231575 Piezo-Mechanics GB

Piezomechanik GmbHPiezo-Mechanics:An Introduction

Table of content1. Generation of motion by piezo-electrical devices1.1. Piezomechanical effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 041.2. Actuator design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 051.3. Actuator characterisation by stroke and force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 061.3.1. Voltage – stroke characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 061.3.2. Voltage – force characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 061.3.3. Intermediate states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 071.4. Energy/Power balances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 081.4.1. Nanopositioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 081.4.2. Power optimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 082. Piezoelectric and electrostrictive ceramics2.1. Low voltage actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 092.2. High voltage actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 092.2.1 Standard PZT ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 092.2.2. High power PZT ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.3. High temperature, high stability PZT ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.3. Electrostrictive PMN ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113. Comments on data sheet3.1. Voltage polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2. Stroke and positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.3. Electrical capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.4. Prestress, preload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.5. Max. compressive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.6. Stiffnesses (inverse compliance) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.7. Blocking force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.8. Resonances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.9. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.10. Lifetime, reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174. Options4.1. Cryogenic operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.2. UHV compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.3. Position detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.4. Internal force detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.5. Internal heat management “Thermostable” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.6. Corrosion resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.7. Special casings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.8. Custom designed actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Table of content5. Comparison of low and high voltage actuators5.1. Materials and dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.2. Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.3. Temperature ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.4. Vacuum compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.5. Noble gas atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.6. Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236. Mounting procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257. Electrical control philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267.1. Piezo-mechanics and electrical charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267.2. Linearisation of piezo motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267.3. Enhancement of stiffness by charge/current control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287.4. Reliability aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287.5. Electronic supplies for piezo-action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

1. Generation of motionby piezo-electrical devices1.1. Piezomechanical effectsPiezo-actuators like stacks, benders, tubes, rings makeuse of the deformation of electro-active PZT-ceramics(PZT: lead (Pb) zirconia (Zr) Titanate (Ti)), when they areexposed to electrical fields. This deformation can beused to produce motions and / or forces.The above effect is the complementary effect to piezoelectricity,where electrical charges are produced uponapplication of mechanical stress to the ceramics. As ananalogy the term “piezo-mechanics” was introduced inthe early 80’s of the past century by L. Pickelmann todescribe the conversion of electricity into a mechanicalreaction by a piezo-material.For piezo-mechanical conversion in the simplest case asingle PZT layer is used.Such a PZT monolayer structure as shown in fig.1aacting as a capacitive element defined by 2 thin conductiveelectrode coatings enclosing the piezo-ceramicas dielectric.Fig. 1a: Schematic of a piezoelectric single layer elementWhen this “piezo-capacitor” is charged by applying avoltage, the deformation is created. PZT actuators are “moving capacitors”Thickness mode (d33 effect)Piezo stack actuators and stacked piezo rings make useof the increase of the ceramic thickness in direction ofthe applied electrical field (d33 effect). Stacking ofseveral layers towards a multilayer structure increasesequivalently the total stroke. In practice, axial strainrates up to 2‰ of stack’s length can be achieved undercertain conditions.Planar mode (d31 effect)Similar to normal elastic deformation of a solid statebody, the thickness expansion (d33) of a PZT layer isaccompanied by an in-plane shrinking (fig.1a). This iscalled the d31 effect, being complementary in motionand showing roughly half linear strain comparedto the d33 effect.The d31 effect is mainly used for bending structures(uni/mono-morphs, bimorphs ect. and piezo-ceramictubes).Fig. 1b:Schematic of an axially acting multilayer piezo-stack4

1.2. Actuator design: Piezostacks andstacked piezo ring actuatorsPiezo-actuators were used preferentially in the past forquasistatic precision positioning tasks, but find nowincreasing interest in completely new fields of applicationlike dynamically actuated mechanics (e.g. valves, fuelinjection devices) or adaptive smart structures (shapetuning, vibration generation and cancellation, modetuning).It is self-evident, that this extreme broad variety ofapplication characteristics cannot be covered by onegeneral purpose type actuator.The main parameters to adapt a piezo actuator to adistinct application are:A selection of proper PZT material defining achievablestrain, stroke, energy balance, temperature range etc.B preparation of a highly reliable and efficient stack structuree.g. shock and vibration resistant electrodingC sealing for corrosion resistanceD packaging of the ceramic stack e.g. under rulesof preloading and heatmanagement for dynamicapplicationsE special actuator system design e.g. antagonisticsystems with a 2-element push-pull arrangementF performance and reliability of actuator systemsdepend further on the electrical driving characteristics(see chapter 7): like voltage / charge / currentcontroloperating strategies and unipolar / semibipolar/ bipolar operationCased stacks with internal preload mechanism(fig. 3)The incorporation of piezo stacks into a metal casinggenerally improve reliability and stability againstmechanical impact and deteriorating environmentalinfluences. The implementation of a preload mechanismcompensates for tensile stress, where the ceramic isextremely vulnerable to (details see section 3.4.).A stainless steel casing is indicated by the addendumVS in the order code.A PSt 1000/10/80 VS 18 actuator is a high voltageactuator for 80 µm stroke, contained in a preloadedstainless steel casing with 18 mm diameter.ceramicpolymercoatingCurrently, the following basic stack designs are usedBare stacksThe mounting and motion transfer is always done viathe endfaces.Never hold a bare stack by sideway clamping.The designation in data sheet is PSt A / B / C withA max. driving voltage in forward polarityB diameter of the ceramicC nominal max. unipolar strokeFig. 2: Schematic of a stacked ring actuatormovingtop endRing stacks (stacks with center hole) fig. 2The need for ring structures is twofold:A when an accessible system’s axis is needed fortransmissive optical setups or the feedthrough ofmechanical parts is requiredB to increase bending stiffness by diameter enlargementof the stack without the need of increase of the operatedceramic volume (e.g. for longstroke elements)Ring actuators provide further the utmost coolingperformance due to potential access of the inner andouter surface by cooling media.The designation of ring actuators is as above withHPSt A/B1-B2/C, where B1 and B2 are the outer andinner diameter of the ceramic stackPiezostackprestressmechanismstainlesssteel casingcoaxialcablebottom piece withthread (fixed end)Fig. 3: Schematic of an actuator with prestress5

1.3. Actuator characterization by strokeand forceLike any other “electrical motor”, a piezo actuator convertselectrical energy into motion and/or force what iscoupled to an external mechanism. This can be in thesimplest case a component, what shall be shifted fromone position to another one. More complex applicationsare dynamic applications like valve control or the setupof adaptive/smart structures where piezo-mechanicalelements are incorporated.In all these cases the interaction between the actuatorand the driven mechanism must be analyzed. Thismechanical interaction is ruled by the stiffnesses(= inverse compliances = spring constants) of the twosystem parts: “actuator” and “actuated mechanics” (seeSection 3.6.).Fig. 4: Schematic of a piezo actuated system:A piezo actuator interacts with a coupled mechanism,where the piezomechanical performance is defined bythe two interacting stiffnesses S piezo and S mech .To characterize the actuator performance two basicexperiments are carried out to determine actuator’sstroke and actuator’s force generation as functions of1.3.1. Voltage – stroke characteristicCondition:The coupled mechanics show stiffness S mech = 0For S mech = 0 a piezo stack shows maximum strokeNo force variation is generated (constant preload)1.3.2. Voltage – blocking force characteristicCondition:S mech = ∞ , actuator cannot expandFor S mech = ∞ , the piezo stack generates maximumchange of force: the blocking force.No stroke is generatedFig. 5:Schematic voltage/stroke diagram of a stack actuatorFig. 6:Schematic voltage/blocking force diagram for a piezostack.The force response shows remarkably lower hysteresisthan the stroke diagram (depends on preload conditions).6

1.3.3. Intermediate states (0 < Smech < ∞)In practice piezo actuators interact mostly withmechanical systems showing an intermediate stiffnessvalue between 0 and ∞.Then the piezoactuator distributes its “activity” partiallyinto generation of stroke and partially into generation offorce, where “partially” depends on the quantitativerelation of actuator’s and mechanic’s stiffnesses.The achievable force-stroke relations in a real systemcan be derived in the following way:Make the following two steps:– draw a line from piezo actuators max. stroke ∆l maxto max. blocking force F B ( ______ ) (at same voltages)– draw a line from origin with a slope according thestiffness S mech of the coupled mechanics:N/µm ( _ _ _ _ )These two lines intersect at point A.The coordinates of A: (∆l A , ∆F A ) show the achievablestroke and force at maximum excitation voltage ofthe actuator.Notice:This schematic represents a linearised actuator’s response.It does not take into account stroke enhancementeffects as described in section 3.4. C. Especially the highpower material HP (section 2.2.2.) shows improved performance.For S mech = S piezo the achievable displacement is 50%of the maximum displacement l max and the achievableforce variation is 50% of the blocking force F B .Under this condition the mechanical energy transfer efficiencyE from the actuator into the mechanical systemis maximized (matching point).Fig. 7: Shows a diagram spanned by piezo actuator’smax. stroke versus blocking force7

1.4. Energy/Power balancePiezo actuators are “motors” converting electricalenergy P into a piezomechanical response.The (non resonant) electrical energy content P of anelectrically charged piezostack isP = 1 /2 CU 2The piezomechanical response areStroke: ∆lForce generation: ∆FMechanical energy E = ∆l x ∆F1.4.1. (Nano)Positioning philosophy:C = actuator’s capacitanceU = applied voltageHere the user of an actuator is mainly interested in thedisplacement/stroke ∆l of the actuator and (maybe) theultrafine positioning capability down to the subnanometerrange. A variation of force or generation ofmechanical energy during piezo actuator’s action is nota priority.In terms of energy efficiency, piezoactuators are thennot well matched for most positioning applications,because only a small part of actuator’s energy content isneeded to be transferred to the coupled mechanics.Optimizing piezo actuators for dynamic positioningpurposes means that the electrical energy input shall beas low as possible to get a certain displacement. Orwith other words, actuator’s capacitance (in relation tothe driving voltage) shall be as low as possible to minimizethe electrical current/power requirements for adistinct dynamic response (e.g. for oscillating arrangements): A PZT material with low dielectric constant εtogether with a high piezoelectric constant d 33is required.For dynamic positioning purposes PIEZOMECHANIKprovides excellently adapted PZT materials like thestandard ceramic and the HS/HT ceramic, showinghigh strain together with low dielectric constant.Low ε materials have other interesting propertiestoo: high operating temperatures and high stabilityagainst depoling. So energy aspects are not theonly one criterion for selecting this type of PZTceramic.Notice:Low dielectric ultrasound PZT materials show rathersmall strain rates under non resonant conditions andare not optimized for actuation.Examples for positioning strategyThe “mother of piezo actuation” were optics, whereactuators are used frequently to shift a distinct optomechanicalcomponent with ultrahigh accuracy to getan optimum adjust e.g. of a laser resonator.Optomechanical components like stages or mirrormounts are usually systems using soft reset springs andshow therefore low stiffnesses.Adjusting of optomechanics by piezo actuators doesnot require a production of high mechanical energies orforces.An example for high dynamic positioning is fast valvecontrol, where the actuator is shifting a mechanical partlike a small ball or a needle to open or to close thevalve.1.4.2. Power optimisation“Positioning” according 1.4.1. ruled piezo actuationnow for nearly 30 years, but new applications areemerging, where a piezo actuator must be adapted toother strategies:In these cases a piezoactuator shall provide high displacementstogether with high force generation toachieve high mechanical energy transfer.Examples are adaptronic/smart structures of high stiffness,which shall be highly deformed by integratedactuators. This is a requirement to ensure large mechanicalenergy transfer.Examples e.g. adaptive frame structures of machines,car bodies, wings of air planes for active vibration excitationand cancellation or shape optimization.⇒ High mechanical energy shall be transferred intoa mechanical structure.⇒ To provide this large mechanical energy output,an increased electrical energy input is required viaa reasonable large electrical capacitance of theactuator to get a high mechanical energy densitywithin the ceramics.⇒ A PZT material is requested showing an elevateddielectric constant ε combined with a very highstrain and force generation rate.Notice: Large ε materials are widely offered,but often do not show the wanted large piezomechanicalresponse. Piezomechanik is offering the high powerPZT material HP, which shows the highestmechanical energy density available for PZTceramics. The energy density is roughlydouble that of standard materials (like PZT 5A)or the HS/HT material. To get the same powerout from “positioning actuators”, the actuatorvolume must be simply doubled with consequencesfor weight, space and price comparedto the energy optimized HP material.With a HP based actuator PSt 1000/25/100 mechanicalpulse powers of more than 100 kWatts have beengenerated.8

2. Piezoelectric andelectrostrictive ceramicsPZT ceramics will be the heart of solid state actuation for the near and mid future.New electroactive materials like highest strain/single crystalline formulations cannot compete for technical and costreasons for broad applications at the moment.Piezomechanik provides a set of piezoelectric PZT ceramic materials for stack manufacturing, which covers a verywide range of applications.This is true both for co-fired low voltage as well as discretely stacked high voltage materials.The standard program has been defined by compromising technical performance and pricing.Nevertheless, special materials are available on request.2.1. Low voltage actuators (cofired monolithic stacks)Operating voltage range: (–)30 V thru (+)150 VOperating temperature: –50 °C thru +100 °CAxial coefficient of thermal expansion (CTE): approx. –3 ppm/°CVacuum compatibleLow voltage actuators for cryogenic applications on request.Low voltage actuators based on HP, HS/HT materials on request (see below)Low voltage actuators for other voltage ranges (e.g. 200 V) on request2.2. High voltage actuatorsFig. 8:Normalized stroke/voltage diagram of low voltage stacks(relative stroke 1 for voltage step 0 V/U max )A. For bipolar operation in the range(–)0.2 U max thru (+)U maxB. For unipolar operation in the range0 V thru (+)U max2.2.1 Standard PZTOperating voltage range: (–)100 V thru (+)1500 V for PSt 1500 … stacks(–)200 V thru (+)1000 V for PSt 1000 … stacksOperating temperature: –60 °C thru +120 °CAxial coefficient of thermal expansion (CTE): approx. +1 ppm/°CVacuum compatibleActuators for cryogenic (–273 °C) applications on request (e.g. tuning of superconducting accelerator resonators)Fig. 9:Normalized stroke/voltage diagram of standard highvoltage stacks PSt 500 and PSt 1000A. For bipolar operation in the range(–)0.2 U max thru (+)U maxB. For unipolar operation in the range 0 V thru (+)U max9

2.2.2 High power PZT HPHigh Power HP PZT material is the choice, where high mechanical power generation is required. Stroke and forcegeneration under high load are remarkably larger than for the standard or HS/HT material. The energy output isroughly doubled.The HP material is used for adaptronic applications, where the mechanical energy transfer to a coupled system shallbe maximized.The electrical capacitance is higher by about 80% compared to the standard configuration.Operating voltage range: (–)100 V thru (+)1500 V for PSt 1500 … stacks(–)200 V thru (+)1000 V for PSt 1000 … stacksOperating temperature: –60 °C thru +120 °CAxial coefficient of thermal expansion (CTE): approx. +1 ppm/°CVacuum compatibleFig. 10:Normalized stroke/force diagram for HP modified highvoltage actuators. Relative stroke = 1 at U maxA. For bipolar operation in the range(–)0.2 U max thru (+)U maxB. For unipolar operation in the range 0 V thru (+)U max2.2.3. High temperature/high stability PZT: HS/HTThe main feature of the HS/HT material is the stability of strain and force characteristics under varying temperaturesor load forces.Electrical capacitance and achievable strain exceed slightly standard PZTOperating voltage range: (–)100 V thru (+)1500 V for PSt 1500 … stacks(–)200 V thru (+)1000 V for PSt 1000 … stacksOperating temperature: –60 °C thru +120 °CAxial coefficient of thermal expansion (CTE): approx. +1 ppm/°CVacuum compatibleOptions:Actuators for cryogenic (–273 °C) applications on request (e.g. tuning of superconducting accelerator resonators)High temperature application (up to 200 °C on request)HS/HT material is used for high frequency/high speed actuation likehigh dynamic valve controlsshaker applicationsscanningFig. 11:Normalized stroke/force diagram for HS modified highvoltage actuators. Relative stroke = 1 at U maxA. For bipolar operation in the range(–)0.2 U max thru (+)U maxB. For unipolar operation in the range 0 V thru (+)U max10

2.3. Electrostrictive PMN ceramicsThis material is used preferentially for static positioning applications.On one side it outperforms PZT stacks by a strongly reduced hysteresis (approx. 2%) and absence of creep.But unfortunately these features are true only in a very narrow temperature range of approx. +/–5 °C around roomtemperature. Further the electrical capacitance of electrostrictors is remarkably higher than for comparable PZTelements.Electrostrictive devices are restricted mainly to laboratory use for certain adjustment tasks. High dynamic operationleads to self-warming and then the systems drifts away from the optimum temperature range.Electrostrictive devices show no defined poling direction, any voltage polarity can be applied resulting in a positivestroke with increase of voltage. Therefore no stroke enhancement by semibipolar operation is achieved.Electrostrictive actuators are available as low voltage and high voltage stack elements.Operating temperature: +20 °C thru +35 °CFig.12a: Normalized stroke/voltage characteristic forelectrostrictive actuators, unipolar operation only.Fig.12 b: Creep of piezoactuators versus electrostrictors.Creep in PZT is noticed only for a few seconds afterapplication of a voltage step and then declines. It variesin the percent range of the initial achieved stroke.11

3. Comments on datasheetThe problem in comparing data of actuators from differentsuppliers is, that specifications and data on piezostacks are often defined in different ways.Therefore it is necessary to understand to some extentthe background of the specified parameters to ensure areasonable decision for and proper use of a certainpiezo-element.Especially for highly sophisticated applications, a profoundconsulting is recommended to check for allrelevant aspects when designing a piezo active arrangement.Contact PIEZOMECHANIK, when you make firststeps towards piezo actuation.3.1. Voltage polarityPiezostacks can be operated by unipolar or semi-bipolarvoltage signals.When special PZT-material is used even a symmetricalbipolar operation is possible (see PIEZOMECHANIKcatalog: “Long life bipolar stacks”).In case of unipolar or semibipolar driving mode, attentionmust be paid for correct voltage polarity, whenconnecting a piezostack to a power supply. Otherwise,the PZT actuator can be deteriorated in its action bydepoling, when the maximum voltage is applied.A simple check for correct polarity is, when a harmlesslow voltage signal is applied and the stack actuatorexpands with increase of voltage.Bipolar actuators are insensitive to polarity reversal bydefinition, but the mechanical response invertse.g. the stacks shrinks upon application of a counterpolarvoltage.PIEZOMECHANIK’s actuators with casing and powersupplies are manufactured for positive polarity withreference to ground.Bare piezostacks wired by two pigtails are manufacturedpotential free, so any power supply polarity can be usedwhen connected correctly to the piezostack.3.2. Stroke and positioningAny PZT material can be operated to some extent with acountervoltage opposing the burned-in polarity. Comparedto unipolar activation, the semibipolar moderesults in wider total voltage swing with the expectedconsequences: increase of stroke, blocking force andenergy density.A 20% countervoltage (of the specified maximum forwardvoltage) is applicable to any material. The yield instroke (and blocking force) can be up to 30% then comparedto the unipolar situation.The energy content canbe enhanced up to 50% and more. Hard PZT materialslike PZT ceramic HS/HT can withstand still higher electricalcounterfields than the above defined 20%.The data sheet show two values for the maximum stroke.These are related to semibipolar operation (higher valueequiv. larger voltage swing) and conventional unipolaroperation (lower value) e.g. the PSt 150/7/40 VS 12 elementis specified with 55 µm/40 µm stroke meaning:40 µm stroke for unipolar 0 V/+150 V activation and55 µm for the semibipolar –30 V/+150 V swing.A piezostack’s maximum stroke is proportional tostack’s length. Strain rates of approx. 1– 2‰ are usuallyachieved.The data sheet show nominal values for “no load” conditionat room temperature.The achievable real stroke depends on the mountingconditions like preload and stiffness of attachedmechanics (see sections 1.3.3.).One of the most important features of piezoactuators isthe ultrafine positioning capability down to the subnanometerrange (provided the electronic supply shows asufficiently high quality with respect to noise and signalresolution).Please take notice, that piezo-elements show aninfinitely large relative positioning sensitivity: This meansthat an infinitely small voltage variation leads to aninfinitely small mechanical shift. Unfortunately the stepwidth for a distinct macroscopic voltage variation is notcalibrated as characterized by the hysteresis in thevoltage stroke/diagrams.To do absolute positioning a position measuring systemsmust be introduced to detect the mechanical shift andto match the voltage signal for setting to the desiredposition.An introduction to the philosophy of feedback controlledpositioning is shown in PIEZOMECHANIK’s catalog“PosiCon feed back control electronics”.Notice: The stroke characteristic becomes morelinear and non-hysteretic when stroke is set intorelation to the electrical charge content of the piezoactuator instead voltage (see chapter 7).12

3.3. Electrical capacitancesPiezo-stacks are a kind of multilayer capacitors. So theelectrical capacitance is the result of the structure(number, thickness and cross section of the layers) andthe used PZT material (dielectric constant ε) on theother side.The large difference in capacitance values of low voltageversus high voltage stacks (of same size) are mainly dueto the structural difference in thickness and number oflayers.It is self-evident, that both types produce the samepiezo-mechanical performance in all aspects, when thesame material and the same driving electrical field isused.The electrical capacitances stated in the data sheet areusually measured as so-called “small signal” values,where a small voltage signal in the Volt range excitesthe capacitance dynamically and the induced current ismeasured. For “normal” capacitors, the derived“capacitance” is valid for larger voltages too, becausecapacitance does not vary with voltage.With “piezo capacitors” this situation changes towardsthe more complex situation, that capacitance of anactuator varies to some extent with voltage, temperature,load. Therefore the stated capacitances are only arough number. Capacitance data are needed to valuethe electrical currents or powers to get a distinctdynamic reaction.Please take therefore into account at least a factor 1.5,when checking for a proper power supply.Notice: approx. 5% of the power consumption of the“piezo capacitor” is dissipated into heat.3.4. Prestress/PreloadIn a lot of applications of piezo-stacks a socalled prestressor preload is applied:This means the application of constant force e.g. by akind of spring mechanism, compressing the stack. In alot of piezo actuated systems, the whole mechanicalsystem is preloaded against the actuator, so that anybacklash by alternating load force directions is avoided.springNotice: Very high accelerations during actuatorswitching (risetimes far below 100 µseconds) byimproper power supplies may create higher internaloscillating modes. Even very high preload maybeinsufficient to prevent ceramic damage.Co-fired stacks are more sensitive to this effect thandiscretely stacked high voltage elements.These problems can be overcome by proper electronicsupplies using pulse current shaping(see chapter 7).C Improvement of piezo-mechanical performanceCertain piezo-ceramics show strain enhancement,when a high load pressure is present.PIEZO-StackFig.13: Schematic of a preloaded stackHPStandardHT/HSThe reasons for preloading are twofold:A Symmetrical push/pull performance:Pure PZT ceramics can only produce high pushingforces, but cannot withstand larger tensile forces. Toget more symmetrical push-pull-performance of anactuator a kind of resetting force must be introducedinto the system: the simplest arrangement is a passivepreload by a kind of spring mechanism.B Improved dynamics“Dynamics” means “acceleration forces”. It is selfevidentfrom basic physics, that dynamic contractionresult in a tensile loading: so compensation for thistensile force is needed by a sufficient high preload.Then the actuator can be operated in a highly dynamicway (high frequency oscillations, pulsed operation.Without prestress, the actuator will fail.Fig.14: Strain enhancement upon prestress for differentPZT materials.The high power HP materials shows the strongest reaction,whereas the stabilized material HS/HT is insensitiveto preload. The standard material is an intermediateformulation.The very high power capability of HP ceramics is relatedto this enhancement effect.13

Preload design criteriaAn optimum preload design fullfills two basic requirements:● The preload mechanism shall provide high forcestogether with an as low as possible stiffness. Onlythen the preload force does not vary with piezoactuator’s motion and maximum stroke is achieved.● The force level of the prestress must be high enoughto reset the moved masses attached to the actuatorsufficiently fast according the mass acceleration lawpreload force F = m · ∆l/t 2An alternative are antagonistic systems, where twopiezo actuators are facing each other and shifting anin-between mounted component. The actuators operatewith complementary push-pull motions.The advantage is, that the actuators must not actagainst a permanent counterforce and no resonancesemerge from reset springs. On the other side, largercapacitances must be handled. The simplest antagonisticarrangement is using bipolar stack actuatorswith opposing polarity setting. So the system can beoperated with one driving signal for both actuators.m: attached mass∆l: actuator’s stroket: minimum reset time defined by desired operationfrequencyPreload forces up to 50% of actuator’s maximum loadcapability are applied sometimes to get symmetricalpush/pull performance.In a lot of applications the complete actuated mechanicsis preloaded against the actuator avoiding push-pull(oscillation of force direction) with risk of backlash.PIEZOPIEZOActive resetting:The above mentioned preloading is a kind of passivetechnique using e.g. reset springs.Fig. 15: Schematic of a bipolar antagonistic actuatorsystem for active reseting.3.5. Max. compressive loadThe max. load or max. compressive force capability isnot a hard limit for the maximum applicable force, wherea damage must be immediately expected, when slightlyexceeded.As shown in fig.14, the achievable maximum strain candepend on the applied compressive force.The maximum applicable load is defined as the forcelevel, where the actuator shows a reduction in performancelower than the zero load performance.Notice: The reduction in performance is reversible.A mechanical damage of the ceramic structure occursunder remarkably higher loads.Attention must be paid to actuators with a criticallength/diameter ratio because of the increasing risk ofbuckling (Typically factor of 15–20). High bendingstability by large diameter designs can also be achievedby using ring actuators instead of bulky stacks. Theadvantages are equivalently lower elctrical capacitances.14

3.6. StiffnessesLike any other solid state body, a piezo actuator showsa distinct elasticity.This elasticity is described by Hooke’s law, where adeformation ∆l is related to the applied ∆F force by∆F = S ∆ldefining thereby thespring constant = stiffness S = inverse complianceIn contrast to “normal” passive materials the uniquefeature of piezo-ceramic is to show a variable stiffness,depending on the electrical driving scheme as it isshown by following simple experiments:A compressive force ∆F shall be applied to the piezostack and the resulting compression ∆l of stack’s lengthis measured.Following situations can now be distinguished:a) the actuator leads are short circuited or connectedto a voltage amplifier with constant output voltagesetting,b) the actuator leads are open,c) the actuator is connected to a position feedbackcontrol unit.In a) a distinct actuator stiffness S volt = const. = ∆F/∆l isderived.In b) roughly the double stiffness compared to a, isobserved (depends on type of PZT material andto some extent on manufacturing technique of thestack)In c) a nearly infinitely high stiffness is seen, nocompression occurs (within the feedback controlrange).The reason for the difference in a) and b) is easily understood:The application of a compressive force to PZT leads tothe generation of electrical charge via the normal piezoelectriceffect.In case a): short circuiting the leads (or holding the voltagelevel constant via a voltage amplifier) the generatedelectrical charges can flow and equilibrate.In case b): the generated charges cannot flow due toopen leads and a voltage is generated creating astabilizing electrical field acting against the mechanicalcompression => higher stiffness than a)The case c) leads self-evidently to virtual infinitely highstiffness, because the feedback makes any compressivedeflection to zero.These basic phenomena find their equivalents in differenttypes of electrical supplies for driving a piezo actuator:a) corresponds to open loop voltage controlb) corresponds to open loop charge (or current)controlc) is realized by closed loop position control.Voltage control fits excellent to the needs of low dynamicprecision positioning, where no high requirementsfor dynamic stiffnesses are given. This was the situationfor the past decades.Motion control by open loop charge or current controlof piezo actuators leads to remarkably higher stiffnessesthan the open loop voltage control. Currently applicationsare becoming more and more important, wherehigh dynamic stiffnesses are requested to modulatehigh stiffness structures (see chapter 7).The data sheet show the actuator stiffnesses forvoltage control.3.7. Blocking forceThe maximum blocking force shown in the data sheet isdefined for maximum semi bipolar voltage swing. Detailsfor blocking forces see section 1.3.2.3.8. ResonancesAs any other solid state body, a piezostack shows resonantmodes. In the data sheet the axial resonances for aone side fix oscillation of the stack are shown.Usually piezostacks are operated broadband non resonantlywell below stack’s resonance frequency.When the piezo actuator is attached to a mechanicalsystem, the resonance of the total system will be lowerfor several reasons:ABDue to the mass m of the attached mechanics,which results in a new actuator resonance frequencyaccording the spring/mass system’s law(where the stack acts as the “spring”)f res = 1 /2π (S/m)The resonant modes of the attached mechanics(e.g. optomechanical components like translationstages have resonances in the 100 Hz region).Mechanical resonances of a piezo-actuated system canbe easily detected by using the piezo-stack as vibrationsensor. When the piezo-actuator is connected to anoscilloscope, the ringing signal can be monitored, whenthe mechanics is excited by a short knock.15

3.9. Thermal propertiesThe stability of piezoactuators at high temperatures isnot only determined by the properties of thepiezoceramic, but also by the used accessory materialssuch as insulating coatings, electroding, adhesives etc...The Curie temperature of the ceramic is in most casesnot the limiting factor. An important aspect for industrialapplication is, that actuators shall not only withstandwide temperature variations, but also shall show onlysmall changes in their properties. Here the materialcomposition HS is unrivaled (see chapter 3.2.).The use of actuators under cryogenic conditions is astandard application.SelfheatingPiezoactuators show self-heating proportional to thereactive power balance during dynamic operation,increasing therefore with actuation frequency and amplitude.The tendency to dramatic self-heating is furtherenhanced by the poor internal heat management forstandard mounted actuators with the sole mechanicalcontacts of the ceramic stacks via the endfaces.Heat-sinking by this arrangement is very poor due to thelow thermal conductivity of PZT ceramic and the poorheat transfer for cased actuators with an airgap betweenceramic stack and casing.The consequences are severe frequency limitations forthe operation of piezoactuators to avoid overheating.Normal low voltage actuators with mid sized diameterstend to overheat for frequencies in the range of about200 Hertz/full stroke operation.To make use of the full dynamic potential of piezoactuatorswith high frequency/amplitude rates inlong-term operation, a consequent heat managementis necessary to remove heat efficiently fromthe piezostack.The solution is the unique ThermoStable configurationprovided to PIEZOMECHANIK’s actuators(option 3.9.).Thermal expansionLow voltage actuators show a thermal axial expansioncoefficient of –3 ppm/°K (measured for short circuitedactuator).Discretely stacked high voltage actuators show highervalues due to the ceramic/metal/adhesive compoundstructure of about +1 ppm/°K. High voltage actuatorsfrom other sources sometimes show very high thermalexpansion due to oversized adhesive layers.For actuators with casing the mechanical endpiecescontribute to the total thermal expansion. This effect canbe minimized by the use of INVAR alloy.Low temperature properties:The piezomechanical and electrical properties of PZTceramic depend on temperature. When piezoactuatorsare cooled down to 77 °K (LN2) or even lower towardsthe 4° Kelvin region, any PZT ceramic behaves like apiezoelectrically very hard piezo material featuring• strong reduction of electrical capacitance• reduction of loss factor / reduced hysteresis• reduced piezoelongation factor d33• strong increase of the coercive field strengthThe last point means, that at low temperatures apiezostack becomes extremely stable against electricaldepoling and other destabilizing effects.So a much wider voltage swing (bipolar operation)compared to room temperature is possible now.Thereby, the loss in stroke for low temperature canbe partially compensated for.In the following the main properties of the standard softPZT PSt 150 actuators are compared for roomtemperatureand cryogenic temperatures.Standard capacitance stroke for 0V/ max. operatingPSt 150 (+)150V voltage rangeroomtemp. 100% 100% (–) 30 V / (+) 150 V77 °K 15 % 20% (–) 150 V / (+) 150 V4 °K 5 % 6 % (–) 300 V / (+) 300 VOther PZT materials show qualitatively the same trend.For hard and semi hard PZT materials the loss in strokeduring cool down is not dramatic as for soft PZT materialsas used with the standard PSt 150 stacks.Fig. 16: IR thermography of a piezostack PSt 1000/16/40conventionally mounted via the endfaces in thermalEquilibrium for 1000 V/150 Hertz excitation.The hotspotting in the stack’s center with about 100 °Cis evident as well as the temperature gradient towardsthe cool endpieces (approx. 30 °C).16

3.10. Lifetime, reliabilityLifetime and reliability of actuator stacks do not onlydepend on inherent features like material, electroding,coating etc., but depends strongly on the quality ofmechanical and electrical coupling. These points aredefined by the system design provided by the user.To characterize the most important aspects for reliabilityand lifetime, two extreme driving conditions shall bediscussed.AHigh dynamic cyclingAn example is the mechanical switching of piezostackswith risetimes of 50 to100 µsec over the fullstroke. This is e.g. true for the newest generation ofDieselfuel injectors.Lifetime and reliability are defined as achievablecycle numbers. Up to 10 10 cycles are achievable atthe current state of the art.This is due to the fact, that life time limitation comesfrom the high acceleration forces and mechanicalstress created inside the stack components like theceramic body and its surface electrodes. Crackingcan lead to insulation break down or to rupturing theelectrodes, when improperly designed. Ceramiccracking can be compensated for by rather high preloadforces (up to 50% of the max. force load ) andwell matched driving electronics.Discretely stacked high voltage elements are a kindof composite structure and show better reliabilitiesthan monolithic cofired low voltage elements underpulsed operation.Especially PIEZOMECHANIK’s high voltage actuatorswith their unique electrode design are highly resistantto stress induced failures and electrode rupturing.High power electronics for actuator switching shallprovide the necessary slew rate in charging currentto get the necessary actuator acceleration. But asuperimposed current jitter shall be strictly avoided,because it creates high mechanical stress by excitinghigher mechanical oscillation modes. These cannotbe compensated for sufficiently by external preload.The consequences are stress induced failures ofthe stack arrangement. Usually electrical pulse generatorsfor actuator switching provide a kind charge orcurrent control instead voltage control to improvecurrent’s quality (see chapter 7.).B Static operationOn the first glance, the application of a constant ornearly constant voltage signal seems to be uncritical.But experience shows, that under certain circumstancesstandard actuators are subject to a kind ofelectro-corrosion in on stack’s surface, leading to along-term degradation and final short-circuiting ofthe stack. This effect is accelerated by high environmentalhumidity, temperature and elevated voltageratings.Counter measures are improved surface coating techniquesup to the use of ceramic surface blocking layers(ask PIEZOMECHANIK for “Buried electrode”-design).Further long-term stability can be improved by properdesign and electrical driving techniques.A Use longer stacks to reduce the necessary drivingvoltage level to get a distinct elongation.B No “stand by” mode. Switch of actuators, when notin use.C Make use of the semibipolar driving mode (see section3.2.) to avoid high peak voltages.D Do not touch bare stacks with fingers nor bring theminto direct contact with water and electrolytes.E Use 100% isopropanol for cleaning.17

4. Options4.1. Cryogenic operationLow voltage and high voltage actuators can be modifiedfor operation under cryogenic conditions even belowLiquidHelium temperatures.4.2. UHV compatibilityPIEZOMECHANIK’s standard actuators can be operatedunder high vacuum conditions without restriction. Foruse in ultra high vacuum (UHV) actuators can be adaptedby the use of accessory materials with very low outgassingrates. UHV modified actuators are bakeable tosome extent.4.3. Position detectionMost of PIEZOMECHANIK’s actuators can be providedwith strain gauges in full and half Wheatstone bridgeconfiguration for position detection. Together with thePosiCon-feedback control the actuators can be usedfor linearizing the expansion characteristic of the actuators,and for absolute positioning tasks. Actuator’s hysteresisand creep are eliminated. External influences on theactuator system are compensated for as long as theseare changing the strain state of the piezoactuator.1R4+R14 2+R33R2sensor output + sensor output -supplygroundsupply+10 VFig. 17: Schematic of a Wheatstone bridge configurationof strain gauges for position detectionFig. 18: Variety of piezoactuating components with surfacemounted strain gauges18

4.4. Internal force detectionEspecially under dynamic load operation of actuators,the actual force balance can be of interest.An elegant arrangement for force detection within anactuator can be obtained using the operating dualism ofpiezo ceramics: it can be also used for force detectiontoo. In practice a small fraction of the ceramic stack iselectrically separated from the actuating part. The smallactuator section is separately contacted for charge orvoltage detection purposes, thereby delivering informationabout force variations within the stack.actuatorsensorsectionFig.19: Schematic of an integrated actuator-force sensorconfiguration4.5. Internal heat management,option “ThermoStable”As shown in chapter 3.9. an efficient heat managementwithin actuators is a prerequisit to achieve veryhigh power levels in the dynamic operation of piezoactuators. This is provided by PIEZOMECHANIK’s“ThermoStable” modification of piezoactuators. Themain aspect is the efficient transfer of heat from theceramic stack’s surface to the metal casing, from whichthe heat can be removed by conventional means. Therebyany hotspotting within the ceramic stack is prevented,and no damages due to overheating occur even forvery high power levels.Measurements show only a minimum temperature differenceof about 10 °C from ceramic to casing.The immediate consequences are:a. The thermal state of the actuator can be measuredsimply via the casing temperature, no internal temperaturemeasurements are necessary.b. No cooling media or liquids circulating within theactuator casing are necessary. Bulky arrangementsare avoided.When the “ThermoStable” modification is combinedwith a copper casing, a rather efficient heat sinking viathe bottom piece to the supporting mechanics isachieved. The dimensions of this version are compatibleto the standard dimensions of the actuators. Existingarrangement can simply be upgraded with respect topower applications. Alternative designs use air coolingfins to the casing for forced air cooling etc.19

ExampleAn actuator PSt 1000/25/80 VS 35 (nominal capacity:approx. 2 µF) with standard ceramics was setup in“ThermoStable” casing with air cooling fins togetherwith forced air cooling (see fig.20):• The operation with a switching amplifier RCV 1000/7(1000 V/7 A) leads to a casing temperature of only95 °C for dynamic cycling with 80 µm/1000 V and900 Hz.• The arrangement is well below any critical temperaturelimit (about 140 °C at casing).• Additional means for heat extraction from the stackarrangement within the casing can be provided, sothat a multikiloHertz/full stroke operation is possible.The ThermoStable application is recommended forall actuator systems, where high power LE or RCVamplifiers or equivalent power sources are used.Fig. 20: Different configurations for the heat managementof actuators using the “ThermoStable” technique.Left side: actuator with copper casing for heat sinking.Right side: actuator casing with air cooling fins forforced air cooling.4.6. Corrosion resistancePiezoactuators are sometimes used in mechanicalarrangements for handling chemical agents such asliquid fuels in piezo operated injection valves. Actuatorssometimes come into direct contact with these liquids,which can cause actuator failure due to the degradationof common surface insulation layers on the ceramicused by other suppliers.PIEZOMECHANIK’s stacks are rather insensitive to suchinfluences due to proper surface modification as shownin section 3.10. reliability.PIEZOMECHANIK offers additional modifications of theactuator stacks to improve the stability of the stacksagainst the attack of organic liquids or agents.4.7. Special casingsPIEZOMECHANIK offers the modification of the casing’sparts using special metals such as INVAR, aluminium,titanium etc. instead of the standard stainless steel.20

4.8. Custom designed actuatorsOn request PIEZOMECHANIK is able to apply a widerange of modifications to actuators to optimize them forspecial applications such as watertight configurations,rugged versions for space applications, shaker setupsfor material testing etc.Fig. 21: Large cross section custom designed actuatorfor active vibration cancellation, 56 mm diameter for veryhigh loads of 100 kNewtonsFig. 22: Actuator stacks with rectangular cross section21

5. Comparison of low voltage andhigh voltage actuators5.1. Materials, dimensionsLow voltage, co-fired stacks:The PZT material situation is no longer a concern foruse in low voltage systems as it was in the past. Inprinciple any PZT material can be used for thin-layeredco-fired systems.Low voltage and high voltage elements show the samepiezo-mechanical performance, when comparable PZTmaterial and driving fields are used.Low voltage actuators are mostly used as small andmedium sized elements with cross sections typically upto 10 x 10 mm. Larger cross sections lead to a dramaticincrease of costs. Low voltage actuators become cheapin large quantities.Piezomechanik standard low voltage actuator rangeis aiming for an as broad as possible compromise inperformance and pricing. When you do not find a properconfiguration: contact PIEZOMECHANIK for specialsolutions.When selecting piezo-actuators, please keep in mindthe availability of driving electronics. Bulky, big volumelow voltage actuator require equivalently huge electricalcurrents in dynamic action compared to the high voltageequivalent.Discretely stacked high voltage elementsManufacturing of actuator prototypes of non-standardsize and performance is much easier for high voltageelements than with low voltage elements. All kinds ofPZT materials can easily be implemented.High voltage elements can be set up as big volumestacks for heavy load /large force/large stroke/highpower applications.5.2. Electrical propertiesThe layer structure of a piezo stack and the used PZTmaterial determine the electrical capacitance.Low voltage elements use a larger number of thinneractive layers compared to high voltage elements to getthe same active stack’s length. This results in an equivalentlyhigher electrical capacitance, but the requiredcharging power to drive a stack is the same in bothcases:When two actuators of same size, shape, and materialare built up as one 200 V element and one 1000 V element,the capacitance of the low voltage stack is largerby a factor of 25.When a driving current e.g. of 1 A is needed to get a distinctresponse from the 1000 V actuator, an equivalent200 V actuator will need 5 Amperes. The power = U · Iis the same for both cases.5.3. Temperature rangesLow voltage elements are available as special designedelements for temperatures up to approx.160 °C –180 °C.With high voltage elements, limiting temperatures of220 °C have been achieved.Cryogenic applications are feasible both for low andhigh voltage types.The important point in practice is, that any lowering ofthe max. driving voltage is accompanied by an equivalentincrease in the current levels to get a distinctdynamic response.Big volume actuators in switching application mayrequire currents up to Hundreds of Amperes, whendesigned as low voltage elements.22

5.4. Vacuum compatibilityThe operation both of high voltage and low voltageactuators is possible without problems in normalvacuum. Modifications towards UHV compatibility areoffered.The only problematic gas pressure range is roughvacuum in the Torr range, where glow discharge can beignited by the applied electrical fields.Low voltage elements can be operated here over the fullvoltage range, whereas to high voltage elements only areduced voltage rating can be applied.5.5. Noble gas atmosphereCare must be taken, when stack actuators are operatedin a gas atmosphere containing He or Ar. Dischargeprocesses can be ignited, leading to an insulationbreakdown of the system. When this is accompanied bylarge power or current, local damages of the ceramicor insulation can take place irreversibly.Therefore, actuators can be operated only with areduced voltage.To handle the self-heating problem of actuators operateddynamically in vacuum, attention must be paid to aproper heat management via heatsinking.5.6. ReliabilityIn practice high quality actuators both in low voltageor high voltage configuration are the result of asequence of carefully carried out manufacturing steps,not only including the ceramic structure, but also theperipheral electroding, sealing etc.Therefore no general statement can be given for reliabilityand lifetime. These aspects must be evaluatedindividually for each manufacturer.One example is the electroding of PIEZOMECHANIK’shigh voltage stack actuators.Most manufacturers prepair the side electrodes of astack as rather rigid solder bars. This works fine in staticapplications. But under dynamic operation, the alternatingacceleration forces lead to fatigue and crackingof the side electrodes, what desactivates the stackpartially or completely.To avoid mechanical stress and fatigue in the sideelectrode configuration by rapid piezo action L. Pickelmannintroduced in the 1970ies the electrode wrappingtechnology as shown in fig. 23 – fig. 26.The result are stacks withstanding high frequencycycling within the Kilohertz/full stroke range or repetitivepulsed operation with mechanical powers of about100 kilowatts over approx. 50 µsec.For high dynamic applications, (e.g. like for Diesel fuelinjection) monolithic co-fired stacks require obviouslymuch more care about the mechanical design and preloadingthan discretely stacked high voltage elements,which are less sensitive due to their compound structure.23

Example: surface supply electroding for high voltage stacksComparison of PIEZOMECHANIK’s stressfree feed through contacting with standard solder strip design.Fig. 23 Fig. 25Fig. 24 Fig. 26A. PIEZOMECHANIK actuator contact design withfeedthrough metalfoil showing low massand high flexibility to ensure stress free cyclingof electrode sectionAdvantage:no material fatiguevery high reliabilityfor high dynamic cyclingB. Standard actuator designwith massive tinsolder strips forside contact of stackRisk of material fatigue during dynamic cyclingdue to high inertial mass and rigidness of thecontact strip24

6. Mounting proceduresBare stacksMounting or adaption of piezo-stacks to mechanicalsystems is done using only the endfaces. The attachedmechanics can be preloaded against the stack by usingsome kind of spring or flexmount. For glueing any kindof adhesive showing rather thin and rigid connectionssuch as epoxies, cyano acrylates etc. can be used.Temperature curing is possible within the specified temperaturerange of the actuators.Forces must only be applied in axial direction, especiallyfor high stack length/diameter ratio.Tensile forces must not exceed a few percent of thespecified maximum compressive load.For smaller cross sections, tensile forces should principallybe avoided. Potential tensile forces are best compensatedby a mechanical prestress mechanism. Shearand bending forces are again only applicable in the percentrange of the stated maximum load force and arebest avoided for small cross section stacks.When actuators have piezoceramic endfaces, high forcesmust be introduced homogeneously distributedacross the actuator’s cross section, to avoid high localpressure. When high local pressure is applied, protectpiezo ceramic endfaces by reinforcing them by a steelor corundum plate (see options).Generally all influences which can potentially damagethe surface coating of actuator’s circumfirence must beavoided. Mechanical clamping at stack’s circumferenceis not allowed.Actuators with casingActuators with casing (esp. preloaded ones) are muchmore robust against mechanical impacts and environmentalinfluences than bare stacks. But the best performanceis again achieved when forces are applied axiallyto the stack. Tensile forces are applicable up to theinternal prestress rate. Overstress by static tensile forcesis principally avoided by the internal design,because the moving top then separates simply from thestack. Shear or bending forces are partially compensatedby the mechanics of the casing, but the actuatorsperformance can be reduced (indicated by reducedstroke or nonsteady motion). After eliminating this mismatch,the actuators will work again properly.Ring actuatorsIn principle the same considerations for ring actuators(bare or with casing) as for stacks are valid. The onlyadditional requirement is, to prevent any mechanicaldamage or attack of unwanted substances to thecoated inner side of the ring. This is important for bothbare rings and actuators with casing.TiltingPiezostack- and ringactuators are widely used in coherentoptics for positioning purposes with subwavelengthaccuracy e.g. in interferometers, tunable etalons, resonantors.Most of these applications imply a pure translationalmotion. Users may assume, that piezoactuatorsdo not show tilting of the endfaces during expansion butthis is not necessarely true. Residual tilting of actuatorscan be caused by some inhomogenities in piezoefficiencyof the ceramic or by other internal stress (e.g.from temperature variations). Stress can be furtherimplied by external procedures e.g. by glueing mirrors tothe endfaces or horizontal mounting of mass loadedactuators. It is up to the user of stacks to verify what tiltis acceptable and how his system behaves.Following general qualitative rules can be given:• Tilt increases with applied stroke in most cases• Tilt is reduced for increasing actuator diameters, sosmall sized elements are rather critical• Preloading improves stability against tilting• Totally tilt-free arrangements for long strokes requiresome external guiding mechanism, so the piezo actsonly as a pusher and parallelism is achieved bymechanical guiding.Typical tilt rates for preloaded ring actuatorsHPSt…/15-8/... VS22 are in the range of 2–3 µrad/µmstroke.25

Consequences of linearization for practice:Pure mode excitationCurrently, the main advantage of linearization via thecharge philosophy is the pure mode excitation indynamic applications:A sinusoidal electrical current will be converted into amonochrome sinusoidal oscillation without sidebandsas it is the case for voltage controlled stroke with itsnonlinearities and hysteresis.Schematic of mechanical frequency spectra of a piezo actuator upon application of a single frequencyFig. 28 a:Voltage signalFig. 28 b:Current signalNotice:Current controls “speed”.In case of harmonic excitation, there is a 90° phase shift to the variation of the position.Precision positioning:On the first glance, charge control seems to be “thetrick” for simple open loop position control. But similarto the situation with electrostrictive elements, the “precision”of charge controlled positioning is only in the1% range. Usually piezo actuators are used for muchbetter resolution (10 –4 to 10 –6 or even higher): this canagain be done only via a kind closed loop position feedbackcontrol. Therefore in most cases, closed looppiezo positioning via voltage control is acceptable.Advantages of charge or current control within feedbackloops can be expected in lowering the response time forhigh speed positioning tasks.27

7.3. Enhancement of stiffness bycharge/current controlAs shown in section 3.6. “Stiffnesses” (= inverse compliances),the effective stiffness of a piezo actuatordepends again on its charge balance. Voltage control isnot able to withstand the piezoelectric “generator effect”when the force load onto the actuator is externallyvaried: the generated electrical charges are equilibratedvia the “voltage amplifier” to keep the voltage constantand this results in lower stiffness.Charge control via a “current amplifier” withstands this“generator effect” under varying load because anyunwanted flow of charges is blocked (by compensatingthe piezo generator effect by increasing correspondinglythe output voltage of the amplifier):The stiffness of a charge controlled system is remarkablyhigher than for voltage control.In practice, the “charge philosophy” is used to realizehigh dynamic stiffnesses e.g. in adaptronic systems(e.g. vibration generation/cancellation) via current amplifiers.7.4. ReliabilityThe actuator acceleration is related to the current slewrate dI/dt (see section 7.1.).For high dynamic = high current applications, a currentjitter superimposed to the regular signal introducesunwanted highest acceleration noise to the movingactuator. This can generate higher axial resonancemodes within the stack actuator length, which cannotbe completely be compensated for by high external preload.The consequence is a desintegration of the ceramicstructure and failure of the actuator due to locallyexcited tensile stress within the stack.So the charging current profile must be optimized onone hand to get the wanted high acceleration, but mustbe kept smooth to avoid unnecessary higher currentvariation.Discretely stacked high voltage actuators are morestable against current jitter than monolithic cofired elements.7.5. Electronic supplies for piezo actionBasics:Any type of amplifier used for piezo control must providevoltage and current to transfer energy into a piezoactuated system.Voltage amplifiers:The input signal is magnified by the amplifier into a proportionaloutput voltage signal.Notice:The voltage applied to a piezo actuator determinesactuator’s position!The variation of the induced current depends on thecapacitance of the actuator, the slew rate and theregulation strategy of the amplifier.When selecting a voltage amplifier following points areimportant:a) proper voltage range, not exceeding the actuatorvoltage limitsb) sufficient high current to get sufficient large bandwidthc) sufficient voltage stability and low noise to ensure adistinct position stability.Voltage amplifiers were mostly used for piezo actuationin the past.Current amplifiers:Current amplifiers convert an input signal into a proportionaljitterfree current output.Notice:Current amplifiers control actuator’s “speed v”.Position is varied with a distinct delay orphase shift.In case of a harmonic oscillation, position varieswith a 90° phase shift in relation to the signal inputto the amplifier.The variation of the induced output voltage dependson the load, frequency and the regulation strategy.Usually the amplifiers operate around a stable centervoltage equivalently a distinct midposition of the piezoactuator.Current amplifiers make sense for dynamic piezo applicationsas described in sections 7.1.–7.4. Therefore,current amplifiers control current above a distinct thresholdfrequency (approx. 5–10 Hz).When selecting a current amplifier following points areimportanta) sufficient high current to get sufficient large frequencybandwidth28

) proper voltage range, not exceeding the actuatorvoltage limitsA wider voltage range will definitely destroy theactuator, because the voltage variation is not externallycontrollable, but is a consequence from thecurrent regulation.U-I-Hybrid amplifiersTo get a widest application spectrum it is reasonable tocombine a voltage amplifier stage and a current amplifierstage into one so-called “U-I-hybrid amplifier”.With the voltage amplifier stage, the position of a piezoactuator can be controlled in a static or low frequencymode. For dynamic motion above a distinct frequencythreshold, the current amplifier stage is used to provideall the advantages of current control.E.g. by this strategy an harmonic sine oscillation can begenerated around a voltage defined center position.Charge amplifiersThe charge content Q of a piezo actuator is related to itsposition (similar to voltage U, but with much betterlinearity and no hysteresis).Charge control of piezo actuators is used mainly for veryhigh dynamic applications in a kind of current control fora well-defined short pulse time transferring so a distinctcharge quantity to the piezo. Such a mode is used forfast proportional Diesel-Fuel injection to provide lowstress activation of a piezostack by the current modeand to reach a well defined final position.PIEZOMECHANIK’s amplifier philosophyPIEZOMECHANIK is offering a wide range of voltageand current controlling amplifiers as well as hybridsystems to get the best match for your application.For low power/high precision positioning, the SVRamplifiers are ongoingly the best choice. Voltage amplifierscontrol primarely actuator’s position, whereas currentamplifiers control primarely actuator’s speed.For higher power application envolving LE amplifiers,voltage-, current- and hybrid-amplifiers configurationsfor low voltage and high voltage actuators are offeredon request.Please notice, that for current amplifiers, piezoactuators with casing must be modified.Please contact PIEZOMECHANIK in any case aboutthe used actuators, when you want to apply currentor charge control.Discuss with us in detail your needs in actuatorapplication to ensure the best-matched system.29

Piezomechanik · Dr. Lutz Pickelmann GmbHBerg-am-Laim-Str. 64 · D-81673 Munich · Phone ++ 49/ 89 / 4 31 55 83 ·Fax ++ 49/89/4 31 64 12e-mail: info@piezomechanik.com · http://www.piezomechanik.comStand: September 2003