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

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Piezo • Nano • PositioningBasic Designs of Piezoelectric Positioning Drives/Systems (cont.)© Physik Instrumente (PI) GmbH & Co. KG 2008. Subject to change without notice.Cat120E Inspirations2009 08/10.18Piezo Actuators withIntegrated Lever MotionAmplifiersPiezo actuators or positioningstages can be designed in sucha way that a lever motionamplifier is integrated into thesystem. To maintain subnanometerresolution with theincreased travel range, thelever system must be extremelystiff, backlash- and frictionfree,which means ball or rollerbearings cannot be used.Flexures are ideally suited aslinkage elements. Using flexures,it is also possibleto design multi-axis positioningsystems with excellentguidance characteristics (seep. 2-211).PI employs finite elementanalysis (FEA) computer simulationto optimize flexurenanopositioners for the bestpossible straightness and flatness(see Fig. 49 and Fig. 51).Piezo positioners with integratedmotion amplifiers have bothadvantages and disadvantagescompared to standard piezoactuators:Advantages: Longer travel Compact size compared tostack actuators with equaldisplacement Reduced capacitance(= reduced drive current)Disadvantages: Reduced stiffness Lower resonant frequencyThe following relations applyto (ideal) levers used to amplifymotion of any primary drivesystem:where:r = lever transmissionratioL 0 = travel of the primarydrive [m]L Sys = travel of the leveramplifiedsystem [m]k sys = stiffness of the leveramplifiedsystem[N/m]k 0= stiffness of the primarydrive system(piezo stack andjoints) [N/m]f res-sys = resonant frequency ofthe amplified system[Hz]f res-0= resonant frequency ofthe primary drive system(piezo stack andjoints) [Hz]Fig. 47. Simple lever motion amplifierNote:The above equations are basedon an ideal lever design withinfinite stiffness and zero mass.They also imply that no stiffnessis lost at the couplinginterface between the piezostack and the lever. In realapplications, the design of agood lever requires long experiencein micromechanics andnanomechanisms. A balancebetween mass, stiffness andcost must be found, whilemaintaining zero-friction andzero-backlash conditions.Coupling the piezo stack to thelever system is crucial. Thecoupling must be very stiff inthe pushing direction butshould be soft in all otherdegrees of freedom to avoiddamage to the ceramics. Evenif the stiffness of each of thetwo interfaces is as high as thatof the piezo stack alone, a 67 %loss of overall stiffness stillresults. In many piezo-drivensystems, the piezo stiffness isthus not the limiting factor indetermining the stiffness of themechanism as a whole.PI piezomechanics are optimizedin this regard as a resultof more than 30 years experiencewith micromechanics,nanopositioning and flexures.2-210
Piezo • Nano • PositioningPiezo Flexure NanopositionersFor applications whereextremely straight motion inone or more axes is neededand only nanometer or microraddeviation from the idealtrajectory can be tolerated,flexures provide an excellentsolution.A flexure is a frictionless, stictionlessdevice based on theelastic deformation (flexing) ofa solid material (e.g. steel).Sliding and rolling are entirelyeliminated. In addition, flexuredevices can be designed withhigh stiffness, high load capacityand do not wear. They arealso less sensitive to shock andvibration than other guidingsystems. They are also maintenance-free,can be fabricatedfrom non-magnetic materials,require no lubricants or consumablesand hence, unlike aircushion bearings, are suitablefor vacuum operation.Parallelogram flexures exhibitexcellent guidance characteristics.Depending on complexityand tolerances, they havestraightness/flatness values inthe nanometer range or better.Basic parallelogram flexurescause arcuate motion (travel inan arc) which introduces anout-of-plane error of about0.1% of the travel range (seeFig. 48). The error can be estimatedby the following equation:(Equation 28)Fig. 48. Basic parallelogram flexure guiding system with motion amplification.The amplification r (transmission ratio) is given by (a+b)/aFig. 49. Zero-arcuate-error multi-flexure guiding systemFor applications where thiserror is intolerable, PI hasdesigned a zero-arcuate-errormulti-flexure guiding system.This special design, employedin most PI flexure stages,makes possible straightness/flatness in the nanometeror microradian range (seeFig. 49).Note:Flexure positioners are farsuperior to traditional positioners(ball bearings, crossedroller bearings, etc.) in terms ofresolution, straightness andflatness. Inherent friction andstiction in these traditionaldesigns limit applications tothose with repeatability requirementson the order of 0.5to 0.1 μm. Piezo flexurenanopositioning systems haveresolutions and repeatabilitieswhich are superior by severalorders of magnitude.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 PositioningNanometrologyMicropositioningIndexwhere:H = out-of-plane error [m]L = distance traveled [m]H= length of flexures [m]2-211
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