Measurement and Control of Vibration

UNIT –V
MEASUREMENT & CONTROL OF
VIBRATION
Prof. Manojkumar.V
Ambalagi
Dr. D Y Patil SOET, Lohegaon, Pune

Vibrations
 Introduction to Vibration Instruments
 Classification of Vibration Measuring Instruments
 Accelerometer
 Frequency Measuring Instruments
 Vibration shakers/exciters
 FFT Spectrum Analyser
 Vibration based condition monitoring
 Vibration Tests (Impact hammer*)
Contents

Vibrations in Everyday Life
Vibrations are an integral part of our day-to-day life

Fundamentals of Vibrations
Vibration – Oscillatory motions of a body
about an equilibrium position
Natural Frequency – Frequency at which
a system oscillates when not subjected to a continuous or repeated
external force
Mode Shapes – A standard manner in which a particular system can
vibrate. Each mode is associated with a particular natural frequency
Resonance – A phenomenon that occurs when the forced frequency
is same as that of the natural frequency

Sources of Vibrations
Some of the sources of vibrations are:
Mass Unbalance
Misalignment
Eccentricity
Parallel Misalignment Angular Misalignment

Why measure vibrations ?
 To verify that frequencies and amplitudes do not exceed the material
limits, which can cause fatigue & therefore reduce the life of the
component
 To avoid excitation of resonances in certain parts of a machine, which
may otherwise cause excessive displacements & cause failure of the
components
 To be able to dampen or isolate vibration sources
 To make conditional maintenance on machines
 To construct or verify computer models of structures (system analysis)

BASIC CONSTRUCTION OF VIBRATION MEASURING
INSTRUMENTS
• The vibration measurement process start by sensing the vibratory motion
from vibrating machine of structure and converting it into an electrical
signal with the help of transducer or pickup
• Output signal from transducer which is in the form of voltage or current is
too small to record it
• Therefore the signal conversion unit is used to amplify the signal from the
transducer to the required value
• The output signals from the signal conversion unit is displayed on display
unit for visual inspection. This information is recorded and stored into
computer for later use
• Finally, the data can be analyzed to know the vibration characteristics of
machine or structure

CLASSIFICATION OF INSTRUMENTS
• The vibratory response of A vibrating system can be
expressed in terms of various parameters such as:
1.Displacement (Amplitude)
2.Velocity
3.Acceleration
4.Frequency
5.induced stresses
• The choice of parameter depends upon the objective
and field of application

Measurement and Control of Vibration

VIBROMETERS
• Vibrometers or seismometer is an instrument which measures
the
displacement of a vibrating machine or structure
• The various type of amplitude measuring instruments are as
follows
1.Stylus recording instrument
2.Seismic instrument or seismometer or vibration pickup
3.Optical recording instrument
4.Capacitance pickup
5.Simple Potentiometer
6.Mutual inductance pickup

1.STYLUS RECORDING INSTRUMENTS
 It consists of a drum which is rotating
about Y-Y axis and a stylus which is
pivoted at a fulcrum ‘O’. To the other
end of the stylish a link is attached
which pickups the vibratory motion
from vibrating machine or structure.
 The motion between the rotating drum
and linear moment of stylus plot an
amplitude of vibratory motion on
paper which is attached on drum

2. SEISMIC INSTRUMENT
 A seismic instrument consists of a
spring-mass-damper system in a
frame or casing which is mounted on
the vibrating machine or structure to
measure the displacement amplitude
of a vibratory motion
 The Mass ‘m’ is supported in a frame
or casing by means of spring having
'K‘ and dashpot of damping
coefficient 'c'. The frame or casing his
fastened to the vibrating body so that
it vibrates along with the vibrating
body. seismometer is equivalent to a
spring-mass-damper system having a
base or support excitation

3. OPTICAL RECORDING INSTRUMENT
 Light source sends the light signal through a lens to on mirror.
The mirror is attached to a vibrating body by means of some
linkage.
 The light which is reflected from the mirror falls on a sensitized
film on the revolving drum and lots the displacement of
vibratory motion

4. SIMPLE POTENTIOMETER
• It consists of a voltmeter a battery and
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 the resistance
is recorded this voltage is proportional
to the amplitude of vibrations

5. CAPACITANCE PICK-UP
• Capacitance pickup is non-contacting active type vibration measuring
instrument which generates an output proportional to the displacement of the
vibratory motion
• Change in capacitance due to variation in the air gap is utilized in an RC
circuit to indicate the amount of the vibratory displacement

6. MUTUAL INDUCTANCE PICK-UP
• The mutual inductance pick up is also
anon contacting active pickup which is
very useful for a non magnetic metallic
vibratory surface
• The mutual inductance Lm changes
due to the variation in the 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 set up by
the primary coil. Output voltage is
modulated by the vibratory motion and
demodulated output is proportional to
the displacement

Frequency-Measuring Instruments
• Most frequency-measuring instruments are of the mechanical type and are based
on the principle of resonance
• Two types : Fullarton tachometer and Frahm tachometer.
Single-Reed Instrument or Fullarton Tachometer.
• This instrument consists of a variable length
cantilever strip with a mass attached at one of its
ends. The other end of the strip is clamped, and its
free length can be changed by means of a screw
mechanism.
• Since each length of the strip corresponds to a
different natural frequency, the reed is marked
along its length in terms of its natural frequency. In
practice, the clamped end of the strip is pressed
against the vibrating body, and the screw
mechanism is manipulated to alter its free length
until the free end shows the largest amplitude of
vibration. At that instant, the excitation frequency is
equal to the natural frequency of the cantilever; it
can be read directly from the strip.

Frequency-Measuring Instruments
Multireed-Instrument or Frahm Tachometer.
• This instrument consists of a number
of cantilevered reeds carrying small
masses at their free ends Each reed
has a different natural frequency and
is marked accordingly. Using a
number of reeds makes it possible to
cover a wide frequency range.
• When the instrument is mounted on a
vibrating body, the reed whose natural
frequency is nearest the unknown
frequency of the body vibrates with
the largest amplitude. The frequency
of the vibrating body can be found
from the known frequency of the
vibrating reed.

Vibration Exciters/ Shakers
 These are used to produce the required cyclic
excitation force at a required frequency. The
cyclic excitation force produced by the exciter
can be applied to machine/structure.
 Classification :
 Mechanical Exciter
 Electrodynamic Exciter
 Hydraulic & Pneumatic exciter

Vibration Transducers
Transducers for Measurement
 Eddy Current Transducers
 Seismic Transducers
 Velocity Pick-up
 Accelerometers
Transducers for Excitation
 Impulse Hammer
 Impedance Head
 Electromagnetic Shaker

Mechanical Exciter (Inertia force & Spring force

Exciting force due to two rotating
masses

 When current passes through a coil placed in a
magnetic field, a force F (in Newtons)
proportional to the current I (in amperes) and the
magnetic flux intensity D (Wb/m2 or teslas). The
magnetic field is produced by a permanent
magnet in small shakers & electromagnet is
placed is used in large shakers. The magnitude of
acceleration of table or component depends on
the maximum current and the masses of
component & moving element of shaker. The
electrodynamic exciters are used to generate
forces up to 30kN, x= 25mm & f= 5 to 20kHz

Vibration Measurements
 To measure the physical phenomenon in terms of a quantifiable
property
System
Conditioning
Recording/Analysis
Display
Transducers
Parameters
Device that converts energy
from one form into another,
usually electrical, electronic,
electro-mechanical,
electromagnetic etc
Conditioning of the received
signal to record/analyse.
Conditioning includes filter,
sampling, amplify etc

Impulse Hammer
 Force excitation device usually
with different impact tips and a
force transducer(load cell)
 Used to input energy in system in
very short time by generating an
impulse (transient signal)
 The tip and size of hammer
decides the frequency range of
excitation
 Available as manual or automated
hammers
 Extensively used for experimental
modal testing
Load cell

Impact Hammer for Modal Testing
https://www.youtube.com/watch?v=tBRjPN8m6zE

Electromagnetic Shaker
 Uses the principle of electromagnetic
induction, ie motion of an electrical
conductor attached to a table (armature or
driving coil) in a magnetic field
 The magnetic field is created by either a
permanent magnet or electromagnet (Field
Coil)
 A Power amplifier is required to provide
sufficient current to the driving coil.
 By using different types of electrical inputs
to the driving coil, sine, swept sine,
random, pseudo random, burst random
motion of the table can be generated
 Frequency range limited by the resonance
of the armature (moving element)
 Besides electromagnetic, hydraulic and
pneumatic shakers can also be used
depending on the frequency range and

Shaker Video
https://www.youtube.com/watch?v=uMSYFtIXnQ4

Experimental Modal Analysis
 Basic Idea :
 Experimental modal analysis, also known as modal analysis or
modal testing, deals with the determination of natural
frequencies, damping ratios, and mode shapes through
vibration testing. Two basic ideas are involved:
 1. When a structure, machine, or any system is excited, its
response exhibits a sharp peak at resonance when the forcing
frequency is equal to its natural frequency when damping is
not large.
 2. The phase of the response changes by 180° as the forcing
frequency crosses the natural frequency of the structure or
machine, and the phase will be 90° at resonance.
 https://www.youtube.com/watch?v=Y3GXy4mQV_o

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

 Determination of Modal Data from Observed Peaks
 Determination of Damping Ratio from Bode Diagram
 Determination of Modal Data from Nyquist Plot

Machine-Condition Monitoring and Diagnosis
 Most machines produce low levels of vibration when designed properly.
During operation, all machines are subjected to fatigue, wear, deformation,
and foundation settlement.
 These effects cause an increase in the clearances between mating parts,
misalignments in shafts, initiation of cracks in parts, and unbalances in
rotors all leading to an increase in the level of vibration, which causes
additional dynamic loads on bearings. As time progresses, the vibration
levels continue to increase, leading ultimately to the failure or breakdown
of the machine.
 The common types of faults or operating conditions that lead to increased
levels of vibration in machines include bent shafts, eccentric shafts,
misaligned components, unbalanced components, faulty bearings, faulty
gears, impellers with faulty blades, and loose mechanical parts.

Vibration Severity Criteria
 The vibration severity charts, given by standards such as ISO 2372,
can be used as a guide to determine the condition of a machine.
 In most cases, the root mean square (RMS) value of the vibratory
velocity of the machine is compared against the criteria set by the
standards.
 Although it is very simple to implement this procedure, the overall
velocity signal used for comparison may not give sufficient warning
of the imminent damage of the machine.

Machine Maintenance Techniques
 The life of a machine follows the classic bathtub curve
 Since the failure of a machine is usually characterized by an increase in vibration
and/or noise level, the vibration level also follows the shape of the same bathtub
curve.
 The vibration level decreases during the initial running-in period, then increases
very slowly during the normal operating period due to the normal wear, and finally
increases rapidly due to excessive wear until failure or breakdown in the wear out
period.
Three types of maintenance schemes can be
used in practice
• Breakdown maintenance
• Preventive maintenance
• Condition-based maintenance

Machine- Condition Monitoring Techniques
Machine vibration monitoring techniques.

Time-Domain Analysis
Time Waveforms. Time-domain analysis uses the time history of the signal
(waveform). The signal is stored in an oscilloscope or a real-time analyzer and
any nonsteady or transient impulses are noted. Discrete damages such as broken
teeth in gears and cracks in inner or outer races of bearings can be identified
easily from the waveform of the casing of a gearbox.
.
The pinion of the gear pair is coupled to a 5.6-kW, 2865-rpm, AC electric motor.
Since the pinion (shaft) speed is 2865 rpm or 47.75 Hz, the period can be noted
as 20.9 m/s. The acceleration waveform indicates that pulses occur periodically
with a period of 20 m/s approximately. Noting that this period is the same as the
period of the pinion, the origin of the pulses in the acceleration signal can be
attributed to a broken gear tooth on the pinion

Time-Domain Analysis
Indices : In some cases, indices such as the peak level, the root mean square (RMS)
level, and the crest factor are used to identify damage in machine-condition monitoring.
The crest factor, defined as the ratio of the peak to RMS level, includes information
from both the peak and the RMS levels. However, it may also not be able to identify
failure in certain cases. For example, if the failure occurs progressively, the RMS level
of the signal might be increasing gradually, although the crest factor might be showing
a decreasing trend.
Orbits : Sometimes, certain patterns known as Lissajous figures can be obtained by
displaying time waveforms obtained from two transducers whose outputs are shifted by
90° in phase. Any change in the pattern of these figures or orbits can be used to identify
faults such as misalignment in shafts, unbalance in shafts, shaft rub, wear in journal
bearings, and hydrodynamic instability in lubricated bearings.

Frequency -Domain Analysis
Frequency Spectrum : The frequency-domain signal or frequency spectrum is a plot of
the amplitude of vibration response versus the frequency and can be derived by using
the digital fast Fourier analysis of the time waveform. The frequency spectrum provides
valuable information about the condition of a machine
As the machine starts developing faults, its vibration level and hence the shape of the
frequency spectrum change. By comparing the frequency spectrum of the machine in
damaged condition with the reference frequency spectrum corresponding to the
machine in good condition, the nature and location of the fault can be detected.
Another important characteristic of a spectrum is that each rotating element in a
machine generates identifiable frequency
A number of formulas can be derived to find the fault frequencies of standard
components like bearings, gearboxes, pumps, fans, and pulleys. Similarly, certain
standard fault conditions can be described for standard faults such as unbalance,
misalignment, looseness, oil whirl, and resonance.

Eliminate or Reduce Vibrations
Vibrations in a system can be eliminated/reduced by:
 Reducing the mechanical disturbance causing the vibration (source)
 Isolate the disturbance from the radiating surface (path)
 reducing the response of the radiating surface (receiver)
A simple example to illustrate how vibration transfer occurs & how to reduce them
• Source – Engine
• Path – Body Structure
• Receiver – Driver ENGIN
E

Vibration Control
 Why vibration control
Linear motion Rotation

Vibration Control
 Vibrations are damped to get
 Less noise to surroundings
-> comfort for users
 Decrease conduction of vibration into the structures
-> comfort for users/operators
 Less wear of parts and need for maintenance
-> less costs

Vibration Control
As seen in the previous slides as to what effect vibration has on the system and
human being, it is important to control the vibration generated/transferred from a
system.
Broadly, there are three types of control methods:
1. Passive – Isolation is achieved by limiting the ability of the vibrations to be
coupled with the system. Ex: Springs, Elastomers, Pneumatic systems etc
2. Active – Equal but opposite forces are created electronically using sensors and
actuators to cancel out the unwanted vibrations.
3. Hybrid – Damper changes the vibration absorption properties based on the
external vibrations. Ex: Shock Absorber with Magneto-rheological Fluid.

System modeling
 How accurate the system modeling should
be?
Finite element modeling
Distributed parameter system
Lumped parameter system

Vibrations in electrical
machines
 Structure of an AC induction motor

Rotor vibrations
 Radial vibrations
 Torsional vibrations
x
y
z
ω
ω

Examples to Eliminate/Reduce
Vibrations
 Mechanical disturbances that produce vibration can be reduced by balancing
rotating parts, improving alignment, reducing clearances, replacing worn parts,
etc., all of which may be considered part of good maintenance.
 Vibration from bearings and sliding and rolling friction may be reduced by
maintaining minimum clearances and proper lubrication.
 Hydraulic and aerodynamic disturbances producing vibration can be
minimized by designing for smooth even flow and avoiding excessive pressure
drop.
 The disturbances can also be isolated from the other parts of the machine or
from adjacent structures by using flexible connectors to reduce the
transmission through rigid connections.
 Vibration may be controlled by reducing the response of the radiating
surface. This may be accomplished by stiffening, increasing the mass, of
damping.

Vibration effects on Human Body
 Humans are exposed to localised vibrations ( arms or legs) or whole body
vibrations.
 If hand-arm vibrations are exposed for a long duration & long period enough, then
the Hand-Arm Vibration Syndrome can develop, which is commonly known as
white finger syndrome. This affects the operators health & touch perception.
 Whole body vibrations affect all parts of the body, usually transmitted through
seat, floor & arms, generally associated with the forms of transportation.
 Almost every person experiences some kind of vibration in their day-to-day life.
This can lead to malfunctioning or even failure of certain organs if exposed for
longer durations.

 Vibrations are produced in machines having unbalanced masses or forces
 High vibration levels can cause machinery failure, as well as objectionable
noise levels
 A common source of objectionable noise in buildings is the vibration of
machines that are mounted on floors or walls

SOURCES OF VIBRATION EXAMPLES
Rotating and reciprocating Unbalance Pumps, Turbines, Electric Generators,
Compressors
Misalignment and wear of tools Manufacturing – Machining
Seismic Vibrations Buildings, Bridges, Chimneys and
Cooling Towers
Wind (Pressure loading) Tall Structures
Small magnitude vibrations E.g.
Pedestal and vehicular traffic
Sensitive systems like optical
instruments, microscopes,
Nanofabrication
Impact and shock loads Hammer and presses, in vehicles due
to bumpy/ irregular nature of road

VIBRATION ISOLATION
 Vibration isolation is the process of isolating
an object, such as a piece of equipment, from
the source of vibration
 The effectiveness of isolation is expressed in
terms of force or motion
 Lesser the amount of force or motion
transmitted to the foundation greater is said to
be the isolation

TYPES OF VIBRATION
ISOLATORS
 PASSIVE VIBRATION ISOLATION
 Refers to vibration isolation or mitigation of
vibrations by passive techniques such as
rubber pads or mechanical springs
 ACTIVE VIBRATION ISOLATION
 Also known as electronic force cancellation
 Employs electric power, sensors, actuators and
control systems for vibration isolation

 The basic objectives of vibration isolation are:
 To protect the delicate machine from excessive
vibration transmitted to it from its supporting
structure
 To prevent vibratory forces generated by machine
from being transmitted to its supporting structure
 The vibration isolation may be obtained by placing
materials, called vibration isolators in between the
vibrating body and the supporting foundation or
structure

PASSIVE VIBRATION
ISOLATORS
METALLIC SPRING ISOLATION PAD
OLATION HANGER PNEUMATIC
ISOLATOR

ACTIVE VIBRATION
ISOLATORS
 FRAME MOUNTABLE ACTIVE ACTIVE VIBRATION
 HARD-MOUNT PIEZOELECTRIC ISOLATION TABLE
VIBRATION CONTROL SYSTEM

VIBRATION ISOLATION
MATERIALS

RUBBER FELT
CORK METALLIC SPRING

Measurement and Control of Vibration

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