UNIT I

INTRODUCTION

PART-A

1. What is standard? What are the different types of standards? (MAY

2008/MAY 2009/MAY 2011)

A standard is a physical representation of a unit of measurement. A known

accurate measure of physical quantity is termed as standard. Types are
International standard, primary standard, secondary standard and working standard.

2. Define calibration.(NOV/DEC 2010)

Calibration is the process of checking the accuracy of instrument comparing

the instrument reading with a standard against a similar meter of known accuracy.

3. Define static error and how is it classified?(NOV 2009)

The static error of a measuring system is the numerical difference between
the true value of a quantity and its value as obtained by measurement. The various
types are gross error, systematic error and random error.

4. What are the various important functional elements of a typical

measurement system? (Apr / May 13)

 Primary sensing element

 Variable conversion element
 Variable manipulation element
 Data transmission element
 Data presentation element
5. Illustrate the difference between precision and accuracy. (Apr / May 15)
Accuracy Precision

Accuracy refers to the degree of closeness Precision refers to the degree of agreement
or conformity to the true value of within a group of measurements and
quantity under measurement where the true instruments or reproducibility of the value
value is the ideal value

Accuracy gives the maximum error which is Precision of a measuring system gives its
maximum departure of the final result from capability to reproduce a certain reading
its true value with a given accuracy

6. Give the international standards of instruments. (Apr /

May 14) International Ohms

International Amperes
7. What is drift? (Nov/Dec 2011)
It is the variation of the measured value with time. Perfect reproducibility means
that the instrument has no drift.

8. The expected value of the voltage across a resistor is 40 volt, however the
measurement gives a value of 39 volt. Calculate the absolute error. (May/June
2013)
Absolute error e= At-Am
=40-39
E=1V

9. Define limiting errors. (Dec 2007)

Instruments having analog meters are usually guaranteed to be accurate within

certain percentage limits called limiting errors or Guarantee errors.

10. Define dynamic characteristics of instrument. (Dec 2008)

The behavior of instrument when inputs vary with time and do the output.

11. Define fidelity (Nov 2009)

It is determined by the fact that how closely the instrument reading follows the
measured variable. i.e. It is the degree to which an instrument indicates the changes in
measured variable without dynamic error.

12. What are the static characteristics of instrument? (May 2008)

Static characteristics of instrument are used to measure unvarying processes of the

instrument. The main static characteristics are accuracy, resolution, precision, drift, static
error, dead zone etc.
PART-B
1. Explain the functional elements of measurement system with eat block
diagram.(DEC 14)

Most of the measurement systems contain three main functional elements. They are:

i) Primary sensing element

ii) Variable conversion element &

iii) Data presentation element.

Primary sensing element:

The quantity under measurement makes its first contact with the primary sensing
element of a measurement system. i.e., the measurand- (the unknown quantity which
is to be measured) is first detected by primary sensor which gives the output in a
different analogous form This output is then converted into an e electrical signal by a
transducer - (which converts energy from one form to another). The first stage of a
measurement system is known as a detector transducer stage’.

Variable conversion element:

The output of the primary sensing element may be electrical signal of any form, it may
be voltage, a frequency or some other electrical parameter For the instrument to
perform the desired function, it may be necessary to convert this output to some other
suitable form.
Variable manipulation element:

The function of this element is to manipulate the signal presented to it preserving the
original nature of the signal. It is not necessary that a variable manipulation element
should follow the vari ble conversion element Some non -linear processes like
modulation, detection, s mpling , filtering, chopping etc., are performed on the signal to
bring it to the desired form to be accepted by the next stage of measurement system
This process of conversion is called μ sig al conditioning’

Data presentation element:

The information about the quantity under measurem nt has to be conveyed to the
personnel handling the instrument or the system for monitor ng, control, or analysis
purposes. This function is done by data presentation element. In case data is to be
monitored, visual display devices are needed. These devices may be analog or digital
indicating instruments like ammeters, voltmeters etc. In case data is to be r corded,
recorders like magnetic tapes, high speed camera & TV equipment, CRT, printers may
be used. The final stage in a measurement system is known as terminating stage’.

2. Explain the static characteristics of measurement system in detail.(APR

2011/MAY 2013)

The set of criteria defined for the instruments, which are used to measure the
quantities which are slowly varying with time or mostly constant, i.e., do not vary with
time, is called ‘static characteristics’.

The various static characteristics are:

i) Accuracy
ii) Precision
iii) Sensitivity
iv) Linearity
v) Reproducibility
vi) Repeatability
vii) Resolution
viii) Threshold
ix) Drift
x) Stability
xi) Tolerance
xii) Range or span
Accuracy:
It is the degree of closeness with which the reading approaches the true value of the
quantity to be measured.
Precision:
It is the measure of reproducibility i.e., given a fixed value of a quantity, precision is a
measure of the degree of agreement within a group of measurements. The precision is
composed of two characteristics:
a) Conformity:
Consider a resistor having true value as 2385692, which is being measured by an
ohmmeter. But the reader can read consistently, a value as 2.4 M due to the non
availability of proper scale. The error created due to the limitation of the scale reading
is a precision error.
b) Number of significant figures:
The precision of the measurement is obtained from the number of significa t figures, in
which the reading is expressed. The significant figures convey the actual informa ion
about the magnitude & the measurement precision of the quantity.
Sensitivity:
The sensitivity denotes the smallest change in the measured variable to which the
instrument responds. It is defined as the ratio of the changes in the output of an
instrument to a change in the value of the quantity to be measured.
Linearity:
The linearity is defined as the ability to reproduce the input characteristics
symmetrically & linearly.
Reproducibility:
It is the degree of closeness with which a given value may be repeatedly
measured. It is specified in terms of scale readings over a given period of time.
Repeatability:
It is defined as the variation of scale reading & random in nature.
Drift:
Drift may be classified into three categories:
a) Zero drift:
If the whole calibration gradually shifts due to slippage, permanent set, or due to
undue warming up of electronic tube circuits, zero drift sets in. b) Span drift or
sensitivity drift:
If there is proportional change in the indication all along the upward scale, the drifts is
called span drift or sensitivity drift.
c) Zonal drift:
In case the drift occur only a portion of span of an instrument, it is called zonal drift.
Resolution:
If the input is slowly incre sed from some arbitrary input value, it will again be found
that output does not change at all until a certain increment is exceeded. This increment
is called resolution.
Threshold:
If the instrument input is increased very gradually from z ro there will be some
minimum value below which no output change can be detected. This m n mum value
defines the threshold of the instrument.
Stability:
It is the ability of an instrument to retain its performance throughout is specified opera
ing life.
Tolerance:
The maximum allowable error in the measurement is specified in terms of some value
which is called tolerance.
Range or span:
The minimum & maximum value of a quantity for which an instrument is designed to
measure is called its range or span.
3. With a suitable illustration elaborate the significance of
calibrations.(APR/MAY14)
 Calibration is the process of making an adjustment or marking a scale so that
the readings of an instrument agree with the accepted & the certified standard.
 In other words, it is the procedure for determining the correct values of
measurand by comparison with the measured or standard ones.
  The calibration offers a guarantee to the device or instrument that it is operating
with required accuracy, under stipulated environmental conditions.
 The calibration procedure involves the steps like visual inspection for various
defects, installation according to the specifications, zero adjustment etc.,
 The calibration is the procedure for determining the correct values of
measurand by comparison with standard ones.
 The standard of device with which comparison is made is called a standard
instrument. The instrument which is unknown & is to be calibrated is called test
instrument.
 Thus in calibration, test instrument is compared with standard instrument.
Types of calibration methodologies:
There are two methodologies for obtaining the comparison between test
instrument & standard instrument. These methodologies are
i) Direct comparisons
ii) Indirect
comparisons Direct
comparisons:
 In a direct comparison, a source or generator appli s a known input to the meter
under test.
 The ratio of what meter is indicating & the known generator values gives the
meters error.

 In such case the meter is the test instrument while the generator is the standard
instrument.

 The deviation of meter from the standard value is compared with the allowable
performance limit.

 With the help of direct comparison a generator or source also can be

calibrated.

Indirect comparisons:

 In the indirect comparison, the test instrument is compared with the response
standard instrument of same type i .e., if test instrument is meter, standard instrument
is also meter, if test instrument is generator; the standard instrument is also generator
& so on. If the test instrument is a meter then the same input is applied to the test
meter as well a standard meter.
 In case of generator calibration, the output of the generator tester as well as
standard, or set to same nominal levels. Then the transfer meter is used which
measures the outputs of both standard and test generator.

4. Discuss the different types of standards of measurements (Apr/May 15)

Standard
All the instruments are calibrated at the time of manufacturer against measurement
standards. A standard of measurement is a physical representation of a unit of
measurement. A standard means known accurate measure of physical quantity.
The different size of standards of measurement are classified as
 International standards
 Primary standards
 Secondary standards
 Working standards

International standards are defined as the international agreement. These

standards, as mentioned above are maintained at the international bureau of weights
an d measures and are periodically evaluated and check d by absolute
measurements in term s of fundamental units of physics.

These international standards are not available to the ordinary users for the
calibration purpose.
For the improvements in the accuracy of absolute measurements the international
units are replaced by the absolute units in 1948. Absolute units are more accurate
than the international units.

These are highly accurate absolute standards, which can be used as ultimate
reference standards. These primary standards are maintained at national standard
laboratories in different countries.

These standards representing fundamental units as well as some electrical and

mechanical derived units are calibrated independently by absolute measurements at
each of the national laboratories
These are not available for use, outside the national laboratories. The main function
of the primary standards is the calibration and verification of secondary standards.
As mentioned above, the primary standards are not available for use outside the
national laboratories. The various industries need some reference standards.

So, to protect highly accurate primary standards the secondary standards are
maintained, which are designed and constructed from the absolute standards.

These are used by the measurement and calibration laboratories in industries and are
maintained by the industry to which they belong. Each industry has its own standards.

Working standards
These are the basic tools of a measurement laboratory and are used to check and
calibrate the instruments used in laboratory for accuracy and the performance.

5.A) i) Explain the dynamic characteristics of measurement system in detail.

( MAY2013)
Dynamic characteristics: The set of criteria defined for the instruments, which
are changes rapidly with time, is called ‘dynamic characteristics’.
The various static characteristics are: i) Speed of response ii)Measuring lag iii)
Fidelity iv) Dynamic error
Speed of response: It is defined as the rapidity with which a measurement
system responds to changes in the measured quantity.
Measuring lag: It is the retardation or delay in the response of a measurement
system to changes in the measured quantity. The measuring lags are of two types: a)
Retardation type: In this case the response of the measurement system begins
 immediately after the change in measured quantity has occurred. b) Time delay
lag: In this case the response of the measurement system begins after a dead
time after the application of the input.
Fidelity: It is defined as the degree to which a measurement system indicates
changes in the measurand quantity without dynamic error.
Dynamic error: It is the difference between the true value of the quantity changing
with time & the value indicated by the measurement system if no static error is
assumed. It is also called measurement error.
ii) A circuit was tuned for resonance by eight different students and the values
of resonant frequency in KHz were recorded as 532, 548, 543, 535, 546, 531, 543,
and
536. Calculate a) Mean b) Average deviation

a) Mean = (532+548+ 543+ 535+ 546+ 531+ 543+ 536)/8

= 4314/8
= 539.25
b) SD:
Deviation from meanD1 = 532-539.25 = -7.25
D2 = 548-539.25= 8.75
D3 = 543-539.25= 3.75
D4= 535-539.25= -4.25
D5= 546-539.25=6.75
D6= 531-539.25= -8.25
D7= 543-539.25=3.75
D8= 536-539.25= -3.25
Average deviation = (7.25+8.75+3.75+4.25+6.75+8.25+3.75+3.25) /8
= 46/8
= 5.75

6.Classify and explain the different errors of measurements. (Nov/Dec 14)

The types of errors are follows
i) Gross errors
ii) Systematic errors
iii)Random errors
Gross Errors:
The gross errors mainly occur due to carelessness or lack of experience of a human
being. These errors also occur due to incorrect adjustments of instruments. These
errors cannot be treated mathematically. These errors are also called personal errors.
Ways to minimize gross errors:
The complete elimination of gross errors is not possible but one can minimize them
by the following ways:
 Taking great care while taking the reading, recording the reading & calculating
the result.
 Without depending on only one reading, at least three or more readings must be
taken preferably by different persons.
Systematic errors:
A constant uniform deviation of the operation of an instrument is known as a
Systematic error. The Systematic errors are mainly due to the short comings of the
instrument & the characteristics of the material use d in the instrument, such as
defective or worn parts, ageing effects, environmental effects, etc.
Types of Systematic errors:
There are three types of Systematic errors as:
i) Instrumental errors
ii)Environmental errors
iii) Observational errors

Instrumental errors:

These errors can be mainly due to the following three reasons:

a) Short comings of instruments:
These are because of the mechanical structure of the instruments. For
example friction in the bearings of various moving parts; irregular spring
tensions, reductio s in due to improper handling , hysteresis, gear backlash,
stretching of spring, variations in air gap, etc .,
b)
Ways to minimize this error:

These errors can be avoided by the following methods:

 Selecting a proper instrument and planning the proper procedure for the
measurement recognizing the effect of such errors and applying the proper
correction factors calibrating the instrument carefully against a standard

b) Misuse of instruments:
A good instrument if used in abnormal way gives misleading results. Poor
initial adjustment, Improper zero setting, using leads of high resistance etc.,
are the examples of misusing a good instrument. Such things do not cause
the permanent damage to the instruments but definitely cause the serious
errors.

c) Loading effects

Loading effects due to im proper way of using the instrument cause the serious
errors. The best ex ample of such loading effect error is connecting a w ell calibrated
volt meter across the two points of high resistance circuit. The same volt meter
connected in a low resistance circuit gives accurate reading.

Ways to minimize this error:

Thus, the errors due to the loading effect can be avoided by using an instrument
intelligently and correctly.
Environmental errors:
These errors are due to the conditions external to the measuring instrument. The
various factors resulting these environmental errors are temperature changes,
pressure changes, thermal emf, ageing of equipment and frequency sensitivity of an
instrument.
 The various methods which can be used to reduce these errors are:
 Using the proper correction factors and using the information supplied by the
manufacturer of the instrument
 Using the arrangement which will keep the surrou ding conditions Constant
Reducing the effect of dust, humidity on the compon nts by hermetically sealing the
components in the instruments
 iv)The effects of external fields can be minimized by using the magnetic or
electrostatic shields or screens
  Using the equipment which is immune to such environmental effects.
v) Observational errors:

These are the errors introduced by the observer. These are many sources of
observational errors such as parallax error while reading a meter, wrong scale
selection, etc.
Ways to minimize this error
To eliminate such errors one should use the instruments with mirrors, knife
edged pointers, etc.,
The systematic errors can be subdivided as static and dynamic errors. The static errors
are caused by the limitations of the measuring device while the dynamic errors are
caused by the instrument not responding fast enough to follow the changes in the
variable to be measured.
Random errors:
Some errors still result, though the systematic and instrumental errors are reduced or
at least accounted for. The causes of such errors are unknown and hence the errors are
called random errors.
Ways to minimize this error

The only way to reduce these errors is by increasing the number of observations and
using the statistical methods to obtain the best approximation of the reading.
 UNIT II

ELECTRICAL AND ELECTRONIC INSTRUMENTS

PART-A

1.What is creeping in energy meter? How it is prevented?(MAY/JUN 2012)

A slow but continuous rotation of the energy meter disc system even when there
is no current flowing though the coil but only the pressure coil is energized is called as
creeping. It can be prevented by drilling two diametrically opposite holes in the disc
which makes the disc comes to rest with one of the holes under the edge of a pole of
the shunt magnet.

2.Why the PMMC instruments are not used for AC measurements?(NOV/DEC 14)

When the PMMC instruments are connected to AAC, the torque reverses as the
current reverses and the pointer cannot follow the rapid reversals. Hence the
deflection corresponding to mean torque is zero thus making the PMMC instrument
not suitable for AC measurements

3.Which torque is absent in energy meter? Why?(NOV 2009)

 In energy meter, there is no controlling torque, as the driving torques is enough
to cause continuous revolution of the disc.

4.State the purpose of shunts in the voltmeter (APR/MAY 11)

When an ammeter is needed to measure curre ts of having large magnitudes a

proportion of the current is diverted through a low value r sistance connected in
parallel with the meter. Such a diverting type of resistor is referred to as shu t. The milli
ammeter is converted into voltmeter by connecting a resister series with the meter
called multiplier.

5.Classify different types of iron loss. (APR/MAY 11)

 Eddy current

 Hysteresis losses

6.Explain with example the term hysteresis. (Nov/Dec 12)

Hysterisis is the phenomenon which depicts different output effects when loading
and unloading in any system, whether it is a electrical system or a mechanical system.
7.How does one extend the range of ammeter and voltmeter? (Nov/Dec 2011)

Voltmeter is extended by adding resistance in series with it whereas ammeter is

extended by connecting resistance in parallel with it.
8.How we do the ballistic test? (Nov/Dec 2011)

These tests are generally employed for the determination of B- H curves and
hysteresis loops of Ferro-magnetic materials.

9.Draw the circuit of a basic dc voltmeter. (May/June 2013)

10.What are the different types of torque produced in PMMC instrument?(Dec

2009)

 Deflection torque

 Controlling Torque

 Damping Torque

11.What are the advantages of digital multimeter?

 Highly accurate

 Loading effect is Nil

 Easily portable

 Very Cheap

 Easy to interface

12.Why ordinary watt meters are not suitable in places of LPF circuits? (Dec
2010)

 The deflecting torque is very small due to low power factor

 Errors due to induction coil is large due to low power factor

PART-B

1.(i)Explain the moving iron instrument in detail with neat sketch.(NOV/DEC 14)

Classification of Moving Iron Instruments

 Moving iron instruments are of two types

(i) Attraction type.

(ii) Repulsion type.
Attraction Type

 The coil is flat and has a narrow slot like opening. The moving iron is a flat disc
or a sector eccentrically mounted.

 When the current flows through the coil, a magnetic field is produced and the
moving iron moves from the weaker field outside the coil to the stronger field inside it
or in other words the moving iron is attracted in.

 The controlling torque is provided by springs hut gravity control can be used for
panel type of instruments which are vertically mounted. Damping is provided by air
friction with the help of a light aluminium piston (attached to the moving system)
which move in a fixed chamber closed at one end.
Repulsion Type

In the repulsion type, there are two vanes inside the coil one fixed and other movable.
These are similarly magnetized when the current flows through the coil a d th re is a
force of repulsion between the two vane s resulting in the movement of the moving
vane. Two different designs are in common use

(I) Radial Vane Type

In this type, the vanes are radial strips of iron. The fixed vane is attached to the coil
and the movable one to the spindle of the instrument.

(a) Radial vane type (b) Co-axial vane type

ii) Co-axial Vane Type

 In this type of instrument, the fixed and moving vanes are sections of co axial
cylinders as shown in Fig. The controlling torque is provided by springs. Gravity
control can also be used in vertically mounted instruments.

 The damping torque is produced by air friction as in attraction type instruments.

The operating magnetic field in moving iron instruments is very weak and
therefore eddy current damping is not used in them as introduction of a
permanent magnet required for eddy current damping would destroy the
operating magnetic field.
  It is clear that whatever may be the d rection of the current in the coil of the
instrument, the iron vanes are so magnetized that there is always a force of attraction
in the attraction type and repulsion in the repulsion type of inst uments. Thus moving
iron instruments are un-polarized instruments i.e., they are independe t of the direction
in which the current passes.

(ii) Explain in detail the instrument transformer with neat sketch.(Nov/Dec 14)

Power measurements are made in high voltage circuits connecting the wattmeter o the
circuit through current and potential transformers as shown. The primary winding of the
C.T. is connected in series with the load and the secondary winding is connected in
series with an ammeter and the current coil of a wattmeter.

The primary winding of the potential transformer is connected across the supply lines
and a voltmeter and the potential coil circuit of the wattmeter are connected in parallel
with the secondary winding of the transformer. One secondary terminal of each
transformer and the casings are earthed.

The errors in good modem instrument transformers are small and may be ignored for
many purposes. However, they must be considered in precision work. Also in some
power measurements these errors, if not taken into account, may lead to very
inaccurate results.
Voltmeters and ammeters are affected by only ratio errors while wattmeter’s are
influenced in addition by phase angle errors. Corrections can be made for these errors if
test information is available about the instrument transformers and their burdens.
2) Obtain B-H curve of a ring specimen.(Apr/May14)

Method of reversals

A ring shaped specimen whose dimensions are known s used for the purpose. After
demagnetizing the test is started by setting the magnetizing current to its lowest test
vane. With galvanometer key K closed, the iron specimen is brought in o a ‘
reproducible cyclic magnetic state’ by throwing the reversing switch S backward and
forward about twenty times.


Key K is now opened and the value of flux corresponding to this value of H is
measured by reversing the switch S and noting the throw of galvanometer. The value
of flux density corresponding to this H can be calculated by dividing the flux by the
area of the specimen. The above procedure is repeated for various values of H up to
the maximum testing point.


The B-H curve may be plotted from the measured values of B corresponding to the
various values of H.
Step by step method
 The circuit for this test is shown in Fig. The magnetizing winding is supplied
through a potential divider having a large number of tapping. The tapings are
arranged so that the magnetizing force H may be increased, in a number of suitable
steps, up to the desired maximum value.
 The specimen before being tested is demagnetized. The tapping switch S is set
on tapping I and the switch S is closed. The throw of the galvanometer corresponding
to this increase in flux density in the specimen, from zero to some value B, is
observed.
Step by step method
After reaching the point of maximum H i.e... when switch S is at tapping 10, the
magnetizing current is next reduced, in steps to zero by moving switch 2 down through
the tapping points 9, 8, 7 3, 2, 1. After reduction of magnetizing force to zero, negative
values of H are obtained by reversing the supply to potential divider and then moving
the switch S up again in order 1, 2, 3 7, 8. 9, 10.
Method of reversals
This test is done by means of a number of steps, but the change in flux density
measured at each step is the change from the maximum value + Bm down to some
lower value. But before the next step is commenced the iron specimen is passed
through the remainder of the cycle of magnetization back to the flux density + Bm.
Thus the cyclic state of magnetization is preserved. The connections for the method of
reversals are shown in Fig.
3) With a neat block diagram explain the construction and operating principle of
digital voltmeter. (8)(April/May 2011)

Digital Voltmeter

 A digital voltmeter (DVM) displays the value of a.c. or d.c. voltage being
measured directly as discrete numerals in the decimal number system. Numerical
readout of DVMs is advantageous since it eliminates observational errors committed
by op rators. The errors on account of parallax and approximations are entirely
eliminated.

 The use of digital voltmeters increases tile speed with which readings can be
taken. A digital voltmeter is a versatile and accurate voltmeter which has many
laboratory applications.

 On account of developments in the integrated circuit (IC) technology, it has

been possible to reduce the size, power requirements and cost of digital voltmeters. In
fact, for the same accuracy, a digital voltmeter now is less costly than its analog
counterpart.

 The decrease in size of DVMs on account of use of ICs, the portability of the
instruments has increased.

 The unknown voltage is applied to the input of the integrator, and the output
voltage starts to rise. The slope of output voltage is determined by the value of input
voltage This voltage is fed a level detector, and when voltage reaches a certain
reference level, the detector sends a pulse to the pulse generator gate. The level
detector is a device similar to a voltage comparator.

 The output voltage from integrator is compared with the fixed voltage of an
internal reference source, and, when voltage reaches that level, the detector produces
an output pulse.

 It is evident that greater then value of input voltage the sharper will be the slope
of output voltage and quicker the output voltage will reach its reference level. The
output pulse of the level detector opens the pulse level gate, permitting pulses from a
fixed frequency clock oscillator to pass through pulse generator.

 The basic block diagram of a typical integrating type of DVM is shown in Fig

 The generator is a device such as a Schmitt trigger that produces an output

pulse of fixed amplitude and width for every pulse it receives. This output pulse,
whose polarity is opposite to that of and has greater amplitude, is fed back of the i put
of the integrator.

 Thus no more pulses from the clock oscillator can pass through to trigger the
pulse generator. When the output voltage pulse from the pulse generator has passed,
is restored to its original value and starts its rise again. When it reaches the level of
reference voltage again, the pulse generator gate is opened.
 The pulse generator is trigger by a pulse from the clock generator and the entire
cycle is repeated. Thus, the waveform of is a saw tooth wave whose rise time is
dependent upon the value of output voltage and the fail time is determined by the
width of the output pulse from the pulse generator.

Thus the frequency of the saw tooth wave is a function of the value of the voltage being
measured. Since one pulse from the pulse generator is produced for each cycle of the
saw tooth wave, the number of pulses produced in a given time interval and hence
the frequency of saw tooth wave is an indication of the voltage being measured.

4. Describe the functional operation of energy meter. (8) (April/May 2011)

Construction of Induction Type Energy Meters: There are four main parts of the
operating mechanism.
(i) Driving system
(ii) Moving system
(iii) Braking system
(iv) Registering system
Driving system
 The driving system of the meter consists of two electro-magnets. The core of
these electromagnets is made up of silicon steel laminations. The coil of one of the
electromagnets is excited by the load current. This coil is called the current coil. The
coil of second electromagnet is connected across the supply and, therefore, carries a
current proportional to the upply voltage.
 This coil is called the pressure coil. Consequently the two electromagnets are
known as series and shunt magnets respectively.
 Copper shading bands are provided on the central limb. The position of these
bands is adjustable. The function of these bands is to bring the flux produced by the
shunt magnet exactly in quadrature with the applied voltage.
Moving System
 This consists of an aluminum disc mounted on a light alloy shaft This disc is
positioned in the air gap between series and shunt magnets.

 The upper bearing of the rotor (moving system) is a steel pin located in a hole in
the bearing cap fixed to the top of the shaft. The rotor runs on a hardened steel pivot,
screwed to the foot of the shaft.
  The pivot is supported by a jewel bearing. A pinion engages the shaft with

the counting or registering mechanism.
Braking System

 A permanent magnet positioned near the edge of the aluminium disc forms the
braking system.
The aluminium disc moves in the field of this magnet and thus provides a braking
torque. The position of the permanent magnet is adjustable, and therefore braking
torque can be adjusted by shifting the permanent magnet to different radial positions
as explained earlier

Registering (counting) Mechanism

 The function of a registering or counting mechanism is to record continuously a

number which is proportional to the revolutions made by the moving system. By a
suitable system, a train of reduction gears the pinion on the rotor shaft drives a series
of five or six pointers. These rotate on round dials which are marked with ten equal
divisions

5. What are the different methods used for the measurement of frequency?
Explain any one method(16)(NOV/DEC2011)
CONSTRUCTION:

This frequency meter is a moving iron type instrument which consists of two
coils A&B mounted perpendicular to each other. Each coil is divided into two sections
whose connections are as shown in figure. The branch circuit of coil A has a resistor
RA series with it and a reactance coil L A parallels it. Whereas the branch circuit of coil
B has a reactance coil LB series with it and a reactance R B parallel to it. The moving
element is a soft iron needle, which is also called magnetic needle. This needle is
pivoted on a spindle, which also carries a pointer and damping vanes. A reactance coil
L is connected in series with the supply to suppress higher harmonics in the current of
the meter, and therefore it tends to minimize the waveform errors in its indication.
There is no controlling force.

PRINCIPLE OF OP RATION:

When the meter is connected across the supply, the two coils carry currents
which setup two magnetic fields at right angles to each other terminal of an AND gate.
A constant gate pulse of I sec is applied to the other input terminal of the AND gate as
shown in the fig. Number of pulses counted at the output terminal for a period of 1 sec
gives the frequency to be measured.

BASIC BLOCK DIAGRAM:

 Fig shows the block diagram of a digital frequency meter. The supply voltage
whose frequency is to be measured is first amplified with the help of amplifier. The
amplified voltage signal is fed to the Schmitt trigger which converts the input signal
into a square wave with fast rise and fall times, which is then differentiated and
clipped in order to produce a train of pulses.

 Each pulse represents a cycle of the voltage signal with unknown frequency.
The output pulses from the Schmitt trigger are fed to a START/STOP gate which is an
AND gate. This AND gate is enabled and disabled by giving a time interval to one of
the input terminals of the gate.

 When this gate is enabled, the input pulses pass through this gate and are fed
directly to an electronic counter, which counts the number of pulses. When this gate
is disabled, the counter stops counting the incoming pulses.

 The counter displays the number of pulses that have passed through it in the
time interval between START and STOP. Using the time interval and the number of
pulses, the unknown frequency c n be calculated.
BASIC ELECTRONIC FREQUENCY COUNTER:
 Both these fields act upon the soft iron needle and deflects it which depends
upon the relative magnitude of the two fields and hence of the current through
the coils.
 The values of RA,LA,RB and LB are so chosen that, at normal frequency equal
currents flow through the coil A and B thereby making the needle a d pointer to
take up the mean position.
  When equal currents flow through the coils A and B, the needle will be at 45° to
both the coils and the pointer will be at the central of the scale as shown in the
figure.
 If the frequency increases above its normal value, reactance LA and LB increase
while the resistances RA and RB remain the same.
 Hence the voltage drop on coil A increases as compared to that of coil B, thereby
increasing the current through coil A as compared to coil B. Thus the magnetic
field of coil A becomes stronger than that of coil B.
 Because of the tendency of the needle to deflect towards the stronger field, it
tends to itself in line with the axis of coil A, thus making the pointer deflects to
the left.

 If the frequency decreases below its normal value, reactance LA and LB decrease
while the resistances RA and RB remain the same. Hence the voltage drop on coil
decreases as compared to that of coil B, thereby decreasing the current through coil
A as compared to coil B.Thus the magnetic field of coil B becomes stronger than that
of A.

 Because of the tedency of the needle to deflect towards the stronger field, it tends to
st itself in line with the axis of coil B, thus making the pointer deflects to the right.

ADVANTAGES:

1. Simple in construction.

2. Simple operation.

3. Gives an accurate measurement.

PART C (15 MARK)

1. Explain about the working principle of electrodynamometer type
instrument. (NOV/DEC 13)
o The necessity for the a.c. calibration of moving iron instruments as well as other
types of instruments which cannot be correctly calibrated requires the use of a
transfer type of instrument
o A transfer instrument is one that may be calibrated with a d.c. source and then
used without modification to measure a.c. This requires the transfer type instrument
to have same accuracy for both d.c. and a.c., which the electrodynamometer
instruments have.

o These standards are precision resistors and the Watson standard cell (which is a
d.c. cell).It is obvious, therefore, that it would be impossible to calibrate an a.c.
instrument directly against the fundamental standards. The calibration of an a.c.
instrument may be performed as follows.

o The transfer instrument is first calibrated on d.c. This calibration is hence

transferred to the a.c. instrument on alternating current, using operating conditions
under which the latter operates properly. Electrodynamic instruments are capable of
service as transfer instruments.

o Indeed, their principal use as ammeters and voltmeters in laboratory and

measurement work is for the transfer calibration of working instruments and as
standards for calibration of other instruments as their accuracy is very high.

o Electrodynamometer types of instruments are used as a.c. voltmeters and

ammeters both in the range of power frequencies and lower part of the audio power
frequency range. They are used as watt-meters, and with some modification as
power factor meters and frequency meters

Operating Principle of Electrodynamometer Type Instrument

 It would have a torque in one direction during one half of the cycle and an
equal effect in the opposite direction during the other half of the cycle.If the
frequency were very lo , the pointer would swing back and forth around the zero
point.
 However, for an ordinary meter, the inertia is so great that on power
frequencies the pointer does not go very far in either direction but merely stays
(vibrates slightly) around zero.
 If, however, we were to reverse the direction of the flux each time the current
through the movable coil reverses, a u idirectional torque would be produced for
both positive and negative halves of the cycle.
 In electrodynamometer instruments the field can be made to reverse
simultaneously with the current in the movable coil if the fi ld (f xed) coil is connected
in series with the movable coil.

Construction of Electrodynamometer type instrument Fixed Coils

The field is produced by a fixed coil. This coil is divided into two sections to give a
more uniform field near the centre and to allow passage of the instrument shaft.

Moving Coil
A single element instrument has one moving coil. The moving coil is wound either as
a self-sustaining coil or else on a nonmetallic former. A metallic former cannot be
used as eddy current would be induced in it by the alternating field. Light but rigid
construction is used for the moving coil. It should be noted that both fixed and
moving coils are air cored.

Control

The controlling torque is provided by two control springs. These springs act
as leads to the moving coil.

Moving System
 The moving coil is mounted on an aluminum spindle. The moving system
also carries the counter weights and truss type pointer. Sometimes a suspension
may be used in case a high sensitivity is desired.

Damping

Air friction damping is employed for these instruments and is provided by a pair
of aluminum vanes, attached to the spindle at the bottom. These vanes move in
sector shaped chambers. Eddy current damping cannot be used in these instruments
as the operating field is very weak (on account of the fact that the coils are air cored)
and any introduction of a permanent magnet required for eddy current damping would
distort the operating magnetic field of the instrument.

Shielding
The field produced by the fixed coils is somewhat weaker than in other types
of instruments It is nearly 0.005 to 0.006 Wb/m In d.c. measurements even the earth
magnetic field may affect the readings. Thus it is necessary to shield an
electrodynamometer type instrument from the effect of stray magnetic fields. Air
cored electrodynamometer type instruments are protected against external magnetic
fields by enclosing them in a casing of high permeability alloy. This shunts external
magnetic fields around the instrument mechanism and minimizes their effects on the
indication.

Cases and Scales

Laboratory standard instruments are usually contained in highly polished
wooden cases. These cases are so constructed as to remain dimensionally stable
over long periods of time. The glass is coated with some conducting material to
completely remove the electrostatic effects. The case is supported by adjustable
leveling screws. A spirit level is also provided to ensure proper leveling. The scales
are hand drawn, using machine sub-dividing equipment. Diagonal lines for fine sub-
division are usually drawn for main markings on the scale. Most of the high-precision
instruments have a 300 mr scale with 100, 120 or 150 divisions.

Errors in Electrodynamometer Instruments

 i) Frequency error
iii) Eddy current error
iv) External magnetic field
v) Temperature changes

Advantages
i) These instruments can be used on both a.c & d.c
ii) Accurate rms value

Disadvantages
(i) They have a low torque/weight ratio and hence have a low sensitivity.
(ii) Low torque/weight ratio gives increased frictional losses.
(iii) They are more expensive than either the PMMC or the moving iron type
instruments.

2. Explain in detail the PMMC instrument with neat sketch. (NOV/DEC2014)

The permanent magnet moving coil instrument is the most accurate type for DC
measurements. The working principle of these instruments is the same as that of the
D’

Arsonval type of galvanometers, the difference being that a direct reading instrument
is provided with a pointer and a scale.
Construction of PMMC Instruments

 The moving coil is wound with many turns of enameled or silk covered copper
wire. The coil is mounted on a rectangular aluminium former which is pivoted on
jeweled bearings. The coils move freely in the field of a permanent magnet.

 Most voltmeter coils are wound on metal frames to provide the required electro-
magnetic damping
 Most ammeter coils, however, are wound on non -magnetic formers, because
coil turns are effectively shorted by the ammeter shunt.

 Old style magnet system consisted of relatively long U shaped permanent

magnets having soft iron pole pieces.

 Owing to development of materials like Alcomax and Alnico, which have a high
coercive force, it is possible to use smaller magnet lengths and high field intensities.

 The flux densities used in PMIMC instruments vary from 0.1 W b/m to 1
Wb/m Control

When the coil is supported between two jewel bearings the control torque is provided
by two phosphor bronze hair springs. These springs also serve to lead current in and
out of the coil. The control torque is provided by the ribbon suspension as shown. This
method is comparatively new and is claimed to be advantageous as it eliminates
bearing friction. Damping

Damping torque is produced by movement of the aluminium former moving in the

magnetic field of the permanent magnet.

Pointer and Scale

 The pointer is carried by the spindle and moves over a graduate d scale. The
pointer is of light-weight construction and, apart from those used in some inexpensive
instruments has the section over the scale twisted to form a fine blade.

 This helps to reduce parallax errors i n the reading of the scale. When the coil
is supported between two jewel bearings the control torque is provided by two
phosphor bronze hair springs.
 Torque Equation:

The torque equation of a moving coil instrument is given

by Deflecting torque Td = NBI dl = GI

where G = a constant = NB dl

The spring control provides a restoring torque Tc =

Kθ where K = spring constant

For final steady deflection Tc = Td

Final steady deflection θ= (G/K) I
As the deflection is directly proportional to the current passing through the meter (K and
G being constants) we get a uniform (linear) scale for the instrument
Errors in PMMC Instruments
The main sources of errors in moving coil instruments are due to
 Weakening of permanent magnets due to ageing at temperature effects.
 Weakening of springs due to ageing and temperature effects.
 Change of resistance of the moving coil with temperature.

1. The scale is uniformly divided.

2. The power consumption is very low
3. The torque-weight ratio is high which gives a high accuracy. Ø A single
instrument may be used for many different current and voltage ranges by using
different values for shunts and multipliers.
4. Self-shielding magnets make the core magnet mechanism particularly useful in
aircraft and aerospace applications.
1. These instruments are useful only for DC. The torque reverses if the current
reverses. If the instrument is connected to AC., the pointer cannot follow the
rapid reversals and the deflection corresponds to mean torque, which is zero.
Hence these instruments cannot be used for AC.
2. The cost of these instruments is higher than that of moving iron instruments.
 3.a) What are the major considerations in selecting a Voltmeter.(MAY/JUNE 13)
INPUT IMPEDANCE:
The input impedance should be at least an order of magnitude higher than
the impedance of the circuit under measurement to avoid loading effects.

VOLTAGE RANGE:
The voltage ranges on the meter scale may be in the 1-3-10 sequence with 10
db of separation, or in the 1.5-5-1.5 sequence or in a single scale calibrated in
decibels. Decibels ;

In case of measurements a wide range of voltages use of the decibel scale can be very
effective. All voltmeter with db scale is calibrated in dbm, referred to some particular
impedance.
Sensitivity Vs Bandwidth:
Noise is a function of bandwidth. A voltmeter with a broad bandwidth will pickup
and generate more noise than one operating over a narrow range of frequencies.

b) Write shot notes on digital multimeter. (MAY/JUNE 13)

The digital multimeter is an instrument which is capable of measuring are voltages,d.c
voltages, a.c and d.c currents and resistances over several ranges.

The current is converted to voltage by passing it through low shunt resistance. The
a.c quantities are converted to d.c by employing various rectifiers and filtering
circuits.

For resistance measurements the meter consists of a precision low current square
that is applied across the unknown resistance which gives d.c voltage.

All the quantities are digitized using analog to digital converter and displayed in the
digital form on the display. In addition, the output digital multimeters can also be
used to interference with some other equipment.

Advantages:
1. The accuracy is very high
2. The input impedance is very high
3. The size is compact, and cost is also low.
 UNIT III
COMPARISON METHODS OF MEASUREMENTS
PART-A
1. Write the necessary balance conditions for a Schering bridge. (Nov/Dec 12)

2.Define transformer Ratio Bridge.

The transformer ratio bridges are replacing the conventional AC bridges which consist
of voltage transformer whose performance approaches that of an ideal transformer.

3.Define interference.
The instruments used for electrical measurements are in an environment which
contains many sources of electrical and magnetic energy. These sources can produce
undesirable signals called interference.

4. What is the need for screening? (Apr/May15)

Screening is a process of preventing EM radiation from coupling into or leaking out of
defined areas, or regions.

5. What are the applications of potentiometers? (Nov/D c13)

a) Calibration of voltmeter

b) Calibration of ammeter

c) Calibration of wattmeter

d) Measurement of resistance

e) Measurement of power
6.How does a Hay’s bridge differ from Maxwell’s bridge? What is its uniqueness?
(May/Jun13)

 The difference in Maxwell’s bridge and Hay’s bridge is that the hay’s bridge
consists of resistance R1 in series with standard capacitor C1 in one of the ratio arms.
 Hence for larger phase angle, R1 needed is very low, which is practicable
 Hence bridge can be used for the coils with high values.
7.Which instrument is used for measuring very high resistance found in cable
insulations? (May/Jun13)

Kelvin bridge were used for measuring very high resistance found in cable insulations.

8.List the applications of ac bridge.(April/May 2011)

Measurement of inductance, capacitance, storage factor, loss factor may be made

conveniently and accurately by employing ac bridge network

9.Enumerate the principle of grounding. .(April/may 2011)

The Shielding and grounding of bridge is one way of reducing the effect of stray
capacitances . But this technique does not eliminate the stray capacitances but makes
them constant in value and hence they can be compensated.

10.Which bridge is used to measure incremental inductance? (Nov/Dec 2011)

Hay’s bridge is used to measure incremental inductance.

11.Define the term standardization of potentiometer.(Nov/Dec 2009)

Before putting the potentiometer in in use it is standardized by the adjustment of

current from the supply battery by direct reading.

12.What are the advantages of Kelvins double Bridge?

 The effect of lead and contact resistance is eliminated.

 It is mainly designed for measurement of very low resistance
 PART-B

1. Explain Maxwell Bridge in detail with neat circuit

diagram.
Definition

A Maxwell Bridge (in long form, a Maxwell-Wien bridge) is a type of Wheatstone

bridge used to measure an unknown inductance (usually of low Q value) in terms of
calibrated resistance and capacitance. It is a real product bridge.

The Maxwell Bridge is used to measure unknown inductance in terms of calibrated

resistance and capacitance. Calibration-grade inductors are more difficult to
manufacture than capacitors of similar precision, and so the use of a simple
"symmetrical" inductance bridge is not always practical.

Circuit Diagram

Explanation
With reference to the picture, in a typical application R1 and R4 are known fixed
entities, and R2 and C2 are known variable entities. R2 and C2 are adjusted until the
bridge is balanced. R3 and L3 can then be calculated based on the values of the other
components.
 As shown in Figure, one arm of the Maxwell bridge consists of a capacitor in
parallel with a resistor (C1 and R2) and another arm consists of an inductor L1 in
series with a resistor (L1 and R4). The other two arms just consist of a resistor each
(R1 and R3). The values of R1 and R3 are known, and R2 and C1 are both
adjustable. The unknown values are those of L1 and R4. Like other bridge circuits, the
measuring ability of a Maxwell Bridge depends on 'Balancing' the circuit.

Balancing the circuit means adjusting C1 and R2 until the current through the
bridge between points A and B becomes zero. This happens when the voltages at
points A and B are equal.

Mathematically,
Z1 = R2 + 1/ (2πfC1); while Z2 = R4 + 2πfL1.
(R2 + 1/ (2πfC1)) / R1 = R3 / [R4 + 2πfL1];
or
R1R3 = [R2 + 1/ (2πfC1)] [R4 + 2πfL1]

To avoid the difficulties associated with determining the precise value of a

variable capacitance, sometimes a fixed-value capacitor will be installed, and more
than one resistor will be made variable. The additional complexity of using a Maxwell
bridge over simpler bridge types is warranted in circumstances where either the mutual
inductance between the load and the known bridge entities, or stray electromagnetic
interference, distorts the measurement results.

The capacitive reactance in the bridge will exactly oppose the inductive
reactance of the load when the bridge is balanced, allowing the load's resistance and
reactance to be reliably determined.

Advantages:
 The frequency does not appear
 Wide range of inductance
 Disadvantages:
 Limited measurement
 It requires variable standard capacitor

2) Explain Schering Bridge with neat circuit diagram in detail.
Definition

A Schering Bridge is a bridge circuit used for measuring an unknown electrical

capacitance and its dissipation factor. The dissipation factor of a capacitor is the ratio
of its resistance to its capacitive reactance. The Schering Bridge is basically a four-arm
alternating-current (AC) bridge circuit whose measurement depends on balancing the
loads on its arms. Figure 1 below shows a diagram of the Schering Bridge.

Diagram

Explanation
In the Schering Bridge above, the resistance values of resistors R1 and R2 are known,
while the resistance value of resistor R3 is unknown. The capacitance values of C1 and
C2 are also known, while the capacitance of C3 is the value being measured. To
measure R3 and C3, the values of C2 and R2 are fixed, while the values of R1 and C1
are adjusted until the current through the ammeter between points A and B becomes
zero. This happens when the voltages at points A and B are equal, in which case the
bridge is said to be 'balanced'.
When the bridge is balanced, Z1/C2 = R2/Z3, where Z1 is the impedance of R1 in
parallel with C1 and Z3 is the impedance of R3 in series with C3.

In an AC circuit that has a capacitor, the capacitor contributes a capacitive reactance to

the impedance.

Z1 = R1/[2πfC1((1/2πfC1) + R1)] = R1/(1 + 2πfC1R1)

while Z3 =1/2πfC3 + R3. 2πfC2R1/ (1+2πfC1R1) = R2/(1/2πfC3 + R3); or 2πfC2
(1/2πfC3 + R3) = (R2/R1) (1+2πfC1R1); or C2/C3 + 2πfC2R3 = R2/R1 + 2πfC1R2.

When the bridge is balanced, the negative and positive reactive components are equal
and cancel out, so

2πfC2R3 = 2πfC1R2 or R3 = C1R2 / C2.

Similarly, when the bridge is balanced, the purely resistive components are equal,
C2/C3 = R2/R1 or C3 = R1C2 / R2.

Advantages:
 Balance equation is independent of frequency
 Used for measuring the insulating properties of electrical cables and
equipment’s

3.Discuss the effects of electrostatic and electromagnetic interference in

instruments (Apr/May15)

INTERFERENCE BY RADIATION.
Interference by electromagnetic radiation becomes important at cable lengths
greater than 1/7 of the wavelength of the signals. At frequencies beyond 30Mhz, most
of the interference occurs by electromagnetic radiation

INTERFERENCE BY ELECTROSTATIC CHARGE.

Charged persons and objects can store electrical charges of up to several micro-
Coulombs, which means voltages of some 10kV in respect to ground. Dry air, artificial
fabrics and friction favor these conditions. When touching grounded equipment, an
instantaneous discharge produces arcing with short, high current pulses and
associated strong changes of the e.m. field.
REDUCTION OF INTERFERENCE

There are several methods to prevent interference. But all of them only reduce
the interference and never fully prevent it. This means there will never be a system
which is 100% safe from interference. Because the efforts and the cost will rise with
the degree of reduction of interference, a compromise must be found between the
effort and the result.

The requirement for the reduction of interference will depend on:

- The strength of the interference source
- The sensitivity of the interference sink

The problems caused by interference

4.How a DC potentiometer is used for the calibration of a voltmeter? Explain it

with a diagram.(Apr/May 15)

An instrument that precisely measures an electromotive force (emf) or a vol age

by opposing to it a known potential drop established by passing a definite current
through a resistor of known characteristics.

o There are two ways of accomplishing this balance: (1) the current I may be held
at a fixed value and the resistance R across which the IR drop is opposed to the
unknown may be varied; (2) current may be varied across a fixed resistance to
achieve the needed IR drop.
p The essential features of a general-purpose constant-current instrument are
shown in the illustration. The value of the current is first fixed to match an IR drop to
the emf of a reference standard cell. With the standard-cell dial set to read the emf of
the reference cell, and the galvanometer (balance detector) in position G1, the
resistance of the supply branch of the circuit is adjusted until the IR drop in 10 steps
of the coarse dial plus the set portion of the standard-cell dial balances the known
reference emf, indicated by a null reading of the galvanometer.

 This adjustment permits the potentiometer to be read directly in volts. Then,

with the galvanometer in position G2, the coarse, intermediate, and slide-wire dials
are adjusted until the galvanometer again reads null. If the potentiometer current has
not changed, the emf of the unknown can be read directly from the dial settings.

 There is usually a switching arrangement so that the galvanometer can be

quickly shifted between positions 1 and 2 to check that the current has not drifted
from its set value.

 Circuit diagram of a general-purpose constant-current potentiometer, showing

essential features Potentiometer techniques may also be used for current
measurement, the unknown current being sent through a known resistance and the
IR drop opposed by balancing it at the voltage terminals of the potentiometer.

 Here, of course, internal heating and consequent resistance change of the

current-carrying resistor (shunt) may be a critical factor in measurement accuracy;
and the shunt design may require attention to dissipation of heat resulting from its I2R
power consumption.

 Potentiometer techniques have been extended to alternating-voltage

measurements, but generally at a reduced accuracy level (usually 0.1% or so).
Current is set on an ammeter which must have the same response on ac as on dc,
where it may be calibrated with a potentiometer and shunt combination.

q Balance in opposing an unknown voltage is achieved in o e of two ways: (1) a

slide-wire and phase-adjustable supply; (2) separate in-phase and quadrature
adjustments on slide wires supplied from sources that have a 90° phase difference.
Such potentiometers have limited use in magnetic testing.
5.Explain about the AC potentiometer. (APR/MAY 15)
This potentiometer is an exceedingly useful one for the accurate measurement
of alternating currents and voltages, since such measurements are not easily carried
out by some other methods.

The basic principle of AC potentiometer is the same as that of DC

potentiometer. The main difference between these two is “whereas in the DC
potentiometer only the magnitudes of the “unknown” emf and slide wire voltage drop
must be made equal to obtain balance. In the AC potentiometer the phases of the two
voltages, as well as then magnitudes, must be equal for balance to be obtained.”

The following factors must be considered for the principle of operation of an AC

potentiometer.

1. In all the types of AC potentiometers, the potentiometer circuit must be supplied

from the same source as the voltage or current being measured.
2. There AC supply should be free from harmonics because in the presence of
harmonic it may not ne possible to achieve balance. The input supply should be
sinusoidal.
3. There being no reference source (the reference source is DC being of standard
cell or a zener source) the absolute accuracy with which an AC voltage can be
measured in an AC potentiometer cannot be comparable with corresponding type of
DC measurement.
 4. Extraneous or stray emfs picked up from fields or coupling between portions of
the potentiometer circuit arrangement, which seriously affect the result, must be
eliminated. Compensated for and or measured (since they may add vectorially to the
emf being measured).

Standardization of AC potentiometers:

The AC type potentiometers are made direct reading type,i.e., the readings are
read off directly from the dial settings. In order to do this, the AC potentiometer must
be standardized as is done in case of DC type instruments.

The standardization is achieved by with the help of standard DC source, i.e, a

standard cell or a zener source and a transfer instrument. This transfer instrument is
an electrodynamometer milliammeter, so constructed that its response to alternating
currents is the same as its DC response. Such an instrument can be calibrated on DC
and then brought to the same setting on AC. Otherwise, we can use thermocouple
type of instrument as a transfer instrument.

Types of AC potentiometer
It can be classified according to the manner in which the instrument dials (or
scales) present the value of unknown voltage. There are two types.

1. Polar potentiometers.

2. Co-ordinate potentiometers

1. Polar potentiometers

In this type of potentiometers, the unknown emf is measured in polar form i.e.,
in terms of its magnitude and relative phase. Here, the magnitude is given by one
scale and its phase angle with respect to some reference vector is given by the second
(other) scale. Provision is made for reading phase angles upto 360°. Figure shows that
the voltage is read in the form V The example for this type of instrument is Drysdale
polar potentiometer.
v
2. Co-ordinate potentiometers

Here, the unknown emf is measured in terms of its components along and
perpendicular to a standard axis There re two unknown components of an emfs.

COMPARISON METHODS OF MEASUREMENTS

These two are given directly by two different scales known as “in phase”
(V1) and “quadrature” (V2) scales.

Provision is made in this type of instruments to read positive and negative

values of voltages so that all angles upto 360° are covered.

2 2
Voltage V = √V1 + V2
-1
θ = tan (V2/V1)
Examples of this instrument are Gall-Tinsley and Campbell-Larsen type instruments.
3. Dryscale polar potentiometer

Figure shows a dryscale polar AC potentiometer.

For AC measurements, the slide wire AB is supplied from a phase shifting circuit so
arranged that the magnitude of the voltage supplied by it remains constant while its
phase can be varied through 360°. Consequently, slide-wire current I 1 is maintained
constant in magnitude but varied in phase.
 The phase shifting circuit consists of two stator windings. These two stator
windings are supplied from the same source in parallel. Their currents are made to
differ by 90° by using phase shifting technique.

The two stator windings produce rotating flux which induces a secondary emf in
the rotor winding which is of constant magnitude but the phase of which can be varied
by rotating the rotor circuit emf is read from the circular graduated dial provided for the
purpose. The ammeter A in the slide wire circuit is of electrodynamic or thermal type
before using it for AC measurements, the potentiometer is first calibrated by using DC
supply for slide wire and standard cell for test terminals T1 and T2.

The unknown input AC voltage to be measured is applied across test terminals

T1 and T2. The balance is effected by the alternate adjustment of the slide wire contact
and position of phase shifting rotor. This slide-wire reading represents the magnitude
of the test voltage and voltage phase shifter reading gives the phase with reference to
an arbitrary reference vector.20

PART C(15 Mark)

1. Give detailed notes on earthing.

This is one of the simplest but most efficient methods to reduce
interference.

Grounding can be used for three different purposes:

1. Protection Ground

Provides protection for the operators from dangerous voltages. Widely used on
mains-operated equipment.

2. Function Ground

The ground is used as a conductive path for signals. Example: in asymmetrical

cables screen, which is one conductor for the signal, is connected to the ground.

3. Screening Ground
  Used to provide a neutral electrical path for the interference, to prevent that the
interfering voltages or currents from entering the circuit. In this chapter we will only

consider the third aspect.

 Grounding of equipment is often required for the cases 1 or 2 anyhow, so that

the screening ground is available "free of charge". Sometimes the grounding
potential, provided by the mains connection, is very "polluted".
This means that the ground potential itself already carries an interfering signal.
This is especially likely if there are big power consumers in the neighborhood or
even in the same building.
Using such a ground might do more harm than good. The quality of the ground
line can be tested by measuring it with a storage scope against some other ground
connection, e.g. a metal water pipe or some metal parts of the construction.
Never use the Neutral (N) of the mains as ground.

 It might contain strong interference; because it carries the load current of all
electrical consumers. The grounding can be done by single-point grounding or by
multi-point grounding. Each method has advantages which depend on the frequency
range of the signal frequencies.
 All parts to be grounded are connected to one central point. This results in no
"ground loops" being produced. This means the grounding conductors do not form
any closed conductive path in which magnetic interference could induce currents.
Furthermore, conductive lines between the equipment are avoided, which could
produce galvanic coupling of interference.

 Central grounding requires consistent arrangement of the grounding circuit and

requires insulation of the individual parts of the circuit. This is sometimes very difficult
to achieve in a system using the single-point rounding.
Multi-point grounding:
 In multi-point grounding all parts are connected to ground at as many points as
possible. This requires that the ground potential itself is as widely spread as possible.
In practice, all conductive parts of the chassis, the cases, the shielding, the room and
the installation are included in the network.

When considering the effect of electrical and magnetic fields, we have to

distinguish between low and high frequencies. At high frequencies the skin effect plays
an important role for the screening. The penetration describes the depth from the
surface of the conductor, where the current density has decayed to 37% compared to
the surface of the conductor.

Screening of cables:

When signal lines run close to interference sources or when the signal circuit is very
sensitive to interference, screening of signal lines will give an improvement. There are
different ways of connecting the cable screen: Three different ways of connecting the
cable screen. Cable screen not connected.

 This screen will not prevent any interference, because the charge on the
screen, produced by interference, will remain and will affect the central signal line.
Also, the current induced by interference in the line will flow through the sink, affecting
the signal. Cable screen grounded on one side only.

 This screen will only prevent interference at low frequency signals. For
electromagnetic interference, where the wavelength is short compared to the length
of the cable, the screening efficiency is poor.

 Cable screen grounded on either side is effective for all kinds of interference.
Any current induced in the screen by magnetic interference will flow to ground. The
inner of the cable is not affected. Only the voltage drop on the screen will affect the
signal in the screen.

2. Explain Hay’s Bridge with neat circuit diagram in detail.

 A Hay Bridge is n AC bridge circuit used for measuring an unknown inductance
by balancing the loads of its four arms, one of which contains the unknown inductance.
One of the arms of a Hay Bridge has a capacitor of known characteristics, which is the
principal component used for determining the unknown inductance value. Figure 1
below shows a diagram of the Hay Bridge.

Explanation

As shown in Figure 1, one arm of the Hay bridge consists of a capacitor in series with
a resistor (C1 and R2) and another arm consists of an inductor L1 series with a
resistor (L1 and R4).

The other two arms simply contain a resistor each (R1 and R3). The values of R1and
R3 are known, and R2 and C1 are both adjustable.

The unknown values are those of L1 and R4.

Like other bridge circuits, the measuring ability of a Hay Bridge depends on 'balancing'
the circuit. Balancing the circuit in Figure 1 means adjusting R2 and C1 until the
current through the ammeter between points A and B becomes zero. This happens
when the voltages at points A and B are equal.

Diagram

When the Hay Bridge is balanced, it follows that Z1/R1 = R3/Z2 wherein Z1 is the
impedance of the arm containing C1 and R2 while Z2 is the impedance of the arm
containing L1 and R4.
Thus, Z1 = R2 + 1/(2πfC) while Z2 = R4 + 2πfL1.
[R2 + 1/(2πfC1)] / R1 = R3 / [R4 + 2πfL1]; or
[R4 + 2πfL1] = R3R1 / [R2 + 1/(2πfC1)]; or
R3R1 = R2R4 + 2πfL1R2 + R4/2πfC1 + L1/C1.
When the bridge is balanced, the reactive components are equal, so 2πfL1R2 =
R4/2πfC1, or R4 = (2πf) 2L1R2C1.

Substituting R4, one comes up with the following equation: R3R1 = (R2+1/2πfC1)
((2πf) 2L1R2C1) + 2πfL1R2 + L1/C1; or L1 = R3R1C1 / (2πf) 2R22C12 + 4πfC1R2 +
1); L1 = R3R1C1 / [1 + (2πfR2C1)2]

After dropping the reactive components of the equation since the bridge is
Thus, the equations for L1 and R4 for the Hay Bridge in Figure 1 when it is balanced
are:
L1 = R3R1C1 / [1 + (2πfR2C1)2]; and
R4 = (2πfC1)2R2R3R1 / [1 + (2πfR2C1)2]

Advantages:
Simple expression

Disadvantages:
It is not suited for measurement of coil

3.Explain wheatstone Bridge with neat circuit diagram in detail.

The term “Wheatstone bridge” is also called as Resistance Bridge that is, invented by
“Charles Wheatstone”. This bridge circuit is used to calculate the unknown resistance
values and as a means of regulating measuring instrument, ammeters, voltmeters, etc.
But, the present digital millimeters offer the easiest way to calculate a resistance. In
recent days, Wheatstone bridge is used in many applications such as; it can be used
with modern op-amps to interface various sensors and transducers to amplifier circuits.
This bridge circuit is constructed with two simple serial and parallel resistances in
between a voltage supply terminal and ground terminals. When the bridge is balanced,
then the ground terminal produces a zero voltage difference between the two parallel
branches. A Wheatstone bridge consists of two i/p and two o/p terminals includes of
four resistors arranged in a diamond shape.

Wheatstone Bridge and Its Working

A Wheatstone bridge is widely used to measure the electrical resistance. This circuit is
built with two known resistors, one unknown resistor and one variable resistor
connected in the form of bridge. When the variable resistor is adjusted, then the
current in the galvanometer becomes zero, the ratio of two unknown resistors is equal
to the ratio of value of unknown resistance and adjusted value of variable resistance.
By using a Wheatstone Bridge the unknown electrical resistance value can easily
measure.

Wheatstone Bridge Circuit Arrangement

The circuit arrangement of the Wheatstone bridge is shown below. This circuit is
designed with four arms, namely AB, BC, CD & AD and consists of electrical
resistance P, Q, R and S. Among these four resistances, P and Q are known fixed
electrical resistances. A galvanometer is connected between the B & D terminals via
an S1 switch. The voltage source is connected to the A &C terminals via a switch S2.
A variable resistor ‘S’ is connected between the terminals C & D. The potential at
terminal D varies when the value of the variable resistor adjusts. For instance, currents
I1 and I2 are flowing through the points ADC and ABC. When the resistance value of
arm CD varies, then the I2 current will also vary.
If we tend to adjust the variable resistance one state of affairs could return once when
the voltage drop across the resistor S that is I2.S becomes specifically capable to the
voltage drop across resistor Q i.e I1.Q. Thus the potential of the point B becomes
equal to the potential of the point D hence the potential difference b/n these two points
is zero hence current through galvanometer is zero. Then the deflection in the
galvanometer is zero when the S2 switch is closed.

Wheatstone Bridge Derivation

From the above circuit, currents I1 and I2 are

I1=V/P+Q and I2=V/R+S

Now potential of point B with respect to point C is the voltage drop across the Q
transistor, then the equation is

I1Q= VQ/P+Q …………………………..(1)

Potential of point D with respect to C is the voltage drop across the resistor S, then the
equation is

I2S=VS/R+S …………………………..(2)

From the above equation 1 and 2 we get,

 VQ/P+Q = VS/R+S

` Q/P+Q = S/R+S

P+Q/Q=R+S/S

P/Q+1=R/S+1

P/Q=R/S

R=SxP/Q

Here in the above equation, the value of P/Q and S are known, so R value can easily
be determined.

The electrical resistances of Wheatstone bridge such as P and Q are made of definite
ratio, they are 1:1; 10:1 (or) 100:1 known as ratio arms and the rheostat arm S is made
always variable from 1-1,000 ohms or from 1-10,000 ohms.
 UNIT –IV
STORAGE AND DISPLAY DEVICES
PART-A
1.What are data loggers? (NOV/DEC 13)
The data loggers are used to automatically make a record of the readings of
instruments located at different parts of the plant.

2.Define the deflection sensitivity of CRT.

The deflection sensitivity of a CRT is defined as the deflection of the screen per unit
deflection voltage.

3.What are the different materials used in LED? Also name the colors emitted.
Gallium Arsenide phosphate, Gallium Arsenide and Gallium phosphide. It emits red,
yellow and green colors.

4.Distinguish between LED & LCD.(Nov/Dec 13)

Dynamic
Parameter scattering LCD Field effect LCD
LED
Power/digit 100µw 1 to 10 µw

10-140 mw depending
on color

Voltage 18V 3 to 7V
5V
Switching speed 1 µs 300 ms 100 to 300ms
Colors Depends on Depends on
Red, orange, Yellow and illumination illumination
Green

5.What are the various components of a recording instrument? (May/Ju 13)

 Recording head
 Magnetic tape
 Reproducing head
 Tape transport mechanism
  Conditioning devices.
6.Reason out why today’s commercial LED monitor have become more popular
than their LCD counterparts.(May/Jun 13)

 Less power consumption

 Low cost
 Uniform brightness with good contrast
7.List any two storage devices.(April/may 2011)

1) Bitable storage oscilloscopes 2) fast storage oscilloscopes 3) Digital storage

oscilloscopes
8.Differentiate the functions of printer and plotter. (April/ May 2011)

Printers are the most commonly used output devices today for producing hard copy
output. Analog X-Y recorders are replaced by digital x-y recorders it is known as
digital plotters .to measure the performance of the digital plotters are used .

9.How does dynamic scattering type LCD work? (Nov/Dec 2011)

When not activated the transmittive type cell simply transmits the light through the cell
in the straight lines. In this condition the cell will not appear bright .When the cell is
activated, the incident light is scattered forward and the cell appears quite bright even
under high intensity light conditions

10) What are the advantage of magnetic tape recorder? (Nov/Dec 2011) i)
Magnetic tape can be recorded over and reused repeatedly.
ii) Large amounts of information is stored.

iii) Magnetic tape is inexpensive and budget friendly.

11) What are the merits of digital storage oscilloscope?(May 2010)

 Infinite storage time
 Easy to operate
 Signal processing is possible
 Cursor measurement is possible
  A number of traces depending on memory size can be stored &
Recalled
12) What are the types of printers? (Dec 2009)
 Drum & Chain printer
 Dot matrix printer
 Inkjet Printer
 Laser Printer
PART-B
1) With neat diagram, explain the parts of CRT in detail.(NOV/DEC 14)

The device which allows, the amplitude of such signals, to be displayed primarily as a
function of time, is called cathode ray oscilloscope. The cathode ray tube (CRT) is the
heart of the C.R.O. The CRT generates the electron beam, accelerates the beam,
deflects the beam and has a screen where beam becomes visible as a spot. The main
parts of the CRT are

i) Electron gun
ii) Deflection system
iii) Fluorescent screen
iv) Glass tube or envelope
v) Base
Electron gun
The electron gun section of the cathode ray tube provides a sharply focused, electron
beam directed towards the fluorescent-coated screen. This section starts from
thermally heated cathode, emitting the electrons. The control grid is given negative
potential with respect to cathode. This grid controls the number of electrons in t beam,
going to the screen. The momentum of the electrons (their number x their speed)
determines the intensity, or brightness, of the light emitted f om the fluorescent screen
due to the electron bombardment. The light emitted is usually of the green colour.
Deflection System
When the electron beam is accelerated it passes through the deflection sys em,
with which beam can be positioned anywhere on the screen.

Fluorescent Screen

 The light produced by the screen does not disappear immediately when
bombardment by electrons ceases, i.e., when the signal becomes zero.

 The time period for which the trace remains on the screen after the signal
becomes zero is known as “persistence or fluorescence”. The persistence may be as
short as a few micro second, or as long as tens of seconds or even minutes.

 Medium persistence traces are mostly used for general purpose applications.
Long persistence traces are used in the study of transients. Long persistence helps in
the study of transients since the trace is still seen on the screen after the transient
has disappeared.
Glass Tube
All the components of a CRT are enclosed in an evacuated glass tube called
envelope. This allows the emitted electrons to move about freely from one end of the
tube to the other end.
Base
The base is provided to the CRT through which the connections are made to the
various parts.

2) Write detailed notes on applications of LCD with necessary

sketches.(NOV/DEC)

 A liquid crystal display (LCD) is a thin, flat electronic visual display that uses the
light modulating properties of liquid crystals (LCs). LCs do not emit light directly. They
are used in a wide range of applications including: computer monitors, television,
instrument panels, aircraft cockpit displays, signage, etc. They are common in
consumer devices such as video players, g ming devices, clocks, watches,
calculators, and telephones. LCDs have displaced cathode ray tube (CRT) displays in
most applications. They are usually more compact, lightweight, portable, less
expensive, more reliable, and easier on the eyes. They are available in a wider ran e
of screen sizes than CRT and plasma displays, and since they do not use phosphors,
they cannot suffer image burn-in. LCDs are more energy efficient and offer safer
disposal than CRTs.
Each pixel of an LCD typically consists of a layer of molecules aligned between
two transparent electrodes, and two polarizing filters, the axes of transmission of which
are (in most of the cases) perpendicular to each other. With no actual liquid crystal
between the polarizing filters, light passing through the first filter would be blocked by
the second (crossed) polarizer. In most of the cases the liquid crystal has double
refraction.

The surfaces of the electrodes that are in contact with the liquid crystal material
are treated so as to align the liquid crystal molecules in a particular direction. This
treatment typically consists of a thin polymer layer that is uni-directionally rubbed
using, for example, a cloth. The direction of the liquid crystal alignment is then defined
by the direction of rubbing. Electrodes are made of a transparent conductor called
Indium Tin Oxide (ITO).

 Before applying an electric field, the orientation of the liquid crystal molecules is
determined by the alignment at the surfaces of electrodes. In a twisted nematic
device (still the most common liquid crystal device), the surface alignment directions
at the two electrodes are perpendicular to each other, and so the molecules arrange
themselves in a helical structure, or twist.
 This reduces the rotation of the polarization of the incident light, and the device
appears grey. If the applied voltage is large enough, the liquid crystal molecules in the
center of the layer are almost completely untwisted and the polarization of the
incident light is not rotated as it p s es through the liquid crystal layer.

This light will then be mainly polarized perpendicular to the second filter, and
thus be blocked and the pixel will appear black. By controlling the voltage applied
across the liquid crystal layer in each pixel, light can be allowed to pass through in
varying amounts thus constituting different levels of gray. This electric field also
controls (reduces) the double refraction properties of the liquid crystal.

o LCD with top polarizer removed from device and placed on top, such that the
top and bottom polarizers are parallel. The optical effect of a twisted nematic device
in the voltage-on state is far less dependent on variations in the device thickness than
hat in the voltage-off state.

o Because of this, these devices are usually operated between crossed polarizers
such that they appear bright with no voltage (the eye is much more sensitive to
variations in the dark state than the bright state).

o These devices can also be operated between parallel polarizers, in which case the
bright and dark states are reversed. The voltage-off dark state in this configuration
appears blotchy, however, because of small variations of thickness across the device.


Both the liquid crystal material and the alignment layer material contain ionic
compounds. If an electric field of one particular polarity is applied for a long period of
time, this ionic material is attracted to the surfaces and degrades the device
performance.
 This is avoided either by applying an alternating current or by reversing the
polarity of the electric field as the device is addressed (the response of the liquid
crystal layer is identical, regardless of the polarity of the applied field).
 When a large number of pixels are needed in a display, it is not technically
possible to drive each directly since then each pixel would require independent
electrodes. Instead, the display is multiplexed.
 In a multiplexed display, electrodes on one side of the display are grouped and
wired together (typically in columns), and each group gets its own voltage source. On
the other side, the electrodes are also grouped (typically in rows), with each group
getting a voltage sink.
The groups are designed so each pixel has a unique, unshared combination of
source and sink The electronics or the software driving the electronics then turns on
sinks in sequence, and drives sources for the pixels of each sink.

3) Discuss in detail about the various types of recorders.(MAY 2012)

The recording procedure performed in magnetic tape recorder can be done
by 3 methods. They are

1. Direct recording
2. FM (frequency Modulation) Recording
3. PDM (Pulse Duration Modulation ) recording

Direct recording
The signal to be recorded modulates the current in the recording head.
Because of current modulation, magnetic flux in the recording gap is linearly
modulated. When the tape is moved under the recording head, the magnetic particles
retain a state of permanent magnetization proportional to the flux in the gap.

The input signal is thus converted to a spatial variation of the magnetization of

the particles on the tape. The reproduce head detects these changes as changes in
the reluctance of its magnetic circuit which will induce a voltage in its winding. This
voltage is proportional to the rate of change of flux. The signal on the exposed tape
can be retrieved and played out at any time.
 Disadvantages:
(i) . This method cannot be used in DC because reproduce head genet & a signal
which is proportional to the rate of change of flux.
(ii) Lower limit is around 100 Hz and upper limit is around 2 Mhz.

FM Recording:

In this FM system, the input signal is used to frequency modulate a carrier which is
then recorded on the tape in the usual way. The central frequency is selected with
respect to the tape speed and frequency deviation selected for the tape recorders is
±40% about the carrier frequency.
The reproduce head reads the tape in the usual way and sends a signal to the FM
demodulator and low pass filter and the ori inal s g al is reconstructed.


The signal to noise ratio (S/N) of an FM record r is of the order of 40-50 db, with an
accuracy of less than ±1%. This ±1 db flat frequency response of FM recorder can go
as high as 80 kHz at 120 in/s tape speed, when using very high carrier frequencies
above 400 kHz.


When high frequency (HF) is not needed and with a View to conserving tape, a
tape speed range selector is generally provided. When the tape speed is changed,
the carrier frequency also changes in the same proportion.

Input to the tape recorders is generally at 1 V level and so most transducers require
amplification before recording. A FM recording system is illustrated in figure. In this
 system, a carrier oscillator frequency f6, called the centre frequency, is modulated by
the level of the input signal.
When there is no input signal, the modulation is at centre frequency fc. If a
positive input signal is applied, the frequency deviates from the centre frequency by
some amount in a certain direction, the application of 8 negative input voltage deviates
the carrier frequency in the opposite direction.

The output of the modulation, which is fed to the tape, is a signal of Constant
frequency for DC inputs and varying frequency for AC inputs. The Variation of
frequency is directly proportional to the amplitude of the input signal. On playback, the
output of the reproduce head is demodulated and fed through a low pass filter which
removes the carrier and other unwanted (frequencies produced due to the modulation
process.

o The operation of FM modulation can be easily checked by applying a known

input voltage and measuring the output frequency with an electronic counter. This
signal is applied to the tape with no further conditioning. as the signal is independent
of the amplitude.

The FM demodulator converts the difference between the centre frequency and the
frequency on the tape, to a voltage proportional to the difference in the frequencies.
This system can thus record frequencies from DC to several thousand Hertz.
Residual carrier signals and out of band noise are removed by a low pass filter.
Advantages:
(i) DC component of the input signal is preserved.
(ii) Wide frequency range.
(iii) No drop out effect due to in homogeneities of the tape material.
(iv) Accurately reproduces the wave form of the input signal.

Disadvantages:

(i) Extremely sensitive to tape speed fluctuations.

(ii) Limited frequency response.
(iii) Requires a high tape speed.
(iv) Requires a high quality of tape transport and speed control.

Pulse Duration Modulation:

Pulse duration modulation allows simultaneous recording of a large number of
slowly changing variables by using Time Division Multiplexing (TDM). The PDM
recording process requires the input signal at the instant of sampling be converted to a
pulse, the duration of which is proportional to amplitude of the signal at that instant.

As an example, for recording sine wave, it is sampled and recorded at uniformly

spaced discrete intervals instead of continuously recording instantaneous values. The
original sine wave can be reconstructed on playback by passing the discrete readings
through an appropriate filter.

4.With the help of a functional block diagram explain the working principle of
digital storage oscilloscope mean its advantages over analog CRO.(Nov/Dec 13)

Ø The input signal is applied to the amplifier and attenuator section.

Ø The oscilloscope uses same type of amplifier and attenuator circuitry as used in the
conventional oscilloscopes.
Ø The attenuated signal is then applied to the vertical amplifier.
Ø To digitize the analog signal, analog to digital (A/D) converter is used.
Ø The output of the vertical amplifier is applied to the A/D converter section.
Ø The successive approximation type of A/D converter is most often used in the digital
storage oscilloscopes.
Ø The sampling rate and memory size are selected depending upon the duration& the
aveform to be recorded.
Ø Once the input signal is sampled, the A/D converter digitizes it.
Ø The signal is then captured in the memory.
Ø Once it is stored in the memory, many manipulations are possible as memory can
be readout without being erased.
 The block diagram of digital storage oscilloscope is shown in the Fig.

Ø The digital storage oscilloscope has three modes:

1. Roll mode
2. Store mode
3. Hold or Save mode
Advantages
i) It is easier to operate and has more capability. ii) The storage time is infinite.
iii) The display flexibility is available. The number of traces that can be stored and
recalled depends on the size of the memory.

iv) The cursor measurement is possible.

v) The characters can be displayed on screen along with the waveform which can
indicate waveform information such as minimum, maximum, frequency, amplitude etc.
vi) The X-Y plots, B-H curve, P-V diagrams can be displayed.
vii) The pretrigger viewing feature allows to display the waveform before trigger pulse.
viii) Keeping the records is possible by transmitting the data to computer system where
the further processing is possible

ix) Signal processing is possible which includes translating the raw data into finished
information e.g. computing parameters of a captured signal like r.m.s. value, energy
stored etc.

5. (i) Draw the block diagram X-Y Recorder and explain ( 8) (Nov/Dec

2011) XY recorder

Each of the input signals is attenuated in the range of 0-5mV, so that it can work
in the dynamic range of the recorder

 In X-Y recorder one variable is plotted against another variable. In this recorder,
pen is moved in either X or Y direction on a fixed graph paper.

 The writing assembly movement is controlled by using either sevo feedback

system or self balancing potentiometer.
 The writing assembly consists of one or two pens depending on this application.
 In practice, X-Y recorder plots one voltage as a function of other voltage.
 Many times X-Y recorder is used to record non electrical physical such as
displacement, pressure, strain etc as a function of another non electrical physical
quantity.
Construction:
 It consists of attenuator which attenuates the input circuit. The balancing signal
and error detector gives error signal.
 This error signal is DC signal. The chopper circuit converts error signal to AC
signal
 The servo amplifier drives servomotor which drives writing assembly on a fixed
graph paper.
 There are two such circuits for two different inputs X and Y. The error signal of
X input is amplified by servo amplifier of X channel driving corresponding servomotor
and pen in X-direction.
 Similar section is performed for Y channel

ii) Explain data loggers in detail.(8) (Nov/Dec 2011)

Definition
Data logger is an electronic device that records data over time or in relation to
location either with a built in instrument or sensor.

Components: 1. Pulse inputs Counts circuit closing 2.Control ports Digital in and out

 Most commonly used to turn things on and off can be programmed as a digital
input excitation outputs. Though they can be deployed while connected to a host PC
over an Ethernet or serial port a data logger is more typically deployed as standalone
devices.

 The term data logger (also sometimes referred to as a data recorder) is

commonly used to describe a self-contained, standalone data acquisition system or
device. These products are comprised of a number of analog and digital inputs that
are monitored, and the results or conditions of these inputs is then stored on some
type of local memory (e.g. SD Card, Hard Drive).

 A data logger is a self-contained unit that does not require a host to operate. It
can be installed in almost any location, and left to operate unattended. This data can
be immediately analyzed for trends, or stored for historical archive purposes. Data
loggers can also monitor for alarm conditions,

 while recording a minimum number of samples, for economy. If the recording is

of a steady-state nature, without rapid changes, the user may go through rolls of
paper, without seeing a single change in the input.
 A data logger can record at very long intervals, saving paper, and can note
when an alarm condition is occurring. When this happens, the event will be recorded
and any outputs will be activated, even if the event occurs in between sample times.
A record of all significant conditions and events is generated using a minimum of
recording hardcopy.

 The differences between various data loggers are based on the way that data is
recorded and stored.

The basic difference between the two data logger types is that one type allows the
data to be stored in a memory, to be retrieved at a later time, while the other type
automatically records the data on paper, for immediate viewing and analysis. Many
data loggers combine these two functions, usually unequally, with the emphasis on
either the ability to transfer the data or to provide a printout of it.

PART C(15 MARKS)

1. Explain about digital plotters and Printers.
PRINTERS
Printers can be classified according to their printing methodology Impact printers and
Non- impact printers.
Impact printers press formed character faces against an inked ribbon onto the paper.

A line printer and dot matrix printer are the examples of an impact printer.

Non impact printer and plotters use laser techniques, inkjet sprays, xerographic
processes, electrostatic methods and e1ectrothermal methods to get images onto the
paper.
A ink-jet printer and laser printer are the examples of non- impact printers.

Line Printers

A line printer prints a complete line at a time. The printing speed of line printer varies
from 150 lines to 2500 lines per minute with 96 to 100 characters on one line. The line
printers are divided into two categories Drum printers and chain printer.

Drum Printers

Drum printer consists of a cylindr ical drum. One complete set of characters i s
embossed on all the print positions on a l ine, as shown in the Fig. The character to
be printed is adjusted by rot ting drum.

In these printers chain with embossed character set is used, instead of drum. Here,
the character to be printed is adjusted by rotating chain.

Dot Matrix Printers

Dot matrix printers are also called serial printers as they print one character at a time,
with printing head moving across a line.
Laser Printer
Ø The line, do t matrix, and ink jet printers need a head movement on a
ribbon to print characters.
 Ø This mechanical movement is relatively slow due to the high inertia
of mechanical elements.
Ø In laser printers these mechanical movements are avoid d.
Ø In these printers, an electronically controlled laser beam traces
out the desired character to be printed on a photoconductive
drum.
Ø The exposed areas of the drum gets charged, which attracts an
oppositely charged ink from the ink toner on to the exposed areas.
Ø This image is then transferred to the paper which comes in contact with
the drum with pressure and heat.
Ø The charge on the drum decides the darkness of the print.
Ø When charge is more, more ink is attracted and we get a dark print.

Ø A colour laser printer works like a single colour laser printer, except that the
process is repeated four times with four different ink colours: Cyan, magenta, yellow
and black.
Ø Laser printers have high resolution from 600 dots per inch upto 1200 per inch.
Ø These printers print 4 to 16 page of text per minute.
Ø The high quality and speed of laser printers make them ideal for office environment.
Advantages of Laser printer
Ø The main advantages of laser printers are speed, precision and economy.
Ø A laser can move very quickly, so it can “ write” with much greater speed than an
inket.
Ø Because the laser beam has an unvarying diameter, it can draw more
precisely, without spilling any excess ink.
Ø Laser printers tend to be more expensive than ink-jet printe s, but it doesn’t cost
as much to keep them running.
Ø Its toner power is cheap and lasts for longer time.

2. Explain the principle of working of a magnetic tape recorder. What are its
basic components and their functions?(Apr/May 15)

The magnetic tape recorders are used for high frequency signal recording. In these
recorders, the data is recorded in a way that it can be reproduced in electrical form
any time. Also main advantage of these recorders is that the recorded data can be
replayed for almost infinite times. Because of good higher frequency response, these
are used in Instrumentation systems extensively.

Basic Components of Tape Recorder:

 Recording Head
 Magnetic Tape
 Reproducing Head
 Tape Transport Mechanism
 Conditioning Devices
 Recording Head
 The construction of the magnetic recording head is very much similar to the
construction of a Transformer having a toroidal core with coil. There is a uniform fine
air gap of 5μm to 15μm between the head and the magnetic tape.

When the current used for recording s passed through coil wound around
magnetic core, it produces magnetic flux. The magnetic tape is having iron oxide
particles. When the tape is passing the head, the flux produced due to recording
current gets linked with iron oxide part ices on the magnetic tape a d these particles
get magnetized. This magnetization particle remains as it is, e vent Hough the
magnetic tape leaves the gap. The actual recording takes place at the trailing edge of
the air gap.

Any signal is recorded in the form of the patterns. These magnetic patterns are
dispersed anywhere along the length of magnetic tape in accordance with the variation
in recording current with respect to time.

Magnetic Tape

 The magnetic tape is made of thin sheet of tough and dimensionally stable plastic
ribbon.
 One side of this plastic ribbon is coated by powdered iron oxide particles (Fe2O3)
thick.
 The magnetic tape is wound around a reel.
 This tape is transferred from one reel to another

 When the tape passes across air gap magnetic pattern is created in accordance with
variation of recording current.
 To reproduce this pattern, the same tape with some recorded pattern is passed
across another magnetic head in which voltage is induced.
 This voltage induced is in accordance with the magnetic pattern.

Reproducing Head
 The use of the reproducing head is to get the recorded data played back.
 The working of the reproducing head is exactly opposite to that of the recording head.
 The reproducing head detects the magnetic pattern recorded on the tape.
 The head converts the magnetic pattern back to the original electrical signal.
 In appearance, both recording and reproducing heads are very much similar.

Tape Transport Mechanism

The tape transport mechanism moves the magnetic tape along the recording head or
reproducing head with a constant speed. The tape transport mechanism must
perform following tasks.

 It must handle the tape without straining and wearing it.

 It must guide the tape across magnetic heads with great precision.
 It must maintain proper tension of magnetic tape.
 It must maintain uniform and sufficientgap between the tape and heads.
 The magnetic tape is wound on reel.
 There are two reels; one is called as supply & other is called as take-up reel.
 Both the reels rotate in same direction.
 The transportation of the tape is done by using supply reel and take-up re l.
 The fast winding of the tape or the reversing of the tape is done by using special
arrangements.
 The rollers are used to drive and guide the tape.
Conditioning Devices
These devices consist of amplifiers and fitters to modify signal to be recorded.
The conditioning devices allow the signals to be recorded on the magnetic tape with
proper format.

Amplifiers allow amplification of signal to be recorded and filters removes

unwanted ripple quantities.

Principle of Tape Recorders

When a magnetic tape is passed through a recording head, the signal to be
recorded appears as some magnetic pattern on the tape. This magnetic pattern is in
accordance with the variations of original recording current. The recorded signal can
be reproduced back by passing the same tape through a reproducing head where the
voltage is induced corresponding to the magnetic pattern on the tape. When the tape
is passed through the reproducing head, the head detects the changes in the
magnetic pattern i.e. magnetization. The change in magnetization of particles
produces change in the reluctance of the magnetic circuit of the reproducing head,
inducing a voltage in its winding. The induced voltage depends on the direction of
magnetization and its magnitude on the tape.

Methods of Recording
The methods used for magnetic tape recording used for instrumentation
purposes are as follows:

i) Direct Recording
ii) Frequency Modulation Recording
iii) Pulse Duration Modulation Recording
For instrumentation purposes mostly frequency modulation recording is used. The
pulse duration modulation recording is generally used in the systems for special
applications where large number of slowly changing variables has to be recorded
simultaneously.

3. i) Compare and Contrast the working, advantages and disadvantages of

LED and LCD.(N/D 2012)
Sl.No LED LCD

1. It consumes more power It consumes less power

2. Life time is more Life time is less

3. High cost Low cost

4. Capable of generating its own light Requires an external or

internal light source

Emitting colour depends upon the

5. material used Monochrome in nature

6. It emits light energy when energized It will alter the externally

available illumination

ii)Write a detailed note on LCD.(N/D2012)

Reflective twisted nematic liquid crystal display.
1. Polarizing filter film with a vertical axis to polarize light as it enters.
2. Glass substrate with ITO electrodes. The shapes of these electrodes will
determine the shapes that will appear when the LCD is turned ON.
3. Vertical ridges etched on the surface are smooth.
4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up
with the horizontal filter.
5. Polarizing filter film with a horizontal axis to block/pass light.
6. Reflective surface to send light back to viewer. (In a backlit LCD, this layer is
replaced with a light source.)

A liquid crystal display (LCD) is a thin, flat electronic visual display that uses the light
modulating properties of liquid crystals (LCs). LCs do not emit light directly.
They are used in a wide range of applications including: computer monitors,
television, instrument panels, aircraft cockpit displays, signage, etc. They are
common in consumer devices such as video players, gaming devices, clocks,
watches, calculators, and telephones. LCDs have displaced cathode ray tube (CRT)
displays in most applications. They are usually more compact,

lightweight, portable, less expensive, more reliable, and easier on the eyes. They are
available in a wider range of screen sizes than CRT and plasma displays, and since
they do not use phosphors, they cannot suffer image burn-in. LCDs are more energy
efficient and offer safer disposal than CRTs. Twisted nematic liquid crystal.
 UNIT-V
TRANSDUCERS AND DATA ACQUISITION SYSTEMS
PART-A
1) Give any two applications of smart sensors. (April/may 2011)
Measuring exposures in cameras, optical angle encoder and optical
arrays
2)Mention the need of ADC and DAC in Digital data Acquisition system.
(Nov/Dec 2011)

ADCs are used to convert analog signals like the output from a temperature
transducer, a radio receiver or a video camera into digital signals for processing.
Conversely, DACs are used to convert digital signals back to analog signals

3) Give the factors to be considered in selecting a transducer.

Operating range, sensitivity, electrical output characteristics, errors, accuracy,
environmental conditions .

4) Define inverse transducer.

An inverse transducer is defined as a device which converts an electrical
quantity into a non – electrical quantity.

5) Define gauge factor.

The gauge factor is defined as the ratio of per unit change in resistance to per
unit change in length.

Gauge factor Gf = (ΔR/R) / (ΔL/L)

6) What is piezo-electric effect?

A piezo electric material is one in which an electric potential appears across
certain surfaces of a crystal if the dimensions of he crystal are cha ged by the
application of a mechanical force.

7) List any four force summing devices.(Nov/Dec 11)

 Bourdon tube
 Bellows
 Capsule
 Diaphragm
8) When do you call an instrument to be intelligent? (May/Jun 13)
The system can able to process and gives their output its own calibration by sensing.
Automatic operation done by all the system by using various sensors. These systems
are called intelligent.
9) What is known as thermocouple effect & how do you use it in a
transducer?(May/Jun13)
It is a thermoelectric transducer which converts the thermal energy into an electrical
energy.
It is mostly used as primary transducer for temperature measurement where
thermocouple directly converts changes in temperature into an electrical
signal.

Thermocouple comes under class of active transducer.

10) What is Transducer? (Dec 2010)

A transducer is a device which converts physical energy in to electrical energy.
Eg. LVDT , Strain guage, thermistor etc.

11) What are the materials used for piezoelectric transducers? (Dec 2009)
Some of the piezoelectric materials are
• Rochelle salt
• Ammonium Dihydrogen Phosphate (ADP)
• Quartz
12) What is an active tr n ducer? (May 2010)
An active transducer generates an electrical signal directly in response to the
physical parameter and does not require external power source for its operation. Eg.
Tachogenerators, piezoelectric crystals

PART-B
1.Explain the classification and characteristics of transducer.
Transducers may be classified according to their structure, method of energy
conversion and application. Thus we can say that transducers are classified
• As active and passive transducer
 • According to transduction principle
• As analog and digital transducer
• As primary and secondary transducer
• As transducer and inverse transducer
Active and Passive Transducer
Active Transducers
Active transducers are self-generating type of transducers. These transducers
develop an electrical parameter (i.e. voltage or current) which is proportional to the
quantity under measurement. These transducers do not require any external source
or power for their operation. They can be subdivided into the following commonly
used types

Passive Transducers
Passive transducers do not generate any electrical signal by themselves. To obtain an
electrical signal from such transducers, an external source of power is essential.
Passive transducers depend upon the change in an electrical parameter (R, L, or C).
They are also known as externally power driven transducers. They can be subdivided
in o the following commonly used types.

According to Transduction Principle

The transducers can be classified according to principle used in transduction.
• Capacitive transduction
• Electromagnetic transduction
• Inductive transduction
• Piezoelectric transduction
• Photovoltaic transduction
• Photoconductive transduction

Analog and Digital Transducers

The transducers can be classified on the basis of the output which may be a
continuous function of time or the output may be in discrete steps.

Analog Transducers
These transducers convert the input quantity into an analog output which is a
continuous function of time.

Digital Transducers
Digital transducers produce an electrical output in the form of pulses which
forms an unique code. Unique code is generated for each discrete value sensed.

Primary or Secondary
 Some transducers consist of mechanical device along with the electrical device.
In such transducers mechanical device acts as a primary transducer and converts
physical quantity into mechanical signal. The electrical device then converts
mechanical signal produced by primary transducer into an electrical signal.
 Therefore, electrical device acts as a secondary transducer. For an example, in
pressure measurement Bourdons tube acts as a primary transducer which converts a
pressure into displacement and LVDT acts as a secondary transducer which converts
this displacement into an equivalent electrical signal.
Transducer and Inverse Transducer
Transducers convert non-electrical quantity into electrical quantity whereas
inverse transducer converts electrical quantity into on- l ctrical quantity. For example,
microphone is a transducer which converts sound signal into an electrical signal
whereas loudspeaker is an inverse transducer which converts electrical s g al into
sound signal.

Advantages of Electrical Transducers

1. Electrical signal obtained from electrical transducer can be easily processed (mainly
amplified) and brought to a level suitable for output device which may be an indicator
or recorder.
2. The electrical systems can be controlled with a very small level of power
3. The electrical output can be easily used, transmitted, and processed for the purpose
of measurement.
4. With the advent of IC technology, the electronic systems have become extremely
small in size, requiring small space for their operation.
5. No moving mechanical parts are involved in the electrical systems. Therefore there
is no question of mechanical wear and tear and no possibility of mechanical failure.
Electrical transducer is almost a must in this modem world. Apart from the merits
described above, some disadvantages do exist in electrical sensors.

Disadvantages of Electrical Transducers

Ø The electrical transducer is sometimes less reliable than mechanical type because
of the ageing and drift of the active components.
Ø Also, the sensing elements and the associated signal processing circuitry are
comparatively expensive.
Ø With the use of better materials, improved technology and circuitry, the range of
accuracy and stability have been increased for electrical transducers.
Ø Using negative feedback technique, the accuracy of measurement and the stability
of the system are improved, but all at the expense of increased circuit complexity,
more space, and obviously, more cost.
2) Explain the construction of RTD and thermistor with neat sketches.
Temperature Sensors
Temperature is one of the fundamental parameters indicating the physical condition of
matter, i.e. expressing its degree of hotness or coldness. Whenever a body is heat’
various effects are observed. They include

• Change in the physical or chemical state, (freezing, melting, boiling etc.)

• Change in physical dimensions,
• Changes in electrical properties, mainly the change in resistance,
• Generation of an emf at the junction of two dissimilar metals.

One of these effects can be employed for temperature measurement purposes.

Electrical methods are the most convenient and accurate methods of temperature
measurement. These methods are based on change in resistance with temperature
and generation of thermal e.m.f. The change in resistance with temperature may be
positive or negative. According to that there are two types

• Resistance Thermometers —Positive temperature coefficient

• Thermistors —Negative temperature coefficient

Construction of Resistance Thermometers

The wire resistance thermometer usually consists of a coil wound on a mica or
ceramic former, as shown in the Fig. The coil is wound in bifilar form so as to make it
no inductive. Such coils are available in different sizes and with different resistance
values ranging from 10 ohms to 25,000 ohms.
Advantages of Resistance Thermometers
1. The measurement is accurate.
2. Indicators, recorders can be directly operated.
3. The temperature sensor can be easily installed and replaced.
4. Measurement of differential temperature is possible.
5. Resistance thermometers can work over a wide range of temperature from -20’ C to
+ 650° C.
6. They are suitable for remote indication.
7. They are smaller in size
8. They have stability over long periods of time.

Limitations of Resistance Thermometers

1. A bridge circuit with external power source is necessary for their operation.
2. They are comparatively costly.

Thermistors
Thermistor is a contraction of a term ‘ thermal-resistors’ .Thermistors are
semiconductor device which behave as thermal resistors having negative temperature
coefficient i.e. their resistance decreases as temperature increases.

Construction of Thermistor
Thermistors are composed of a sintered mixture of metallic oxides, manganese,
nickel, cobalt, copper, iron, and uranium. Their resistances at temperature may range
from 100 to 100k. Thermistors are available in variety of shapes and sizes. Smallest in
size are the beads with a diameter of 0.15 mm to 1.25 mm. Beads may be sealed in
the tips of solid glass rods to form probes. Disks and washers are made by pressing
thermistor material under high pressure into flat cylindrical shapes. Washers can be
placed in series or in p r llel to increase power dissipation rating. Thermistors are well
suited for precision temperature measurement, temperature control, and temperature
compensation, because of their very large change in resistance with temperature.
They are widely used for measurements in the temperature range -100 C to +100 C

Advantages of Thermistor
1. Small size and low cost.
2. Comparatively large change in resistance for a given change n temperature
3. Fast response over a narrow temperature range.

Limitations of Thermistor
1. The resistance versus temperature characteristic is highly non-linear.
2. Not suitable over a wide temperature range.
3. Because of high resistance of thermistor, shielded cables have to be used to
minimize interference.

Applications of Thermistor
1. The thermistors relatively large resistance change per degree change in
temperature
2. The high sensitivity, together with the relatively high thermistor resistance that may
be selected [e.g. 100k .], makes the thermistor ideal for remote measurement or
control. Thermistor control systems are inherently sensitive, stable, and fast acting,
and they require relatively simple circuitry.
3. Because thermistors have a negative temperature coefficient of resistance,
thermistors are widely used to compensate for the effects of temperature on circuit
performance.
4. Measurement of conductivity.

3) Explain in detail the principle, working and features of LVDT with neat sketch.

 An LVDT, or Linear Variable Differential Transformer, is a transducer that

converts a linear displacement or position from a mechanical reference (or zero) into
a proportional electrical signal containing phase (for direction) and amplitude
information (for distance).
 The LVDT operation does not require electrical contact between the moving
part (probe or core rod assembly) and the transformer, but rather relies on
electromagnetic coupling; this and the fact that they operate without any built-in
electronic circuitry are the primary reasons why LVDTs have been widely used in
applications where long life and high reliability under severe environments are a
required, such as Military/Aerospace applications.
 The LVDT consists of a primary coil (of magnet wire) wound over the whole
length of a non-ferromagnetic bore liner (or spool tube) or a cylindrical coil form. Two
secondary coils are wound on top of the primary coil for “long stroke” LVDTs (i.e. for
actuator main RAM) or each side of the primary coil for “Short stroke” LVDTs (i.e. for
electro-hydraulic servo-valve or EHSV).


The two secondary windings are typically connected “opposite series”
(or) wound in opposite rotational directions). A ferromagnetic core, which length is a
fraction of the bore liner length, magnetically couples the primary to the secondary
winding urns that are located above the length of the core.

 When the primary coil is excited with a sine wave voltage (Vin), it generate a
variable magnetic field which, concentrated by the core, induces the secondary
voltages (also sine waves).
 While the second ry windings are designed so that the differential output
voltage (Va-Vb) is proportional to the core position from null, the phase angle
(close to 0 degree or close to 180 degrees depending of direction) determines
the direction away from the mechanical zero. The zero is defined as the core
position where the phase angle of the (Va-Vb) differential output is 90 degrees.
 The differential output between the two seco dary outputs (Va-Vb) when the
core is at the mechanical zero (or “Null Position”) is called the Null Voltage; as
the phase angle at null position is 90 degrees, the Null Voltage is a “quadrature”
voltage.
This residual voltage is due to the complex nature of the LVDT electrical model, which
includes the parasitic capacitances of the windings.

3) With block diagram, explain DAS in detail.

Definition
Data acquisition is the process of real world physical conditions and conversion
of the resulting samples into digital numeric values that can be manipulated by a
computer. Data acquisition and data acquisition systems (abbreviated with the
acronym DAS) typically involves the conversion of analog waveforms into digital
values for processing. The components of data acquisition systems include:

i) Sensors that convert physical parameters to electrical signals.

ii) Signal conditioning circuitry to convert sensor signals into a form that can be
converted to digital values.
iii) Analog-to-digital converters, which convert conditioned sensor signals to digital
values.
Explanation
Data acquisition is the process of extracting, transforming, and transporting
data from the source systems and external data sources to the data processing
system to be displayed, analyzed, and stored.

A data acquisition system (DAQ) typically consist of transducers for asserting

and measuring electrical signals, signal conditioning logic to perform amplification,
isolation, and filtering, and other hardware for receiving analog signals and providing
them to a processing system, such as a personal computer.

Data acquisition systems are used to perform a variety of functions, including

laboratory research, process monitoring and control, data logging, analytical
chemistry, tests and analysis of physical phenomena, and control of mechanical or
electrical machinery.

Data recorders are used in a wide variety of applications for imprinting various
types of forms, and documents.

Data collection systems or data log ers ge erally include memory chips or strip
charts for electronic recording, probes or sensors which m asure product
environmental parameters and are connected to the data logger.

Hand-held portable data collection systems permit in field data collection for up-
to-date information processing.

Source
Data acquisition begins with the physical phenomenon or physical property o be
measured. Examples of this include temperature, light intensity, gas pressure, fluid
flow, and force. Regardless of the type of physical property to be measured, the
physical state that is to be measured must first be transformed into a unified form that
can be sampled by a data acquisition system.

The task of performing such transformations falls on devices called sensors. A

sensor, which is a type of transducer, is a device that converts a physical property into
a corresponding electrical signal (e.g., a voltage or current) or, in many cases, into a
corresponding electrical characteristic (e.g., resistance or capacitance) that can easily
be converted to electrical signal.

The ability of a data acquisition system to measure differing properties depends

on having sensors that are suited to detect the various properties to be measured.
There are specific sensors for many different applications.
 DAQ systems also employ various signal conditioning techniques to adequately
modify various different electrical signals into voltage that can then be digitized
using an Analog-to-digital converter (ADC).

Transducer 1 Sensor 1

Transducer 2 Sensor 2

MUX A/D

Transducer 3
Sensor 3

Signals
 Signals may be digital (also called lo ic s gnals sometimes) or analog
depending on the transducer used. Signal conditioning may be necessary if the signal
from the transducer is not suitable for the DAQ hardware being us d.
 The signal may need to be amplified, filtered or demodulated. Various other
examples of signal conditioning might be bridge completion, providi curre t or voltage
excitation to the sensor, isolation, and linearization.

 For transmission purposes, single ended analog signals, which are more
susceptible to noise can be converted to differential signals. Once digitized, the signal
can be encoded to reduce and correct transmission errors.
DAQ hardware

 DAQ hardware is what usually interfaces between the signal and a PC. It could
be in the form of modules that can be connected to the computer's ports (parallel,
serial, USB, etc.) or cards connected to slots (S-100 bus, Apple Bus, ISA, MCA, PCI,
PCI-E, etc.) in the mother board.
Usually the space on the back of a PCI card is too small for all the connections
needed, so an external breakout box is required. The cable between this box and the
PC can be expensive due to the many wires, and the required shielding DAQ cards
often contain multiple components (multiplexer, ADC, DAC, TTL-IO, high speed
timers, RAM).

 These are accessible via a bus by a microcontroller, which can run small
programs. A controller is more flexible than a hard wired logic, yet cheaper than a
CPU so that it is alright to block it with simple polling loops.

 The fixed connection with the PC allows for comfortable compilation and
debugging. Using an external housing a modular design with slots in a bus can grow
with the needs of the user.
 Not all DAQ hardware has to run permanently connected to a PC, for example
intelligent stand-alone loggers and oscilloscopes, which can be operated from a PC,
yet they can operate completely independent of the PC.

DAQ software
DAQ software is needed in order for the DAQ hardware to work with a PC. The
device driver performs low-level register writes and reads on the hardware, while
exposing a standard API for developing user applications. A standard API such as
COMEDI allows the same user applications to run on different operating systems, e.g.
a user application that runs on Windows will also run on Linux and BSD.

Advantages
 Reduced data redundancy
 Reduced updating errors and increased consistency
 Greater data integrity and independence from applications programs
 Improved data access to users through use of host and query languages
 Improved data security
 Reduced data entry, storage, and retrieval costs
 Facilitated development of new applications program

Disadvantages
 Database systems are complex, difficult, and time-consuming to design
 Substantial hardware and software start-up costs
 Damage to database affects virtually all applications programs
 Extensive conversion costs in moving form a file-based system to a database
system
 Initial training required for all programmers and users
5. Explain the principle of operation a) Piezoelectric transducer.(16) (April/may
2011) (May/June 2013)

Piezoelectric Transducers

Piezoelectric transducers produce an output voltage when a force is applied to

them. They are frequently used as ultrasonic receivers and also as displacement
transducers, particularly as part of devices measuring acceleration, force and
pressure. In ultra- sonic receivers, the sinusoidal amplitude variations in the ultrasound
wave received are translated into sinusoidal changes in the amplitude of the force
applied to the piezoelectric transducer. In a similar way, the translational movement in
a displacement transducer is caused by mechanical means to apply a force to the
piezoelectric transducer. Piezoelectric transducers are made from piezoelectric
materials. These have n asymmetrical lattice of molecules that distorts when a
mechanical force is applied to it. This distortion causes a reorientation of electric
charges within the material, resulting in a relative displacement of positive and
negative charges. The charge displacement induces surface charges on the material
of opposite polarity between the two sides. By implanting electrodes i to the surface of
the material, these surface charges can be measured as an output voltage. For a
rectangular block of material, the induced voltage is given by:

V = kFd/A
 Where F is the applied force in g, A is the area of the material in mm, d is the
thickness of the material and k is the piezoelectric constant. The polarity of the
induced voltage depends on whether the material is compressed or stretched.

 Where F is the applied force in g, A is the area of the material in mm, d is the
thickness of the material and k is the piezoelectric constant. The polarity of the
induced voltage depends on whether the material is compressed or stretched.

 Materials exhibiting piezoelectric behavior include natural ones such as quartz,

synthetic ones such as lithium sulphate and ferroelectric ceramics such as barium
titanate. The piezoelectric constant varies widely between different materials.
 Typical values of k are 2.3 for quartz and 140 for barium titanate. Applying
equation (13.1) for a force of 1 g applied to a crystal of area 100 mm2 and thickness 1
mm gives an output of 23 µV for quartz and 1 .4 mV for barium titanate. The
piezoelectric principle is invertible, and therefore distortion in a piezoelectric material
can be caused by applying a voltage to it.
 This is commonly used in ultrasonic transmitters, where the application of a
sinusoidal voltage at a frequency in the ultra- sound range causes a sinusoidal
variation in the thickness of the material and results in a sound wave being emitted at
the chosen frequency.

PART C(15 MARKS)

1. Give a short notes on smart sensors.(A/M2016)
A smart sensor is a sensor with local processing power that enables it to react to local
conditions without having to refer back to a central controller. Smart sensors are
usually at least twice as accurate as non-smart devices, have reduced maintenance
cos s and require less wiring to the site where they are used. In addition, long-term
stability is improved, reducing the required calibration frequency.

The functions possessed by smart sensors vary widely, but consist of at least some of
the following:

Remote calibration capability Self-diagnosis of faults Automatic calculation of

measurement accuracy and compensation for random errors Adjustment for
measurement of non-linearity’s to produce a linear output Compensation for the
loading effect of the measuring process on the measured system.

Calibration capability
Self-calibration is very simple in some cases. Sensors with an electrical output can use
a known reference voltage level to carry out self-calibration. Also, load-cell types of
sensor, which are used in weighing systems, can adjust the output reading to zero
when there is no applied mass. In the case of other sensors, two methods of self-
calibration are possible, use of a look-up table and an interpolation technique.
Unfortunately, a look-up table requires a large memory capacity to store correction
points. Also, a large amount of data has to be gathered from the sensor during
calibration. In consequence, the interpolation calibration technique is preferable. This
uses an interpolation method to calculate the correction required to any particular
measurement and only requires a small matrix of calibration points (van der Horn,
1996).

Self-diagnosis of faults
 Smart sensors perform self-diagnosis by monitoring internal signals for
evidence of faults. Whilst it is difficult to achieve a sensor that can carry out self-
diagnosis of all possible faults that might arise, it is often possible to make simple
checks that detect many of the more common faults.
 One example of self-diagnosis in a sensor is measuring the sheath capacitance
and resistance in insulated thermocouples to detect breakdown of the insulation.
Usually, a specific code is generated to indicate each type of possible fault (e.g. a
failing of insulation in a device).
 One difficulty that often arises in self-diagnosis is in differentiating between
normal measurement deviations and sensor faults. Some smart sensors overcome
this by storing multiple measured values around a set-point, calculating minimum and
maximum expected values for the measured quantity.

 Uncertainty techniques can be applied to measure the impact of a sensor fault

on measurement quality. This makes it possible in certain circumstances to continue
to use a sensor after it has developed a fault.
 A scheme for generating a validity index has been proposed that indicates the
validity and quality of a measurement from a sensor (Henry, 1995).

Automatic calculation of measurement accuracy and compensation for

random errors

Many smart sensors can calculate measurement accuracy on-line by computing

the Mean over a number of measurements and analyzing all factors affecting accuracy.
This averaging process also serves to greatly reduce the magnitude of
random measurement errors.

Adjustment for measurement non-linearities

In the case of sensors that have a non-linear relationship between the
measured quantity and the sensor output, digital processing can convert the output to
a linear form, providing that the nature of the non-linearity is known so that an
equation describing it can be programmed into the sensor.

General Architecture of smart sensor:

One can easily propose a general architecture of smart sensor from its definition,
functions. From the definition of smart sensor it seems that it is similar to a data
acquisition system, the only difference being the presence of complete system on a
single silicon chip. In addition to this it has on–chip offset and temperature
compensation. A general architecture of smart sensor consists of following important
components:

o Sensing element/tr n duction element,

o Amplifier,
o Sample and hold,
o Analog multiplexer,
o Analog to digital converter (ADC),
o Offset and temperature compensation,
o Digital to analog converter (DAC),
o Memory,
o Serial communication
o Processor

The generalized architecture of smart sensor is shown below:

 Architecture of smart sensor is shown. In the archit ctu e shown A1, A2…An
and S/H1, S/H2…S/Hn are the amplifiers and sample and hold c rcuit corresponding
to different sensing element respectively.
 So as to get a digital form of an analog signal the analog signal is p riodically
sampled (its instantaneous value is acquired by circuit), and that constant value is
held and is converted into a digital words. Any type of ADC must contain or
proceeded by, a circuit that holds the voltage at the input to the ADC converter
constant during the entire conversion time.

 Conversion times vary widely, from nanoseconds (for flash ADCs) to

microseconds (successive approximation ADC) to hundreds of microseconds (for
dual slope integrator ADCs). ADC starts conversion when it receives start of
conversion signal (SOC) from the processor and after conversion is over it gives end
of conversion signal to the processor. Outputs of all the sample and hold circuits are
multiplexed together so that we can use a single ADC, which will reduce the cost of
the chip.

 Offset compensation and correction comprises of an ADC for measuring a

reference voltage and other for the zero. Dedicating two channels of the multiplexer
and using only one ADC for whole system can avoid the addition of ADC for this.
  This is helpful in offset correction and zero compensation of gain due to
temperature drifts of acquisition chain. In addition to this smart sensor also include
internal memory so that we can store the data and program required.
2) i) Discuss any one method of A/D converter.(Nov/Dec14)

Analogue-To-Digital Converters
Important factors in the design of an analogue-to-digital converter are the speed
of conversion and the number of digital bits used to represent the analogue signal
level. The minimum number of bits used in analogue-to-digital converters is eight.

 The use of eight bits means that the analogue signal can be represented to a
resolution of 1 part in 256 if the input signal is carefully scaled to make full use of the
converter range.
 However, it is more common to use either 10 bit or 12 bit analogue-to-digital
converters, which give resolutions respectively of 1 part in 1024 and 1 part in 4096.
Several types of analogue-to-digital converter exist. These differ in the technique
used to effect signal conversion, in operational speed, and in cost.
 The simplest type of analogue-to-digital converter is the counter analogue-to-
digital converter, as shown in Figure 5.23. This, like most types of analogue-to-digital
converter, does not convert continuously, but in a stop-start mode triggered by
special signals on the computer’s control bus.
 At the start of each conversion cycle, the counter is set to zero. The digital
counter value is converted to an analogue signal by a digital- to-analogue converter (a
discussion of digital-to-analogue converters follows in the next
section), and comparator then compares this analogue counter value with the
unknown analogue signal. The output of the comparator forms one of the inputs to an
AND logic gate.

 The other input to the AND gate is a sequence of clock pulses. The comparator
acts as a switch that can turn on and off the passage of pulses from the clock through
the AND gate. The output of the AND gate is connected to the input of the digital
counter.
 Following reset of the counter at the start of the conversion cycle, clock pulses
are applied continuously to the counter through the AND gate, and the analogue
signal at the output of the digital-to-analogue converter gradually increases in
magnitude.
 At some point in time, this analogue signal becomes equal in magnitude to the
unknown signal at the input to the comparator. The output of the comparator changes
state in consequence, closing the AND gate and stopping further increments of the
counter. At this point, the value held in the counter is a digital representation of the
level of the unknown analogue signal.

2)ii)Explain the working of D/A converter with a neat diagram(Apr/May14)

Digital-To-Analogue (D/A) Conversion

Digital-to-analogue conversion is much simpler to achieve than analogue-to-digital

conversion and the cost of building the necessary hardware circuit is considerably
less. It is required wherever a digitally processed signal has to be presented to an
analogue control actuator or an analogue signal display device. A common form of
digital-to-analogue converter is illustrated in Figure 5.24. This is shown with 8 bits for
simplicity of explanation, although in practice 10 and 12 bit D/A converters are used
more frequently. This form of D/A converter consists of a resistor-ladder network on
the input to an operational amplifier

V0 to V7 are set at either the reference voltage level Vref or at zero volts according to
whether an associated switch is open or closed. Each switch is controlled by the logic
level of one of the bits 0 – 7 of the 8 bit binary signal being converted. A particular
switch is open if the relevant binary bit has a value of 0 and closed if the value is 1.
Consider for example a digital signal with binary value of 11010100. The values of V7
to V0 are therefore:
3.Explain about the characteristics and selection of transducers.
Characteristics of Transducer
1. Accuracy: It is defined as the closeness with which the reading approaches an
accepted standard value or ideal value or true value, of the variable being measured.
2. Ruggedness: The transducer should be mechanically rugged to withstand
overloads. It should have overload protection.
3. Linearity: The output of the transducer should be linearly proportional to the input
quantity under measurement. It should have linear input - output characteristic. -
4. Repeatability: The output of the transducer must be exactly the same, under same
environmental conditions, when the same quantity is applied at the input repeatedly.
5. High output: The transducer should give reasonably high output signal so that it
can be easily processed and measured. The output must be much larger than noise.
Now-a-days, digital output is preferred in many applications;
6. High Stability and Reliability: The output of the transducer should be highly stable
and reliable so that there will be minimum error in measurement. The output must
remain unaffected by environmental conditions such as change in temperature,
pressure, etc.
7. Sensitivity: The sensitivity of the electrical transducer is defined as the electrical
output obtained per unit change in the physical parameter of the input quantity. For
example, for a transducer used for temperature measurement, sensitivity will be
expressed in mV/’ C. A high sensitivity is always desirable for a given transducer.
8. Dynamic Range: For a transducer, the operating range should be wide, so that it
can be used over ideal range of measurement conditions.
9. Size: The transducer should have smallest possible size and shape with minimal
weight and volume. This will make the measurement system very compact.
10. Speed of Response: It is the rapidity with which the transducer responds to
changes in the measured quantity. The speed of response of the transducer should be
as high as practicable.

Selection of Transducer:
1.Operating range: The range of the transducer should be large enough to
encompass all the expected magnitudes of the measurand.

2.Sensitivity: The transducer should give a sufficient output signal per unit of m
assured input in order to yield meaningful data.

3.Electrical output characteristics: The electrical characteristics of transducer viz.

output impedance, 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.

4.Environmental conditions: The transducer should be immune to environmental

conditions such as change in temperature, humidity, vibration and shock ,acceleration
and corrosive chemicals.
5.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.

6.Accuracy: Accuracy of the transducer should be within the specified range so that
input in the specified range can be reliably measured.