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PrrzoSvsrems, lttc. 65 Tower Office Park Woburn. MA 01801 Tef(781) 933-4850 . Fax(781) 933-4743 www.piezo.com. email:sales @ piezo.com PIEZOELECTRICMOTOR / ACTUATORKIT MANUAL
INTRODUCTIONTO PIEZOELECTRICITY Motor/ActuatorKit Purposeof Kit Contentsof Kit PiezoelectricPhenomenon Piezoelectricand MaterialProperties of Piezoceramic Terminologyand Relationships Piezoceramic& MaterialProperties of PSI-5A-S4-ENHPiezoceramic
DESIGNINGPIEZOELECTRIC ACTUATORS The Spectrumof PiezoelectricMotor Transducers BasicEngineering Considerations PerformanceConsiderations Deflectionand Force ResponseTime Stability Hysteresis Creep MechanicalConsiderations Mounting Resonance OperatingFrequency Band StrengthLimitations DriveCircuit Considerations Quasi-StaticOperation NearResonance Operation Charge/DischargeProtection OutputStage Protection Electricallsolation Electricalbreakdown ElectricalLosses ThermalConsiderations CurieTemperature Propertiesas a Functionof Temperature PyroelectricEffects ThermalExpansion CryogenicOperation VacuumConsiderations ElectricField Considerations StressConsiderations Aging
PIEZOELECTRICBENDING MOTORS Principlesof Operation StandardPolarization Configurations SeriesOperation ParallelOperation SinglePlate Operation StandardMounts For Bending Motors
PrezoSYsreus, lwc. 65 Tower Office Park Woburn, MA O1BO1 BendingMotor Equations BendingMotors Mounted as Cantilevers Freedeflection Blockedforce ResonantfrequencV MaximumSurface Stress Electricalcapacitance BendingMotors Mounted as SimpleBeams Freedeflection Blockedforce Resonantfrequency MaximumSurface Stress Electricalcapacitance MeasuredPerformance of KitStock 2-Layer Bending Motors
BUILDINGPIEZOELECTRIC ACTUATORS WorkingWith Piezoceramic SingleSheet Piezoceramic Stock 2-LayerMotor Stock Durability Cuttingand Shaping Accessingthe Center Shim Bondingand Attaching to Piezoceramic Soldering& AttachingLeads to the Electrodesand Center Shim PerformanceTesting PiezoSystem's Professional Engineering Services CustomDesign & DevelopmentEngineering ConsultingServices
TABLES Table-1.Piezoelectric and Material Properties of pS|-SA-S4-ENH Table-2.The Spectrum of PiezoelectricMotor Transducers
2 PrezoSvsrenns, lNc. 65 Tower Office Park Woburn, MA 01 801 INTRODUCTIONTO PIEZOELECTRICITY
MOTOR/ACTUATORKIT
PURPOSEOF THE KIT The Motor/ ActuatorKit is a developmenttool for those who wish to rapidlyprove the feasibility of a piezoelectricapproach to a specializedapplication. This self contained package makes it possibleto establishand verify the performancespecifications ( i.e. total motion, force, response time, etc. ) necessary for a successfulactuator. Once performance is defined,the numberof materialand construction options availablefor productionare considerable. However, the advantages of theseoptions lie primari! in costat thispoint. PiezoSystems recognizes that prototyping material and practicaltechnical information pertaining to piezoceramicmotor applications are difficult to obtain.The piezoelectrictransducers provided in thiskit representthe basicbuilding blocks employed in manypiezoelectric systems. The stock supplied is easyto cutto size,center access, and voltage, temperature, and stress limitations are minimal. Theliterature will informthe user of basicprinciples, terminology, actuator design and fabrication techniques,and point out certain practical limitations .
CONTENTSOF THE KIT PiezoceramicSingle Sheets; PSI-5A-S4-ENH Poledthrough the thickness; (Red stripe-on one side only) 2.5"x 1.25"x .0075"(63 mm x 31.8mm x 0,19mm); 2 pieces 2-LayerPiezoelectric Motor Elements; PSI-5A-S4-ENH Poledfor SeriesOperation (Redstripe-both sides) 2.50"x 1.250"x .020"(63 mm x 31.B mm x 0.51mm) 1 piece 1.25"x 0.500'x .020"(31.8 mm x 12.7mm x 0.51mm) 1 piece 1.25"x 0.250"x .020"(31.8 mm x 6.35mm x 0.51mm) 1 piece 1.25"x0.125" x .020"(31.8 mm x 3.18mm x 0.51mm) 1 piece Poledfor Parallel Operation (Redstripe-on one side only) 2.50"x 1.250"x .020"(63 mm x 31.8mm x 0.51mm) 1 piece; 1.25"x 0.500'x .020"(31.8 mm x 12.7mm x 0.51mm) 1 piece 1.25"x 0.250'x .020"(31.8 mm x 6.35mm x 0.51mm) 1 piece 1.25"x0.125" x .020"(31.8 mm x 3.18mm x 0.51mm) 1 piece PiezoelectricMotor/Actuator Kit Manual Solder& Flux Wires(5 Red & 5 Black:30 gauge)
PIEZOELECTRICPHENOMENON
Piezoelectricityis a propertyof certainmaterials to physicallydeform in the presenceof an electric field, or conversely,to producean electricalcharge when mechanically deformed. There are a widevariety of materialswhich exhibit this phenomena to somedegree, including natural quartz crystals, semi-crystalline polyvinylidenepolymer, polycrystalline piezoceramic, and even human bone. Piezoelectricityis due to the spontaneousseparation of chargewithin certain crystal structures under the rightconditions. This phenomenon, referred to as spontaneouspolarization, is causedby a displacement of the electronclouds relative to theirindividual atomic centers, i.e. a displacementof the positiveions relative to the negativeions within their crystal cells. Sucha situationproduces an electricdipole. Polycrystallineceramic, one of the mostactive piezoelectric materials known, is composedof randomlyoriented minute crystallites. Each crystallite is furtherdivided into tiny "domains", or regionshaving similardipole arrangements. The overall effect of randomlyoriented polar domains is an initiallack of piezoelectricbehavior. However, the materialmay be inducedto exhibitmacroscopic polarization in any
PrezoSvsreus, lruc. 65 Tower Office Park Woburn, MA 01BO'l givendirection by subjectingit to a strongelectric field, as shownin Figure-'1.Such induced materials are termedferroelectric. Polarization is accomplishedby applyinga fieldof -2350 volts/mm(60 V/mil) across electrodesdeposited on outersurfaces.
BEFOREPOLARIZATION AFTERPOLARIZATION
Figure-1.Inducing macroscopic polarization in a polycrystallinepiezoceramic by applyinga strongelectric fieldacross randomly oriented microscopic polar domains.
Duringelectrical polarization the materialbecomes permanently elongated in the directionof the polingfield ( polaraxis ) andreduced in thetransverse direction. When voltage is subsequentlyapplied in the samedirection as the polingvoltage, the pieceexperiences further elongation along the polaraxis and transversecontraction as stipulatedby Poisson'sratio. Whenthe voltage is removed,the piecereverts to itsoriginal poled dimensions. When voltage is appliedopposite to thepoled direction (depoling direction), the piececontracts along the polaraxis and expands in thetransverse direction. Again, it revertsto its original poleddimensions after removing the voltage.These distortions are illustratedin Figure-2for a rectangularly shapedpiece. lf toolarge a voltageis appliedin thedepoling direction, the original polarization will be degraded(partially or fullydepolarized). Or, the electric dipoles may be partiallyor completelyflipped around 180', causingthe piece to be repoledin theopposite direction. The maximum depoling field a piececan withstandwithout experiencing depolarization is its coercive field, Ec.
+ + + lP lP Y V VlP
Figure-2. Physicaldeformation of a rectangularpiezoelectric body underthe influenceof an appliedelectric field.
4 PrezoSYsreus, lruc. 65 Tower Office Park Woburn. MA 01801 PIEZOELECTRICAND MATERIAL PROPERTIES OF PIEZOCERAMIC TERMINOLOGYAND RELATIONS Thissection describes the terminology commonly used in thediscussion of piezoceramics andnotes the fundamental relationships useful in motorapplications. lt also defines commonly used notationsand sign conventions. Relationshipsbetween applied electric fields and the resultant responses depend upon the piezoelectricproperties of the ceramic,the geometryof the piece,and the directionof electricalexcitation. The propertiesof piezoceramicvary as a functionof bothstrain and temperature. lt shouldbe recognized thatthe datacommonly presented represent values measured at verylow strain levels at -20" C. Directionsare identified using the three axes, labeled 1,2, and3, shownin Figure-3.The 3
"polar"or 3-axisis chosenparallel to the directionof polarization.
Figure-3.Definition of axesfor a rectangularpiezoelectric body showing the polaror 3-axis,and the transverseor 1 and 2- axes.
The polarizationvector, establishedduring manufacture by a high DC voltageapplied to the electrodes,is representedby an arrow pointingfrom the positiveto the negativepoling electrode. This informationis conveyedby a dot or stripeon the electrodesurface held at high voltageduring the poling process. Piezoelectriccoefficients, used to relateinput parametersto output parameters,use double subscripts.The piezeelectrle strain eeefficients, d;; , correlatethe strain producedby an appliedelectric fieldaccording to the relation:
S=dE
where d;; is expressedin meters/volt. The first subscriptgives the directionof the electricfield associated withthe appliedvoltage. The secondsubscript gives the directionof mechanicalstrain. d33 relatesthe ratio of the the strainalong the 3-axisto the electricfield appliedalong the 3-axis,assuming the piece is free to distortin any direction. d31 relatesthe strainalong the 1-axis( or 2-axis) to the excitationalong the 3-axis undersimilar conditions.
PrezoSvsrenlls, lNc. 65 Tower Office Park Woburn. MA 01801 Thecoupling coefficient, k ( lowercase ), is an indicationof thematerials ability to convert electricalenergy to mechanicalenergy. Specifically, the squareof the couplingcoefficient equals the ratioof mechanicalenergy output to the electricalenergy input. lt is morerelevant to the maximumwork output of solidceramic devices than to bendingelements because a practicalbending element stores a portionof its mechanicalenergy in themount and metal shim center layer. For a bendingelement, keffective - 314ky . Therelative dielectric constant, K ( uppercase ), is the ratioof the permitivityof piezoceramicto thatof emptyspace, eo (eo = 8.85x10-12farads/meter). K3 , the relativedielectric constant betweenthe polingelectrodes, determines the capacitanceof the pieceaccording to the relationship,
C=K3eoA/t
whereA is thearea of the surfaceelectrode , and t is thethickness of the ceramiclayer or layersbetween electrodes. Certainpiezoceramic material constants are written with superscripts to specifythe mechanicalor electricalmeasurement conditions shown below:
T = ConstantStress ( mechanicallyfree ) S = ConstantStrain ( mechanicallyclamped ) E = ConstantElectric Field ( electrodesshort-circuited ) D = ConstantElectric Displacement ( electrodes open-circuited )
Forexample, KT3 , is the dielectricconstant measured across the polingelectrodes of a mechanicallyfree piece. Young'sModulus, Y , theratio of stressrequired to producea unitof strain,describes the material stiffnessof piezoceramicin unitsof newtons/squaremeter. Because mechanical stressing of the ceramic producesan electricalresponse opposing the resultantstrain, the effectiveYoung's Modulus with electrodes shortcircuited is lowerthan with the electrodesopen circuited. Furthermore, the stiffnessdiffers in the 3 directionfrom that of the 1 or2 direction.Thus, YE11, is the ratio of stressin the 1 directionto strainin the 1 directionwith the electrodesshorted.
PIEZOELECTRICAND MATERIALPROPERTIES FOR PSI.sA.S4.EN H PIEZOCERAMIC The piezoelectricand material properties of PSI-SA-S4-ENHpiezoceramic are listedin Table-1on page22.
6 PrezoSvsrrnns, lwc. 65 Tower Office Park Woburn. MA 01801 DESIGNING PIEZOELECTRIC ACTUATORS
THESPECTRUM OF PIEZOELECTRICMOTOR TRANSDUCERS
Transducerswhich convert electrical energy to mechanicalenergy ( i.e.motors ) comein a widerange of shapesand sizes, each having their own characteristic force-displacement capabilities. Stiff (lowcompliance) transducers provide tremendous force but tiny motion, On the other hand, highly compliant transducersprovide substantial motion but smallforce. As a generalpurpose reference guide,Table-2 shows the spectrumof motortransducers commonlyconsidered in piezoelectricapplications. The equations for displacement,force, and resonant frequency,are basedon linearrelationships and low signal values of theirpiezoelectric strain coefficients.
BASICENGINEERING CONSIDERATIONS
PERFORMANCE DEFLECTIONAND FORCE: Piezoelectric actuators are usually specified in termsof their freedeflection and blocked force. Freedeflection, X1, refers to displacementattained at the maximum recommendedvoltage level when the actuatoris completelyfree to moveand is notasked to exertany force. Blockedforce, Fg, refers to theforce exerted at the maximumrecommended voltage level when the actuator is totallyblocked and not allowed to move. Deflectionis at a maximumwhen the force is zero, andforce is at a maximumwhen the deflection is zero. Allother values of simultaneousdisplacement and force are determinedby a linedrawn between these points on a forceversus deflection line, as shownin Figure-4. Generally,a piezomotor must move a specifiedamount and exert a specifiedforce, which determines its operatingpoint on theforce vs. deflectionline. Workis maximizedwhen the deflectionperformed permits onehalf the blockedforce to be developed.This occurs when the deflectionequals one half the free deflection.
uJ O t Operatingpoint o produce Lr optimizedto maximumwork
DEFLECTION
Figure-4. The force versus displacementdiagram for a piezoelectricmotor element.
PrEzoSvsrrus, lruc. 7 65 Tower Office Park Woburn, MA 01801 Forcantilevered bending motors, Xl andFg are approximated by observingthe the tip deflection afterenergizing, and by holdinga forcegauge against the tip during energization.
RESPONSETIME: Typically, it is usefulto knowhow fast an actuatorwill operate. The fundamental resonantfrequency, Fr, is a guidepost towards answering this question. A piezoactuator can follow a sinusoidalsignal up to itsresonant frequency. Beyond this point, its inertia inhibits it fromkeeping up with electricalexcitation. From a practicalstandpoint, it's a goodidea to limitoperation to - 3t4Fr The response time,tr, for an actuatorto travelthrough its full range, for bipolaroperation (from 0 to positivevoltage, then to negativevoltage, and back to 0) is 1/4cycle. Thus,
t'"(bipolar operation) = 4x(.75Fr)
Forexample, an actuatordriven by a bipolarpower supply, and having a resonant frequencyof 500Hertz, has a responsetime of 0.67milliseconds. Any mass added to theend of the actuatorwill lengthen the response time.
STABILITY:For those applications which require the piezoactuator to holdits positionaccurately fora longtime, an understandingof hysteresis and creep is important.For dynamic applications, where positionis changingcontinuously, these issues are less critical. Hysteresis:When a polycrystallinepiezoelectric body is deformed,part of the mechanicalenergy is storedas elasticstrain energy, and part is dissipatedas heatduring small internal slidingevents. Hysteresis appears as an offsetbetween the the positionpath traveled during the application andremoval of the excitationfield. Thesize of the offsetdepends on thefield level, the cycletime, and the materialsused. lt is oftenspecified as a percentageof thetotal deflection achieved, and ranges from .1% to 10%. Hysteresisis a considerationwherever high frequencies ( > l KHz) areconcerned because of heat accumulationwithin the pieceafter each cycle of operation.This is especiallythe casefor lowvoltage piezo stacksmade of highstrain material. This can leadto excessivetemperatures if care is nottaken in the design.Other portions of thepiezoelectric system also contribute to losses,such as adhesivebonds, mounts,and attachments. These show up in ultrasonicdesigns. Creep(or Drift): Creep is the result of timedependent plastic deformation. Usually it is nota concernunder oscillating drive conditions. But, anyhigh level DC voltage application should pay closeattention to creep. Aftervoltage has been applied to a piezoelectricactuator, the deflectionincreases withtime. Formoderate drive levels( <10 volts/mil ), the creeprate decreases with time. However,as the drivelevel increases ( >20 volts/mil ), the creeprate accelerates. Upon removal of the drivevoltage, the piece retainsa set,a portionof whichis irreversibleeven after a longrelaxation period. At highdrive levels (>30 volts/mil), creepmay proceed to the pointwhere the piecefinally cracks. Increased temperature exacerbatescreep. Whenexcessive creep is encountered,certain strategies may be required,such as stops to limitexcessive travel, or closedloop feedback control to lockin a desiredposition. Figure-Sdemonstrates the typical hysteresis and creep behavior of a bendingelement. Whena piecehas been at restfor some length of time ( - 1 day), it willreside at itsequilibrium position, 0. Uponinitial energization, it will move to position1. Afterde-energization it will go to position 2. lf it is allowed to restfor a sufficientlength of time( -1 day) it willrevert back to position0 again. However,if it is re- energizedimmediately, it will follow path 2-1. Whenthe piece is energizedand left on, it willcreep along the path1-1', and come to restat position2' afterde-energization. A piece operating in AC modewillfollow path 1-2-3-4-1.
I PrezoSYsrenrts, lruc. 65 Tower Office Park Woburn. MA O'l801 z I (.)F tJJ J ll- trJ o
Y ELEcrRrcFEILD 4
3l
Figure-S.Typical hysteresis and creep behavior of a singlebending element.
MECHANICALCONSIDERATIONS MOUNTING:An idealmount permits the normal distortion of theentire active portion of the motorelement, while at thesame time preventing motion in certaindirections at themounting point or points. Generallypiezo motors are either bonded, clamped, or springloaded to theirmounting points. Mounts introducesome mechanical damping into the systemsince some of the energyfrom the motordistorts the mountitself, This may or maynot be desirable. RESONANCE:Mechanical resonance is a manifestationof thetrading back and forth of kineticenergy (moving mass) and potential energy (elasticity)in an oscillatingbody. At certainfrequencies, knownas resonances,the amountof storedenery becomes very large compared to the excitationenergy. Thisphenomena can be usefulfor achieving large deflections at lowvoltages, and for obtaining high efficiency.Piezo fans and ultrasonic devices utilize this property. Because of the highamplitudes exhibited at resonance,care must be takennot to overstrainand crack the actuator. Theresonant frequencies of a piezomotor depend on itsdimensions, material properties, andthe manner of mounting.The cantilever beam element has the lowest fundamental ( first ) resonant frequencyper unit length of allconfigurations and mounting schemes. Equations for determining the fundamentalresonant frequencies for severalmotor configurations are shown in Table-2. Thesefrequencies applyto unloadedelements only. Attachmentsto the elementwill add to the resonatingmass and lower the resonantfrequency. OPERATINGFREQUENCY BAND: Up to resonantfrequency, the deflection of a piezoelectricbending element is nearlyindependent of frequencyand proportional to the operatingvoltage. Aroundthe resonantfrequency, deflection rises rapidly to a multipleof its normalvalue. The amplitude and narrownessof the resonancedepend on the internaland external losses acting on the actuator.Above resonance,deflection decreases steadily with the squareof thefrequency. First resonance marks the limitof the usablefrequency band for quasi-staticactuators. For resonant applications, the useablefrequency rangeis limitedto a smallband around the useful resonant modes. STRENGTHLIMITATIONS: Piezoceramic isvery strong in compressionbut weak in tension.Bending elements always have one side in compressionand the other side in tension,where the magnitudeof stressincreases linearly from the midplaneto the outsidesurface. Therefore, the elementis alwayslimited by the maximum recommended tensile strength, generally considered to be in therange of 20-35x 106NeMons/meter2. From a strainpoint of view, the ceramicshould not be allowedto strainmore than500x1 0-6 meter/meter.
PrezoSvsrrnas, lNc. I 65 Tower Office Park Woburn, MA 01801 DRIVECIRCUIT CONSIDERATIONS QUASI-STATICOPERATION: A piezoelectricactuator operating below its fundamental resonancecan be treatedsimply as a capacitiveload. Thecircuit must supply charge to causea motion,and mustwithdraw charge to causea retraction( i.e. chargeapplied to the devicedoes not bleed off internally). Whenheld motionless in anyposition, piezoelectric actuators draw negligible current, typically much less thana microamp. NEARRESONANCE OPERATION: A piezoelectricactuator operating near resonance can be modeledas a capacitor(having a valueequal to thetransducer capacitance) with a resistorin parallel( typically10 to 100ohms ). Thepower dissipated by this resistance represents the work which the actuator doeson itsenvironment. The drive circuit must have sufficient current capacity to maintainthe desired voltageon the resistor. CHARGE/ DISCHARGE PROTECTION: Instantaneous charging or dischargingof piezoelectricactuators causes acoustic shockwaves within the piezoceramicwhich can leadto localized stressconcentrations and fracture. Therefore, the peakcurrent to anyactuator must be limited.One simple methodplaces a protectionresistor in serieswith the actuator,the value of whichcan be estimatedusing the followingrelation: 5R^C> 4 i Fora seriesoperated cantilevered bending element, substituting for C andFr:
Rp= (51/eoK3b)( p lYtt)1t2
Thisessentially limits operation to a frequencyregion below the fundamental resonance. OUTPUTSTAGE PROTECTION: Piezoelectric bending elements can generate high voltages( >100 volts ) underexternal vibration, shock, or temperatureshifts. lf theseconditions are expected,the drive circuitry of the outputstage mustbe protectedagainst transient voltages of all polarities. ELECTRICALISOLATION: The outer electrode surfaces of certainmotor elements are electrically"live" in manyconfigurations. For product or experimentalsafety, consideration should be givento insulatingor sheildingthe electrodes, mount, and power take-off sections of themotor element. ELECTRICALBREAKDOWN: The highest value of appliedelectric field is determinedby electricalbreakdown occurring either through the body of thepiezoceramic sheet or overthe its edges. Piecesof dustand debris adhering to edgescan initialize edge discharge at fieldsas lowas 400-800 volts/mm.However, the discharge arc vaporizes the debris, thereby cleaning itself. A numberof theseedge- debrisarcs may occur during the initial energization of thebending motor, but they will not occur again. Continuousbreakdown occurs around 3,000-4,000 volts/mm, usually at impurityor defectregions within the bulkof thematerial. This can lead to a shortcircuit across the sheet due to vapordeposition of electrodeor shimmaterial near the site of arcing.A currentlimiting resistor or in-linefuse is recommendedwhen excessiveelectric fields are used. ELECTRICALLOSSES: The bulk resistivity of piezoceramicis - 1012Q-cm. Therefore, electricallosses are minimalunder static or lowfrequency operation. However, dielectric losses are significantunder cycled operation and can lead to heatingunder high frequency /high power operation. The losstangent, the ratio of seriesresistance to seriesreactance, for PSI-5A-S4-ENHis -0.015.
THERMALCONSIDERATIONS CURIETEMPERATURE: For each piezoceramic material there is a criticaltemperature, knownas itsCurie point, which represents its maximumoperating temperature before suffering a permanent andcomplete loss of piezoelectricactivity. In practice,the operatingtemperature must be limitedto some valuesubstantially below the Curiepoint because at elevatedtemperatures depoling is greatlyfacilitated, the agingprocess is accelerated,electrical and mechanical losses increase, and the maximumsafe stress is reduced.As a ruleof thumb,a temperatureequal to onehalf the Curie temperature is considered the maximumsafe operating temperature. PIEZOELECTRICAND MATERIAL PROPERTIES AS A FUNCTIONOF TEMPERATURE: Piezoceramicproperties are strongly temperature dependent, and thermal dependence varies markedly from onematerialto the next. Figure-6 demonstrates the temperature dependence of K3and d31for PSI-5A-S4- ENH.
10 PrrzoSYsrerus, lruc. 65 Tower Office Park Woburn, MA ol BO'l 100
Ers I zF z{. -l 96 art 4.= ka +- -
Figure-6.The relative dielectric constant and transverse strain coefficient as a functionof temperature.
PYROELECTRICEFFECTS: An electricfield is inducedon theelectrodes of a piezomotor whenit is exposedto a thermalchange. The induced field is
E- cr(AT) eoK3 wherea is the pyroelectriccoefficient in unitsof coulombsI M2"C, AT is thetemperature change, K3 is the relativedielectric constant in the polingdirection, and eois the permitivityof freespace. lt shouldbe noted thata depolingvoltage will be developedacross a layerof piezoceramicwhen the temperaturedrops. This canhappen during processing, testing, or normaluseage. lf a temperaturedrop is sufficient,over a time intervalwhich is too shortto allow chargeto leakaway, a voltagegreater than the coercivefield can result. Thiscan degrade the originalpolarization causing reduced performance. lt is alwaysgood practice to short circuitthe electrodesof anypiezo device during a cooldown procedure. THERMALEXPANSION: One must account for thermal displacements over the temperature rangeanticipated. The actuator must be capableof compensatingfor thermaldisplacements and still have a usefulmotionraffi Inaddition, differential thermal expansions of adjacentassembly parts will cause moments andwarping. The standard bending motor element has a symmetricalconstruction. Distortion due to thermal excursionshould be negligible.However, care should be taken in the design of themount or anyother attachmentsnot to introducethermal distortion. This is facilitatedby properlymatching the thermal expansioncoefficients of adjacentmembers to thatof the ceramicelement. The coefficient of thermal 'C. expansionof PSI-5A-S4-ENHis - 4 pM/ M CRYOGENICOPERATION: The lowsignal values of the straincoefficients for operationat 4.2 "K arereduced by a factorof 5-7times. The value of the coercivefield increases substantially however. Cyc|ingthetran@ratureextremesdoesnotseemtoaffectthemadverse|y.
VACUUMCONSIDERATIONS Becausepiezo actuators are solid state devices, they lend themselves to highvacuum operation.However, several issues should be understood.First, voltage should not be appliedto the
PrezoSYsreus, lruc. 11 65 Tower Office Park Woburn, MA O'l8O'l electrodesduring the vacuum pump-down process because of the lowinsulation resistance of air and nitrogenbetween the rangeof 10to 0.1torr. Arcingbetween electrodes is possiblewithin this pressure range.Secondly, outgassing of thepart is possibledepending on constructionmaterials. Motors to be used in highvacuum environments should have small cross sections of outgassingmaterials (primarily the adhesive).lf bake-outis necessary,the transducerwill have to be builtto withstandsolvents and bake-out temperatures.
ELECTRICFIELD CONSIDERATIONS As mentionedearlier, under adverse conditions piezoelectric properties may degrade, vanishcompletely, or beflipped around 180'. A strongelectric field applied to a piezoceramicin a sense oppositeto theoriginal poling voltage will tend to causedepoling. The field strength necessary to initiate depolingdepends on thematerial, duration of application,and temperature, but is typicallyin therange of 475volts/mm at 20' C for PSI-SA-S4-ENHunder static conditions. Alternating fields may also degrade the piezoceramic,but the peakfield level is higherbecause the durationis shorterbefore the field is reversed.A peakfield of 600volts/mm may be toleratedfor 60 Hzoperation at20" C.
STRESSCONSIDERATIONS Whenthe mechanical stress on a piezoceramicelement becomes too high,again there is a dangerof degradingthe piezoelectricproperties. Generally, compressive or hydrostaticstress levels of -50 x 106N/V2 arerequired to degradePSI-5A-S4-ENH if no otherdegrading influences are present.
AGING Piezoelectricproperties change gradually with time. The changes tend to be logarithmicwith timeafter the original polarization. Therefore, the rate reduces rapidly with time. Aging depends on the ceramicmaterial, manufacturing process, and ambient conditions such as temperature, vibration or shock, Piecesmay be heatedfor a specifiedtime to acceleratethe agingprocess.
12 PrezoSvsreus, lruc. 65 Tower Office Park Woburn. MA 01 801 PRINCIPLESOF OPERATION The most commontype of piezoelectricbending motor is composedof two layersof piezoceramicbonded to a thin metalshim sandwiched in the middle.The constructionand typical dimensionsof the 2-layerelements provided in this kit are shownin Figure-7.
OUTERNICKEL ELECTRODE.2 PLCS. PIEZOCERAMIC,2 PLCS. .0075' INNERNICKEL _-.-A-+ ELECTRODE,2 PLCS. .005' v CENTERSHIM CONDUCTIVE -T-+ BOND,2 PLCS. \0 .5pM
Figure-7.Construction of laminated2)ayer motorelement.
The applicationof voltageto the elementis analogousto the applicationof heatto a bimetallicstrip. The voltageacross the benderelement forces one layerto expand,while the othercontracts, as depictedin Figure-8.The resultof thesephysicalchanges is a strongcurvature and largedeflection at the tip when the otherend is clamped.The tip deflectionis muchgreater than the changein lengthof either ceramiclayer. Bendingmotors exhibit unique properties. They may be energizedproportionately and be heldin the energizedposition with negligibleconsumption of energyor generationof heat.They may be operatedover billionsof cycleswithout wear or deterioration.Their low profileallows their use in very restrictedlocations.
V
L+AL + Figure-8.The curvature of a bendingmotor is dueto theexpansion of onelayer and contraction of theother
PrezoSYsrrnns, lruc. 13 Tower Office Park Woburn, MA 01801 STANDARDPOLARIZATION CONFIGURATIONS There are three standardpolarization configurations for the two layer benderconstruction: series, parallel,and singleplate ( monomorph). Theseare illustratedin Figure-9. Boththe seriesand parallel elementsare limitedto electricfields below the coercivefield ( - 475Ylmmfor PSI-SA-S4-ENH). SERIESOPERATION The benderpoled for seriesoperation is the simplestand mosteconomical. lt requirestwo connectionsto the outsidesurfaces of the piezoceramiclayers which are electricallyin series. lt is characterizedby a lowercapacitance, lower current, and highervoltage. PARALLELOPERATION The benderpoled for paralleloperation requires three electrical connections. The thirdconnection accesses the centershim of the bender, requiringan extramanufacturing step and thereforea highercost. Voltageis appliedacross the individuallayers. The advantageof the parallelbender is that its deflectionper volt is twicethat of a seriesbender. However,the maximumdeflection is the same for both. The parallelbender is characterizedby highercapacitance, higher current, and lowervoltage. SINGLEPLATE OPERATION In motorapplications requiring extra deflection, a thirdalternative is available.ln this casea benderpoled for seriesoperation is usedwith three electrical connections. Voltage is appliedto eitherlayer of piezoceramicin the directionof polarization.Usually only one side is energized at a time. The excitationfield may exceedthe coercivefield since there is no concernfor depolarization. Fieldsas highas 1,500-2,000volts/mm ( the levelwhere arcing starts to occurnear the edges)may be applied.
SERIES OPERATION
PARALLEL OPERATION
@1
SINGLE PLATE OPERATION
@2
Figure-9.The threestandard polariazation configurations for 2-layerbending motors
14 PrezoSvsrentts, lruc. 65 Tower Office Park Woburn. MA O1801 STANDARDMOUNTS FOR BENDING MOTORS Standardmounts for bendingmotors are illustrated in Figure-10and fall into two general categories.The first category has power taken off at oneend and is mountedat the other. Knownas the cantilevermount, it providesmaximum compliance and deflection. The second category has power taken off at thecenter and is mountedat theends. The simple beam mount allows the ends to movein andout as wellas rotate,but fixes their vertical position. Compared to the cantilevermount, the simplebeam mount producesreduced deflections, increased forces, and increased frequency. For high frequency-resonant applications,power dissipation at themounts can be minimizedby usingnodal mounts. The nodes are evenlyspaced, .55L apart, where L is the lengthof the beam.The beam may also be rigidlyclamped at both ends,although this results in a significantproportion of the beam being constrained.
I CANTILEVER ^X MOUNT -T-
SIMPLE BEAM
1M NODAL Y MOUNT k_.551 _+1
ENDS CLAMPED
Figure-10.Standard mounting techniques for bending motors.
BENDINGMOTOR EQUATIONS The "BendingMotor Equations" describe non-linear behavior and takeinto account the center shim. They canbe usedto: 1.)verify that the bending motor is operatingproperly, and 2.) scale the dimensions of the experimentalactuator to definethe dimensionsof thefinal design, or viceversa. The relations below were developedby C. P, Germanoof VernitronPiezoelectric, with minor modifications by PiezoSystems.
Definitionof terms: L = Total lengthof the bendingmotor ( M ) Lc = Cantileverlength( M ) T = Total thicknessof bendingmotor ( M ) d = Thicknessof centershim and adhesivelayers ( M ) tc = Thicknessof a singlelayer of piezoceramic( M ) [ = Widthof bendingmotor ( M ) d31 = Piezoelectrictransverse strain coefficient ( MA/ ) - E Electricfield strength (V/M ) E (seriesoperation) = Voltage/ 2 tc E (paralleloperation) = Voltage/ tc Y = Young'smodulus of elasticity( N/M2 ) K = Relativedielectric constant of piezoceramic eo = Permitivityof free space (8.85x 10-12 Farads/ Meter )
PtezoSYsrenas, ltuc. 15 65 Tower Office Park Woburn, MA 01801 p = Averagedensity of bendingmotor I = A non-linearityconstant related to the electricfield strength T = A non-linearityconstant related to the electricfield strength
E (V/mm) E(V/mil) I
0 0 1.0 1.0 200 5 1.5 1.2 400 10 1.8 1.3 800 20 2.0 1.4 1200 30 2.1 1.3 1600 40 2.0 1.3 2000 50 1.9 1.3
BENDINGMOTOR MOUNTED AS A CANTILEVER a.t tl Tlv ^X IY --J-
Y| lc' Lc
Figure-11.Terminology used for cantilevered bending motor equations
FREEDEFLECTION: The free deflection, X1, is usuallytwice the required operating deflection.
Xf= 3Fdsr(1"2 tT2)(1+6/T)tcE
wheretct6 0
BLOCKEDFORCE: The blocked force, F5, is usuallytwice the required operating force.
Fb= (3 | 4)TYrr dsr(bT/LcX 1 +6/T )tcE
wheretc>6>0
RESONANTFREQUENCY:Fr= (.16r lLc2)(YrrI p)1t2
-6 MAXIMUMSURFACE STRAIN: - 500X 10 is themaximum recommended strain limit in tension. S= XfTlLc2
CAPACITANCE: C (seriesoperation) = K3eo Lb l2tc C (paralleloperation) = 4 x seriesoperation
16 PtezoSYsreus, lNc. 65 Tower Office Park Woburn. MA 018O1 BENDINGMOTOR MOUNTED AS A SIMPLEBEAM
_l_ ^X t
7 +
Figure-12.Terminology used for simple beam bending motor equations
FREEDEFLECTION: Multiplythe cantilever Xlby 114
BLOCKEDFORCE: Multiplythe cantileverFg by 4
RESONANTFREQUENCY: Multiplythe cantileverF,. by r/8
SURFACESTRAIN: Multiplythe cantilever S by4
CAPACITANCE: Sameas cantileverfor bothseries and parallel operation
PrezoSvsrrvrs,Iruc. 17 65 Tower Office Park Woburn, MA 0'r801 MEASUREDBENDING MOTOR PERFORMANCE OF 2.LAYER KIT ELEMENTS As a referencepoint, the chartbelow provides measured bender performance for the 2Jayer elementssupplied with the motor/actuatorkit (PN:KMA-005). The data corresponds to the performanceat thetip of a cantileveredelement driven at t180 VDCin theseries mode (See Figure-9).
1 T220-44-303 W I r220-44-203
PRnrNunrerR WucHr CRpRcrrRrucr RnrroVorrncr REsporusrTrur Fnee BLOCKED (Fur-lArvrer-ruor) Drrrrcrroru Foncr
Series Parallel Series Parallel Active Lenqth, mm OperationOperation OperationOperation 25.4 50.8 25.4 50.8 25.4 50.8 grams nF nF tVp tVp MS MS +mm +mm tN tN T220-44-103 .40 3.6 14. t1 B0 t90 1.2 x.25 t.07 T220-44-203 .80 7.4 29. t1 B0 t90 1.2 t.25 t.14 T220-A4-30:t.b 14.5 58. t1 B0 t90 1.2 t .25 t.28 T220-A4-503 8.0 72.5 290. t 180 t90 1.2 4.8 x .25 +1 A t.70 +?6
* lnches 61.0 .002 .004 .006 .008 .010 .012
25.5 0.5 50.9
c 20.4 Eoq 40.8 E o E o) E o c z ,lE p a Z nt JU.b ; o (, o TI Y TI I L 10.2 " 0.2 20.4 TI TI
5.1 0.1 10.2
0
+ Deflection(mm)
18 PrrzoSYsreus, lwc. 65 Tower Office Park Woburn, MA 01801 BUILDI NG PIEZOELECTRICACTUATORS
WORKINGWITH PIEZOCERAMIC
THIN.SHEETPIEZOCERAMIC STOCK Unlikelaminated bending motor stock, thin sheet piezoceramic is extremely fragile and difficultto handle.However, with proper care and practice it canbe manipulatedquite easily. Rough cutting is accomplishedby lightlyscribing a ruledline along it's surface with a sharprazor blade untilthe piece cracks.lt shouldthen be pickedup by insertingthe razor blade under one edge and lifting until it caneasily be graspedwith one's fingers. Subsequent operations should not be performeduntil after the piecehas been bondedto its intendedsurface.
2.LAYERMOTOR STOCK DURABILITY:The two layer motor stock is muchmore rugged than generally assumed. lt canbe handledwithout special care and oftentimes dropped without damage. The ceramic is nonporous andis imperviousto moistureas well as chemicallyinert with acids and solids. The adhesives used for lamination,the center shim, and the nickel electrodes, however, are susceptible to particularsolvents and acids. CUTTINGAND SHAPING: For prototyping purposes, the motor stock can be roughcut on a bandsaw (having-14 teeth/inchor more)as longas it is supportedunderneath by a back-upplate (plexiglass,metal, etc,). This is notrecommended for dimensions less lhan 114". Rough cutting usually producesburrs at the centershim which may make electrical contact to oneof the outerelectrodes. The burrscan be removedby filing or sandingthe edge. Chipping will occur along the edge, but this is seldom greatenough to affectperformance. With some practice, the motorstock can be trimmedwith scissors when onewants to removethin slivers. Highquality cuts, necessary for long term stable performance, require the use of a highspeed diamondwheelsaw. ACCESSINGTHE CENTER SHIM: A millingmachine can be usedto removeceramic in orderto accessthe center shim electrode. Removal of -1 milper pass is recommended.A hand held grindingtool (i,e. Dremel Tool) is suitablefor quick center shim access. It is alsopossible to contactthe centershim with a tazotblade or pushpin when only temporary accessis needed(for example, repoling).
BONDINGAND ATTACHING TO PIEZOCERAMIC Attachmentsfor powertake off or mechanicalgrounding are usually accomplished by bondingto thepiezoceramic at its ends or middle.Holes or fastenersare put in thesesecondary members. Almostany adhesive bonds wellto the piezoceramic nickel surface. These include epoxies, anaerobics, and cyanoacrylates.For quick mounting, the bendingelement is oftenclamped between two surfaces.
SOLDERING& ATTACHINGLEADS TO THE ELECTRODES & CENTER SHIM Piezoceramicelectrodes will be eitherfired silver or nickel.Silver electrodes are flat white in colorwhile nickel electrodes are grey. Electricalconnections are usually made to theseelectrodes by soldering,but one may also use conductive adhesive, or clipsto attachwires. Soldering materials in the Motor/ActuatorKit are for solderingto nickelelectrodes unless specifically requested othenrvise. Silverelectrodes are notrecommended for highelectric field DC applications where the silveris likelyto migrateand bridge the two electrodes. lt is oftenused in AC applications.Silver used as an electrodeis in the formof flakessuspended in a glassfrit. lt is generallyscreened onto the ceramicand fired. Theglass makesthe bond between the ceramic and the silver particles. Silver is solublein tinand a silverloaded soldershould be usedto preventscavenging of silverin the electrode.Nickel has good corrosion resistance andis a goodchoice for both AC andDC applications. lt can usually be soldered to easilywith tin/lead solder.Electroless nickel, used for plating piezoceramic, contains phosphor. Sometimes the phosphor
PrrzoSvsrenas, lNc. 19 65 Tower Office Park Woburn. MA 01 801 contentin a platingrun can make it hardto solder.Vacuum deposited nickel electrodes are usually very thin, makingsoldering tricky. Choiceof the correctflux (to removesurface oxidation) makes soldering to electrodesurfaces easy evenunder adverse conditions. A wireis attachedto thecenter shim if theelement is usedin paralleloperation. Generally, the centershim layer of a 2-Layerpiezoelectric bending elements is eitherbrass or stainlesssteel. Shimsare solderedto in the sameway as the nickelelectrode.
Tools& MaterialsFor Soldering . Solderingiron set - 550'-600'F . 60 SnI 40 PbSolder . Supersafe# 67 DSA LiquidFlux . Wires(preferably #30 gauge or smaller) . Pencileraser and paper clip
ProcedureFor Soldering . Cleansurface to besoldered with an abrasive (pencil eraser) and wipe with alcohol. This step can usually beskipped when using the proper flux. . Dipthe tip of a paperclip into the flux and apply a smalldot of SupersafeLiquid Flux to theelectrode area to be soldered. . Applysmall amount of solderto irontip and transfer solder to thepiezoceramic electrode by touching iron tipto fluxdot. A goodsolder joint should flow rapidly ( < 1 second)and look shiny. Metalshims take longer dueto theincreased thermal mass (-2 seonds). . Applyanother small dot of SupersafeLiquid Flux to thesolder dot on electrode. . Positionpre{inned wire on solderdot and apply soldering iron to thewire until the solder melts. Remove ironquickly after the solder melts and hold the wire still until the solder solidifies. A 30 gaugewire or smaller is recommendedto minimize strain on the solder joint during wire handling. . RemoveSupersafe Liquid Flux residue with clean running water. This flux residue is electricallyconductive andmust be removedfor properfunctioning of a piezodevice. Any rosinresidue may be removedwith alcohol. . Whereverfeasible, the wire-solder joint should be strainrelieved with a dropof adhesive.
PERFORMANCETESTING Generally,measurements of freedeflection, blocked force, resonant frequency, and capacitanceare easy to makeand should be recordedfor eachdesign configuration explored. Capacitance is a goodmeasure of thehealth of the element. lf thecapacitance of thepiece decreases during operation or testing,from that of theits initial value, then the element has probably depoled, cracked, or lostelectrical contactduring operation. Forfundamental performance testing, a powersupply and switching box with an adequate seriesprotection resistor are suggested. Any frequency generator can be usedto measureresonant frequency.The resonance patterns may be observedby sprinklingsugar on thepiece and sweeping through thefrequency band. At resonance,sugar will be tossedoff the anti-nodesand collect at the nodes.Suitable meansfor measuring force and deflection, such as anX-Y table and high compliance force gage, are needed.Such a systemis shownin Figure-13.
20 PrrzoSYsreus, lruc. 65 Tower Office Park Woburn, MA 01801 AC/ DC On/ Off
Amplifier
Frequency Generator
Figure-13.System for measuringoutput performance ( free deflection, blocked force, and resonant frequency) of a bendingmotor element.
PROFESSIONALENGINEERING SERVICES
PiezoSystems Inc. (PSl) recognizes that in manyinstances, after the principles and feasibility of a piezoelectricapproach have been demonstrated, timely and professional development is essential, PSI offersthe supportservices listed below.
CUSTOMPIEZOELECTRIC DESIGN AND DEVELOPMENT ENGINEERING Forcustomers who have demanding longterm projects employing piezoelectric motor or generator technology,PSI offers a phasedprogram for customdevice and product development. The program enhancesdevelopment success and product reliability by eliminating potentialdesign flaws which might plaguethose unfamiliar with piezoelectric technology, especially in theareas of: laminationbonding, hinge/flexuredesign, ceramic stress and fatigue criteria, high temperature stability and performance, mountingand power take-off attachment, drive circuit design, testing, and evaluation. Using extensive in- housesoftware, PSI can movedirectly from your requirements to a completedactuator or systemdesign. Developmenttime is reducedfrom years to monthsand incorporates realistic economic and processing assumptions.
CONSULTINGSERVICES Discussionswith the PiezoSystem's technical staff concerning a widerange of technicaland economicissues are available.
PrrzoSYsreus,Iruc. 21 65 Tower Office Park Woburn. MA 01BO1 TABLE.1. PIEZOELECTRICAND MATERIAL PROPERTIES FOR PSI.5A.S4.STD PIEZOCERAMIC
PIEZOELECTRIC Composition LeadZirconate Titanate, NavyType-ll
MaterialDesignation PSI-5A-S4-ENH
RelativeDielectric Constant (@1 KHz) Kts 1800 Kr,' 1900
PiezoelectricStrain Coefficient dir 390 x 10-12 Meters/ Volt der -190x 10-12 Meters/ Volt drs -550 x 10-12 Meters/ Volt
PiezoelectricVoltage Coefficient gll 24,0x 10-3 Volt Meters/ Newton gy -11.8x 10-3 VoltMeters / Newton grs -26.0 x 10-3 VoltMeters / Newton
CouplingCoefficient klr 0.72 k:r 0.32 krs 0,59
PolarizationFietd Ep 2 x 106 Volts/ Meter
CoerciveField (DC) Ec 5 x 105 Volts/ Meter (@ 60 Hz) 6 x 10s Volts/ Meter
MECHANICAL Density p 7750 Kg/ Meterr
ElasticModulus YE.. 4,9x 1010 Newtons/ Meter2 Yr;; 0,2x 1010 Newtons/ Meter2
Poisson'Ratio D 0.31
CompressiveStrength 5.2x 108 Newtons/ Meter2
TensileStrength (Static) 7.5x 107 Newtons/ Meter2 (Dynamic) 2,0x 107 Newtons/ Meter2
MechanicalQ BO
THERMAL 'c CurieTemperature 350
PyroelectricCoefficient -420x 10-6 Coulombs/ Meter2 "C 'C ThermalExpansion Coefficient -4 x 10-6 Meters/ Meter 'C SpecificHeat Cp 440 Joules/ Kg
22 PrezoSvsrerus, lNc. 65 Tower Office Park Woburn. MA 01801 TABLE.2.SPECTRUM OF COMMON PIEZOELECTRIC TRANSDUCERS
PIEZOELECTRIC FREE BLOCKED RESONANT GENERAL CONFIGURATION DEFLECTION FORCE FREOUENCY FEATURES CANTILEVERBENDING MOTOR
o 3dsrL2E 3derYbT2E Yrr ll p 8?s 2T 8L (rx3 I SIMPLEBENDING MOTOR b-n --T LI 3dgrL2E 3dsrYbf2e 8T 2L 2 o n = m o TRANSVERSE(D31 CONTRACTIONMOTOR n z L ) m o z 2 n o o a m n z n m o a m z @ (t dgrYA E 1 I z dsrLE a .Tt z o ll z 2L t n -l o whereA=bT o 11 @ rn m n i m m o z c -l m ( MOTOR z LONGITUDTNALD33 ) EXTENSION o -* L _l_ d33LE dgsYA E whereA=ab j l f -T SHEARMODE MOTOR EJJ@ ,6$o +L dtsTE dtsGAE whereA= b L ;+ PrrzoSvsreus, lruc. 23 65 Tower Office Park Woburn, MA 01 BO'l