APPLICATION NOTE
PHASE CHOPPING AND INHIBIT CHOPPING
First let's examinethe situationwhenthe phase li-nes are chopped.
The choppercan act on eitherthe phase lines (ABCD) oronthe inhibitlinesINHl andINH2.An in-put namedCONTROL decides which. Inhibitchop-pingis used for unipolar motorsbut you can choose between phasechopping and inhibitchoppinqforbi-polarmotors.The reasons for this choice are best explainedwith another example.
age on the windingis low(Vcsset01+VD3)(figure 16).
WhyisBpulledhigh,whypushAlow ? The reason is to avoidthe current decayingthrough Rs. Since the currentrecirculatesin the upper half of the brid-ge,current onlyflows in the sensingresistorwhen thewindingis driven.Less poweris theretoredissi-pated in
Rs
andwe can get awaywithacheaperreo
sistor.This explain why phasechoppingis not suitablefor unipolar motors:whenthe A windingis driventhe chopperactsonthe B winding.Clearly,thisis no use at all for a variablereluctance motorand wouldbe As before,we are drivinga
two
phase bipolarmotor slowandinefficient for a bifilar wound permanent and Ais high,Blow (figure15).Currenttherefore magnet motor.flowsthrough 01,winding,04 and Rs.When the The alternativeis to tiethe CONTROL input to voltage across Rs reaches Vrefthe chopperbrings ground so that the chopper acts onINH1 and INH2.
B high to switch off the winding. Looking at the same example,A is high and Blow.
Theenergy stored in the windingis dissipatedby 01 and 04 are thereforeconductingandcurrent currentreci rculatingth rough01and03. Currentde- flows through 01,the winding,04 and Rs, (fig-cay throughthis pathis ratherslowbecausethevolt - ure 17).
Fig ure 15: Phase Chopping. Inthis examplethe currentXis interrupted by activatingB, givingthe recir-culationpathY. The alternative, de-activatingA, would give the recirculationpathZ,increasing dissipationin
As.
CNC Roboti cs
APPLICATION NOTE
Figure 16 :PhaseChoppingWaveforms.Theexample showsASwinding energizedwith Apositive with respecttoB.Controlishigh.
A
SET BYosc.. FUPBY COMPARATORFLOP RESET
SlOWISH DECAY Tt1NOUut1TI ANDoJ
Figlre17:Inhibit Chopping.The drive current(01,wirding, 04)in this case isintemptedbyactivating INH1. The decay path through02 and 03 isfaster thanthe path Y ofFigure15.
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Chapter 2 / E lectron ics
In thiscase whenthevoltage seeress Asreaches VREF the chopperflipflop is resetand INH1activated (broughtlow). lNH1,remember,turns off allfour tra ns istors thereforethe current rec irculatesfro m ground,through 02,the winding and 03to Vs. Di-schargedacrossthe supply,whichcan be up to 46V, the currentdecaysvery rapidly(figure18).
The usefulnessof this second faster decayoptionis fairlyobvious;itallowsfastoperatio nwith bipolar motorsanditis the onlychoiceforunipolarrroto rs.
But why dowe offer theslower alternative,phase chopping?
The answeris thatwe might be obligedtouse alow chopper ratewitha motorthat does not storemuch energyin the windings.Ifthe decayis veryfast the average motorcurrent maybetoo lowtogivean usefultorque. Low chopperrates may,for example, beirrpo sedifthereis alarger rrotor inthe same sy·
stem. To avoidswitchingnoiseon the ground plane alldriversshouldbesynchronizedand the chopper rate is therefore determined by the largestmotor in the system.
Multiple L297saresynchronizedeasilyusingthe SYNC pin.This pin is the squarewave output of the on-chip oscillatorandthe clock inputfor the chop-pers.The firstL297isfittedwith the oscillator com-ponentsa nd outputs a sqarewavesignalonthispin (figure 19).SubsequentL29 7sdonot needtheoscil-lator components and use SYNC as a clock input.
An externalclockmay alsobeinjected atthis termi-nalifan L297 must be synchronizedtoothersystem components.
Figure 18: InhibitChopperWaveforms.Winding ABis energizedand CONTROLislow.
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Figure 19: The Chopperosci1latorof multiple L297sare synchronizedby connecting
TheL297A is a specialversionof theL297 develo-pedoriginally forheadpositioning in floppy disk dri-ves. It can,however,be used inotherapplications.
Comparedtothe standardL297thedifferenceare the addition of a pulse doubleronthe step clock in·
put and the availability oftheoutput of the direction flipflop(block diagram,figure20).Toadd these fun-ctions whilekeeping the low-cost 20·pinpackage the CONTROL and SYNC pinsare notavailable on this version (theyarenote neededanyway). The chopperacts on the ABCD phase lines.
The pulse doubler generates aghostpulse internal-Iyfor eachinputclock pulse.Consequentlythe tran-slator moves two steps foreachinput pulse.An external AC networksets thedelay timebetween the inputpulseand ghostpulseand should be cho-sen so that the ghostpulsesfall roughlyhalfway betweeninput pulses,allowing timefor the rrotorto step.
This feature is usedtoimprove positioningaccura-cy. Sincethe angularposition error ofast eppermo-tor is noncumulative(itcancelsoutto zeroeveryfour stepsinafourstep sequence motor)accuracyis im-provedby steppingtwo offourstepsata time.
l.
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