a simple shaft encoder for modified rc servos

7/31/2019 A Simple Shaft Encoder for Modified RC Servos

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MOTOR

A Simple Shaft Encoder for Modified RC ServosRussianversion

ModiiedRCservosareperhapsthemostpopularmotorsusedinhobbyrobotics.Nootherkindofmotorhas

similaravailability,priceandpossibilities.Someusethemunmodiied,someonlymodifythemforcontinuous

rotation(whichissometimescalledhalf-lobotomy),butthebestcontrolbecomespossibleifthemotorssuffer

fulllobotomyirst-whenyouremoveeverythingexceptthemotorandgears.OfcourseaproperH-bridge

andcontrolcircuitisnecessaryinthiscase.

Foranyadvancedtaskthatinvolveswheelrotationitisnecessarytohavesomefeedbackfromthewheels.

Thecontrolprogrammustknowhowfarthewheelrotatestoadjustrotationspeedofdifferentwheels,orjust

tobesurethatthevehicleisstandingstillandnotbeingpulledsomewherebyanunknownforce.MyAkipaki

robotneedsthemtobeabletomoveinastraightlineandtobeabletodetectthatthemaincontrolloopis

undercompensatingthefallanditkeepsrollinginsteadofstandingorrockingaroundoneplace.

Therearedifferenttypesofshaftencodershobbyiststendtouse.I'veseenalotofdesignswhereapatternis

printedoutonthewheelitselfandaphotosensingcircuitismountedsomewhereonthechassis-thiskindof

designseemsweirdtome,afterallyoudon'tcarryyourgutsaround,youkeeptheminthebelly.Somemore

advanceddesignsimplementquadratureencoders,sometimesevenittedinsideofanoriginalgearhead

motorchassis.Althoughquadratureencodersgivehigherresolutionandallowtodetectthedirectionofspin,

theyusuallydependheavilyonrarelyavailableparts,becausethecircuitisboundtobeextremelysmall.Also,

quadratureencoderisnotrequiredincaseofamodiiedservobecausethegearreductionratioissohighthat

it'salmostimpossibletoturnthemotorbyrotatingtheoutputshaft.Wecanverysafelyassumethatthe

directionofrotationalwaysfollowscontrolandifitdoesn'twecouldn'tpossiblyhelpitanyway.

AdesignthatIfoundtobemostsuitableformyneedsbelongstoAlanMacek.Heusedservosverysimilarto

thosethatIhaveandheimplementedaverysimpleshaftencoderusingonlyonepreciousunavailablepart.

Thefollowingdesignisafurtherdevelopmentofhisideas,there'sverylittleoriginalinmyproject.Iuseno

unavailablepartsbecauseifididuseany,I'dnotbeabletomakeit.IfyouhavenoproblemsindingQRB1114

ortheirlikes,justfeelfreetoentertainyourselfbywatchingmesuffering.

Icouldnotindanyofsooftenmentionedonthe

netconvenientandusefulintegratedIRemitter-

receiverpairsspeciallydesignedforrelectivesensors.NeithercouldIindanypartsofthis

sortthathaveintegratedampliiersinthem.All

thatIhadavailablewereregularIRemittersand

phototransistors.SoIhadtoadoptthecircuita

littlebittomakeauseofwhatIhave.Hereismy

circuit.Itexplicitlyhaspoweriltertoavoidhard

noisethatmaybecausedbylongwires,

interferenceandwhatnotisthereinamotor.R4

is680ohminmycase,butitmustbechosen

experimentallyforyourIRemitter/receiver

combination.Italsoheavilydependsonthekind

ofnon-relectoryou'reusing(moreonthat

later).Alternatively,sensitivitycanbechanged

byalteringR1/R2,butIhaven'ttriedthat.

Providedyou'recapableofmakingexperiments

toindsuitablevalues,anykindof

emitter/transmittercanbeused-pickthe

cheapestonesyourretailerhastooffer.And

sincetheentirethingwillsitinsideofablack

case,it'snotnecessarytousepartswithvisible

lightilters.

Fig.1 Circuit (click)

Idesignedatemplatefor

PCBthatwoulditinsideof

aservomotor.

UnfortunatelyIthrewthe

originalservobrainsout

(aftereatingthemost

deliciouspartsofcourse),

soIwasn'tsureoftheir

mple Shaft Encoder for Modified RC Servos http://sensi.org/~svo/shaft_encoder/

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Fig.3 Just etched

Fig.5 Harness

Fig.2 Phototemplate (click for PDF)

originalsize.Butthis

designitsinprettywell,

withahelpofaileanda

hammeranyway.Youcan

usetheprovidedPDFileto

makeyourownPCB's.Just

liketheonepicturedright.

ThisboardissosmallthatI

couldnotindplacetoput

anyobnoxioustextsonit...

N.B.TheboardsImade

havea(non-fatal)error,a

trainedeyewillspotthem

immediately.Thetemplate

givenherehasthiserror

corrected.

Fig.4 Populated board

Here'swhatapopulatedboardlookslike.Ididn'twashit

aftersolderinganddebugging,soitmaylooknotasfancyas

afactorymadeone,butitdeinitelylooksauthentic,

especiallywithmismatchedsizesofsalvagedSMDcomponents.Notethatalthoughthisboardissingle-sided,

thereversesideisstillpresentandisusedagroundplane.It

isnecessaryforinterconnectionsanditalsoservesasanEMI

shield.Onthispictureitisvisiblethatsmallsquareofthe

backplane(totheleft-top)isremovedtoavoidpossibleshort

circuitofwires.

Itisnowtimetothink

abouthowtomount

discretepartsintothe

casingofaservo.Ifwe

hadonesender-receivermodule,we'd

justcutaholeinthe

caseandglueitin

somewhere.Butwith

twoseparateparts,itis

reallyimportantto

keepthemaligned

together.Iusedsomeof

remainingcutsof

copper-laminated

iberglasstomakea

harness.Youwould

havetomaketwo

markingsatproper

distancestoittwo

3-mmled-likethings,

thendrillverysmall

holesforalignment,

thentakeyourbigdrill

andmake3mmholesto

itintheledandthe

receiver.Imadethe

holessothatthe

emitterandthe

receiverarelookingtowardseachother,kind

ofsimulatingthem

being"focused"toa

certaindistance.But


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Fig.7 Encoder wheel

I'mreallynotsureif

thisworksand

importantatall.The

emitterledseemstobe

shiningprettymuch

omindirectionallyata

distancethissmall.

Aftersomesimple

testing(thecircuit

outputmustgodownfromhighwhen

somethingrelectiveis

nearthesensor)Iglued

bothcomponentstothe

harnessandsoldered

shortwirestoattach

themtothePCB

permanently.

Fig.6 Sensor window

Ontheleft

pictureyou

canseea

windowinthecasing,

thesensor

will"look"

throughiton

thegearwith

interrupter

(thisisa

viewwith

reductor

gears

removed,but

youmust

knowthatif

youwent

thisfarinto

servo

anatomy).

Theright

picture

showsthe

bottomside

ofthe

"encoder

wheel".Therealbeneit

ofthisdesign

isthatthe

encoder

wheelis

reallysimple

anddoesn't

require

complicated

printed

patterns

(although

youcantrythat,Idon't

recommend

becausethis

wheelis


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reallysmall,I

barelycould

photograph

thepest).

Youcansee

tracesofall

kindsof

torturesI

triedonthis

poorgear.Intheendof

thisordealI

went

completely

medieval

andglued

ontoitatiny

patchof

insulation

tapemadeof

pitchblack

fabric.Paintingthe

gearwith

marker

didn'tgive

enough

contrast.

Evenglueing

apieceof

paperall

blackened

withapen

didn'tgive

enoughcontrast-at

distancethis

close

everything

exceptpitch

blackblastic

isvery

relectivefor

IR.Actually

glueinga

tinybitof

plasticfrom

SMDreel

packaging

wouldwork

too.

Fig.8 Gooed in

Leftpicture

showshow

theharness

isitted

insideofthe

servocasing.

Iusedsome

kindof

thermalglue.It'snot

exactlythe

kindusedin

gluepistols,


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Fig.9 Peek-a-boo

Fig.11 Fat boy

it'snot

transparent

andmessy

andsmells

funny.Any

gluewilldo

though.

Rightpicture

showshowthe

makeshift

sensoris

peeking

throughthe

peepholein

thecasing.

Justalittle

moretime

andallitwill

everseewill

bethe

torture

encoder

wheel.Har.

Fig.10 Packing

Everything

mustbe

clearfrom

nowon.I

hadtocut

theboarda

littleonthe

sidestolet

thescrews

itin.

Protectedit

withtwo

layersof

insulation

tapetoavoid

anycontact

withthe

motor,then

itteditin.

The

capacitor

wasthemostprotruding

partonthe

board,Ieven

hadtocuta

littlewindow

inthecover

forittoo.But

still,the

boardisa

littlethicker

thanthe

casing

allows,afterIscrewed

backthe

coverthe

servolooksa


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little

oversatiated.

Withthis

reductor

(theoriginal

servowas

Hi-Tec

HS-311,they

sayit'sthemost

unmodiiable

servoever

made,butI

don't

understand

why-I

modiiedthe

helloutofit

anditstill

works),Iget

42pulses

peroutput

shaft

revolution.

Theduty

cycleisnot

perfectly

50%asyou

mightguess

bylookingat

theencoder

wheelIuse.

Numberof

pulsesperrevolution

canbe

increasedat

least

two-foldby

adding

another

nonrelective

stripeof

blackgoo.

Conclusion:thisdesignisrelativelysimpletoimplementanditworkswiththemostunmodiiableservoin

theworld.Numberofpulsesperrevolutionhassacredmeaningwhichcannotbeabadthing.Afterthevalue

ofcurrent-limitingresistorforcertaintypeofemitter/receiverisfound,theentirethingmustbeeasyto

reproduceforasmanymotorsasneeded.Nohard-toindorexpensivecomponentsarenecessary,andthe

entiremodiicationitscompletelyintotheoriginalservocase.

* * *

2005 Viacheslav Slavinsky. Please do not republish without permission. Send all comments and criticisms to svofski a t gmail dot

c0m.


a simple shaft encoder for modified rc servos

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