http://kluniversity.in/bes/pdfs/test3.pdf
KL University
Department of Basic Engineering Science
I B.TECH , A.Y 2014-15
TEST -III
Course Title: MEASUREMENTS
Course Code: 13-ES-102
Branches: ECE,ME,PE
Semester:II
Date: 16-03-2015
Time:90 min
Max. Mark=30
Mark Cog. Level
SECTION: A
Answer ALL questions
1 a Why Detector is used in an Electrical bridge.
b What is the necessity of Delay line in CRO
1
1
Understand
Understand
In following Lissajous pattern frequency of
signal given to Horizontal deflection plate is 300 Hz.
c
What is the frequency of the signal given to vertical
deflection plates.
1
Apply
d Why two secondary coils are used in LVDT
1
Understand
1
Apply
Com
. No
2
4
4
3
In an interferometer 100 fringes are counted during movement of
e mirror, calculate displacement if wave length of light used is 650nm
1
Remember
3
3
Explain construction and working of wheatstones bridge. Derive
expression to find out unknown resistance
4
Understand
2
Suggest a suitable bridge for measurement of unknown inductance with
b neat sketch and derive the balancing condition to get the unknown
inductance.
4
Understand
2
4
Understand
2
4
Apply
4
4 a Explain construction and working of Cathode ray tube.
Explain following components of a CRO
i)
ii) Time base generator iii) Trigger circuit iv)
b Vertical amplifier
Horizontal amplifier
Describe the construction & working of Michelson interferometer and
5 a Mention how it is utilized for the measurement of linear displacement.
4
Understand
4
4
Understand
4
4
Understand
3
b Explain the construction and working of pneumatic gauge.
6 a Sketch and explain the working of a centrifugal tachometer
4
4
Understand
Understand
3
3
4
Understand
3
f what is the working principle of Drag cup tachometer.
SECTION: B
Answer any THREE questions
2a
Describe how an unknown capacitance can be measured with the
3 a help of D’Sauty’s bridge. Derive the equation of balance with
bridge diagram?
Calculate the Peak voltage, peak-peak voltage and RMS voltage of
the waveform shown when the vertical amplifier value is set at
b 100mV/division.
b
How principle of Resonance vibration can be used for measuring
speed
Answer key
Measurements (13-ES-102)
1.
a. To identify balanced condition of bridge
b. The delay line is used to delay the signal for some time in the vertical section. When the
delay line is not used, the part of the signal gets lost. Thus the input signal s not applied
directly to the vertical plates but is delayed by some time.
c.
FV 2

FH 3
FV 
2
X 300  200 Hz
3
d. When the displacement is given to LVDT voltage in one secondary increases at the same time
voltage in the second secondary decreases. As the out put of LVDT is taken as the difference
between these two voltages, it results in two times the sensitivity than using a single coil, also
error in the one secondary is nullified by same error in the second.
N  100 X 650
e.  

 32500nm  32.5 m
2
2
f. Eddy current.
2.
a. The Wheatstone bridge circuit is used to precisely measure resistances in the range of 1Ω
to 1 MΩ. The bridge circuit consists of four resistors, a dc voltage source, and a detector.
The resistance of one of the four resistors can be varied. The dc voltage source is usually
a battery, which is indicated by the battery symbol for the voltage source. The detector is
generally a D’Arsonval movement in the microampere range and is called a
galvanometer. Fig. shows the circuit arrangement of the resistances, battery, and detector.
b. Maxwell’s Bridge:
In this, values of the unknown inductance L1 and its internal resistance R1 are obtained by
comparison with the standard inductor and resistance, i.e. L3 and R3. In this bridge R2 is chosen as
the inductive balance control and R4as the resistance balance control. Balance is obtained by
alternately varying L3 or R3.
Z1 = R1 + jX1 = R1 + jωL1
Z3 = R3 + jX3 = R3 + jωL3
The balance condition is that Z1Z4 = Z2Z3
(R1 + jωL1)R4 = (R3 + jωL3)R2
Equation the real and imaginary parts on both sides, we have
R1R4 = R2R3 or R1/R3 = R2/R4
(i.e. products of the resistances of opposite arms are equal).
And ωL1R4 = ωL3R2
Or L1 =L3R2/R4
We can also write that L1 = L3R1/R3
Anderson Bridge:
Anderson’s bridge is the most accurate bridge used for the measurement of self – inductance over a
wide range of values, from a few micro-Henries to several Henries. In this method the unknown selfinductance is measured in terms of known capacitance and resistances, by comparison. It is a
modification of Maxwell’s L - C bridge. In this bridge, double balance is obtained by the variation of
resistances only, the value of capacitance being fixed.
AC bridges are often used to measure the value of unknown impedance (self/mutual inductance of
inductors or capacitance of capacitors accurately). A large number of AC bridges are available and
Anderson's Bridge is an AC bridge used to measure self inductance of the coil. It is a modification of
Wheatstones Bridge. It enables us to measure the inductance of a coil using capacitor and resistors
and does not require repeated balancing of the bridge. The connections are shown in Fig.
3.
a. It is a fundamental capacitance comparison bridge for the measurement of unknown
capacitance. Fig. represents a basic capacitance measuring bridge called De sauty’s bridge. It
comprises two fixed resistances and two capacitances in which one capacitance is unknown.
Resistances R3 or R4 is varied to get balance condition. If C1 is unknown capacitance,
b.
Peak to peak voltage 100 X 4  400mV
400
Peak voltage 
 200mV
2
200
RMS Voltage 
 141mV
2
4.
a.
Electron Gun:
In the electron gun of the CRT electrons are emitted, converted into a sharp beam and focused upon
the fluorescent screen. The electron beam consists of an indirectly heated cathode, a control grid, an
accelerating electrode and a focusing anode. The electrodes are connected to the base pins. The
cathode emitting the electrons is surrounded by a control grid with a fine hole at its centre. The
accelerated electron beam passes through the fine hole. The negative voltage at the control grid
controls the flow of electrons in the electron beam, and consequently, the brightness of the spot on
the CRO screen is controlled.
Deflection Systems
Electrostatic deflection of an electron beam is used in a general purpose oscilloscope. The deflecting
system consists of a pair of horizontal arid vertical deflecting plates. The beam is focused at point 0
on the screen in the absence of a deflecting plate voltage. The deflection is proportional to the
deflecting voltage between the plates.
Fluorescent screen: coated with fluorescent material
b.
Vertical Amplifier
The input signals are generally not strong to provide the measurable deflection on the screen. Hence
the vertical amplifier stage is used to amplify the input signals. The amplifier stages used are
generally wide band amplifiers so as to pass faithfully the entire band of frequencies to be measured.
Trigger Circuit
It is necessary that horizontal deflection starts at the same point of the input vertical signal. Each time
it sweeps. Hence to synchronize horizontal deflection with vertical deflection a synchronizing or
triggering circuit is used. It converts the incoming signal into the triggering pulses which are used for
the synchronization.
Time Base Generator
The time base generator is used to generate saw tooth voltage, required to deflect the beam in the
horizontal section. This voltage deflects the spot at a constant time dependent rate. Thus the x-axis on
the screen can be represented as time, which helps to display and analyze the time varying signals.
Horizontal Amplifier
The saw tooth voltage produced by the time base generator may not be of sufficient strength. Hence
before giving it to the horizontal deflection plates it is amplified using the horizontal amplifier.
5.
a.
A traditional interferometer is a device where we shoot a laser at a half transparent mirror, which
splits the laser into two beams, reflecting half the light at a 900 angle and allowing the other half to
continue on a straight path. After traveling some distance those beams are then reflected off mirrors
and recombined at the half transparent mirror where they travel as a single beam to an observing
screen. If the separated beams of light have traveled the same distance they recombine and form
constructive interference and a bright spot will appear on the screen, however if they have traveled
different lengths while separated they recombine and form destructive interference and a dark or dim
spot will appear on the screen. One of the mirror is connected on the part to which we have to find
the displacement. If this mirror moves a distance δ, the path of the light beam increases by 2δ. If the
number of successive occurrence of destructive interference, or dark spots, at the photo detector
screen during this motion is equal to ‘N’ then the displacement of the movable reflector is given by
the following equation:
2δ = Nλ Where, δ = Distance moved by the part
N = Number of successive occurrences of destructive interference, or dark spots; λ = Wave length of
the beam.
b.
In pneumatic type of devices, the displacement signal is converted to pressure signal. The device
shown below is pneumatic displacement gauge and this is also known as flapper nozzle device. A
pneumatic displacement gauge system operates with air. One of the basic building blocks of a
pneumatic displacement gauge system is the flapper nozzle amplifier. It converts very small
displacement signal (in order of microns) to variation of air pressure. The basic construction of a
flapper nozzle amplifier is shown in above figure. Constant air pressure (20psi) is supplied to one end
of the pipeline. There is an orifice at this end. At the other end of the pipe there is a nozzle and a
flapper. As the flapper moves closer to the nozzle, there will be less airflow through the nozzle and
the air pressure inside the pipe will increase. On the other hand, if the flapper moves further away
from the nozzle, the air pressure decreases. At the extreme, if the nozzle is open (flapper is far off),
the output pressure will be equal to the atmospheric pressure. If the nozzle is blocks, the output
pressure will be equal to the supply pressure. A pressure measuring device in the pipeline can
effectively show the pressure variation. The characteristic is inverse and the pressure decreases with
the increase in distance.
6.
a.
Centrifugal tachometers utilize the fact that the centrifugal force on a rotating mass depends on the
speed of rotation and can be used to stretch or compress a mechanical spring.
Where, M = Rotating mass
R = Radius of rotation
ω = Angular speed (rad/s)
b. The instrument consists of set of vertical reeds each having its own natural frequency of
vibration. The reeds are lined up in their order of natural frequency and are fastened to a base
plate at one end, with other end free to vibrate. When the tachometer base plate is placed in
mechanical contact with frame of rotating machine, a reed tuned to resonance with machine
vibration responds more.