary transducer.

Transducer
Capacitive Transducers/Sensors are
extensively used in industrial and
scientific applications.
They are based on changes in
capacitance in response to physical
variations.
These sensors find many diverse
applications but main are related to the
measurements of displacement.

Transducer

Principle Of Capacitive Type
Transducer
Capacitive Transducers measure the
displacement by measuring the change in
capacitance.
Where capacitance is given by,
C= €A/D.
Where C= Capacitance in Farads
A=Area of cross section of plates in
square meters
D=Distance between two plates in meters
€=Dielectric Constant

Transducer
Means change in capacitance occurs
due to:
Change in overlapped area of plates
Change in distance between the two
plates
Change in dielectric medium.
Capacitance may be changed either
mechanically or electronically.

Transducer
o The variable capacitance used to
convert physical phenomena into
electrical signals is called a variable
capacitance transducer.
o As there are no moving is no wear and
the life expectancy quoted is 200 years.
o The major problem with these
transducers is their high impedence
which makes them very suspectible to
noise.

Advantages Of Capacitive
Transducer
Low Hysteresis
Good linearity
Highly sensitive
Stability
Repeatability
Resolution as small as 0.003mm can be
obtained
Static pressure measurement
capability.

Disadvantages Of Capacitive
Transducer
High impedance output.
High temperature sensitivity.
Need for complex electronics.
Dirt may vary the output.

Capacitive Transducer(For
Thickness Measurement)
Capacitive transducer can also be used
to measure the thickness of the
material. This method of measuring
thickness is used in case the material to
be tested is an insulator.
Material whose thickness is to be
measured is placed in between the two
metal plates forming a parallel plate
capacitor.

Capacitance of the parallel plate
capacitor is given by C = €A/D
where C= Capacitance in farads
A=Area of cross section of plates in
square meters
D=Distance between two plates in
meters
€=Dielectric Constant

Dielectric Gauge(For Liquid Level
Measurement)
Capacitive transducers can be used in
liquids and free flowing solids for
continuous level measurement.
Materials placed between the plates of
a capacitor increase the capacitance by
the factor € known as the dielectric
constant of the material.

Measurement)
When two capacitor plates are partially
immersed in a nonconductive liquid, the
capacitance CD is given by:
CD = (Ca*€*d + Ca )/r
Where Ca = Capacitance with no liquid
€ = Dielectric constant of liquid between
the plates
r = Height of the plates
d = Depth or level of the liquid between
the plates

Measurement)
There are large variations in dielectric
constant with temperature, so that
temperature correction may be needed.
The liquid level is given by:
d = (Cd-Ca)r/€Ca
This type of sensors are widely used for
chemical and petrochemical industries;
and can be used for a wide range of
temperature(-40 to 200 0
C) and pressure
variation (25 to 60 kg/cm2
).

Variable Inductance Type
Transducer
Inductance is another property of
electric circuits which can be used in
transducers. If a length of wire is wound
into a coil and an electrical current
passes through it a magnetic field is
generated.
If the current varies the magnetic field
varies and this induces other currents in
the coil which tend to oppose the
original.

Variable Inductance Type
Transducer
Self inductance is a measure of this effects
and is given by:
L= N2
u0urA/l henries
Where, N is the number of turns in the coil
A is the cross section area of the coil in
metres2
l is the length of the coils in metres
u0 is the permeability of free space
(vacuum)=4∏*10-7
H/m

Linear Variable Differential
Transducer (LVDT)
LVDT is used for the measurement of
displacement.
It is the passive transducer.
Supply is provided to the middle coil
and outputs is taken across the other
two coils.
Differential amplifier is used to
measure the differential output from
the windings.
The output is an AC waveform.

Linear Variable Differential
Transducer(LVDT)

Why use LVDT?
Friction – Free Operation
NO mechanical contact between core
and coil (usually)
Infinite Mechanical Life
Infinite Resolution
Electromagnetic coupling
Limited only by electrical noise
Low risk of damage
Most LVDT’s have open bore holes

Why use LVDT?
Null Point Repeatability
Zero displacement can be measured
Single Axis Sensitivity
Effects of other axes are not felt on
the axis of interest
Environmentally Robust
Stable/Strong sensors – good for
structural engineering tests.

Underlying Principle Of LVDT
In the null position, the magnetic field
generates currents of equal magnitude
in both secondary coils.
When the core is moved, there will be
more magnetic flux in one coil than the
other resulting in different currents and
therefore different voltages.
This variation in voltages is linearly
proportional to displacement.

Underlying Principle Of LVDT

Advantages Of LVDT
Measurement resolution is almost
infinite.
Negligible hysteresis.
High signal to noise ratio and low output
impedance.
Relative to low cost.
No permanent damage to the LVDT if
measurements exceed the designed
range.

Disadvantages Of LVDT
Presence of harmonics in the excitation
voltage.
Stray capacitance also affects output.
Non zero output of low magnitude when
the core is in the null position.
The core must contact directly or
indirectly with the measured surface
which is not possible or desirable.

Bourdon Tube Pressure Gauge
Perhaps the most common device
around today is the pressure gauge
which utilizes a bourdon tube as its
sensing elements.
Bourdon : A bourdon tube is a curved,
hollow tube with the process pressure
applied to the fluid in the tube. The
pressure in the tube causes the tube to
deform or uncoil. The pressure can be
determined from the mechanical
displacement of the pointer connected to
the Bourdon tube. Typical shapes for
the tube are “C” (normally for local
display), spiral and helical.

Bourdon Tube Pressure Gauge
Bourdon tubes are generally
are of three types;
1. C-type
2. Helical type
3. Spiral type

SUMMARY
There are some other types of
transducers also , such as:
Solid state temperature sensor
PWM Sensors
Thermocouple
Piezoelectric Transducer
Photoelectric Devices
Bimetallic Thermometer
Ultrasonic Flow-meters

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