ATypicalSmallDCMotor
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GORESET
ElectricalMotors
ElectricalMotorsarecontinuousactuatorsthatconvertelectricalenergyintomechanicalenergyintheformofa
continuousangularrotationthatcanbeusedtorotatepumps,fans,compressors,wheels,etc.Aswellasrotarymotors,
linearmotorsarealsoavailable.Therearebasicallythreetypesofconventionalelectricalmotoravailable:ACtype
Motors,DCtypeMotorsandStepperMotors.
ACMotorsaregenerallyusedinhighpowersingleormulti-phaseindustrial
applicationswereaconstantrotationaltorqueandspeedisrequiredto
controllargeloads.Inthistutorialonmotorswewilllookonlyatsimplelight
dutyDCMotorsandStepperMotorswhichareusedinmanyelectronics,
positionalcontrol,microprocessor,PICandroboticcircuits.
TheDCMotor
TheDCMotororDirectCurrentMotortogiveititsfulltitle,isthemost
commonlyusedactuatorforproducingcontinuousmovementandwhose
speedofrotationcaneasilybecontrolled,makingthemidealforusein
applicationswerespeedcontrol,servotypecontrol,and/orpositioningis
required.ADCmotorconsistsoftwoparts,a"Stator"whichisthestationary
partanda"Rotor"whichistherotatingpart.TheresultisthattherearebasicallythreetypesofDCMotoravailable.
BrushedMotor-Thistypeofmotorproducesamagneticfieldinawoundrotor(thepartthat
rotates)bypassinganelectricalcurrentthroughacommutatorandcarbonbrushassembly,
hencetheterm"Brushed".Thestators(thestationarypart)magneticfieldisproducedbyusing
eitherawoundstatorfieldwindingorbypermanentmagnets.GenerallybrushedDCmotors
arecheap,smallandeasilycontrolled.
BrushlessMotor-Thistypeofmotorproduceamagneticfieldintherotorbyusing
permanentmagnetsattachedtoitandcommutationisachievedelectronically.Theyare
generallysmallerbutmoreexpensivethanconventionalbrushedtypeDCmotorsbecausethey
use"Halleffect"switchesinthestatortoproducetherequiredstatorfieldrotationalsequence
buttheyhavebettertorque/speedcharacteristics,aremoreefficientandhavealonger
operatinglifethanequivalentbrushedtypes.
ServoMotor-ThistypeofmotorisbasicallyabrushedDCmotorwithsomeformof
positionalfeedbackcontrolconnectedtotherotorshaft.Theyareconnectedtoandcontrolledby
aPWMtypecontrollerandaremainlyusedinpositionalcontrolsystemsandradiocontrolled
models.
NormalDCmotorshavealmostlinearcharacteristicswiththeirspeedofrotationbeingdeterminedbytheappliedDC
voltageandtheiroutputtorquebeingdeterminedbythecurrentflowingthroughthemotorwindings.Thespeedof
rotationofanyDCmotorcanbevariedfromafewrevolutionsperminute(rpm)tomanythousandsofrevolutionsper
minutemakingthemsuitableforelectronic,automotiveorroboticapplications.Byconnectingthemtogearboxesorgear-
trainstheiroutputspeedcanbedecreasedwhileatthesametimeincreasingthetorqueoutputofthemotoratahigh
speed.
The"Brushed"DCMotor
AconventionalbrushedDCMotorconsistbasicallyoftwoparts,thestationarybodyofthemotorcalledtheStatorandthe
innerpartwhichrotatesproducingthemovementcalledtheRotoror"Armature"forDCmachines.
Themotorswoundstatorisanelectromagnetcircuitwhichconsistsofelectricalcoilsconnectedtogetherinacircular
configurationtoproducetherequiredNorth-polethenaSouth-polethenaNorth-poleetc,typestationarymagneticfield
systemforrotation,unlikeACmachineswhosestatorfieldcontinuallyrotateswiththeappliedfrequency.Thecurrent
whichflowswithinthesefieldcoilsisknownasthemotorfieldcurrent.
Theseelectromagneticcoilswhichformthestatorfieldcanbeelectricallyconnectedinseries,parallelorbothtogether
(compound)withthemotorsarmature.AserieswoundDCmotorhasitsstatorfieldwindingsconnectedinserieswith
thearmature.Likewise,ashuntwoundDCmotorhasitsstatorfieldwindingsconnectedinparallelwiththearmatureas
shown.
SeriesandShuntConnectedDCMotor
TherotororarmatureofaDCmachineconsistsofcurrentcarryingconductorsconnectedtogetheratoneendto
electricallyisolatedcoppersegmentscalledthecommutator.Thecommutatorallowsanelectricalconnectiontobe
madeviacarbonbrushes(hencethename"Brushed"motor)toanexternalpowersupplyasthearmaturerotates.
Themagneticfieldsetupbytherotortriestoalignitselfwiththestationarystatorfieldcausingtherotortorotateonits
axis,butcannotalignitselfduetocommutationdelays.Therotationalspeedofthemotorisdependentonthestrength
oftherotorsmagneticfieldandthemorevoltagethatisappliedtothemotorthefastertherotorwillrotate.Byvaryingthis
appliedDCvoltagetherotationalspeedofthemotorcanalsobevaried.
Conventional(Brushed)DCMotor
Permanentmagnet(PMDC)brushedmotorsaregenerallymuchsmallerandcheaperthantheirequivalentwoundstator
typeDCmotorcousinsastheyhavenofieldwinding.InpermanentmagnetDC(PMDC)motorsthesefieldcoilsare
replacedwithstrongrareearth(i.e.SamariumCobolt,orNeodymiumIronBoron)typemagnetswhichhaveveryhigh
magneticenergyfields.Thisgivesthemamuchbetterlinearspeed/torquecharacteristicthantheequivalentwound
motorsbecauseofthepermanentandsometimesverystrongmagneticfield,makingthemmoresuitableforusein
models,roboticsandservos.
AlthoughDCbrushedmotorsareveryefficientandcheap,problemsassociatedwiththebrushedDCmotoristhat
sparkingoccursunderheavyloadconditionsbetweenthetwosurfacesofthecommutatorandcarbonbrushesresulting
inselfgeneratingheat,shortlifespanandelectricalnoiseduetosparking,whichcandamageanysemiconductor
switchingdevicesuchasaMOSFETortransistor.Toovercomethesedisadvantages,BrushlessDCMotorswere
developed.
The"Brushless"DCMotor
ThebrushlessDCmotor(BDCM)isverysimilartoapermanentmagnetDCmotor,butdoesnothaveanybrushesto
replaceorwearoutduetocommutatorsparking.Therefore,littleheatisgeneratedintherotorincreasingthemotorslife.
Thedesignofthebrushlessmotoreliminatestheneedforbrushesbyusingamorecomplexdrivecircuitweretherotor
magneticfieldisapermanentmagnetwhichisalwaysinsynchronisationwiththestatorfieldallowsforamoreprecise
speedandtorquecontrol.ThentheconstructionofabrushlessDCmotorisverysimilartotheACmotormakingitatrue
synchronousmotorbutonedisadvantageisthatitismoreexpensivethananequivalent"brushed"motordesign.
ThecontrolofthebrushlessDCmotorsisverydifferentfromthenormalbrushedDCmotor,inthatitthistypeofmotor
incorporatessomemeanstodetecttherotorsangularposition(ormagneticpoles)requiredtoproducethefeedback
signalsrequiredtocontrolthesemiconductorswitchingdevices.Themostcommonposition/polesensoristhe"Hall
EffectSensor",butsomemotorsalsouseopticalsensors.
UsingHalleffectsensors,thepolarityoftheelectromagnetsisswitchedbythemotorcontroldrivecircuitry.Thenthe
motorcanbeeasilysynchronizedtoadigitalclocksignal,providingprecisespeedcontrol.BrushlessDCmotorscanbe
constructedtohave,anexternalpermanentmagnetrotorandaninternalelectromagnetstatororaninternalpermanent
magnetrotorandanexternalelectromagnetstator.
AdvantagesoftheBrushlessDCMotorcomparedtoits"brushed"cousinishigherefficiencies,highreliability,low
electricalnoise,goodspeedcontrolandmoreimportantly,nobrushesorcommutatortowearoutproducingamuch
higherspeed.Howevertheirdisadvantageisthattheyaremoreexpensiveandmorecomplicatedtocontrol.
TheDCServoMotor
DCServomotorsareusedinclosedlooptypeapplicationswerethepositionoftheoutputmotorshaftisfedbacktothe
motorcontrolcircuit.Typicalpositional"Feedback"devicesincludeResolvers,EncodersandPotentiometersasusedin
radiocontrolmodelssuchasairplanesandboatsetc.Aservomotorgenerallyincludesabuilt-ingearboxforspeed
reductionandiscapableofdeliveringhightorquesdirectly.Theoutputshaftofaservomotordoesnotrotatefreelyasdo
theshaftsofDCmotorsbecauseofthegearboxandfeedbackdevicesattached.
DCServoMotorBlockDiagram
AservomotorconsistsofaDCmotor,reductiongearbox,positionalfeedbackdeviceandsomeformoferrorcorrection.
Thespeedorpositioniscontrolledinrelationtoapositionalinputsignalorreferencesignalappliedtothedevice.
Theerrordetectionamplifierlooksatthisinputsignalandcomparesitwiththefeedback
signalfromthemotorsoutputshaftanddeterminesifthemotoroutputshaftisinanerror
conditionand,ifso,thecontrollermakesappropriatecorrectionseitherspeedingupthe
motororslowingitdown.Thisresponsetothepositionalfeedbackdevicemeansthatthe
servomotoroperateswithina"ClosedLoopSystem".
Aswellaslargeindustrialapplications,servomotorsarealsousedinsmallremotecontrol
modelsandrobotics,withmostservomotorsbeingabletorotateuptoabout180degreesin
RCServoMotorbothdirectionsmakingthemidealforaccurateangularpositioning.However,theseRCtype
servosareunabletocontinuallyrotateathighspeedlikeconventionalDCmotorsunless
speciallymodified.
Aservomotorconsistofseveraldevicesinonepackage,themotor,gearbox,feedbackdeviceanderrorcorrectionfor
controllingposition,directionorspeed.Theyarewidleyusedinroboticsandmodelsastheyareeasilycontrolledusing
justthreewires,Power,GroundandSignalControl.
DCMotorSwitchingandControl
SmallDCmotorscanbeswitched"On"or"Off"bymeansofswitches,relays,transistorsormosfetcircuitswiththe
simplestformofmotorcontrolbeing"Linear"control.ThistypeofcircuitusesabipolarTransistorasaSwitch(A
Darlingtontransistormayalsobeusedwereahighercurrentratingisrequired)tocontrolthemotorfromasinglepower
supply.
Byvaryingtheamountofbasecurrentflowingintothetransistorthespeedofthemotorcanbecontrolledforexample,if
thetransistoristurnedon"halfway",thenonlyhalfofthesupplyvoltagegoestothemotor.Ifthetransistoristurned"fully
ON"(saturated),thenallofthesupplyvoltagegoestothemotoranditrotatesfaster.Thenforthislineartypeofcontrol,
powerisdeliveredconstantlytothemotorasshownbelow.
UnipolarTransistorSwitch
Thesimpleswitchingcircuitontheleft,showsthecircuitfor
aUni-directional(onedirectiononly)motorcontrolcircuit.A
continuouslogic"1"orlogic"0"isappliedtotheinputofthe
circuittoturnthemotor"ON"(saturation)or"OFF"(cut-off)
respectively.
Aflywheeldiodeisconnectedacrossthemotorterminalsto
protecttheswitchingtransistororMOSFETfromanyback
emfgeneratedbythemotorwhenthetransistorturnsthe
supply"OFF".
Aswellasthebasic"ON/OFF"controlthesamecircuitcan
alsobeusedtocontrolthemotorsrotationalspeed.By
repeatedlyswitchingthemotorcurrent"ON"and"OFF"ata
highenoughfrequency,thespeedofthemotorcanbevaried
betweenstandstill(0rpm)andfullspeed(100%).Thisis
achievedbyvaryingtheproportionof"ON"time(t)tothe
"OFF"time(t)andthiscanbeachievedusingaprocess
knownasPulseWidthModulation.
PulseWidthSpeedControl
TherotationalspeedofaDCmotorisdirectlyproportionaltothemean(average)valueofitssupplyvoltageandthe
higherthisvalue,uptomaximumallowedmotorvolts,thefasterthemotorwillrotate.Inotherwordsmorevoltagemore
speed.Byvaryingtheratiobetweenthe"ON"(t)timeandthe"OFF"(t)timedurations,calledthe"DutyRatio",
"Mark/SpaceRatio"or"DutyCycle",theaveragevalueofthemotorvoltageandhenceitsrotationalspeedcanbevaried.
Forsimpleunipolardrivesthedutyratioβisgivenas:
andthemeanDCoutputvoltagefedtothemotorisgivenas:Vmean=βxVsupply.Thenbyvaryingthewidthof
pulsea,themotorvoltageandhencethepowerappliedtothemotorcanbecontrolledandthistypeofcontrolis
ON
OFF
ONOFF
calledPulseWidthModulationorPWM.
Anotherwayofcontrollingtherotationalspeedofthemotoristovarythefrequency(andhencethetimeperiodofthe
controllingvoltage)whilethe"ON"and"OFF"dutyratiotimesarekeptconstant.ThistypeofcontroliscalledPulse
FrequencyModulationorPFM.Withpulsefrequencymodulation,themotorvoltageiscontrolledbyapplyingpulsesof
variablefrequencyforexample,atalowfrequencyorwithveryfewpulsestheaveragevoltageappliedtothemotorislow,
andthereforethemotorspeedisslow.Atahigherfrequencyorwithmanypulses,theaveragemotorterminalvoltageis
increasedandthemotorspeedwillalsoincrease.
Then,TransistorscanbeusedtocontroltheamountofpowerappliedtoaDCmotorwiththemodeofoperationbeing
either"Linear"(varyingmotorvoltage),"PulseWidthModulation"(varyingthewidthofthepulse)or"PulseFrequency
Modulation"(varyingthefrequencyofthepulse).
H-bridgeMotorControl
WhilecontrollingthespeedofaDCmotorwithasingletransistorhasmanyadvantagesitalsohasonemain
disadvantage,thedirectionofrotationisalwaysthesame,itsa"Uni-directional"circuit.Inmanyapplicationsweneedto
operatethemotorinbothdirectionsforwardandback.Oneverygoodwayofachievingthisistoconnectthemotorinto
aTransistorH-bridgecircuitarrangementandthistypeofcircuitwillgiveus"Bi-directional"DCmotorcontrolasshown
below.
BasicBi-directionalH-bridgeCircuit
TheH-bridgecircuitabove,issonamedbecausethebasicconfigurationofthefourswitches,eitherelectro-mechanical
relaysortransistorsresemblesthatoftheletter"H"withthemotorpositionedonthecentrebar.TheTransistoror
MOSFETH-bridgeisprobablyoneofthemostcommonlyusedtypeofbi-directionalDCmotorcontrolcircuits.Ituses
"complementarytransistorpairs"bothNPNandPNPineachbranchwiththetransistorsbeingswitchedtogetherin
pairstocontrolthemotor.
ControlinputAoperatesthemotorinonedirectionie,ForwardrotationwhileinputBoperatesthemotorintheother
directionie,Reverserotation.Thenbyswitchingthetransistors"ON"or"OFF"intheir"diagonalpairs"resultsin
directionalcontrolofthemotor.
Forexample,whentransistorTR1is"ON"andtransistorTR2is"OFF",pointAisconnectedtothesupplyvoltage(+Vcc)
andiftransistorTR3is"OFF"andtransistorTR4is"ON"pointBisconnectedto0volts(GND).Thenthemotorwillrotate
inonedirectioncorrespondingtomotorterminalAbeingpositiveandmotorterminalBbeingnegative.Iftheswitching
StepperMotor
statesarereversedsothatTR1is"OFF",TR2is"ON",TR3is"ON"andTR4is"OFF",themotorcurrentwillnowflowin
theoppositedirectioncausingthemotortorotateintheoppositedirection.
Then,byapplyingoppositelogiclevels"1"or"0"totheinputsAandBthemotorsrotationaldirectioncanbecontrolled
asfollows.
H-bridgeTruthTable
InputAInputBMotorFunction
TR1andTR4TR2andTR3
00MotorStopped(OFF)
10MotorRotatesForward
01MotorRotatesReverse
11NOTALLOWED
Itisimportantthatnoothercombinationofinputsareallowedasthismaycausethepowersupplytobeshortedout,ie
bothtransistors,TR1andTR2switched"ON"atthesametime,(fuse=bang!).
Aswithuni-directionalDCmotorcontrolasseenabove,therotationalspeedofthemotorcanalsobecontrolledusing
PulseWidthModulationorPWM.ThenbycombiningH-bridgeswitchingwithPWMcontrol,boththedirectionandthe
speedofthemotorcanbeaccuratelycontrolled.CommercialofftheshelfdecoderIC''ssuchastheSN754410QuadHalf
H-BridgeICortheL298Nwhichhas2H-Bridgesareavailablewithallthenecessarycontrolandsafetylogicbuiltinare
speciallydesignedforH-bridgebi-directionalmotorcontrolcircuits.
TheStepperMotor
LiketheDCmotorabove,StepperMotorsarealsoelectromechanicalactuatorsthatconvertapulseddigitalinputsignal
intoadiscrete(incremental)mechanicalmovementareusedwidelyinindustrialcontrolapplications.Asteppermotoris
atypeofsynchronousbrushlessmotorinthatitdoesnothaveanarmaturewithacommutatorandcarbonbrushesbut
hasarotormadeupofmany,sometypeshavehundredsofpermanentmagneticteethandastatorwithindividual
windings.
Asitnameimplies,asteppermotordoesnotrotateinacontinuousfashionlikea
conventionalDCmotorbutmovesindiscrete"Steps"or"Increments",withtheangle
ofeachrotationalmovementorstepdependantuponthenumberofstatorpolesand
rotorteeththesteppermotorhas.
Becauseoftheirdiscretestepoperation,steppermotorscaneasilyberotatedafinite
fractionofarotationatatime,suchas1.8,3.6,7.5degreesetc.Soforexample,lets
assumethatasteppermotorcompletesonefullrevolution(360inexactly100steps.
Thenthestepangleforthemotorisgivenas360degrees/100steps=3.6degrees
perstep.ThisvalueiscommonlyknownasthesteppermotorsStepAngle.
Therearethreebasictypesofsteppermotor,VariableReluctance,Permanent
MagnetandHybrid(asortofcombinationofboth).AStepperMotorisparticularlywell
suitedtoapplicationsthatrequireaccuratepositioningandrepeatabilitywithafast
responsetostarting,stopping,reversingandspeedcontrolandanotherkeyfeatureof
thesteppermotor,isitsabilitytoholdtheloadsteadyoncetherequirepositionisachieved.
Generally,steppermotorshaveaninternalrotorwithalargenumberofpermanentmagnet"teeth"withanumberof
electromagnet"teeth"mountedontothestator.Thestatorselectromagnetsarepolarizedanddepolarizedsequentially,
causingtherotortorotateone"step"atatime.
Modernmulti-pole,multi-teethsteppermotorsarecapableofaccuraciesoflessthan0.9degsperstep(400Pulsesper
Revolution)andaremainlyusedforhighlyaccuratepositioningsystemslikethoseusedformagnetic-headsin
floppy/harddiscdrives,printers/plottersorroboticapplications.Themostcommonlyusedsteppermotorbeingthe200
stepperrevolutionsteppermotor.Ithasa50teethrotor,4-phasestatorandastepangleof1.8degrees(360
degs/(50x4)).
StepperMotorConstructionandControl
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Inoursimpleexampleofavariablereluctancesteppermotorabove,themotorconsistsofacentralrotorsurroundedby
fourelectromagneticfieldcoilslabelledA,B,CandD.Allthecoilswiththesameletterareconnectedtogethersothat
energising,saycoilsmarkedAwillcausethemagneticrotortoalignitselfwiththatsetofcoils.Byapplyingpowerto
eachsetofcoilsinturntherotorcanbemadetorotateor"step"fromonepositiontothenextbyanangledeterminedby
itsstepangleconstruction,andbyenergisingthecoilsinsequencetherotorwillproducearotarymotion.
Thesteppermotordrivercontrolsboththestepangleandspeedofthemotorbyenergisingthefieldcoilsinaset
sequenceforexample,"ADCB,ADCB,ADCB,A..."etc,therotorwillrotateinonedirection(forward)andbyreversing
thepulsesequenceto"ABCD,ABCD,ABCD,A..."etc,therotorwillrotateintheoppositedirection(reverse).
Soinoursimpleexampleabove,thesteppermotorhasfourcoils,makingita4-phasemotor,withthenumberofpoles
onthestatorbeingeight(2x4)whicharespacedat45degreeintervals.Thenumberofteethontherotorissixwhichare
spaced60degreesapart.Thenthereare24(6teethx4coils)possiblepositionsor"steps"fortherotortocompleteone
fullrevolution.Therefore,thestepangleaboveisgivenas:360/24=15.
Obviously,themorerotorteethandorstatorcoilswouldresultinmorecontrolandafinerstepangle.Alsobyconnecting
theelectricalcoilsofthemotorindifferentconfigurations,Full,Halfandmicro-stepanglesarepossible.However,to
achievemicro-stepping,thesteppermotormustbedrivenbya(quasi)sinusoidalcurrentthatisexpensivetoimplement.
Itisalsopossibletocontrolthespeedofrotationofasteppermotorbyalteringthetimedelaybetweenthedigitalpulses
appliedtothecoils(thefrequency),thelongerthedelaytheslowerthespeedforonecompleterevolution.Byapplyinga
fixednumberofpulsestothemotor,themotorshaftwillrotatethroughagivenangleandsotherewouldbenoneedfor
anyformofadditionalfeedbackbecausebycountingthenumberofpulsesgiventothemotorthefinalpositionofthe
rotorwillbeexactlyknown.Thisresponsetoasetnumberofdigitalinputpulsesallowsthesteppermotortooperatein
an"OpenLoopSystem"makingitbotheasierandcheapertocontrol.
Forexample,letsassumethatoursteppermotorabovehasastepangleof3.6degsperstep.Torotatethemotor
throughanangleofsay216degreesandthenstopagainattherequirepositionwouldonlyneedatotalof:216
degrees/(3.6degs/step)=80pulsesappliedtothestatorcoils.
TherearemanysteppermotorcontrollerIC''savailablewhichcancontrolthestepspeed,speedofrotationandmotors
direction.OnesuchcontrollerICistheSAA1027whichhasallthenecessarycounterandcodeconversionbuilt-in,and
canautomaticallydrivethe4fullycontrolledbridgeoutputstothemotorinthecorrectsequence.Thedirectionofrotation
canalsobeselectedalongwithsinglestepmodeorcontinuous(stepless)rotationintheselecteddirection,butthis
putssomeburdenonthecontroller.Whenusingan8-bitdigitalcontroller,256microstepspersteparealsopossible
SAA1027StepperMotorControlChip
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InthistutorialaboutRotationalActuators,wehavelookedatthebrushedandbrushlessDCMotor,theDCServo
MotorandtheStepperMotorasanelectromechanicalactuatorthatcanbeusedasanoutputdeviceforpositionalor
speedcontrol.InthenexttutorialaboutInput/OutputdeviceswewillcontinueourlookatoutputdevicescalledActuators,
andoneinparticularthatconvertsaelectricalsignalintosoundwavesagainusingelectromagnetism.Thetypeofoutput
devicewewilllookatinthenexttutorialistheLoudspeaker.
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