单片机 外文翻译 外文文献 英文文献 基于单片机的超声波测距系统的研究与设计.docx
《单片机 外文翻译 外文文献 英文文献 基于单片机的超声波测距系统的研究与设计.docx》由会员分享,可在线阅读,更多相关《单片机 外文翻译 外文文献 英文文献 基于单片机的超声波测距系统的研究与设计.docx(30页珍藏版)》请在冰豆网上搜索。
单片机外文翻译外文文献英文文献基于单片机的超声波测距系统的研究与设计
单片机外文翻译外文文献英文文献基于单片机的超声波测距系统的研究与设计
附录
附录A
外文翻译
theequivalentdcvalue.Intheanalysisofelectroniccircuitstobeconsideredinalatercourse,bothdcandacsourcesofvoltagewillbeappliedtothesamenetwork.Itwillthenbenecessarytoknowordeterminethedc(oraveragevalue)andaccomponentsofthevoltageorcurrentinvariouspartsofthesystem.
EXAMPLE13.13DeterminetheaveragevalueofthewaveformsofFig.13.37.
FIG.13.37
Example13.13.
Solutions:
a.Byinspection,theareaabovetheaxisequalstheareabelowoveronecycle,resultinginanaveragevalueofzerovolts.
b.UsingEq.(13.26):
asshowninFig.13.38.
26
Inreality,thewaveformofFig.13.37(b)issimplythesquarewaveofFig.13.37(a)withadcshiftof4V;thatisv2=v1+4V
EXAMPLE13.14Findtheaveragevaluesofthefollowingwaveformsoveronefullcycle:
a.Fig.13.39.
b.Fig.13.40.
27
Solutions:
Wefoundtheareasunderthecurvesintheprecedingexamplebyusingasimplegeometricformula.Ifweshouldencounterasinewaveoranyotherunusualshape,however,wemustfindtheareabysomeothermeans.Wecanobtainagoodapproximationoftheareabyattemptingtoreproducetheoriginalwaveshapeusinganumberofsmallrectanglesorotherfamiliarshapes,theareaofwhichwealreadyknowthroughsimplegeometricformulas.Forexample,
theareaofthepositive(ornegative)pulseofasinewaveis2Am.Approximatingthiswaveformbytwotriangles(Fig.13.43),weobtain(usingarea
1/2baseheightfortheareaofatriangle)aroughideaoftheactualarea:
Acloserapproximationmightbearectanglewithtwosimilartriangles(Fig.13.44):
28
whichiscertainlyclosetotheactualarea.Ifaninfinitenumberofformswereused,anexactanswerof2Amcouldbeobtained.Forirregularwaveforms,thismethodcanbeespeciallyusefulifdatasuchastheaveragevaluearedesired.Theprocedureofcalculusthatgivestheexactsolution2Amisknownasintegration.Integrationis
presentedhereonlytomakethemethodrecognizabletothereader;itisnotnecessarytobeproficientinitsusetocontinuewiththistext.Itisausefulmathematicaltool,however,andshouldbelearned.Findingtheareaunderthepositivepulseofasinewaveusingintegration,wehave
where?
isthesignofintegration,0andparethelimitsofintegration,Amsinaisthe
functiontobeintegrated,anddaindicatesthatweareintegratingwithrespecttoa.
Integrating,weobtain
Sinceweknowtheareaunderthepositive(ornegative)pulse,wecaneasilydeterminetheaveragevalueofthepositive(ornegative)regionofasinewavepulsebyapplyingEq.(13.26):
ForthewaveformofFig.13.45,
29
EXAMPLE13.15DeterminetheaveragevalueofthesinusoidalwaveformofFig.13.46.
Solution:
Byinspectionitisfairlyobviousthat
theaveragevalueofapuresinusoidalwaveformoveronefullcycleiszero.
EXAMPLE13.16DeterminetheaveragevalueofthewaveformofFig.13.47.
Solution:
Thepeak-to-peakvalueofthesinusoidalfunctionis16mV+2mV=18mV.Thepeakamplitudeofthesinusoidalwaveformis,therefore,18mV/2=9mV.Countingdown9mVfrom2mV(or9mVupfrom-16mV)resultsinanaverageordclevelof-7mV,asnotedbythedashedlineofFig.13.47.
EXAMPLE13.17DeterminetheaveragevalueofthewaveformofFig.13.48.
Solution:
30
EXAMPLE13.18ForthewaveformofFig.13.49,determinewhethertheaverage
valueispositiveornegative,anddetermineitsapproximatevalue.
Solution:
Fromtheappearanceofthewaveform,theaveragevalueispositiveandinthevicinityof2mV.Occasionally,judgmentsofthistypewillhavetobemade.Instrumentation
Thedcleveloraveragevalueofanywaveformcanbefoundusingadigitalmultimeter(DMM)oranoscilloscope.Forpurelydccircuits,simplysettheDMMondc,andread
thevoltageorcurrentlevels.Oscilloscopesarelimitedtovoltagelevelsusingthesequenceofstepslistedbelow:
1.FirstchooseGNDfromtheDC-GND-ACoptionlistassociatedwitheachverticalchannel.TheGNDoptionblocksanysignaltowhichtheoscilloscopeprobemaybeconnectedfromenteringtheoscilloscopeandrespondswithjustahorizontalline.Settheresultinglineinthemiddleoftheverticalaxisonthehorizontalaxis,asshowninFig.13.50(a).
2.Applytheoscilloscopeprobetothevoltagetobemeasured(ifnotalreadyconnected),andswitchtotheDCoption.Ifadcvoltageispresent,thehorizontallinewillshiftupordown,asdemonstratedinFig.13.50(b).Multiplyingtheshiftbytheverticalsensitivitywillresultinthedcvoltage.Anupwardshiftisapositivevoltage(higher
31
potentialattheredorpositiveleadoftheoscilloscope),whileadownwardshiftisanegativevoltage(lowerpotentialattheredorpositiveleadoftheoscilloscope).Ingeneral,
1.UsingtheGNDoption,resetthehorizontallinetothemiddleofthescreen.2.SwitchtoAC(alldccomponentsofthesignaltowhichtheprobeisconnectedwillbeblockedfromenteringtheoscilloscope—onlythealternating,orchanging,
componentswillbedisplayed).
Notethelocationofsomedefinitivepointonthewaveform,suchasthebottomofthehalf-waverectifiedwaveformofFig.13.51(a);thatis,noteitspositionontheverticalscale.Forthefuture,wheneveryouusetheACoption,keepinmindthatthecomputerwilldistributethewaveformaboveandbelowthehorizontalaxissuchthattheaveragevalueiszero;thatis,theareaabovetheaxiswillequaltheareabelow.3.ThenswitchtoDC(topermitboththedcandtheaccomponentsofthewaveformtoentertheoscilloscope),andnotetheshiftinthechosenlevelofpart2,asshowninFig.13.51(b).Equation
(13.29)canthenbeusedtodeterminethedcoraveragevalueofthewaveform.ForthewaveformofFig.13.51(b),theaveragevalueisabout
TheprocedureoutlinedabovecanbeappliedtoanyalternatingwaveformsuchastheoneinFig.13.49.InsomecasestheaveragevaluemayrequiremovingthestartingpositionofthewaveformundertheACoptiontoadifferentregionofthescreenorchoosingahighervoltagescale.DMMscanreadtheaverageordclevelofanywaveformbysimplychoosingtheappropriatescale.
32
13.7EFFECTIVE(rms)VALUES
Thissectionwillbegintorelatedcandacquantitieswithrespecttothepowerdeliveredtoaload.Itwillhelpusdeterminetheamplitudeofasinusoidalaccurrentrequiredtodeliverthesamepowerasaparticulardccurrent.Thequestionfrequentlyarises,Howisitpossibleforasinusoidalacquantitytodeliveranetpowerif,overafullcycle,thenetcurrentinanyonedirectioniszero(averagevalue0)?
Itwouldalmostappearthatthepowerdeliveredduringthepositiveportionofthesinusoidalwaveformiswithdrawnduringthenegativeportion,andsincethetwoareequalinmagnitude,thenetpowerdeliverediszero.However,understandthatirrespectiveofdirection,current
ofanymagnitudethrougharesistorwilldeliverpowertothatresistor.Inotherwords,
duringthepositiveornegativeportionsofasinusoidalaccurrent,powerisbeingdeliveredateach
instantoftimetotheresistor.Thepowerdeliveredateachinstantwill,ofcourse,varywiththemagnitudeofthesinusoidalaccurrent,buttherewillbeanetflowduringeitherthepositiveorthenegativepulseswithanetflowoverthefullcycle.Thenetpowerflowwillequaltwicethatdeliveredbyeitherthepositiveorthenegativeregionsofsinusoidalquantity.AfixedrelationshipbetweenacanddcvoltagesandcurrentscanbederivedfromtheexperimentalsetupshowninFig.13.52.Aresistorinawaterbathisconnectedbyswitchestoadcandanacsupply.Ifswitch1isclosed,adccurrentI,
determinedbytheresistanceRandbatteryvoltageE,willbeestablishedthroughthe
resistorR.Thetemperaturereachedbythewaterisdeterminedbythedcpowerdissipatedintheformofheatbytheresistor.
Ifswitch2isclosedandswitch1leftopen,theaccurrentthroughtheresistorwillhaveapeakvalueofIm.Thetemperaturereachedbythewaterisnowdeterminedbytheacpowerdissipatedintheformofheatbytheresistor.Theacinputisvarieduntilthetemperatureisthesameasthatreachedwiththedcinput.Whenthisisaccomplished,theaverageelectricalpowerdeliveredtotheresistorRbytheacsourceisthesameas
thatdeliveredbythedcsource.Thepowerdeliveredbytheacsupplyatanyinstantoftimeis
33
Theaveragepowerdeliveredbytheacsourceisjustthefirstterm,sincetheaveragevalueofacosinewaveiszeroeventhoughthewavemayhavetwicethefrequencyoftheoriginalinputcurrentwaveform.Equatingtheaveragepowerdeliveredbytheacgeneratortothatdeliveredbythedcsource,
which,inwords,statesthat
theequivalentdcvalueofasinusoidalcurrentorvoltageis1/2or0.707ofits
maximumvalue.
Theequivalentdcvalueiscalledtheeffectivevalueofthesinusoidalquantity.
Insummary,
Asasimplenumericalexample,itwouldrequireanaccurrentwithapeakvalueof2(10)14.14AtodeliverthesamepowertotheresistorinFig.13.52asadccurrentof10A.Theeffectivevalueofanyquantityplottedasafunctionoftimecanbefoundbyusingthefollowingequationderivedfromtheexperimentjustdescribed:
34
which,inwords,statesthattofindtheeffectivevalue,thefunctioni(t)mustfirstbe
squared.Afteri(t)issquared,theareaunderthecurveisfoundbyintegration.ItisthendividedbyT,thelengthofthecycleortheperiodofthewaveform,toobtaintheaverageor