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PMN Co mp are d to PZT Electrostrictive Actuators(PMN) L Electrostrictive actuators oper-ate on a principle similar to that
of PZT actuators. The elec-
trostrictive effect can beobserved in all dielectric mate-rials, even in liquids. Electrostrictive actuators aremade of an unpolarized lead
magnesium niobate (PMN)
ceramic material. PMN is aceramic exhibiting displace-ment proportional to the
square of the applied voltage
under small-signal conditions.
Under these conditions PMN
unit cells are centro-symmetric
at zero volts. An electrical fieldseparates the positively andnegatively charged ions,
changing the dimensions of the
cell and resulting in an expan-
sion. Electrostrictive actuators
must be operated above theCurie temperature, which istypically very low when com-
pared to PZT materials. The quadratic relationshipbetween drive voltage and dis-
placement means that PMNactuator are intrinsically non-linear, in contrast to PZT actua-
tors. PMN actuators have an
electrical capacitance several
times as high as piezo actua-
tors and hence require higherdrive currents for dynamicapplications. However, in a lim-
ited temperature range, elec-
trostrictive actuators exhibit
less hysteresis (on the order of
3 %) than piezo actuators. An
additional advantage is theirgreater ability to withstandpulling forces. PZT materials have greatertemperature stability than elec-
trostrictive materials, especial-
ly with temperature variations
of over 10 °C. As temperature increases,available travel is reduced; at
low temperatures where travel
is greatest hysteresis increases(see Fig. 53b). PMN actuators
are thus best for applications
with little or no temperaturevariations of the ceramic, bethey caused by dynamic opera-
tion or by environmental fac-
tors. PMN
PZTV Fig. 52. Comparison of PMN and PZT material: displacementas a function of field strength (generalized). L 16 PMN Fig. 53 a. Comparison of PMN and PZT material: displacementas a function of temperature. PMN Fig. 53 b. Comparison of PMN and PZT material: hysteresis as a
function of temperature. © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-46
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