APPLICATION NO T E
Figure 5 :A realmotor. Multiplepolesare norma-lly employedtoreducethe stepangleto apracticalvalue.Theprincipleof opera-tion and driveseq uencesremainthe same.
Figure6 :A unipolarPMrrotorusesMilar win-dingstoreversethe flux ineach phase.
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VARIABLE RELUCTANCEMOTORS
Avariablereluctancemotor has a non-magnetized soft iron rotorwithfewer polesthanthestator (fig-ure7).Unipolardriveis used and the motoris step-pedbyenergizing stator pole pairs toalignthe rotor with thepole piecesofthe energizedwinding.
Onceagain threedifferentphasesequercescanbe used.The wave drivesequence isNCIBID;
two-phase-on isAClCBlBDIDAand thehalfstep se-quence is NAC/C/BCI8JBO/OtDA. Note that the stepangleforthemotorshownabove is15°,not450.
As before.pratica l motor snormally employ multiple poles to giveamuch sma llerstepangle.Thisdoes not, however,affect theprlrc oleof operationof the dri ve sequences.
Figure7 :Avariab lereluctance motor has asoft iron rotorwithfewerporesthan the sta-tor.The stepangleis15°forthis motor.
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c
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GENERATINGTHE PHASE SEQUENCES The heartof the l297 bloc k diagram,figure8,is
a
bloc k calledthe tran slator whichgeneratessuilable phase sequences for half step,one-phase-onfuU step and two-phase-on full step operation.This blockis controlled by
two
modeinputs- directionICWICCW)and HALFIFULL - andastep clock whic hadvances the translatorfromone step to the next .
Fouroutputsareprovdedbythetrans latorfor sub-sequen tproce ssing by theoutputlogicblockwhich implementsthe inhibitandchopper functi ons. Internal lythetranslator consists ofa 3-bitcounte r plussomecomb inatio nallo gic which gener ates a basic eight-stepgra y code sequence asshow n in figure9.An threedrive sequencescanbegenerated easilyfrom this master sequence.This state se-quencecorrespondsdirecttyto half~mode.
se-lectedbya high level on theHALFIFULL irout.
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51'8
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-Chapter 2 / El ectroni cs
APPLICAnON NOTE
The outputwaveformsforthis sequenceareshown in figure1O.
Notethattwoothersignals,
TNR1
andINH2 are ge-neratedin this sequence.The purposeof these si-gnalsis explained a little furtheron.Thefull ste p mode sare bothobtainedbyskipping alternate states intheeig ht-step seq uence.What happensisthatthestepclockbypassesthe first sta-ge of the 3-bilcounterin the translator.The least si-gnificantbit 01thiscounteris not affectedtherefore
thesequencegenerateddependsonthestate ofthe translatorwhenfullstep mode is selected (the HAlFIFULLinputbroughtlow).
Iffull stepmodeisselected whenthe translatorisat anyodd-numberedstatewe getthetwo-phase-on fullstepsequence shown in figure11.
Bycontrast ,one-phase-on full step modeis obtai-nedby selectingfullstepmode 'When thetranslator is at an even-numberedstate (figure 12).
Figure8 :TheL297 containstranslator(phase sequencegenerator),a dualPWM chopper andoutput controllogic.
Figure 9:Theeightstep mastersequence ofthe translator.Thiscorresponds tohalfstepmode.
Clockwiserotationisindicated.
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APPLICATION NOTE
Figure 10 :The outputwaveformscorrespondingtothe half step sequence.The chopperactionin not shown.
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Figure11:Statesequenceand outputwaveformsfor thetwo phaseon sequence.INHl andINH2 remainhighthroughout.
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L...-_I"""l..--~
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0--- _
~ 0 _
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-Chapter 2 / El ectr onics
APPLICATIONNOTE
Fig ure 12: State Sequence and Output Waveforms for WalleDrive (one phase on).
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Inhalfstep andone-phase<ln fullstep modes
two
othersi gnals aregeneraled:INHI ancIINH2.These are inhibit signalswhicharecoupled tothe 1298N's enableinputs andserve to speedthe currentdecay when a windingis switchedoff.
Sinceboth windings areenergized continuously in two-phase-onfull step modeno windingis ever swit-ched off andthese signalsare not generated.
To seewhat these signals do let's lookatone half ofthe L298Nconnectedto thefirslphase of a
two-phasebipo la r motor(figure 13).Reme mber thatthe L298N's A andBnputs determinewhichtransistor in eachpush pull pair willbeon.INH1,on the other hand,turns off all fourtransistors.
Assume that A is high,B low and current flowing through 01, 04 and the motorwinding. If A isnow brought low the current would recirculate through 02,04 and Rs. giving a slow decay andincreased dissipationin Rs.If,onaotherhand, Ais brought b w andINH1is activated,all fourtransistorsareturned off.The currentrecirculatesinthiscase fromground to Vs viaD2 and03, givingafasterdecaythus al-lowing faster operation ofthemotor. Also,sincethe recirculationcurrentdoes notflowthroughAs,aless expensive resistor can be used.
Exactlythe samethinghappenswiththesecond winding,theother half of theL298and thesignals C,D andINH2.
TheINHl andINH2 signals are generated by OR functions:
A+B=INH l C+ D=INH2 However,theoutput logic is morecomplexbecause inhibitlines are alsousedbythe chopper, aswe will seefurther on.
OTHERSIGNALS
TIM) other signals are connected to the translator block : theRESETinputandtheHOMEoutput RESETisanasynchronousreset inputwhich resto-resthetranslatorblocktothe homeposition(state 1,ABCD=0101).TheHOMEoutput(open collec-tor)signalsthis conditionandis intendedtothe AN-Ded wlh the output of a mechanical
rome
positionsensor.
Fnally, thereis anENABLE iflJut connected to the output logic.A low level onthis input bringsINH1, INI-l2,A,B, C and D low.Thisinputisuseful to di-sable therrotor driverwhen the system is initialized.
LOAD CURRENTREGULATION
Someform ofloadcurrentcontrolis essential to ob-taingoodspeedandtorquecharacteristics.There are severalways in which this can be done- swit-ching thesupplybetweentwo voltages, pulse rate modulationchoppingorpulse width modulation chopping.
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APPLICATIONNOTE Figure13 :Whena windingis switchedoffthe inhibitinputis activated to speed currentdecay. Ifthis
were notdonethe currentwouldrecirculatethrough02 and04inthis example. Dissipation inRs isalsoreduced.
TheL297 providesloadcurrentcontrolintheform oftwoPWMchoppers,oneforeach phase ofa bi-polarmatororonefor eachpairef windingsforauni-polar motor. (Ina unipolarmotorthe A and 8 wirdings areneverenergizedtogether so thaycan sharea chopper;the same applies to CandO).
Each chopper consists of a comparator,aflipflop and an extemal sensing resistor.Acomrron on chip oscillatorsuppliespulsesat the chopperrateto both choppers.
Ineachchopper(figure14) the tl~flopisset by each pulsefrom the oscillator,enablingtheoutputand al-lowing theload currenttoincrease.Asit increases the voltageacrossthesensingresistorincreases, and whenthisvoltagereachesVrefthe flip flopis re-set,disablingthe output untilthenextosciilator pul-searrives.Theoutputof this circuit(theflipflop'sQ output)istherefore a constantratePWM signal.
Note that Vrefdetermines the peak load current.
Figure14:Eachchoppercircuil consists ofa comparator, flipflop and external sense resistor.A commonoscillatorclocks