Piezoelectric vibration sensors
Piezoelectric vibration sensors
Introduction
piezoelectric sensor is
used for flex, touch, vibration and shock measurement. Its basic principal, at
the risk of oversimplification, is as follows: whenever a structure moves, it
experiences acceleration. A piezoelectric shock sensor, in turn, can generate a
charge when physically accelerated. This combination of properties is then used
to modify response or reduce noise and vibration. Why is that important?
Because vibration and shock can shorten the life of any electronic and
electromechanical system. Delicate leads and bond wires can be stressed,
especially after exposure to long term vibration. Solder joints can break free
and PCB traces can ever so slightly tear from impact and impulse shock,
creating the hardest type of system failure to debug; an intermittent failure.
This article discusses piezoelectric shock and vibration sensors and sensor
technology, focusing on available products (all parts mentioned here can be
found on the Digi-Key website - links are provided), as well as design issues
and design techniques.
1. How it works
piezoelectric effect was
discovered by Pierre and Jacques Curie in the latter part of the 19th century.
They discovered that minerals such as tourmaline and quartz could transform
mechanical energy into an electrical output. The voltage induced from pressure
(Greek: piezo) is proportional to that applied pressure, and piezoelectric
devices can be used to detect single-pressure events as well as repetitive
events. Still, the ability of certain crystals to exhibit electrical charges
under mechanical loading was of no practical use until very-high-input
impedance amplifiers enabled engineers to amplify the signals produced by these
crystals. Several materials can be used to make piezoelectric sensors,
including tourmaline, gallium phosphate, salts, and quartz. Most electronic
applications use quartz since its growth technology is far along, thanks to
development of the reverse application of the piezoelectric effect; the quartz
oscillator. Sensors based on the piezoelectric effect can operate from
transverse, longitudinal, or shear forces, and are insensitive to electric
fields and electromagnetic radiation. The response is also very linear over
wide temperature ranges, making it an ideal sensor for rugged environments. For
example, gallium phosphate and tourmaline sensors can have a working
temperature range of 1,000˚C. The physical design of the piezoelectric
sensor depends on the type of sensor you wish to create. For example, the
configuration of a pressure sensor, or a shock (impulse) sensor, would arrange
a smaller, but well-known mass of the crystal in a transverse configuration,
with the loading deformation along the longest tracks to a more massive base.
This assures that the applied pressure will load the base from only one
direction.accelerometer based on the piezoelectric effect, would use a known
mass to deform the sensing crystal part in either a positive or negative
direction depending on the excitation force . It should be noted that you need
a known modulus of elasticity in the sensor substrate.
Because the modulus of
elasticity is known for a substrate material, the unconstrained mass is allowed
to move with vibration making this type of piezoelectric sensor ideal for
detecting shock and vibration.
2. Designing with
piezoelectric sensors
piezoelectric sensor piezo vibration
Piezoelectric sensors
require some precautions when connecting to sensitive electronic components.
First and foremost, the voltage levels created by hard shock can be very high,
even around 100-V spikes. More than likely, an op amp will be used to interface
these sensors to an A/D converter, either discrete or on a microcontroller. One
tip is to choose a high-input-impedance op amp to minimize current. One
possible candidate is the Linear Technology JFET input dual op amp. It has 10№І
Ω input resistance and a 1 MHz gain bandwidth product, good enough to
easily handle the vibration ranges of piezoelectric sensors. Another suitable
part is the TLV2771 from Texas Instruments. This rail-to-rail low-power op-amp
also has a 10№І Ω differential input resistance and a 5 MHz unity-gain
bandwidth. Signal conditioning in a single stage can prepare the input from the
shock sensor directly into an A/D converter.amp circuits can be designed to operate
in voltage mode or charge mode. Charge mode is used when the amplifier is
remote to the sensor. Voltage mode is used when the amplifier is very close to
the sensor. Another tip is to attenuate the input signal and use the op amp’s
gain to bring into the desired range. Be aware that you may need snubbing
protection on the inputs of the op amp, especially if the design could be
subjected to harsh hits. Also note that you may think that a pressure sensor
would generate only a positive voltage, but, in reality, the signal from the
sensor can ring and introduce negative voltage spikes. This means that you may
need to squelch negative voltage levels on the op-amp inputs, especially if
using only a single rail power supply on the op amp.
3. Vibration sensor Parallax 605-00004
Many off the shelf
piezoelectric sensors are readily available to use in your designs. A case in
point is the Parallax 605-00004, which is a piezo vibra tab sensor capable of
acting as a switch, or as a vibration sensor . A polymer film laminate uses
crimped contacts and features a sensitivity of 50 mV/g. Figure 5. The flexible
through-hole LDTO polymer film piezoelectric sensors can be hard mounted or
free floating to detect strain, shock, or vibration. You should be aware that
adding mass to a piezoelectric sensor can change its resonant frequency as well
as change its baseline sensitivity. Many piezoelectric sensors like the
605-00004 are characterized to be used this way and provide supporting tables
and graphs.part worth considering is the Measurement Specialties 0-1002794-0
cantilever piezo film sensor. This is also a vibra tab sensor capable of hard
mounting to a surface, floating in an axis of inertia, or mass loaded to
prebias and calibrate. The output voltage swings can directly trip a FET or
CMOS input, and a multiaxis response can be obtained by offsetting the mass
center.LDT0 is a flexible component comprising a28 µ m thick piezoelectric PVDF
polymer film with screen-printed Ag-ink electrodes, laminated to a0.125 mm
polyester substrate, and fitted with two crimped contacts. As the piezo film is
displaced from the mechanical neutral axis, bending creates very high strain
within the piezo polymer and therefore high voltages are generated. When the
assembly is deflected by direct contact, the device acts as a flexible
«switch», and the generated output is sufficient to trigger MOSFET or CMOS
stages directly. If the assembly is supported by its contacts and left to
vibrate «in free space» (with the inertia of the clamped/free beam creating
bending stress), the device will behave as a form of accelerometer or vibration
sensor. Adding mass, or altering the free length of the element by clamping,
can change the resonant frequency and sensitivity of the sensor to suit
specific applications. Multi-axis response can be achieved by positioning the
mass off center.different experiments serve to illustrate the various
properties of this simple but versatile component.
. LDT0 as Vibration
Sensor - with the crimped contacts pushed through a printed-circuit board, the
LDT0 was soldered carefully in place to anchor the sensor. A charge amplifier
was used to detect the output signal as vibration from a shaker table was
applied (using a charge amplifier allows a very long measurement time constant and
thus allows the «open-circuit» voltage response to be calculated). Small masses
(approximately 0.26g increments) were then added to the tip of the sensor, and
the measurement repeated. Results are shown in Table 1 and the overlaid plots
in Fig 1. Without adding mass, the LDT0 shows a resonance around 180 Hz. Adding
mass to the tip reduces the resonance frequency and increases «baseline»
sensitivity.
. LDT0 as Flexible
Switch - using a charge amplifier to obtain «open-circuit» voltage sensitivity,
the output was measured for controlled tip deflections applied to the sensor
(supported by its crimped contacts as described above). 2 mm deflection was
sufficient to generate about 7 V. Voltages above 70V could be generated by
bending the tip of the sensor through 90° (see Table 2).
3. LDT0 Electrical
Frequency Response - when the source capacitance of around 480 pF is connected
to a resistive input load, a high-pass filter characteristic results. Using an
electronic noise source to generate broad-band signals, the effect of various
load resistances were measured and the -3 dB point of the R-C filter determined
(see Table 3).
4. LDT0 Clamped at
Different Lengths - using simple clamping fixture, the vibration sensitivity
was measured (as in (1) above) as the clamp was moved to allow different «free»
lengths to vibrate. The sensor may be «tuned» to suit specific frequency
response requirements (see Table 3).Piezo Film Vibra Tab Sensor is the LDT0
Solid State Switch/Vibration Sensor manufactured by Measurement Specialties.
The LDT0 is a piezoelectric film device capable of acting as switch or
vibration sensor. Characteristics of this device allow even more possibilities
for use.
Start Circuit
The circuit above allows
you to start using the LDT0 as a switch or shock detector. You can test for
functionality by checking the pin for a HIGH signal on the connected I/O pin
when the sensor is tapped, flicked or snapped..a vibration sensor the LDT0 has
a sensitivity of 50 mV/g. As mass is added to the device, sensitivity
decreases, as does it’s resonant frequency. Please see the manufacturer’s
datasheet for further details about how adding mass to the device affects these
characteristics.LDT0 is a flexible film piezoelectric device laminated to a
polymer substrate and includes two crimped contacts for mounting and electrical
connections. As the device is bent or displaced from its neutral axis, a very
high strain is generated by the piezo-polymer and high voltage is generated.
This device can generate voltages of ~70 volts. Always be sure to clamp, buffer
or filter the signal going to the I/O pin to keep it within acceptable
voltage/current limits.
4. Piezo-vibration
sensor Bosch 608-00112
sensors of this type are
suitable for the detection of structure-borne acoustic oscillations as can
occur for example in case of irregular combustion in engines and on machines.
Thanks to their ruggedness, these vibration sensors can be used even under the
most severe operating conditions. On account of its inertia, a mass exerts compressive
forces on a ring-shaped piezo-ceramic element in time with the oscillation
which generates the excitation.the ceramic element, these forces result in
charge transfer within the ceramic and a voltage is generated between the top
and bottom of the ceramic element. This voltage is picked-off using contact
discs - in many cases it is filtered and integrated - and made available as a
measuring signal. In order to route the vibration directly into the sensor,
vibration sensors are securely bolted to the object on which measurements take
place. Every vibration sensor has its own individual response characteristic
which is closely linked to its measurement sensitivity. The measurement
sensitivity is defined as the output voltage per unit of acceleration due to
gravity (see characteristic curve). The production-related sensitivity scatter
is acceptable for applications where the primary task is to record that
vibration is occurring, and not so much to measure its severity. The low
voltages generated by the sensor can be evaluated using a high-impedance AC
amplifier.
instructionssensor’s
metal surfaces must make direct contact. No washers of any type are to be used
when fastening the sensors. The mounting-hole contact surface should be of high
quality to ensure low-resonance sensor coupling at the measuring point. The
sensor cable is to be laid such that there is no possibility of sympathetic
oscillations being generated. The sensor must not come into contact with
liquids for longer periods.
Conclusion
is the electric charge
that accumulates in certain solid materials (notably crystals, certain
ceramics, and biological matter such as bone, DNA and various proteins) in
response to applied mechanical stress. The word piezoelectricity means
electricity resulting from pressure. It is derived from the Greek piezo or
piezein (рйЭжейн), which means to squeeze or press, and electric or electron
(Юлекфспн), which stands for amber, an ancient source of electric charge.
Piezoelectricity was discovered in 1880 by French physicists Jacques and Pierre
Curie. The piezoelectric effect is understood as the linear electromechanical
interaction between the mechanical and the electrical state in crystalline
materials with no inversion symmetry. The piezoelectric effect is a reversible
process in that materials exhibiting the direct piezoelectric effect (the
internal generation of electrical charge resulting from an applied mechanical
force) also exhibit the reverse piezoelectric effect (the internal generation
of a mechanical strain resulting from an applied electrical field). For
example, lead zirconate titanate crystals will generate measurable
piezoelectricity when their static structure is deformed by about 0.1% of the
original dimension. Conversely, those same crystals will change about 0.1% of
their static dimension when an external electric field is applied to the
material. The inverse piezoelectric effect is used in production of ultrasonic
sound waves. Piezoelectricity is found in useful applications such as the
production and detection of sound, generation of high voltages, electronic
frequency generation, microbalances, and ultrafine focusing of optical
assemblies. It is also the basis of a number of scientific instrumental
techniques with atomic resolution, the scanning probe microscopies such as STM,
AFM, MTA, SNOM, etc., and everyday uses such as acting as the ignition source
for cigarette lighters and push-start propane barbecues.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
different 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 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.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/mІ citation needed. Even though
piezoelectric sensors are electromechanical systems that react to compression,
the sensing elements show almost zero deflection. This is the reason why
piezoelectric sensors are so rugged, have 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
(especially gallium phosphate [2] or tourmaline) have an extreme stability even
at high temperature, 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. 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 through measurements of
resonance and capacitance simultaneously.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 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.
Frequency response of a piezoelectric sensor; output voltage vs applied force 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. 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. 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. In the flat region, the sensor can be modeled as a
voltage source in series with the sensor's capacitance or a charge source in
parallel with the capacitance 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. [7] [8] 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. 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.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 (released from the compensation element) 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. Two main
groups of materials are used for piezoelectric sensors: piezoelectric ceramics
and single crystal materials. The ceramic materials (such as PZT ceramic) 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 unfortunately their high sensitivity degrades over time. The
degradation is highly correlated with temperature. The less sensitive 'natural'
single crystal materials (gallium phosphate, quartz, tourmaline) have a much
higher - when carefully handled, almost infinite - long term stability. There
are also new single crystal materials commercially available such as Lead
Magnesium Niobate-Lead Titanate (PMN-PT). These materials offer greatly
improved sensitivity (compared with PZT) but suffer from a lower maximum
operating temperature and are currently much more expensive to manufacture. 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. Frequency response of a piezoelectric sensor;
output voltage vs applied force 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. 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. 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.
In the flat region, the sensor can be modeled as a voltage source in series
with the sensor's capacitance or a charge source in parallel with the
capacitance.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.
Reference
1. http://en.wikipedia.org/wiki/Piezoelectricity
.
http://en.wikipedia.org/wiki/Piezoelectric_sensor
. Technical manual
for piezo vibration sensor Parallax 605-00004
. Technical manual
for piezo vibration sensor Bosch 608-00112