Piezoelectrics in Positioning - PZT & Piezo Actuators: Sub ...

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Piezo • Nano • PositioningQuick Facts (cont.)© Physik Instrumente (PI) GmbH & Co. KG 2008. Subject to change without notice.Cat120E Inspirations2009 08/10.182-180Open- and Closed-Loop OperationIn contrast to many other types ofdrive systems, piezo actuators canbe operated without servo-control.The displacement is approximatelyequal to the drive voltage.Hysteresis, nonlinearity and creepeffects limit the absolute accuracy.For positioning tasks which requirehigh linearity, long-term stability,repeatability and absolute accuracy,closed-loop (servo-controlled) piezoactuators and systems areused (see p. 2-199). With suitablecontrollers, closed-loop operationenables reproducibilities in thesub-nanometer range.High-Resolution Sensors forClosed-Loop OperationLVDT (linear variable differentialtransformer), strain gauge andcapacitive sensors are the mostcommon sensor types used forclosed-loop operation. Capacitivesensors offer the greatest accuracy.For more information, see p. 2-187 ff.Dynamic BehaviorA piezo actuator can reach its nominaldisplacement in approximatelyone third of the period of its resonantfrequency. Rise times on theorder of microseconds and accelerationsof more than 10,000 g arepossible. This feature makes piezoactuators suitable for rapid switchingapplications such as controllinginjector nozzle valves, hydraulicvalves, electrical relays, opticalswitches and adaptive optics. Formore information, see p. 2-192 ff.Power RequirementsPiezo actuators behave as almostpure capacitive loads. Static operation,even holding heavy loads,consumes virtually no power. Indynamic applications the energyrequirement increases linearly withfrequency and actuator capacitance.At 1000 Hz with 10 μmamplitude, a compact piezo translatorwith a load capacity ofapprox. 100 N requires less than10 W, while a high-load actuator(> 10 kN capacity) would use severalhundred watts under the same conditions.For more information, seep. 2-195 ff.Protection from MechanicalDamagePZT ceramics are brittle and cannotwithstand high pulling or shearforces. The mechanical actuatordesign must thus isolate theseundesirable forces from the ceramic.This can be accomplished bymeasures such as spring preloads,use of ball tips, flexible couplings,etc. (for more mounting guidelines,see p. 2-216). In addition, theceramics must be protected frommoisture and the intrusion of foreignparticles. Close contactbetween the piezo mechanics manufacturerand the user facilitatesfinding an optimal match betweenthe piezo system and the applicationenvironment.Fig. 4. Piezo actuator with water-proof case and connection for flushing/cooling air
Piezo • Nano • PositioningFundamentals of PiezoelectricityMaterial PropertiesNotesThe following pages give adetailed look at piezo actuatortheory and their operation. Forbasic knowledge read “QuickFacts”, p. 2-178. For definitionof units, dimensions and terms,see “Symbols and Units”,p. 2-217 and “Glossary”, p.2-175.Since the piezo effect exhibitedby natural materials such asquartz, tourmaline, Rochellesalt, etc. is very small, polycrystallineferroelectric ceramicmaterials such as bariumtitanate and lead (plumbum)zirconate titanate (PZT) withimproved properties have beendeveloped.PZT ceramics (piezoceramics)are available in many variationsand are still the mostwidely used materials for actuatorapplications today.Before polarization, PZT crystalliteshave symmetric cubicunit cells. At temperaturesbelow the Curie temperature,the lattice structure becomesdeformed and asymmetric. Theunit cells exhibit spontaneouspolarization (see Fig. 5), i.e. theindividual PZT crystallites arepiezoelectric.thermal and electrical limits ofthe material). The ceramic nowexhibits piezoelectric propertiesand will change dimensionswhen an electric potentialis applied.Linear Actuators & MotorsNanopositioning / PiezoelectricsPiezo Flexure Stages /High-Speed Scanning SystemsLinearVertical & Tip/Tilt2- and 3-Axis6-AxisFast Steering Mirrors /Active OpticsPiezo Drivers /Servo ControllersSingle-ChannelMulti-ChannelModularAccessoriesPiezoelectrics in PositioningNanometrologyMicropositioningIndexGroups of unit cells with thesame orientation are calledWeiss domains. Because of therandom distribution of thedomain orientations in theceramic material no macroscopicpiezoelectric behavior isobservable. Due to the ferroelectricnature of the material,it is possible to force permanentalignment of the differentdomains using a strong electricfield. This process is called poling(see Fig. 6). Some PZTceramics must be poled at anelevated temperature. The materialnow has a remnant polarization(which can be degradedby exceeding the mechanical,Fig. 5. PZT unit cell:1) Perovskite-type lead zirconatetitanate (PZT) unit cell in the symmetriccubic state above the Curietemperature2) Tetragonally distorted unit cell belowthe Curie temperatureFig. 6. Electric dipoles in domains; (1) unpoled ferroelectricceramic, (2) during and (3) after poling (piezoelectric ceramic)2-181
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