department of technical education unit 1: basics...
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
Note: This is only Basic Information for students. Please refer “Reference Books” prescribed as per syllabus
Unit 1: Basics of measurements
07 Hours
Necessity of measurements-direct and indirect methods, basic terminology, dynamic characteristics of
an instrument, generalized electronic measurement system, Errors–gross, systematic and random errors,
sources of errors. Statistical analysis–problems involving arithmetic mean, deviation, average deviation,
standard deviation. Limiting errors and probable errors. Standards-primary, secondary, working and
IEEE standards. Comparison of AC and DC bridges. Principle of Wheatstone bridge and mention its
applications.
1.1 Necessity of measurements
Measurements play a very important role in every branch of scientific research and engineering. The
whole area of automation is based on measurements. The very concept of control is based on the
comparison of the actual condition and the desired performance. The measurement confirms the validity
of a theory and also adds to its understanding. This eventually leads to new discoveries. Through
measurement a product can be designed or a process be operated with maximum efficiency, minimum
cost and with desired degree of reliability and maintainability.
1.1.1 Methods of Measurement
Measurement of any quantity involves two parameters, the magnitude of the value and unit of
measurement. For example if we have to measure voltage we can say it is 10volts. Here “10” is the
magnitude and “volts” is the unit of measurement.
There are two methods of measurement:
1. Direct comparison method
2. Indirect comparison method
Direct Comparison method:
In the direct comparison method of measurement, we compare the quantity directly with the
primary or secondary standard. For example, if we have to measure the height of a person, we do it with
the help of the measuring tape or scale that acts as the secondary standard. Here we are comparing the
quantity to be measured (height) directly with the standard.
Indirect comparison method:
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In the indirect comparison method of measurement, the quantity to be measured is not measured
directly but other parameters related to the quantity are measured. For example if you want to measure
power we find voltage (V) and current (I) first and then calculate power using formula P=V*I.
1.1.2 Basic terminology
Instrument: It is defined as a device for determining the value or magnitude of a quantity or variable.
Accuracy: It is defined as the closeness with which an instrument reading approaches the true value of
the quantity being measured.
Precision: It is defined as how exactly the result is determined. i.e given a fixed value of the quantity,
precision is a measure of the degree of agreement within a group of measurements.
Sensitivity: It is defined as the ratio of the magnitude of output signal to the input signal or response of
measuring system to the quantity being measured.
Resolution: It is defined as the smallest change in measured quantity that causes a visible change in its
output.
1.1.3 Dynamic characteristics of an instrument
Dynamic characteristics of a measuring instrument refer to the case where the measured quantity
changes rapidly with time.
The dynamic characteristics of any measurement system are:
1. Speed of response
2. Measuring Lag
3. Fidelity
4. Dynamic error
Speed of Response (desirable): It is defined as the speed with which an instrument or measurement
system responds to changes in measured quantity.
Response Time (desirable): It is the time required by instrument or system to settle to its final steady
position after the application of the input.
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Measuring lag: It is the delay in the response of a measurement system to changes in measured quantity.
It is of two types,
i) Retardation type: In this type of measuring lag the response begins immediately after a
change in measured quantity has occurred.
ii) Time delay: In this type of measuring lag the response of the measurement system begins after
a dead zone after the application of the input.
Fidelity: Fidelity of a system is defined as the ability of the system to reproduce the output in the same
form as the input. It is the degree to which a measurement system indicates changes in the measured
quantity without any dynamic error.
Dynamic error: It is difference between the true value of the quantity changing with time and the value
indicated by the measurement system if no static error is assumed.
**Note: Static error is the difference between the true value and the measured value of a quantity.
1.1.4 Generalized electronic measurement system
The measurement of a given quantity is the result of comparison between the quantity (whose
magnitude is unknown) & a predefined Standard. Since two quantities are compared, the result is
expressed in numerical values.
Figure 1. Shows a generalised measurement system with different elements.
Figure 1.1.4.1 Generalised measurement system
Primary sensing element:
The unknown quantity under measurement makes its first contact with the primary sensing
element of a measurement system. The sensing elements sense the condition, state or value by taking out
a small part of energy from the measured (the unknown quantity which is to be measured), and then
produce an output.
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Variable conversion element:
The output from the primary sensing element may require to be converted to a more suitable form
while saving its information contents. This conversion is performed by the variable conversion element
called transducer.
Variable manipulation element:
The function of this element is to manipulate the signal presented to it preserving the original
nature of the signal. Some non-linear processes like modulation, detection, sampling, filtering, etc., are
performed on the signal to bring it to the desired form to be accepted by the next stage of measurement
system.
Data transmission element:
This element transmits the signal from one location to another without changing the physical
nature of the variable.
Data Presentation element:
This element presents a display record or indication of the output from the manipulation elements to
the person handling the instrument.
1.2 Errors
Error is defined as the difference between the actual value of a quantity and the value obtained by a
measurement.
A study of errors is the first step in finding ways to reduce them. Errors may arise from different
sources and they are mainly classified as shown below,
1.2.1 Gross errors
This type of error occurs due to human mistakes, while reading, recording and calculating
measurement results. For example the observer due to an oversight, may read the temperature as 30.50C
while the actual reading may be 30.20 C, there is 0.3
0C error in the reading.
Gross errors may be of any amount and therefore their mathematical analysis is impossible. But, the
following precautions can be taken to avoid such errors. They are:
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1. Proper care should be taken while reading and recording the data.
2. More than one reading should be taken for the quantity under measurement preferably by
different observers.
1.2.2 Systematic errors
Systematic errors occur usually from the measuring instruments. They may occur because there is
something wrong with the instrument or its data handling system, or because the instrument is wrongly
used by the experimenter.
These errors can be found by conducting repeated measurements under different conditions or with
different equipment and if possible by entirely different method.
These errors are further classified as follows,
Instrumental errors:
These errors arise due to following reasons:
1. Due to inbuilt shortcomings in the instruments.
2. Due to misuse of the instruments.
3. Due to loading affects the instruments.
These errors can be minimised by using the following methods,
1. Measurement procedure must be carefully planned.
2. Correction factors should be adopted after finding the instrumental errors.
3. Instrument must be re-calibrated carefully.
Environmental errors:
These errors arise due to conditions external to the measuring device (e.g. effects of temperature,
pressure, humidity, dust etc.)
These errors can be minimised by using the following methods,
1. Temperature controlled enclosure can be used to avoid temperature variations.
2. The effect of humidity, dust etc. Can be entirely eliminated by sealing the equipment in an
airtight container.
3. By providing shields the instrument can be protected against external magnetic and electrostatic
fields.
Observational errors:
These errors arise due wrong observations. The Observational errors arise due to following
reasons:
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1. Parallax error occurs on account of the pointer and the scale not being in the same plane (shown
in figure 1.2.2.1.
2. Wrong scale reading and wrong recording of data.
3. Incorrect conversion of units in between consecutive readings.
These errors can be eliminated by using digital display systems.
Figure 1.2.2.1 Parallax error
1.2.3 Random errors
Random errors are accidental, small and independent. These errors arise due to following reasons:
1. Parallax: when an observer reads a scale from an incorrect direction
2. Variation in environmental conditions
3. Friction in instrument movement
4. Mechanical vibrations
These errors can be minimised by using the following methods,
1. Taking repeated readings to obtain an average value.
2. Maintaining good experimental technique (e.g. reading from a correct position).
1.2.4 Sources of errors
1. Insufficient knowledge of process parameters and design conditions.
2. Selection of improper instrument for measurement.
3. Poor design.
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4. Human error caused by person operating the instrument.
1.3 Statistical analysis
Statistical analysis of measurement is a procedure of collection, analysis, interpretation,
presentation, and organization of data.
Arithmetic mean:
It is the ratio of sum of readings taken to the total no. of readings.
Arithmetic Mean = (Sum of readings)
/ (Number of readings)
Where 1, 2, .... n are the readings taken, N is the no. of readings,
̅ is the symbol of the arithmetic mean.
Deviation:
Deviation is the departure of the given reading from the arithmetic mean of the group of readings.
Let the deviation of the first reading 1 be d1 and that of second reading 2 be d2 and so on.
Then the deviation from the mean is expressed as
Average deviation:
It is the ratio of sum of the absolute values of deviations to the no. of readings.
Where d1, d2, d3...................... dN are the deviations of readings 1, 2,.........................xN
https://en.wikipedia.org/wiki/Analysishttps://en.wikipedia.org/wiki/Data
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Standard deviation:
The standard deviation also called as mean square deviation of N no. of data is defined as the
square root of the sum of individual deviations squared(d12
, d22
, ..................... dN2) divided by the no. of
readings(N).
Limiting errors:
Limiting error is used to indicate the accuracy of an instrument. The limiting error (or guarantee error) is
given by the manufacturer to define the maximum limit of the error that may occur in the instrument. For example,
if the resistance of a resistor is given as 50Ω ± 5%, it means that the resistance value falls between the limits 45Ω
and 55Ω.In other words the manufacturer of the resistor guarantees its value lie between 45 Ω to 55 Ω.
Probable errors:
It defines the half-range of an interval about a central point for the distribution, such that half of
the values from the distribution will lie within the interval and half outside. Thus it is equivalent to half
the interquartile range, or the median absolute deviation. The probable error can also be expressed as a
multiple of standard deviation σ,
i.e. Probable error ϒ= 0.675x σ
https://en.wikipedia.org/wiki/Half-rangehttps://en.wikipedia.org/wiki/Central_tendencyhttps://en.wikipedia.org/wiki/Interquartile_rangehttps://en.wikipedia.org/wiki/Median_absolute_deviation
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Figure 1.3.1 Guassian distribution curve
Variance:
The square of the standard deviation is called variance.
i.e. V= (standard deviation)2
V
= σ
2
Problem 1:
A circuit was tuned for resonance by eight different students and the values of resonance
frequency in KHz were recorded as 532,548,543,535,546,531,543 and 536. Calculate the arithmetic
mean, average deviation, standard deviation and variance.
SI.NO RESONANT
FREQUENCIES(RF)
DEVIATION(d)
d=xi- ̅ 1 532(x1) d1= x1- ̅= -7.25 2 548(x2) d2= x2- ̅= 8.75 3 543(x3) d3= x3- ̅= 3.75 4 535(x4) d4= x4- ̅= -4.25 5 546(x5) d5= x5- ̅= 6.75 6 531(x6) d6= x6- ̅= -8.25 7 543(x7) d7= x7- ̅= 3.75 8 536(x8) d8= x8- ̅= -3.25 ∑RF=4314
**Note N =no. of resosonant frequencies=8
1. Arithmetic mean ( ̅) =532+548+543+535+546+531+543+536 8
=4314
8
= 539.25
2. Average deviation ( ̅) = d1+d2+d3+d4+d5+d6+d7+d8 8
=
=5.75KHZ
3. Standard deviation (σ) = N-1
= d1+d2+d3+d4+d5+d6+d7+d8)2 8-1
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= 7
=6.54KHZ
4. Variance (V) = σ2
= (6.54)
2
=42.77KHZ
1.4 Standards
A standard is a physical representation of unit of measurement. Standards have been developed for all the
fundamental units as well as some of the derived mechanical and electrical units.
Standards are classified as follows:
1. Primary standards:
Primary standards are standards of such high accuracy which can be used as ultimate reference
standards. These standards are maintained by national standard laboratories in different parts of the world.
2. Secondary standards:
Secondary standards are basic reference standards used by measurement and calibration
laboratories. It is obtained by comparing with primary standard. For measurement of a quantity using
secondary standard instrument, pre-calibration is required. Calibration of a secondary standard is made by
comparing the results with a primary standard instrument or with an instrument having high accuracy or
with a known input source.
3. Working standards:
These standards are used to check and calibrate general laboratory instrument for their accuracy
and performance. Working standards are checked against the secondary standards.
4. International standards:
The Institute of Electrical and Electronics Engineers Standards Association (IEEE) is an
organization within IEEE that develops global standards in a broad range of industries,
including: power and energy, biomedical and healthcare, information
technology and robotics, telecommunication etc.
https://en.wikipedia.org/wiki/IEEEhttps://en.wikipedia.org/wiki/Technical_standardhttps://en.wikipedia.org/wiki/Electrical_powerhttps://en.wikipedia.org/wiki/Energy_developmenthttps://en.wikipedia.org/wiki/Medical_researchhttps://en.wikipedia.org/wiki/Health_carehttps://en.wikipedia.org/wiki/Information_technologyhttps://en.wikipedia.org/wiki/Information_technologyhttps://en.wikipedia.org/wiki/Roboticshttps://en.wikipedia.org/wiki/Telecommunication
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These standards are not physical items that are available for comparison and checking of
secondary standards but are standard procedure, nomenclature, definitions etc. These standards have been
kept updated and some of the IEEE standards have been adopted by other organizations as standards.
One of the most important standards is the IEEE 4888 digits interface for programmable
instrumentation for test and other equipment standardising.
1.5 Comparison of AC and DC bridges
AC BRIDGES DC BRIDGES
The AC bridges are used to
measure the impedances
consisting of capacitance and
inductances.
The DC bridges are used to
measure resistances.
The AC bridges use the
alternating voltage as the
exciting voltage.
The DC bridges use the DC voltage
as exciting voltage.
The four arms of bridge
consists of resistors, inductors,
capacitors or their
combinations.
The four arms of bridge consists of
pure resistors.
The balancing equation for AC
bridges are
1. Z1Z4=Z2Z3, for magnitude
balance.
2. ɸ1+ɸ3= ɸ2+ɸ4, for phase
angle balance
The balancing equation for DC
bridges is
R1R4=R2R3
Examples of AC bridges are,
Maxwell’s bridge, Wein
bridge, etc,.
Examples of AC bridges are,
Wheatstone bridge, Kelvin bridge,
etc,.
1.6 Wheatstone bridge
The Wheatstone bridge was developed by Charles Wheatstone to measure unknown resistance. A
schematic of a Wheatstone bridge is shown below:
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Figure 1.6.1 Wheatstone bridge
The bridge has four arms together with a source of EMF (V0) and a null detector i.e galvanometer
(G). The unknown resistor is Rx, the resistor Rk is known value, and the resistors R1 and R2 have a
known ratio R2/R1.A galvanometer (G) measures voltage difference VAB between points A and B.
When VAB=0 the bridge is said to be “balanced” and no current flows through the galvanometer (G).
Since VAB=0, the voltage drop from C to A must be equal to the voltage drop from C to B.VCA =
VCB. Likewise, we must have VAD = VBD. So we can write,
(1)
(2) .
Dividing (2) by (1), we have
(3) .
Thus, the unknown resistance Rx can be computed from the known resistance Rk and the known
ratio R2/R1. The resistors R1 and Rk are called Ratio arms, while the resistor R2 is called standard arm of
the bridge.
Applications of Wheatstone bridge:
1. To measure very low resistance values accurately.
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2. Along with operational amplifiers it is used to measure physical parameters like temperature,
strain, light etc.
3. It is used by telephone companies to locate cable faults.
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Unit 2: Transducers
08 Hours
Necessity of electrical transducers, selection of a transducer, active, passive, analog and digital
transducers. Strain gauge-principle, gauge factor, features of bonded, unbonded, wire and foil type strain
gauges, load cell. Principle of working & features of capacitive transducer, Hall effect type, LVDT,
thermistor, thermocouple, piezoelectric, proximity sensors, digital optical encoders & PIR sensors.
2.1 Necessity of electrical transducers
In a measurement system all the quantities being measured, could not be displayed. In such
situation, the accurate measurement of a quantity is usually done by converting the related information or
signal to another form which is more conveniently or accurately displayed. This is achieved with the help
of a device which is known as transducer.
Definition: It is a device which converts the energy of one form to another.
OR
It is a device which converts non electrical quantity (e.g. sound, light, heat) into an electrical
quantity (Voltage, current or frequency).
Non electrical quantity Electrical quantity
Figure 2.1.1 Transducer
Benefits of electrical transducer:
1. Electrical amplification and attenuation can be done easily that too using static device.
2. The effect of friction is minimised.
3. The electric or electronic system can be controlled with a very small electrical power.
4. The power can be easily used, transmitted and processed for purpose of measurement.
2.2 Selection criteria of a transducer
1. Operating principle: The transducers are so many times selected on the basis of operating principle
used by them. The operating principles used in transducer may be resistive, inductive, capacitive, opto-
electronic, and so on.
2. Sensitivity: The transducer should give a sufficient output signal per unit of measured input in order
to yield meaningful data.
Transducer
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3. Operating range: The transducer should maintain the range requirements and have a better resolution
over its entire range.
4. Accuracy: High degree of accuracy is necessary for measurement.
It should not alter the quantity to be measured.
5. Stability and reliability: The transducer should exhibit a high degree of stability during its operation
and storage life. Reliability should be assured so that the instrument continues uninterrupted.
6. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signal
and highly sensitive to wanted signal.
7. Cost: The transducer should be easily available at reasonable prices.
8. Loading Effects: To avoid loading effect, it is necessary that a transducer has high input impedance
and low output impedance.
9. Physical Environment: The transducer selected should be able to withstand any change in
environmental conditions and maintain its output-input relationship.
2.3 Active and passive transducers
On the basis of methods of energy conversion used the transducers are classified into following two
categories:
A transducer, which develops its output in the form of electrical current or voltage without any
auxiliary source, is called active transducer. The energy required for this is absorbed from the physical
quantity which is being measured. Therefore, active transducers are also called as self generating type
transducers.
Examples are thermocouples, piezo-electric transducers, photovoltaic cell etc.
A transducer, which derives the power required for energy conversion from an external power
source is called as a passive transducer. Therefore, passive transducers are also called as externally
powered transducers.
Examples are Resistance thermometers and thermistors, photoemission cell etc.
2.4 Analog transducers and digital transducers
On the basis of type of output, the transducers are classified into following two categories:
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A transducer which converts the input physical quantity into an analog output which is a
continuous function of time is known as analog transducers.
Examples are linear variable differential transformer (LVDT), thermo-couple, strain gauge,
thermistor etc.
A transducer which converts the input physical quantity into an digital output which is in the form
of pulses is known as digital transducers.
Examples are digital tachometers, digital optical encoders etc.
2.5 Strain gauge
2.5.1 Principle
Strain Gauge is a passive transducer. It is a type of sensor whose resistance varies with applied
force. It converts force, pressure, tension etc., into a change in electrical resistance which can be
measured.
The basic principle of operation of a strain gauge is simple: when strain is applied to a thin metallic
wire, its dimension changes, thus changing the resistance of the wire. The value of resistivity of conductor
also changes.When it is strained its property is called piezo-resistance. Therefore, resistance strain gauges
are also known as piezo-resistive gauges.
2.5.2 Gauge factor
It is defined as the ratio of per unit change in resistance to per unit change in length.
Gauge factor (Gf) =
⁄
Where, R = change in resistance R,
= change in length per unit length L.
The resistance of the wire of strain gauge, R is given by
R =
Where, ρ = Resistivity of the material of wire ( of strain gauge ),
L = Length of the wire.
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A = Cross-sectional area of the wire.
Problem 1:
A strain gauge has an unstrained length of 10cm and resistance of 100KΩ. When its length
reduces to 9.9cm, the resistance decreases to 98KΩ. Estimate its gauge factor.
Solution: R= Initial Resistance = 100KΩ.
L= Initial length=10Cm
ΔR= change in initial resistance
=100x103-98x10
3
=2KΩ
ΔL = The change in length
= 10-9.9 = 0.1 cm
Therefore, gauge factor = Gf =
=
= 20
2.5.3 Bonded resistance strain gauge
These strain gauges are directly bonded (that is, pasted) onto the surface of the structure under
study. Hence they are termed as bonded strain gauges.
Figure 2.5.3.1 Bonded type strain gauge
Features of bonded resistance strain gauge:
1. They are reasonably inexpensive.
2. They can pull off overall accuracy of better than +/-0.10%.
3. They are available in a short gauge length and have small physical size.
4. These strain gauges are only moderately affected by temperature changes.
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5. They are extremely sensitive and have low mass.
6. Bonded resistance strain gages can be employed to measure both static and dynamic strain.
7. These types of strain gauges are appropriate for a wide variety of environmental conditions.
2.5.4 Unbonded strain gauge
These strain gauges are not directly bonded (that is, pasted) onto the surface of the structure under
study. Hence they are termed as unbonded strain gauges.
Figure 2.5.4.1 Unbonded type strain gauge
Features of unbounded strain gauge:
1. They are able to measure strains of ±1μm/m.
2. They are small in size and light in weight.
3. They are able to respond to high frequency signals.
4. They have wide range of frequency response.
5. They have stable calibration constant (gauge factor).
6. They are flexible in use and are used in wide range of applications.
7. They are low in cost.
2.5.5 Wire type strain gauge
These strain gauges consists of grid of fine resistance wire of about 0.025mm in diameter or
less directly bonded (that is, pasted) onto the surface of the structure under study.
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Figure 2.5.5.1 Wire type strain gauge
Features of wire type strain gauges:
1. The wire type strain gauge should have a high value of gauge factor.
2. The resistance of wire type strain gauge should be as high as possible.
3. The wire type strain gauge should have a low resistance temperature coefficient.
4. The wire type strain gauge should not have any hysteresis effect in its response.
5. The wire type strain gauge should have linear characteristics.
6. The wire type strain gauge should have a good frequency response.
2.5.6 Foil type strain gauge
Foil type strain gauges use similar materials to wire strain gauges but have greater heat dissipation
capacity on account of greater surface area. Due to this reason they can be employed for higher operating
temperature range.
Features of foil type strain gauge:
The features of foil type strain gauge are similar to those of wire wound strain gauges except
resistance value of foil gauge are available in between 50 and 1000ohms, and maximum gauge current is
about 30mamps.
2.6 Load cells
Load cell is a passive transducer or sensor which converts applied force or load into electric
signals. These electric signals can be voltage change, current change or frequency change depending on
the type of load or circuit used.
Strain gauge load cells:
Figure shows a strain gauge load cell. It consists of a steel cylinder, on which four identical strain
gauges. The gauges R1 and Rg are along the direction of applied load and the gauges R2 and R3 are
attached circumferentially to gauges R1 and Rg. All the four gauges are connected electrically to the four
limbs of a wheat stone bridge circuit.
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Figure 2.6.1 Load cell
When there is no load on the cell, all the four gauges have the same resistance
(R1=R2=R3=Rg).under these conditions the A and B terminals are at the same potential, the bridge is
balanced and the output voltage is zero.
Applications:
1. Road vehicle weighing devices.
2. Draw bar and tool-force dynamometers.
3. Crane load monitoring.
2.7 Capacitive transducers
Capacitive transducers are passive transducers with a variable capacitance. These are mainly
used for the measurement of displacement, pressure etc,.
The capacitive transducer comprises of two parallel metal plates that are separated by a dielectric
material.
2.7.1 Principle of working
The principle of operation of capacitive transducers is based upon the equation for capacitance of
a parallel plate capacitor.
Where, A = Overlapping area of plates; m2,
d = Distance between two plates; m,
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= 0rPermittivity (dielectric constant); F/m.
0 = Permittivity of free space=8.854*10-12
F/m.
r = Relative Permittivity.
Capacitive transducers using change in Area of the plates:
Figure (a) shows a capacitive transducer, where capacitance changes due to change in area (A) of
the plates. Since capacitance is directly proportional to the effective area of the plates, response of such
system is linear.
Capacitive transducers using change in distance of the plates:
Figure (b) shows a capacitive transducer, where capacitance changes due to change in distance(D)
between the plates. Here, one is a fixed plate and another is a movable plate. The displacement to be
measured is applied to the movable plate. Since the capacitance varies inversely as the distance between
the plates the response of the transducer is not linear.
Capacitive transducers using change in dielectric medium:
Figure (c) shows, if the area (A) and the distance (D) between the plates of a capacitor remain
constant, capacitance will vary only as a function of the dielectric constant (e) of the substance filling the
gap between the plates. Physical variables such as displacement, force or pressure can cause the
movement of dielectric material in the capacitor plates, resulting in changes in the effective dielectric
constant which in turn will change the capacitance.
Figure 2.7.1.1 Capacitive transducers
2.7.2 Features of capacitive transducer
1. Accuracy - provide accuracies as high as ±0.02% Full Scale (FS).
2. Minimal mechanical motion.
3. Range capabilities.
4. Long term stability.
5. High-level output.
6. Electromagnetic compatibility.
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7. Resistant to harsh environments.
2.7.3 Advantages of capacitive transducer
1. Requires extremely small force for operation.
2. Requires small power for operation.
3. Frequency response is good.
4. A resolution of the order of 2.5*10-3
.
5. Higher input impedance. Therefore loading effects are minimum.
2.7.4 Disadvantages of capacitive transducer
1. Sensitivity to temperature variations.
2. The possibility of erratic or distortion signals due to long lead length.
3. They show nonlinear behavior on account of Edge effects. This can be eliminated using guard
rings.
2.7.5 Applications
1. As frequency modulator in RF oscillator.
2. In capacitance microphone.
3. Use the capacitance transducer in an ac bridge circuit.
4. To measure force and pressure.
5. To measure humidity in gases.
2.8 Hall effect transducers
2.8.1Hall effect
When a magnetic field is applied at right angles to the direction of electric current, an electric field
is setup which is perpendicular to both the direction of electric current and the applied magnetic field.
This phenomenon is called Hall effect.
Fig 2.8.1.1 Current carrying semiconductor bar subject to transverse magnetic field
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In the above figure, A thin sheet of semiconductor bar (called Hall element) is carrying a current (I)
and is placed into a magnetic field (B) which is perpendicular to the direction of current flow. Due to the
presence of force, the distribution of current is no more uniform across the Hall element and therefore a
potential difference is created across its edges perpendicular to the directions of both the current and the
field. This voltage is known Hall voltage and its typical value is in the order of few microvolts. The Hall
voltage is directly proportional to the magnitudes of I and B. So if one of them (I and B) is known, then
the observed Hall voltage can be used to estimate the other.
VH
VH =
Where, RH = Hall co-efficient.
B = Magnetic field strength.
I = Current carried by the semiconductor bar.
B = width of the specimen along the magnetic field.
The Hall effect may be used:
To find whether semiconductor is N-type or P-type.
To determine charge carrier concentration.
Hall effect transducer:
These are transducers in which “Hall Effect” is used to measure various electrical and non-electrical
quantities.
The following are the applications of Hall effect transducers:
2.8.2 Hall effect displacement transducer
Hall effect element may be used for measuring a linear displacement.
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Figure 2.8.2.1 Hall effect displacement transducer
The Hall Effect element is located in the gap, adjacent to the permanent magnet and the field
strength produced in the gap, due to the permanent magnet, is changed by changing the position of the
ferromagnetic plate. The voltage output of the Hall effect element is proportional to the field strength of
the gap which is a function of the position of ferromagnetic plate with respect to the structure.Thus
displacement can be measured by the Hall effect transducer. Very small displacements (as small as 0.025
mm) can be measured by this method.
2.8.3 Current measurement
Hall effect transducer is used to measure current in a conductor, without disturbing the circuit and
without making electrical connection between the conductor circuit and the meter.
Figure 2.8.3.1 Measurement of current using Hall effect In the above figure when a DC or AC current flows through the conductor wound around a core it
sets up a magnetic field. This magnetic field is proportional to the current. A Hall effect sensor is placed
in the slot which acts as a magnetic concentrator. The voltage produced at the output terminals is
proportional to the magnetic field strength and hence is proportional to the current, flowing through the
conductor.This method is used to measure current from less than a mA to thousands of amperes.
2.8.4 Fluid level measurement
The below figure shows how a Hall effect transducer can be used to measure fluid(fuel) level in an
automobile fuel tank.
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Figure 2.8.4.1 Measurement of fluid level using Hall effect
A fuel level indication can be obtained with a Hall-effect sensor by attaching a magnet to the float
assembly. As the float moves up and down with the fuel level, the gap between the magnet and the Hall
element will change. The gap changes the Hall-effect and thus the output voltage.
2.9 LVDT (Linear Variable Differential Transducer)
LVDT works under the principle of mutual induction. It is used to translate the linear motion into
electrical signals.
2.9.1 Construction
Figure 2.9.1.1 shows the construction of LVDT. LVDT consists of a cylindrical former where it is
surrounded by one primary winding in the centre of the former and the two secondary windings at the
sides. The number of turns in both the secondary windings are equal, but they are opposite to each other,
i.e., if the left secondary windings is in the clockwise direction, the right secondary windings will be in
the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the
two secondary coil. The two secondary coils are represented as S1 and S2. An iron core is placed in the
centre of the cylindrical former which can move in to and fro motion as shown in the figure. The AC
excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.
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Figure 2.9.1.1 LVDT construction and circuit
2.9.2 Working of LVDT
As shown in the above figure, an ac voltage with a frequency between (50-400Hz) is supplied to the
primary winding. Thus, two voltages VS1 and VS2 are obtained at the two secondary windings S1 and S2
respectively. The output voltage will be the difference between the two voltages (VS1-VS2) as they are
combined in series. Let us consider three different positions of the soft iron core inside the former.
Null Position - This is also called the central position as the soft iron core will remain in the exact
center of the former. Thus the linking magnetic flux produced in the two secondary windings will be
equal. The voltage induced because of them will also be equal. Thus the resulting voltage VS1-VS2 = 0.
Right of Null Position - In this position, the linking flux at the winding S2 has a value more than
the linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS2.
Left of Null Position - In this position, the linking flux at the winding S2 has a value less than the
linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS1.
From the working it is clear that the difference in voltage, VS1-VS2 will depend on the right or left
shift of the core from the null position. Also, the resulting voltage is in phase with the primary winding
voltage for the change of the arm in one direction, and is 180 degrees out of phase for the change of the
arm position in the other direction.
The magnitude and displacement can be easily calculated or plotted by calculating the magnitude
and phase of the resulting voltage.
2.9.3 Advantages of LVDT
1.High resolution.
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2. High output.
3. High sensitivity.
4. Very good linearity.
5. Ruggedness.
6. Less friction.
7. Low hysteresis.
8. Low power consumption.
2.9.4 Disadvantages of LVDT
1. Very high displacement is required for generating high voltages.
2. Shielding is required since it is sensitive to magnetic field.
3. The performance of the transducer gets affected by vibrations
4. It is greatly affected by temperature changes.
2.9.5 Applications of LVDT
LVDT is used to measure displacement ranging from fraction millimetre to centimetre. Acting as a
secondary transducer, LVDT can be used as a device to measure force, weight and pressure, etc,.
Multiple LVDT‟s are used for measurement of pressure or weight applied by liquid in a tank.
2.10 Thermistors
Thermistors are transducers which are thermally sensitive variable resistance made of
semiconducting materials. A thermistor is a type of resistor whose resistance is dependent on temperature.
It exhibits high negative temperature coefficient of resistance.
Figure 2.10.1 Types of thermistors
2.10.1 Applications of thermistors
1. Measurement of temperature.
2. Measurements of level, flow and pressure of liquids.
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3. Voltage Regulation.
4. Circuit Protection.
5. Volume Control.
6. Time Delay.
2.11 Thermocouple
A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs
made from different metals. The wires legs are welded together at one end, creating a junction. This
junction is where the temperature is measured. When the junction experiences a change in temperature, a
voltage is created.
Figure 2.11.1 Thermocouple
The thermocouple works on the principle of Seebeck effect. Seebeck effect is the phenomenon in
which a voltage difference is produced between two dissimilar electrical conductors or semiconductors
due to temperature difference between the two substances. When heat is applied to one of the two
conductors or semiconductors, heated electrons flow towards the cooler one. Direct current will flow
through an electric circuit, if the pair is connected through it. Seebeck effect usually produces small
voltages that are a few microvolts per kelvin of temperature difference at junction.
2.11.1 Advantages
1. Low cost.
2. Small size.
3. Robust.
4. Wide range of operation.
5. Provide fast response.
6. Accurate for large temperature changes.
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2.11.2 Disdvantages
1. Very weak output.
2. Limited accuracy for small temperature.
3. Sensitive to electrical noise.
2.11.3 Applications
1. Temperature measurement for kilns, gas turbine exhaust, diesel engines.
2. Steel industry.
3. Heating appliance safety.
4. Power production in thermoelectric generation.
5. Thermoelectric cooling.
2.12 Piezoelectric Transducers
Piezoelectric transducer is based on principle of piezoelectric effect
Piezoelectric effect:
Piezoelectric effect states that when mechanical stress or forces are applied on quartz, crystal,
produce electrical charges on quartz crystal surface. The rate of charge produced will be proportional to
rate of change of mechanical stress applied on it. Higher will be stress higher will be voltage. Certain
crystals namely Quartz, Rochelle salt and tourmaline, which exhibits piezoelectric effect are called
piezoelectric crystals.
There are two main groups of piezoelectric crystals,
Natural crystals: Quartz and tourmaline.
Synthetic crystals: Rochelle salt, lithium sulphate etc.
Working of piezoelectric transducer:
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In the above figure, Piezo electric crystal is used for measuring varying force applied to a simple
plate. The magnitude and polarity of the induced charge on the crystal surface is proportional to
magnitude and direction of applied force.
The charge at the electrode gives rise to voltage (E) is given by,
E=
= gtP
Where, g = Voltage sensitivity
T = Thickness of the crystal
F = Force in Newton
A = Area of the crystal
P = pressure =
2.12.1 Advantages
1. No need of external force.
2. Easy to handle due to small size.
3. High frequency response.
4. High output.
5. They can be cut into variety of shapes and sizes.
2.12.2 Disadvantages
1. It is not suitable for measurement in static condition.
2. It is affected by temperatures.
2.12.3 Applications
1. Microphones.
2. Medical diagnostics.
3. It is used in electric lighter used in kitchens.
4. They are used for studying high speed shock waves and blast waves.
5. Used in Inkjet printers.
6. It is also used in restaurants or airports where when a person steps near the door the door opens
automatically. In this the concept used is when person is near the door a pressure is exerted
persons weight on the sensors due to which the electric effect is produced and the door opens
automatically.
7. Under water detection system.
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2.13 Proximity sensors
A proximity sensor is a sensor able to detect the presence of nearby objects without any physical
contact. The object being sensed is often referred to as the proximity sensor's target. The maximum
distance that this sensor can detect is defined "nominal range".
Types of proximity sensors:
The Eddy current proximity sensor:
This uses the effect of eddy (circular) currents to sense the proximity of non-magnetic but
conductive materials. A typical eddy current transducer contains two coils: an active coil (main coil) and
a reference coil. When a coil is supplied with an alternating current, an alternating magnetic field is
produced. If there is metal object in close proximity to this alternating magnetic field, then eddy currents
are induced in it.Therefore impedance of the coil changes thereby changing the amplitude of the
alternating current.
Figure 2.13.1 Eddy current proximity sensor
Advantages of Eddy current proximity sensor:
1. Non-contacting measurement.
2. High resolution.
3. High frequency response.
Disadvantages of Eddy current proximity sensor:
1. Effective distance is limited to close range.
2. The relationship between the distance and the impedance of the coil is nonlinear and
temperature dependent.
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3. Only works on conductive materials with sufficient thickness.
Capacitance proximity sensors:
Figure 2.13.2 below shows capacitance proximity sensor. Capacitive Proximity Sensors detect
changes in the capacitance between the sensing object and the sensor. The amount of capacitance varies
depending on the size and distance of the sensing object. An ordinary capacitive proximity sensor is
similar to a capacitor with two parallel plates, where the capacity of the two plates is detected. One of the
plates is the object being measured (with an imaginary ground), and the other is the Sensor's sensing
surface. The changes in the capacity generated between these two poles are detected. The objects that can
be detected depend on their dielectric constant.
Figure 2.13.2 Capacitive proximity sensor
Inductive proximity sensors:
In the below figure a coil is wound around a core. When the end of a inductive coil is close to a
metal object its inductance changes. This change can be monitored by its effect on a oscillator circuit, and
this change is used to trigger a switch.
Figure 2.13.3 Inductive proximity sensor
Pneumatic sensors:
Pneumatic sensors are used to measure displacement, as well as sense the objects close to it. The
displacement and proximity are transformed into change in air pressure. Figure below shows a schematic
of such a sensor. It comprises of three ports. Low pressure air is allowed to escape through port A. In the
absence of any obstacle / object, this low pressure air escapes and in doing so, reduces the pressure in the
port B. However, when an object obstructs the low pressure air (Port A), there is rise in pressure in output
port B. This rise in pressure is calibrated to measure the displacement.
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Figure 2.13.4 Pneumatic sensor
2.14 Digital optical encoder
A digital optical encoder is a device that converts motion into a sequence of digital pulses. By
counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute
position measurements.
Encoders have both linear and rotary configurations, but the most common type is rotary.
Rotary encoders are manufactured in two basic forms:
Absolute encoder where a unique digital word corresponds to each rotational position of the shaft,
The incremental encoder, which produces digital pulses as the shaft rotates, allowing
measurement of relative position of shaft. Most rotary encoders are composed of a glass or plastic code
disk with a photographically deposited radial pattern organized in tracks. As radial lines in each track
interrupt the beam between a photo emitter-detector pair, digital pulses are produced.
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Figure 2.14.1 Digital optical encoder
Figure above shows the construction of an optical encoder. It comprises of a disc with three
concentric tracks of equally spaced holes. Three light sensors are employed to detect the light passing
through the holes. These sensors produce electric pulses which give the angular displacement of the
mechanical element e.g. shaft on which the Optical encoder is mounted. The inner track has just one hole
which is used locate the „home' position of the disc. The holes on the middle track offset from the holes of
the outer track by one-half of the width of the hole. This arrangement provides the direction of rotation to
be determined. When the disc rotates in clockwise direction, the pulses in the outer track lead those in the
inner; in counter clockwise direction they lag behind.
2.15 PIR sensors
All objects with a temperature above absolute zero emit heat energy in the form of radiation.
Usually this radiation isn't visible to the human eye because it radiates at infrared wavelengths, but it can
be detected by electronic devices designed for such a purpose. A passive infrared sensor (PIR sensor) is
an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are
often referred to as PIR, "Passive Infrared", "Pyroelectric", or "IR motion" sensors.
PIRs are basically made of a pyroelectric sensor (which is shown in figure below as the round metal
can, with a rectangular crystal in the centre), which can detect levels of infrared radiation. The PIR sensor
itself has two slots in it, each slot is made of a special material that is sensitive to IR. The lens used here
consists of two slots that can 'see' out past some distance. When the sensor is idle, both slots detect the
same amount of IR. When a warm body like a human or animal passes by, it first intercepts one half of
the PIR sensor, which causes a positive differential change between the two halves. When the warm body
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leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change.
These change pulses are what is detected.
Figure 2.15.1 PIR sensors
2.15.1 Applications
1. All outdoor Lights.
2. Lift Lobby.
3. Multi Apartment Complexes.
4. Common staircases.
5. For Basement or Covered Parking Area.
6. Shopping Malls.
2.15.2 Features
1. Low Noise and High Sensitivity.
2. Supply Voltage – 5V.
3. Delay Time Adjustable.
4. Standard TTL Output.
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Unit 3: Analog meters 11 Hours
Principle of PMMC meters, DC ammeters and voltmeters using PMMC. Shunt and series resistors, multi
range voltmeters/ammeters, loading effect and voltmeter sensitivity, problems on extending range.
Working of electrodynamometer type voltmeter, ammeter and wattmeter.
Electronic voltmeters: Pros and cons, working of FET input, chopper type DC amplifier voltmeter,
solid-state voltmeter using op-amp, AC voltmeter using full-wave rectifier, Peak responding and true
RMS voltmeters. Ohmmeters series and shunt type. Concept of Calibration of meters.
3. ANALOG METERS:
An instrument which measures and indicates values by means of a continuous scale and a movable
pointer are called analog meters.
“Ammeters”, are connected in series in the circuit whose current is to be measured. “Voltmeters”
are connected in parallel with the circuit whose voltage is to be measured. “Ohmmeters” are used for
measurement of resistance.
3.1 PMMC meters
Figure 3.1.1 PMMC meter and D’Arsonval movement
Principle of PMMC meters:
When current carrying conductor is placed in a magnetic field, a mechanical force acts on the
conductor, if it is attached to a moving system, with the coil movement, the pointer moves over the scale.
Thereby, the basic PMMC movement is called as D‟Arsonal movement. It can be used for D.C
measurements.
Construction:
It consists of a permanent horse shoe magnet with soft iron pole pieces attached to it.
A cylinder shaped soft iron core, is placed in between two pole pieces around which a coil of fine
wire moves wound on a light metal frame. A light pointer attached to the moving coil moves up-scale as
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the coil rotates when the current is passed through it. The rotating coil is prevented from continuous
rotation by a spring which provides restoring torque.
Working:
This meter movement works on “MOTOR PRINCIPLE” (when a current carrying conductor is
placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the
field).When the instrument is connected in the circuit to measure current or voltage, the operating current
flows through the coil. Since the current carrying coil is placed in the magnetic field of the permanent
magnet, a mechanical torque acts on it. As a result of this torque, the pointer attached to the moving
system moves in clockwise direction over the graduated scale to indicate the value of current or voltage
being measured.
The deflecting torque is given by,
Td = NBldI
where N is number of turns,
B is magnetic flux density in air gap,
l is the length of moving coil, d is the width of the moving coil,
I is the electric current.
Thus, Td α I
The instrument is spring controlled so that,
Tc α θ
The pointer will comes to rest at a position, where
Td =Tc
Therefore,
Thus, the deflection is directly proportional to the operating current. Hence, such instruments have
uniform scale.
Advantages:
1. Uniform scale.ie, evenly divided scale.
2. Very effective eddy current damping.
3. High efficiency.
4. Require little power for their operation.
5. No hysteresis loss (as the magnetic field is constant).
6. Very accurate and reliable.
Disadvantages:
1. Cannot be used for a.c measurements.
θ α I
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2.More expensive (about 50%) than the moving iron instruments because of their accurate design.
3. Some errors are caused due to variations (with time or temperature) either in the strength of
permanent magnet or in the control spring.
Applications:
1. In the measurement of direct currents and voltages.
2. In DC galvanometers to detect small currents.
3.2 DC ammeters and voltmeters using PMMC
3.2.1 DC Ammeters
The basic movement of DC ammeter is the PMMC D‟Arsonal movement. Since the coil winding
in PMMC meter is small and light, they can carry only small currents (μA-1mA). Measurement of large
current requires a shunt external resistor to connect with the meter movement, so only a fraction of the
total current will passes through the meter.
Figure 3.2.1.1 DC ammeters
Let, Rm=Internel resistance of the meter
Rsh=shunt resistance
Im= current through the meter
Ish= shunt current
I=Current of the circuit to be measured.
Thus, I= Im+Ish
Since voltage drop across the shunt and the meter is same,
Vsh=Vm
Ish*Rh=Im*Rm
Rsh=
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Therefore,
3.2.2 D.C Voltmeter
A voltmeter is always connect in parallel with the element being measured, and measures the
voltage between the points across which it is connected. Most d.c voltmeter use PMMC meter with series
resistor as shown in figure below. The series resistance should be much larger than the impedance of the
circuit being measured, and they are usually much larger than Rm.
Figure 3.2.2.1 DC voltmeter
Let, Rm=Internel resistance of the meter
Rs=series resistance
Im= current through the meter
V=Voltage of the circuit to be measured
Now, V=Im*(Rs+Rm)
Rs =
Therefore,
3.3 Multirange ammeters
The current range of the DC ammeter can be extended by a number of shunts selected by a
switch(S).Such meter is called multirange ammeters.
Rs=
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Figure 3.3.1 Multirange ammeters
Let, Rm = Internel resistance of the meter
R1, R2, R3, R4 = shunt resistors
Im = current through the meter
I1, I2, I3, I4 = shunt currents
I = Current of the circuit to be measured.
We know that, Rsh =
Ish=I-Im
We can write, Rsh =
- 1=
But,
= m = multiplying power,
Therefore, Rsh =
Let m1, m2, m3, m4 be the shunt multiplying powers for current I1, I2, I3, I4.
R4=
R2=
R1=
R3=
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3.3.1 Multirange DC ammeter using universal shunt (Ayrton shunt)
The universal shunt or the Ayrton shunt eliminates the possibility of having the meter in the
circuit without shunt.
Figure 3.3.1.1 Aryton shunt
The selector switch S, selects the appropriate shunt required to change the range of the meter.
When the position of the switch is '1' then the resistance R1 is in parallel with the series combination
of R2 , R3 and Rm. Hence current through the shunt is more than the current through the
meter, thus protecting the basic meter.
When the switch is in the position '2', then the series resistance of R1 and R2, is in parallel with the
series combination of R3 and Rm. The current through the meter is more than through the shunt in this
position.
When the switch is in the position '3', the resistances R1 , R2 and R3 are in series and acts as the
shunt. In this position, the maximum current flows through the meter. This increases the sensitivity of the
meter.
The voltage drop across the two parallel branches is always equal.
Thus, Ish Rsh = Im Rm
But in position 1, R1 is in parallel with R2 + R3 + Rm
Where, I1 is the first range required.
In position 2, R1 + R2 is in parallel with R3 + Rm .
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where I2 is the second range required.
In position 3, R1 + R2 + R3 is in parallel with Rm .
where I3 is the third range required.
The current range I3 is the minimum while I1 is maximum range possible. Solving the equations
(1), (2) and (3) the required Ayrton shunt can be designed.
3.3.2 Multirange DC voltmeter
A DC voltmeter is converted into a multirange voltmeter by connecting a number of resistors
(multipliers) in series with the meter movement.
Figure 3.3.2.1 Multirange DC voltmeter
In the above figure, the multipliers are connected in series with the meter. The selector switch is
used to select the required voltage range.
When the switch S is at position V1, R1 + R2 + R3 + R4 acts as a multiplier resistance. While when the
switch S is at position V4 then the resistance R4 only acts as multiplier resistance. The V4 is
the lowest voltage range while V1 is the maximum voltage range.
The multiplier resistances can be calculated as :
In position V4, the multiplier is R4 only. The total resistance of the circuit is say RT.
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In position V3, the multiplier is R3 + R4.
In position V2, the multiplier is R2 + R3 + R4.
In position V1, the multiplier is R1 + R2 + R3 + R4.
Using equations (1), (2), (3) and (4) multipliers can be designed.
3.3.3 Loading effect and voltmeter sensitivity
Loading effect:
When selecting a meter for certain voltage measurement it is important to consider the sensitivity
of a DC voltmeter. A low sensitivity meter may give a correct reading if the circuit resistance is low but it
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
Note: This is only Basic Information for students. Please refer “Reference Books” prescribed as per syllabus
will produce defective readings in a high resistance circuit because the meter acts as a shunt which in turn
reduces the total equivalent circuit. Than the meter indicates low readings. This is called loading effect.
Voltmeter sensitivity:
In a, the ratio of the total resistance RT to the voltage range remains same. This ratio is nothing but
the reciprocal of the full scale deflection current of the meter i.e. 1/Im. This value is called sensitivity of
voltmeter.
Thus the sensitivity of voltmeter is defined as,
S =
Ω/V
The sensitivity range is specified on the meter dial and it indicates the resistance of the meter for a
one volt range.
Where, S=sensitivity of the voltmeter (Ω/V)
V=Voltage rang, as set by the range switch
Rm=internal resistance of the meter
Rs=Resistance of the multiplier
RT=Total circuit resistance
RT =
------------------------------------------------------------(1)
S =
---------------------------------------------------------------(2)
Sub (2) in (1)
RT = S*V-------------------------------------------------------------(3)
Since Rs and Rm are in series, RT=Rs+Rm---------------------(4)
Sub (4) in (3), Rs+Rm = S*V ------------------------------------(5)
Solving eq (5) for Rs,
3.4 Electrodynamometer
Rs=(S*V)-Rm
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
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Electrodynamometer type instruments are used as AC voltmeters and ammeters both in the range
of power frequencies and lower part of the audio frequency range. In order that the instrument should be
able to read a.c. quantities, the magnetic field in the air gap must change along with the change in current.
This principle is used in the electrodynamometer type instrument.
Figure 3.4.1 Electrodynamometer
Construction:
The necessary field required for the operation of the instrument is produced by the fixed coils. A
uniform field is obtained near the centre of coil due to division of coil in two sections. These coils are air
cored. Fixed coils are wound with fine wire for using as voltmeter, while for ammeters and wattmeter‟s it
is wound with heavy wire. Ceramic is usually used for mounting supports. If metal parts would have been
used then it would weaken the field of the fixed coil. The moving coil is wound either as a self-sustaining
coil or else on a non-metallic former. If metallic former is used, then it would induce eddy currents in it.
The construction of moving coil is made light as well as rigid. It is air cored. The moving coil is mounted
on a aluminium spindle. The moving system carries a pointer.
The electrodynamometer is used as Ac voltmeter, ammeter and with a slight modification it can
also be used as a wattmeter, power factor meter, frequency meter.
3.4.1 Electrodynamometer type ammeter
Figure 3.4.1.1 Electrodynamometer type ammeter
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
Note: This is only Basic Information for students. Please refer “Reference Books” prescribed as per syllabus
In the above circuit arrangement, when the instrument is used as an ammeter, fixed coils and
moving coil are connected in series and therefore carry the same current
i.e.I1=I2=I
Hence, angular deflection,
To measure heavy currents shunt is used to limit current through the moving coil. For small currents
shunt is not needed.
3.4.2 Electrodynamometer type voltmeter
Figure 3.4.2.1 Electrodynamometer type voltmeter
In the above circuit arrangement, when the instrument is used as an voltmeter, fixed coils and
moving coil are connected in series along with a high resistance.
i.e I1=I2=I
I =
in DC circuits
And I =
in AC circuits
Hence, angular deflection,
3.4.3 Electrodynamometer type wattmeter
Figure 3.4.3.1 electrodynamometer type wattmeter
ɵ
ɵ V2
ɵ
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
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The fixed coils are connected in series with the circuit, while the moving coil is connected
in parallel. Also, on analog wattmeters, the moving coil carries a needle that moves over a scale to
indicate the measurement. A current flowing through the fixed coil generates an electromagnetic
field around the coil. The strength of this field is proportional to the line current and in phase with it.
For a dc circuit the deflection of the needle is proportional to both current(I) and the voltage(V).
Thus power, P=VI
For an ac circuit the deflection is proportional to the average instantaneous product of voltage and
current,
Thus power, P=VI cos φ. Here, cosφ represents the power factor.
Advantages of electrodynamic instruments:
1. As the coils are air cored, these instruments are free from hysteresis and eddy current losses. These
instruments can be used on both a.c. and d.c.
2. Electrodynamometer voltmeters are very useful where accurate r.m.s values of voltage, irrespective
of waveforms, are required.
3. Low power consumption
4. Light in weight.
Disadvantages of electrodynamic instruments:
1. These instruments have a low sensitivity also it introduces increased frictional losses. To get
accurate results, these errors must be minimized.
2. They are more expensive than other type of instruments.
3. These instruments are sensitive to overload and mechanical impacts.
4. The operation current of these instruments is large due to the fact that they have weak magnetic
field.
3.5 Electronic voltmeters
Pros:
1. Low level signal detection.
2. Low power consumption.
3. Less loading effect.
4. High sensitivity and high input impedence.
5. High frequency response.
https://en.wikipedia.org/wiki/Series_and_parallel_circuitshttps://en.wikipedia.org/wiki/Series_and_parallel_circuitshttps://en.wikipedia.org/wiki/Analog_signalhttps://en.wikipedia.org/wiki/Electromagnetic_fieldhttps://en.wikipedia.org/wiki/Electromagnetic_fieldhttps://en.wikipedia.org/wiki/Alternating_current
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
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6. Parallax error is eliminated.
Cons:
1. Special circuits are needed to convert analog to digital
2. These are expensive.
3. Effected by temperature changes
4. Large bandwidth is required
3.5.1 FET voltmeter using differential amplifier
Figure 3.5.1.1 FET voltmeter using differential amplifier
FET Voltmeter using differential amplifier is as shown in figure above. It consists of two identical
FET‟s Q1 and Q2. Increase in the current of one FET is offset by corresponding decrease in source current
of the other. The two FET‟s form the lower arms of the balanced bridge circuit where as the two drain
resistors RD form the upper arms.
The circuit is balanced under zero input voltage condition provided the tw FET‟s are identical.
Under such conditions there would be no current through the PMMC meter. Current flows through the
meter when positive voltage is applied to the gate of FET Q1. The magnitude of this current is found to be
proportional to the voltage being measured.
3.5.2 Chopper type DC amplifier voltmeter
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
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Figure 3.5.2.1 Chopper type DC amplifiervoltmeter
Figure above shows the circuit diagram of a Chopper type DC amplifier voltmeter.
Two photo diodes are used in input stage which acts as half-wave modulators because of its
alternate switching action by the neon lamps at the frequency of oscillator.
In dc voltmeter circuit two neon lamps are used, these are supplied by an oscillator for alternate half
cycles. Output of chopper modulator is a square wave voltage (proportional to the input signal) which is
supplied to the ac amplifier through a capacitor and the amplified output is again passed through a
capacitor and then fed to chopper demodulator. The capacitor is used to smooth the output from the
amplifier.
The Chopper demodulator gives a dc output voltage (proportional to the input voltage) which is
passed through the low pass filter to remove any residual ac component and this dc output voltage is
supplied to the PMMC meter for measurement of input voltage.
Advantages:
1. It has very high input impedance of the order of 10MΩ.
2. It allows input signal in the range of 0.01mV.
3.5.3 Solid-state voltmeter using op-amp
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
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Figure 3.5.3.1 Solid state voltmeter using op-amp
Figure above shows an electronic voltmeter using op-amp 741. This is directly coupled very high
gain amplifier. The gain can be adjusted to any suitable value by providing appropriate resistance
between its output terminal, Pin No.6, and inverting input pin No.2, to provide negative feedback. The
ratio R2/R1 determines the gain. The 0.1μF capacitor across the 100KΩ resistance R2 is used for stability
under stray pickups.
A 10KΩ potentiometer is connected between the offset null terminal 1 and 5 with its centre tap
connected to -5V supply for adjusting zero output for zero input conditions. The two diodes used are for
IC protection. If an excessive voltage say more than 100mV appears across them then depending on the
polarity of the voltage one of the diode conducts and protects the IC. A μA scale 50-1000μA full scale
deflection can be used as an indicator. R4 is adjusted to get maximum full scale deflection.
3.5.4 AC voltmeter using full-wave rectifier
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
Note: This is only Basic Information for students. Please refer “Reference Books” prescribed as per syllabus
Figure 3.5.4.1 AC voltmeter using full-wave rectifier
A full wave rectifier type AC voltmeter consists of four diodes and a PMMC meter as shown in
above fig. The current through the moving coil instrument flows in the same direction for both cycles of
the input voltage. The indication of instrument depends upon the mean value of the current flowing
through it.
Here, the meter reading would be 90% RMS i.e., 90% of the DC value. When the input is positive
D1 and D3 conducts, and the current flows through the meter from top to bottom. When the input is
negative D2 and D4 conducts through the meter from top to bottom. In both the cycles of the input
voltage current flows in same direction.
Advantages:
1. The frequency range extends from about 20Hz to high audio frequencies.
2. They have much lower operating current.
3. They have practically uniform scale for most ranges.
3.5.5 AC voltmeter using peak responding voltmeter
Figure 3.5.5.1 DC and AC coupled peak voltmeter respectively
Figure3.5.5.1 shows the two most common types of peak responding voltmeters. The capacitor
charges through the diode to the peak value of the applied voltage.
In both the circuits, the capacitor discharges very slowly through the high impedance input of DC
amplifiers, so that a negligible small amount of current supplied by the circuit under test keeps the
capacitor charged to the peak AC voltage. In DC coupled peak voltmeter the reading of the meter is
affected by the presence of DC with AC voltage.
Advantages:
The rectifying diode and the storage capacitor may be taken out of the instrument and placed in
probe when no pre-amplification is needed. The measured AC signal then travels no further than the
diode. The meter is then able to measure frequencies upto hundreds of MHz with a minimum of circuit
loading.
Disadvantages:
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
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1. Error caused due to harmonic distortion.
2. Limited sensitivity.
3.5.6 True RMS reading voltmeter
Figure 3.5.6.1 Block diagram of true RMS reading voltmeter
Figure above shows the block diagram of a RMS Reading Voltmeter. It consists of two
thermocouples called main thermocouple (MT) and balancing thermocouple (BT). BT is used in the
feedback loop to cancel out the non-linear effects of the MT.
The unknown A.C voltage is amplified and fed to the heating element of the main thermocouple.
The heat produced by the wire is sensed by the measuring thermocouple which produces a proportional
DC voltage. This DC voltage upsets the bridge balance. The unbalance voltage is amplified by the DC
amplifier and fed back to the heating element of the balancing thermocouple.
Bridge balance is reestablished when the two thermocouples produce the same output voltages. At
this point the DC current in the heating element of the feedback thermocouple is proportional to the AC
current in the input thermocouple i.e., the DC is proportional to the rms value of the input AC signal. This
DC value is indicated by the meter movement in the output circuit.
Advantages:
1. Sensitivities in the mV range is possible.
2. The non-linear behaviour is avoided.
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DEPARTMENT OF TECHNICAL EDUCATION E-CONTENT
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3. Complex waveforms are accurately mea