The document discusses the piezoelectric effect, which is the generation of electric voltage in materials subjected to mechanical stress, and how piezoelectric accelerometers leverage this effect to measure dynamic changes in acceleration and vibration. It details the working principle, advantages, and limitations of piezoelectric accelerometers, emphasizing their high frequency and transient response but also the challenges of low output and high impedance. Additionally, it highlights various applications in dynamic measurement scenarios, including automotive and medical fields.

2. Introduction
Piezoelectric effect:-
The piezoelectric effect refers to a change in electric polarization that
is produced in certain materials when they are subjected to
mechanical stresses. This stress-dependent change in polarization
manifests as a measurable potential difference across the material.
Referred to as the direct piezoelectric effect, this phenomenon is
observable in many naturally available crystalline materials, including
quartz, Rochelle salt, and even human bone. Engineered materials,
such as lithium niobate and lead zirconate titanate (PZT), exhibit a
more pronounced piezoelectric effect.

3.  An accelerometer is an electromechanical device used to measure
acceleration forces. Such forces may be static, like the continuous force
of gravity or, as is the case with many mobile devices, dynamic to sense
movement or vibrations. Acceleration is the measurement of the
change in velocity, or speed divided by time.

4. piezoelectric accelerometer
 A piezoelectric accelerometer is an accelerometer that employs the piezoelectric
effect of certain materials to measure dynamic changes in mechanical variables (e.g.,
acceleration, vibration, and mechanical shock). As with all accelerometer,
piezoelectric accelerometers convert one form of energy into another and provide an
electrical signal in response to a quantity, property, or condition that is being
measured.
 As with all accelerometer, piezoelectric accelerometers convert one form of energy
into another and provide an electrical signal in response to a quantity, property, or
condition that is being measured. Using the general sensing method upon which all
accelerometers are based, acceleration acts upon a seismic mass that is restrained by a
spring or suspended on a cantilever beam, and converts a physical force into an
electrical signal. Before the acceleration can be converted into an electrical quantity it
must first be converted into either a force or displacement. This conversion is done via
the mass spring system shown in the figure to the right.

5. Working Principle
 The piezoelectric accelerometer work on the principle of piezoelectric effect.
When mechanical stress or forces are applied to some materials along certain
planes, they produce electric voltage. This electric voltage can be measured
easily by the voltage measuring instruments, which can be used to measure the
stress or force.
 The physical quantities like stress and force cannot be measured directly. In
such cases the material exhibiting piezoelectric accelerometer can be used. The
stress or the force that has to be measured is applied along certain planes to
these materials. The voltage output obtained from these materials due to
piezoelectric effect is proportional to the applied stress or force. The output
voltage can be calibrated against the applied stress or the force so that the
measured value of the output voltage directly gives the value of the applied
stress or force. In fact the scale can be marked directly in terms of stress or
force to give the values directly.
 The voltage output obtained from the materials due to piezoelectric effect is
very small and it has high impedance. To measure the output some amplifiers,
auxiliary circuit and the connecting cables are required.

7. Materials used for the Piezoelectric Accelerometer
 There are various materials that exhibit piezoelectric effect as
mentioned above. The materials used for the measurement purpose
should posses desirable properties like stability, high output,
insensitive to the extreme temperature and humidity and ability to be
formed or machined into any shape. But none of the materials
exhibiting piezoelectric effect possesses all the properties. Quartz,
which is a natural crystal, is highly stable but the output obtained from
it is very small. It also offers the advantage of measuring very slowly
varying parameter as they have very low leakage when they are used
with high input impedance amplifiers.

8. Advantages of Piezoelectric accelerometer
Every devise has certain advantages and limitations. The piezoelectric
accelerometer offer several advantages as mentioned below:
1) High frequency response: They offer very high frequency response
that means the parameter changing at very high speeds can be sensed
easily.
2) High transient response: The piezoelectric accelerometer can detect
the events of microseconds and also give the linear output.
3) High output: They offer high output that be measured in the
electronic circuit.
4) The piezoelectric accelerometer are small in size and have rugged
construction.

9. Limitations of Piezoelectric Accelerometer
Some of the limitations of piezoelectric accelerometer are:
1) Output is low:- The output obtained from the piezoelectric
accelerometer is low, so external electronic circuit has to be connected.
2) High impedance:- The piezoelectric crystals have high impedance so
they have to be connected to the amplifier and the auxiliary circuit,
which have the potential to cause errors in measurement. To reduce
these errors amplifiers high input impedance and long cables should
be used.
3) Forming into shape:- It is very difficult to give the desired shape to
the crystals with sufficient strength.

10. Applications of the Piezoelectric accelerometer
1) The piezoelectric accelerometer are more useful for the dynamic
measurements, i.e. the parameters that are changing at the fast rate. This is
because the potential developed under the static conditions is not held by the
instrument. Thus piezoelectric crystals are primarily used measurement of
quantities like surface roughness, and also in accelerometers and vibration
pickups.
2) For the same reasons they can be used for studying high speed phenomenon
like explosions and blast waves. They are also used in aerodynamic shock tube
work and seismograph (used for measurement of acceleration and vibration in
rockets).
3) Many times the piezo sensors or accelerometer are used along with the strain
gauges for measurement of force, stress, vibrations, etc.
4) The automotive companies used piezoelectric accelerometer to detect
detonations in the engine blocks.
5) Piezoelectric accelerometer are used in medical treatment, Sonchemistry and
industrial processing equipments for monitoring the power.

11. Reference
1) Book: Mechanical and Industrial Measurements by R. K. Jain, Khanna
Publishers
2) Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck
3) http://en.wikipedia.org/wiki/Piezoelectricity

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