Institute of Aeronautics and applied Mechanics Final Project Mechanics and Machine Design Modelling and Computer Simulations in Mechanics Modelling and Simulation of resonance flapping wing resonance propulsion Aurelio García Lloris Krzysztof Sibilski Warsaw, 2017
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Institute of Aeronautics and applied Mechanics
Final Project
Mechanics and Machine Design Modelling and Computer Simulations in Mechanics
Modelling and Simulation of resonance flapping wing resonance propulsion
Aurelio García Lloris
Krzysztof Sibilski
Warsaw, 2017
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ABSTRACT
This Project describes an analysis and design of aMAVMicro AirVehicle.Theaimoftheprojectistofindsomestructurethatcanallowtheresonanceoftheflapping-wingmovement.
Thestudy is focus inthestructureoftheconceptswheredifferentspringmaterialsarestudiedtoknowwhichonefitsbetterwiththeflexiblestructure desired. The concepts studied are based in the insect thorax-wingssystemwhichisatunedresonantsystem.
Theprojectisdividedintofiveparts.Thefirstonepresentsbrieflytheobjective of the project. Then, insect flight is described. The next partdescribes flapping wing MAVs where our two concepts are presented.Sectionbelowcontainssimulationsandcalculations.Lastpart,comprisesconclusionsbasedonexaminedinstances.
Throughout history of evolution, the nature has optimized itsresources in order to maximize the performance of existing biologicalstructuresonourplanet.This facthasmotivated thesearchof conceptsbasedinanimals.Forexample,LeonardoDaVincidesignedaflyingmachinefromthestudyonthebirdanatomy,theOrnithopter.
Last years, there have been great advances in different areas ofscience related to the design of artificial devices from the study andobservation of organisms, systems and processes existing in nature. Inmanycases,acompletebiological imitationofaninsect,bird,orfishhasbeen attempted, resulting in disappointing results. It is important todistinguishbetweenwhatismeantbybiologicalimitation,biomimetic,andinspirationinbiology.Thebiomimeticorcopyofanorganismisverydifficult
Figure1:LeonardoDaVinci’sOrnithopter.
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to achieve with current technology, on the other hand, inspiration inbiology,isagoodstartingpointtotakeafirststeptowardsthedesignandmanufactureofrobotsthathaveaperformanceequalorgreaterthanthebiologicalentitiesexistinginnature.
Nowadays, scientific community is focus in the development ofMAVs,MicroAirVehicle,inspiredintheflightoftheinsectsandlittlebirds,likethehummingbird,thatareabletoperformcomplexmovements.
1.2 MICROAIRVEHICLE
The United States Defence Advanced Research Projects Agency,DARPA,hasdefinedtheMAVasanyflyingvehiclewhichislimitedto150mmorsmallerinanylineardimension.
The fixed-wing is themost efficient kind ofMAV; it is able to runthroughlongerdistancesbutithaslessmobilitythantherest.Therotary-wingMAV,standsoutbecauseitseaseofmovements.Theflappingwingflyers, are inspiredby thenature. Theadvantagesof this typeof insect-inspiredvehiclesaregreaterversatility toperformcomplexmanoeuvres,greaterflightstabilityandlowerenergyconsumption.Itmeanslongerflighttimes.Amongthemanoeuvres, it isabletoperformlow-speedflights,aswellastake-offsandlandingsovershortdistances.Also,itcanproducethesameliftwithsmallerwingbytheincreasingofflappingfrequency.
Inthisproject,wearedesigningaflappingwingMAV.WetrytofulfiltherequirementsfromtheAtalantaProject.OurMAVshouldbeabletoflyautonomously,hoverandflyslowly,communicatewithothersandabasestation, manage power, provide payload capacity and be adaptable formanysituations.
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1.3 BACKGROUND
Theimplementationofflappingflightatthesmallerscales ismuchmore difficult than fixed-wing and rotary-wing implementations.Nevertheless,itisinterestingtostriveforinsectflapping-wingMAVsduetosomereasons.
Thefirstreason,itistheflyingspeed.Duetoflappingwingsystem,westayairborneinlowvelocitiesproducingliftwiththewingsmovement.Secondly,insectsandbirdscombinecontrolandpropulsionintheirflappingwing systems while rotary-wings and fixed-wingsMAVs need additionalactuatorsforcontrol,additionalrotorsinrotary-wingsandcontrolsurfacesinfixed-wings.
Becauseoftheirabilitytohover,wecanlookbirds,batsandinsectstogetsomeideas.ButthesizerangeofflappingwingMAVsmakesuspayclose attention in insects. Firstly, we notice the system that drives thewings. In insects the whole thorax structure functions as a spring andcombinedwith thewings essentially forms a tunedmass-spring-dampersystem. The wings in insects are predominantly passive structures. Theabsence of active elements in thewingsmake insectsmore suitable forinspirationofflapping-wingMAVs.
Along history, attempts have been made to imitate the insectsflapping-wing flight. In spite of the advances in this kind of MAVs lastdecade, the biological entities stay ahead of manmade prototypes. Themostcommonproblemsthatwecanfindaresizeproblemsandautonomyoftheflapping-wingMAV.
When you are designing and constructing a MAV, an importantaspect is to understand how insects and small birds flap their wings toproduceenoughliftandthrusttoflyandtodrivearoundtheair.
There are a lot of experimental works about the study of insectaerodynamics,whichprovideagooddescriptionofthenon-conventionalaerodynamicmechanismsusedbythesecreaturestostayinflight.Ontheother hand, a precise quantitative description of such phenomena isdifficult to achieve due to the errors in the techniques used to performmeasurementsofthemagnitudesofinterest.
The insect consists of three body parts, the head, thorax andabdomen. The head carries the brain and perceive vision, olfactory andhearing. The abdomen contains the reproductive organs, breathingapparatusandmostofthedigestivesystem.Aswementionedbefore,thethorax is one of themost interesting aspects in the flight of the insectsbecause it produces the power and drives the wings. It has the wingactuationmechanism.
From an engineering viewpoint the wing-thorax structure is acompliant amplification mechanism. Two different thorax designs existwhichdifferinthewaythewingsaredriven:directandindirect.
Thewingsare,as seen fromanengineeringviewpoint,a structurewhich consistsof loadbearingmembers, longitudinal veins, crossbeams,cross-veins, and spanning between the veins and cross-veins is a sheet,membrane.
Beingpassivestructures,thewingsaredrivenatthebasewheretheyconnect to the thorax through the wing joint. The wings constitute themajorpartofeffectiveinertiaofthemovingthorax-wingsystem.
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Theresonantpropertieshelptoreducetheinertialcostofthewingmovement.Aswementioned,theinsect-thoraxwingsystemcanthenbeseenasatunedmass-spring-dampersystem.Themassisdominatedbytheinertia of thewings. The spring function is a combination of the elasticthorax shell and themuscleproperties. Theaerodynamic loadingon thewingscanbeseenasthedampingforce.
2.2 WINGMOVEMENT
The insectswingmovementcanbeseparated in twoparts:a rigidbodymotionandanelasticdeformation.Therigidbodymotionconsistsofthreerotationsfacilitatedbythewing-rootjoint.Theelasticdeformationconsistsmostlyoftorsionaldeformationandbending.Themagnitudeoftheelasticdeformationissmallcomparedtotherigidbodymovement.
Thethreerotationsthatcompoundtherigidbodymotionsarethemain sweeping angle, the wing pitching motion and the out of planemotion.
Thesecondmotion,thewingrotationorwingpitching,isdefinedasthe rotationof thewingabout its radialaxis.Thismotion, togetherwithdirectioninducedflow,definestheangleofattackofthewing.Thewingpitching is generally a motion close a pure sinus. The frequency of thepitchingmotion is always the same as themain flappingmotionwhit aphaseshiftbetweenboth.Theoriginofwingpitchinghasbeenthesubjectofargumentamongbiologists.Analysisoftheinsectthoraxsuggestthatthecontrolmuscles are too small to generate the required power for wingpitching.However,pitchingcouldbetheresultofasmartwing-roothinge
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configuration.Thesecondoptionispassivepitchingcontrolledbyinertialand aerodynamics loads. The general consensus among biologists andengineers is that during steady-state flying or hovering wing pitching ispassive. However, when the insects perform extreme manoeuvresindicationsarethatthereconfigurationofthewingjointissuchthatwingpitchingisatleastpartiallyactivelycontrolled.
Atypicalwingstrokecyclevanberoughlydividedinfourphases.Thefirst are the up and downstroke. In these phases the wing moves withrespect to the body with approximately constant wing rotation. Theremainingtwophasesarethetwostrokereversals,calledsupinationafterthedownstrokeandpronationaftertheupstroke.
While bending is present in insect-wings, the aspect of torsion ismore important. Torsionandmore specifically the timingof torsionanddeformation during thewing stroke can directly influence the timing oftorsiondeformationduringthewingstrokecandirectlyinfluencethelocalwingpitchingangleand,consequently,thelocalangleofattack.
FlappingflightbyinsectsischaracterizedbyunsteadyincompressibleflowatintermediateReynoldsnumber.ForlargeinsectstheReynoldsliesbetween 5000 and 10000. The smallest flying insects have Reynoldsnumberswhich cangoas lowas10. The range is largedue to thewidevariationinthebodymassoftheflyinginsects.
Theproductionofliftandthrustfollowsfromthecomplexinteractionbetween the compliant wings, wing kinematics and the aerodynamics.Aerodynamictheoryiswelldevelopedforlargerscalesandcanbeappliedatinsectscalewiththeincorporationofeffectsthatarepresentatthesesmaller scales and, consequently, lower velocities. The flapping winginducesunsteadyflowwithacyclicnature.
The production of lift and drag in insects is based on threemechanisms, the leading edge vortex, the rotational circulation and thewakecapture.
Two other effects are present in insects but are not directlyresponsibleforliftproduction.Thefirstistheeffectofaddedmassorvirtualmass.Thevirtualmasscanbeseenasaregionofinfluencewheretheairaround the wing moves with the wing and contributes to the effectiveinertiaofthewing.Theothereffectisknownasclapandflight.Theeffectconsistsofexpandingthewingsweepinganglesofarthewingstouchattheendoftheupstrokeabovethebodyoftheinsect.WhenthedownstrokestartsthewingsarerolledopeninordertojumpstarttheLEV.Sincethe
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formationoftheLEVis independentofwingsize, jumpstartingmightbebeneficial for very small insects which have a very small wing stroke inabsolutesense.
2.4 KEYPOINTS
As we mentioned, direct copy of insects is not possible withnowadays science. However, to produce a great flapping-wingMAVweneedtolookindetailtheinsectsbecausetheyaresuchsuccessfulflyers.
Thewingsweepingfrequencybecomesboundedbyhighsensitivetochangesindrivefrequency.Thismechanicalequivalentcouldbeanelasticelementtostorethekineticenergy.Besides that,weneedamechanicalequivalent for the thorax structure in insects that participates in theresonant thorax-wing system, it should strive for a very high degree ofintegration.
Wing pitching also finds its origin in the wing root. MAVs thoraxwouldbeamendabletoaccomplishthesamefeatures.
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Thewingistheplacewhereliftisproduced.Itwillposeasignificantchallengetodesignamechanicalequivalentiftheinsectwing.Asweseebefore, thewingpitchingmaybeofpassiveorigin. In the idealcase, thetiming and amplitude of wing pitching will have to be obtained by acombinationofahighlycompliantwingrootwithawingwhichisabletosupportabeneficialtorsionalwavetoaugmentliftproduction.Thesystemhastobedesignedinahighlyintegratedfashion.
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3 Flapping Wing MAVs
3.1 BACKGROUND
The design of flapping wing MAVs is still a very active area ofresearch. While the model plane world has known small rubber bandpoweredornithopterssincethe1870s,thefirstelectricpoweredflappingwingMAV,namedtheMicroBat,onlyflewin1998.Thedesignofflappingwing MAVs until now mainly progressed by means of trial-and-error.Automatic optimization is still very unreliable due to a lack of accuratetheoreticalmodels.Especiallythedesigndecisionsconcerningtheshape,tension,andmaterialsofthewingscannotbemadepurelyonthebasisofsimulationduetoalackinknowledgeontheaerodynamicsaroundflexibleairfoil.
Historically,flapping-wingMAVshavebeenbasedonbird-likeflightfocussingonforwardflight.Nowadays,thesedesignsareinspiredbysmallbirds and insects because hovering and slow moving is interesting forpossibleindoorusage.
An aircraft design is highly dependent on the intended use of theplatform.Thedrivingforceinthedesigncanbetooptimizeformaximumenduranceononehand,oronthecontrarytooptimizeforminimumsize.Typically,thegoalalsoincludesotheraspectssuchasstabilityorpayloadcapability. Different combinations of goals can lead to very differentdesigns. Common to almost all of these goals is that they are harder toattainatsmallerscales.Therearecoarselytwoapproachestofinallyarriveatfullyfunctioningfly-sizedflappingwingMAVs:bottom-upandtop-down.
The top-downapproach startswith relatively larger scalebut fullyfunctioning flapping wing MAVs. The idea behind the approach is thatstudyingsuchMAVscanleadtoinsightsfortheconstructionofafollowing,smallerorsmarterversion.Oneadvantageofthisapproachisthatitallowsinterplaybetweentheoryandpractice.Foraerodynamicsresearch,havinga flyingsystemensuresthat theresearch isdirectedtoaspects thatalsohave a practical relevance. For artificial intelligence research, having aphysicalandfullyfunctioningMAVisofgreatvalue:real-worldtestsforcetheexperimenterstotakeintoaccountallaspectsoftheroboticsystem.Inaddition,theyrevealphysicalpropertiesofthesystemthatcanbeexploitedbythealgorithms.
The bottom-up and top-down approach have complementaryadvantagesandrisks.Forexample,ariskofthetop-downapproachisthattheresearchwillfocustoomuchonincrementalmodificationsoftheMAV,ignoringpossibledisruptiveimprovements.Ontheotherhand,ariskofthebottom-up approach is that research may focus too much on detailedaspectsthatmightturnoutirrelevantforafullyflyingsystem.Thebottom-upapproachcanleadtofundamentalnewunderstandingandtechniques,whilethepractical ‘surprises’of thetop-downapproachgive insight intopressing problems of lacking scientific knowledge or technology. Webelievethatprogressinflappingwingresearchrequiresbothapproachestoexist.
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3.2 WINGS
Variousmethodsexisttomanufacturewingsforflapping-wingMAVs.Themost commonmethodof constructing is themethodof spanning amembrane by stiff rod type structures. Other methods for constructingwings usually rely on using a membrane and stiffening structure whichrepresentsaveinlikestructure.
Passive rotation characteristics have a strong dependence on thedetailedshapeandmassdistributionofawing.Naturalandartificialwingsdisplay great variation in size, platform, material composition, and veinstructure.Developmentofaconcisewingparameterizationisrequired.
Among the existing and functional MAVs, three types of wingconfigurations,monoplanes,biplanesandfoldingwingsaredistinguished.The monoplanes consist of a pair of wings that oscillate in phase. Theadvantagetheypresentisalowerfinalweightandasadisadvantageitismoreinstableandithasmoreamplitudeofrolling.
Biplanes consist of two pairs of wings thatmove in phase. Thesewingscanbeonebehindtheotheroroneovertheother.Theadvantageofthis configuration is a lower rolling range and greater stability. Thedisadvantageisanincreaseintheweightofthevehicle.
Figure4:TheMicroBat,exampleofmonoplane.
Figure5:TheDelfly,exampleofbiplane.
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Finally,thefoldingwingsarefoldedtosimulatetheflightofbirds.Thewingsextendwhenthemovement isdownwardand fold to thebodyastheyrise,thusreducingtheareawhichminimizesthenegativeliftingeffect.ThedisadvantageofthisconfigurationistheexcessiveweightofactuatorsmakingitdifficulttoimplementinMAV.
Thewingdesignisveryimportantfortheaerodynamicperformance,and hence the lift that can be generated by the flapping wingMAV. Itinvolvesa choiceof thematerials for the structuringelementsandwingmembrane,andthewaytocombinethesetoformthewing’sshapeandstructure.AtraditionalchoiceforthewingmaterialsconsistsofPET-foilandcarbonfibberreinforcedpolymer(CFRP)rods.Thesematerialshaveproventheirworth,arewidelyavailable,anddonotrequirespecificinfrastructureforconstruction.Thedownsideisthattheytypicallyrequiresomemanualwork, which can limit repeatability. Moreover, the design options withthesematerialsarerelativelylimited.MostdesignswithPET-foilandrodsarelimitedtogeometricshapeswithastiffleadingedgeandafewstiffenersaddedtothewing.Inordertoallowformoreintricate,andyetrepeatabledesigns,othermaterialsandfabricationmethodshavebeeninvestigated.
Thecurrentlyavailablewingtechnologiesaresuitableforthecurrentinsect sized flapping-wings MAVs, however, improvements can beexpected.
Inordertochooseoneactuatortechnologyoranother,theactuatorshavetobechecked if they liveupto theboundaryconditionswhichareimposedbyaflapping-wingMAV.Thedesignoftheflapping-wingMAVishighlyinfluencedbythechoiceoftheactuator.
Magneticactuatorsarepulsewidthmodulation(PWM)drivenwithadutycycleproportionaltothetransmittercontrolstick,whichresultsinaproportionalcurrent.Thiscurrentproducesamomentinthemagnetwhichagain translates toaproportional force thatgoes to thecontrol surface.However,theforceissmallandwiththeairpressureonthecontrolsurfacesbeingproportionaltotheairvelocitysquared,thecontrolthrowgetsmuchlower at higher airspeed. This can cause inability to pull up from fastdescendingflight.
Analternativeconsistsofconventionalservosthatuseasmallelectricmotor,gearingandapotentiometerormagnetic-hall sensor forpositionfeedback. This type of actuator has more force and a higher accuracycomparedtothemagneticactuator.
Due to the trend of miniaturization and the availability ofmanufacturing techniques, piezoelectric actuators have been used formicro devices. Application in flapping-wing MAVs is complex due tomediumtohighvoltagesneededforactuation,andthecorrespondingneedfor high voltage electronics. Piezoelectric actuators allow a high velocitymovementofthewingswithasmallamplitude.
Not all mentioned actuator technologies are equally feasible.Implementationeffortfortheapplicationofaspecificactuatortechnologydependsonthesizeoftheintendedflapping-wingMAV.
The requirements for the current developments of the wingactuationmechanismfor the flapping-wingMAVaregearedtowards theimplementation of resonant principles while maintaining a very simpledesign.
Theapplicationof resonance in flapping-wingMAV isbased in thestorageofpotentialenergyinadeformablestructure.Thisisfocusinfourproperties:Thematerialused,thevolumeofthematerial,thedeformationmodeandamountofdeformation.
The deformation mode used to store potential energy has largeinfluence of the thorax structure. The deformationmodeswhich can beusedarecompression,bending,torsionandshearofamaterialwhichcanstoreenergyefficiently.Combinationsofdeformationmodesarepossible.
In order to put boundaries on the designs, four main aspectsinfluencethedesignandprovidedesigndirections.
• Simplicity:Basedonthefactthatsimplerdesignsseemtobemore successful. Low complexity designs are usually of lowmassandthereforeprovidebetterchancesoflift-off.
• Resonance: The design has to fully exploit resonance formaximumreductionofthepowerexpenditureinacceleratinganddeceleratingthewings.
Due to that, we need to differentiate between the two differentconcept sizing. After an exhaustive search, we do not find informationaboutthesizingofthespring,soweassumeit.
Concept1Firstsizing
SpringLength 4cm
SpringArea 8𝑐𝑚2
BerylliumCooperK 2’48*10hiK
StainlessSteelAISI302K 3’6*10hiK
Table4:YoungisModulus
Table5:Firstsizingstiffness
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Concept1Secondsizing
SpringLength 5cm
SpringArea 15𝑐𝑚2
BerylliumCooperK 3’72*10hiK
StainlessSteelAISI302K 5’4*10hiK
After our calculations, we compare the results with the springpropertiesusedbyBolsmanandwenotifythatourresultsarenotcoherent.As we have not many information about the springs, we will use thestiffnessusedbyBolsman.
K1 381’1iK
K2 403’4iK
4.3 TRANSMISSIONRATIO
First simulation in the project is the analysis of transmissionmechanism.Theequationis:
The first comparison is between the application of the naturalfrequencyandotherfrequency.Therestofthevalueswillbecommonforbothsimulationsandwecanlooktheminthefollowingtable:
𝐦𝐭𝐨𝐭𝐚𝐥 10’5g
k 381’1iK
𝐂𝐝,𝐜𝐂𝐥,𝐜
0’1
L 0’1m
g 9’8Ktu
Thenaturalfrequencyis:
𝑤v =𝑘𝑚
𝑤v = 190^51𝑟𝑎𝑑𝑠
Table8:Commonvaluesforfrequencyrepresentations.
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Figure14:w=10rad/s
Figure15:w=80rad/s
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-8
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0
2
4
6
8
Des
plaz
amie
nto
u(t),
m
10-6 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
u(t)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Tiempo, s
0
1
2
3
Fuer
za F
(t), N
10-5 Excitación
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-6
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0
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Des
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amie
nto
u(t),
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10-6 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
u(t)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Tiempo, s
0
1
2
3
Fuer
za F
(t), N
10-5 Excitación
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Looking the different simulations, we can verify the sinusoidalresponse.
Aswecansee,whenthesystemisexcitedatthenaturalfrequency,figure 16,weobtain the phenomenonof resonance. This is thatwe arelookingforbecausewearetryingtoimitatetheflyoftheinsectandaswesaw,theyusethisphenomenontomovetheirwings.Duetothat,forthenextsimulationsweusethenaturalfrequency.
Second comparison is for different values of K, this represent thedifferencebetweentwokindofspringmaterials.BecauseofdifferentK,wehave different values of natural frequency. As we saw before the firstsimulation,thenaturalfrequencydependsonthemassandthestiffness.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-4
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0
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Des
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amie
nto
u(t),
m
1010 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
u(t)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Tiempo, s
0
1
2
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Fuer
za F
(t), N
10-5 Excitación
Figure16:w=naturalfrequency.
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𝐦𝐭𝐨𝐭𝐚𝐥 10’5g
𝐂𝐝,𝐜𝐂𝐥,𝐜
0’1
L 0’1m
g 9’8Ktu
The values of K studied are 381,1 and 403,4 iK so the natural
1010 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
u(t)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Tiempo, s
0
1
2
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Fuer
za F
(t), N
10-5 Excitación
Figure17:k=381’1iK.
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Betweenthis twostiffness,wecannotseethedifference,sobothmaterials for the spring can be a good idea for our concepts. To seedifferencewiththevaluesofK,weprovevaluesthatwehavenotconsidertoourconcept.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-4
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0
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Des
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amie
nto
u(t),
m
1010 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
u(t)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Tiempo, s
0
1
2
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Fuer
za F
(t), N
10-5 Excitación
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-8
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0
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6
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Des
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amie
nto
u(t),
m
1010 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
u(t)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Tiempo, s
0
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Fuer
za F
(t), N
10-5 Excitación
Figure18:k=403’4iK.
Figure19:k=100iK.
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Aswecanconclude fromthe figures, less stiffnessproducesmoredisplacement.Inspiteofthat,maybewecannotobtainaspringwiththisproperties,ormaybea springwith this stiffnesshas lessduration.Morestiffness produces less movement as far as reverse movement of thesystem.
Third simulations are for compare the different values of the
The rest of the values are the same as we use in the rest ofsimulationswiththenaturalfrequency.
𝐦𝐭𝐨𝐭𝐚𝐥 10’5g
k 381’1iK
𝐰𝐧 190’51�'-K
L 0’1m
g 9’8Ktu
As we can see down below, the increment of the value of therelationshipbetweenliftanddragproducesagrowthofthevalueoftheexternal force. It produces in turn an increment in the value of thedisplacementbecausethevalueoftheamplitudehasgrown.
Table11:Commonvaluesfor|},~|�,~
representations.
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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-4
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0
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Des
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nto
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m
1010 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
u(t)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Tiempo, s
0
1
2
3
Fuer
za F
(t), N
10-5 Excitación
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-1
-0.8
-0.6
-0.4
-0.2
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0.4
0.6
0.8
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1011 Respuesta a excitación armónica de un sistema de 1 gdl amortiguado
The frequency response is a characteristic of a system that has ameasuredresponsethatistheresultofaknownappliedinput.Inthecaseofamechanicalstructure,thefrequencyresponseisthespectrumofthevibrationofthestructure,dividedbythespectrumoftheinputforceintothesystem.Tomeasurethefrequencyresponseofamechanicalsystem,wemustmeasure the spectra of the input force to the system and thevibrationresponse.ThisisdonemoreeasilywithaTRFanalyser.Frequencyresponse measurements are widely used in the modal analysis ofmechanicalsystems.
The frequency response function is a three-dimensional quantityconsistingofamplitudevs.phasevsfrequency.Thatiswhyatruegraphofitneedsthreedimensions,whichisdifficulttorepresentonpaper.Onewayto do this is the Bode diagram, which consists of two curves, one ofamplitudevsfrequency,andoneofphasevsfrequency.
Lookingatthephasegraphic,thelowerplot,wecanseeachangeofphaseof180ºat thenatural frequency.This isdue toour system isnotdumped.Ifoursystemweredumped,thechangeofphasewouldhavethesamemagnitudebutthechangewouldbesofter.
The H(w) representation shows us a maximum in the naturalfrequencyofthesystem.At lowerfrequencieswecanobservethestaticdeflexionduetoatthesefrequenciesthestiffnesshasmoreimpact.