RVDT

9AEI 306.19 2

It produces an electrical output which is proportional to the

angular displacement of the magnetic movable core.

Principle

9AEI 306.19 3

Uex = excitation voltage,

Uo = output voltage,

1 = excitation coil,

2 = output coil,

3 = moving core

or armature, 4 = sensing shaft

9AEI 306.19 4

9AEI 306.19 5

9AEI 306.19 6

9AEI 306.19 7

Rotary Variable Differential Transformer (RVDT)

Fig 1

9AEI 306.19 8

• 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.

Operation

9AEI 306.19 9

• 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

9AEI 306.19 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

9AEI 306.19 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

9AEI 306.19 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

9AEI 306.19 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.

9AEI 306.19 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

vibration

9AEI306.11 15

9AEI 306.20 16

• 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

Vibration

9AEI 306.20 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

9AEI 306.20 18

9AEI 306.20 19

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

Vibration

9AEI 306.20 20

Vibration defines the motion in structure and machine

components.

Vibration can be due to

• unbalance of rotating parts,

• misalignment,

• external forces.

Vibration

9AEI 306.20 21

Vibrations

9AEI 306.20 22

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.

Vibration Measurement Are Made In General For

Three Major Reasons

9AEI 306.20 23

9AEI 306.20 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.

9AEI 306.20 25

9AEI 306.20 26

(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.

Monitoring and control of a system

9AEI 306.20 27

Moving Coil type Velocity Transdcuer

9AEI 306.21 29

This transducer utilizes the voltage produced in a coil on

account of change in flux linkages resulting from change in

reluctance.

Principle

9AEI 306.21 30

Output wires

case

coil

Magnet

diaphragm

9AEI 306.21 31

Moving Coil Type Velocity Transducer

Fig 1

9AEI 306.21 32

• 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.

Operation

9AEI 306.21 33

9AEI 306.21 34

• Therefore the magnitude of the voltage is a measure of velocity and is given by

WhereN = number of turns of the coilR = reluctance of the coil

= rate change of flux

N dV

R dt

d

dt

9AEI 306.21 35

• High stability at temperature varying conditions.

• Less effective to stray magnetic field

Advantages

Moving Iron Type Velocity Transducer

9AEI 306.22 37

Principle

This Transducer Utilizes

• The voltage produced in a coil

• On account of change in flux linkages

• Resulting from change in reluctance.

9AEI 306.22 38

Magnet coilcoil

O/p

9AEI 306.22 39

• 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.

Operation

9AEI 306.22 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.

9AEI 306.22 41

• Maintenance free due to absence of mechanical surfaces

or contacts.

• Output voltage is linearly proportional to velocity.

• Used as event markers

• Less expensive

Advantages

9AEI 306.22 42

• 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

Disadvantages

AC Tachogenerator

9AEI306.23 43

9AEI306.23 44

It operates on the principle that…

•The relative motion between a magnetic field and a conductor results the voltage

generation in that conductor.

Principle

9AEI306.23 45

A.C Tacho Generator

•The Fig. shows the A.C Tacho Generator.

9AEI306.23 46

9AEI306.23 47

Construction

It consists of

• Permanent magnet (rotor)

• Coil (stator)

• Rectifier bridge

• Moving Coil (MC) voltmeter

9AEI306.23 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

9AEI306.23 49

Advantages

• Maintenance free due to the absence of brushes and

commutators

9AEI306.23 50

Disadvantages

• Large number of poles are required

• Requires high input impendence display devices

DC Tacho Generator

9AEI 306.24 52

It operates on the principle that..

• The relative motion between a magnetic field and a conductor

results the voltage generation in that conductor.

Principle

9AEI 306.24 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

9AEI 306.24 54

DC Tacho Generator

Fig 1

9AEI 306.24 55

• 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

Construction

9AEI 306.24 56

9AEI 306.24 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.

9AEI 306.24 58

• When the armature is stationary there is no relative

motion between the magnetic field and the armature

winding.

• Hence the output voltage is ZERO.

Operation

9AEI 306.24 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

9AEI 306.24 60

Where ω = angular velocitye0 = DC output voltageNp = No. of polesNc = No. of conductors in armatureøp = flux per poleNpp = parallel paths between positive and negative brushes.

p c p 8

0pp

N Ne 10 volts

60N

0 pp 8

p c p

e 60N10 rpm

N N

9AEI 306.24 61

•The direction of rotation is indicated by the polarity of the

output voltage.

Advantages

9AEI 306.24 62

• The commutator and brushes required periodic

maintenance.

• The output voltage is non-linear

Disadvantages

Photo Electric Tachometer

9AEI 306.25 64

• It converts speed of rotation into an electrical signal.

• This is used to determine angular speed of a rotating

device.

Principle

9AEI 306.25 65

Light sourceshaft

Light sensor

Opaque Disc

Photo Electric Tachometer

9AEI 306.25 66

• 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.

Construction

9AEI 306.25 67

• 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.

Operation

9AEI 306.25 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.

9AEI 306.25 69

Advantages and Disadvantages

Advantages :-• Digital output , requires no ADC

• Simple electronic circuitry

Disadvantages :-• Light source must be replaced from time to time

Toothed Rotor Variable Reluctance Transducer

9AEI 306.26 70

9AEI 306.26 71

• It converts speed of rotation into an electrical signal.

• This is used to determine angular speed of a rotating device.

Principle

9AEI 306.26 72

Toothed Rotor Variable Reluctance Transducer

FIG 1

9AEI 306.26 73

Induced pulses

Toothed rotor

magnet

To timer/counter/frequency meter

Shaped amplifier

Shaped pulses

9AEI 306.26 74

• 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.

Construction

9AEI 306.26 75

• 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.

Operation

9AEI 306.26 76

• 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.

9AEI 306.26 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

PN rps

T

PN 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.

9AEI 306.26 78

Advantages

• Simple and rugged construction

• Maintenance free

• Easy to calibrate

• The information from the device can be transmitted easily

Hall Probe

9AEI306.27-28 80

• It converts speed of rotation into an electrical signal.

• This is used to determine angular speed of a rotating device.

Principle

9AEI306.27-28 81

9AEI306.27-28 82

Hall Probe

I

VH Hall

Voltage

9AEI306.27-28 83

• 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.

Operation

9AEI306.27-28 84

9AEI306.27-28 85

9AEI306.27-28 86

• 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.

9AEI306.27-28 87

• A linear relationship exists between the rotation and the

output voltage can be obtained up to ± 60 of the rotation.

The hall voltage is given by

Where

KH =Hall coefficient

I = electric current

B= flux density

t= thickness of strip

HH

K I BV

t

9AEI306.27-28 88

1. Small size.

2. High resolution.

3.It is a non contact type device

Advantages

9AEI306.27-28 89

Applications

Used for measurement of

• Velocity

• rpm

• Non contact current

• Magnetic field