Def:

•A transducer is a device that converts energy from one form to another.

• Any device or element or component that converts an input signal of one form to an output signal of another
form.
• Restriction to electrical instrumentation: A device which converts a physical quantity or a physical condition
into electrical signal.
Example:
1. Mechanical to Electrical – Capacitive and Inductive (LVDT)
2. Electrical to magnetic – Antenna, hall effect, magnetic cartridge
3. Electrical to mechanical – Pressure flow, Accelerometer, Potentio meter
4. Chemical to Electrical – pH probes, Electro galvanic sensor
5. Electrical to Acoustic – Piezo electric, sonar, ultra sonic
6. Electrical to optical – LED, Laser, Photodiode, PV cell, LCD, Opto couplers
7. Radio to acoustic – Geiger muller tubes, Radio receiver and transmitter
Electromechanical transducers
LVDT – Linear variable differential transformer

• LVDT is an electromagnetic induction type displacement transducer.

• Linear motion (displacement) to electrical output.
• Passive transducer
Construction of LVDT:

• The LVDT consists of three individual coils wound sequentially around a hollow, non-magnetic insulated tube.
• One coil of magnetic wire is classed as the primary coil and the other coils forming two identical secondaries. The
primary winding P and two secondary winding S1 and S2 wound on a cylindrical former.
 The primary winding is connected to an AC source which produces a flux in the air gap and voltages are induced in
secondary windings.
 Both the secondary windings have equal number of turns and are identically placed on the either side of primary winding.
• The two secondary coils are connected together in a series-opposing configuration, that is they are electrically 180o out-
of-phase with each other. Hence the name “Differential” .
• A soft iron ferromagnetic core, called a “core”, “slug”, “plunger” or “armature”, is allowed to move freely inside the
central hollow tube in a straight line.
•The iron core is generally of high permeability which helps in reducing harmonics and high sensitivity of LVDT.

•The LVDT is placed inside a stainless steel housing because it will provide electrostatic and electromagnetic shielding.

•As a result of the displacement of the connected object increasing or decreasing the mutual inductance between the
primary and the secondary coils which in turn increases or decreases the voltage induced in each secondary coil.
Working and construction of an LVDT
LVDT consists of a primary coil and two exactly similar secondary coils with a rod shaped magnetic core positioned
centrally, inside the coil. An alternating current is fed into the primary, and voltages Vo1 and Vo2 are induced in the secondary coils.
As these coils are connected in series opposition, the output voltage is Vo = Vo1 -Vo2 . The working of the LVDT circuit diagram can
be divided into three cases based on the position of the iron core in the insulated former.

In Case-1: When the core of the LVDT is at the null location, then both the minor windings flux will equal, so the induced e.m.f is
similar in the windings. So for no dislocation, the output value (eout) is zero because both the e1 & e2 are equivalent. Thus, it
illustrates that no dislocation took place.
In Case-2: When the core of the LVDT is shifted up to the null point. In this case, the flux involving minor winding S1 is additional as
contrasted to flux connecting with the S2 winding. Due to this reason, e1 will be added as that of e2. Due to this eout (output voltage)
is positive.
In Case-3: When the core of the LVDT is shifted down to the null point, In this case, the amount of e2 will be added as that of e1.
Due to this eout output voltage will be negative plus it illustrates the o/p to down on the location point.

Output characteristics of LVDT

Merits and demerits:

Merits
• LVDT provides continuous resolution and show low hysteresis
• Repeatability is excellent under all condition
• No sliding contact, so less friction and less noise
Demerits
• Sensitive to vibrations and temperature
• Receiving instrument must be selected to operate on ac signals or demodulator network must be used if a dc output
is required.
•LVDT is sensitive to stray magnetic fields so they always require a setup to protect them from stray magnetic fields.
•They are affected by vibrations and temperature.
Applications:
1. Used to measure displacement with ranging from a few mm to cm.
2. Can be used as primary and secondary transducers.
3. Used in combination with Bourdon tube to measure pressure.
4. Mostly used in servomechanisms and other industrial applications.
Strain gauge:

The strain gauge is an example of a passive transducer the; uses electrical resistance variation in wires to sense the strain produced
by a force on the wires. It is a very versatile detector and transducer for measuring weight pressure mechanical force, or
displacement.

The construction of a bonded strain gauge Figure, it shows a fine-wire element looped back and forth on a mounting plate, which
is usually cemented to the member undergoing stress. A tensile stress tends to elongate the wire and there by increase its length
and decrease its cross-sectional area.The combined effect is an increase in resistance as seen from Eq. (1)
 L
R 
A
Where
R = the specific resistance of the conductor material in ohm
L = the length of the conductor in meters
A = the area of the conductor in square meters

 As a consequence of strain two physical qualities are of particular interest: (1) the change in gauge resistance and (2) the change in
length. The relationship between these two variables expressed as a ratio is called the gauge factor (K).

 R / R
K 
 L / L

R = the initial resistance in ohms (without strain)

 R = the change in initial resistance in ohms
L = the initial length in meters (without strain)
 L = the change in initial length in meters
 Strain gauge bridge circuit shows the measured stress by the degree of
applied strain, in proportion to the strain the unbalance created in the
bridge causes deflection in the voltmeter.

The strain gauge is made of constantan alloy (zero temperature

coefficient) to cancel out the effect of temperature on the resistance.

Wheatstone bridge strain gauge

In this circuit, R1 and R3are the ratio arms equal to each other, and R2 is the rheostat arm has a value equal to the strain gage
resistance. When the gauge is unstrained, the bridge is balanced, and voltmeter shows zero value. As there is a change in resistance of
strain gauge, the bridge gets unbalanced and producing an indication at the voltmeter. The output voltage from the bridge can be
amplified further by a differential amplifier.
The other factor that affects the resistance of the gauge is temperature.
If the temperature is more resistance will be more and if the
temperature is less the resistance will be less. It can be overcome this
problem by using strain gauges that are self- temperature-compensated
or by a dummy strain gauge technique.

Whenever temperature changes, the resistance will change in the same

proportion in the both arms of the rheostat, and the bridge remains in
the state of balance. Effect of temperature get nullifies.
Electromechanical transducers:
Pressure, flow, Potentiometer and Accelerometer

Electrical Mechanical

Measurement of Pressure:

Pressure is represented as force per unit area. There are primary and secondary transducers for the measurement of
pressure.

In primary transducer the force can be converters into a displacement by elastic elements.
Ex: flat diaphragms, Bellows, Bourdon tube etc…

In secondary transducers, the displacement created by the action of force summing member is converted into a change of
some electrical parameter. The force summing member actuates a transducer which converts the displacement into an
output of electrical format.

The transducer may uses any one of the phenomenon to get the applied pressure.
(1) Resistive (2) Inductive (3) Capacitive
Resistive transducer for measuring the pressure

1. The change in the resistance of the strain gauge

breaks the balance of the Wheatstone’s bridge and
change the voltage V. The voltage V is proportional to
the pressure change in the strain gauge.
2. Attaching a strain gauge to a diaphragm results in a
device that changes resistance with applied pressure.
Pressure forces the diaphragm to deform, which in
turn causes the strain gauge to change resistance. By
measuring this change in resistance, we can infer the
amount of pressure applied to the diaphragm.

Note: A diaphragm is a sheet of a semi-flexible material anchored at its periphery. The physical dimensions will change
based on the applied pressure.
 Inductive transducer for measuring the pressure

Pressure measurement using diaphragm and strain gauge

 This type of transducers is used as secondary transducer for the measurement of pressure.
 Figure shows an arrangement which uses two coils; an upper and a lower coil which form the two arms of an ac bridge.
The coils have equal number of turns. The other two arms of the bridge are formed by two equal resistances each of
value R. The diaphragm is symmetrically placed with respect to the coils and so when P 1 = P 2 , The reluctances of the path
of magnetic flux for both the coils are equal and hence the inductances of the coils are equal.
 Based on the pressure difference applied on diaphragm causes pressure difference leads unbalances the bridge. The output
voltage obtain in linear relation with applied pressure.
 The free end of the bourdon tube senses the pressure
and converts it into a displacement.

LVDT converts the displacement into electrical

signal analogous to applied pressure.

Pressure measurement using Bourdon tube and LVDT

Applied pressure on the bellow gives a linear
displacement.
LVDT converts into electrical signal equivalent to
the pressure.
Note: Bellows are elastic elements that can be
compressed when pressure is applied to the on it,

Pressure measurement using Bellows and LVDT or extended under vacuum.

Electro acoustic transducers
• The membrane acts as the diaphragm and is electrically conductive.

• This construction creates a capacitor which may be in the region of 10 to 50pF.

• For the condenser microphone to operate it requires a DC voltage to be applied. This can be supplied by
battery.

• When sound waves hit the microphone, the diaphragm moves backwards and forwards.

• This changes the level of capacitance and as a result small voltage changes are seen across a high load resistor
connected across the microphone element.
Speaker

The electric current flows through the speaker’s voice coil, creating a magnetic field that causes it to move toward or away from
the magnet as it changes from positive to negative. This moves the speaker cone that creates sound waves as the air moves rapidly.
Speakers use alternating current.

As music is in kilohertz range, the cone moves thousands of times per second to produce sound your ears hear and that your brain
can recognize.
Piezoelectric speaker:
Piezoelectric effect:

1. Certain materials will generate a measurable potential difference when they are made to expand or shrink in a
particular direction.

2. Increasing or decreasing the space between the atoms by squeezing, hitting, or bending the crystal can cause
the electrons to redistribute themselves and cause electrons to leave the crystal, or create room for electrons
to enter the crystal. A physical force on the crystal creates the electromotive force that moves charges around a
circuit.

3. The opposite is true as well: Applying an electric field to a piezoelectric crystal leads to the addition or
removal of electrons, and this in turn causes the crystal to deform and thereby generate a small physical force.
Piezoelectric speaker:

The piezoelectric effect can be employed in the construction of thin-form-factor speakers that are valuable alternatives to
traditional electrodynamic speakers in space-constrained applications. These devices are referred to as both piezo speakers
and ceramic speakers.
Working of Piezoelectric speaker:

 Apply an electric field to a piezoelectric material and it will change size. The
piezoelectric material will shrink or grow as charges are introduced or removed, but the
base material will not.

 This causes elastic deformation of the material toward or away from a direction that is
perpendicular to the surface of the speaker. As soon as the electric field is removed from
the piezoelectric material, it will return to its original shape.

 As the speaker flexes and strikes air molecules, it causes a chain reaction of collisions
that eventually reaches your ear. If enough air molecules strike your ear, the nerve cells
send a signal to your brain that you interpret as sound.
SONAR: SOund NAvigation and Ranging

• A sonar system consists of an ac pulse generator, an acoustic

emitter (Piezoelectric), a receiver (Piezoelectric), a delay timer,
and an indicating device such as LCD.

• The transmitter sends out acoustic waves through the medium,

usually water or air.

• These waves are reflected by objects, and the echo is picked up

by the receiver.

• The distance to an object is determined on the basis of the echo

delay, provided the speed of the acoustic waves in the medium is
known.

• Sonar make use of ultrasound waves.

• Ultrasound has a frequency too high to hear, ranging from about

20 kHz to more than 100 kHz
Thermoelectric transducers
RTD: (Resistance temperature detector)
It is a temperature sensor used to measure temperature by changing its resistance based on the
temperature. It is a positive temperature co-efficient device. Most commonly platinum, nickel and
copper elements are used for RTD.

Working:
1. A Resistance Temperature Detector (RTD) functions on the resistance and
temperature relationship in metals.

2. The resistance of Resistance Temperature Detector (RTD) changes

constantly with respect to the applied temperature and so the temperature
is quite predictable by measurement of its resistance.

3. By supplying the constant electric current to the bridge circuit and

measuring the resulting voltage drop across the resistor, the RTD resistance
can be calculated.

4. Thereby, the temperature can be also determined.

Thermocouple:

Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two
junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for
different combinations of the metals.

Peltier effect: As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is
generated within the circuit due to the different temperatures of the two junctions of the circuit.

Thomson effect: As per the Thomson effect, when two unlike metals are joined together forming two junctions, the
potential exists within the circuit due to temperature gradient along the entire length of the conductors within the
circuit.
Working:
• The thermocouple is a temperature measuring device.

• It is a sensor used for measuring the temperature in the form of an electric current or the EMF.

• A thermocouple is comprised of two metals joined together to form two junctions.

• One is connected to the body whose temperature is to be measured; this is the hot or measuring junction.

• The other junction is connected to a body of known temperature; this is the cold or reference junction.

• If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net
current flowing through the junction is zero.

• If the junctions are maintained at different temperatures, there will be a net current flowing through the circuit. The emf
generated in the circuit depends on the metals used within the circuit as well as the temperature of the two junctions.
 Applications

•Temperature sensors in thermostats in offices, homes.

•Measure temperature in the chemical plants, petroleum plants.

Advantages

• Capable of being used to directly measure temperatures up to 2600ºC

• Fast response
• Cheaper

Disadvantages

•Requires cold junction compensation

Thermistor:
It is a temperature sensing element composed of semiconductor material which exhibits large change resistance in a small
variation in temperature. Generally, it is having a negative temperature co-efficient which means that the resistance of the
thermistor decreases as the temperature increases.

Working:

The thermistor acts as the temperature sensor and it is placed on the body
whose temperature is to be measured. It is also connected in the electric
circuit. When the temperature of the body changes, the resistance of the
thermistor also changes, which is indicated by the circuit directly as the
temperature since resistance is calibrated against the temperature. The
thermistor can also be used for some control which is dependent on the
temperature. Main calculations of thermistors are done using a sophisticated
formulae.
Applications:

Temperature sensing: The most obvious application for a thermistor is to measure temperature.
They are used to do this in a wide range of products such as thermostats.

 In rush current limiting: In this application the thermistor is used to initially oppose the flow of current (by having a high resistance)
into a circuit. Then as the thermistor warms up (due to the flow of electricity through the device) it resistance drops letting current flow
more easily.
Advantages:
•Small size
•Very high sensitivity
•Inexpensive
•Fast response

Disadvantages:
• Non linear
•Unstable at high temperatures
•Narrow span
•Fragile
Electromagnetic energy conversion
Hall transducer:

• Hall-effect sensors are the linear transducers that are used to measure the magnitude of the magnetic field. Working
on the principle of Hall Effect, these sensors generate a Hall voltage when a magnetic field is detected.
Hall Effect:
If an electric current flows through a conductor in a magnetic
field, the magnetic field exerts a transverse force on the moving
charge carriers which tends to push them to one side of the
conductor. This is most evident in a thin flat conductor as
illustrated. A buildup of charge at the sides of the conductors will
balance this magnetic influence, producing a measurable voltage
between the two sides of the conductor. The presence of this
measurable transverse voltage is called the Hall effect after E. H.
Hall who discovered it in 1879.
Working

• On a thin strip of a conductor, electrons flow in a straight line

when electricity is applied.

• When this charged conductor comes in contact with the magnetic

field which is in a perpendicular direction to the motion of
electrons, the electrons get deflected.

• Some electrons get collected on one side while some on another

side. Due to this, one of the conductor’s plane behaves as negatively
charged while the other behaves as positively charged.

• This creates potential difference and voltage is generated. This

voltage is called Hall voltage.