Electronic Instrumentation and control systems

Electronic
Instrumentation and
Control
By: Dr. Ankita Malhotra
Dr. Ankita Malhotra
SVKM's D J Sanghvi College of
Engineering

Unit 1:
PRINCIPAL OF MEASUREMENT,
TESTING AND MEASURING
INSRUMENTS
Dr. Ankita Malhotra
Engineering

Measurement
The measurement of a given quantity is essentially an act or the result of
comparison between the quantity (whose magnitude is unknown) and a
predefined standard. Since two quantities are compared, the result is
expressed in numerical values.
In order that the results of the measurement are meaningful, there are two
basic requirements :
(i) The standard used for comparison purposes must be accurately defined
and should be commonly accepted, ·
And
(ii)The apparatus used and the method adopted must be provable
Dr. Ankita Malhotra
Engineering

Significance of measurement-
 To validate and understand the new hypothesis, phenomenon
and relations between them
 To pave the way for new discoveries backed by earlier
measurements.
Dr. Ankita Malhotra
Engineering

Methods of measurement
• In these methods, the unknown quantity (also called
the measurand) is directly compared against a
standard.
• Direct methods are used to measure physical
quantities like length, mass and time.
Direct
Methods
• Used when using direct methods is not feasible.
Also they are less accurate (dependent on human
factors)
• Electronic measurements are based on indirect
methods where transducer converts the physical
quantity to be measured in an analogous
form(electrical form) and then measured.
Indirect
Methods
Dr. Ankita Malhotra
Engineering

Generalized Measurement System
*Data/signal conditioning includes :
A/D converter, amplifiers, filters, modulators, attenuators and processes like
addition, subtraction, integration, differentiation etc.
*Data presentation element includes:
analog/digital display, recorders, storage devices, processors, printers etc.
Fig: Block diagram of generalized measurement system
Dr. Ankita Malhotra
Engineering

Performance Characteristics of
Measuring Systems/Instruments
 Measurement involves using an instrument as a physical means of
determining a quantity or variable. In simple cases, an instrument consists
of a single unit which gives an output signal according to the unknown
variable applied to it.
 The knowledge of performance characteristics of an instrument/measuring
device is essential for minimizing the errors and to select the most suitable
instrument for specific measurements.
 It can be divided into two distinct categories
Static characteristics
Dynamic Characteristics
Dr. Ankita Malhotra
Engineering

Static Characteristics of
Instruments
1. Accuracy: It is the closeness with which an instrument
reading approaches the true value of the quantity being
measured.
i) Point accuracy,
ii) Accuracy as percentage of scale range,
iii) Accuracy as percentage of true value.
2. Precision : It is a measure of the· reproducibility of the
measurement, that is given a fixed value of variable. Precision
is a measure of the degree to which successive measurements
differ from each other.
Dr. Ankita Malhotra
Engineering

3. Linearity : The characteristic of the instrument which defines
that the output is linearly proportional to the input. This is the most
desirable characteristic of an instrument because the conversion
from a linear scale reading to the corresponding measured value of
input quantity is most convenient if one merely has to multiply by a
fixed constant rather than consult a non-linear calibration curve or
compute from non-linear calibration equations. Also when the
instrument is part of a large data or complex system, linear behavior
of the part often simplifies the design and analysis of the whole
system.
Dr. Ankita Malhotra
Engineering

4.Sensitivity: Sensitivity of an instrument or an instrumentation
system is the ratio of the magnitude of the output signal/response to
the magnitude of input. signal /quantity being measured.
It is the ratio of change in output of an instrument to the change in
input.
 Sensitivity=
 Inverse sensitivity (deflection factor)=
Fig : sensitivity of an instrument
Dr. Ankita Malhotra
Engineering

5- Hysteresis: Hysteresis is a phenomenon which depicts different
output effects while loading and unloading.
Hysteresis takes place due to the fact, that all the energy put into
the stressed parts when loading is not recoverable while unloading.
When the input of an instrument is varied from zero to its full scale
and then if the input is decreased from its full scale value to zero,
the output varies.
Fig: Hysteresis effects
Dr. Ankita Malhotra
Engineering

 Threshold: Threshold is the smallest
measurable input, below which no output
change can be identified.
 Dead Time: It is the time required for the
instrument to respond to the change in output.
For a certain range of inputs, the output value
does not change. Dead time is the time before
which the instrument starts to respond after the
output has been changed.
 Dead Zone: It is the largest change of input
quantity for which there is no output.
 Resolution:
It is the smallest change in the measured value to which the instrument will
respond. It is smallest increment in the input value that can be detected by
the instrument. Thus, resolution defines the smallest measurable input
change while threshold defined the smallest measurable input.
Dr. Ankita Malhotra
Engineering

Many measurements are concerned with rapidly varying quantities
and therefore, for such cases we must examine the dynamic relations
which exist between the output and input. This is normally done
with the help of differential equations. Performance criteria based
upon dynamic relations constitute the dynamic characteristics.
 Dynamic Response Transient and Steady-state response
Dr. Ankita Malhotra
Engineering

Dynamic response cntd.
When an input is applied to an instrument or a measurement
system, the instrument or the system cannot take up its final
steady state position immediately. On the other hand, the
system go through a transient state before it finally settles to
its final 'steady state’ position.
Some measurements are made under conditions that sufficient
time is available for the instrument or the measurement
system to settle to its final steady state conditions. Under such
conditions the study of behavior of the system under transient
state, called 'transient response' is not of much of importance ;
only steady state response of the system need be considered.
Dr. Ankita Malhotra
Engineering

 Invariably measurement systems are subjected to inputs which are
not static but dynamic in nature. i.e. the inputs vary with time
 Since the input varies from instant to instant, so does the output.
The behavior of the system under such conditions is described by
the dynamic response of the system.
 The variations in the input, that are used practically to achieve
dynamic behavior are :
i)Step input, ii) Ramp input ,iii) Parabolic input iv)Sinusoidal input:
 *Note: The systems exhibit a characteristic sluggishness on account of
presence of energy storage elements like capacitance, inductance etc.
Dr. Ankita Malhotra
Engineering

 The dynamic characteristics of any measurement system are :
(i) Speed of response (ii) Lag
(iii) Fidelity (iv) Dynamic error
 Speed of Response. It is the rapidity with which an instrument responds to
changes in the measured quantity. Response Time is defined as the time
required by instrument or system to settle to its final steady position after
the application of the input.
 Lag: Measuring lag is defined as the delay in the response of an
instrument to a change in the measured quantity.
 The lags are of two types :
1. Retardation lag : As soon as there is a change in the measured
quantity, the measurement system begins to respond.
2. Time delay : The response of the measurement system starts after a
dead time, once the input is applied. They cause dynamic error.
Dr. Ankita Malhotra
Engineering

 Fidelity: Fidelity of a system is defined as the ability of the
system to reproduce the output in the same form as the input.
 Dynamic Error. It is the difference between , the true value of
the quantity changing with time and the value indicated by the
instrument if no static error is assumed. However, the total
dynamic error of the instrument is the combination of its
fidelity and the time lag or phase difference between input and
output of the system.
Dr. Ankita Malhotra
Engineering

Calibration
 To ensure proper functioning of closed loop feedback systems,
calibration is one of the essential requirements.
 Calibration is a condition and also a process.
 Calibration is the condition of an electronic instrument when its
output or response is as close as possible to the specified value.
 Calibration is a process of examination. To calibrate an instrument
means to determine how close to the true value its indication or
output really is. This process also includes the correction and/or
documentation of degree of error.
The signals produced by various instruments may be altered and
create undesirable results due to wearing or aging of components.
These instruments are designed so that adjustments can be made to
compensate for the deterioration of their internal components. The
process of making these adjustments is called 'calibration'.
Dr. Ankita Malhotra
Engineering

Need of calibration
 The value of internal· electronic components of the instruments
changes as well as its mechanical parts wear. This results in
malfunctioning of the instruments. Calibration is required to
overcome this problem.
 Calibration procedures are performed for other reasons, such as :
◦ Before new installation
◦ After repair
◦ After extended shut down
◦ After product repair
Dr. Ankita Malhotra
Engineering

Types of calibration
 Three-point calibration: The measurements are made at three
points and compared with the defined standards. The three points
chosen are:
Starting point, end point and centre point of the measurement scale.
Disadvantage: cannot accurately calibrate the instrument over
the entire scale, as it cannot ensure proper linear response of the
system.
 Five-point calibration: Measurements are made at Test point
values of 10,30, 50, 70 and 90 percent of the instruments input
ranges. It is more accurate.
Dr. Ankita Malhotra
Engineering

Errors
 The error of an instrument is the algebraic difference between the observed
value and the true value of the quantity being measured.
 Error may be expressed either as absolute or as percentage of error.
TYPES OF ERRORS:
 Gross errors :human errors
 Systematic errors: Fixed errors due to components
 Schematic errors : Static and Dynamic
Static errors due to limitations of instrument
Dynamic errors due to delay in instrument’s response
 Random errors: Due to unknown causes. These errors differ in
values when repeated measurements of the same variable is made.
Dr. Ankita Malhotra
Engineering

Limiting Errors
 The accuracy of an instrument, we know is specified by the
manufacturer within a certain percentage of the full scale reading.
The components for e.g. resistor, capacitor, inductor are guaranteed
to be within some percentage of their rated value. This percentage
shows the deviation of that quantity from its specified value. This
deviation from the specified value is called as limiting error.
 These errors are also called as guarantee errors.
 For e.g. : if the value of a capacitor specified by the manufacture is
4.7 μF with a tolerance of ± 5% then the actual value of capacitance
is guaranteed to be within these limits.
Dr. Ankita Malhotra
Engineering

Measurement of resistance
 Low resistance: All resistance, of the order of 1 ohm and
below are classified as low resistance. In practice such
resistance, are· present in armature and series windings of
large machines, ammeter shunts, cable lengths, contacts etc.
 Medium resistance :This class includes resistances from about
1 ohm upwards to about 1,00,000 ohms. The majority of
electrical apparatus have resistance in this range.
 High resistance: The resistances of the order of 1,00,000 ohm
and above are classified as high resistance. The following
methods are explained in this section for measurement of low,
medium and high resistances.
a) Ohmmeter b) Wheatstone's bridge
c)Kelvin's double bridge d)Megger
Dr. Ankita Malhotra
Engineering

Kelvin’s Double bridge
This method is one of the best
available method for the precise
measurement of low resistance.
It is known as double bridge as
it incorporates two sets of ratio
arms. One set of ratio arms is
composed R1,R2 and the other
set is composed of Ra, Rb.
Rx is the resistor to be
measured· and R is adjustable
resistor having very small
tolerance.
The ratio R1/R2 is kept same Ra/Rb
Dr. Ankita Malhotra
Engineering

 For zero deflection of galvanometer
Ry is resistance of connecting lead between points m and n.
Substituting value of E:
Similarly:
But
k
l p
o
m n
E
I
k
I1
I2
Dr. Ankita Malhotra
Engineering

Dr. Ankita Malhotra
Engineering

The known resistance R3 is adjusted such that galvanometer indicates
zero and then using above equation Rx can be calculated.
Dr. Ankita Malhotra
Engineering

Advantages of Kelvin’s bridge
 In a typical Kelvin's double bridge the range of resistance covered
is lΩto 10μΩ with an accuracy of± 0.05% to± 0.2%.
 The resistance of the connecting lead R, has no effect on
measurement
Dr. Ankita Malhotra
Engineering

Wheatstone’s Bridge
 This is the best and commonest method of measuring medium
resistance.
The galvanometer is a sensitive micro ammeter with a zero centre scale.
When there is no current through the meter, the galvanometer pointer rests
at 0, i.e. mid scale. Current in one direction causes the pointer to deflect on
one side and current in the opposite direction deflects it to the other side.
Dr. Ankita Malhotra
Engineering

 The resistors R1 R2 and R3 are known and Rx is unknown resistor
to be measured.
 The bridge is balanced when there is no current through
galvanometer or when the potential at points C and D are equal i.e.
the potential across galvanometer is zero.
 When bridge is balanced:
Dr. Ankita Malhotra
Engineering

 For measurement of unknown resistor Rx, R3 is adjusted such that
bridge is balanced and galvanometer current is zero. Then using
above equation, the unknown resistor Rx is calculated.
R3 is precision resistor having very small tolerance.
 When the bridge is in unbalanced condition, current flows through
the galvanometer, causing deflection of its pointer.
To determine current flowing through galvanometer when bridge is
unbalanced, Thevenin' s theorem can be used.
Dr. Ankita Malhotra
Engineering

Thevenin’s equivalent to find current
flowing through Galvanometer
Dr. Ankita Malhotra
Engineering

Thevenin's equivalent circuit for
unbalanced Wheatstone's bridge
If Rg is the internal resistance of
galvanometer, then Ig is given by:
Dr. Ankita Malhotra
Engineering

Mega ohm Bridge
 This method is used for measuring high resistance above 50MΩ
 It is called as mega ohmmeter
 Megger is a portable ohmmeter with built-in high voltage source.
Working Principle of Ohmmeter
The instrument is connected with a
battery, a series adjustable resistor and an
instrument which gives the reading. The
resistance to be measured is connected at
terminal ab. When the circuit is completed
by connecting output resistance, the
circuit current flows and so the deflection
is measured.
Dr. Ankita Malhotra
Engineering

D’arsonval Movement
 This type of instruments consists of a
permanent magnet and a coil which
carries current and is placed in between
them.
 Due to high intensity magnetic fields the
deflecting torque produced is of large
value due to which sensitivity of the
meter is also increased.
 If the direction of current is reversed in
these types of instruments, then torque
direction will also be reversed so these
types of instruments are applicable in
DC measurements only. The deflecting
torque is directly proportional to the
deflection angle hence these types of
instruments have the linear scale.
Dr. Ankita Malhotra
Engineering

 There are many advantages due to which we use D’Arsonval type
instrument. They are-
 They have uniform scale.
 Effective eddy current damping.
 Low power consumption.
 No hysteresis loss.
 They are not affected by stray fields.
However they suffer from drawbacks such as-
 It cannot be used with AC signal.
 Costlier compared to MI instruments.
 There may be error due to ageing of springs by which we may not
get accurate result.
Dr. Ankita Malhotra
Engineering

Basic types of ohmmeter
Dr. Ankita Malhotra
Engineering

Mega ohmmeter (Megger)
Megger is essentially a portable ohmmeter with built-in high voltage source.
Dr. Ankita Malhotra
Engineering

 It mainly has two elements, a magnet type dc generator to supply
current for making measurement and an ohmmeter which measures
the resistance value.
 The meter used differs slightly from the standard D' Arsonoval
movement. It has two windings.
 Coil A is the current coil with one terminal connected to negative
output and other connected in series with R1 to the test lead P2• It is
wound in such a way that it moves the pointer towards the zero end
of the scale when current flows through it from generator.
 Coil B is the voltage coil and is connected across the generator
output through resistance R. This coil is wound such that the pointer
moves towards the high resistance end of the scale when current
flows through it from generator.
Dr. Ankita Malhotra
Engineering

 The two coils are mounted on the same shaft but at right angles to each
other.
 The unknown resistance Rx is connected between test leads P1 and P2.
 If the test leads are left open, no current flows in coil A and coil B
alone moves the pointer as current flows through it. Coil B moves a
pointer such that it indicates infinity or open.
 When the unknown resistor Rx is connected across P 1 and. P 2,
current flows from the generator· though coil A, resistors R1 and. Rx·
The corresponding torque developed moves the pointer away from the
infinity position, into a field of gradually increasing strength, until the
torque fields between coils A and B are equal.
 The value of R1 is so chosen that even if the line terminals are a short
circuited, coil A does not get damaged.
Dr. Ankita Malhotra
Engineering

Measurement of inductance
Dr. Ankita Malhotra
Engineering

Maxwell’s Inductance Bridge
 This bridge circuit measures an inductance by comparison with a
variable standard self inductance.
The condition for balance of bridge
require that there should be no current
through the detector. This requires,
that the potential difference between
points b and d should be
zero i.e. E1 = E3.
Dr. Ankita Malhotra
Engineering

In the circuit:
Dr. Ankita Malhotra
Engineering

 Comparing real and imaginary part:
 Note – To measure unknown Lx; R2 and L2 are adjusted to keep the
bridge balanced
Dr. Ankita Malhotra
Engineering

Maxwell's Inductance-Capacitance
Bridge
 It measures unknown inductance in terms of known capacitance
Dr. Ankita Malhotra
Engineering

 When bridge is balanced
 Equating real and imaginary parts:
Dr. Ankita Malhotra
Engineering

 Thus the measurement of unknown inductance is independent of the
excitation frequency. The scale of the resistance can be calibrated to
read inductance directly.
 Quality factor (Q) for inductance is,
 For high values of Q, R 1 becomes excessively large and it is impractical
to obtain a satisfactory variable standard resistance in the range. Therefore
Maxwell's bridge is suitable for measurement of inductance with low Q
values.
 For measurement of inductance,. unknown inductor is connected in one of
the arms and resistor R1 and capacitor C1 are adjusted to balance the
bridge.
Dr. Ankita Malhotra
Engineering

Advantages of Maxwell’s bridge
 The equation for Rx and Lx are independent.
 The measurement is independent of frequency.
 The bridge yields simple expression for inductance Lx and
resistance Rx·
 It is very useful for measurement of a wide range . of inductance at
power and audio frequencies.
 It can measure inductance from 1 H to 1000 H with-± 2 % error.
Dr. Ankita Malhotra
Engineering

Disadvantages of Maxwell’s bridge
 This bridge requires a variable standard capacitor which may be
expensive if calibrated to a high degree of accuracy.
 The bridge is limited to measurement of low Q vaIues {1 < Q. <
10). The measurement of high Q coils demand a large value for
resistance R1.
 The Maxwell's bride is also unsuited for coils with very low Q
values i.e. Q < 1.
Dr. Ankita Malhotra
Engineering

Hay's Bridge
 This method of measurement is particularly suited for the
measurement of inductance having high Q Values
Hay's bridge differs
from Maxwell's bridge
by having a resistance
R1 in series with a
capacitor C1 instead of
being parallel
Dr. Ankita Malhotra
Engineering

 When bridge is balanced,
Dr. Ankita Malhotra
Engineering

 Equating real and imaginary parts:
 Putting the value of Lx from equation 2) in equation 1), we get:
1)
2)
3)
Dr. Ankita Malhotra
Engineering

 Putting the value of Rx from equation 3) to equation 2)
 As ω appears in the expression for Lx, this bridge is frequency
sensitive.
 Quality factor (Q) for capacitor is
 Substituting in equation 4):
4)
Dr. Ankita Malhotra
Engineering

For Q>10,1/Q2 can be neglected, thus:
(Similar to Maxwell's bridge)
For Q less than 10, the 1/Q2 term can not be neglected. Hence this
bridge is not suited for measurement of inductors having Q less than 10.
Dr. Ankita Malhotra
Engineering

 Advantages :
 (1) It can measure inductances with high Q i.e. Q > 10.
 (2) A commercial bridge measures inductance from 1 J1H to 100 H
with± 2% error.
 (3) It can be used for measuring incremental inductance.
 Disadvantages·:
 (1)The unknown value of inductance depends on loss of
inductor(Q) and also on operating frequency.
 (2)It can not measure inductance with low Q i.e. Q < 10
Dr. Ankita Malhotra
Engineering

Measurement of Capacitance
Dr. Ankita Malhotra
Engineering

Schering's Bridge
 It measures capacitances and their
insulating properties precisely.
 This bridge is widely used for testing
small capacitors at low voltage with very
high precision.
 The standard capacitor c3 is a high
quality mica capacitor with low loss for
general measurements or an air capacitor
having a very stable value and a very
small electric field for insulation-
measurement.
V
 For measuring unknown capacitor, it is connected to one of the arms
of the bridge and C1 and R2 are adjusted to obtain bridge balance.
Dr. Ankita Malhotra
Engineering

 When bridge is balance,
Thus
Dr. Ankita Malhotra
Engineering

 Comparing real and imaginary parts:
Thus, once bridge is balanced, unknown capacitance can be calculated
using above equation
The dissipation factor D, which is inverse of quality factor of a series
RC circuit is given by
Dr. Ankita Malhotra
Engineering

Advantages
 Commercial bridge measure from 100 pf -1 μF with± 2% accuracy.
 The direct reading for Cx can be obtained by graduating C3
accordingly, if resistance ratio is maintained at a fixed value.
 It can measure small capacitors at low voltage precisely.
Dr. Ankita Malhotra
Engineering

UNIT -2
SENSORS and TRANSDUCERS
Dr. Ankita Malhotra
Engineering

Transducers and their
characteristics
 Transducer: A device which converts one form of energy into the
other form.
Characteristics of Transducers:
 Ruggedness : It is the· ability of a transducer to withstand
overloads. A good transducer must have a high degree of
ruggedness.
 Linearity : It is the ability of transducer to reproduce input-output
characteristics symmetrically and linearly. Overall linearity is the
main factor considered.
 Repeatability : It is. the· ability of a transducer to reproduce the
output signal exactly when the same input is applied repeatedly
under the same environmental conditions.
Dr. Ankita Malhotra
Engineering

 Accuracy : It is defined as closeness of the actual output produced
by a transducer to the ideal or true value of the quantity being
measured. It should be high for any transducer.
 High stability and reliability :There should be a minimum·
amount of error in measurement and it should be unaffected by
temperature, vibrations and other environmental variations.
 Speed of response : It shows how quickly a transducer responds to
the changes in the quantity being measured. The speed of response
should be as high as possible.
 Sensitivity : The sensitivity of a transducer is defined as the output
produced per unit change in the input quantity being measured. For
example the _sensitivity of a thermocouple is expressed in m V /°C.
The sensitivity should be as high as possible.
 Size : A transducer must have small size, proper shape and
minimum volume so that it can be placed at any location for
measurements . Dr. Ankita Malhotra
Engineering

Classification of transducers
Dr. Ankita Malhotra
Engineering

Mechanical and electrical
transducers
 Output is either mechanical or electrical in
nature
 E.g. of mechanical transducer: springs,
pendulum etc.
 E.g. of electrical transducer:
potentiometer, electromagnet etc.
Dr. Ankita Malhotra
Engineering

Active Transducers
These transducers do not need any external source of power for
their operation. Therefore they are also called as self generating
type transducers.
Dr. Ankita Malhotra
Engineering

Passive Transducers
 These transducers need external power supply for their operation.
So they are not self-generating type" transducers.
 A DC power supply or an ,audio frequency generator is used as an
external power source.
Dr. Ankita Malhotra
Engineering

Classification Based on the
Quantity to be Measured
Dr. Ankita Malhotra
Engineering

Analog and Digital Transducers
 The output is either analog or digital in nature.
 E.g. of analog transducer: thermocouple, strain gauge etc.
 E.g. of digital transducer: signal encoders
Dr. Ankita Malhotra
Engineering

Primary and secondary
transducers
 Primary transducers: Which convert the primary input, which
is mostly the physical quantity into other form.
 Secondary transducers: which convert secondary input,
processed by primary transducer into the desired form
Dr. Ankita Malhotra
Engineering

Transducer and Inverse
Transducer
 Transducers convert non-electrical quantity to electrical
quantity while inverse transducers convert electrical quantity
to a non electrical quantity;
Dr. Ankita Malhotra
Engineering

According-to Transduction Principle
 1. Capacitive transduction: In
capacitive transduction transducers, the
measurand is converted to a change in
the capacitance.
 2. Electromagnetic transduction: In
electromagnetic transduction, the
measurand is converted to voltage
induced 1n conductor by change in the
magnetic flux, in absense of excitation.
 The electromagnetic transducers are
self generating active transducers.
Dr. Ankita Malhotra
Engineering

 3. Inductive induction :In inductive
transduction, the measured is
converted into a change in the self
inductance of a single coil. It is
achieved by displacing the core of
the coil that is attached to a
mechanical sensing element
 4. Piezo electric induction: In
piezoelectric induction the
measurand is converted into a
change· in· electrostatic charge q or
voltage V generated by crystals when
mechanically· it is stressed
Dr. Ankita Malhotra
Engineering

5. Photovoltaic transduction:In photovoltaic
transduction ·the measurand is converted to
voltage generated when the junction
between dissimilar materials is illuminated
6.Photo conductive transduction:In
photoconductive transduction the
measurand is converted to change in
resistance of semiconductor material by the
change in light incident on the material.
Dr. Ankita Malhotra
Engineering

Transducer Selection Factor
 1. Nature of measurement: The selection of a transducer depends
on the nature of quantity to be measured e.g. for measuring
capacitance the capacitance sensors need to be used .
2. Cost and availability
3. Measuring system compatibility : The transducer selected must
be compatible with the electrical system used. The output
impedance of the transducer and the measuring system must not
affect each other.
4. Loading effect-: If the transducer affects the value to be
measured, errors may be introduced in the measured. Hence, in
order to ·minimize. the errors the loading effect should be
minimum.
Dr. Ankita Malhotra
Engineering

5. Environmental considerations: output can be affected by
electromagnetic interference, shock , temperature etc.
6. Operating range: The transducer must be selected such that it
provides good resolution and range.
7. Sensitivity: It should be as high as possible.
Dr. Ankita Malhotra
Engineering

Sensors
 Sensors are the devices used to sense changes in a physical quantity in the
surrounding environment in which they are kept.
 Sensors detect the presence of energy, changes in or the transfer of energy.
Sensors detect by receiving a signal from a device such as a transducer, then
responding to that signal by converting it into an output that can easily be read
and understood. Typically sensors convert a recognized signal into an electrical
– analog or digital – output that is readable. In other words, a transducer
converts one form of energy into another while the sensor that the
transducer is part of converts the output of the transducer to a readable
format.
 Transducers convert one form of energy to another, but they do not
quantify the conversions. The light bulb converts electrical energy into
light and heat; however, it does not quantify how much light or heat. A
battery converts chemical energy into electrical energy but it does not quantify
exactly how much electrical energy is being converted. If the purpose of a
device is to quantify an energy level, it is a sensor.
Dr. Ankita Malhotra
Engineering

Examples of sensors
Thermal sensors:
Thermometer, Thermocouple gauge etc.
Mechanical Sensors : Pressure sensor ,Barometer ,Altimeter , Gas
flow sensor , Accelerometer etc.
Electrical Sensors: Ohmmeter, Voltmeter, galvanometer etc.
Optical sensors: photo detectors, photo resistors, infra-red detectors
etc.
Others: motion sensors, chemical sensors etc.
Dr. Ankita Malhotra
Engineering

Inductive Sensor
 Inductive sensors use currents induced by magnetic fields to detect
nearby metal objects.
 The inductive sensor uses a coil (an inductor) to generate a high
frequency magnetic field as shown below. If there is a metal object
near the changing magnetic field, induced current (eddy current) will
flow in the object.
This resulting eddy current
flow sets up a new magnetic
field that opposes the original
magnetic field. The net effect
is that it changes the
inductance of the coil in the
inductive sensor.
Dr. Ankita Malhotra
Engineering

 Single-ended eddycurren~ type inductive
sensor
Single-ended eddy current type inductive sensor
Push-pull eddy current type inductive sensor Dr. Ankita Malhotra
Engineering

Inductive sensor
 Inductive sensors are used in the industry to avoid
collisions/detect collision or to detect part position.
 Their range is limited by the magnetic field of the sensor.
Dr. Ankita Malhotra
Engineering

Capacitive sensors
 Capacitive sensors are primary sensors for measurement of
displacements. They are also known as proximity sensors in the
sense that they measure the nearness of an object without any
mechanical coupling between them.
 They can detect displacement of metallic and non-metallic objects
both.
 They are commonly used in touch switches.
 It is non-loading, noncontact and non-invasive type sensor for
displacement measurements.
Dr. Ankita Malhotra
Engineering

 Displacements conveyed to the movable plate may be so arranged as to
either vary A which is the area common to the plates or alter the extent of
penetration of a dielectric material inside the plates. The capacitance of
such sensors is as low as 1 PF and with the help of suitable electrical
circuitry, it is possible to detect very small variations in capacitance and
measure displacements of the order of 25 nm.
 Capacitive sensors with cylindrical electrodes are popular for
measurements of pressure of fluids and level of fluids and granular
materials.
 For composition measurement, variation effect of dielectric .90nstant due
to variation in composition, absorption of moisture and other effects is
used. The dielectric constant of certain insulators and semiconductors
varies with temperature. This effect is used for measurement of
temperature. Dr. Ankita Malhotra
Engineering

Capacitive Pressure Sensors
 It consists of a fixed plate and a movable plate. The movable plate
is either a metallic diaphragm or the membrane.
 Capacitive pressure sensor using diaphragm is shown in diagram.
Due to deformation of the clamped diaphragm for the deflection y
at any radius r from the centre of the diaphragm, the capacitance
changes.
Dr. Ankita Malhotra
Engineering

 Consider an annular element of width dr at a distance r from the
centre. Its capacitance can be given by
 For small deflections with y/d << l,
d = initial spacing between the electrodes without any pressure
difference Δp
y = displacement of the annular element from its initial position
Dr. Ankita Malhotra
Engineering

The total capacitance due to the deflected diaphragm is given by
For a diaphragm, the deflection y at any radius r from the centre is
given by:
The fractional change in capacitance can be given by
Dr. Ankita Malhotra
Engineering

Cylindrical capacitive sensor for
pressure measurements
The capacitance Co of the sensor is given by
Dr. Ankita Malhotra
Engineering

 The dielectric constants of gases and liquids vary
with pressure. To measure pressure, of such fluids,
the cylindrical electrode arrangement shown in figure
is used so that the pressure is continuously measured.
under flowing conditions. The walls of the metallic
pipe are used as the electrode and a· solid
cylindrical rod running along the pip serve as the
inner electrode .
Dr. Ankita Malhotra
Engineering

Capacitive Displacement sensor
Single ended capacitive displacement sensor
The capacitive displacement sensor is fundamentally a proximity sensor
If the sensor has ·a solid insulating material of dielectric constant ε, the
capacitance is given by
Dr. Ankita Malhotra
Engineering

 If the air gap is decreased by Δx, the capacitance increases by ΔC
which is given by,
 Fractional change in capacitance:
Dr. Ankita Malhotra
Engineering

Push-pull type capacitive displacement
The range of linearity can be extended to a large extent by using
push-pull sensor.
Unity-ratio arm Wheatstone bridge circuit may be used for
measuring ΔC/Co.
Dr. Ankita Malhotra
Engineering

Potentiometers
 A resistive potentiometer (Pot) consists of a resistance element with
a sliding contact, called a wiper. The motion of sliding contact may
be translatory or rotational or a combination of both.
Dr. Ankita Malhotra
Engineering

 Under no load condition:
Where Vo=Output voltage under no load condition
Ei = Input voltage
Rx =Resistance after displacement of contactor
Rp = Full resistance of potentiometer
For the same position of contactor, output voltage will get reduced if
voltmeter has finite input resistance. In that case, fraction value x' is
given by:
Voltmeter with
resistance RL
Voʹ = voltage under
loading
Dr. Ankita Malhotra
Engineering

Let,
Dr. Ankita Malhotra
Engineering

 The error is given by:
Percentage error =
Dr. Ankita Malhotra
Engineering

Advantages:
 1.They are inexpensive.
 2. They are simple to operate and are very useful for applications
where the requirements are not particularly severe.
 3. They are useful for the measurement of large amplitudes of
displacement.
 4. Electrical efficiency is very high and they provide sufficient
output to allow control operations.
Disadvantages:
 1. When using a linear potentiometer, a large force is required to
move the sliding contacts.
 2. The sliding contacts can wear out, become misaligned and
generate noise.
Dr. Ankita Malhotra
Engineering

Linear Variable Differential Transformers
(LVDT) for Measurement of Pressure and
Displacement
It is a variable inductance displacement transducer.
The LVDT consists of a primary winding and two identical secondary
windings. These windings are axially spaced. A rod shaped magnetic core
is positioned centrally inside the coil assembly. This rod provides a low
reluctance path for the magnetic flux linking the coils (windings). The
moving object, displacement of which is to be measured is coupled to this
movable rod.
Dr. Ankita Malhotra
Engineering

Equivalent circuit
The output voltage is given by:
Transfer characteristics of LVDT
Dr. Ankita Malhotra
Engineering

Operation of LVDT :
 The primary winding is connected to the ac source.
 Assume that the core is exactly at the centre of the coil assembly.
Then the flux linked to both the secondary windings will be equal.
Due to equal flux linkage, secondary induced voltages are equal but
they have opposite polarities.
eo= 0
 Now if the core is displaced from its null position towards
secondary-1 then the flux linked to secondary-1 increases and flux
linked to secondary-2 decreases.
eo1 > eo2
When core is at centre Null position
eo = positive
Dr. Ankita Malhotra
Engineering

 If the core is displaced towards the secondary-2 then flux liked to
secondary-2 will be more than secondary-1
eo2 > eo1
 Thus the magnitude of output signal Is made to vary "linearly'' with
the mechanical displacement. Hence the word "Linear," is used in
LVDT. The output is obtained "differentially" between the two
secondary windings. Hence the word "differential" is used in LVDT.
eo = negative
Dr. Ankita Malhotra
Engineering

Operation of LVDT
Dr. Ankita Malhotra
Engineering

Performance characteristics of LVDT
 i) Null voltage: Ideally, the output of L VDT should be zero when
its core is at null position. But practically a small residual voltage
(called as null voltage) is observed at the null position of the core.
This is due to the presence of harmonics in the output of excitation
source and stray capacitance between the primary and secondary
windings.
 ii) Resolution: If we assume that the L VDT is frictionless then it
can respond to very minute movement of the core to produce a
proportional output voltage. This makes the resolution of the LVDT
ideally infinite.
Dr. Ankita Malhotra
Engineering

 iii) Linearity : As is clear from transfer characteristics of, the
LVDT response is quite linear.
 iv) Sensitivity :it is typically 1 to 2 m V /0.01 mm for the LVDT.
 v) Excitation voltage and excitation frequency :Maximum
excitation voltage is limited by the maximum primary current
which can be allowed to flow.
The excitation frequency should be precisely adjusted to a
frequency where the best possible Sensitivity of detection can be
obtained. This frequency is between 1 kHz to 10 kHz.
 vi) Dynamic response: This indicates how fast the LVDT responds
to the displacement of the core. Dynamic response is 'dependent on
the excitation frequency and the core weight.
Dr. Ankita Malhotra
Engineering

Advantages of LVDT
 1. Very fine resolution
 2. High accuracy
 3. Very good stability
 4. Linearity of transfer characteristic(better than 0.25 %) .
 5. Ease of fabrication and installation
 6. Ability to operate at high temperature_
 7. High sensitivity (2mVNolt/10 microns at 4kHz excitation).
Dr. Ankita Malhotra
Engineering

Disadvantages:
 1. LVDT is sensitive to the external magnetic fields. To minimize
this effect magnetic shielding is necessary.
 2. Complicated circuitry is needed.
 3. Due to mass of the core, LVDT is not suitable for dynamic
measurement (fast displacements).
 4. Larger displacements are needed to get appreciable differential
output.
Applications: In addition to displacement measurement the L VDT
is used in measurement of pressure, load, acceleration, force, weight
etc.
Dr. Ankita Malhotra
Engineering

Rotary Variable Differential
Transformer (RVDT) :
 It is used to sense angular displacement. It is similar to L VDT
except the shape of core and may be rotated between its windings
by means of a shaft.
Dr. Ankita Malhotra
Engineering

 At the null position of its core, the output voltage of secondary
windings S1 and S2 are equal and in opposition. Therefore the net
output is zero.
 Any angular displacement from the null position will result in a
differential voltage output. Hence the response of transducer is
linear.
 Clockwise rotation produces an increasing voltage of a secondary
winding of one phase while anticlockwise rotation produces an
increasing voltage of opposite phase.
Dr. Ankita Malhotra
Engineering

Strain Gauges Dr. Ankita Malhotra
Engineering

Electronic Instrumentation and control systems

More Related Content

Similar to Electronic Instrumentation and control systems (20)

Recently uploaded (20)

Electronic Instrumentation and control systems