Upload
dinhanh
View
214
Download
0
Embed Size (px)
Citation preview
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 1/9
Piezo Actuators (Motors)Single Layer Actuators (Sheets)2Layer Actuators (Benders & Extenders)MultiLayer Actuators (Stacks)Actuator Performance
Piezo Sensors (Generators)Single Layer Generators (Sheets)2Layer Generators (Benders & Extenders)MultiLayer Generators (Stacks)Generator PerformanceStatic & Dynamic Operation
Polarization & WiringSeries & Parallel OperationX & Y Poling Configurations
Actuator EquationsGenerator EquationsThermal Dependence of Piezo Properties
Transducers convert one form of energy to another. Piezo actuators convert electricalenergy to mechanical energy. This is why they are referred to as "motors" (often linearmotors). Piezo sensors convert mechanical energy into electrical energy. This is whythey are referred to as "generators". In most cases, the same element can be used toperform either task.
Single sheets: can be energized to produce motion in the thickness, length, and widthdirections. They may be stretched or compressed to generate electrical output.
Thin 2layer elements are the most versatile configuration of all. They may be usedlike single sheets (made up of 2 layers), they can be used to bend, or they can beused to extend. "Benders" achieve large deflections relative to other piezotransducers. "Extenders", being much stiffer, produce smaller deflections but higherforces.
Multilayered piezo stacks can deliver and support high force loads with minimalcompliance, but they deliver small motions.
SINGLELAYER MOTORS(Sheets & Plates)When an electric field having thesame polarity and orientation asthe original polarization field isplaced across the thickness of asingle sheet of piezoceramic, thepiece expands in the thickness or"longitudinal" direction (i.e. alongthe axis of polarization) as shownin Figure1. At the same time, thesheet contracts in the "transverse"direction (i.e. perpendicular to theaxis of polarization) as shown inFigure2. When the field isreversed, the motions are reversed.
Sheets and plates utilize this effect.The motion of a sheet in thethickness direction is extremelysmall (on the order of tens ofnanometers). On the other hand,since the length dimension is oftensubstantially greater than thethickness dimension, thetransverse motion is generallylarger (on the order of microns totens of microns) . The transversemotion of a sheet laminated to thesurface of a structure can induce itto stretch or bend, a feature oftenexploited in structural controlsystems.
Figure1: Single Layer Longitudinal (d33) MotorGetting Thicker
Figure2: Single Layer Transverse (d31) MotorWith Sides Contracting
2LAYER MOTORS(Benders & Extenders)2 layer elements can be made toelongate, bend, or twist dependingon the polarization and wiringconfiguration of the layers. A centershim laminated between the two
PIEZO SYSTEMS, INC. 65 Tower Office Park Woburn, MA 01801 USATel: 7819334850 Fax: 7819334743 email: [email protected]
Find Search for a product or category
HOME PRODUCTS CUSTOM OEM CATALOG TECHNICAL / FAQ's ORDERS CONTACT INFO VIEW BASKET
INTRODUCTION TO PIEZO TRANSDUCERS
Print this Page: from © Piezo Systems, CATALOG #8 (2011), pages 22, 23, 24, 25, 62,63 & 64.
PIEZO ACTUATORS (MOTORS) Piezo motors convert voltage and charge to force and motion.
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 2/9
piezo layers adds mechanicalstrength and stiffness, but reducesmotion.
"2layer" refers to the number ofpiezo layers. A "2layer" elementactually has nine layers, consistingof: four electrode layers, twopiezoceramic layers, two adhesivelayers, and a center shim. The twolayers offer the opportunity toreduce drive voltage by half whenconfigured for parallel operation.
Extension Motors:A 2layer element behaves like asingle layer when both layersexpand (or contract) together. If anelectric field is applied whichmakes the element thinner,extension along the length andwidth results. Typically, only motionalong one axis is utilized, asdemonstrated in Figure3. Extendermotion on the order of microns totens of microns, and force from tensto hundreds of Newtons is typical.
Bending Motors:A 2layer element producescurvature when one layer expandswhile the other layer contracts.These transducers are oftenreferred to as benders, bimorphs,or flexural elements. Bender motionon the order of hundreds tothousands of microns, and benderforce from tens to hundreds ofgrams, is typical. Figures4, 5 and 6show several common bendingconfigurations. The variety ofmounting and motion options makebenders a popular choice of designengineers.
Figure3: 2Layer Extension (d31) MotorWith sides Extending
For extension motors of the same thickness:Free Deflection (Xf) LBlocked Force (Fb) WResonant Frequency (Fr) I / LCapacitance (C) L x W
Figure4: 2Layer Bending MotorMounted as a Cantilever
For standard cantilevered benders of the same thickness:Free Deflection (Xf) L2Blocked Force (Fb) W / LResonant Frequency (Fr) I / L2Capacitance (C) L x WCharacteristics: End takes on an angle. Easy to mount.
Figure5: 2Layer "S" Bending MotorMounted as a Cantilever
To convert standard cantilever performance to "S" bender performance:Free Deflection (Xf) = 1 / 2 x cantilever motionBlocked Force (Fb) = 1 / 2 x cantilever forceResonant Frequency (Fr) = same as cantilever frequencyCapacitance (C) = same as cantilever capacitanceCharacteristics: end moves up and down in a parallel plane
Figure6: 2Layer Bending MotorMounted as a Simple Beam
To convert cantilever performance to simple beam performance:Free Deflection (Xf) = 1 / 4 X cantilever motionBlocked Force (Fb) = 4 X cantilever forceResonant Frequency (Fr) = 3 X cantilever frequencyCapacitance (C) = same as cantilever capacitance
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 3/9
Characteristics: center moves up and down in a parallel plane.
MULTILAYER MOTORS(Stacks)Any number of piezo layers may bestacked on top of one another.Increasing the volume ofpiezoceramic increases the energythat may be delivered to a load. Asthe number of layers grows, sodoes the difficulty of accessing andwiring all the layers.
Stack Motors:The cofired stack shown in Figure7 is a practical way to assembleand wire a large number of piezolayers into one monolithic structure.The tiny motions of each layercontribute to the overalldisplacement. Stack motion on theorder of microns to tens of microns,and force from hundreds tothousands of Newtons is typical.
Figure7: Cofired MultiLayer Stack Motor
MOTOR PERFORMACEPiezoelectric actuators are usuallyspecified in terms of their freedeflection and blocked force. Freedeflection (Xf) refers todisplacement attained at themaximum recommended voltagelevel when the actuator iscompletely free to move and is notasked to exert any force. Blockedforce (Fb) refers to the force exertedat the maximum recommendedvoltage level when the actuator istotally blocked and not allowed tomove. Deflection is at a maximumwhen the force is zero, and force isat a maximum when the deflectionis zero. All other values ofsimultaneous displacement andforce are determined by a linedrawn between these two points ona force versus deflection line, asshown in Figure8. Generally, apiezo motor must move a specifiedamount and exert a specified force,which determines its operatingpoint on the force vs. deflectionline. An actuator is consideredoptimized for a particularapplication if it delivers the requiredforce at one half its free deflection.All other actuators satisfying thedesign criteria will be larger,heavier, and consume more power.
Figure8: Piezo Motor Performance(Force versus Deflection Diagram)
SINGLELAYER GENERATORS(Sheets & Plates)When a mechanical stress isapplied to a single sheet ofpiezoceramic in the longitudinaldirection (parallel to polarization), avoltage is generated which tries toreturn the piece to its originalthickness. Similarly, when a stressis applied to a sheet in a transversedirection (perpendicular topolarization), a voltage is
PIEZO SENSORS (GENERATORS) Piezo sensors convert motion and force to charge and voltage.
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 4/9
generated which tries to return thepiece to its original length andwidth. A sheet bonded to astructural member which isstretched or flexed will induceelectrical generation. Figure9 andFigure10 show longitudinal andtransverse generators respectively.
Figure9: Longitudinal (d33) GeneratorBeing Compressed from the Top and Bottom
Figure10: Transverse (d31) GeneratorBeing Compressed from the Sides
2LAYER GENERATORS(Benders & Extenders)Applying a mechanical stress to alaminated two layer element resultsin electrical generation dependingon the direction of the force, thedirection of polarization, and thewiring of the individual layers.
Extension Generators:When a mechanical stress causesboth layers of a suitably polarized2layer element to stretch (orcompress), a voltage is generatedwhich tries to return the piece to itsoriginal dimensions. Essentially,the element acts like a single sheetof piezo. The metal shimsandwiched between the two piezolayers provides mechanicalstrength and stiffness whileshunting a small portion of theforce.
Bending Generators:When a mechanical force causes asuitably polarized 2layer elementto bend, one layer is compressedand the other is stretched. Chargedevelops across each layer in aneffort to counteract the imposedstrains. This charge may becollected as observed here.
Figure11: Transverse GeneratorCompressed Lengthwise
For extension generators of the same thickness and force loading:Deflection Limit (Xl) LOpen Circuit Voltage (Voc) (Xl) / L = IClosed Circuit Current (Icc) L x W
Figure12: Bending GeneratorCantilever Mount
For Bending Generators of the same thickness and force loading:Deflection Limit (xl) L2
Voc, Open Circuit Voltage (xl) / L2 = IClosed Circuit Current (Icc) L x W
Figure13: Bending GeneratorSimple Beam Mount
To convert cantilever to simple beam generator performance(for the same thickness and force load):Voc = 1/4X cantilever voltageIcc = 1/4X cantilever currentTo convert cantilever to simple beam performance(for the same thickness and deflection):Voc = 4X cantilever voltageIcc = 4X cantilever current
MULTILAYER GENERATORS
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 5/9
(Stacks)Applying a mechanical stress to alaminated two layer element resultsin electrical generation dependingon the direction of the force, thedirection of polarization, and thewiring of the individual layers.
Stack Generators:The stack,shown in Figure14,comprises a large number of piezolayers, and is a very stiff structurewith a high capacitance. It issuitable for handling high force andcollecting a large quantity of chargeefficiently. Figure14: MultiLayer Stack Generator
GENERATOR PERFORMANCEPiezoelectric generators areusually specified in terms of theirclosedcircuit current (or charge)and opencircuit voltage. Closedcircuit current, ICC, refers to the totalcurrent developed, at the maximumrecommended strain level andoperating frequency, when thecharge is completely free to travelfrom one electrode to the other, andnot asked to build up voltage.Opencircuit voltage, Voc, refers tothe voltage developed at themaximum recommended strainlevel, when charge is prohibitedfrom traveling from one electrode tothe other. Current is at a maximumwhen the voltage is zero, andvoltage is at a maximum when thecharge transfer is zero. All othervalues of simultaneous current andvoltage levels are determined by aline drawn between these points ona voltage versus current line, asshown in Figure15.Generally, a piezo generator mustdeliver a specified current andvoltage, which determines itsoperating point on the voltage vs.current line. Maximum powerextraction for a particularapplication occurs when thegenerator delivers the requiredvoltage at one half its closed circuitcurrent. All other generatorssatisfying the design criteria will belarger, heavier, and require morepower input.
Figure15: Piezo Generator Performance(Voltage versus Current Diagram)
DYNAMIC VERSUS STATIC SENSOR OPERATIONPiezo elements are excellent for dynamic or transient motion and force sensing. They are used as strain gages for easy and rapid determinationof dynamic strains in structures due to their extremely high signal/noise ratios (on the order of 50 times that of wire strain gages). They require nopower input since they generate their own power. In fact, this is why they are now considered useful as energy harvesting and scavengingdevices. They are small enough that they will not materially affect the vibrational characteristics of most structures.On the other hand, piezo elements are generally poor at measuring static or slowly changing inputs due to charge leakage across theirelectrodes or through monitoring circuits.
Making a 2layer piezo element either bend or extend isdetermined by how it is polarized and wired.
SERIES AND PARALLEL OPERATION
Series Operation: Series operation refers to the casewhere supply voltage is applied across all piezo layers atonce. The voltage on any individual layer is the supply
Figure17: 2Layer Bending Element Poled for Series Operation (2wire)
POLING AND WIRING Poling and wiring determine how the same transducer can be used many ways.
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 6/9
voltage divided by the total number of layers. A 2layerdevice wired for series operation uses only two wires(one attached to each outside electrode), as shown inFigure17.
Parallel Operation: Parallel operation refers to the casewhere the supply voltage is applied to each layerindividually. This means accessing and attaching wires toeach layer. A 2layer bending element wired for paralleloperation requires three wires (one attached to eachoutside electrode and one attached to the center shim),as shown in Figure18. For the same motion, a 2layerelement poled for parallel operation needs only half thevoltage required for series operation.
Figure18: 2Layer Bending Element Poled for Parallel Operation (3wire)
"X" AND "Y" POLING CONFIGURATIONS
XPoled: refers to the case where the polarization vectorsfor each of the 2 layers point in opposite directions,generally, towards each other.
YPoled: refers to the case where the polarization vectorsfor each of the 2 layers point in the same direction.
Figure19: XPoled Element
Figure20: YPoled Element
SIMPLE LINEAR EQUATIONS FOR PIEZO ACTUATORS (MOTORS)
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 7/9
SIMPLE LINEAR EQUATIONS FOR PIEZO SENSORS (GENERATORS)
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 8/9
TYPICAL THERMAL DEPENDENCE OF PIEZOELECTRIC PROPERTIES
2/21/2014 Piezo Systems: Introduction to Piezoelectric Transducers
http://www.piezo.com/tech2intropiezotrans.html 9/9
piezoelectric, piezoelectricity, piezoceramic, piezo, pzt, piezoelectric materials, piezoelectric transducers, piezoelectric effect, piezoelectric education, piezoelectric tutorial
© Piezo Systems, Inc. . Contact . Site Map