Measurement of Acceleration & Vibration : Different simple Instruments Principle of seismic-instruments Geometric COSMOLOGY/PACIFIC RING OF FIRE Vibro meter and Accelerometer

1. SUDHEER NANDI
MECHANICAL MEASUREMENTS
19MET12

2. Topic covered –III
Measurement of Acceleration & Vibration :
 Different simple Instruments
 Principle of seismic-instruments
 Geometric COSMOLOGY/PACIFIC RING OF FIRE
 Vibro meter and Accelerometer

3. Measurement of Acceleration & Vibration
Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed
and a direction, there are only two ways for you to accelerate: change your speed or change your
direction—or change both.
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”Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium
point.”
Vibrations fall into categories: free , forced and Damped
Free vibration occurs when a mechanical system is set in motion with an initial input and allowed
to vibrate freely.Eg: swing , tuning fork.
Forced vibration is when a time-varying disturbance (load, displacement or velocity) is applied to
a mechanical system. Eg: washing machine shaking due to an imbalance, earthquake
Damped vibration: When the energy of a vibrating system is gradually dissipated by friction and
other resistances, the vibrations are said to be damped
Eg: vehicular suspension dampened by the shock absorber

4. Sensor(s)
Cables Signal Conditioning
Data Acquisition
& Storage
Communications Remote
Analysis and
Diagnostics
Measuring Acceleration and Vibration

5. 5
Measuring Acceleration and Vibration
Vibrometer
• An instrument that is used to measure the ground motion in earthquakes and sometimes to measure
vibration in machines is called the vibrometer.
Although the basic components are the same as the
piezoelectric or strain-gage accelerometers, the
mode of operation is different.
• In the vibrometer, the spring is quite soft and as the
housing moves, the mass remains approximately
stationary. The relative motion, y, is large and sensed
with a potentiometer.
• These devices are used to measure vibrations with frequencies that are high relative to the natural frequency of
the spring-mass system, which is often less than 1 Hz.
• The vibrometer effectively measures the displacement of the base rather than the acceleration.
• Thus, these devices are most sensitive to vibrations with moderate frequencies and fairly large displacement
amplitudes.
• High frequency vibrations usually have small values of displacement amplitude and are better measured with
accelerometers.

7. 7
Measuring Acceleration and Vibration
Accelerometers using Piezoelectric Sensing Elements
• An accelerometer using a piezoelectric material as the sensing element is shown below:
• It consists of a housing, a mass called the seismic mass,
and a piezoelectric sensing element, which typically uses
the longitudinal piezoelectric effect.
• An initial force between the mass and sensor is
obtained with a preloading spring sleeve.
• As the housing for the accelerometer is subject to an acceleration, the force exerted by the mass on the
quartz crystal is altered. This generates a charge on the crystal, which can be sensed with a charge
amplifier.
• Piezoelectric accelerometers are available in many ranges up to ±1000g, where g is the acceleration
due to gravity. Quartz crystal accelerometers can have very high values of natural frequency up to 125
kHz. This allows them to measure frequencies as high as 25 kHz.

8. Acceleration Sensors
 Pro’s and Con’s
 Pro’s
 Measures Accel.
 Small Size
 Easily Installed
 Large Frequency Range (1-10,000 Hz)
Con’s
•Measures Acceleration (requires
integration to Vel.)
•Susceptible to Shock & Requires Power

9. Machine Speed Sensors
 Displacement Probes
 Active or Passive Magnetic Probes
 Optical Permanent
 Stroboscopes
 Laser Tachometer
Voltage or Current?
 Current Output Accelerometers
 4-20 mA Output
 Proportional to Dynamic Signal and/or
Overall
 Voltage Output Accelerometers
 Preferred in U.S.
 Generally 100mV per g Sensitivity
Grounds
A Potential Problem Source
Ground Loops
Caused when two or more grounds are at
different potentials
Sensors should be grounded only at the
sensor, not the monitoring rack!

10. Sensor Cables
 Coaxial with BNC Connectors
 Long Coaxial can become antennas!
 Twisted, Shielded Pair
 Teflon Shield – ground at only one end!
Driving Long Cables
 Under 90 feet, cable capacitance no problem –Cable Capacitance spec’d in Pico-farads per foot of cable
length
 Over 90 feet or so, CCD must supply enough current to charge the cable as well as the sensor amplifier.
 May result in amplifier output voltage becoming “Slew Rate Limited”
Output of Sinusoid looks like this:
 What’s Happening?
 The + part of the signal is being limited by the current available to drive the cable capacitance.
 In the – part of the sin wave, the op-amp must “sink” the current being discharged by the cable capacitance.

11. Sensor Cables
 Practical Effect:
 Signal distortion produces harmonics
 May lead to vibration signals being misinterpreted.
 To calculate the maximum frequency for a length of cable:
Signal Conditioning Gain
 Integration (Hardware)
 AC/DC Coupling
 Anti-Aliasing Filter(s)
 Sample and Hold Circuit

12. AC/DC Coupling
 Normally, Systems are AC coupled
 Means that there is a DC blocking Capacitor that only allows AC signal through to the system
 MAARS(Metro Addiction Assessment Referral Service ) Innovation
 DC Switch that allows AC and DC to work on the same data channel without contaminating phase
 Allows use of same channel to record data for shaft centerline (DC) and Transient data (AC)
Data Acquisition and Storage
 Analog to Digital Converter
 Hard disk vs. Flash Memory
 Physical download vs. Ethernet file Transfer
 FFT Conversion(Fast Fourier transform)

13. Generation and measurement of seismic waves

14. Mechanical seismograph
Mass
Damping
Spring
Measure of mass
displacement
Recording

15. Electromagnetic seismometer
Voltage is proportional with the velocity of the
coil in the magnetic field
Geophone
Frequency (Hz) Type of measurements
0.00001-0.0001 Earth tides
0.0001-0.001 Earth free oscillations, earthquakes
0.001-0.01 Surface waves, earthquakes
0.01-0.1 Surface waves, P and S waves, earthquakes with M > 6
0.1-10 P and S waves, earthquakes with M> 2
10-1000 P and S waves, earthquakes, M< 2
*Mach Number M
Where u-Local flow velocity
c-speed of sound in the medium
c
u
M 

16. Accelerometer, the hart of the broad band seismometer and the
accelerometer
Simplified principle behind Force Balanced Accelerometer. The displacement transducer normally
uses a capacitor C, whose capacitance varies with the displacement of the mass. A current,
proportional to the displacement transducer output, will force the mass to remain stationary
relative to the frame.
mass
R
C
Force
coil
spring
Displacement
transducer
Volt out ~
acceleration

17. Sensor frequency response
All seismometers have a natural resonance frequency f0 below which the output is no longer
linearly proportinal with the ground velocity
Short period seismometer
Filter responseLeft: An RC high cut filter consisting
of a capacitor C and a resistor Rc. The resistance of
the capacitor decreases with increasing frequency so
the effect of the RC combination is to filter out
higher frequencies. The input signal is x(t) and the
output signal y(t). Right: The amplitude response
(output y(f) divided by input x(f)) of the RC filter.

18. Displacement, velocity and acceleration
A fault is displaced a given distance D
A standard seismometer measures the velocity of the ground V
A force is proportinal to acceleration measure by an accelerometer A
The relation between these measures are: V = 2 f D ; A = 2 f A
f : frequency in Hz
Measuring one, we can therefore calculate the others
Seismologists like to use nm displacement
Sensor output
All sensore give an output in volts
The output is linearly proportianl to velocity for seismometers
The output is linearly proportional to acceleration for accelerometers

19. Conjecture
 Astrology is a pseudoscience that claims to divine information about human affairs and terrestrial
events by studying the movements and relative positions of celestial objects.
Studies area
1. Michel de Nostredame (depending on the source, 14 or 21 December 1503 – 1 or 2 July 1566),
usually Latinised as Nostradamus, was a French astrologer, physician and reputed seer, who is best known
for his book Les Prophéties, a collection of 942 poetic quatrains allegedly predicting future events. The book
was first published in 1555.
2. Johannes Kepler (27 December 1571 – 15 November 1630) was a German astronomer, mathematician,
and astrologer. He is a key figure in the 17th-century scientific revolution, best known for his laws of
planetary motion, and his books Astronomia nova, Harmonices Mundi, and Epitome Astronomiae
Copernicanae .These works also provided one of the foundations for Newton's theory of universal
gravitation.
3. Galileo has been called the "father of observational astronomy", the "father of modern physics", and the
"father of science".His contributions to observational astronomy include the telescopic confirmation of the
phases of Venus, the discovery of the four largest satellites of Jupiter (named the Galilean moons in his
honour), and the observation and analysis of sunspots.
 Horology the study and measurement of time.
Universal Clock" at the Clock Museum
in Zacatlán, Puebla, Mexico

20. Seismic -CHARADE

21. Seismic -CHARADE

22. Seismic -CHARADE

23. Seismic -CHARADE

24. Seismic -CHARADE

25. Seismic -CHARADE

26. Seismic -CHARADE

27. Seismic -WAVES

28. Quake-Catcher Network
Sensores can be a mobile phone or better, a very inexpensive
accelerometer with built in digitizers ($100). Blue dots are stations,
red dots are events.
Equator
Prime Meridian
N
W E
S

29. RING OF FIRE

32. GSN network, all have public access
A large number of seismic stations are open to the public so any user can, using e.g. the free SeisComP,
build his own seismic network.

33. Seismic network is a set of interconnected seismic stations that work together to detect the seismic
waves in space and time with the main purpose of locating the earthquakes.
Location
Thunder and lightning both occur at the same instant. If you are observing them from a distance, then
you perceive the lightning first, because the light travels to you much faster than the sound does.
Lightning. Its ionization of air is what makes the boom(Sound).

34. Networked seismic stations
Computer with
data collection
software
Seismic
recorder
Communication network
Seismic
recorder
Seismic
recorder
Seismic
recorder
The SeisComP data collection software.
To the right is seen the traces for a
trigger and to the left the location is
shown together with the arrival times.

35. Seismic sensors for different frequencies
Different sensors are distinguised by the lowest frequecy they can record linearly
Geophone: 4.5 Hz →
Short period: 1 Hz →
Broadband: 30 s (0.03 Hz) →
Very broadband: 120 s (0.008 Hz) →
Inreasing
price
Correction for frequency response
Seismologists like to work with displacment or velocity Within certain frequency limits it is possible
to correct for the instrument response and generate displacment or velocity
The top trace shows the original digitally recorded signal. The bottom trace shows the signal converted to
true ground displacement in nm. The seismometer is a 1 Hz sensor with an output proportional to ground
velocity.

36. Computer Data storage
Analog to
Digital
converter
Power
supply
GPS
Communication
Sensor
input
Main units of a seismic recorder. The GPS can be connected to the digitizer or the recorder. The power
supply may be common for all elements or each may have its own regulator, but usually the power
source is unique (e.g. a battery).
Time
Amplitude
Δt
Digitizer: The analog to digital conversion
process. The arrows show the location and
values (amplitudes) of the samples and the
signal is thus approximated with a sequence
of numbers available at time intervals Δt.
Normally a signal is sampled 100 times a
second.

37. We record in 3 directions to get the 3D earth movement
Seismic recorder
•The seismometer gives out an electric signal proportinal to ground movement
•The signal is saved in a seismic recorder
•The recorder migh also retransmit the signal to a center

38. Seismic station
The seismic station has the sensor, the digitzer and may also have a recorder. The most crititcal part of
the installation is the broad band sensor which must be well shielded from ambient noise and
temperature changes
High ambient noise can ruin the signals from a good sensor !
Microseismic noise
Seismic noise in different filter bands. The short period station (1 Hz) is situated about 40 km
from the North Sea and the unfiltered trace clearly shows the high level of low frequency noise
(~0.3 Hz) generated by the sea.

39. MEMS accelerometer
Principal elements of a MEMS (micro electro mechanical systems) accelerometer with capacitive
transducer. The mass is the upper mobile capacitor plate which can rotate around the torsion bars. The
displacement, proportional to acceleration, is sensed with the variance in the capacitance.
The size of the sensor above is about 2 mm. Found in mobile phones
A smart phone has a built in accelerometer and a digitizer. So it can work as a complete seismic station
with the appropriate software. Some smartphones are actually connected to global seismic networks

40. Sensor to use
•All sensors can, within a given frequecy range, produce the same result
•Depending on task to be solved, a low price sensor, like the geophone might be suitable, e.g. for location
and magnitude
•Senstive accelerometers may ofen be sufficient in noisy environments
•Broad band sensors are needed for doing advanced analysis for larger events
•Geophone, one componnent: 100 $
•Accelerometer, three component; 3000 $
•Short period, one component: 500-1000 $
•Broadband, three component : 10000-150000 $
Kinemetrics accelerometer
13 cm

41. Geophone and short period seismometers, commenly used in
local seismic networks
Nanometrics and Guralp

42. Old and new seismic SeisComP and recording drums
Potential use of a small seismic array
•Improved detection of weak signals
•Automatic detection of P and S-wave arrivals
•Determination of azimuth
•Automatic location
•Location of weak emergent arrivals like volcanic
tremor
•Building a regional location capability in a small
area
Future
•New communcations will make local
recording redundant
•Some networks will consist of only low cost
acceelrometers connected in a global or local
network
•Broad band seismometers become better and
smaller but not cheaper in the short run
•MEMS technology, cheaper

43. Broadband station insulation Broad band on rock

44. Vibro meter and Accelerometer
What is Vibro meters ……?
The instruments or equipments which are used for measure the displacement, velocity, frequency,
phase distortion and acceleration of a vibrating g body are called vibration measuring instruments
named as vibro meters.
How it works
Vibration measuring devices having mass , spring , dashpot etc are called seismic instruments .the
quantity which is measure by the instruments are displayed on the screen in the form of electric
signal which can be readily amplified and recorded. The output of electric signal of the instrument is
proportional to the quantity which is measured. The input is reproduced as output very precisely.
Types of vibration measuring device
1. Vibro meter
• A Vibro meter or seismometer is used for measuring displacement of a vibrating body.
• Vibro meter is design with the low natural frequency transducers

45. Vibro meter and Accelerometer
Its natural frequency has ranged between 1 Hz to 5
Hz and useful natural frequency range of 10 Hz to
2000 Hz. The sensitivity of this instruments is in the
range between the 20 to 350 mV/cm/s. The
maximum displacement is ranged between the 0.5
peaks to one peaks.
Application-:
The instrument is used to record building vibrations. also used for measuring vibration of the huge
structure like railway bridge.
Disadvantage of vibrometer-:
It is large in size because of its relative motion of the seismic mass must be of the same arrangement
of the magnitude as that of the vibration to be measured.

46. Vibro meter and Accelerometer
Accelerometer
Accelerometer is used for measure the acceleration of the
vibrating body.
Accelerometer is a instrument used for measuring the acceleration
of the vibrating body. The accelerometer is design with the high
natural frequency and it is said to be high frequency transducer.
Several different type of accelerometer is used-:
Electromagnetic type of accelerometer use damper to extend the
useful natural frequency range. it is also using for the prevents
phase distortion.
Piezoelectric crystal accelerometer having zero damping is
operating without distortion. It is used for measuring high
frequency.
Seismic mass accelerometer is used for low frequency vibration.
The supporting springs are four electric strain gauges wires which
is connected with the bridge circuit.

47. Vibro meter and Accelerometer
Accelerometer Mounting Procedure
When measuring vibration we must always attach the accelerometer as close as possible to the bearing.
More specifically, we must attach it as close as possible to the centerline of the bearing to avoid
picking up distorted signals.

48. Vibro meter and Accelerometer
Understand how vibration sensors should be mounted

49. Vibro meter and Accelerometer
Fullarton Tachometer:
This instrument is known as single reed instrument. • It consists
of a thin strip carrying small mass attached at one of its free
ends.
• The strip is treated as a cantilever the length of which is
changed by means of a screw mechanism as shown in figure.
The strip of the instrument is pressed over the vibrating body to
find its natural frequency. We go on changing the length of the
strip till amplitude of vibration is maximum.
At the instant, the excitation frequency equals the natural
frequency of cantilever strip which can be directly seen from
the strip itself.
• The strip has different frequencies for its different lengths.
• The natural frequency can be determined with the help of
this formula

50. Vibro meter and Accelerometer
Fruhm Tachometer
This is also known as multi reed instrument.
• It consists of several reed of known different natural frequencies.
• There may be a known series of frequencies for the reeds.
• Small difference in the frequencies of successive reeds will show more accurate results.
• The instrument is brought in contact with the vibrating body whose frequency is to be measured
and one of the reeds will be having maximum amplitude and hence that reed will be showing the
frequency of the vibrating body.
• The mathematical analysis involved in the calculation of the natural frequency of the vibrating
body with the help of a Fruhm's Reed Tachometer is discussed below

51. REFERENCES
1. Experimental Methods for Engineers / Holman/McGraw Hill.
2. Mechanical Measurements / Sirohi and Radhakrishna / New Age.
3. Instrumentation & Mech. Measurements /A.K. Tayal /Galgotia Publications.
4. Instrumentation and Control systems! S.Bhaskar/Anuradha Agencies.
5. Instrumentation, measurement & analysis IB.C.Nakra & K.K.Choudhary/
TMH.
6. Principles of Industrial Instrumentation and Control Systems Chennakesava R
Alavala/ Cengage Learning.
7. Measurement systems: Application and design, Doeblin Earnest. O. Adaptation
by Manik and Dhanesh/ TMH.
8. Mechanical and Industrial Measurements / R.K. Jain/ Khanna Publishers.
9. Mechanical Measurements / BeckWith, Marangoni,Linehard, PHI / PE.

52. Thank you