PiezoelectricFoilSensorsandActuators

Category:Sensors&MeasuringTechniquesReference:TDO0213BrokerCompanyName:cesahGmbHBrokerName:FrankZimmermannTelephone:+49(0)6151392156-12Email:zimmermann@cesah.com

AbstractPiezoelectric films made of PVDF(Polyvinylidene fluoride) are very thin andflexibleactiveelectromechanicaltransducers.They can convert about 12%ofmechanical energy into electrical energy, and viceversa.Thustheyareusefulasasensoraswellasanactuator.

DescriptionThepiezofoiltechniqueallowsthedesignofextremelyflexibleandthinsensorsaswell as actuators, which may be placed on 3D-contoured surfaces or inserted insmallgapsaswellasembeddedinGRP/CRPstructures.

Due to the piezo-active measurement principle only very small contact wires arerequired, allowing the integration of a high number of sensors with a spatialresolution down to sub-mm scales. Due to anisotropic properties of mono-axialoriented foils the charge produced by different strain components is amplified ordampened, resulting in a multitude of specific sensor applications. Several multi-layer and multi-element actuators can be realized, producing longitudinal ortorsionalmotionortravelingwavesinbeamelements.

FurtherInformation

Thepiezofilmisatransparentfilmofhighlypolarpoly-vinylidene-fluoride(PVDF).Inproportion to the influence of mechanical stress or strain the film develops an

electrical charge that could be shuntedover the metal-coated surfaces.Otherwise the application of electriccurrent leads to a proportionalmechanicdeformation.The direct piezoelectric effect is used insensor applications and the inversepiezoelectric effect in actuatorapplications.

Fig.1:Numericalclassificationofaxes(1-length,2-width,3-thickness)

Inelectronictermsthepiezofilmisacapacitorthatreactswithachangeinelectriccharge proportional to the change of applied force. Piezoelectric materials areanisotropic.Thismeans,thattheirelectricalandmechanicalpropertiesvarywiththedirectionofappliedforce,strainorelectricalfield.Thefilm´saxesareidentifiedbythenumeralsindicatedinfig.1.ThesymbolslistedinTechnicalData forstrain-andstressconstantdiiandgiiaredeclaredas follows.Thefirstindexidentifiesthedirectionofappliedelectricalfield(alwaysthedirectionofthethicknessofthefilmwhenusingpiezoelectricfilms).Thesecondindexreferstotheaxisofinducedmechanicalstrainorappliedstress.UsagePiezofilm sensors are well qualified for detecting dynamic events, like pressurefluctuations,vibrationsor forcechanges.Especiallyunderspace-limitedconditions,theextremethinsensormaterialisadvantageous.Becauseofthehighdynamic range, the greatbandwidthandthehighflexibility,manymeasurementproblemscanbe solved, which is impossible todowithothertechniques.SensorandactuatordesignThepiezofilmcouldbedesignedasa single- or multi sensor array innearly any form and dimension.Single point sensors with aminimum diameter of 0.25 mmcould be realized. Multi layersensorswithdifferentorientationscould be used as filters forseparatingspecificforcedirectionsorforscreeningthesensorsignals.Actingasanactuator, really complex bending-torsion motions could be realized, i.e. foradjustablemirrors.Thepiezofilmmetalcoatusuallyconsistsofaluminiumornickel.Othermetallizationsuchas copper, goldor silver couldbedeliveredon special request. Thepiezofilmsensor signals are enhanced by special adapted amplifiers for specific measuringtasksandcanbestoredandcomputedwithcommondatarecordingandprocessingunits.ApplicationThesensorfoilsaresuitableforapplicationsintheareasofdynamicforce,pressure,motion and temperature measurements up to high frequencies. Some realizedexamples are: fluid, IR, vibration and pulse pressure sensors, keyboards, robotics,tactile and ultrasonic sensors. Piezoelectric actuators are suitable for largedisplacements with small forces as for example the damping or amplification of

oscillationsof lightweightstructures,opticalcomponentsandminiaturepositioningdevicesorfans.

DescriptionofSpaceHeritagePiezofilmsensorswithamplifiersareusedinspaceapplicationsforthedetectionofstructurevibrationsonanaerodynamictestbody,foractivevibrationdampingandstimulating.Furtherspaceapplicationsare:

- Development and construction of piezofilm sensor arrays with amplifiersystemsforflowsensing inairplanewingresearch,pressurefluctuationsonwingparts.

- Aerodynamicmeasurementsinwindtunnele.g.simulationofre-entry

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