INTERNAL ASSESSMENT EXAMINATION II ANSWER KEY …

INTERNAL ASSESSMENT EXAMINATION – II ANSWER KEY

COURSE: B.E – CSE

OAN551 – SENSORS AND TRANSDUCERS

IAE 1 ANSWER KEY Page 1

PART A - (10 X 1 = 10 marks)

1. _____ is the time required to come to an output value within the specified error level.

(a) Settling time (b) Rise time (c) Response time (d) Delay time

2. A piezo-electrical crystal generates voltage when subjected to ____ force.

(a) Electrical (b) Mechanical (c) Gravity (d) Magnetic

3. An instrument that is capable of measuring only DC is _______.

(a) Moving Coil (b) Moving iron (c) Thermocouple (d) Thermistor

4. Magnetic materials can be tested by _______.

(a) Self inductance bridge (b) Cambell Bridge (c) AC Pot (d) LVDT

5. The household energy meter is ________ instrument.

(a) Indicating (b) Recording (c) Integrating (d) Digital

6. Resistances can be measured with the help of ________.

(a) Ohm meter (b) Ammeter (c) Voltmeter (d) Wattmeter

7. Thermocouple generate output voltage according to _________.

(a) Temperature (b) Humidity (c) Voltage (d) Current

8. Following is (are) the type(s) of Light sensor(s) ________.

(a) Ultrasonic (b) LVDT (c) Photodiode (d) RVDT

9 Smallest change which a sensor can detect is _________.

(a) Resolution (b) Accuracy (c) Precision (d) Stability

10 The sensors are classified on the basis of _________.

(a) Function (b) Performance (c) Output (d) All the above

PART B - (10 X 2 = 20 marks)

11. Label out primary and secondary standards.

Primary standards are made to the highest metrological quality and are the definitive definition

or realization of their unit of measure. The next quality standard in the hierarchy is known as

a secondary standard. Secondary standards are calibrated with reference to a primary standard.

12. List out the type of instrumental errors.

Loading error

Environmental Error

13. Outline the elements of a generalized measurement system.

Primary Sensing Element

Variable Conversion Element

Data Presentation Element

14. Recall the performance evaluation techniques of sensors.

Sensing

Planning

Acting


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15. Write down the advantages of electronic instruments over mechanical instruments.

They are less prone to failure meaning they are reliable at all times

They are more power efficient

They are more portable in the sense that :

They can be moved around easily

They can be used in a variety of power supply all over the world

16. Draw LVDT and list out the parts of LVDT.

17. How will you justify potentiometer as motion sensor?

The continuous coils were then cut to length and assembled into Helipot 10 turn potentiometers.

This was an instrument that would measure inches of Helipot coil windings coming off the continuous

winding machines. It would check that the coils had the proper resistance via the wire spacing on the

core wire.

18. Classify the different types of POT.

Linear potentiometer

Rotational potentiometer

Multi-turn potentiometer

19. Infer the periodical calibration of measuring instrument necessary.

Calibration is a comparison between a known measurement (the standard) and

the measurement using your instrument. Typically, the accuracy of the standard should be ten times the

accuracy of the measuring device being tested. It checks the accuracy of the instrument and it

determines the traceability of the measurement.

20. The output voltage of a LVDT is 1.5V at maximum displacement. At a load of 0.5 MΩ, the deviation

from linearity is maximum and it is ±0.003V from a straight line through origin. Find the linearity at

given load.

PART C - (5 X 14 = 70 marks)

21. (a) Describe the general measurement system of an instrument with neat block diagram.

Generalized measurement system is a system that is comprised of the typical elements of a

measurement system. It helps to understand how a measurement system works.

A generalized measurement system consists of the following components:

1. Primary Sensing Element

2. Variable Conversion Element

3. Variable Manipulation Element

4. Data Processing Element

5. Data Transmission System

6. Data Presentation Element


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In addition to the above components, a measurement system may also have a data storage

element to store measured data for future use. As the above six components are the most common

ones used in many measurement systems, they are discussed in detail below:

1. Primary Sensing Element:

The primary sensing element receives signal of the physical quantity to be measured as input. It

converts the signal to a suitable form (electrical, mechanical or other form), so that it becomes easier

for other elements of the measurement system, to either convert or manipulate it.

2. Variable Conversion Element:

Variable conversion element converts the output of the primary sensing element to a more suitable

form. It is used only if necessary.

3. Variable Manipulation Element:

Variable manipulation element manipulates and amplifies the output of the variable conversion

element. It also removes noise (if present) in the signal.

4. Data Processing Element:

Data processing element is an important element used in many measurement systems. It processes

the data signal received from the variable manipulation element and produces suitable output.

Data processing element may also be used to compare the measured value with a standard value to

produce required output.

5. Data Transmission System:

Data Transmission System is simply used for transmitting data from one element to another. It acts

as a communication link between different elements of the measurement system. Some of the data

transmission elements used is cables, wireless antennae, transducers, telemetry systems etc.

6. Data Presentation Element:

It is used to present the measured physical quantity in a human readable form to the observer. It

receives processed signal from data processing element and presents the data in a human readable

form. LED displays are most commonly used as data presentation elements in many measurement


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systems.

OR

(b) (i) Classify the standards and give example for each level of standard.

Here is a three-level hierarchy of physical measurement standards

Primary standards are made to the highest metrological quality and are the definitive

definition or realization of their unit of measure

The next quality standard in the hierarchy is known as a secondary standard. Secondary

standards are calibrated with reference to a primary standard

The third level of standard, a standard which is periodically calibrated against a secondary

standard, is known as a working standard

Working standards are used for the calibration of commercial and industrial measurement

equipment

Primary Standard:

An example of a primary standard was the international prototype kilogram (IPK) which

was the master kilogram and the primary mass standard for the International System of

Units (SI)

Secondary reference standard:

Secondary reference standards are very close approximations of primary reference

standards

For example, major national measuring laboratories such as the US's National Institute of

Standards and Technology (NIST) will hold several "national standard" kilograms, which

are periodically calibrated against the IPK and each other

Working standard:

A machine shop will have physical working standards (gauge blocks for example) that are

used for checking its measuring instruments

Working standards and certified reference materials used in commerce and industry have a

traceable relationship to the secondary and primary standards

Working standards are expected to deteriorate, and are no longer considered traceable to a

national standard after a time period or use count expires

Laboratory standard:

National organizations provide calibration and private industrial laboratories with items,

processes and/or certification so they can provide certified traceability to national standards

(In the United States, NIST operates the NVLAP program)

These laboratory standards are kept in controlled conditions to maintain their precision,

and used as a reference for calibration and creating working standards

Sometimes they are (incorrectly) called "secondary standards" because of their high quality

and reference suitability

https://en.wikipedia.org/wiki/Realisation_(metrology)

https://en.wikipedia.org/wiki/Kilogram#International_prototype_kilogram

https://en.wikipedia.org/wiki/Kilogram

https://en.wikipedia.org/wiki/International_System_of_Units

https://en.wikipedia.org/wiki/International_System_of_Units

https://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technology

https://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technology

https://en.wikipedia.org/wiki/Gauge_block

https://en.wikipedia.org/wiki/Certified_reference_materials


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(ii) Elaborate in detail about the statistical methods of error analysis with example.

Accuracy is the closeness of agreement between a measured value and a true or accepted

value. Measurement error is the amount of inaccuracy.

Precision is a measure of how well a result can be determined (without reference to a

theoretical or true value). It is the degree of consistency and agreement among independent

measurements of the same quantity; also the reliability or reproducibility of the result

The uncertainty estimate associated with a measurement should account for both the

accuracy and precision of the measurement

Random errors are statistical fluctuations (in either direction) in the measured data due to

the precision limitations of the measurement device. Random errors can be evaluated

through statistical analysis and can be reduced by averaging over a large number of

observations

Systematic errors are reproducible inaccuracies that are consistently in the same direction.

These errors are difficult to detect and cannot be analyzed statistically

If a systematic error is identified when calibrating against a standard, applying a correction

or correction factor to compensate for the effect can reduce the bias

Unlike random errors, systematic errors cannot be detected or reduced by increasing the

number of observations

Gross personal errors, sometimes called mistakes or blunders, should be avoided and

corrected if discovered. As a rule, personal errors are excluded from the error analysis

discussion because it is generally assumed that the experimental result was obtained by

following correct procedures. The term human error should also be avoided in error

analysis discussions because it is too general to be useful.

The uncertainty of a single measurement is limited by the precision and accuracy of the

measuring instrument, along with any other factors that might affect the ability of the

experimenter to make the measurement.

The number of significant figures implies an approximate relative uncertainty:

1 significant figure suggests a relative uncertainty of about 10% to 100%

2 significant figures suggest a relative uncertainty of about 1% to 10%

3 significant figures suggest a relative uncertainty of about 0.1% to 1%

Type A evaluation of standard uncertainty - method of evaluation of uncertainty by the

statistical analysis of a series of observations. This method primarily

includes random errors.

Type B evaluation of standard uncertainty - method of evaluation of uncertainty by

means other than the statistical analysis of series of observations. This method

includes systematic errors and any other uncertainty factors that the experimenter believes

are important.

22. (a) Briefly explain the categories of sensors.

There are several classifications of sensors made by different authors and experts. Some are

very simple and some are very complex. The following classification of sensors may already be

used by an expert in the subject but this is a very simple classification of sensors.

In the first classification of the sensors, they are divided in to Active and Passive. Active

Sensors are those which require an external excitation signal or a power signal.

Passive Sensors, on the other hand, do not require any external power signal and directly


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generates output response.

The other type of classification is based on the means of detection used in the sensor. Some

of the means of detection are Electric, Biological, Chemical, Radioactive etc.

The next classification is based on conversion phenomenon i.e. the input and the output.

Some of the common conversion phenomena are Photoelectric, Thermoelectric,

Electrochemical, Electromagnetic, Thermooptic, etc.

The final classification of the sensors are Analog and Digital Sensors. Analog Sensors

produce an analog output i.e. a continuous output signal with respect to the quantity being

measured.

Digital Sensors, in contrast to Analog Sensors, work with discrete or digital data. The data in

digital sensors, which is used for conversion and transmission, is digital in nature.

Different Types of Sensors

The following is a list of different types of sensors that are commonly used in various applications.

All these sensors are used for measuring one of the physical properties like Temperature,

Resistance, Capacitance, Conduction, Heat Transfer etc.

Temperature Sensor

Proximity Sensor

Accelerometer

IR Sensor (Infrared Sensor)

Pressure Sensor

Light Sensor

Ultrasonic Sensor

Smoke, Gas and Alcohol Sensor

Touch Sensor

Color Sensor

Humidity Sensor

Tilt Sensor

Flow and Level Sensor

(b) Summarize the classification of transducers in detail.

A device which converts a physical quantity into the proportional electrical signal is called a

transducer. The electrical signal produced may be a voltage, current or frequency. A transducer uses

many effects to produce such conversion. The process of transforming signal from one form to other

is called transduction. A transducer is also called pick up. The transduction element transforms the

output of the sensor to an electrical output, as shown in the Fig.

A transducer will have basically two main components. They are

1. Sensing Element The physical quantity or its rate of change is sensed and responded to by this

part of the transistor.

2. Transduction Element The output of the sensing element is passed on to the transduction element.

This element is responsible for converting the non-electrical signal into its proportional electrical


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signal.

There may be cases when the transduction element performs the action of both transduction

and sensing. The best example of such a transducer is a thermocouple. A thermocouple is used to

generate a voltage corresponding to the heat that is generated at the junction of two dissimilar

metals.

Classification of Transducers:

The Classification of Transducers is done in many ways. Some of the criteria for the

classification are based on their area of application, Method of energy conversion, Nature of output

signal, According to Electrical principles involved, Electrical parameter used, principle of operation,

& Typical applications. The transducers can be classified broadly,

i. On the basis of transduction form used

ii. As primary and secondary transducers

iii. As active and passive transducers

iv. As transducers and inverse transducers

Resistive Transducers:

1. Resistance Strain Gauge – The change in value of resistance of metal semi-conductor due to

elongation or compression is known by the measurement of torque, displacement or force.

2. Resistance Thermometer – The change in resistance of metal wire due to the change in

temperature known by the measurement of temperature

3. Resistance Hygrometer – The change in the resistance of conductive strip due to the change of

moisture content is known by the value of its corresponding humidity

4. Hot Wire Meter – The change in resistance of a heating element due to convection cooling of a

flow of gas is known by its corresponding gas flow or pressure

5. Photoconductive Cell – The change in resistance of a cell due to a corresponding change in light

flux is known by its corresponding light intensity

6. Thermistor – The change in resistance of a semi-conductor that has a negative co-efficient of

resistance is known by its corresponding measure of temperature

7. Potentiometer Type – The change in resistance of a potentiometer reading due to the movement of

the slider as a part of an external force applied is known by its corresponding pressure or

displacement

Capacitance Transducers:

1. Variable capacitance pressure gage - Principle of operation: Distance between two parallel plates

is varied by an externally applied force Applications: Measurement of Displacement, pressure

2. Capacitor microphone Principle of operation: Sound pressure varies the capacitance between a

fixed plate and a movable diaphragm. Applications: Speech, music, noise

3. Dielectric gauge Principle of operation: Variation in capacitance by changes in the dielectric.

Applications: Liquid level, thickness

Inductance Transducers:

1. Magnetic circuit transducer Principle of operation: Self inductance or mutual inductance of ac-

excited coil is varied by changes in the magnetic circuit. Applications: Pressure, displacement

2. Reluctance pickup Principle of operation: Reluctance of the magnetic circuit is varied by

changing the position of the iron core of a coil. Applications: Pressure, displacement, vibration,

position

3. Differential transformer Principle of operation: The differential voltage of two secondary

windings of a transformer is varied by positioning the magnetic core through an externally

applied force. Applications: Pressure, force, displacement, position


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4. Eddy current gage Principle of operation: Inductance of a coil is varied by the proximity of an

eddy current plate. Applications: Displacement, thickness

5. Magnetostriction gauge Principle of operation: Magnetic properties are varied by pressure and

stress. Applications: Force, pressure, sound

Voltage and current Transducers:

1. Hall effect pickup Principle of operation: A potential difference is generated across a

semiconductor plate (germanium) when magnetic flux interacts with an applied current.

Applications: Magnetic flux, current

2. Ionization chamber Principle of operation: Electron flow induced by ionization of gas due to

radioactive radiation. Applications: Particle counting, radiation

3. Photoemissive cell Principle of operation: Electron emission due to incident radiation on

photoemissive surface. Applications: Light and radiation

4. Photomultiplier tube Principle of operation: Secondary electron emission due to incident radiation

on photosensitive cathode.

Applications: Light and radiation, photo-sensitive relays

23. (a) A linear resistance potentiometer is 50 mm long and is uniformly wound with a wire having a resistance

of 10000 Ω. Under normal conditions, the slider is at the centre of the potentiometer. Find the linear

displacement when the resistance of the potentiometer as measured by a Wheatstone bridge for the two

cases are:

(i) 3850 Ω (ii)7560 Ω

Are the two displacements in the same direction?

If it is possible to measure a minimum value of 10 Ω resistance with the above arrangement , find the

resolution of the potentiometer in mm.


COURSE: B.E – CSE



OR

(b) A Capacitive transducer uses two quartz diaphragms of area 75 mm2 separated by a distance of 3.5 mm.

A pressure of 900 kN/m2 when applied to the top diaphragm produces a deflection of 0.6 mm. The

capacitance is 370 pF when no pressure is applied to the diaphragms. Find the value of capacitance after

application of pressure 900 kN/m2.

24. (a) Give an overview of the inductive transducer explaining the principle of operation with neat sketch.

Inductance Transducers:

1. Magnetic circuit transducer Principle of operation: Self inductance or mutual inductance of ac-

excited coil is varied by changes in the magnetic circuit. Applications: Pressure, displacement

2. Reluctance pickup Principle of operation: Reluctance of the magnetic circuit is varied by

changing the position of the iron core of a coil. Applications: Pressure, displacement, vibration,

position

3. Differential transformer Principle of operation: The differential voltage of two secondary

windings of a transformer is varied by positioning the magnetic core through an externally

applied force. Applications: Pressure, force, displacement, position

4. Eddy current gage Principle of operation: Inductance of a coil is varied by the proximity of an

eddy current plate. Applications: Displacement, thickness


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5. Magnetostriction gauge Principle of operation: Magnetic properties are varied by pressure and

stress. Applications: Force, pressure, sound

Inductive transducers work on the principle of inductance change due to any appreciable

change in the quantity to be measured i.e. measured.

For example, LVDT, a kind of inductive transducers, measures displacement in terms of

voltage difference between its two secondary voltages.

Secondary voltages are nothing but the result of induction due to the flux change in the

secondary coil with the displacement of the iron bar.

Anyway LVDT is discussed here briefly to explain the principle of inductive transducer.

LVDT will be explained in other article in more detail.

Hence inductive transducers use one of the following principles for its working.

1. Change of self inductance

2. Change of mutual inductance

3. Production of eddy current

OR

(b) Draw the input output characteristics of LVDT. How displacement is being sensed by LVDT with

neat sketch.

When an externally applied force moves the core to the left-hand position, more magnetic flux

links the left-hand coil than the right hand coil. The magnitude of the output voltage is then equal to

the difference between the two secondary voltages and it is in phase with the voltage of the left-hand

coil.


COURSE: B.E – CSE



Now three cases arise according to the locations of core which explains the working of LVDT are

discussed below as,

CASE I When the core is at null position (for no displacement) When the core is at null position

then the flux linking with both the secondary windings is equal so the induced emf is equal

in both the windings. So for no displacement the value of output eout is zero as e1 and e2

both are equal. So it shows that no displacement took place.

CASE II When the core is moved to upward of null position (For displacement to the upward of

reference point) In the this case the flux linking with secondary winding S1 is more as

compared to flux linking with S2. Due to this e1 will be more as that of e2. Due to this

output voltage eout is positive.

CASE III When the core is moved to downward of Null position (for displacement to the downward

of reference point) In this case magnitude of e2 will be more as that of e1. Due to this

output eout will be negative and shows the output to downward of reference point.

Output VS Core Displacement A linear curve shows that output voltage varies linearly with

displacement of core.

Some important points about magnitude and sign of voltage induced in LVDT

The amount of change in voltage either negative or positive is proportional to the amount of

movement of core and indicates amount of linear motion

By noting the output voltage increasing or decreasing the direction of motion can be

determined

The output voltage of an LVDT is linear function of core displacement

25. (a) Demonstrate the linear and angular displacement of Potentiometer.


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OR

(b) Explain the methods of encoders based on

(i) Tachometer Encoder

(ii) Incremental Encoder


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(iii) Absolute Encoder

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INTERNAL ASSESSMENT EXAMINATION II ANSWER KEY …