this is help you to learn how to measure vibration

1. VIBRATION MEASUREMENT
MECHANICAL ENGINEERING DEPARTMENT
INDIAN INSTITUTE TECHNOLOGY, KHARAGPUR
WEST BENGAL
SESSION 2016-2018
NAME – RINTU SASMAL
SPECIALIZATION – MECHANICAL SYSTEM DESIGN

2. INTRODUCTION
• Why we need to measure vibrations:
 To measure natural frequencies for selecting operational speed
to avoid resonance.
 To verify theoretical values it may be different from measured
values.
 To design active vibration isolation systems.
 To identify mass, stiffness and damping of a system.
 To detect shifts in natural frequency which indicates possible
failure.
 To verify the approximated model.

3. BASIC STEP OF VIBRATION MEASUREMENT
Vibrating machine or structure
Vibration transducer or pickup
Signal conversion instrument
Display unit recorder, or computer
Data analysis

4. NECESSARY EQUIPMENT
The measurement of vibration requires the following
hardware:
1. An exciter or source of vibration to apply a known input
force to the structure or machine.
2. A transducer to convert the physical motion of the structure
or machine into an electrical signal.
3. A signal conditioning amplifier to make the
transducer characteristics compatible with the input
electronics of the digital data acquisition system.
4. An analyzer to perform the tasks of signal processing
and modal analysis using suitable software.

5. VIBRATION EXCITERS
 A vibration exciter is a machine which produces mechanical
vibratory motion to test object.
 It is designed to produce a given range of harmonic or time
dependent excitation force and displacement through a given range
of frequencies.
Types of Exciters :
The three major types of vibration exciters are commonly used in several
applications as:
1. Mechanical exciters
2. Electro-dynamic exciter
3. Electro-hydraulic exciter

6. MECHANICAL EXCITERS
 Mechanical exciters which vibrate the structure or machine by
mechanical system.
Types of Mechanical Exciters :
According to applied force mechanical exciters are three types.
1. Inertia force mechanical exciters
2. Elastic spring force mechanical exciters
3. Unbalanced force mechanical exciters

7. INERTIA FORCE MECHANICAL EXCITERS
• Forced applied as an inertia
force.
• Scotch yoke mechanism
used to produce harmonic
vibrations.
• It is used for applied
harmonic force.

8. ELASTIC SPRING FORCE MECHANICAL EXCITERS
• Force applied as an
elastic spring force.
• Scotch yoke mechanism
used to produce
harmonic vibrations.
• It is used for applied
harmonic force.
• It is generally used for
frequencies less than 30
Hz and loads less than
700 N

9. UNBALANCED FORCE
MECHANICAL EXCITERS
• Forced is applied as rotating
unbalance mass.
• The rotating unbalance
generates an oscillating force
which vibrate structure or
machine.
• It is used to generate relatively
large loads between 250 and
25,000 N.

10. ADVANTAGES OF MECHANICAL
EXCITERS
• The generated forces are transmitted directly to the table
without dependence upon a reactionary force against a heavy
base or rigid ground connection.
• The cost of mechanical exciter is less compared to other
exciters.
• There are no leakage problems as in hydraulic exciters.
• Defective electrical components and connections fail under the
induced vibration.
• This exciter eliminates embarrassing and costly repair and
difficult tracing of circuits in the field.

11. DISADVANTAGES OF
MECHANICAL EXCITERS
• Mechanical exciter cannot be used in high temperature,
humidity and altitude environments.
• It can only be used for small applications.
• The frequency range is small compared to hydraulic and
pneumatic

12. ELECTRO-DYNAMIC EXCITER
• It also known as electromagnetic
exciter.
• Electro-dynamic force applied to
vibrate the structure.
• The magnitude of the
acceleration of the component
depend on maximum current and
masses of component and moving
element.
• It generate force up to 30,000N
and displacement 25mm.
• The operating-frequency range of
the exciter lies between 5 to 20
kHz.

13. ELECTRO-DYNAMIC EXCITER
Advantages :
• It can produced purely harmonic and constant force.
• It can used wide frequency range.
• Easily measurement the excitation force.
Disadvantages :
• It has great mass and volume with respect to the amplitudes of
the generating exciting force.

14. ELECTRO-HYDRAULIC EXCITER
• Fluid pressure is used as
power source of hydraulic
vibration exciter.
• In this arrangement an
electrically actuated servo
valve operates a main
control valve, in turn
regulating flow to each
end of a main driving
cylinder.
• Large capacities (up to 2
MN) and relatively high
frequencies (to 400 Hz),
with amplitudes as great as
46 mm, have been
attained.

15. COMPARISON OF EXCITER

16. IMPACT HAMMER
• Impact hammer is the one of the
popular exciter.
• The equipment consists of an
hammer, usually with a set of
different heads and tips.
• Magnitude of the impact force
detect by transducer.
• Manually excited the system by
impact hammer.

17. IMPACT HAMMER
Advantages :
 Impact hammer is simple, portable, inexpensive.
 It is much faster to use than a shaker.
Disadvantages :
 it is often not capable of imparting sufficient energy to obtain
adequate response signals in the frequency range of interest.
 It is also difficult to control the direction of the applied force.

18. TRANSDUCERS
• Translates motion or forces into electrical signals
Size of transducer is important (micro)
 Ideally does not influence the structure’s dynamics through
added mass or stiffness.
 Analytical models often include effects of transducer mass.

19. VARIABLE RESISTANCE
TRANSDUCER

20. VARIABLE RESISTANCE TRANSDUCER
• An electrical resistance
strain gage consists of a
fine wire.
• Resistance changes the
wire when it is subjected
to mechanical
deformation.
• It shows as output
voltage.

21. PIEZOELECTRIC TRANSDUCER

22. PIEZOELECTRIC TRANSDUCER
• It measure the motion using piezoelectric effect of the
material.
Advantages :
 It is compactness and ruggedness.
 High sensitivity.
 It can used high frequency range.

23. ELECTRO-DYNAMIC TRANSDUCER

24. VIBRATION PICKUP
• When a transducer is used in conjunction with another device
to measure vibrations, it is called a vibration pickup.
• The vibratory motion is measured by finding the displacement,
velocity and acceleration of the test object.
• The commonly used vibration pickups are known as seismic
instruments.

25. VIBROMETER
• A vibrometer or a seismometer is an
instrument that measures the
displacement of a vibrating body.
• It consists of a mass-spring-damper
system mounted on the vibrating body.
• The relative displacement between the
mass and the base sensed by the
transducer.
• Relative displacement analysis by this
equation.

26. VIBROMETER
Disadvantages:
 At large frequency ratio vibormeter give accurate result.
 So, for fixed operating frequency , natural frequency need to
be low that means mass must be large and the spring must
have a low stiffness.
 As a result instrument become bulky which is not desirable in
many applications.

27. ACCELEROMETER
• Accelerometer used to measure the
acceleration of vibrating body.
• It is widely used to vibration
measurement and also to record
earthquakes.
• From the accelerometer record, the
velocity and displacements are
obtained by integration.
• Two type of accelerometer are
commonly used compression type
and shear type.
• Mostly piezoelectric transducer used
in accelerometer.

28. ACCELEROMETER
Advantages:
 Frequency ratio is small so, instrument required large natural
frequency.
 It need to small mass and high spring stiffness, so the
instrument will be small size.
 It exhibits better all-round characteristics than any other type
of vibration transducer.
 It has very wide frequency and dynamic ranges with good
linearity throughout the ranges.
 Additionally, the piezoelectric accelerometer is self-
generating, so that it doesn't need a power supply.

29. VELOMETER
• A velometer measures the velocity of a vibrating body.
• Frequency ratio must be very large.
• Velocity of vibrating body computed by

30. SIGNAL CONVERSION INSTRUMENT
• In signal conversion capture vibration signal from vibration
pickup device and present it in convenient form.
• Signal conversion done by two step.
1. Signal conditioning
2. Signal analyzer

31. SIGNAL CONDITIONING
• The direct output of a transducer not
usually well suited for input into an
analyzer.
• Impedance miss matched, voltage or
current levels too low.
• SC is a charge or voltage amp
designed to take signal and match it
to the input requirements of the
analyzer.

32. SIGNAL ANALYZER
• These devices analyze a signal in the frequency domain by
separating the energy of the signal into various frequency
bands.
• The separation of signal energy into frequency bands is
accomplished through a set of filters.
• The analyzers are usually classified according to the type of
filter employed.
• In recent years, digital analyzers have become quite popular
for real-time signal analysis.
• There are two types of real-time analysis procedures the digital
filtering method and the fast Fourier transform (FFT) method.

33. DIGITAL FILTERING METHOD
• The digital filtering method is best suited for constant-percent
bandwidth analysis.
• In signal processing, a digital filter is a system that performs
mathematical operations on a sampled, discrete-time signal to
reduce or enhance certain aspects of that signal.
• It consists of an analog-to-digital converter to sample the input
signal.
• followed by a microprocessor and some peripheral
components such as memory to store data and filter
coefficients etc.

34. FAST FOURIER TRANSFORM(FFT)
• FFT method best suited for constant-bandwidth analysis.
Working step of FFT:
This is based on three steps
1. Decompose N point time domain into N signals each
containing a single point.
2. Find the spectrum of each of the N point signal.
3. Synthesize N frequency spectra into a single frequency
spectrum.

35. BLOCK DIAGRAM OF
EXPERIMENTAL SETUP