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

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Piezo • Nano • PositioningEnvironmental Conditions and Influences (cont.)Lifetime of Piezo Actuators© Physik Instrumente (PI) GmbH & Co. KG 2008. Subject to change without notice.Cat120E Inspirations2009 08/10.18The lifetime of a piezo actuatoris not limited by wear and tear.Tests have shown that PI piezoactuators can perform billions(10 9 ) of cycles without anymeasurable wear.As with capacitors, however,the field strength does have aninfluence on lifetime. The averagevoltage should be kept aslow as possible. Most PI piezoactuators and electronics aredesigned for semi-bipolaroperation.There is no generic formula todetermine the lifetime of apiezo actuator because of themany parameters, such as temperature,humidity, voltage,acceleration, load, preload,operating frequency, insulationmaterials, etc., which have(nonlinear) influences. PI piezoactuators are not only optimizedfor maximum travel, butalso designed for maximumlifetime under actual operatingconditions.The operating voltage rangevalues in the technical datatables are based on decades ofexperience with nanomechanismsand piezo applicationsin industry. Longer travel canonly be obtained with highervoltages at the cost of reducedreliability.Example:An P-842.60 LVPZT actuator(see p. 1-76 in the “PiezoActuators & Components” section)is to operate a switch witha stroke of 100 μm. Of its operatingtime, it is to be open for70 % and closed for 30 %.Optimum solution: The actuatorshould be linked to theswitch in such a way that theopen position is achieved withthe lowest possible operatingvoltage. To reach a displacementof 100 μm, a voltage2-206amplitude of approximately110 volts is required (nominaldisplacement at 100 V is only90 μm).Since the P-842.60 can be operateddown to -20 volts, theclosed position should beachieved with 90 V, and theopen position with -20 volts.When the switch is not in use atall, the voltage on the piezoactuator should be 0 volts.Statistics show that most failureswith piezo actuators occurbecause of excessive mechanicalstress. Particularly destructiveare tensile and shearforces, torque and mechanicalshock. To protect the ceramicfrom such forces PI offers avariety of actuators with preloads,ball tips, flexible tips aswell as custom designs.Failures can also occur whenhumidity or conductive materialssuch as metal dust degradethe PZT ceramic insulation,leading to irreparable dielectricbreakdown. In environmentspresenting these hazards,PICMA ® actuators with theirceramic-only insulation arestrongly recommended. PI alsooffers hermetically sealed actuatorsand stages.Fig. 39 c. PICMA ® piezo actuators (lower curve) compared with conventional multilayerpiezo actuators with polymer insulation. PICMA ® actuators are insensitve tohigh humidity in this test. In conventional actuators, the leakage current beginsto rise after only a few hours—an indication of degradation of the insulation andreduced lifetime.Test conditions: U = 100 VDC, T = 25 °C, RH = 70%Fig. 39 d. P-885.50 PICMA ® actuators with 15 MPa preload in dynamic motion test at116 Hz. No observable wear after 1.2 billion (10 9 ) cycles
Piezo • Nano • PositioningBasic Designs of Piezoelectric Positioning Drives/SystemsStack Design (Translators)The active part of the positioningelement consists of a stackof ceramic disks separated bythin metallic electrodes. Themaximum operating voltage isproportional to the thickness ofthe disks. Most high-voltageactuators consist of ceramiclayers measuring 0.4 to 1 mmin thickness. In low-voltagestack actuators, the layers arefrom 25 to 100 μm in thicknessand are cofired with the electrodesto form a monolithicunit.Stack elements can withstandhigh pressures and exhibit thehighest stiffness of all piezoactuator designs. Standarddesigns which can withstandpressures of up to 100 kN areavailable, and preloaded actuatorscan also be operated inpush-pull mode. For furtherinformation see “MaximumApplicable Forces”, p. 2-189.Laminar Design(Contraction-Type Actuators)The active material in the laminaractuators consists of thin,laminated ceramic strips. Thedisplacement exploited inthese devices is that perpendicularto the direction of polarizationand electric field application.When the voltage isincreased, the strip contracts.The piezo strain coefficient d 31(negative!) describes the relativechange in length. Itsabsolute value is on the orderof 50 % of d 33 .The maximum travel is a functionof the length of the strips,while the number of stripsarranged in parallel determinesthe stiffness and force generationof the element.Displacement of a piezo contractionactuator can be estimatedby the following equation:Fig. 40. Electrical design of a stack translatorLinear 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 PositioningNanometrologyMicropositioningIndexDisplacement of a piezo stackactuator can be estimated bythe following equation:(Equation 25)(Equation 24)where:L = displacement [m]where:L = displacement [m]d 33 = strain coefficient (fieldand displacement inpolarization direction)[m/V]d 31 = strain coefficient(displacement normalto polarization direction)[m/V]L= length of the piezoceramicsin the electricfield direction [m]Fig. 41. Mechanical design of a stack translatorn= number of ceramic layersU= operating voltage [V]U= operating voltage [V]d= thickness of one ceramiclayer [m]Example:P-845, p. 1-76, etc. (see the“Piezo Actuators & Components”section)Fig. 42. Laminar actuator design2-207
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