Post on 14-Apr-2018
7/29/2019 Lecture Lab2 PartB
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Lab2,PartB:RobotDCMotor
ECEN2270 1ElectronicsDesignLaboratory
Speedmeasurementcircuitdesign
7/29/2019 Lecture Lab2 PartB
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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
ElectronicsDesignLaboratory 4ECEN2270
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PartB:SpeedSensorCircuit
ElectronicsDesignLaboratory 5ECEN2270
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
ElectronicsDesignLaboratory 6ECEN2270
Oneshot
Fixedton output
Oneshotout
Oneshotout
Inputtrigger
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SpeedSensor(Tachometer)Circuit
ElectronicsDesignLaboratory 7ECEN2270
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Howtoapproachtheanalysis(ordesign,ordebugging)
ofacomplexcircuit
1.DONT:diverightinandstartwritingalotofloopand
nodeequations
Youwillmakeanalgebraicmessandgetnowhere
Whendebugging:dontjustbuildthewholethingandturniton,expectingittoworkfirsttime
2.DO:Breakthecircuitdownintosmallerfunctionalblocks
thatcanbeseparatelyunderstood
Firsttrytoexplaininwords(aparagraph)howeachblockworks
Getthefirstblocktoworkbeforemovingontothenext
ElectronicsDesignLaboratory 8ECEN2270
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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
ElectronicsDesignLaboratory 9ECEN2270
7/29/2019 Lecture Lab2 PartB
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Breakingthetachometercircuitintoblocks
ElectronicsDesignLaboratory 10ECEN2270
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
ElectronicsDesignLaboratory 12ECEN2270
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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Breakingthetachometercircuitintoblocks
ElectronicsDesignLaboratory 14ECEN2270
Trigger
Block
Oneshot
Block
Lowpassfilter
Block
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Triggercircuit
ElectronicsDesignLaboratory 15ECEN2270
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
Inputandoutputwaveforms
ttrig
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TriggerCircuit
ElectronicsDesignLaboratory 16ECEN2270
DiodeD1clampstrignodetoVCConrisingedgeofencsquarewave
CapC1pullstrignodedownonfalling
edgeofencsquarewave
ResistorR1chargesC1andpullstrig
backuptocompletetriggerpulse
ttrig
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Breakingthetachometercircuitintoblocks
ElectronicsDesignLaboratory 17ECEN2270
Trigger
Block
Oneshot
Block
Lowpassfilter
Block
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Lowpassfiltercircuit
ElectronicsDesignLaboratory 18ECEN2270
Input:PulsewidthmodulatedsignalOUT
Output:speedsignalhavingaDCvalue
proportionaltof
Thiscircuitisalowpassfilterthatpasses
theDCcomponentofOUTbutremoves
thefundamentalandhigherfrequency
components
Thingswewanttoknow:
Howdoesthiscircuitoperateonthepulse
widthmodulatedOUTsignaltoproducethe
speedsignal? HowshouldwechooseR3 andC3?
Equivalentcircuit
Inputandoutputwaveforms
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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
ElectronicsDesignLaboratory 20ECEN2270
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Appendix:MoreDetails
TriggerCircuit
OneShotCircuit
LowPassFilterCircuit
ElectronicsDesignLaboratory 21ECEN2270
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Triggercircuit
ElectronicsDesignLaboratory 22ECEN2270
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
Inputandoutputwaveforms
ttrig
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Triggercircuitanalysis:preliminaries
ElectronicsDesignLaboratory 23ECEN2270
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
ElectronicsDesignLaboratory 24ECEN2270
vC1 chargestoVCC throughR1
vC1 discharges
to
0
throughD1
D1 doesnotallowTRIG >VCCsovC1 =0
D1 OND
1OFF
ttrig
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Triggercircuit:analysistofindttrig
ElectronicsDesignLaboratory 25ECEN2270
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
ElectronicsDesignLaboratory 26ECEN2270
Input:Setpulsefromtriggercircuit
Output:Outsignalhavingpulsewidthton
Thiscircuitresetsthelatchwhenthe
capacitorvoltagevC2(t)reachesthethresholdvoltageCTRL
Thingswewanttoknow:
Howdoesthiscircuitgeneratethetonpulse?
Howlongiston,andhowshouldwechooseR2 andC2?
Equivalentcircuit
Inputandoutputwaveforms
ttrig
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Oneshotcircuit:waveforms
ElectronicsDesignLaboratory 27ECEN2270
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
ElectronicsDesignLaboratory 28ECEN2270
Exerciseforthestudent:deriveanexpressionfortonShowthestepsof:
TheLaplacetransformedcircuittobesolved
Deriverelevantequationandtakeinversetransform
SolvefortonThenuseyourresulttoselectvaluesforR2 andC2.
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Lowpassfiltercircuit
ElectronicsDesignLaboratory 29ECEN2270
Input:PulsewidthmodulatedsignalOUT
Output:speedsignalhavingaDCvalue
proportionaltof
Thiscircuitisalowpassfilterthatpasses
theDCcomponentofOUTbutremoves
thefundamentalandhigherfrequency
components
Thingswewanttoknow:
Howdoesthiscircuitoperateonthepulse
widthmodulatedOUTsignaltoproducethe
speedsignal? HowshouldwechooseR3 andC3?
Equivalentcircuit
Inputandoutputwaveforms
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Lowpassfiltercircuit:analysis
ElectronicsDesignLaboratory 30ECEN2270
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.