Piezoelectricity

Piezoelectricity was discovered in Rochelle Salt in 1880 by Pierre and Jacques Curie. Piezoelectricity is the ability of certain materials to develop an electric charge that is proportional to a direct applied mechanical stress. These materials also have the ability to do the opposite - they will deform proportionally to an applied electric field.

Piezoelectricity is named after the greek word“piezein”, which means “press”1).

Piezoelectric materials are used as both sensors and actuators. This page focuses on the use of piezoelectric materials as actuators; the Piezoelectric page on sensorwiki deals with piezoelectric materials used as sensors.

Principles of Piezoelectricity

Piezoelectric materials such as crystals and ceramics.are nonconductive and non-centrosymmetric. While the piezoelectric effect is natural in certain crystals (quartz, rochelle salts, etc.), it can also be induced in certain ceramics using a process known as poling. Common ceramic piezoelectric materials include Lead Zirconium Titanate (PZT) and Barium Titanate.

Placing stress on a piezoelectric material alters the separation between the positive and negative charge sites leading to a net polarization at the crystal surface. Similarly, a deformation of these materials is effected by placing them under an electric charge. When used as both sensor and actuator a piezoelectric device is referred to as a “transducer”.

The electrical polarization in a piezoelectric material due to mechanical stress is known as the direct piezoelectric effect, and is characterized by: Pm = Dmij*Xij. The mechanical deformation of a piezoelectric material due to an electric field applied to it is known as the converse piezoelectric, and is characterized by: Xij = Dmij*Em.

Where: Pm = electrical polarization Dmij = piezoelectric tensor components Xij = Components of elastic strain Em = Components of the electrical field

Properties of Piezoelectric Materials

Piezoelectric charge and voltage coefficients express the polarization and electric field, respectively, generated by a piezoelectric material when a mechanical stress is applied to it. Large values correspond to large changes in dimension and voltage. 2)

The electromechanical coupling coefficient reflects efficiency with which a piezoelectric material converts mechanical energy to mechanical energy. Output energy / Input energy = (Coupling coefficient)^2

Curie point: The Curie point is the temperature above which the structure of the piezoelectric material is centrosymmetric, i.e. becomes non-piezoelectric. Below this temperature the structure is noncentrosymmetric, a requirement for piezoelectric behaviour.

The Curie temperature is the temperature at which material has its highest dielectric constant. It is always below, but often within 10º of, the Curie point.

The Discharge Time Constant (DTC) is based on RC constants prior to amplifier. Typically it is very short, but it can be made longer to enable non-static measurements

While well below their Curie point, piezoelectric materials act like capacitors. They are also typically characterized by extremely high impedance. They are insensitive to magnetic and electrical fields3).

Piezoelectric Actuator Applications

Piezoelectric actuators are used to control vibrations in buildings: C.M.A. Vasques, J. Dias Rodrigues. “Active vibration control of smart piezoelectric beams: Comparison of classical and optimal feedback control strategies.” Computers & Structures, Volume 84, Issues 22–23, September 2006, Pages 1402-1414.

Squiggle motors are piezoelectric linear motors smaller than a penny.

Two piezoelectric actuators are used to create a smooth vibrotactile flow: Jeonggoo Kang, Jongsuh Lee, Heewon Kim, Kwangsu Cho, Semyung Wang, Jeha Ryu, “Smooth Vibrotactile Flow Generation Using Two Piezoelectric Actuators,” IEEE Transactions on Haptics, pp. 21-32, Jan.-March, 2012.

External Links

References

1) Webster, John G. “The Measurement, Instrumentation and Sensors Handbook.” CRC Press, Boca Raton, FL: 1999, pp. 53-65.
2) APC International, Ltd. "Piezoelectric Ceramics:Principles and Applications." APC International, LTD, Mackeyville, PA:2011.
3) Gautschi, G. Piezoelectronic Sensorics. Springer-Verlag, New York: 2002.
actuators/piezoelectric.txt · Last modified: 2016/04/08 04:34 by bogason
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