1. RVDT

2. Principle
It produces an electrical output which is proportional to the angular displacement of the magnetic movable core.
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3. Uex = excitation voltage, Uo = output voltage, 1 = excitation coil,
2 = output coil,
3 = moving core
or armature, 4 = sensing shaft
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5. 9AEI

6. 9AEI

7. Rotary Variable Differential Transformer (RVDT)
Fig 1
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8. Operation
It consists of a single primary winding P1 and two secondary windings S1 and S2.
The secondary windings have an equal number of turns.
They are identically placed on either side of the primary winding connected in series opposition.
A shaft whose angular velocity is to be measured is connected to the core.
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9. The out put of RVDT is given by eo= es1-es2
When primary winding is excited by an a.c signal, voltages are induced in each secondary section.
The out put of RVDT is given by
eo= es1-es2
Where
es1 is induced voltage in secondary s1
es2 is induced voltage in secondary s2
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10. when the core is at null position
When the core at null position the output voltage in secondary windings S1 and S2 are equal.
Therefore the differential output is e0=0
Fig 2
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11. When The Core Rotates In Clockwise Direction
When the core rotates in clockwise direction es1>es2.
The output voltage e0 is positive and in phase with input signal.
Fig 3
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12. When the core rotates in Anticlockwise Direction
When the core is in anticlockwise direction es2>es1.
The output voltage e0 is negative and 1800 out of phase with the input signal.
Fig 4
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13. The amount of the angular displacement and its direction may be ascertained from the magnitude and the phase of output voltage of the transducer.
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14. Advantages
RVDT provide an extremely reliable solution for precision angular displacement (position) measurements.
They are used wherever a physical quantity can be converted to rotary displacement.
The construction of the device prevents direct contact between the moving and the stationary part
Therefore insures a long operational life
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15. vibration
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16. Vibration
Vibration refers to mechanical oscillations about an equilibrium point .
The oscillations may be periodic such as the
motion of a pendulum
random such as the movement of a tire on a gravel road
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17. Importance
The need for making measurement of vibrations has arisen mainly because of
The growth of environmental testing
Specifications
Many a times requires that the equipment should withstand stated levels of vibrations
This can be done quantitatively only through vibration measurements
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18. 9AEI

19. Vibration Vibration Monitoring is Important in power stations Turbines
generators
to give an early warning of impending conditions
which may develop and lead to complete failure and destruction of equipment
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20. Vibration
Vibration defines the motion in structure and machine components.
Vibration can be due to
unbalance of rotating parts,
misalignment,
external forces.
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21. Vibrations
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22. Vibration Measurement Are Made In General For Three Major Reasons
A) Obtaining the response of a body or structure, such as the response of an aircraft wing to various load conditions.
It requires the analysis of signals in addition to actual measurements.
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23. 9AEI

24. (b) Defining the vibratory environment surrounding a vibratory source.
Like floor vibrations surrounding a high speed compressor or generator.
A judicious selection of the number of measurement stations and their location at which vibration is measured is important.
The investigation includes a number of fields tests are carried out under varying environmental conditions.
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25. 9AEI

26. Monitoring and control of a system
(c) Such as in maintaining acceleration at a desired level in electromagnetic exciters or in an inertial navigational system.
Measurements made are mainly on the acceleration levels, acceleration-time waveforms, spectral density distribution.
All measurements are carried out with suitable velocity or acceleration transducer.
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27. 9AEI

28. Moving Coil type Velocity Transdcuer

29. Principle
This transducer utilizes the voltage produced in a coil on account of change in flux linkages resulting from change in reluctance.
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30. diaphragm
coil
Magnet
case
Output wires
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31. Moving Coil Type Velocity Transducer
Fig 1
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32. Operation It consists of an arm on which coil is mounted
A mass is attached to the end of the arm.
The velocity to be measured is applied to the arm
Therefore the coil moves in the field of a permanent magnet
When the coil moves a voltage is generated which is proportional to the velocity of the coil.
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33. 9AEI

34. N = number of turns of the coil R = reluctance of the coil
Therefore the magnitude of the voltage is a measure of velocity and is given by
Where
N = number of turns of the coil
R = reluctance of the coil
= rate change of flux
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35. Advantages High stability at temperature varying conditions.
Less effective to stray magnetic field
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36. Moving Iron Type Velocity Transducer

37. Principle This Transducer Utilizes The voltage produced in a coil
On account of change in flux linkages
Resulting from change in reluctance.
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38. M a g n e t
coil
coil
O/p
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39. Operation Fig.1 shows a moving iron (magnet ) type linear velocity
transducer.
It consists of a permanent magnet which is rigidly coupled to the device whose velocity is to be measured.
There is a coil surrounding the permanent magnet.
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40. Operation The motion of the magnet induces voltage in the coil and
Amplitude of the voltage is directly proportional to the velocity.
For a coil placed in a magnetic field, the voltage induced in the coil is directly proportional to the velocity.
The polarity of the output voltage determines the direction of motion.
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41. Advantages
Maintenance free due to absence of mechanical surfaces or contacts.
Output voltage is linearly proportional to velocity.
Used as event markers
Less expensive
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42. Disadvantages
Performance is adversely affected by stray magnetic fields. These fields cause noise.
Frequency response is usually limited and is stated
Susceptible to vibrations, it leads to demagnetization
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43. AC Tachogenerator
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44. Principle It operates on the principle that…
The relative motion between a magnetic field and a conductor results the voltage generation in that conductor.
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45. A.C Tacho Generator
The Fig. shows the A.C Tacho Generator.
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46. 9AEI306.23

47. Construction It consists of Permanent magnet (rotor) Coil (stator)
Rectifier bridge
Moving Coil (MC) voltmeter
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48. Operation
When the magnet rotates in a stationary coil , an AC voltage is generated
The amplitude and the frequency of this voltage are both proportional to the speed of rotation
Thus either amplitude or frequency of induced voltage may be used as a measure of rotational speed
The output voltage of ac tacho generator is rectified and is measured with MC voltmeter
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49. Advantages
Maintenance free due to the absence of brushes and commutators
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50. Disadvantages Large number of poles are required
Requires high input impendence display devices
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51. DC Tacho Generator

52. Principle It operates on the principle that..
The relative motion between a magnetic field and a conductor results the voltage generation in that conductor.
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53. For measurement of angular velocity tachogenerators are used
There are two types of tacho generators
1) D.C tacho generator
2) A.C tacho generator
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54. DC Tacho Generator
Fig 1
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55. Construction
An armature is rotating type and this magnet is a fixed type.
The armature is coupled to the machine whose velocity is to be measured.
It consists of commutator and brushes is connected to the armature
Output is connected to a Moving Coil (MC) type voltmeter
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56. 9AEI

57. As the armature speed increases the relative motion also increases.
The output voltage is induced in the armature winding.
The magnitude of this voltage is proportional to the speed of the armature.
A commutator and brushes are connected in the armature to give the DC output voltage.
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58. Operation
When the armature is stationary there is no relative motion between the magnetic field and the armature winding.
Hence the output voltage is ZERO.
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59. The polarity of output voltage indicates the direction of rotation
This output voltage is measured with the help of moving coil voltmeter calibrated in terms of speed.
The relationship between the DC output voltage and angular velocity is given by
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60. Nc = No. of conductors in armature øp = flux per pole
Where
ω = angular velocity
e0 = DC output voltage
Np = No. of poles
Nc = No. of conductors in armature
øp = flux per pole
Npp = parallel paths between positive and negative
brushes.
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61. Advantages
The direction of rotation is indicated by the polarity of the output voltage.
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62. Disadvantages
The commutator and brushes required periodic maintenance.
The output voltage is non-linear
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63. Photo Electric Tachometer

64. Principle It converts speed of rotation into an electrical signal.
This is used to determine angular speed of a rotating device.
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65. Photo Electric Tachometer
Light source
shaft
Opaque Disc
Light sensor
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66. It consists of an opaque disc mounted on a rotating shaft.
Construction
It consists of an opaque disc mounted on a rotating shaft.
The disc has a number of equidistant holes on its circumference.
At one side of the disc a light source is fixed
At another side of the disc a light sensor is placed on line with the light source.
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67. Operation
When the opaque portion of the disc is between the light source and light sensor ,
The light sensor is unilluminated and produces no output.
But when a hole appears between the light source and the light sensor , the light falls upon the sensor and produces an output pulse.
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68. The frequency of output pulses depends upon the number of holes in the disc and its speed of rotation.
Since the number of holes is fixed then the pulse rate is a function of speed rotation.
The pulse rate can be measured by an electronic counter which can be directly calibrated in the terms of speed in rpm.
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69. Advantages and Disadvantages
Digital output , requires no ADC
Simple electronic circuitry
Disadvantages :-
Light source must be replaced from time to time
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70. Toothed Rotor Variable Reluctance Transducer
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71. Principle It converts speed of rotation into an electrical signal.
This is used to determine angular speed of a rotating device.
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72. Toothed Rotor Variable Reluctance Transducer
FIG 1
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73. Induced pulses To timer/counter /frequency meter Shaped amplifier
magnet
Shaped pulses
Toothed rotor
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74. Construction
It consists of a small permanent magnet with a coil wound around it.
This magnet is placed near a metallic toothed rotor
Rotor is made with Ferro magnetic material
It is connected to shaft whose speed is to be measured.
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75. Operation
When toothed rotor is rotating the air gap will change between the rotor and the permanent magnet.
Due to change in the air gap the field expanses or collapses.
The voltage is induced in the coil in the form of pulses.
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76. The pulses are amplified and fed to a counter or frequency meter.
The frequency of pulses depend upon the number of teeth on wheel and its speed of rotation.
The pulses are amplified and fed to a counter or frequency meter.
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77. Let T is the number of teeth on rotor.
N is the number of revolutions per second.
P is the number of pulses per second.
Then
If the rotor has 60 teeth and the counter counts the pulses in one Second. Then the counter will directly display the speed in rpm.
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78. Advantages Simple and rugged construction Maintenance free
Easy to calibrate
The information from the device can be transmitted easily
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79. Hall Probe

80. Principle It converts speed of rotation into an electrical signal.
This is used to determine angular speed of a rotating device.
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81. 9AEI

82. Hall Probe I
VH Hall
Voltage
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83. Operation
The hall probe is rigidly suspended between the poles of permanent magnet.
The magnet is connected to the shaft whose angular velocity is to be measured.
As the shaft rotates the hall probe remains stationary.
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84. 9AEI

85. 9AEI

86. It is done by means of a constant current source
A constant current is applied to the electrical contacts at the end of the probe
It is done by means of a constant current source
A voltage (Hall voltage ) is generated across the probe
The voltage generated across the probe is directly proportional to the sine of the angular displacement of the shaft.
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87. The hall voltage is given by Where KH =Hall coefficient
I = electric current
B= flux density
t= thickness of strip
A linear relationship exists between the rotation and the output voltage can be obtained up to ± 60 of the rotation.
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88. Advantages 1. Small size. 2. High resolution.
3.It is a non contact type device
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89. Applications Used for measurement of Velocity rpm Non contact current
Magnetic field
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