An 01 Electrical Encoder 1

8/2/2019 An 01 Electrical Encoder 1

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,Jan2009

isionMoti

ustrialPark

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ation

coder

nSensors

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comcision.com

Note

;DSpro

Ltd.

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2/7www.netzerprecision.com AN01,TheElectricEncoder 2/7

1. TheElectricEncoderaperspectiveThepatented,stateofart,ElectricEncodertechnologyprovidesamixofperformancefeatures

unmatchedbyanycompetingtechnology,optical,magneticorotherwise.TherotaryElectric

Encoderis

implemented

using

either

of

two

topologies:

3

plate

and

2

plate,

both

include

a

space/timemodulatedelectricfieldinsideashieldedspace.Thetotalfieldisintegratedand

convertedintoasignalcurrentwhichisprocessedbyonboardelectronicstoprovideDCoutput

signalsproportionaltothesineandcosineoftherotationangle.Inthe3platetopology,shownin

Figure1aadielectricrotorwhoserotationangleinfluencesthefieldbetweenstationary

transmitterandreceiverplates.Inthe2platetopology,showninFigure1b,thefieldisconfined

betweenastationarytransmitter/receiverplateandaconductivelypatternedrotor.

TheElectricEncoderhastwooperationmodesselectablebyalogiclevelcommand;TheCoarse

modehasMsine/cosineperiods/revolution,andgeneratesacorrespondingnumberofElectrical

Cycle/Revolution(EC/R).Thismodeisaccurateenoughtohelpidentifyonsystemturnon in

combinationwiththeFinemodedata,theinitialabsoluteposition.TheFinechannelhasNEC/Rs

andprovidestheaccuracyandresolutionofthecomputedabsoluteangledata.Mtypicallyranges

fromoneto7andNfrom16to128 dependingonthespecificdesign.Inordertoprovidean

unambiguousreadingover360mechanicaldegrees(i.e.,eachangularpositionhasauniquepair

ofFineandCoarseelectricalangles)MandNhavenocommondenominator.Inpracticethe

coarsemodeisneededonlyuponsystemturnon,afterwhichtheencoderispermanently

switchedto

the

Fine

mode.

Signal

conditioning

is

based

on

digitizing

and

processing

the

output

analogsignals,usingfactorysuppliedcodeandprovideshighaccuracyabsoluteposition

informationseeapplicationnotesAN02,AN03.

-1.5

-1

-0.5

0

0.5

1

1.5

1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253 265 277 289 301 313 325 337 349 36

Figure 2

Figure

1(a)

(b)



Figure2illustratestheoutputsignalsanElectricEncoderwithM=1andN=16,asafunctionof

therotationangle.ComparedtoanopticalSin/CosencodertheElectricEncodergenerates

muchlessEC/Rs,however,theSine/Cosineoutputsignalsarenearperfect,resultinginhigh

accuracyandresolution.OtheradvantagesoftheElectricEncoderare:

Hollow,

floating,

(no

bearings)

shaft

Verylowpowerconsumption

Immunitytomagneticandelectricinterference

Lowprofile

Highmechanicaldurability

Hightolerancetomechanicalinstallationerrors

Widetemperatureoperationrange

Simplemechanicalandelectricalinterface

2.TheHolisticrotor

ManyoftheadvantagesoftheElectricEncoderresultfromitsholisticrotor seeFigure3.

Unlikeinotherencoders,thewholeareaoftheElectricEncoderrotorparticipatesinsignal

generation,i.e.,multiplespatialperiodsareintegrated;thisresultsintwopowerfulmechanisms:

Figure 3

1.Geometricalcompensation eachtwoopposingregionsoftheencoderreactoppositelytotilt

andtranslationoftherotorplaneshownbywhitearrows,theElectricEncoderisalso

insensitiveto

axial

motion

shown

by

the

red

arrow;

it

thus

approaches

closer

than

any

other

encodertheidealinbeingsensitivetorotationonly.Onepracticaloutcomeisthat,unlike

opticalencoderswithcomparableaccuracy,nointernalballbearingsareneededandtherotor

canbedirectlymountedonthehostshaftwithoutflexibleshaftcouplingorsoftstatormounting.

Thisuniquefloatinghollowshaftresultsinthelowestpossibleaxialspacerequirementsbutalso

enhancereliabilitybyeliminatingthemajorlongtermdegradationmechanism.Figure4illustrates

thesignificantlylowererrorinducedbyrotoreccentricityoftheElectricEncodercomparedto

anopticalencoderofthesamediameter.



Figure 4

2.Averaging

the

output

signals

are

the

integrated

sum

of

multiple

periods,

any

effect

due

to

geometricalerrors,temperaturevariations,contamination,etc,tendstoaverageout,in

proportiontothenumberofpoles.

3.Commonsignalprocessingchain therotationinducedsignalsareprocessedinananalog

channel shownasablockdiagraminFigure5wheremostoftheblocksarecommontotheSine

andtheCosinesignalsandthenseparatedbyunitygaindemodulatorsandfilteredbyunitygain

lowpassfilters.Thetwooutputamplitudesarethereforetightlymatched,irrespectiveof

componenttolerances,ortemperatureinfluence.Inaddition,sincetheprocessedsignalsareAC,

offsetvoltagesarenearlyeliminated.

Figure 5



3. EnvironmentalcompatibilityFigure6illustrates(inred)thebuiltinelectrostaticshieldoftheElectricEncoder,theshield

comprisesgroundlayersandagroundedconductivelabyrinthwhichallowsmechanical

attachmenttotherotorwhileblockingpotentialparasiticcouplingfromthehostshaft.The

physicsoftheElectricEncoderprovidesinherentinsensitivitytomagneticfields;thereforenomagneticshieldisrequired,evenwhenmountedinsideelectricmotors.TheElectricEncoder

alsohasvirtuallynomagneticsignature;acriticalfeatureinspecialapplications.

AnotheruniquefeatureoftheElectricEncoderisthelowfrequencyrange(typicallyunder1

kHz)oftheoutputsignals.ThisisduetothelownumberofEC/Rs,andenablesthesignalstobe

carriedoverlongdistances,eveninnoisyenvironments,sincecoupledinterferences,suchas

PWM,areinmostcasesofmuchhigherfrequenciesandcanbeeasilyfilteredoutpriorto

digitization.

TheElectricEncoderisunusuallyimmunetocontaminantsandmoistureconditionsandhas

proventoperformnotonlyimmersedinoilbuteveninhumidandcondensingconditions.This

immunityisespeciallynoticeableinthe3platetopologywhich,inadditiontohavingaholistic

rotor,has

athree

dimensional

construction

which

is

largely

uninfluenced

by

foreign

material

deposit.

AsaresultofitsrobustconstructiontheElectricEncodernotonlysurvivesextremeshockand

vibrations,butactuallyperformsunderharshconditions.Thetemperaturestabilityoftheoutput

signalsresultsfromitsconstructionmaterials,theholisticrotor,andtheuniquesignal

conditioning,asdescribedabove.Theencodercanbedesignedtooperatefromcryogenic

temperatureupto125Candabove.Figure7illustratesatypicalplotofthereadoutangleofa

premiumDS58encodercycledovertemperature.

Figure 6



Figure

7

4.Comparisonwiththeresolver

TheoutputsignalsoftheElectricencoderareproportionaltothesine/cosineoftherotation

angle,i.e.,theyareDCsignalsatanyfixedangleandaresinusoidalonlyatconstantrotation

speeds.ThisisincontrastwiththeresolverwherethesignalsaremodulatedonanACcarrier

whosefrequencyislimitedbytheinductancetotypically10kHz;theeffectiveservobandwidthis

usuallyfurtherlimitedbytheResolvertoDigitalConverter(R/DC).Theexcitationfrequencyofthe

ElectricEncoderisnearlyunlimited,whichresultsinpotentiallyunlimitedservobandwidth 1

kHzinthestandardproducts,andmuchhigherincustomizedversions.Thefollowingtable

comparesthe

two

technologies.

Parameter Resolver Electricencoder

1 Operatingtemp.range 55to+150 55to+125

2 Weight/Diameter Larger Smaller

3 Profile Larger Smaller

4 Rotor Active Passive

5 Electricallyfloatingrotor Addsaxiallength Inherent

6 Sensitivitytomagneticfield

Onlyifshielded Inherentlyinsensitive

7 Powerconsumption Severalwatts Typically30mW

8 Mountingtolerance Relativelytight Relativelyloose

9 Powersupply AC DC

10 Cost/performance Higher Lower

11 Accuracy/diameter Lower Higher

12 Servobandwidth Medium High

13 Absolutepositionoutpu Yes Yes

14

Number

of

wires

6

6

15 Redundancyoption Yes Yes



5 ElectricalEncoderoutputsTheDSproductlineprovidesSine/CosineoftheCoarseandFinesignals(electricangle)however,severaladditionaldigitaloutputsavailablebyselectedmodelsexecutedbyexternalinterpolatoror

onboardversions.

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An 01 Electrical Encoder 1