Piezoelectric ceramic & film
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Summary
Introduction
Piezoelectric devices are pervasive in our high-tech world. Quartz crystal time keeping devices, flint-free electric butane lighters, and "intelligent" shock absorbing sports equipment all make use of piezoelectric materials. Piezoelectric materials are widely used in a variety of other sensors such as accelerometers, contact mics, and ultrasound transducers, and as actuators in speakers, motors, and buzzers. The piezoelectric effect is the ability of a material (crystal) to expand or contract along its primary axis under an externally applied voltage. The effect is reversible, thus an externally applied deformation will cause a charge (polarization) to appear across the surface of the material. The material is available mainly in two forms, ceramic crystals and polyvinylidene fluoride (PVDF) film, though many materials exhibit the piezoelectric effect to a much lesser degree.
Devices
Mesurement Specialties Inc. (http://www.msiusa.com) make a wide range of piezo film (and many other disparate) devices. Their piezo sensors can be used to sense vibration, pressure, shock, and flexion. The devices come in pre-cut sensor elements, coaxial cable, and piezo film sheets. For details on implementing these devices in sensors and actuators refer to their technical manual. (http://www.meas-spec.com/myMSI/download/pdf/english/piezo/techman.pdf) Many of these sensors are available through digikey.
Piezoelectric ceramic elements are found in watches, alarm clocks, old portable video games, and many other commercial products which contain sound generating devices. These inexpensive transducers can be salvaged from old products or obtained from the usual sources (i.e. digikey, etc.) with specific electrical properties (operating voltage range, sensitivity, capacitance, resonant frequency, etc.) for optimal operation.
As mentioned previously these sensors can be used in sensing or actuating modes. They are commonly modeled electrically as a parallel capacitance, leakage resistance, and pressure dependant current source (or under source transformation: a series resistance, capacitance, and voltage source). It should be noted that piezoelectric materials have strong pyroelectric (temperature dependant) properties. These properties can either be harnessed to sense temperature, or the unwanted effect must be taken into account or reduced when designing with these materials. Filtering, averaging, and differential sensing are the common approaches to reduce band-specific noise, random noise on periodic signals, and common-mode noise respectively. Other design problems include electromagnetic interference and electrical or acoustical cross-talk.
Piezoelectric sensors can be used as force-sensitive switches or threshold detectors, or as analog pressure sensors. Care must be taken in the design of piezoelectric sensing circuitry. Application-specific load resistance matching and buffering are a must.
Source Country Price Digikey (http://www.digikey.ca) Canada CAN$ 1.44
[edit]Measurement Specialties Inc. 0-1002794-0
0.005"(125µm) polyester layer laminated to a 28µm piezo film element
Variants: MSI 0-1005447-1 piezo film with mass
Datasheet: LDTO Vibration Sensor.pdf (http://rocky.digikey.com/WebLib/Measurement%20Specialties%20Inc/Web%20Data/LDTO%20%20Vibration%20Sensor.pdf)
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