Transducers notes for beginners mechatronics syllabus

Introduction
 Basically transducer is defined as a device, which
converts energy or information from one form to
another.
 These are widely used in measurement work because
not all quantities that need to be measured can be
displayed as easily as others.
 A better measurement of a quantity can usually be
made if it may be converted to another form, which is
more conveniently or accurately displayed.

Introduction(cont’d)
 For example, the common mercury thermometer
converts variations in temperature into variations in the
length of a column of mercury. Since the variation in
the length of the mercury column is rather simple to
measure, the mercury thermometer becomes a
convenient device for measuring temperature.
 Manometer, which detects pressure and indicates it
directly on a scale calibrated in actual units of
pressure.

Introduction(cont’d)
 Thus the transducer is a device, which provides a
usable output in response to specific input measured,
which may be physical or mechanical quantity,
property or condition.
 The transducer may be mechanical, electrical,
magnetic, optical, chemical, acoustic, thermal nuclear,
or a combination of any two or more of these.

Mechanical transducers
Advantages:-
 simple and rugged in construction
 cheaper in cost
 accurate and
 operate without external power supplies
Disadvantage:-
 poor frequency response
 requirement of large forces to overcome mechanical
friction
 in compatibility when remote control or indication is
required

ELECTRICAL TRANSDUCERS
 Mostly quantities to be measured are non-electrical
such as temperature, pressure, displacement, humidity,
fluid flow, speed etc., but these quantities cannot be
measured directly. Hence such quantities are required
to be sensed and changed into some other form for
easy measurement.
 Electrical quantities such as current, voltage,
resistance. inductance and capacitance etc. can be
conveniently measured, transferred and stored, and
therefore, for measurement of non-electrical quantities
these are to be converted into electrical quantities first
and then measured.

ELECTRICAL TRANSDUCERS(cont’d)
 The function of converting non-electrical quantity into
electrical one is accomplished by a device called the
electrical transducer.
 Basically an electrical transducer is a sensing device
by which a physical, mechanical or optical quantity to
be measured is transformed directly, with a suitable
mechanism, into an electrical signal (current, voltage
or frequency).
 The production of these signals is based upon electrical
effects which may be resistive, inductive, capacitive
etc in nature.

BASIC REQUIREMENTS OF A TRANSDUCER
The main function of a transducer is to respond only for the
measurement under specified limits for which it is designed. It
is, therefore, necessary to know the relationship between the
input and output quantities and it should be fixed.
Transducers should meet the following basic requirements:-
 Ruggedness: It should be capable of withstanding overload
and some safety arrangement should be provided for
overload protection.
 Linearity: Its input-output characteristics should be linear
and it should produce these characteristics in symmetrical
way.

Basic Requirements Of a Transducer
(cont’d)
 Repeatability: It should reproduce same output signal when
the same input signal is applied again and again under fixed
environmental conditions e.g. temperature, pressure,
humidity etc.
 High Output Signal Quality: The quality of output signal
should be good i.e. the ratio of the signal to the noise
should be high and the amplitude of the output signal
should be enough.
 High Reliability and Stability: It should give minimum
error in measurement for temperature variations, vibrations
and other various changes in surroundings.

(cont’d)
 No Hysteretic: It should not give any hysteretic during
measurement while input signal is varied from its low
value to high value and vice-versa.
 Residual Deformation: There should be no
deformation on removal of local after long period of
application.
 Range: The range of the transducer should be large
enough to encompass all the expected magnitudes
of the measurand.

(cont’d)
 Sensitivity: The transducer should give a sufficient
output signal per unit of measured input in order to
yield meaningful data.
 Electrical Output Characteristics: The electrical
characteristics-the output impedance, the frequency
response, and the response time of the transducer
output signal should be compatible with the recording
device and the rest of the measuring system
equipment.
 Physical Environment: The transducer selected should
be able to withstand the environmental conditions to
which it is likely to be subjected while carrying out
measurements and tests.

(cont’d)
 Errors: The errors inherent in the operation of the
transducer itself, or those errors caused by
environmental conditions of the measurement, should
be small enough or controllable enough that they allow
meaningful data to be taken.
 Good Dynamic Response: Its output should be faithful
to input when taken as a function of time. The effect is
analyzed as the frequency response.

Classification Of Transducers
 The transducers may be classified in various ways such
as on the basis of electrical principles involved, methods
of application, methods of energy conversion used,
nature of output signal etc.
1. On the basis of transduction principle
involved:
• Resistive
• Inductive
• Capacitive etc.

Classification Of Transducers(cont’d)
 Primary and Secondary Transducers:
Transducers, on the basis of methods of applications,
may be classified into primary and secondary
transducers. When the input signal is directly sensed
by the transducer and physical phenomenon is
converted into the electrical form directly then such a
transducer is called the primary transducer.

2-Primary and Secondary
Transducers(cont’d)
 For example a thermistor used for the measurement of
temperature fall in this category. The thermistor senses
the temperature directly and causes the change in
resistance with the change in temperature. When the
input signal is sensed first by some detector or sensor
and then its output being of some form other than input
signals is given as input to a transducer for conversion
into electrical form, then such a transducer falls in the
category of secondary transducers.

Primary and Secondary Transducers(cont’d)
For example, in case of pressure measurement,
bourdon tube is a primary sensor which converts
pressure first into displacement, and then the
displacement is converted into an output voltage by
an LVDT. In this case LVDT is secondary transducer.

3-Active and Passive Transducers.
 Transducers, on the basis of methods of energy
conversion used, may be classified into active and
passive transducers.
Active transducers:-
 Self-generating type transducers i.e. the transducers,
which develop their output the form of electrical
voltage or current without any auxiliary source, are
called the active transducers. Such transducers draw
energy from the system under measurement. Normal
such transducers give very small output and, therefore,
use of amplifier becomes essential. E.g.
Thermocouple.

Active and Passive Transducers(cont’d)
Passive transducers:-
 Transducers, in which electrical parameters i.e.
resistance, inductance or capacitance changes with the
change in input signal, are called the passive
transducers. These transducers require external power
source for energy conversion. In such transducer
electrical parameters i.e. resistance, inductance or
capacitance causes a change in voltages current or
frequency of the external power source. These
transducers may draw sour energy from the system
under measurement. Resistive, inductive and
capacitive transducer falls in this category.

4-Analog and Digital Transducers
 Transducers, on the basis of nature of output signal,
may be classified into analog and digital transducers.
Analog transducer:-
 converts input signal into output signal, which is a
continuous function of time such as thermistor, strain
gauge, LVDT, thermo-couple etc.
Digital transducer:-
 converts input signal into the output signal of the form
of pulse e.g. it gives discrete output.

Analog and Digital transducers(cont’d)
 These transducers are becoming more and more
popular now-a-days because of advantages associated
with digital measuring instruments and also due to the
effect that digital signals can be transmitted over a
long distance without causing much distortion due to
amplitude variation and phase shift. Sometimes an
analog transducer combined with an ADC (analog-
digital convector) is called a digital transducer.

5. Transducers and Inverse Transducers.
Transducer:-
 Transducer, as already defined, is a device that
converts a non-electrical quantity into an electrical
quantity. For example a thermo-couple,
photoconductive cell, pressure gauge, strain gauge etc.
Inverse transducer:-
 An inverse transducer is a device that converts an
electrical quantity into a non-electrical quantity. It is a
precision actuator having an electrical input and a low-
power non-electrical output.

Transducers and Inverse Transducers(cont’d)
 For examples a piezoelectric crystal and transnational
and angular moving-coil elements can be employed as
inverse transducers. Many data-indicating and
recording devices are basically inverse transducers. An
ammeter or voltmeter converts electric current into
mechanical movement and the characteristics of such
an instrument placed at the output of a measuring
system are important. A most useful application of
inverse transducers is in feedback measuring systems.

DEFINITION
• A strain gauge is an example of passive transducer
that converts a mechanical displacement into a
change of resistance.
• A strain gauge is a thin wafer like device that can
be attached to a variety of materials to measure
applied strain.

WORKING
• - The strain gauge is
connected into a
Wheatstone Bridge
circuit. The change in
resistance is
proportional to applied
strain and is measured
with Wheatstone
bridge.

TYPES OF STRAIN GAUGES
• Unbonded metal strain gauges
• Bonded metal wire strain gauges
• Bonded metal foil strain gauges
• Vacuum deposited thin metal film strain gauges
• Sputter deposited thin metal film strain gauges
• Semiconductor strain gauges

ADVANTAGES &
DISADVANTAGES
• Advantages: There is no moving part. It is small
and inexpensive.
• Disadvantages: It is non-linear. It needs to be
calibrated.

APPLICATIONS
• Residual stress and Vibration measurement,
Torque measurement, Bending and deflection
measurement, Compression and tension
measurement, Strain measurement

LINEAR VARIABLE
DIFFERENTIAL TRANSFORMER
(LVDT)

CONTENTS:
 LVDT
 CONSTRUCTION
 PRINCIPLE
 OPERATION
 USES

LINEAR VARIABLE DIFFERENTIAL
TRANSFORMER (LVDT)
 Linear variable-differential transformer
is the
most widely used Inductive transducer to
translate linear motion into electrical
signal.

 A differential transformer consists of
a primary winding and two
secondary windings. The windings
are arranged concentrically and
next to each other.
 A ferromagnetic core(armature) in
the shape of a rod or cylinder is
attached to the transducer sensing
shaft.
 The core slides freely within the
hollow portion of

PRINCIPLE :
 Any physical displacement of the core causes
the voltage
of one secondary winding to increase while
simultaneously
reducing the voltage in the other secondary
winding.
 The difference of the two voltages appears
across the output
terminals of the transducers and gives a measure
of the physical
position of the core and hence displacement.

working:
 When the core is in the neutral or zero
position and two secondary windings are equal
and opposite and the net output is negligible.
 By comparing the magnitude and phase of
output with input source, the amount and
direction of movement of core and hence
displacement may be determined.

USES
 The LVDT can be used in all applications
where displacements ranging from fraction
of a mm to few cm have to be measured.
 Acting as a secondary transducer it can be
used as a device to measure force, weight, pressure
etc.

Transducers notes for beginners mechatronics syllabus

It is defined as the force acting on a body which tends to
produce rotation.
Mathematically ,torque is given as :
T = F x D
Where, T = torque
F = force
D = perpendicular distance from the axis of rotation of the line of
action of force
A number of devices which can be used for the
measurement of torque is :
1. Strain gauge torque meter
2. Inductive torque transducer
3. Magnetostrictive transducer
4. Digital methods

1. STRAIN GAUGE TORQUE
METER :-
The torque is given by the relation ;
Where, G = modulus of rigidity
measured in N/m².
R = outer radius of the shaft measured
in m.
r = inner radius of the shaft measured
in m.
L = length of the shaft measured in m.

ADVANTAGE
1. They are fully temperature compensated
2. they give a maximum sensitivity for a given torque
A strain gauge is generally measured by electrical means, In this
arrangement , two strain gauges are subjected to tensile stresses
while the other two experience Compressive stresses to indicated
the torque.
THEORY:-
The gauges must be at 45 with the shaft axis. Gauge 1 and 2 must
be diametrically opposite , as must gauge 3 and 4 .

2. Inductive torque transducer
In inductive torque transducer ,flange A carries a coil and
flange B, an iron core. This core is move IN an OUT of
the coil according to the relative displacement of the two
flanges.

The coil used an arm of A.C bridge .
The output of the A.C bridge is depend upon the
inductance of the coil which in turn depends upon the
position of core and hence on the displacement.
The displacement is depend upon torque.
The bridge output can be directly calibrated to read the torque.
THEORY:

3. Magnetostrictive transducers
These are based on the principle that the permeability of
magnetic material changes when they are subjected to strain .
The permeability decreases with positive strain and increases
with Negative strain .

THEORY :-
Magnetostrictive transducer is used for the measurement of
torque.
The inductance of one of the coil increases due to the
increase in permeability.
When an torque is applied, then bridge is balanced and
two coil have equal inductance.
When torque is applied , inductance of one coil increases
whereas inductance of another coil is decreases and hence ,
bridge is unbalanced..

4. Digital methods Types:-
1. single toothed wheel system
2. Multi- toothed wheel system
Single toothed wheel system :-
Digital timing techniques are generally used for the
determination of relative displacement between two flanges
A and B .

THEORY :-
When a torque is applied to the shaft, there is a relative
there is a relative displacement between the two flanges ,
and a phase shift is produced between the pulses in the
inductive transducer C and D .
when these pulses are compared with the help of an
electronic timer, a time interval will be displayed between
the two pulses. these time interval is proportional to the
relative displacement of the two flanges which in turn
proportional to the torque.
ADVANTAGE :-
1. Errors are eliminated.
2. There is no noise problem.

2.Multi toothed wheel system
Multi –toothed wheels will replace the single –toothed wheels
.

THEORY :-
The transducer are generally magnetic or photoelectric. In
this case, the output is perfectly sinusoidal .
.
The two outputs are exactly in phase of the two wheels
are correctly.
The output voltage progressively becomes out of phase as
the torque increases because an increases in torque
results in relative displacement of the two flanges.

Introduction
 A Electromagnetic flow meter is a volumetric
flow meter which does not have any moving parts
and is ideal for wastewater applications or any
dirty liquid which is conductive or water based.

Working Principle
 The operation of a Electromagnetic flow meter is
based upon Faraday’s Law, which states that:
 “The voltage induced across a conductor as it
moves at right angles through the magnetic field is
proportional to the velocity of that conductor.”

Faraday’s Law
According to this law the induced voltage
E is proportional to VxBxD
where:
E= The voltage generated in a conductor
V= The velocity of the conductor
B= The magnetic field strength
D= The length of the conductor (in this instance distance between the electrodes)

• Electromagnetic flowmeter use Faraday’s Law
of electromagnetic induction to determine the
flow of liquid in a pipe. In a magnetic
flowmeter, a magnetic field is generated and
channeled into the liquid flowing through the
pipe. Following Faraday’s Law, flow of a
conductive liquid through the magnetic field
cause a voltage signal to be sensed by
electrodes located on the flow tube walls.

Uses…
 Pipelines…
 Refineries

Advantages & Disadvantages
Advantages:
 Minimum obstruction in the flow path yields minimum
pressure drop.
 It can measure forward as well as reverse flow with equal
accuracy.
 Low maintenance cost because of no moving parts.
 Corrosive or slurry fluid flow.
Disadvantage:
 Requires electrical conductivity of fluid.
 Zero check can only be done when there is no flow.

PH:-
 The solution of alkalinity or acidity of
aqueous solution is determine by the
relative concentration of hydrogen and
hydroxyl ions in solution.
 Solution is acidic when hydrogen ions are
in majority and alkaline when hydroxyl ions
are in majority .
 Hydrogen ion concentration is measured
on a scale known as pH scale
 pH= -log10(H+)

ELECTRICAL METHOD OF PH
MEASUREMENT
 This method is based upon the measurement
of electrical potential.

Theory of ELECTRICAL METHOD
 In this method Electrode is immersed in the
solution .An electric potential is produced at
the Electrode which forms an electrolytic
half cell.
 This is a measuring cell. A second electrode
is required to provide the a standard
potential and
to complete the cell. This is the reference
cell.
 The algebraic sum of the potential of the
two half

ELECTRONIC METHOD OF PH
MEASUREMENT
 In this type, pH value is read on a digital meter
or indicating type meter.

Theory of ELECTRONIC METHOD
 In this method there are two electrodes, one as
a Reference electrode and other is measuring
electrode.
 The voltage difference between two is depend
on the pH value of solution . the output voltage
obtained here is not linear and is of very low
value.
 A pH amplifier is used to amplify and line arise
this voltage.
 The output of the amplifier can be sent to any
indicating instrument or can be sent to recorder
or to computer for storage

Transducers notes for beginners mechatronics syllabus

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