The Piezoelectric Effect

Many crystals generate an electric charge when subjected to a mechanical load. All over the world, this correlation has become known as the piezoelectric effect. Piezoelectric measuring technology is the perfect tool for carrying out measurement tasks with extreme requirements in terms of geometry, temperature range and dynamics.

The piezoelectric effect was discovered in 1880 by the Curie brothers. The prefix “piezo” comes from the Greek “piezein”, to press. The two physicists found that the surfaces of certain crystals become electrically charged when the crystal is mechanically loaded. This electric charge is exactly proportional to the force acting on the crystal. It is measured in pico-coulombs (pC).

Charge generation
Application of force to a crystal deforms the its lattice structure. For a crystal to exhibit the piezoelectric effect, this structure has to lack a center of symmetry. Synthesized quartz (SiO2) is therefore very suitable. The deformation forces its positive silicon and negative oxygen ions towards each other. The resultant shift in the center of positive and negative charge generates an electric charge on the surface of the crystal.

Orientation
The orientation of the polar axes of the crystal with respect to the acting force determines the magnitude of the charge. Three different effects can accordingly be discerned:

• Longitudinal Effect
• Shear Effect
• Transverse Effect

The piezoelectric effect can only happen in non-conducting materials. Piezoelectric materials for sensor elements must exhibit very high mechanical strength and rigidity above all else. Other requirements include stable mechanical and electrical properties over a wide temperature range and a long service life. High sensitivity, good linearity, negligible hysteresis (that is identical rising and falling calibration curves) and high electrical insulation resistance are further advantageous characteristics. The most important materials used by Kistler for its measurement solutions are quartz, the quartz-like PiezoStar crystal and piezoceramics.

Signal conditioning
As electric charge is rather difficult to access experimentally, Walter P. Kistler's charge amplifier patent in 1950 represented a practical breakthrough that enabled accurate measurement despite extreme geometric, temperature range and dynamic requirements.

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