Transducers
Presented By:
Er.Abinash Singh
Ph.D Scholar
Deptt. Of Electrical Engg.
Introduction
• A transducer can be defined as a device capable of
converting energy from one form into another.
Transducers can be found both at the input as well as
at the output stage of a measuring system.
• The input transducer is called the sensor, because it
senses the desired physical quantity and converts it
into another energy form.
• The output transducer is called the actuator, because it
converts the energy into a form to which another
independent system can react, whether it is a
biological system or a technical system.
Contd…
• A transducer is a device that converts one
type of energy to another. The conversion can
be to/from electrical, electro-mechanical,
electromagnetic, photonic, photovoltaic, or
any other form of energy. While the term
transducer commonly implies use as a
sensor/detector, any device which converts
energy can be considered a transducer.
Classification of Transducers
• The transducers may be classified on basis of
electrical principles involved, methods of
application, methods of energy conversion
used, nature of output signal etc.
• a) Primary and secondary transducers.
• b) Active and passive transducers
• c) Analog and digital transducers
• d) Transducers and inverse transducers.
Resistive Transducers
• In such a transducer, the resistance between
the output terminals of a transducer gets
varied according to the measurand.
• Resistive transducers are preferred over the
other transducers because dc and ac both are
suitable for resistance measurement.
• Examples: Potentiometer, Strain gauge,
Thermistor, Thermocouple.
Potentiometer
• A position or displacement transducer may be built
with a linear or rotary potentiometer or a pot for short.
It follows that the resistance linearly relates to the wire
length. Thus, by making an object to control the length
of the wire, as it is done in a pot, a displacement
measurement can be performed.
• Because a resistance measurement requires passage of
an electric current through the pot wire, the
potentiometric transducer is of a passive type; that is,
it requires an excitation signal, (e.g., dc current).
• It converts linear or rotational displacement into a
voltage.
Contd…
• The voltage across the wiper of a linear pot is proportional to the
displacement d:
• where D is the full-scale displacement and E is the voltage across the pot
(excitation signal). The force which is required to move the wiper comes
from the measured object, and the resulting energy is dissipated in the
form of heat. When the motion of wiper are translational as well as
rotational, the potentiometer is called helipot
Contd…
• Types of Potentiometers:
a) Wire wound potentiometers
b) Carbon film potentiometer
c) Thin film Potentiometer
d) Hot moulded Carbon potentiometer
e) Cermet potentiometer(In this, the resistance
element is made of ceramic base)
Contd…
• Merits:
• They are cheap, easy to
operate, simple in
construction and very useful
for simple applications
• They have very good
frequency response except
for the wire wound type.
• They can measure large
amplitude of displacement.
• They have high electrical
efficiency.
• Demerits:
• The main drawback is the
wear and tear caused of
wiper and its effect on the
life of transducer.
• They require large force for
wiper movement
• The output is insensitive to
variation in displacement of
wiper b/w two consecutive
turns of pot.
Strain Gauge
• A strain gauge is a device used to measure the
strain of an object. A strain gauge is a resistive
elastic sensor whose resistance is a function of
applied strain (unit deformation).
• Because all materials resist deformation, some
force must be applied to cause deformation.
Hence, resistance can be related to applied
force. That relationship is generally called the
piezoresistive effect
Contd…
• A strain gauge takes advantage of the physical property
of electrical conductance and its dependence on the
conductor's geometry. When an electrical conductor is
stretched within the limits of its elasticity such that it
does not break or permanently deform, it will become
narrower and longer, changes that increase its
electrical resistance end-to-end.
• Conversely, when a conductor is compressed such that
it does not buckle, it will broaden and shorten, changes
that decrease its electrical resistance end-to-end. From
the measured electrical resistance of the strain gauge,
the amount of applied stress may be inferred.
Contd…
• Types of strain gauge
a) Wire strain gauge
b) Thin film strain gauge
c) Foil strain gauge
d) Semiconductor strain gauge
• (Learn the derivation from class notes.)
Contd…
• Merits:
• They permit the measurement of very small strain i.e
as small as 0.01 micron
• The semiconductor strain gauges can be manufactured
in very small sizes ranging from 0.7 to 7 mm.
• They have got frequency response upto 1012 Hz.
• They are almost free from hysteresis effect
• Their fatigue life is much higher.
• Demerits:
• They are expensive brittle and highly sensitive to
temperature variations
Thermistor
• A thermistor is a resistor whose value varies with temperature. the
thermistor is also a resistive device that changes its resistance predictably
with temperature. Its benefit is a very large change in resistance per
degree change in temperature, allowing very sensitive measurements over
narrow spans. Due to its very large resistance, lead wire errors are not
significant. However, there are several disadvantages to the thermistor:
1. It is a very nonlinear device and reasonable accuracy
is obtained only over narrow spans (see Figure).
2. It is quite small and will exhibit errors due to self
heating.
3. Exposure to high temperature will cause a dramatic
and permanent shift in its output characteristics.
• Most applications of the thermistor are in commercial and
laboratory applications. Few are used in industrial process
control.
Contd…
• Commercial forms of thermistors:
a)Bead type
b)Disc Type
c)Rod Type
d)Probe type e)Washer Type
Thermocouple
• A thermocouple consists of two pieces of
dissimilar metals with their ends joined
together (by twisting, soldering or welding).
When heat is applied to the junction, a
voltage, in the range of milli-volts (mV), is
generated. A thermocouple is therefore said
to be self-powered.
Contd…
Principle of operation
In 1821, the German–Estonian physicist Thomas Johann
Seebeck discovered that when any conductor is subjected
to a thermal gradient, it will generate a voltage. This is now
known as the thermoelectric effect or Seebeck effect. Any
attempt to measure this voltage necessarily involves
connecting another conductor to the "hot" end. This
additional conductor will then also experience the
temperature gradient, and develop a voltage of its own
which will oppose the original. Fortunately, the magnitude
of the effect depends on the metal in use. Using a
dissimilar metal to complete the circuit creates a circuit in
which the two legs generate different voltages, leaving a
small difference in voltage available for measurement. That
difference increases with temperature, and is between 1
and 70 microvolts per degree Celsius (µV/°C) for standard
metal combinations.
The voltage is not generated at the junction of the two
metals of the thermocouple but rather along that portion
of the length of the two dissimilar metals that is subjected
to a temperature gradient. Because both lengths of
dissimilar metals experience the same temperature
gradient, the end result is a measurement of the
temperature at the thermocouple junction.
Contd…
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Advantages:
Thermocouples are used on most
transformers. The hot junction is inside the
transformer oil and the cold junction at the
meter mounted on the outside. With this
simple and rugged installation, the meter
directly reads the temperature rise of oil
above the ambient temperature of the
location.
In general, thermocouples are used
exclusively around the turbine hall because
of their rugged construction and low cost.
A thermocouple is capable of measuring a
wider temperature range
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Disadvantages:
If the thermocouple is located some distance
away from the measuring device, expensive
extension grade thermocouple wires or
compensating cables have to be used.
Thermocouples are not used in areas where
high radiation fields are present (for
example, in the reactor vault). Radioactive
radiation (e.g., Beta radiation from neutron
activation), will induce a voltage in the
thermocouple wires. Since the signal from
thermocouple is also a voltage, the induced
voltage will cause an error in the
temperature transmitter output.
Thermocouples are slower in response
If the control logic is remotely located and
temperature transmitters (milli-volt to milliamp transducers) are used, a power supply
failure will of course cause faulty readings.
Inductive Transducers
• These can be classified as:
• Linear variable differential transformer
• Inductive transducer utilising change in selfinductance
• Variable Reluctance inductive transducer
• Inductive transducer working on the principle
of variation of mutual inductance
• Eddy current Sensors
LVDT
• Linear variable differential transformer(LVDT)
• A linear variable differential transformer (LVDT),which can also be
called a mutual-inductance element, that operates on the
inductance ratio principle. Here three coils are wound onto the
same insulating tube containing the high-permeability iron core.
Alternating current is applied to the primary coil (P) in the center,
and if the core is in the center, equal voltages are induced in the
secondary coils (#1 and #2) by the magnetic flux. Usually the
secondary coils are wired in series, and therefore when the core is
centered, the resulting output is zero. As the core moves to the left
or right, the voltage induced in coil #1 or #2 will predominate.
• The LVDT-type transducers are also available in therotary form and
can be obtained with ranges from 0–30 to 0–10,000 PSIG (0–210
kPa to 0–70 MPa). Their limitations include mechanical wear and
vibration sensitivity.
Contd…
•The Linear Variable Differential Transformer (LVDT) is
a type of electrical transformer used for measuring
linear (i.e. translational) displacement. A counterpart
to this device that is used for measuring rotary
displacement is called a Rotary Variable Differential
Transformer (RVDT). The linear variable differential
transformer has three solenoidal coils placed end-toend around a tube. The center coil is the primary, and
the two outer coils are the secondaries. A cylindrical
ferromagnetic core, attached to the object whose
position is to be measured, slides along the axis of the
tube.
•An alternating current is driven through the primary,
causing a voltage to be induced in each secondary
proportional to its mutual inductance with the
primary. The frequency is usually in the range 1 to 10
kHz.
•As the core moves, these mutual inductances change,
causing the voltages induced in the secondaries to
change. The coils are connected in reverse series, so
that the output voltage is the difference (hence
"differential") between the two secondary voltages.
Contd…
Advantages of the LVDT are:
Disadvantages of LVDT are:
• The sensor is a non contact
device with no or very little
friction resistance with small
resistive forces;
• Hysteresis (magnetic and
mechanical) are negligible;
• Output impedance is very low;
• There is low susceptibility to
noise and interferences;
• Its construction is solid and
robust,
• Infinitesimal resolution is
possible.
• Sometimes the transducer
performance is affected by the
vibrations.
• The receiving instrument must be
selected on ac signals or a
demodulator must be used if a dc
output is required.
• Relatively large displacements are
required for appreciable
differential output.
• These devices are sensitive to
stray magnetic fields but its
problem is overcome by using
magnetic shields.
OPTICAL TRANSDUCERS
•The movement of elastic pressure
sensors can also be used to operate
optical sensors. As the process
pressure moves a diaphragm sensor,
which in turn lifts a vane in front of an
infrared light beam, the amount of
light impinging on the measuring
diode 2 varies (Figure).
• A reference diode is also provided to
compensate for the aging of the light
source (LED) or for dirt buildup on the
optics. This transducer is insensitive to
temperature variations, as such
variations affect the measuring and
reference diodes in the same way.
• Because the amount of movement in
the sensor is very small (0.5 mm), both
the hysteresis and the repeatability
errors are negligible.
Infrared Detectors
• In general, IR detectors are good for detecting hydrocarbon
based fires (i.e., fires that have strong IR emissions). IR
detectors are generally not as fast as UV detectors to
respond to a fire.
• A disadvantage of IR is that ice buildup can desensitize the
detector (lessen its ability to detect a fire), but this can be
overcome with heated optics. IR does not respond to
electric arc welding unless the welding is very close to the
detector, in which case the detector may alarm due to
seeing the burning from the welding process.
• Some IR detectors have flicker and statistical analysis
algorithms to minimize the effects of black body sources, a
false alarm source.
Ultrasonic Sensors
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For noncontact distance measurements, an active sensor which
transmits some kind of a pilot signal and receives a reflected
signal can be designed. The transmitted energy may be in the
form of any radiation—for instance, electromagnetic in the
optical range electromagnetic in the microwave range, acoustic,
and so forth. Transmission and reception of the ultrasonic energy
is a basis for very popular ultrasonic-range meters, and velocity
detectors.
Ultrasonic waves are mechanical acoustic waves covering the
frequency range well beyond the capabilities of human ears (i.e.,
over 20 kHz). However, these frequencies may be quite
perceptive by smaller animals, like dogs, cats, rodents, and
insects. Indeed, the ultrasonic detectors are the biological ranging
devices for bats and dolphins.
When the waves are incident on an object, part of their energy is
reflected. In many practical cases, the ultrasonic energy is
reflected in a diffusemanner; that is, regardless of the direction
from which the energy comes, it is reflected almost uniformly
within a wide solid angle, which may approach 180◦. If an object
moves, the frequency of the reflected waves will differ from the
transmitted waves. This is called the Doppler effect.
The distanceL0 to the object can be calculated through the speed
v of the ultrasonic waves in the media, and the angle
Capacitive transducers
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Capacitive sensors are more often used for linear than angular proximity
measurements. Either the dielectric or one of the capacitor plates is movable for
displacement measurement. Capacitive proximity sensors use the measured object
as one plate, and the sensor contains the other plate. The capacitance changes
according to the question.
where k = is a constant, depending on the area of the plates and the dielectric
constant
d = the distance between the plates
• Capacitive transducers are available with packaged signal-conversion circuitry for
DC output operation. Capacitive sensors are widely used for dimensional
inspections in large-volume manufacturing operations, such as the filling of
containers or the monitoring of the wearing of moving surfaces. The capacitive
displacement sensors have very broad applications, they are employed directly to
gauge displacement and position and also as building blocks in other sensors
where displacements are produced by force, pressure, temperature, and so forth.
Contd…
•The proximity switches illustrated in
Figure can detect liquids, glass, plastic,
wood, or metallic objects. For the
proximity switches shown, the sensing
distance can be fixed or adjustable
between 0.1 and 1.0 in. (3 to 25 mm).
•Proximity switches provided with sensing
plates can operate over a range of 0.2 to
5 in. (5 to 127 mm), can detect
capacitance changes down to 0.02 pF,
and can detect more than 100
operations/s.
•The switch is operated when the
capacitance caused by the approaching
object exceeds the reference level set to
trigger the switch. Types of Capacitive
transducers( on basis of the method of
variation of pressure):
•C=εA/d
•By varying the overlapping area of
plates(A)
•By varying the distance between the
plates(d)
•By varying the relative permittivity of
dielectric material between the two
plates.
Piezoelectric Sensors
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The piezoelectric sensor is a self-generating device that is
ideal for the measurement of such dynamic events as shock
and vibration. They are basically motion transducers with
large output signals and comparatively small sizes. They are
available with very high natural frequencies and are suitable
for high-frequency applications and shock measurements.
Their sensing element is sandwiched between the transducer
body and a seismic mass (usually tungsten). Because the
seismic mass is constant, the force acting on the sensing
element (F = ma) is proportional to acceleration. If the sensing element is a quartz crystal or lead-zirconate-titanate
(PZT), an electric charge is generated (150 times more for
PZT than for quartz) that is in proportion to the experienced
force and therefore to acceleration.
Contd…
These devices utilize a mass in direct contact with
the piezoelectric component or crystal. A
separation of charge is produced on the opposite
faces of the crystal when it is subjected to
acceleration forces. The magnitude of the voltage
produced is in proportion to mechanical
deformation and, hence, acceleration.
Piezoelectric crystals are affected in their output
by temperature variations; however, quartz and
some of the newer piezoelectric ceramics are
superior to such ceramics as bar ium titanate or
PZT in their reduced sensitivity to temperature.
Two commonly used piezoelectric crystals are PZT
and crystalline quartz. They are both selfgenerating materials and produce a large electric
charge for their size. As a result, PZTs are more
sensitive and smaller in size than quartz
counterparts. These accelerometers are useful for
high-frequency applications. Fig. shows
impedance characteristics of a piezoelectric
transducers
Humidity & Moisture
Sensors
Humidity refers to the water vapor
contained in the air at a particular
temperature. Warm air has a greater
capacity for water vapor than does cold
air.
Relative humidity (RH) is the ratio of the
actual partial pressure of the water vapor
to the saturation vapor pressure at a
particular temperature. Fig shows
Capacitive Moisture Sensing System
A hygrometer is an instrument used for
measuring the moisture content in the
environmental air, or humidity. Most
measurement devices usually rely on
measurements of some other quantity
such as temperature, pressure, mass or a
mechanical or electrical change in a
substance as moisture is absorbed. From
calculations based on physical principles,
or especially by calibration with a
reference standard, these measured
quantities can lead to a measurement of
humidity. Modern electronic devices use
temperature of condensation, or changes
in electrical capacitance or resistance to
measure humidity changes.
Contd…
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For applications where cost, space, or fragility are relevant, other types of
electronic sensors are used, at the price of a lower accuracy. In capacitive
humidity sensors, the effect of humidity on the dielectric constant of a polymer
or metal oxide material is measured. With calibration, these sensors have an
accuracy of ±2% RH in the range 5–95% RH. Without calibration, the accuracy is
2 to 3 times worse. Capacitive sensors are robust against effects such as
condensation and temporary high temperatures. Capacitive sensors are subject
to contamination, drift and aging effects, but are suitable for many applications.
In resistive humidity sensors, the change in electrical resistance of a material
due to humidity is measured. Typical materials are salts and conductive
polymers. Resistive sensors are less sensitive than capacitive sensors - the
change in material properties is less, so they require more complex circuitry.
The material properties also tend to depend both on humidity and
temperature, which means in practice that the sensor must be combined with a
temperature sensor. The accuracy and robustness against condensation vary
depending on the chosen resistive material. Robust, condensation-resistant
sensors exist with an accuracy of up to ±3% RH.
In thermal conductivity humidity sensors, the change in thermal conductivity
of air due to humidity is measured. These sensors measure absolute humidity
rather than relative humidity