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Week 13
1
Vibration Measurement
Week 13
2
Vibration Measurement Schemex, v, a, F
electrical signal
amplified 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
Week 13
3
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
Week 13
4
Dynamic Testing of Machine/Structure
Measurement Hardware
Week 13
5
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
Week 13
6
Variable Resistance Transducer
Mechanical motion produces a change in electrical resistance of the strain gage, which in turn causes a change in the output voltage or current.
R ∞ ε ∞ x(t)
Week 13
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Variable Resistance Transducer
R ∞ ε ∞ x(t) Wheatstone bridge, sensitive to small changes in R
Week 13
8
Piezoelectric Transducer
E
Piezoelectric material, such as quartz and Rochelle salt, generate electrical charge when given deformation or mechanical stress.
crystal
Piezoelectric accelerometer
Week 13
9
Electrodynamic Transducer
The movement of the coil in a magnetic field produces a voltage Vout.
Voltage ~ Velocity
Current ~ Force
Week 13
10
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
Week 13
11
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
Week 13
12
Vibration Pick-ups
Week 13
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)
Week 13
14
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)
Week 13
15
Accelerometer
Works in this range
Week 13
16
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’.
Week 13
17
Mechanical Exciter
Week 13
18
Mechanical Exciter
Week 13
19
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)
Week 13
20
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
Week 13
21
Signal Conditioning
Week 13
22
Signal Analyzer
Week 13
23
Signal Analyzer
Week 13
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Impulse Testing
Week 13
25
Relationship between components and vibration spectrum