Basic Designs of Piezoelectric PositioningDrives/Syste m s

Stack Design (Translators) Laminar Design (Contraction-Type Actuators) The active part of the position-ing element consists of a stack of ceramic disks separated by thin metallic electrodes. The maximum operating voltage is proportional to the thickness of the disks. Most high-voltage actuators consist of ceramiclayers measuring 0.4 to 1 mmin thickness. In low-voltage stack actuators, the layers are from 25 to 100 µm in thickness and are cofired with the elec- trodes to form a monolithic unit.Stack elements can withstandhigh pressures and exhibit the highest stiffness of all piezo actuator designs. Standard designs which can withstand pressures of up to 100 kN areavailable, and preloaded actua-tors can also be operated in push-pull mode. For further information see “Maximum Applicable Forces”, p. 4-21.Displacement of a piezo stackactuator can be estimated bythe following equation:(Equation 24)

Piezo actuators Piezo Actuators

where: The active material in the lami-nar actuators consists of thin, laminated ceramic strips. The displacement exploited in these devices is that perpendi- cular to the direction of polar- ization and electric field appli- cation. When the voltage isincreased, the strip contracts.The piezo strain coefficient d

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(negative!) describes the rela- tive change in length. Its absolute value is on the order of 50 % of d

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33

.The maximum travel is a func-tion of the length of the strips, while the number of strips arranged in parallel determines the stiffness and force genera-tion of the element.Displacement of a piezo con-traction actuator can be esti- mated by the following equa- tion:(Equation 25)

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Fig. 40. Electrical design of a stack translator. Motion controllers Motion Controllers Ceramic Linear motors & Stages Ceramic Linear Motors & Stages Index Index

where: L = displacement [m]d

Fig. 41. Mechanical design of a stack translator. 31

= strain coefficient (displacement normal to polarization direction) [m/V]L = length of the piezoce-ramics in the electricfield direction [m]U = operating voltage [V] d = thickness of one ceramiclayer [m] L = displacement [m]d

= strain coefficient (fieldand displacement in polarization direction) [m/V]n = Examples: Laminar piezos are used in theP-280 and P-282 nanoposition- ing systems, (see pp. 2-30and 2-31). number of ceramic layers U = operating voltage [V] Example: P-845, p. 1-36, etc. (see the“Piezo Actuators” section)

Fig. 42. Laminar actuator design.

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