A piezoelectric sensor is a device that uses the piezoelectric effect, to measure changes
in pressure, acceleration, strain or force by converting them to an electrical charge.
The prefix piezo- is Greek for 'press' or 'squeeze'.
Applications Image:Piezoelectric pickup1.jpg|thumb|Piezoelectric
disk used as a guitar pickup Piezoelectric sensors have proven to be versatile tools
for the measurement of various processes. They are used for quality assurance, process
control and for research and development in many industries. Although the piezoelectric
effect was discovered by Pierre Curie in 1880, it was only in the 1950s that the piezoelectric
effect started to be used for industrial sensing applications. Since then, this measuring principle
has been increasingly used and can be regarded as a mature technology with an outstanding
inherent reliability. It has been successfully used in various applications, such as in medical,
aerospace, nuclear instrumentation, and as a tilt sensor in consumer electronics or a
pressure sensor in the touch pads of mobile phones. In the automotive industry, piezoelectric
elements are used to monitor combustion when developing internal combustion engines. The
sensors are either directly mounted into additional holes into the cylinder head or the spark/glow
plug is equipped with a built-in miniature piezoelectric sensor.
The rise of piezoelectric technology is directly related to a set of inherent advantages. The
high modulus of elasticity of many piezoelectric materials is comparable to that of many metals
and goes up to 106 N/m². Even though piezoelectric sensors are electromechanical systems that
react to compression, the sensing elements show almost zero deflection. This gives piezoelectric
sensors ruggedness, an extremely high natural frequency and an excellent linearity over
a wide amplitude range. Additionally, piezoelectric technology is insensitive to electromagnetic
fields and radiation, enabling measurements under harsh conditions. Some materials used
are extremely stable at high temperatures, enabling sensors to have a working range of
up to 1000 °C. Tourmaline shows pyroelectricity in addition to the piezoelectric effect; this
is the ability to generate an electrical signal when the temperature of the crystal changes.
This effect is also common to piezoceramic materials. Gautschi in Piezoelectric Sensorics
offers this comparison table of characteristics of piezo sensor materials vs other types:
One disadvantage of piezoelectric sensors is that they cannot be used for truly static
measurements. A static force will result in a fixed amount of charges on the piezoelectric
material. While working with conventional readout electronics, imperfect insulating
materials, and reduction in internal sensor resistance will result in a constant loss
of electrons, and yield a decreasing signal. Elevated temperatures cause an additional
drop in internal resistance and sensitivity. The main effect on the piezoelectric effect
is that with increasing pressure loads and temperature, the sensitivity is reduced due
to twin-formation. While quartz sensors need to be cooled during measurements at temperatures
above 300 °C, special types of crystals like GaPO4 gallium phosphate do not show any twin
formation up to the melting point of the material itself.
However, it is not true that piezoelectric sensors can only be used for very fast processes
or at ambient conditions. In fact, there are numerous applications that show quasi-static
measurements, while there are other applications with temperatures higher than 500 °C.
Piezoelectric sensors can also be used to determine aromas in the air by simultaneously
measuring resonance and capacitance. Computer controlled electronics vastly increase the
range of potential applications for piezoelectric sensors.
Piezoelectric sensors are also seen in nature. The collagen in bone is piezoelectric, and
is thought by some to act as a biological force sensor.
Principle of operation Depending on how a piezoelectric material
is cut, three main modes of operation can be distinguished: transverse, longitudinal,
and shear. Transverse effect
A force is applied along a neutral axis and the charges are generated along the direction,
perpendicular to the line of force. The amount of charge depends on the geometrical dimensions
of the respective piezoelectric element. When dimensions apply,
,
where is the dimension in line with the neutral axis, is in line with the charge generating
axis and is the corresponding piezoelectric coefficient.[2]
Longitudinal effect The amount of charge produced is strictly
proportional to the applied force and is independent of size and shape of the piezoelectric element.
Using several elements that are mechanically in series and electrically in parallel is
the only way to increase the charge output. The resulting charge is
,
where is the piezoelectric coefficient for a charge in x-direction released by forces
applied along x-direction. is the applied Force in x-direction [N] and corresponds to
the number of stacked elements. Shear effect
The charges produced are strictly proportional to the applied forces and are independent
of the element’s size and shape. For elements mechanically in series and electrically in
parallel the charge is
.
In contrast to the longitudinal and shear effects, the transverse effect opens the possibility
to fine-tune sensitivity on the force applied and the element dimension.
Electrical properties
A piezoelectric transducer has very high DC output impedance and can be modeled as a proportional
voltage source and filter network. The voltage V at the source is directly proportional to
the applied force, pressure, or strain. The output signal is then related to this mechanical
force as if it had passed through the equivalent circuit.
A detailed model includes the effects of the sensor's mechanical construction and other
non-idealities. The inductance Lm is due to the seismic mass and inertia of the sensor
itself. Ce is inversely proportional to the mechanical elasticity of the sensor. C0 represents
the static capacitance of the transducer, resulting from an inertial mass of infinite
size. Ri is the insulation leakage resistance of the transducer element. If the sensor is
connected to a load resistance, this also acts in parallel with the insulation resistance,
both increasing the high-pass cutoff frequency.
For use as a sensor, the flat region of the frequency response plot is typically used,
between the high-pass cutoff and the resonant peak. The load and leakage resistance need
to be large enough that low frequencies of interest are not lost. A simplified equivalent
circuit model can be used in this region, in which Cs represents the capacitance of
the sensor surface itself, determined by the standard formula for capacitance of parallel
plates. It can also be modeled as a charge source in parallel with the source capacitance,
with the charge directly proportional to the applied force, as above.
Sensor design
Based on piezoelectric technology various physical quantities can be measured; the most
common are pressure and acceleration. For pressure sensors, a thin membrane and a massive
base is used, ensuring 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 . The main difference in the working principle
between these two cases is the way forces are applied to the sensing elements. In a
pressure sensor a thin membrane is used to transfer the force to the elements, while
in accelerometers the forces are applied by an attached seismic mass.
Sensors often tend to be sensitive to more than one physical quantity. Pressure sensors
show false signal when they are exposed to vibrations. Sophisticated pressure sensors
therefore use acceleration compensation elements in addition to the pressure sensing elements.
By carefully matching those elements, the acceleration signal is subtracted from the
combined signal of pressure and acceleration to derive the true pressure information.
Vibration sensors can also be used to harvest otherwise wasted energy from mechanical vibrations.
This is accomplished by using piezoelectric materials to convert mechanical strain into
usable electrical energy. Sensing materials
Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics
and single crystal materials. The ceramic materials have a piezoelectric constant/sensitivity
that is roughly two orders of magnitude higher than those of the natural single crystal materials
and can be produced by inexpensive sintering processes. The piezoeffect in piezoceramics
is "trained", so their high sensitivity degrades over time. This degradation is highly correlated
with increased temperature. The less-sensitive, natural, single-crystal
materials have a higher – when carefully handled, almost unlimited – long term stability.
There are also new single-crystal materials commercially available such as Lead Magnesium
Niobate-Lead Titanate. These materials offer improved sensitivity over PZT but have a lower
maximum operating temperature and are currently more expensive to manufacture.
See also Charge amplifier
List of sensors Piezoelectricity
Piezoresistive effect References
External links Material constants of gallium phosphate
Langatate - new material for high-temperature piezoelectric sensors
Spark-Plug with integrated miniaturized piezoelectric pressure sensor [3]
Using the Piezoelectric Effect in Sensors