力、力矩、压力测量

力、力矩、压力测量7.1Introduction7.1.1Force(1)Whatistheforce?Forceisoneofthemostimportantphysicalparameters.Variousmechanicalmotionisvirtuallythetransmissionofforceortorque.Forcepossessesthecapabilityinbothstaticanddynamicphenomenon:itcanchangethebody’smotionstatusorcausebody’sdeformation.(2)unitofforceN(Newton):istheforcewhichwhenappliedtoamassofonekilogramgivesitanaccelerationof1m/s2.(3)Weight:istheforceexertedonabodyofmassmduetoearthgravity.G=mg;gisthelocalgravityacceleration,typicallyabout9.8m/s2.Weightmeasurementisusuallyspecifiedintermsofthemass.(4)Torque(力矩、扭矩、转矩）Torqueissometimescalledmoment.Itistheproductofaforceanditsarm.Forarotatingshaftofradiusr,thetorqueTappliedtotheshafttocauseittorotateisF×r,whereFisthetangentialforceattheradiusr.Whenashaftistwistedasaresultoftheapplicationofatorque,shearstressesandstrainareproducedintheshaftmaterial.(5)PressureAforceFperpendiculartoanareaA,thepressurePisP=F/Aunit:Pascal(Pa)1Pa=1N/m2Atmosphericpressure:theatmospheresurroundingtheearthexertsaforceontheunitearthsurface,thatisthepressurecausedbytheweightoftheatmosphere.Itvarieswithweathercondition,theheightabovesealevel,latitude.Positivepressure:thegaugepressurehigherthantheatmosphericpressure.Gaugepressure:thepressuremeasuredrelativetotheatmosphericpressure.Thedifferentialpressurebetweentheabsolutepressureandtheatmosphericpressure.Absolutepressure:usedforthepressuremeasuredrelativetozeropressure.Negativepressure:thegaugepressurelowerthantheatmosphericpressure.Vacuum:theabsolutepressurelowerthanatmosphere.7.2Measurementmethods7.2.1Forcemeasurementmethods(1)AccordingtostaticordynamiceffectStaticeffect-----thestaticforcecausesmechanicaldeformation.Bymeasuringthedeformationorusingsomeinnerstressorforcerelatedphysicaleffects.Dynamiceffect----accordingtotheNewton2ndlaw,theforceisassociatedwithacceleration.Knowingmassandaccelerationmeasured,theforcecanbedefined:F=maForcemeasurement1.LeverbalancemethodsBasedontheprincipleofmomentsi.e.atstaticequilibriumthealgebraicsumoftheclockwisemomentsaboutanaxisequalstheanticlockwisemoments.Themomentofaforceaboutanaxisistheproductoftheforceandtheperpendiculardistancefromitslineofactiontotheaxis.Thismethodistypicallyusedforweightsupto1000Kg.2.Forcebalancemethods3.Elasticelementmethods4.Pressuremethods(2)accordingtothemethodsusingdifferentstructure&instruments一种伺服式测力系统。无外力作用时，系统处于初始平衡位置，光线全部被遮住，光敏元件无电流输出，力发生器不产生力矩。当被测力F作用在杠杆上时，系统失去平衡，杠杆发生偏转，窗口打开相应的缝隙。光线通过缝隙，照射到光敏元件上，光敏元件输出与光照成比例的电信号，经放大后加到磁电力矩发生器的旋转线圈上。载流线圈与磁场相互作用而产生电磁力矩，用于平衡被测力F与配重(标准质量m)力的力矩之差；使杠杆重新处于平衡状态。当杠杆处于新的平衡位置时，其转角与被测力F成正比，放大器输出电信号在采样电阻R上的电压U0与被测力F成比例。磁电式力平衡测力系统ForcebalancemethodsElasticelementmethodsSensorsthatareusedformeasurementofforce,torqueorpressureoftencontainanelasticelementthatconvertsthemechanicalquantityintoadeflectionorstrainwhichcanthenbetransformedusinganothersensorintoanelectricalsignal.Electricalresistancestraingaugesarewidelyusedinthiscapacity.22()()()()dytdytmcKytftdtdtIngeneral,elasticelementsusedforthemeasurementofforcecanbeconsideredtorepresentedbythemodelshowninFig.Whenaforcef(t)actsonthemassofelement,adisplacementisproducedwhichvarieswithtimebeforeobtainingasteady-statevalue.Thedisplacementvariationwithtimeisdescribedbyasecondorderdifferentialequation.•Inpractice,oftenitisnotthedisplacementitselftobemeasuredbutthestrainexperiencedbyastraingaugeattachedtowhatisequivalentofthespring.dampingspringDiagramofstraingaugeVariousformsofelasticmembersareused.Thesimplestisjustaspringtomakeadevicecalledspringbalance.Theextensionofthespringrepresentstheforceapplied.Loadcells,i.e.elasticmemberswhichtransformforceintodisplacementorstrains,cantakemanyforms.Thestructureoftypicalelasticelementanditsdesigningcalculationasfollows:ColumnarloadcellProvingring测力环BendingbeamAppliedforceTherelativeextensionofstraingaugeinelasticelement:lFlAEEΔl-------Totalextensionofstraingaugel--------theoriginallengthofstraingaugeF------appliedforceA------workingareaofelasticelementE------Yangmodelofelasticelementσ------stressofelasticelementSensitivity:///RRRRkll31132424RRFkkRRAERRFkkRRAE----poisonconstantofelasticelementTotalstrainofelasticelement:012342(1)FAEOutputofelectricalbridge:000(1)2(1)24iiUkUFAEUkkFUAEIngeneral,selectk=2,Voltagesensitivity(mv/v):Otherprincipleofforcemeasurement(1)Piezoelectricdynamometry压电式测力仪利用压电材料(石英晶体、压电陶瓷）的压电效应，将被测力经弹性元件转换为与其成正比的电荷量输出，通过测量电路测出输出电荷，从而实现对力值的测量。弹性元件感受力F时压电材料产生电荷Q输出ijQdFdij:Piezoelectricconstant适用于动态力的测量由工业纯铁、硅钢等铁磁材料制成的铁心在机械力的作用下磁导率发生变化称为压磁效应，或称磁弹性效应。如图a所示，在铁心上安置一对线圈一励磁线圈和测量线圈。两线圈的平面相互垂直,若无外力作用，励磁线圈中心交流电流所建立的磁场对测量线圈没有输出，见图b。若外力作用在铁心上，铁心磁导率改变，则测量线圈被励磁线圈中的磁场交链而输出比例于外力大小的信号，见图c。Piezomagneticdynamometry012/iUkUFNNTorquemeasurementTorquemeasurementareusedforrotatingshaftstodeterminethepowertransmittedandalsotomonitoragainstfailureasaresultofshearstress.Themeasurementoftorqueisalwayslinkedwiththepowerandrotatingspeed.Theworkdoneistheproductoftheforceandthedistancetraveledinthedirectionoftheforce.Workdonepersecond=F×2rn=(Powertransmittedbyarotatingshaft=