Lecture Lab2 PartB

7/29/2019 Lecture Lab2 PartB

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Lab2,PartB:RobotDCMotor

ECEN2270 1ElectronicsDesignLaboratory

Speedmeasurementcircuitdesign


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Discussion

Comepreparedtothelab Reviewlectureslides,preparetoaskquestions

Reviewlabtasks

LTspice simulationscanbecompletedoutsidethelab

FinishPartA:LTspice simulationsStartPartB:paperdesignandLTspice simulations,before youstartbuildingyourcircuit

Inthelab

Attendanceismandatory

Donotrushthroughexperiments,stopifyouhavequestionsaboutwhyorwhatyouaredoing

Reports Donotleavereportforlater,workonitasyoudolabtasks

Followreportguidelines

Makesureyouincludeeverythingstatedinthelabtask

ElectronicsDesignLaboratory 2ECEN2270


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ECEN2270 3ElectronicsDesignLaboratory

Robotplatform

TwoDCmotors,eachdrivingawheel

+

_

VDC

IDC

wheel

64:1

gearOptical

encoder:

12 pulses

permotor

shaft

rotation

DCmotor

1 2 3 4

1EncoderpowersupplyVCC=+710V

2Encoderground

3EncoderoutputpulsesA,frequencyfenc4EncoderoutputpulsesB,frequencyfenc

DCsupply

10


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Lab2objectives

PartA

MeasureDCmotorcharacteristics

DevelopaSpicecircuitmodelfortheDCmotoranddetermine

modelparametersbasedonexperiments

Validatethemodel:compareexperimentalandsimulationresults

PartB

Designaspeedsensorcircuit(tachometer)thatoutputsvoltage

proportionaltowheelspeed

UseLTspice simulationstoverifyanddebugthedesign

Build,testanddemothespeedsensorcircuit



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PartB:SpeedSensorCircuit


Encoderoutputpulses,frequencyfenc [Hz]isproportionalto speed

Goal:Convertfrequencytovoltage

Approach:

Convertencoderoutputfrequencytodutycycleusingaoneshotcircuit

Lowpassfiltertheoneshotcircuitoutputtoproduceavoltageproportional

toencoderfrequency

CanuseLab2PartAdatatodeterminetheexpectedrange

ofencoderoutputfrequencies,particularlythemaximum

frequency

Althoughfrequencyvaries,thedutycycleisconstant

(50%),thustheaverageencoderoutputvoltageisconstant


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Oneshotcircuit

Frequencyoftheoneshotoutputtracksthefrequencyoftheencoder

Thedutycycleoftheoneshotoutputisproportionaltofrequency

Theaveragevalue(lowpassfiltered)oftheoneshotoutputisproportionaltofrequency


Oneshot

Fixedton output

Oneshotout

Oneshotout

Inputtrigger


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SpeedSensor(Tachometer)Circuit



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Howtoapproachtheanalysis(ordesign,ordebugging)

ofacomplexcircuit

1.DONT:diverightinandstartwritingalotofloopand

nodeequations

Youwillmakeanalgebraicmessandgetnowhere

Whendebugging:dontjustbuildthewholethingandturniton,expectingittoworkfirsttime

2.DO:Breakthecircuitdownintosmallerfunctionalblocks

thatcanbeseparatelyunderstood

Firsttrytoexplaininwords(aparagraph)howeachblockworks

Getthefirstblocktoworkbeforemovingontothenext



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3.DO:Foreachblock,decidewhatyouneedtoknow,

andwhatanalysiswillbetractableandfeasible

Identifytheinputandoutputsignals.Thegoalistofindthe

functionalrelationshipbetweenthese.

Writesimpleequationsthatyoucanreasonablysolve

Solvetheequationsfortheelementvalues;oftenthereis

morethanonevalidanswer

Develop

additional

constraints

based

on

your

understanding

of

how

thecircuitissupposedtowork

Choseimpedancelevelssothatcurrentsandpowerconsumption

arereasonable,e.g.mAnotA



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Breakingthetachometercircuitintoblocks


Trigger

Block

Oneshot

Block

Lowpassfilter

Block


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Oneshotblockpreliminaries:insidethe555timer

11

Includestwocomparators,aflipflop,dischargetransistor,

outputbufferandaninternal

resistordivider

Upperthreshold,THRESH,is2/3VCC usedforinternal

Reset

Lowerthreshold,TRIG_BAR,is

1/3VCC usedforinternalSet

TransistorshortsDISCHpinto

groundwhileoutputisLOW

Canbeeasilysetupas

monostable (oneshot)or

astable (oscillator)

Viewtheyoutube 555tutorialpostedontheLab2page


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Oneshotcircuit


Input:Setpulsefromtriggercircuit

Output:Outsignalhavingpulsewidthton

Thiscircuitresetsthelatchwhenthe

capacitorvoltagevC2(t)reachesthe

thresholdvoltageCTRL

Thingswewanttoknow:

Howdoesthiscircuitgeneratethetonpulse?

Howlongiston,andhowshouldwechoose

R2 andC2?

Equivalentcircuit

Inputandoutputwaveforms


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555Timer:Monostable oneshotoperation

Oneshottiming

ttrig:triggermustdropbelowthreshold(1.6VatVcc=5V)longenoughfor

reliabletrigger(>1s),butshorterthanton Vtrigger_low


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Trigger

Block

Oneshot

Block

Lowpassfilter

Block


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Triggercircuit


Input:venc(t),squarewavefromencoder

Output:Setpulsegoingtolatch

Thiscircuitsetsthelatchatthefalling

edgeofvenc(t)

Soonafterthelatchisset,wewantthesetpulsetogoaway:thesetpulseneeds

toterminatebeforeitistimetoresetthe

latch.Butthedatasheetforthe555timer

specifiesaminimumpulsewidthof

approximately1sec.

Thingswewanttoknow:

Howisfrelatedtothegroundspeedofthe

wheels?(leftasexerciseforstudents)

Howdoesthiscircuitgeneratetheset

pulse?

Howlongisttrig,andhowshouldwechoose

R1 andC1?

Equivalentcircuit


ttrig


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TriggerCircuit


DiodeD1clampstrignodetoVCConrisingedgeofencsquarewave

CapC1pullstrignodedownonfalling

edgeofencsquarewave

ResistorR1chargesC1andpullstrig

backuptocompletetriggerpulse

ttrig


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Trigger

Block

Oneshot

Block

Lowpassfilter

Block


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Lowpassfiltercircuit


Input:PulsewidthmodulatedsignalOUT

Output:speedsignalhavingaDCvalue

proportionaltof

Thiscircuitisalowpassfilterthatpasses

theDCcomponentofOUTbutremoves

thefundamentalandhigherfrequency

components

Thingswewanttoknow:

Howdoesthiscircuitoperateonthepulse

widthmodulatedOUTsignaltoproducethe

speedsignal? HowshouldwechooseR3 andC3?

Equivalentcircuit



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TimeConstantsintheTachometercircuit

Threetimeconstantstoselect R1,C1

1s>Rencoder R1,C1determinesttrig

R2,C2

Setton todetermineoutputvoltageofspeedsensoratmaximumspeed(basedonVCCanddutycycle)

R3,C3

LowpassfilterPWMoutput;determinesvoltagerippleonthe

speedsensoroutputvoltage

R3*C3>>lowestexpectedPWMperiod

R3*C3


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Lab2,PartB

DesignandsimulatethespeedsensorcircuitBuild,testanddemothespeedsensorcircuit

Seethe

Lab

2Tasks,

Part

B,for

task

specifications



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Appendix:MoreDetails

TriggerCircuit

OneShotCircuit

LowPassFilterCircuit



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Triggercircuit


Input:venc(t),squarewavefromencoder

Output:Setpulsegoingtolatch

Thiscircuitsetsthelatchatthefalling

edgeofvenc(t)

Soonafterthelatchisset,wewantthesetpulsetogoaway:thesetpulseneeds

toterminatebeforeitistimetoresetthe

latch.Butthedatasheetforthe555timer

specifiesaminimumpulsewidthof

approximately1sec.

Thingswewanttoknow:

Howisfrelatedtothegroundspeedofthe

wheels?(leftasexerciseforstudents)

Howdoesthiscircuitgeneratethesetpulse?

Howlongisttrig,andhowshouldwechoose

R1 andC1?

Equivalentcircuit


ttrig


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Triggercircuitanalysis:preliminaries


1. V+

isdetermined

by

voltage

divider:

V

+

=VCC (5

k

)/(15k

)=VCC/3

=1.67

V

2.DiodeD1 behavesasanonoffswitch(aonewayvalve).Theconductingstate

dependsonthevoltageandcurrentappliedtothediodebythecircuit:

Ifthecircuitmakesthevoltageattheanodeterminalgreaterthanthevoltageatthecathode(or

ifthecircuitappliespositivecurrenttothediode),thenthediodeturnson.Thisclampstheanodetoavoltageslightlygreaterthanthecathodevoltage(i.e.,thediodeisapproximatelya

shortcircuit).

Ifthecircuitattemptstoapplyreversecurrent(fromcathodetoanode)orreversevoltage,then

thediodeturnsoff.Thiscausesthediodetobecome(approximately)anopencircuit.

Characteristics of silicon pn diode


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Triggercircuit:waveforms


vC1 chargestoVCC throughR1

vC1 discharges

to

0

throughD1

D1 doesnotallowTRIG >VCCsovC1 =0

D1 OND

1OFF

ttrig


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Triggercircuit:analysistofindttrig


Duringthesetpulseinterval: DiodeD1 isOFF(opencircuit)

venc(t)=0

vC1 isinitiallyzero,andC1 charges

throughR1

Therefore,forthisintervalthecircuitreducesto:

Letsdefine

t=

0atthe

beginning

of

thisinterval,andtaketheLaplace

transformofthecircuittosolvefor

thecapacitorvoltage:

Nowusepartialfractionexpansiontotake

inverseLaplacetransform(exercisefor

thestudent);theresultis:

Thetriggercircuitcomparatorcausesthe

setpulsetoendwhenvC1 =VCC/3,attime

t=ttrig.

Hence:

Solveforttrig: ttrig

11/

1 1)(CRt

CCC

trigeVtv


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Oneshotcircuit


Input:Setpulsefromtriggercircuit

Output:Outsignalhavingpulsewidthton

Thiscircuitresetsthelatchwhenthe

capacitorvoltagevC2(t)reachesthethresholdvoltageCTRL

Thingswewanttoknow:

Howdoesthiscircuitgeneratethetonpulse?

Howlongiston,andhowshouldwechooseR2 andC2?

Equivalentcircuit


ttrig


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Oneshotcircuit:waveforms


Thetransistoroperatesasanon/offswitch: WhenQishighthenthetransistorisoff,

andviceversa.

Whenon,thetransistorshortsoutC2andholdsvC2 =0.

Whenthetransistorisoff,capacitorC2chargesthroughR2.

vC2 chargestowardsVCC throughR2

TransistorON OFF

ComparatorresetslatchwhenvC2 reachesCTRL

(=2VCC/3)

ttrig


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Oneshotcircuit:analysis


Exerciseforthestudent:deriveanexpressionfortonShowthestepsof:

TheLaplacetransformedcircuittobesolved

Deriverelevantequationandtakeinversetransform

SolvefortonThenuseyourresulttoselectvaluesforR2 andC2.


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Lowpassfiltercircuit


Input:PulsewidthmodulatedsignalOUT

Output:speedsignalhavingaDCvalue

proportionaltof

Thiscircuitisalowpassfilterthatpasses

theDCcomponentofOUTbutremoves

thefundamentalandhigherfrequency

components

Thingswewanttoknow:

Howdoesthiscircuitoperateonthepulse

widthmodulatedOUTsignaltoproducethe

speedsignal? HowshouldwechooseR3 andC3?

Equivalentcircuit



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Lowpassfiltercircuit:analysis


ThepulsewidthmodulatedsignalOUT(t)canberepresentedbyFourieranalysisasa

DCcomponentV0 plusasumofsinusoidscalledharmonics:

Theharmonicshavefrequenciesthatareintegralmultiplesofthefundamental

frequencyf.TheDCcomponentisgivenbytheaveragevalue:

Theamplitude spectrum isaplotoftheharmonicamplitudesvs.frequency:


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Filterdesign

ElectronicsDesignLaboratory 31ECEN2270,

TheeffectoftheR3C3 filteroneachindividualharmoniccanbefoundbyphasor

analysisofthecircuit:usephasors tosolvethecircuitandfindhowtheamplitudeofa

sinusoidischangedbythecircuit,asafunctionoffrequency.

WewanttochooseR3 andC3 sothatthefilterpassestheDCcomponentandanyverylowfrequencyvariationsthatoccurasaresultofthechangingspeedoftherobot.But

wewantthefiltertorejectthecomponentsofOUTatthefundamentalfrequencyf

anditsharmonics.Sothefiltershouldhaveatransferfunction (i.e.,theratioofitsoutputvoltageamplitudetoitsinputvoltageamplitude,vs.frequency)thatlookslike

this:

Exerciseforstudents:usephasor analysistosolveforthetransferfunctionoftheR3

C3 filter.SelectappropriatevaluesforR3 andC3.

Lecture Lab2 PartB

Documents

Transcript of Lecture Lab2 PartB