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Electronic Pressure Element - Piezoelectric |
When pressure, force or acceleration is applied to a quartz crystal, a
charge is developed across the crystal that is proportional to the force
applied. The fundamental difference between these crystal sensors and
static-force devices such as strain gages is that the electric signal
generated by the crystal decays rapidly. This characteristic makes these
sensors unsuitable for the measurement of static force or pressures but
useful for dynamic measurements.
Pieoelectric device can further be classified according to whether the
crystal's electrostatic charge, its resistivity, or its resonant
frequency electrostatic charge is measured. Depending which phenomenon
is used, the crystal sensor can be called electrostatic, piezoresistive
or resonant. The desirable features of peizoelectric sensor include
their rugged construction, small size, high speed, and self-generated
signal. Piezoelectrics are sensitive to temperature variations and
require special cabling and amplification.
Piezoelectric (Working Principle)
When pressure
is applied to a crystal, it is elastically deformed. This deformation
results in a flow of electric charge (which lasts for a period of a few
seconds). The resulting electric signal can be measured as an indication
of the pressure which was applied to the crystal. This sensor cannot
detect static pressures, but are used to measure rapidly changing
pressures resulting from blasts, explosions, pressure pulsations or
other sources of shock or vibration.
Piezoelectric Materials
Many polymers, ceramics, and molecules such as water are permanently
polarized: some parts of the molecule are positively charged, while
other parts of the molecule are negatively charged. When an electric
field is applied to these materials, these polarized molecules will
align themselves with the electric field, resulting in induced dipoles
within the molecular or crystal structure of the material. Furthermore,
a permanently-polarized material such as quartz (SiO2) or barium
titanate (BaTiO3) will produce an electric field when the material
changes dimensions as a result of an imposed mechanical force. These
materials are piezoelectric, and this phenomenon is known as the
piezoelectric effect.
Conversely, an applied electric field can cause a piezoelectric material
to change dimensions. This phenomenon is known as electrostriction, or
the reverse piezoelectric effect. Piezoelectric materials are used in
acoustic transducers, which convert acoustic (sound) waves into electric
fields, and electric fields into acoustic waves. Transducers are found
in telephones, stereo music systems, and musical instruments such as
guitars and drums. Quartz, a piezoelectric material, is often found in
clocks and watches. An oscillating electric field makes the quartz
crystal resonate at its natural frequency. The vibrations of this
frequency are counted and are used to keep the clock or watch on time. A
manufacturer has recently embedded piezoelectric materials in skis in
order to damp out the vibrations of the skis and help keep the ski edges
in contact with the snow. More...
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