This document discusses vibration measurement and analysis. It describes various types of vibration transducers that convert mechanical vibrations into electrical signals, including variable resistance, piezoelectric, electrodynamic, and LVDT transducers. It also discusses vibration pickups like vibrometers, accelerometers, and exciters. Dynamic testing is used to determine natural frequencies, damping, and mode shapes through modal testing or measure in-service loads through operational testing. The measurement signals are then conditioned and analyzed to study the vibration spectrum of machines and structures.

1. Week 13
Vibration Measurement
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2. Week 13
Vibration Measurement Scheme
x, v, a, F amplified signal
electrical signal signal data
The type of vibration measuring instrument used will depend on:
- The expected range of frequencies and amplitudes
- The size of machine / structure involved
- Conditions of operation of the machine / structure
- Type of data processing used
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3. Week 13
Dynamic Testing of Machine/Structure
Dynamic testing is a valuable complement to dynamic analysis
Modal testing:
Natural frequencies
Damping
Mode shapes
Operational testing:
In-service loads
Various cases
Often random
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4. Week 13
Dynamic Testing of Machine/Structure
Measurement Hardware
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5. Week 13
Transducer
A transducer translates changes in mechanical quantities (such
as displacement, velocity, acceleration or force) into electrical
signals (such as voltage or current).
The size of the transducer is importatn
Especially when measuring small structures
Ideally should not influence the structure’s dynamics
through added mass or stiffness
Analytical models often include effects of transducer mass
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6. Week 13
Variable Resistance Transducer
R ∞ ε ∞ x(t)
Mechanical motion produces a change in
electrical resistance of the strain gage, which
in turn causes a change in the output voltage
or current.
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7. Week 13
Variable Resistance Transducer
R ∞ ε ∞ x(t) Wheatstone bridge, sensitive
to small changes in R
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8. Week 13
Piezoelectric Transducer
Piezoelectric material, such
as quartz and Rochelle salt,
E generate electrical charge
when given deformation or
mechanical stress.
crystal
Piezoelectric accelerometer 8

9. Week 13
Electrodynamic Transducer
Voltage ~ Velocity
Current ~ Force
The movement of the coil in a
magnetic field produces a
voltage Vout.
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10. Week 13
LVDT
Linear Variable Differential
Transformers:
- coils and magnetic core
- output voltage depends on
the axial displacement of
the core
- Displacement transducer
- Insensitivity to temperature
- Mass of core restricts application
- Best for low frequency, high displacement application
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11. Week 13
Vibration Pick-ups
Instruments that measure vibrations, e.g. vibrometer, accelerometer.
Most commonly used pick-ups are known as seismic instruments
- a mass-spring-damper system in a cage inertial sensor
- a certain type of transducer is integrated in the instrument.
- fastened onto the vibrating body
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12. Week 13
Vibration Pick-ups
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13. Week 13
Vibrometer
- Measures the displacement of a vibrating body
- Works in the right (blue) range where r >= 3 Z/Y = 1
if
Recorded z(t) lags from
measured y(t)
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14. Week 13
Accelerometer
Measures the acceleration of a vibrating body, from which the velocity and
displacement are obtained by integration.
recorded z(t)ωn2 lags from the
acceleration of the base ytt(t) 14

15. Week 13
Accelerometer
Works in this range
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16. Week 13
Vibration Exciter
Application:
- in the determination of the dynamic characteristics of
machines and structures;
- In fatigue testing of material.
Types: mechanical, electromagnetic/electrodynamic, hydraulic.
Also known as ‘shakers’.
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17. Week 13
Mechanical Exciter
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18. Week 13
Mechanical Exciter
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19. Week 13
Electrodynamic Shaker
Also known as electromagnetic exciter.
A reverse of the electrodynamic transducer.
Apply a force through a range of frequencies (harmonic or
random input)
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20. Week 13
Electrodynamic Shaker
- Current (A) passes through coil
- Generates force F (N) ~ I (A)
- Generates magnetic flux
intensity B (Tesla)
F = BIl
l : length of coil
This accelerates the component
on the shaker table.
AC: F harmonic
DC: F constant
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21. Week 13
Signal Conditioning
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22. Week 13
Signal Analyzer
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23. Week 13
Signal Analyzer
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24. Week 13
Impulse Testing
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25. Week 13
Relationship between components and
vibration spectrum
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