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BOARD EXAMINATONS OCTOBER2019

992 – MEASURING INSTRUMENTS AND SENSORS

PART A (Each question carries two marks)

1. What are primary instruments?

Tangent Galvanometer

Rayleigh current balance

Absolute electrometer..

2. Mention the three operating forces in instruments.

Deflecting force

Controlling force

Damping force

3. Write the balance equation of Maxwell’s bridge.

and

L1 = unknown inductance

L2 = variable inductance of fixed inductance

R2 = variable resistance connected in series with inductor

R3, R4 = known non-inductive resistances

4. What is meant by auto-zeroing?

Automatic zeroing eases the operation of an instrument. After a measurement is

made successfully the instrument comes back to zero, zeroing offers optimum accuracy for

low valued reading.

5. List out various recorders.

There are two types of recorders. They are

1. Analog recorders

2. Digital recorders

Analog recorders are classified into XY recorder, Strip Chart recorder.

Digital recorders are classified into CD Recorders, Hard disk based recorders.

6. Define Sensor.

It is a device that detects a change in physical stimulus and turns it into a signal which can

be measured or recorded.

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7. What is the function of flow rate sensors in automobiles?

For the upcoming electronic fuel injection engines, these sensors are made use of as the air

volume input to the engine is estimated by flow-rate or pressure sensing. The estimation is

done on the basis of engine revolution and the negative pressure measure and at its intake.

8. List the types of capacitive sensors.

a. Parallel plate capacitance type

b. Capacitance type with serrated electrodes

c. Capacitance type with varying dielectric type material

PART B (Each question carries three marks)

9. What is meant by static error?

Error is defined as the difference between the measured value and the true value of

the quantity.

Error= Measured value- True value

Error= Em-Et

Em= Measured value of the quantity

Et= True value of the quantity.

10. What is the advantage of digital storage oscilloscope?

1) Storage Capability is high

2) We can change the voltage and time scale display after the recording.

3) 0.25% resolution is possible with 0.1% accuracy

11. Differentiate between sensors and transducers.

S.

No. Sensors Transducers

1.

A device that responds to a physical

stimulus (as heat, sound, pressure) and

transmits a resulting impulse

A transducer is a device that is used to

convert a physical quantity into its

corresponding electric signal.

2. Characteristics of a sensor are

tolerance, range, resolution.

Characteristics of a transducer are

linearity, sensitivity.

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Applications of a sensors are

a. Inductive proximity sensors

b. Limit switches

Applications of a transducers are

a. Potentiometric transducer

b. LVDT

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12. What is seven segment display?

A display consisting of seven LED arranged in seven segments is called seven

segment displays.

Seven segment displays are widely used in digital clocks, electronic meters, basic

calculators, and other electronic devices that display numerical information.

13. What is a passive sensor?

Passive sensors are the sensors that detect reflected electromagnetic radiation from natural

source.

14. Define smart sensor.

A sensor producing an electrical output when combined with interface electronic circuits is

said to be an intelligent sensor if the interfacing circuits can perform (a) ranging, (b)

calibration, and (c) decision making for communication and utilization of data.

15. What is a nano-sensor?

Nano sensors are nano scale devices that measure physical quantities and convert those

quantities to signals that can be detected and analyzed.

16. What is meant by auto ranging?

o The aim of automatic ranging is to get optimum resolution in

all circumstances. The instrument will be switched to more sensitive range to

measure all type of values from very minimum to maximum.

o The range of the instrument is automatically switched over to the appropriate range

for near perfect measurements.

PART -C

17.(a) Describe the dynamometer type wattmeter.

[Dia – 3 marks; Construction – 3 marks; operation: 4 marks]

These types of instruments have two coils. The two coils are connected in different

circuits for measurement of power. The fixed coil or “field coils” are connected in series with

the load and so carry the current in the circuit. The fixed coils therefore form the current coil

or simply C.C of the wattmeter. The moving coil is connected across the voltage and,

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therefore, carries a current proportional to the voltage. So it is called the pressure coil or

simply called P.C of the wattmeter.

Construction of dynamometer wattmeter:

The wattmeter consists of two coils namely (a) one set of fixed coil (b) single

moving coil as shown in fig 1.10.1.1. The fixed coil is made of two parts and the

moving coil is placed between the two fixed coils.

The fixed coil is used as a current coil. The current coil are made of few turns

of thick copper wire and are connected in series with the load to carry the load

current. The moving coil is used as a pressure coil. The pressure coil is made of

several turns of fine copper wire. A high non inductive resistance is connected in

series with the moving coil in order to limit the current.

The jewel supported spindle carrying the pressure coil also carries a pointer, a

damping vane and control springs. The pointer is fixed to the pressure coil spindle and moves over a

suitably calibrated scale. A cast iron cylinder is placed around the coil to protect against stray

magnetic fields. The moving system of the wattmeter is usually spring controlled. Air friction

damping is used. The pointer is made of aluminium. The pointer is made thin and the front part is

flattened.

Operation:

The two parts of the fixed coil carry load current and create a flux in the air gap between

them. The pressure coil carries another small current proportional to load voltage and hence produces

a flux in the air gap. The current coil produces a flux, which is in phase with the load current.

The flux produced by the pressure coil is very nearly in phase with the applied

voltage. The magnetic fields of fixed and moving coils react on one another causing the moving coil

to turn about its axis and a torque is produced on the moving system. This torque is proportional to the

product of voltage and current.

Td ∞ V I

Td ∞ power

Wattmeter reads both a.c and d.c circuit powers.

17.(b) Discuss the types of secondary instruments. (10 marks)

Types of secondary instruments:

Secondary instruments are further classified into three types namely.

Indicating instruments

Recording instruments

Integrating instruments

Indicating instruments

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This type of instruments indicates the instantaneous value of quantity to be measured at the time

only. The quantity to be measured is indicated by a pointer moving over a calibrated scale. A dial and

a pointer are used in such instruments.

Ex: Ammeter, Voltmeter, Wattmeter

Recording instruments:

Recording instruments give a continuous record of the electrical quantity being measured over a

definite period of time.

The recording is generally made by a pen on a graph paper which is rotated on a disc or drum at

a uniform speed. The amount of the quantity at the instant may be read from the traced chart. Any

variation in the quantity with time is recorded by these instruments.

The X-axis denotes time and y axis denotes electrical quantity. Such instruments are generally

used in power houses where the current, voltage and power etc, are to be maintained within a certain

specified values.

Ex: Recording pyrometer, Recording M.D. meters, Recording KVA meters.

Integrating instruments:

These instruments record the consumption of the total quantity of electricity. Energy etc,

during a particular period of time. These instruments measure the value of product of electrical

quantity and time. The integration is generally given by a register consisting of a set of pointers and

dials.

Ex: Ampere-hour meters, Kilo watt-hour meter, KVARH meters (sine meters) are integrating

instruments.

18. (a) Discuss about the applications of CRO (Each Application award 2marks)

Applications:

a) Meaurement of frequency: The simplest method of comparing two frequencies is by a Lissajous

pattern. This method requires the use of a variable frequency sinusoidal oscillator. The output of the

oscillator is applied to one set of deflecting plates and the voltage whose frequency is to be

determined is applied to the second set of deflecting plate. From the observed Lissajous pattern, the

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unknown frequency can be determined.

Lissajous patterns

b. Phase measurement:

Phase measurement

For measuring of relative phase angle of two sinusoidal voltages ofequal amplitude and same

frequency they are applied to x and y deflecting plates of a CRO an eclipse is obtained. It is

shown in the above fig.

c. Study of waveform: For this purpose, the signal under study is appliedto vertical input

terminal of the CRO. The sweep circuit is set to internal so that “saw tooth wave” is applied

to the horizontal input. Then various controls are adjusted to obtain sharp and well defined

signal waveform on the screen.

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d. Voltage measurement: If the signal is applied to the vertical deflection plates only, a

vertical line appears on the screen. The height of the line is proportional topeak to peak

voltage of the applied voltage. Multiplying the height of the line by the deflection sensitivity,

the peak voltage value is obtained.

e. Current measurement: To measure current, the signal is passed through a known resistor

and the potential developed across it is measured by CRO as explained above. Then by

applying ohm’s law, the current may be calculated

(Or)

18.(b) Illustrate the use of wheat stone bridge. [Dia -5marks; Equation&Uses:5marks]

It is the most commonly used DC bridge for measurement of resistance. A

Wheatstone bridge has been in use longer than almost any electrical measuring instrument. It

is still an accurate and reliable instrument and is extensively used in industry.

Fig 2.2.1 shows the basic circuit of a Wheatstone bridge. It has four resistive arms, consisting of

resistances P, Q, R and S together with source of emf (a battery) and a null detector, usually a

galvanometer G or other sensitive current meter. The current through the galvanometer depends on

the potential difference between point B and D. The bridge is said to be balanced when there is

no current through the galvanometer or when the potential difference across the galvanometer is

zero. For bridge balance, we can write:

I1P = I2R

For the galvanometer current to be zero, the following conditions also exist

I1 = E / (P + Q) (1)

and

I2 = E / (R + S) (2)

Where E = emf of the battery

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Combining and simplifying (1) and (2), we obtain

P / (P + Q) = R / (R + S)

From which

QR = PS

The above equation is the well known expression for the balance of Wheatstone bridge.

If three of the resistance is known, the fourth may be determined from equation

Where R is the unknown resistance S is called ‘standard arm’ of the bridge and P and Q are called the

‘ratio arms’.

Applications:

(i) It is used to measure the resistance value of unknown resistors of the range 0.1Ω to 1, 00,000Ω

(ii) It is used for measurement of small resistance changes that occur in passive resistive

transducers Like strain gauges, thermistors and resistive thermometers

(iii) It is used to measure the inductance, capacitance and frequency by suitably arranging the

components of different arms.

(iv) It is used to measure Q factor, dissipation factors of coils and capacitors

19.(a) Draw the block diagram of digital multimeter. (Diagram-5marks; Working-5marks)

Digital multi-meter: Digital multimeter has high input impedance with high accuracy. They

are in size.

Digital multimeter

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A basic multimeter is made up of many A/D converters with attenuation circuits. The

current is to be measured is applied to summing junction at input of operational amplifier. The

current at input is zero because of high inout impedance.

The current Ir nearly equal to Ii causing a voltage drop across the resistors. The voltage

drop is the input to A/D converter, which gives a reading value proportional to unknown

current.

To measure a resistance a known current from a constant current source is passed to that

resistor. The voltage drop across the resistor is applied to the A/D converter. The A/D

converter indicates the value of unknown resistance.

Digital Meters have the capabilities of displaying half digits. A full digit display

displays all numbers from 0 to9. A half digit display displays only the number 1. The half

digit is always the first digit shown.

A 3.5 digit display has four segments with 3 full digits and one half digit. It will read

maximum of 1999. For example if we like to display 10KV on 3.5 digit meter we use the

leading half digit. We have four digits resolution. Placing the decimal point properly the

reading would be 10.00KV will be displayed in front panel overlay.

A 4.5 digit display has five segments. 4 full digits with one half digit. Maximum display

capability of 19999.

(Or)

19. b. Write brief notes on: (i) Liquid vapour display (ii) CD recording

(Diagram: 3marks; Theory: 2marks)

(i) Liquid vapour display:

These are economical displays based on the principle of the pressure depends on ambient

light. The construction of LVD is shown in above fig. It consists of two glass plates with a

transparent volatile liquid enclosed in it. The background of a rear glass is totally black. While the

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front surface is roughened so that the liquid can wet it. A voltage drive is used to heat a

transparent electrode. Under OFF condition of the display, only the black background can be

viewed through transparent glass electrode. To make display ON, a voltage is applied to the

electrode, generating heat in electrode. This evaporates liquid which is in contact with it. Thus

around the roughened surface of glass the vapour films and vapour bubbles are formed. With this

vapour bubbles formation, there is a discontinuity between the glass plate and liquid interface.

Due to which light scattering takes place.

While selecting organic liquids the precaution must be taken such as the refractive index of

the liquid should be close to that of glass, less requirement of energy for vaporizing the liquid.

The drawback of LVD is its speed of operation is low.

(ii)CD recording: (Diagram: 3marks; Theory: 2marks)

The signal to be recorded on CD is first amplified and the converted into digital signal by

using sample and hold circuit, and ADC. The ADC output is also applied to Laser beam

generator. The signal from the crystal oscillator and Laser beam generator are applied to the

control circuit and for controlling the servo system.

Block diagram of CD recording

The servo system controls the disc rotation through motor, also controls the track and focus

of the Laser beam generator. The block diagram of CD recording system is shown in the

above fig.

After recording, the unexposed photo-resist material is chemically removed, thereby

producing a helical track across the surface of the glass disc. This becomes the glass

master to produce large volumes of CD’s.

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20. a. (i) Explain the working of LVDT.

(Diagram: 5marks; Working: 5marks)

Working: When an AC voltage is applied to the primary winding, voltages are induced in the

two secondary windings which are functions of the relative position of the iron

core with respect to the coil. The output voltage is equal to the difference between

the two input voltages.

Eo = Es1 - Es2

Case1: With the core in the centre position, the induced emfs in the two secondaries are

equal, and since they oppose each other, the output voltage will be 0v.This is the balance

position or null position.

Case2: When the core moves upwards, the voltage induced in upper winding is more than the

voltage induced in the lower winding. Hence the net output is positive.

Case3: When the core moves downwards, the voltage induced in upper winding is less than

the voltage induced in the lower winding. Hence the net output is negative.

(Or)

20. b. Explain the working of pyro - electric sensor.

(Diagram: 3marks; Working: 7marks)

Pyroelectric sensors: The Pyroelectric thermal sensor is comparatively a new entrant in the

area of thermal/temperature detection. It comprises a type of ferroelectric material. Ferroelectric

materials are non-centrosymmetric and their ferroelectricity is attributed to the spontaneous

electric polarization on a polar axis. The direction of this polarization can, however, be changed

by the application of electric field. Also, there occurs a remanent polarization because of

permanent electric dipole in the primitive unit cell of the crystal.

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If the permanent dipoles in the material exhibit electric polarization with temperature, the

characteristic property is called pyroelectricity. Materials with polar point symmetry are likely

to exhibit this property. There are 32 possible point group symmetries; materials which possess

10 out of these 32, exhibit pyroelectricity.

Materials of this category are mainly ceramics. The dipoles, normally, are in random

orientation in the material and net electric output is zero, and at ambient temperature these

orientations are also fixed. If the temperature is now raised above a certain value, often called

the Curie temperature or ‘critical’ temperature, which is again a characteristic of the material,

the molecules with the dipoles are free to rotate.

Sketch of a pyroelctric detector

When a slice of pyroelectric ceramic is placed between a pair of electrodes and the electrodes

given an electric field, with its temperature raised above Curie point, the molecules in the

material orient themselves in the direction parallel to the applied field with opposite polarity of

the dipoles. This state however persists even when the field is removed amount of polarization

thus affected is proportional to the applied field. Thus, if P is the polarization and E is the

applied field, then

P=σE

where σ is a constant, which is a function of the material. The pyroelectric sensors have good

spectral response covering 0.001 - 1000µm, that is, soft x-rays through infrared rays and have

fast response time, sometimes of the order of nanoseconds.

21. a. Explain the advantages of fiber optic sensor.

(Any ten points) (advantages: 10 marks)

Advantages of fiber optic sensors:

The bandwidth of optical fiber is very high when compared with other communications.

1. To increase bandwidth in fiber optics is very easy.

2. Data transmission is very fast in fiber optics.

3. We can transmit data to longer distances without any noise.

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4. It is very difficult to tap the information because it is much secured.

5. It has high reliability.

6. It has high sensitivity.

7. Due to small size and less weight of the sensors it effective in cost.

8. It is safe and suitable to be used in extreme vibration and harse environments.

9. It is tolerant against high temperature (>1450C) and corrosive environments.

10. It can be used in multifunctional sensing capabilities for mechanical measurements, electric

measurements, magnetic measurements, chemical and biological sensing and so on.

11. It can measure nearly all of the physical measurands. Example: Temperature, Presssure,

Flow.

12. It does not have any disturbance from electromagnetic interference.

21. b. What is MEMS accelerometer? Explain. (Theory-10marks)

MEMS accelerometer:

A typical accelerometer has been produced as a device with a polysilicon proof mass

suspended above a single crystal silicon substrate. This requires the removal of a sacrificial

layer of Sio2 making perforations in the mass. The mass remains attached to the substrate at

two anchor points. This allows the mass to bend in the plane of the wafer on one axis. The

position is detected by capacitive effect and the electrostatic force is feedback to operate the

device as an accelerometer. For obtaining a large proof mass, it is laterally extended to several

microns. This also provides an appreciable capacitance value. The processing is carried out as

in the case of surface micromachining. However, such accelerometer structure can be

developed through the BSOI technique as well.

Single crystal silicon is taken as he proof mass in this case which is supported by two silicon

beams attached to the substrate. The sensitive axis is now perpendicular to the plane of the

wafer. Instead of capacitive sensing, this design employs electron tunnelling method for

sensing. Tunnelling takes place across a narrow oblique gap formed by focused-ion-beam

(FIB) which machines through a third beam.

Sensors based on vibrating elements have also been produced by BSOI process to measure

many physical and chemical parameters. This technique can also reduce the size of the

resonant sensors by the miniaturization of the sensors producing them on silicon substrate and

in the process, making them resistant to shock. Internal losses can also be reduced by using

quality materials. The design is possible in the single crystal or the polysilicon type in both the

cases high Q factor is obtainable.

Basically, the idea is to arrange the microresonators such that they affect the mass or stiffness

of the vibrating system. In pressure sensors, the resonant system is mounted/supported on the

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top of the diaphragm. As the external pressure bends the diaphragm, the microresonator is

strained with a consequent change in its resonance frequency.

Prepared by

A.Nagarani / Nanda kumar J

Lecturer,

Department of Electronics (Robotics)

222, Arulmigu Palaniandavar Polytechnic college,

Palani.