磁流变阻尼器-pptPPT文件格式下载.ppt
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easilyredispersed(重分散).Non-Abrasive(零磨损)formulatedtonotabrade(损伤)thedevicesinwhichtheMRfluidisused.,FeaturesandBenefits,Whataremagnetorheologicalfluids?
Magnetorheologicalfluidsbelongtothegroupofsocalledcontrollablefluids.Thismeansthattheyexhibitasignificantchangeintheirrheologicalbehaviourwhenanexternalmagneticfield(外部磁场)isappliedtothem.Theyareindeedcomposedofmicron-sizedmagneticparticles(微米级磁粒),locatedinsidealiquidcarrier,thatformchain-likestructureswhentheexternalmagneticfieldisapplied,resultinginanincreaseoftheapparentviscosity(粘性)ofthefluid(Fig.1and2).,Fig.1:
Chain-likestructuresformationintheMRfluidunderanexternallyappliedmagneticfield(fromLORDcorporation),Fig.2:
ChainsformationinadropofMRfluid(fromISCFraunhofer),TherheologicalbehaviourofMRfluidsisoftenrepresentedasaBinghamplasticmodelwithavariableyieldstrengthdependingontheappliedmagneticfieldH(Fig.3aand3b).Theflowisgovernedbytheequation:
whereistheshearstress,istheshearstrainandistheviscosityofthefluid.TheoperatingrangeistheshadedareainFig.3c.Belowtheyieldstress(atstrainsoftheorderof10-3),theMRfluidbehavesviscoelastically:
WhereGisthecomplexmaterialmodulus.ThismodelisalsoagoodapproximationforMRdevices.However,theactualbehaviourismorecomplicatedandincludesstiction(静摩擦)andhysteresis(磁滞现象)suchsuchasshowninFig.3.d,Fig.3:
(a)and(b)Binghamplasticmodel,(c)Operatingrange,(d)HystereticbehaviourinMRfluiddevices,磁滞现象在铁磁性材料中是被广泛认知的。
是指在磁化和去磁过程中,铁磁质的磁化强度不仅依赖于外磁场强度,还依赖于原先磁化强度的现象。
当外加磁场施加于铁磁质时,其原子的偶极子按照外加场自行排列。
即使当外加场被撤离,部分排列仍保持:
此时,该材料被磁化。
在该材料中,磁场强度(H)和磁感应强度(B)之间的关系是非线性的。
如果在增强场强条件下,此二者关系将呈曲线上升到某点,到达此点后,即使场强H继续增加,磁感应强度B也不再增加。
该情况被称为磁饱和(magneticsaturation)。
如果此时磁场线性降低,该线性关系将以另一条曲线返回到0场强的某点,该点的B将被初始曲线的磁感应强度量BR叫做剩磁感应强度或剩磁(remanentfluxdensity)相抵消。
如果绘制以外加磁场的全部强度的二者关系图,将为S形的回路。
S的中间厚度描述了磁滞量,该量与材料的矫顽力相关。
该现象的实际影响可为,例如,当通过磁芯的外加电流被撤离,由于残留磁场继续吸引电枢,而引起滞后从而延迟磁能的释放。
磁滞现象Hysteresis,磁流变阻尼器的力学模型虽然多种多样,而且有些能很精确模拟阻尼器的动态特性,但是无法直接反映阻尼器的逆向动态特性。
由于磁流变阻尼器的研究涉及到电磁学、流体力学、热力学以及机械学等多学科,这些学科的交叉和融合为研究带来了挑战,但这些研究是使磁流阻尼在工程应用中不可缺少的关键技术。
B-Hloop,对于晶粒取向电材料的一组B-H环路(BR表示剩磁,而HC为矫顽力。
),Workingmodesofmagnetorheologicalfluids,Magnetorheologicalfluidscanbeoperatedinthreedistinctmodes,Valvemode,Directshearmode,Squeezemode,Fig.4WorkingmodesofMRfluids,Thefluidislocatedbetweenapairofstationarypoles.Theresistancetothefluidflowiscontrolledbymodifyingthemagneticfieldbetweenthepoles,inadirectionperpendiculartotheflow(Fig.4a).Devicesusingthismodeofoperationincludeservo-valves,dampers,shockabsorbersandactuators.,(a)Valvemode阀,valvemodeisthemostwidelyusedofthethreemodes.AnMRdeviceissaidtooperateinvalvemodewhentheMRfluidisusedtoimpede阻碍theflowofMRfluidfromonereservoir储存仓toanother.,Figure5.MRfluidusedinvalvemode,Thefluidislocatedbetweenapairofmovingpoles(translationorrotationmotion).Therelativedisplacementisparalleltothepoles(Fig.4b).Theapparentviscosity,andthusthe“dragforce”appliedbythefluidtothemovingsurfacescanbecontrolledbymodifyingthemagneticfieldbetweenthepoles.Devicesusingthismodeofoperationincludeclutches离合器,brakes,lockingdevicesanddampers.,(b)Directshearmode剪切,AnMRfluiddeviceissaidtooperateinshearmodewhenathinlayer(0.005to0.015in.)ofMRfluidissandwichedbetweentwoparamagnetic顺磁性movingsurfaces.Shearmode(seeFigure6)isusefulprimarilyfordampersthatarenotrequiredtoproducelargeforcesandforclutchesandbrakes.,Figure6.MRfluidusedinshearmode,Thefluidislocatedbetweenapairofmovingpoles.Therelativedisplacementisperpendiculartothedirectionofthefluidflow(Fig.4c).Thecompressionforceappliedtothefluidisvaryingperiodically间歇性.Displacementsaresmallcomparedtotheothermodes(intheorderofmillimetres)butresistiveforcesarehigh.Asforthetwoothermodes,themagnitudeoftheseresistiveforcescanbecontrolledbymodifyingthemagneticfieldbetweenthepoles.Whilelesswellunderstoodthantheothermodes,thesqueezemodehasbeenexploredforuseinsmallamplitudevibrationandimpactd