2 Transducer Operation & Application

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VIBRATION MEASURING

INSTRUMENTS

OVERVIEW


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Choosing Your Instrumentation

What do you want to achieve?

What is your present and future.

Budget for equipment & Training?

Person power? Knowledge level?

Number of machines to be monitored?

Type of machines to be monitored?

Environment?


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INSTRUMENT TYPES

Overall Level Meters

Quick Check Analysers

FFT Data Collector/Analysers

Full Feature Analysers

Real Time Spectrum Analysers

Instrument Quality Tape Recorders

Dedicated Balancing instruments


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Real Time Rate

The Highest rate at which

data can be captured anddisplayed without leaving

any gaps in the analysis.


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ON-Line or Permanent Monitors

Due to cable lengths and possible signal

contamination, SNR considerations remain

more important in current permanent / on-line monitoring than they are in a

contemporary data collector context


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Vibration Transducers

Overviewand

Selection


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Transducer TypesSeismic:- Bearing relative to space.

Velocity Pickups Accelerometers

Piezoelectric velocity pickups

Relative:- Shaft relative to bearing. Non-contact Eddy Current Displacement

Probes

Absolute:- Shaft relative to space. Shaft Contact Displacement Probes (including

Shaft Sticks and Shaft Riders)


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Seismic Transducer

VELOCITY


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Velocity Pickups

Note :- There are two types of velocity pickups the aboveadvantages do not apply to piezoelectric velocitytransducers.

ADVANTAGES

Self- Generating

No On-board Electronics Strong signal


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SESMIC TRANSDUCERSACCELEROMETER

AMPAMP


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Accelerometers - advantages

No moving parts, no wear.

Rugged.

Very large dynamic range.

Wide frequency range.

Compact, often low weight.

High stability.

Can be mounted in any orientation.


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Accelerometer Types

The three most common are :-

Compression Type Inverted Compression Type

Shear Type


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Compression type accelerometer

Electric connector

Seismic Mass

Preload Stud

Acoustic Shield

Piezoelectric Material

ICP AmplifierMounting StudReceptacle

Base


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Compression Type Accelerometers

Advantages

Relatively low cost

Disadvantages

Sensitive to base strain

Sensitive to Thermal transients

Can cause over-saturation and

transducer settling problemsWidely used


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Inverted compression type

Piezoelectric MaterialICP Circuit

Mounting stud receptacle

Seismic Mass

Preload Sleeve


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Shear Type Accelerometer

Electric connector

Seismic Mass

Post

Acoustic ShieldPiezoelectric Material

ICP CircuitMounting StudReceptacle

Base


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Advantages - Shear Type

Lower sensitivity to base strain

Large dynamic range

Much less sensitive to temperaturetransients

Stabilizes quickly when takingmeasurements at low frequencies.

Disadvantage: -Generally higher cost due to added

components


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Mounting Methods

BEST GOOD POOR VERY GOOD


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Mounting Methods

Mounting

Maximum

Acceptable

Frequency (cpm)

Rating

Stud 975 000 Best

Adhesive 540 000 Very good

Magnet 450 000 Good

Quick Connect 360 000 Quite good

Hand-held 48 000 Poor


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Piezoelectric Velocity Pickup

Mass

Piezoelectric Disks

Base

Connector

INTEGRATOR


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Non-contact Eddy Current Probe(Relative)


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PICKUPCOIL

MAGNETIC

FIELD

SHAFT

GAP

METER

DC SIGNAL SENSOR

DISPLACEMENT

SIGNAL - TO

ANALYSER ORMONITOR

NON-CONTACT PICKUP

DETECTOROSCILLATOR AMPLIFIER

Non-contact Pickups


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Non-contact Eddy Current

Displacement Probes

USED FOR:-

Relative Shaft Vibration.

Radial & axial shaft position.

Differential expansion between case and rotor.

Especially effective on machinery with high mass

rigid casings and relatively low mass rotorssupported in journal type bearings.


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N.C.P.

Problems & precautions

Only Measures Displacement - SensitiveOnly to low frequency defects.

Subject to Mechanical and Electrical Run-out.

Units must be pre calibrated for specific shaftmaterials.


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Shaft Contact Displacement Probes

(Absolute)

Shaft Sticks

Hardwood, fish-tail, fixed toaccelerometer or velocity pickup

Measures vibration amplitude &

phase Shaft Riders


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SHAFT SURFACE

NON-METALLIC TIP

MACHINE HOUSING

SHAFT RIDER ASSEMBLY

PICKUP MOUNTING STUD

Shaft Rider


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Direct Contact : Absolute Measurements

Shaft Riders (permanently installed) Shaft Sticks or Fishtails

safety issue

VERY useful below coupling of vertical pumps

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2 Transducer Operation & Application