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

Piezo • Nano • PositioningNote:PI piezo actuators and stagesare designed for high reliabilityin industrial applications. Thetravel, voltage and load rangesin the technical data tables canactually be used in practice.They have been collected overmany years of experience inpiezo actuator production andin numerous industrial applications.In contrast to many other piezosuppliers, PI has its own piezoceramic development and productionfacilities together withthe necessary equipment andknowhow. The goal is alwaysreliability and practical usefulness.Maximizing isolatedparameters, such as expansionor stiffness, at the cost of piezolifetime might be interesting toan experimenter, but has noplace in practical application.When selecting a suitablepiezo actuator or stage, considercarefully the fact that “maximumtravel” may not be theonly critical design parameter.Hysteresis(Open-Loop Piezo Operation)Hysteresis is observable inopen-loop operation; it can bereduced by charge control andvirtually eliminated by closedloopoperation (see p. 2-199 ff ).widenstoamaximumof10%to 15 % under large-signal conditions.The highest values areattainable with shear actuatorsin d 15 mode.For example, if the drive voltageof a 50 μm piezo actuatoris changed by 10 %, (equivalentto about 5 μm displacement)the position repeatabilityis still on the order of 1%offull travel or better than 1 μm.The smaller the move, thesmaller the uncertainty.Hysteresis must not be confusedwith the backlash of conventionalmechanics. Backlashis virtually independent of travel,so its relative importanceincreases for smaller moves.For tasks where it is not theabsolute position that counts,hysteresis is of secondaryimportance and open-loopactuators can be used, even ifhigh resolution is required.LIn closed-loop piezo actuatorsystems hysteresis is fullycompensated. PI offers thesesystems for applications requiringabsolute position information,as well as motion withhigh linearity, repeatability andaccuracy in the nanometer andsub-nanometer range (see p.2-199 ff ).Example: Piezoelectrically drivenfiber aligners and trackingsystems derive the control signalfrom an optical powermeter in the system. There, thegoal is to maximize the opticalsignal level as quickly as possible,not to attain a predeterminedposition value. An openlooppiezo system is sufficientfor such applications. Advantageslike unlimited resolution,fast response, zero backlashand zero stick/slip effectare most welcome, even withoutposition control.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 PositioningNanometrologyMicropositioningIndexOpen-loop piezo actuatorsexhibit hysteresis in theirdielectric and electromagneticlarge-signal behavior. Hysteresisis based on crystallinepolarization effects and moleculareffects within the piezoelectricmaterial.The amount of hysteresisincreases with increasing voltage(field strength) applied tothe actuator. The “gap” in thevoltage/displacement curve(see Fig. 11) typically beginsaround 2%(small-signal) andFig. 11. Hysteresis curves of an open-loop piezo actuator for variouspeak voltages. The hysteresis is related to the distance moved, notto the nominal travel rangeV2-185