Vibration Measuring Instrument

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VIBRATION MEASURING INSTRUMENT

Assignment of subject NVH

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CLASSIFICATION OF VIBRATION MEASURING INSTRUMENTS

Vibratory response can be expressed in terms of various parameters such as: Displacement Velocity Acceleration Frequency & Induced stress

choice of parameters depends upon the objective & the field of application

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CLASSIFICATION OF VIBRATION MEASURING INSTRUMENTS

Classification Of Vibration Measuring InstrumentBased On Contact Between Vibrating System &

InstrumentConta

ct Type

Non-conta

ct Type

Based On Power Source Requirement

Active

Instrument

Passive

Instrument

Based On Method Of

MeasurementsIndicating Type

Instrument

Recording Type

Instrument

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VIBROMETERS (AMPLITUDE MEASURING INSTRUMENT)

an instrument which measures the displacement (i.e. amplitude)

Types of vibrometers are:+ Stylus Recording Instrument+ Seismic Instrument or Seismometer or

Vibration Pickup+ Optical Recording Instrument+ Simple Potentiometer+ Capacitance Pickup+ Mutual Inductance Pickup

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STYLUS RECORDING INSTRUMENT A drum - rotating about Y-Y

axis Stylus - pivoted at a fulcrum,

other end is attached which pickups the vibratory motion

Rotating drum & linear movement of stylus plots amplitude of vibratory motion on paper

Very cheap & simple Cannot be used for higher frequency & high acceleration vibratory sys. At the natural frequency of vibration of stylus recording instrument, the

resonance occurs

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SEISMIC INSTRUMENT OR SEISMOMETER OR VIBRATION PICKUP

A spring-mass damper system in a casing

Mounted on the vibrating machine or structure

Measures the displacement or amplitude of vibratory motion

Hence is a seismic instrument

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SEISMIC INSTRUMENT OR SEISMOMETER OR VIBRATION PICKUP

A frame or a casing - mass ‘m’ is supported by means of spring ‘K’ & dashpot ‘c’

The frame or casing is fastened to the vibrating body

So that frame vibrates along with vibrating body

Dial ‘Z’ records relative amplitude = vibrating body’s amplitude ‘Y’

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SEISMIC INSTRUMENT OR SEISMOMETER OR VIBRATION PICKUP

Strain sensing transducer - rigidly fixed to the seismic mass - used to measure relative amplitude ‘Z’

The output voltage -proportional to - the relative displacement ‘Y’

Hence, called as vibration pick-ups

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OPTICAL RECORDING INSTRUMENT:

: A light source sends a light signal through a lens to a mirror. The mirror is attached to a vibrating body by means of some linkages.

The light which is reflected from the mirror falls on a sensitized film on the revolving drum and plots the displacement of vibratory motion.

Such instrument have the advantage of wide range of frequency because of the less mass and negligible inertia.

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SIMPLE POTENTIOMETER

It consists of a voltmeter, a battery and a resistance.

A needle is connected to the vibrating body and it is allowed to slide on the resistance.

The change in voltage due to movement of needle on resistance is recorded. This voltage is proportional to amplitude of vibrations.

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CAPACITANCE PICK-UP The capacitance pick-up is a non-contacting active type

vibrating measuring instrument, which generates an output proportional to the displacement of the vibrating body.

It has one plate of capacitor attached to the vibrating body and the other being kept at some distance from the vibratory surface as shown in fig.

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CAPACITANCE PICK-UP

•The change in capacitance due to variation in the air gap is utilised in an RC circuit to indicate the amount of the vibratory displacement.•The size of the probe to be used depends upon

the range of amplitude to be measured. The range of amplitude covered by such a pick-up is 0.025 to 10mm.

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MUTUAL INDUCTANCE PICK-UP The mutual

inductance pick-up is also a non-contacting active pick-up, which is very useful for a non-magnetic metallic vibratory surface which is shown in fig.

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MUTUAL INDUCTANCE PICK-UP If the mutual inductance between the primary

and secondary coils is Lm, then the voltage induced across the secondary coil will be,

e = Lm ω ipwhere, e= voltage induced across secondary

coil. Lm = mutual inductance between

primary and secondary coil. ω = the frequency of ip.

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MUTUAL INDUCTANCE PICK-UP The mutual inductance Lm changes due to the

variation in air gap between the vibrating surface and the pick-up. This changes the field due to eddy current in the vibrating body. This field opposes the field setup by primary coil. The output voltage is modulated by the vibratory motion and demodulated output is proportional to the displacement.

Such a pick-up can be used for a very wide range of frequency.

VELOMETER Velometer is an instrument which

measures the velocity of a vibrating body.

There are two types of velocity pick ups. Active type pick –ups Passive type pick –ups

ACTIVE TYPE PICK-UPS-

ACTIVE TYPE PICK-UPS-Fig. Shows the active type pick-up which is similar seismometer in construction, with slight in modification. It consists of seismic mass ’m’, with wire coil, is supported by spring ‘k’ & dashpot ‘c’. The magnet is fixed to the frame & frame is fitted on vibrating body. But to relative motion between coil & magnet, a voltage is induced across the coil. The output voltage across the coil is proportional to the relative velocity. The output voltage signal is calibrated to give the velocity of vibration. Just like a seismometer in velometer also the damping factor is kept about 0.7 less than 0.7

PASSIVE TYPE PICK-UPS-

PASSIVE TYPE PICK-UPS-

BASIC PRINCIPAL-The change in air gap between the

vibrating surface & pick –up changes the magnetic reluctance of the path .As a result

the magnetic flux cutting across the coil changes thus inducing voltage across coil which is given by,

PASSIVE TYPE PICK-UPS-

where N= number of coils

X’- velocity of oscillation or vibration - rate of change of flux Now for a given range of air gap, remains constant & if number of coils remains fixed then,The induced output voltage is calibrated to give the velocity of vibration of body. The air gap should be kept within 3 to 5 mm & should be several times the amplitude of vibration.

- change of flux with respect to distance Now for a given range of air gap, remains

constant & if number of coils remains fixed then,

The induced output voltage is calibrated to give the velocity of vibration of body. The air gap should be kept within 3 to 5 mm & should be several times the amplitude of vibration.

A TYPICAL PLOT OF THE VOLTAGE INDUCED PER UNIT VELOCITY VERSUS THE AIR GAP IS SHOWN IN FIGURE.

ACCELEROMETER (ACCELERATION PICK-UPS) :

1. measures the acceleration of a vibrating body.

2. If the frequency ratio is very small,relative amplitude z is directly proportional to the acceleration of vibrating body.

3. Piezoelectric crystal is used in accelerometer which can measure high frequency of vibration.4. A signal produced is proportional to deformation.5. If the signal is weak it is amplified with the help amplifier.

TYPES OF ACCELEROMETERSPiezoelectric Uni-axial Accelerometers

Wide frequency and dynamic range, good linearity. They are of two types:

Compression type & Shear type

Piezoelectric Tri-axial Accelerometer :Along 3 co-ordinate axes the measurement is taken.three input channels are required.

Piezoresistive (PR) Accelerometer : piezo-resistive substrate is used.Force exerted by the seismic mass

changes the resistance.measure accelerations down to DC .

Variable Capacitance(VC) Accelerometer: The change in capacitance between two fixed plates is measured .voltage is taken as function. Due to acceleration the diaphragm vibrates and results in

change in capacitance.