Study of vibration measurement

1. VIBRATION
MEASUREMENT
BY,
TUSHAR GHAG
ME-MACHINE DESIGN
ROLL NO.221603

2. INTRODUCTION
• The instruments which are used to measure the displacement, velocity or
acceleration of a vibrating body are called vibration measuring instruments.
• Vibration measuring devices having spring, mass, dashpot etc. Are known
as seismic instruments.
• The quantities to be measured are displayed on a screen in the form of an
electric signal which can be readily amplified and recorded.

3. .
• The output of electric signal will be proportional to the quantity which is
to be measured.
• Two types of seismic transducer known as vibrometer and accelerometer
are widely used.
• A vibrometer or a seismometer is a device used to measure the displacement
of a vibrating body.
• Accelerometer is an instrument to measure the acceleration of a vibrating
body.
• Vibrometer is designed with low natural frequency & accelerometer with
high natural frequency.

4. Vibration Measurement Scheme

5. What is an accelerometer?
• An accelerometer is a transducer that is used to measure the physical or
measurable acceleration that is made by an object.
• An accelerometer is an electro-mechanical device that is used to measure
the specific force of an object, a force obtained due to the phenomenon of
weight exerted by an object that is kept in the frame of reference of the
accelerometer.

6. How do accelerometers work?
• Accelerometers are electromechanical devices that sense either static or dynamic
forces of acceleration. Static forces include gravity, while dynamic forces can
include vibrations and movement.
• Most accelerometers are Micro-Electro-Mechanical Sensors (MEMS).
• Accelerometers can measure acceleration on one, two, or three axes. 3-axis units are
becoming more common as the cost of development for them decreases.
• Accelerometers contain capacitive plates internally. Some of these are fixed, while
others are attached to miniscule springs that move internally as acceleration forces
act upon the sensor. As these plates move in relation to each other,
the capacitance between them changes. From these changes in capacitance, the
acceleration can be determined.

7. Accelerometer Specifications
A typical accelerometer has the following basic specifications:
1. Analog/digital
2. Number of axes
3. Output range (maximum swing)
4. Sensitivity (voltage output per g)
5. Dynamic range
6. Bandwidth
7. Amplitude stability
8. Mass

8. Accelerometer Specifications
(Continued)
o ANALOG VS. DIGITAL: First and foremost, you must choose between
an accelerometer with analog outputs or digital outputs. This will be
determined by the hardware that you are interfacing the accelerometer
with. Analog style accelerometers output a continuous voltage that is
proportional to acceleration.
e.g. 2.5V for 0g, 2.6V for 0.5g, 2.7V for 1g.
Digital accelerometers usually use pulse width modulation (pwm) for their
output. This means there will be a square wave of a certain frequency, and
the amount of time the voltage is high will be proportional to the amount
of acceleration.

9. Accelerometer Specifications
(Continued)
o Number Of Axes: Accelerometers are available that measure in one, two,
or three dimensions. The most familiar type of accelerometer measures
across two axes. However, three-axis accelerometers are increasingly
common and inexpensive.
o Maximum Swing: If you only care about measuring tilt using earth's
gravity, a ±1.5g accelerometer will be more than enough. If you are going
to use the accelerometer to measure the motion of a car, plane or robot, ±2g
should give you enough headroom to work with. For a project that
experiences very sudden starts or stops, you will need one that can handle
±5g or more.

10. Accelerometer Specifications
(Continued)
o Sensitivity: Generally speaking, the more sensitivity the better. This
means that for a given change in acceleration, there will be a larger change
in signal. Since larger signal changes are easier to measure, you will get
more accurate readings.
o Dynamic Range: The range between the smallest acceleration detectable
by the accelerometer to the largest before distorting or clipping the output
signal.

11. Accelerometer Specifications
(Continued)
o Bandwidth: This means the amount of times per second you can take a
reliable acceleration reading. For slow moving tilt sensing applications, a
bandwidth of 50 hz will probably suffice. If you intend to do vibration
measurement, or control a fast moving machine, you will want a bandwidth
of several hundred hz.
o Amplitude Stability: This is not a specification in itself, but a description
of several. Amplitude stability describes a sensor's change in sensitivity
depending on its application, for instance over varying temperature or time.

12. Accelerometer Specifications
(Continued)
Mass: The mass of the accelerometer should be significantly smaller than
the mass of the system to be monitored so that it does not change the
characteristic of the object being tested.

13. Applications
1) Engineering
2) Biology
3) Industry
4) Building And Structural Monitoring
5) Medical Applications
6) Navigation
7) Transport
8) Volcanology
9) Consumer Electronics

14. Types
Of Accelerometer
The following are the different types of accelerometers:
1) Capacitive accelerometer
2) Piezoelectric accelerometer
3) Potentiometric accelerometer
4) Laser accelerometer
5) Servo accelerometer
6) Strain gauge accelerometer

15. Types Of Accelerometer
➢ Capacitive Accelerometer:

16. Types Of Accelerometer
(Continued)
Potentiometric Accelerometer:

17. Types Of Accelerometer
(Continued)
 Piezometer Accelerometer:

18. • The active elements of the accelerometer are the piezoelectric elements.
• These act as springs connecting the base of the accelerometer to the seismic masses
via the rigid triangular center pot.
• When the accelerometer is vibrated a force, equal to the product of the acceleration
of a seismic mass and its mass, acts on each piezoelectric element.
• The piezoelectric elements produce a charge proportional to the applied force.
• The seismic masses are constant and consequently the elements produce a charge
which is proportional to the acceleration of the seismic masses.
• As the seismic masses accelerate with the same magnitude and phase as the
accelerometer base over a wide frequency range, the output of the accelerometer is
proportional to the acceleration of the base and hence to the acceleration of the
surface onto which the accelerometer is mounted.

19. Piezoelectric Materials:
• A piezoelectric material is one which develops an electrical charge when subjected
to a force.
• Materials which exhibit this property are intrinsic piezoelectric monocrystals such
as quartz and Rochelle salt, and artificially polarized ferroelectric ceramics which
are mixtures of different compounds such as barium titanate, lead zirconate and lead
meta niobate.

20. Calibration Of Accelerometer
• The accelerometer, either alone or with other electrical components,
produces an electrical output signal related to the applied motion.
• Accurate accelerometer calibration is a way to provide physical meaning to
this electrical output and it is a prerequisite for quality measurements.
• When we talk about calibration we are essentially referring to the sensitivity
calibration.

21. Calibration Of Accelerometer
(Continued)
Calibrations are divided into three distinct methods;
1) Absolute Methods: These include laser interferometry and reciprocity
techniques.
2) Comparison Methods: This refers to the back-to-back method.
3) Calibrators: This involves the use of a vibration exciter of known
vibration level.

22. Calibration Of Accelerometer
(Continued)
Laser Interferometry:
• This absolute method of calibration involves the use of a very specialized
equipment and it is therefore unlikely that average users of accelerometers
would ever carry out their own calibration on this type.
• The equipment used is a Michelson interferometer.

23. MICHELSON INTERFEROMETER

24. BACK-TO-BACK METHOD.

25. CALIBRATION EXCITER
CALIBRATORS