Piezoelectric crystal force transducers

The source and detector of ultrasound in an ultrasound medical imager is called a transducer. The transducer is a piezoelectric crystal which physically changes its dimensions when a potential is appHed across the crystal (38). The appHcation of a force to the piezoelectric crystal which changes its dimensions creates a voltage in the crystal. AppHcation of an oscillating potential to the crystal causes the dimensions of the crystal to oscillate and hence create a sound at the frequency of the oscillation. The appHcation of an oscillating force to the crystal creates an alternating potential in the crystal. 

Acceleration is perhaps the best method of determining the force resulting from machine vibration. Accelerometers use piezoelectric crystals or films to convert mechanical energy into electrical signals and Figure 43.23 is a schematic of such a device. Data acquired with this type of transducer are relative... 

In comparison to ordinary dielectrics, the permittivities of the so-called ferroelectric materials are about 103 times larger. The ferroelectric material can be transformed into a new type of material called piezoelectric material by heating the ferroelectric above its Curie temperature and then cooling it in a powerful electric field. A piezoelectric crystal changes its polarization once subjected to a mechanical strain. As a result, it can deform mechanically under an electric field or produce electric impulses as a result of mechanical impulses. Currently, piezoelectric materials are widely used as force or pressure transducers with fast response times and very sensitive output. Permittivities of common dielectric and ferroelectric materials are given in Table 1.9. 

Static force measurements (in which there is no apparent motion) usually rely on measurement of strain due to the applied force. Figure 18.6(c) illustrates the typical construction of a common force transducer called a load cell. The applied force produces a proportional strain in the S-shaped structural member, which is measured with a sensitive displacement transducer, usually a strain gauge resistor or a piezoelectric crystal. 

Quartz and piezoelectric ceramic crystals have more temperature independent constants than PVDF, so they are used for force and acceleration transducers. However, PVDF films can be used for large area flexible transducers. Their sensitivity to stress or strain allows the construction of pressure sensors (using the J33 coefficient), and accelerometers by mounting a seismic mass on the film. PVDF electrets are particularly suited for large area hydrophones (Fig. 12.21) that detect underwater signals. Their... 

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