22-02-2013, 09:46 AM
INTRODUCTION TO PIEZOELECTRIC SENSORS
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INTRODUCTION
Over the past 50 years piezoelectric sensors have
proven to be a versatile tool for the measurement of
various processes. Today, they are used for the
determination of pressure, acceleration, strain or
force in quality assurance, process control and
development across many different industries.
Piezoelectric sensors rely on the piezoelectric
effect, which was discovered by the Curie brothers
in the late 19th century. While investigating a
number of naturally occurring materials such as
tourmaline and quartz, Pierre and Jacques Curie
realized that these materials had the ability to
transform energy of a mechanical input into an
electrical output. More specifically, when a pressure
[piezo is the Greek word for pressure] is applied to
a piezoelectric material, it causes a mechanical
deformation and a displacement of charges. Those
charges are highly proportional to the applied
pressure [Piezoelectricity].
PRINCIPLE OF OPERATION
Depending on the way a piezoelectric material is
cut, three main types of operations can be
distinguished 1. transversal 2. longitudinal 3. shear.
A gallium phosphate crystal is shown with typical
sensor elements manufactured out of it. Depending
on the design of a sensor different ”modes” to load
the crystal can be used: transversal, longitudinal
and shear (arrows indicate the direction where the
load is applied). Charges are generated on both ”xsides”
of the element. The positive charges on the
front side are accompanied by negative charges on
the back.
SENSOR DESIGN
Based on piezoelectric technology various physical
dimensions can be measured, the most important
include pressure and acceleration. Figure 3 shows
schematic configurations of those sensors in the
transverse configuration. In both designs, the
elements are thin cuboids that are loaded along their
longest extension. For pressure sensors, a thin
membrane with known dimensions and a massive
base is used; assuring that an applied pressure
specifically loads the elements in one direction. For
accelerometers, a seismic mass is attached to the
crystal elements. When the accelerometer
experiences a motion, the invariant seismic mass
loads the elements according to Newton’s second
law of motion F=ma.
MATERIALS
Two main groups of materials are used for
piezoelectric sensors: piezoelectric ceramics and
single crystal materials. The ceramic materials (e.g.
PZT ceramic) have a piezoelectric constant /
sensitivity that is roughly two orders of magnitude
higher than those of single crystal materials and can
be produced by an inexpensive sintering processes.
Unfortunately, their high sensitivity is always
combined with a lack of long term stability.
Therefore, piezoelectric ceramics are very often
used wherever the requirements for measuring
precision are not too high.
CONCLUSION
Piezoelectric sensors offer a unique set of
capabilities that cannot be found in other sensing
principles. As discussed, the inherent temperature
stability, the amplitude range and the signal quality
make it very interesting, in particular where no static
information is needed.