RVDT
Principle
It produces an electrical output which is proportional to the
angular displacement of the magnetic movable core.
9AEI 306.19
2
Uex = excitation voltage,
Uo = output voltage,
1 = excitation coil,
2 = output coil,
3 = moving core
or armature, 4 = sensing shaft
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Rotary Variable Differential Transformer (RVDT)
Fig 1
9AEI 306.19
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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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• 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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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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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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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
9AEI 306.19
12
•
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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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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vibration
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Vibration
•
Vibration refers to mechanical oscillations about an
equilibrium point .
•
The oscillations may be periodic such as the
a)
motion of a pendulum
b)
random such as the movement of a tire on a gravel road
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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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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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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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Vibrations
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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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(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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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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Moving Coil type Velocity
Transdcuer
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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diaphragm
coil
Magnet
case
Output wires
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30
Moving Coil Type Velocity Transducer
Fig 1
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31
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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•
Therefore the magnitude of the voltage is a measure of
velocity and is given by
N d
V
R dt
Where
N = number of turns of the coil
R = reluctance of the coil
d
= rate change of flux
dt
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Advantages
• High stability at temperature varying conditions.
• Less effective to stray magnetic field
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Moving Iron Type Velocity
Transducer
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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37
coil
M
a
g
n
e
t
coil
O/p
9AEI 306.22
38
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.
9AEI 306.22
39
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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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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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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AC Tachogenerator
9AEI306.23
43
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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A.C Tacho Generator
•The Fig. shows the A.C Tacho Generator.
9AEI306.23
45
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46
Construction
It consists of
• Permanent magnet (rotor)
• Coil (stator)
• Rectifier bridge
• Moving Coil (MC) voltmeter
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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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Advantages
• Maintenance free due to the absence of brushes and
commutators
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Disadvantages
• Large number of poles are required
• Requires high input impendence display devices
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DC Tacho Generator
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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• 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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DC Tacho Generator
Fig 1
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54
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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55
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•
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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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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•
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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59
e0 

NpNc p 
60Npp
e0 60Npp
NpNc p
 108 volts
 10 8 rpm
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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Advantages
•The direction of rotation is indicated by the polarity of the
output voltage.
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Disadvantages
•
The commutator and brushes required periodic
maintenance.
•
The output voltage is non-linear
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Photo Electric Tachometer
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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Photo Electric Tachometer
Light source
shaft
Opaque Disc
Light sensor
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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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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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•
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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Advantages and Disadvantages
Advantages :• Digital output , requires no ADC
• Simple electronic circuitry
Disadvantages :• Light source must be replaced from time to time
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Toothed Rotor Variable
Reluctance Transducer
9AEI 306.26
70
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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Toothed Rotor Variable Reluctance Transducer
FIG 1
9AEI 306.26
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Induced
pulses
magnet
To timer/counter
/frequency meter
Shaped amplifier
Shaped pulses
Toothed rotor
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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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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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• 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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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
P
N  rps
T
P
N   60 rpm
T
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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Advantages
• Simple and rugged construction
• Maintenance free
• Easy to calibrate
• The information from the device can be transmitted easily
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Hall Probe
Principle
• It converts speed of rotation into an electrical signal.
• This is used to determine angular speed of a rotating device.
9AEI306.27-28
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81
Hall Probe
I
VH Hall
Voltage
9AEI306.27-28
82
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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85
• 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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The hall voltage is given by
Where
KH I B
VH 
t
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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Advantages
1. Small size.
2. High resolution.
3.It is a non contact type device
9AEI306.27-28
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Applications
Used for measurement of
• Velocity
•
rpm
• Non contact current
• Magnetic field
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