Electrostrictive Actuators
Electrostrictive actuators are solid state actuators similar to PZTs. The electrostrictive effect can be observed in all dielectric materials, even in liquids. Although sometimes advertised as a recent discovery, the material used has been around for 20 years. Electrostrictive actuators are made of a lead magnesiumniobate(PMN) ceramic material. PMN is a ceramic exhibiting displacement proportional to the square of the applied voltage under small-signal conditions, for certain compositions and temperature ranges. Under these conditions PMN unit cells are centro-symmetric at zero volts. An electrical field separates the positively and negatively charged ions, changing the dimensions of the cell and resulting in an expansion. Electrostrictive actuators must be operated above the Curie temperature, which is typically very low when compared to PZT materials.
In a limited temperature range, electrostrictive actuators exhibit less hysteresis (on the order of 3%) than PZT actuators. Despite the reduced hysteresis, they provide highly nonlinear motion because of the quadratic relationship between voltage and displacement. They are also unable to take advantage of the reduced electric field strength of bipolar mode operation, because reversing the electric field does not result in contraction (see Fig. 51). Furthermore, PMN actuators show an electrical capacitance four to five times as high as piezo actuators and hence require significantly higher drive currents for dynamic applications.
PZT materials have much greater temperature stability than electrostrictive materials, especially over large (10°C) temperature variations. Both displacement and hysteresis of PMN materials are strongly dependent on the actuator temperature. When the temperature increases, displacement decreases (see Fig. 52.;) at low temperatures, where displacement is at a maximum, hysteresis also reaches a maximum (see Fig. 53.), greatly restricting application.
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