2. INTRODUCTION OF TRANSDUCERS
•A transducer is a device that converts one form of energy to other form. It
converts the measurand ( physical Quantity) to a usable electrical signal.
•In other word it is a device that is capable of converting the physical
quantity into a proportional electrical quantity such as voltage or current.
Pressure Voltage
3. BLOCK DIAGRAM OF TRANSDUCERS
•Transducer contains two parts that are closely related to each other i.e. the
sensing element and transduction element.
•The sensing element is called as the sensor. It is device producing
measurable response to change in physical conditions.
•The transduction element convert the sensor output to suitable electrical
form.
4. Basic requirements of transducer(characteristics of transducers)
1.Ruggedness
2.Linearity
3.Repeatability
4.Accuracy
5.High stability and reliability
6.Speed of response
7.Sensitivity
8.Small size
9.Range
10.Loading effects
11.Flat frequency response
12.Power requirements
13.Environmental effects
14.Repeatability
5. TRANSDUCERS SELECTION FACTORS
1. what is the physical quantity to be measured ?
2.Operating Principle: The transducer are selected on the basis of
operating principle used by them. The operating principle used may be
resistive, inductive, capacitive , optoelectronic, piezo electric etc.
3.Sensitivity: The transducer must be sensitive enough to produce
detectable output.
4.Operating Range: The transducer should maintain the range requirement
and have a good resolution over the entire range.
4.Accuracy: High accuracy is assured.
5.Errors: The transducer should maintain the expected input-output
relationship as described by the transfer function so as to avoid errors.
6. 7.Loading Effects: The transducer should have a high input impedance and low
output impedance to avoid loading effects.
9.Environmental Compatibility: It should be assured that the transducer
selected to work under specified environmental conditions maintains its input-
output relationship and does not break down.
10.Insensitivity to unwanted signals: The transducer should be minimally
sensitive to unwanted signals and highly sensitive to desired signals.
7. CLASSIFICATION OF TRANSDUCERS
The transducers can be classified as:
I. Active and passive transducers.
II. Analog and digital transducers.
III. On the basis of transduction principle used.
IV. Primary and secondary transducer
V. Transducers and inverse transducers.
8. Active transducers :
These transducers do not need any external source of power for their operation.
Therefore they are also called as self generating type transducers.
The active transducer are self generating devices which operate under the energy
conversion principle.
At the output of active transducers we get an equivalent electrical output signal.
e.g. temperature or strain to electric potential, without any external source of
energy being used.
Examples: thermocouple, moving coil generator, photovoltaic cell, piezo-electric
transducer
ACTIVE AND PASSIVE TRANSDUCERS
11. • Passive Transducers :
These transducers need external source of power
for their operation. So they are not self generating
type transducers.
A DC power supply or an audio frequency
generator is used as an external power source.
These transducers produce the output signal in the
form of variation in resistance, capacitance,
inductance or some other electrical parameter in
response to the quantity to be measured.
PASSIVE TRANSDUCERS
13. PRIMARY AND SECONDARY TRANSDUCERS
• Some transducers contain the mechanical as well as electrical device. The mechanical
device converts the physical quantity to be measured into a mechanical signal. Such
mechanical device are called as the primary transducers, because they deal with the physical
quantity to be measured.
•The electrical device then convert this mechanical signal into a corresponding electrical
signal. Such electrical device are known as secondary transducers
14. PRIMARY AND SECONDARY TRANSDUCERS
•Ref fig in which the diaphragm act as primary transducer. It convert
pressure (the quantity to be measured) into displacement(the mechanical
signal).
•The displacement is then converted into change in resistance using strain
gauge. Hence strain gauge acts as the secondary transducer.
16. INDUCTIVE TRANSDUCER:
•In inductive transduction, the measurand is converted into a change in the self
inductance of a single coil. It is achieved by displacing the core of the coil that is
attached to a mechanical sensing element
CLASSIFICATION OF TRANSDUCERS
According to Transduction Principle
17. CLASSIFICATION OF TRANSDUCERS
According to Transduction Principle
PIEZO ELECTRIC INDUCTION :
•In piezoelectric induction the measurand is converted into a
change in electrostatic charge q or voltage V generated by
crystals when mechanically it is stressed as shown in fig.
18. CLASSIFICATION OF TRANSDUCERS
According to Transduction Principle
PHOTOVOLTAIC TRANSDUCTION :
•In photovoltaic transduction the measurand is converted to
voltage generated when the junction between dissimilar
material is illuminated as shown in fig.
20. CLASSIFICATION OF TRANSDUCERS
According to Transduction Principle
PHOTO CONDUCTIVE TRANSDUCTION :
•In photoconductive transduction the measurand is converted to
change in resistance of semiconductor material by the change in
light incident on the material.
21. CLASSIFICATION OF TRANSDUCERS
Transducer and Inverse Transducer
TRANSDUCER:
•Transducers convert non electrical quantity to
electrical quantity.
INVERSE TRANSDUCER:
• Inverse transducers convert electrical quantity to a
non electrical quantity
22. PASSIVE TRANSDUCERS
• Resistive transducers :
– Resistive transducers are those transducers in which the
resistance change due to the change in some physical
phenomenon.
– The resistance of a metal conductor is expressed by a
simple equation.
– R = ρL/A
– Where R = resistance of conductor in Ω
L = length of conductor in m
A = cross sectional area of conductor in m2
ρ = resistivity of conductor material in Ω-m.
23. RESISTIVE TRANSDUCER
There are 4 type of resistive transducers.
1. Potentiometers (POT)
2. Strain gauge
3. Thermistors
4. Resistance thermometer
5. Thermocouple
24. POTENTIOMETER
• The potentiometer are used for voltage division. They consist of a
resistive element provided with a sliding contact. The sliding contact
is called as wiper.
• The contact motion may be linear or rotational or combination of the
two. The combinational potentiometer have their resistive element in
helix form and are called helipots.
• Fig shows a linear pot and a rotary pot.
25. STRAIN GAUGE
•The strain gauge is a passive, resistive transducer which converts the mechanical elongation and
compression into a resistance change.
•A strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it
is stretched or compressed
•This change in resistance takes place due to variation in length and cross sectional area of the gauge wire,
when an external force acts on it.
26. TYPES OF STRAIN GAUGE
When a gauge is subjected to a positive strain, its length increases where as its area of cross
section decreases. Therefore resistance varies according to variation in length or Area.
The type of strain gauge are as
1. Resistance Wire gauge
a) Unbonded
b) Bonded
c) Foil type
2. Semiconductor gauge
27. UNBONDED STRAIN GAUGE
•An unbonded strain gauge consist of a wire stretched between two point in an insulating
medium such as air. The wires may be made of various tungssten-platinum,copper, nickel,
chrome nickel or nickel iron alloys.
•The element is connected via a rod to diaphragm which is used for sensing the pressure. The
wire are tensioned to avoid buckling when they experience the compressive force.
•At initial preload the strain and resistance of the four arms are nominally equal with the
result the output voltage of the bridge is equal to zero.
•Application of pressure produces a small displacement , the displacement increases a tension
in two wire and decreases it in the other two thereby increase the resistance of two wire which
are in tension and decreasing the resistance of the remaining two wire .
•This causes an unbalance of the bridge producing an output voltage which is proportional to
the input displacement and hence to the applied pressure .
28.
29. BONDED STRAIN GAUGE
The bonded metal wire strain gauge are used for both stress analysis and for construction of transducer.
A resistance wire strain gauge consist of a grid of fine resistance wire made up of nichrome, constantum, nickel
or platinum.
The grid is cemented to carrier which may be a thin sheet of paper bakelite or teflon.
The wire is covered on top with a thin sheet of material so as to prevent it from any mechanical damage.
The carrier is bonded with an adhesive material to the specimen which permit a good transfer of strain from
carrier to grid of wires.
Types: wire grid, rossette, torque,helical
30. BONDED METAL FOIL STRAIN GAUGE
It consists of following parts:
1.Base (carrier) Materials: several types of base material are used to support the wires.
Impregnated paper is used for room temperature applications.
2.Adhesive: The adhesive acts as bonding materials. Like other bonding operation, successful
strain gauge bonding depends upon careful surface preparation and use of the correct bonding
agent.
In order that the strain be faithfully transferred on to the strain gauge, the bond has to be
formed between the surface to be strained and the plastic backing material on which the gauge is
mounted .
It is important that the adhesive should be suited to this backing and adhesive material should be
quick drying type and also insensitive to moisture.
3.Leads: The leads should be of such materials which have low and stable resistivity and also a low
resistance temperature coefficient.
31. •This class of strain gauge is only an extension of the bonded metal wire strain gauges.
•The bonded metal wire strain gauge have been completely superseded by bonded metal foil strain
gauges.
•Metal foil strain gauge use identical material to wire strain gauge and are used for most general
purpose stress analysis application and for many transducers.
32. SEMICONDUCTOR GAUGE
•Semiconductor gauge are used in application where a high gauge factor is desired. A high gauge
factor means relatively higher change in resistance that can be measured with good accuracy.
•The resistance of the semiconductor gauge change as strain is applied to it. The semiconductor
gauge depends for their action upon the piezo-resistive effect i.e. change in value of resistance due
to change in resistivity.
•Silicon and germanium are used as resistive material for semiconductor gauges.
33. Gauge factor
Sensitivity of the strain gauge is called gauge factor(K)
K=∆R/R/∆L/L
Where,
∆R: change in resistance due to strain
R: resistance before strain
∆L: change in length due to strain
L: original length.
34. THERMISTOR
Thermistor is a contraction of a term “thermal resistor”.
Thermistor are temperature dependent resistors. They are made of semiconductor material which
have negative temperature coefficient of resistivity i.e. their resistance decreases with increase of
temperature.
Thermistor are widely used in application which involve measurement in the range of 0-60º
Thermistor are composed of sintered mixture of metallic oxides such as manganese, nickel, cobalt,
copper, iron and uranium
The thermistor may be in the form of beads, rods and discs.
The thermistor provide a large change in resistance for small change in temperature. In some cases
the resistance of themistor at room temperature may decreases as much as 6% for each 1ºC rise in
temperature
35.
36.
37. Thermistor
5/9/2012 PUNJAB EDUSAT SOCIETY 37
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, Characteristics of material
T, T0 Absolute temperatures, K
39. Thermistor: temperatureCompensation
The NTC of thermistors are opposite to the PTC of metals used as electrical conductors. Therefore, this property of
thermistors can be used to nullify the effects of temperature . A properly mounted thermistor mounted near a circuit
element ( such as copper meter coil) experiencing the same ambient conditions can be connected in a such a way
that the total circuit resistance is constant over a wide range of temperatures.
40. The compensator consists of thermistor, shunted by a resistor. The –ve negative temperature coefficient
of this combination equals the +ve temperature coefficient of the copper meter coil. Better compensation
can achieved by increasing the number of compensation networks
45. When the core is exactly at the center of the coil then the flux linked to both the secondary
winding will be equal. Due to equal flux linkage the secondary induced voltages (E01 & E02) are
equal but they have opposite polarities. Output voltage E0 is therefore zero. This position is called
“null position”
Now if the core is displaced from its null position toward secondary coil 1 then flux linked to
secondary coil 1 increases and flux linked to secondary coil 2 decreases. Therefore E01 >E02 and
the output voltage of LVDT E0 will be positive
Similarly if the core is displaced toward secondary coil 2 then the E02 > E01 and the output
voltage of LVDT E0 will be negative.
46. Uses of LVDT
Widely used in electronic comparators for thickness measurement units, level indicators, creep testing
machines and numerically controlled machines.
Advantages
Fine resolution, good stability and high accuracy.
47. Thermocouples
See beck Effect
When a pair of dissimilar metals are joined at one end, and there is a
temperature difference between the joined ends and the open ends,
thermal emf is generated, which can be measured in the open ends.
This forms the basis of thermocouples.
48. •Thermocouples operate under the principle that a circuit made by connecting two
dissimilar metals produces a measurable voltage (emf-electromotive force) when a
temperature gradient is imposed between one end and the other.
•They are inexpensive, small, rugged and accurate when used with an understanding of
their peculiarities.
49. Capacitive Transducers
The principle of operation of capacitive transducers is based on equation of
capacitance.
C=ɛA/d
C-capacitance of parallel plate capacitor.
A=over lapping area of plates; m2.
d=distance between two plates; m,
ɛ=Permittivity; F/m.
50. Capacitive Transducers
Changing Distance between the Plates of Capacitive Transducers
a capacitive transducer has a static plate and a flexible diaphragm with a dielectric in between.
When a force is exerted to the outer side of the diaphragm the distance between the diaphragm and the
static plate changes.
This produces a changes in capacitance which is measured using an alternating current bridge or a tank
circuit.
51. Capacitive Transducers
Changing Distance between the Plates of Capacitive Transducers
The capacitance of the variable capacitance transducer also changes with the area of the two plates. This
principle is used in the torquemeter, used for measurement of the torque on the shaft. This comprises of the
sleeve that has teeth cut axially and the matching shaft that has similar teeth at its periphery.
52. Capacitive Transducers
Changing Dielectric Constant type of Capacitive Transducers
In these capacitive transducer the dielectric material between the two plates changes, due to which the
capacitance of the transducer also changes.
When the input quantity to be measured changes the value of the dielectric constant also changes so the
capacitance of the instrument changes. This capacitance, calibrated against the input quantity, directly gives
the value of the quantity to be measured.
This principle is used for measurement of level in the hydrogen container, where the change in level of
hydrogen between the two plates results in change of the dielectric constant of the capacitance transducer.
Apart from level, this principle can also be used for measurement of humidity and moisture content of the air.
53. Hall effect Sensors
A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic
field.
Used to provide a noncontact means to detect and measure a magnetic field
Named based on their use of the Hall Effect, discovered by Edwin Hall in
1879
Hall effect: When a current(I) carrying conductor or semiconductor is placed
in a transverse magnetic field(B), an electric field (emf) is induced in the
direction perpendicular to both I and B.
The hall effect can be used to find whether the semiconductor is n-type or p-
type and can also be used to determine carrier concentration.
54. Hall effect: Theory
A static magnetic field has no effect on a charged particle unless it is moving.
When charges flow, a mutually perpendicular force (Lorentz force) is induced on
the charge.
Now electrons and holes are separated by opposite force.
This produces a electric field which depends upon cross product of magnetic
intensity [H] and current density [J]
Eh=R (J x H)
R is called Hall Coefficient
Consider a Semiconductor bar along X-axis, Magnetic field along Z-axis. Thus Eh
will be along Y-axis.
55. Presence of magnetic field deflects electrons flowing through a conductive
material
As electrons move to one end of a conductive material, a potential is developed
in the direction perpendicular to gross current flow
This potential indicates the strength of the magnetic field
Depiction of the Hall Effect
56. Hall effect transducer
The transducer below is used to detect changes in magnetic field to which its exposed.
When a n- type Germanium semiconductor carrying current is exposed to a magnetic field normal to its
surface, the magnetic field exerts a force on the electrons drifting them towards the edges.
the potential between the edge surfaces is called Hall potential and the electric field due to build up of charges
on the edges is called hall field.
The hall voltage is given by
Where:
VH is the Hall Voltage in volts
RH is the Hall Effect co-efficient
I is the current flow through the sensor in amps
t is the thickness of the sensor(semiconductor plate) in mm
B is the Magnetic Flux density in Teslas
RH=1/N.q
N:concentration of electron
q: charge of electron
“output voltage will directly proportional to the magnetic field passing through the Hall sensor. ”
57. Applications: Hall effect Sensors
Smart phones like iPhone 3GS are equipped with magnetic compass. These compass measure Earth‘s magnetic
field using 3-axis magnetometer. These magnetometer are sensors based on Hall Effect. These sensors produce a
voltage proportional to the applied magnetic field and also sense polarity.
Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications
60. Piezoelectric transducer
Works based on the principle of piezo-electric effect.
Peizoelectric materials: ammonium dihydrogen phosphate, dipotassium tartarate,
potassium dihydrogen phospahte,lithium sulphate, rochelle salts.
Piezoelectric effect is sensitive to direction.
Compressive force produces voltage of one polarity , whereas tensile force produces
voltage of opposite polarity.
Various types: piezoelectric force transducer, piezoelectric strain transducer,
piezoelectric torque transducer, piezoelectric pressure transducer, piezoelectric
acceleration transducer
