低成本陀螺.docx

低成本陀螺.docx

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低成本陀螺

低成本陀螺-加速度混合传感器

作者：

EarlW.Bollinger  文章来源：

http:

//www.dprg.org/projects/  更新时间：

2006年04月17日  打印此文  浏览数：

9053

Clickforfull-sizedimage

ProjectSummary

Inbuildingabalancingrobotyouneedagyroscopeunitandanaccelerometerunitinordertogetittobalance.ButIalsowantedittobereasonablylowincostwithhighperformanceandeasilyinterfacedtoaMCU.Someoftheothersolutionsareprohibitivelyexpensive,eveniftheymayworkwell.AlsothesolutionIhadinmindhadtobefairlysmallanduseuplittlespaceanduseuplittlepowertoo.SowhenAnalogDevicesrecentlycameoutwiththeirADXRS150andADXRS300piezo-gyroscopeintegratedcircuit,Inowhadasolutionthathadlowcostandlowpowerwithhighperformancetoo.Thegyroisverysmallatabout5mmsquare（about0.3inchessquare）.YoucanseethesmallcombogyroPCBmountedverticallyinthecentersectiononthebalancingrobotphotobelow.

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IalsotookadvantageofthesmallsizeandcombinedanAnalogDevicesaccelerometerintegratedcircuitonthesamePCBtomaximizeperformance.Plustherearenomovingpartspersetocomplicatetheissue.Allthegoodstuffiscontainedinsidethechips.UsingthetwoAnalogDevicesADXRS150andADXL202wecanbuildacomboPCBforunder$100.00inparts.InthiscaseattheAnalogDeviceswebsite,theyaresellingthegyroforabout$39.90insinglesandtheaccelerometerforabout$29.90insingles.IknowthattheExpressPCBservicechargesabout$62forthreemini-boards,sooneboardcostsabout$20,thenwehaveabout$20foralltheotherparts,sincethePCBusesuponlyasmallportionofthewholePCBblank,youcanputinafewotherneededlayoutsforotherthingsonthesamePCBandcutthemoutlaterasyouneedthem.IusuallyuseascrollsawtocutoutthesmallPCBs.Insomecasesyoucouldobtainsamplesofthesechips,orastheystarttofillinthevendorpipeline,you'llbeabletogetthematbetterpricesthroughthevendorsanddistributors.

TraditionalGryoSolutions

Youcan,offandon,getniceMurataPiezoGyrosfor$40eachfromsomevendors.Butthesetendtogotothemanufacturersfirst,sosometimesitisdifficulttogetone.Gyrationsellsamicrogyrofor$450.Thereisalsoaspecialacademicversionofthekitfor$35.Ithasthreegyrosandsupportingchipsincludedinthekit.

Microstrainsellsanumberofdifferentgyromodulesstartingat$295uptoaround$1495.

TheoldclassicapproachistoobtainRChobbymodelaircraftgyrosandusethem.Thesegyrosrunincostfromabout$69.00onuptoseveralhundreddollars,dependingonthemodelortype.AsourcetocheckoutisTowerHobbiesormaybeaRCspecialtystorelikeHelicopterWorld.SometimesServoCityhasRCmodelplaneGyrostoo.

Thenthemostbasicmethodistouseapotentiometerwithapoleorpendulumattachedtoit.YoufeedthewiperoutputfromthepotentiometertoanADCandthenyoucandetermineyourpositionortiltangle.Thisisthestandardclassicapproachtobalancingrobotsthatbalanceapoleontopofthem.Butyoucouldinvertitaswell.Usuallyyouwouldstillhavetohavesomekindofareferencetothefloorortabletopfortherobottoknowwheretheflooris.Butyoucouldtrailapotentiometerwithasmallarmtouchingthefloorbehindtherobotforthispurposeaswell.

AlthoughaRChobbygyroscopeiscosteffective,itisabitdifficulttointerfacetoaMCU.ThegyroisdesignedforRCmodelairplaneservosthatworkwitha1to2ms-widthpulsetrain.Thus1.5msisneutral,1msfullonewayand2msfulltheotherway.UsingaMCUyouhavetotransmitthe1.5msneutralsignaltothegyro,thenreceivethepulsesfromthegyroandtranslatethemintosomethingmoremeaningfulforyoutouse.ThenextproblemisyoubasicallyhavetohavetheMCUstopforupto2mstoreadapulsewidth,beforeyoucandosomethingwithit.DependingontheMCUyoumighthavetohavetheMCUsendthethe1.5mspulseaswell,eatingupevenmoretime.That2msmaybeverycriticaltootherthingsgoingon.Butifweusethegyrochip,wecanuseaADC（analogdigitalconverter）toconverttheanalogvoltageleveltoanumericalvaluemuchfaster,plusyoucould,insomecases,starttheADCtogetavalue,andcomebacklaterfortheresult.Wecannowreducethetimetogetanumericalvaluedowntomicrosecondsinsteadofmilliseconds.Thuswegetbetteraccuracy,andfasterperformance.IfyouuseahighperformanceDSPprocessor,youmaybeabletogetthisdowntolessthanamicrosecondtoreadavoltagelevel.

Themoreexpensivegyroscopesystems,unfortunately,tendtobeprohibitivelyexpensive.Soalthoughtheyarereallyneatsolutions,Ididn'tpursuethemanyfurther.

ADXRSGryoscope

Withoutgoingintoalltheengineeringgobblygook,thebasicwaytheADXRS150or300gyroscopeIC'swork,istheyoutputa2.5voltlevelatrest.Ifthegyroistiltedonewayortheother,thenitwilloutputuptoa+/-voltagechangefromreference.Thisisananaloglevelthatyouneedtofeedintoananalogdigitalconverter（ADC）foraMCUtouse.TheADXRS150hasabouta12.5mvperdegreeofsensitivity,foruptoabout150degreespersecondoftilting.Thiscanbeadjustedsomemoreasneededtoo.Thevoltagechangeisproportionaltothespeedorrateofthetiltangle.Thefastertherobottiltsthelargerthechangeinvoltage.TheADXRS300,asahigherperformancedevice,givesabout300degreespersecondoftilting.

Thefollowingsequenceofphotosillustratethevoltagechangewhenthegryoistitled.（thephotosareanexaggerationasIcouldn'ttiltandsnapthephotofastenough）

Above:

Gryoisatrest

Above:

Gryotitledforward

Above:

Gryotitledbackwards

ThisshortMPEGvideoalsoshowstheoscilliscopemeasurementsofthegyrocircuitatrest,tiltedoneway,andthentiltedtheotherway.

Ifyouwantmoreinformationaboutthechips,pleasegototheAnalogDeviceswebsiteanddownloadthespecsheetsanddocumentationonthechips.Alsogetthedocumentationontheaccelerometerevaluationboardaswell.IexpectAnalogDevicestohaveanevaluationboardforthegyroscopeICprettysoonaswell.

ADXL202accelerometer

TheADXL202accelerometerisareallyneatdeviceinitself.Itbasicallyoutputsapulsestring,andchangesthedutycycleastheunitistilted.Plusthechangestaysatthetiltedangleuntilyouchangeittoadifferentangle.Thusitmakesforareallyhandyreferencepointtypeofdevice,suchaswhereis"up"versuswhereis"down".

Usingagyroscopebyitselfseemsreasonable,butthegyrodevelopsaslowcreepingtilterrorthatcontinuestoincrease.Anexampleofthisiswhenyoutiltthegyroyou'llseeachangeintheoutputvoltage,butitquicklyreturnstoit'srestinglevel,ifthetiltdoesn'tchange.Thusifyourrobotwastotiltforwardafewdegreesandstayatthatangle,thegyrowillreportthattherobotisstandingstraightupafterasecond.Soeventuallytheerrorsaddupandtherobotfallsover.Usinganaccelerometerhelpstosolvethisproblem.Theaccelerometeroutputsasteadydutycyclewhentherobotisstandingstraightup,andiftherobottiltsonewayortheotherthedutycyclechanges,andstayschanged,untilyouhavetherobot'scomputercorrectforthetilt.Nowonthesurfaceitlookslikeyoucoulduseonlytheaccelerometer,butthegyroreactsmuchfastertotiltchangesthantheaccelerometercan.Sothegyrogivesyouthefastresponseandtheaccelerometerhelpstocorrectforthecreepinggyroerrorsthatwilloccurandtellsyouwhenyourlevelornot,orwhereupanddownis.

IbasicallyfollowedthedesignsasputforthbyAnalogDevices,butIdidaddmorefilteringintothepowerlineforthegyroscopechip.Withouttheadditionalfilteringyouruntheriskofnoisecausingtoomanyspuriousreadings,whichwouldcausemoreproblems.Ialsokeptthetwocircuitssomewhatseparateinordertoensurethatthenoisegeneratedbytheaccelerometerdoesn'tinterferewiththegyroscopeorviceversa.PlustheadditionalfilteringisneedsastheMCUorelectricmotorswillgenerateadditionalpowerlinenoiseaswell.

PCBoard

ExceptformountingthegyroscopeIC,buildingthePCBisfairlystraightforwardandsimple.Butdobeextremelycarefulwiththesurfacemountcapacitorsastheyarenotmarkedandtheytypicallyalllookalike.Iwouldsuggesttakingthecapacitorsfromtheirpackageoneatatimeandsolderingthemdownoneatatimetoensureyoudon'tmixanyup.AlsonotethatthetwoICchipsarecomplexandsensitivetostaticelectricitysoyouneedtobecarefulwiththem.IuseasmallX-ActoknifetohelpholdthelittlepartsdownasIamsolderingthem.Sometimesabitofairgetstrappedunderthepartandit'llpopofffromtheheatasyousolderit.WhenthepartgoespoofandpopsoffthePCByoucan'tfinditonthedeskorfloor,soyouhavetogogetanotherone.

MountingthegyroscopeICisabigproblemasitisaBGA（BallGridArraypackage）thatisabout1/4"squareinsize.The32littlepinsare.020"indiameterandareabout.015"apart.Inlookingatthechip,ithadmespookedforquiteawhile.Theremaybeotherwaysofdoingit,butthethreethatIknowofare:

One）youcanuseaIRovendesignedforsolderingthesepartsontoPCBs;Two）youcanuseahotairSMDreworkstationtoheatupthepartsandsolderthem;Three）,youcouldheatupatoasterovento300degreesorsoandthenstickthePCBwiththegyroonitintotheovenandcookitforafewminutesorso（Ihavenotevertriedthismethodthough）.Intheovenmethods,I'veseenthemusesomekindofaPCBpre-heatertoheatthePCBuptonear300degrees,butjustshortofwherethesolderstart