Solar Car Pwr Monitoring System

8/6/2019 Solar Car Pwr Monitoring System

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Solar Car

Pow er Monitor ing SystemFinal Technical Report

Submitted by: Team 4431

Timothy BaumgartnerMartin Bouvet Bours

Jessica FalkBenjamin Rawlins

Michael Sunday

Submitted to:

Mentor: Clayton GranthamSponsors: Sean Martinez, Wei Ren Ng

UA Solar Racing Team

Submitted on:

April 29, 2010

Interdisciplinary Engineering Design Program

University of Arizona


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Team4431SolarCarPowerMonitoringSystemFinalTechnicalReport

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Team 4431Solar Car Power Monitoring System

Pictured from left to right:

Martin Bouvet Bours, Mechanical Engineer,

Benjamin Rawlins, Mechanical Engineer,

Jessica Falk, Electrical Engineer,Michael Sunday, Systems Engineer,

Timothy Baumgartner, Computer Engineer


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Abstract

ThisreportdetailstheeffortsoftheSolarCarPowerMonitoringSystemSeniorDesign

Teamtodesign,build,testanddeliveraworkingprototypepowermonitoringsystemfortheUA

Solar Racing Team. Included in this document are the details of the design process, from

concept to analysis, development to build, and finally, to test the components and overallsystem. This engineering challenge was proposed by the UA Solar Racing Team to provide a

meansofanalyzingtheirsolarvehicleduringthebuildandoperationalphasesofthevehiclefor

thepurposeofmaximizingefficiencytomeettheobjectiveofvarioussolarraces. Theprocess

of creating such a system was to discover requirements from customer attributes, develop

concretefunctionalrequirementstosatisfyeachcustomerneed,designhardwareandsoftware

solutionstomeettherequirementsandtotesteachcomponentaswellastheoverallsystemto

validatethatoursystemmeetsorexceedsthecustomerneeds. Thoughthedevelopmentphase

of the project, validation and verification methods were noted to ensure each system

component could be individually tested for performance and compliance to the overall

requirements. The system as a whole was tested to ensure each component was compatible

and worked asdesigned when fully integrated intothevehicle. Througha series of analyses,

comparisons and testing with known data elements, each part of the system was verified to

meet the lower level requirements as well as validated to ensure the solution satisfied the

intended use of the product. The overall results of this team effort are a power monitoring

system design with working components that create a system that can evaluate vehicle

performance,bothmechanicalandelectrical,overawiderangeofparameterstobeusedinUA

SolarRacingTeamprojects. Twostandalonesystemsweredeveloped: Analignmentfixtureto

evaluatethemechanicalaspectofrollingresistancethroughmeansofwheelalignment,andan

electricalmonitoringsystemtoevaluatethepowersystemofthevehicle. TheUASolarRacing

Teamnowhasthemeanstoevaluatetheirvehiclesbothmechanicallyandelectricallytomeet

thegoalsandobjectivesofeachracetheyenter.


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TableofContents1 Introduction.......................................................................................................................................... 9

1.1 SponsorandMentor..................................................................................................................... 9

1.2 CustomerBackground................................................................................................................... 9

1.3 MeetingtheCustomerNeed........................................................................................................ 9

2 SystemRequirements......................................................................................................................... 10

2.1 DiscoveringRequirements.......................................................................................................... 10

2.2 CustomerAttributes................................................................................................................... 10

2.3 FunctionalRequirements............................................................................................................ 10

2.4 Constraints.................................................................................................................................. 15

2.5 RequirementsSummary............................................................................................................. 16

3 PreliminaryDesignsandSelectingaFinalDesign............................................................................... 17

3.1 Developingafinaldesign............................................................................................................ 17

3.2 IdealFinalResult......................................................................................................................... 17

3.3 PreliminaryDesigns..................................................................................................................... 18

3.4 PreliminaryDesignScoringandSelection................................................................................... 19

3.5 FinalDesign................................................................................................................................. 20

4 FinalDesign......................................................................................................................................... 21

4.1 SystemArchitecture.................................................................................................................... 21

4.2 SensorsandProcessing............................................................................................................... 24

4.3 Telemetry.................................................................................................................................... 52

4.4 Software...................................................................................................................................... 53

4.5 Mechanical.................................................................................................................................. 55

4.6 Analysis....................................................................................................................................... 62

4.7 FMEA........................................................................................................................................... 66

4.8 Risk.............................................................................................................................................. 68

5 SystemBuild........................................................................................................................................ 70

5.1 V/ISensor.................................................................................................................................... 70

5.2 SpeedSensor............................................................................................................................... 70

5.3 TiltSensor/Accelerometer.......................................................................................................... 70

5.4 SensorHub.................................................................................................................................. 71


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5.5 CommunicationsHub.................................................................................................................. 71

5.6 Telemetry.................................................................................................................................... 72

5.7 LabViewSoftware....................................................................................................................... 72

5.8 Mechanical.................................................................................................................................. 73

5.9 Analysis....................................................................................................................................... 75

6 ResultsfromTestPlan........................................................................................................................ 79

6.1 ProjectData................................................................................................................................ 79

6.2 Purpose....................................................................................................................................... 79

6.3 VerificationofFunctionalRequirements.................................................................................... 79

6.4 ValidationoftheSolarCarPowerMonitoringSystem............................................................... 80

6.5 VerificationandValidationSummary......................................................................................... 82

7 BudgetandScheduleDocumentation................................................................................................ 84

7.1 Budget......................................................................................................................................... 84

7.2 Schedule...................................................................................................................................... 84

8 Conclusion........................................................................................................................................... 86

9 Acknowledgement.............................................................................................................................. 87

10 References...................................................................................................................................... 88

11 TeamContribution.......................................................................................................................... 89

12 Appendix......................................................................................................................................... 91


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TableofEquationsEquation1:PreAmplifierGain................................................................................................................... 28

Equation2PreAmplifierResistorSelection............................................................................................... 28

Equation3:CurrentthroughDiodes........................................................................................................... 29

Equation4:CurrentthroughDiodesinTermsofR..................................................................................... 29

Equation5:PassiveFilterTransferFunction............................................................................................... 30

Equation6:PassiveFilterAngularFrequency............................................................................................. 31

Equation7:PassiveFilterTransferFunction............................................................................................... 31

Equation8:PassiveFilterAngularFrequency............................................................................................. 31

Equation9:TransferFunctionCombinedEffects....................................................................................... 31

Equation10:MagnitudeofCombinedEffects............................................................................................ 31

Equation11:CutoffFrequency................................................................................................................... 32

Equation12:BiasResistor........................................................................................................................... 34

Equation13:VoltageDivider...................................................................................................................... 34

Equation14:VoltageDividerResistance.................................................................................................... 35

Equation15:BufferInputProtection.......................................................................................................... 36

Equation16:LowPassFilterCutoffFrequency.......................................................................................... 36

Equation17:LowPassFilterCutoffFrequencyintermsofR..................................................................... 37

Equation18Safetyfactorduetostaticloading.......................................................................................... 55

Equation19FrictionalForce....................................................................................................................... 58

Equation20MaximumShearStressDuetoBending................................................................................. 59

Equation21:GearRatio.............................................................................................................................. 62

Equation22:DrivenWheelPower,Pengine.............................................................................................. 62

Equation23:Frolling.................................................................................................................................. 62

Equation24:AerodynamicDrag................................................................................................................. 62

Equation25:Force...................................................................................................................................... 63

Equation26:Torque................................................................................................................................... 63

Equation27:MassFactor........................................................................................................................... 63

Equation28:DragCoefficient..................................................................................................................... 64

Equation29:CarvelocityfromfinaldriveshaftRPM.................................................................................. 76

Equation30:RPMofnextshaft,usinggearratio....................................................................................... 76


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TableofFiguresFigure1:FunctionalDecompositionofCA1.............................................................................................. 11

Figure2:FunctionalDecompositionofCA2.............................................................................................. 12

Figure3:FunctionaldecompositionofCA3............................................................................................... 13

Figure4:IdealFinalResult.......................................................................................................................... 17

Figure5:PreliminaryDesigns...................................................................................................................... 18

Figure6:PughChart.................................................................................................................................... 19

Figure7:FinalDesign.................................................................................................................................. 20

Figure8:DataLoggingArchitecture........................................................................................................... 22

Figure9:Voltage/CurrentSensorHighSideShunt..................................................................................... 25

Figure10:V/ISensorSchematic................................................................................................................. 26

Figure11:IS1101SingleChannelSensorwithDigitalOutput.................................................................... 45

Figure12:OverallConnectionforAccelerometer...................................................................................... 46

Figure13:SensorHubSchematic............................................................................................................... 47

Figure14:CommunicationHubSchematic................................................................................................. 50

Figure15:SystemBlockDiagramforTelemetry......................................................................................... 52

Figure16:ModuleAssemblyfromtheProductManual............................................................................. 52

Figure17:VisualizationSoftware............................................................................................................... 53

Figure18:FinalDigitalAlignmentToolDesign........................................................................................... 55

Figure19Wheelalignmentbottomarms................................................................................................... 56

Figure20:ToparmforDigitalTool............................................................................................................. 57

Figure21WheelAlignmentBeam.............................................................................................................. 59

Figure22:Componentcase........................................................................................................................ 61

Figure23:RollingResistance...................................................................................................................... 64

Figure24:CoastdownTest.......................................................................................................................... 65

Figure25:RiskPlot...................................................................................................................................... 69

Figure26:ElectricMotorGraph(xaxisisTorqueininlb)......................................................................... 78

Figure27:V/ISensorPrintedCircuitBoard................................................................................................ 91

Figure28:CommunicationsHubPrintedCircuitBoard.............................................................................. 92


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TableofTablesTable1:SystemFunctionalRequirements................................................................................................. 14

Table2:Constraints.................................................................................................................................... 15

Table3:V/ISensorInput/OutputRatings................................................................................................... 27

Table4:V/ISensorShuntResistor.............................................................................................................. 28

Table5:PreAmplifierResistorSelection................................................................................................... 29

Table6:ResistorSelection.......................................................................................................................... 30

Table7:ResistorandCapacitorSelection................................................................................................... 33

Table8:ResistorandDiodeSelection......................................................................................................... 34

Table9:ResistorSelection.......................................................................................................................... 35

Table10:ResistorSelection........................................................................................................................ 36

Table11:ResistorandCapacitorSelection................................................................................................. 37

Table12:CapacitorSelection..................................................................................................................... 37

Table13:ResistorandCapacitorSelection................................................................................................. 38







Table20:Input/OutputRatings.................................................................................................................. 48

Table21:RiskRegister................................................................................................................................ 68

Table22:GearRatioCalculation................................................................................................................. 77

Table23:FunctionalRequirements............................................................................................................ 80

Table24:TestResultsSummary................................................................................................................. 83

Table25:Budget......................................................................................................................................... 84

Table26:Ganttchart.................................................................................................................................. 85


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1 Introduction1.1 SponsorandMentor

TheUniversityofArizonaSolarRacingteamhasproposedaseniordesignprojectto

designasolarvehiclepowermonitoringsystemforuseontheirsolarracevehicle. Team4431,

SolarPowerMonitoringSystem,hasacceptedthechallengetodesignandbuildsuchasystem.

Thedesignprojectwillbeguidedthroughoursponsors:SeanMartinez,amechanical

engineerandgraduatestudent,andWeiRenNg,anelectricalengineerandgraduatestudent.

Additionallywehaveafacultymentor,ClaytonGrantham,whowillhelpguideusthroughout

thedesignprocess.

1.2 CustomerBackgroundOurcustomer,theUniversityofArizonaSolarRacingTeam,hasbeencompetinginsolar

vehicleeventssince1999. TheseeventsincludetheSunrayce,FormulaSun,NorthAmerican

SolarChallengeandmostrecentlytheShellEcoMarathon. Eachoftheseraceshasaspecific

goal. Thegoalsofeacheventcanbecategorizedintotwomaintypesofracing:mostefficient

vehicleandquickestvehicletothefinishline. Inordertobestpreparefortheraces,theUA

SolarRacingTeamdesirestoevaluatevehiclepowerandefficiencytobestconfigurethevehicle

forthespecificeventtheyarecompetingin. Inordertoevaluatethevehicle,manyfactors

mustbemonitoredandcharacterized.

1.3 MeetingtheCustomerNeedInordertomeetthecustomersneeds,Team4431willdesignasystemtomonitorthe

powerusageoftheUASolarCartosupportanefforttoimprovetheelectricalandmechanical

efficiencyofthevehicle. Toaccomplishthismission,bothelectricalandmechanicalsystems

willneedtobeanalyzed. Electricalsystemstobeconsideredarethesolararrays,batteries,

motorperformancebothconsumingandregenerating,andonboardelectronics. Collectionof

theelectricaldatawillbeachievedthrougharealtimetelemetrysystemwithagoalofminimal

powerconsumptionduetothemonitoringsystemcomponentdraws. Mechanicalaspectsthat

willbeconsideredarevehiclemechanicalefficiencyoftherollingresistancethroughevaluation

ofalignment,appliedhorsepowerandtorquethoughevaluationofgearratiosandvehicle

speedmonitoring.


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2 SystemRequirements2.1 DiscoveringRequirements

Therequirementsforthissystemwerederivedfromtheoriginalproposal,meetings

withthecustomer,anddiscussionwithourmentor. Theprocessofgatheringrequirements

beganwiththeinitialcustomerneedsstatementfortheprojectdescription. Theteammetand

discussedtherequirementstoformquestionsforthesponsorsinanattempttodiscover

additionalrequirementsthatmaynothavebeenspecifiedinitially. Oncethequestionswere

addressedbythesponsorstheteambegangenerationofthefunctionalrequirements. The

functionalrequirementswerereviewedwiththesponsorandscoredtoestablishahierarchyof

importance. Functionalrequirementswerefurtherbrokendownintosubrequirementsand

finallyintodesignparameterstodefinethesubsystems.

2.2 CustomerAttributesCustomerattributesarethestartofrequirementsdefinitionfortheproject. Each

customerattributeisdevelopedintofunctionalrequirementstobetterdescribehowthe

systemshallfunctionallymeetthecustomerneeds.

CustomerattributeCA1,MonitorSolarVehiclePower,isastatedrequirementfromthe

sponsorsdefiningmuchofthescopeoftheproject.

CustomerAttributeCA2,DataAcquisitionSystemwithTelemetry,isastatedrequirementfrom

thesponsorswhichaddstothescopeofCA1.

CustomerAttributeCA3,EvaluateMechanicalAttributes,isadiscoveredrequirementfromthecustomerthatwasformedaftermeetingwiththecustomerandlearningaboutthemechanical

capabilitiesofourteam.

Customerattributesandtheirdecompositionintofunctionalrequirementsareillustratedin

Figure1,Figure2,andFigure3.

2.3 FunctionalRequirementsFunctionalrequirementsarederivedfromcustomerattributes. Thefunctional

requirementsareusedtodefinesubrequirements,knownasdesignparameters,forwhich

eachofthesubsystemdesignsaredevelopedtocompletetheoveralldesignintent. Fromthefunctionalrequirements,preliminarydesignsaredevelopedinanattempttosatisfythe

customerneeds. Fromallofthepreliminarydesignideas,anidealfinalresultisformed.

Functionalrequirementsarederivedfromcustomerattributes. Thesummaryofderived

functionalrequirementsisillustratedinTable1.


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Figure 1: FunctionalDecomposition ofCA 1


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Figure 2: FunctionalDecomposition ofCA 2


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Figure 3: Functionaldecomposition ofCA 3


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Table 1: SystemFunctionalRequirements

FunctionalRequirement

CustomerImportance

CTQ(metric) TargetValue

Monitorelectrical

subsystems

10 Voltage,Amperageand

Power

95%accuracy(minimum)

Measuredata

withaminimum

accuracyof95%

9 MeasurementAccuracy 95%accuracy(minimum)

Recordand

accessremote,

digitaltelemetryinrealtime

8 Datarecording

Datatransmission

MeetsRequirementCriteria

MonitorSolarCar

speed

7 Groundspeed 95%accuracy(minimum)

Beportable

betweenvarious

platforms

7 Portability FunctionproperlywithinSolarCar

andbeyond

Willnotdegradeelectrical

performanceof

SolarCar

6 Powerconsumption Minimalpowerlossduetomonitoring

Calculate

equivalentmpgs

basedonenergy

used

5 CalculationAccuracy 95%accuracy(minimum)

EvaluateGear

RatioandWheel

alignment

5 Torque,Wheelangle 95%accuracy(minimum)


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2.4 ConstraintsConstraintsaretheenvelopeinwhichthesystemmustoperate. Throughthe

developmentofthefunctionalrequirements,constraintsbecomeapparent. Constraintscanbe

handeddownbythecustomer,bylaw,regulation,orsimplycommonsense. Theseconstraints

typicallyarecategorized(butarenotlimitedto)asperformance,materials,safetyorbudget.

Constraintsdiscoveredthroughoutthedevelopmentofthefunctionalrequirements

wereacombinationofcustomerdrivenandcommonsense. Alistingofspecificconstraintscan

befoundinTable2.

Table 2: Constraints

Constraints PartsoftheDesignAffected

Materials

Weightshouldbelessthan1%oftotal

carweight(~8lbs)

Shouldconsumelessthan1cubicfootof

space

Weight:Entiresystem(sensors,wiring,communicationsbox,antenna)

Size:Maincommunicationsbox

Performance

Rangeofsignaltransmissionshouldbe

atleast1.5miles

Systemwillbeselfpowered

Thesystemshouldbeoperableover

Arizonaseasonaltemperature

Range:Transceiver

Power:Entiresystem

Temperature:Entiresystem

Safety

Willnotinterferewithsafeoperationof

vehicle

Power:voltage,currentlevels

Budget

Willbedevelopedforunder$2000

Cost:Entiresystem


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2.5 RequirementsSummaryTheoverallsystemdefinitioncanbedescribedthroughthefunctionalrequirementsand

constraints. (TeoriyaResheniyaIzobretatelskikhZadatch)orTRIZisoftenusedtoensuredesignparametersmeetfunctionalrequirements

howeverwefeltthiswasnotnecessaryduetotheclearbreakdownofcustomerattributesthroughtofunctionalrequirementsanddowntolowerleveldesignparameters.

Insummary,thesystemshall:

Monitorelectricalsubsystemsandvehiclespeedwithinanaccuracyof95%oftheactual

values,recordandaccessremotedigitaltelemetryinrealtime,beportablebetween

variousplatformsandwillnotdegradetheelectricalperformanceofthevehiclein

whichitisinstalled

Readoutdateonaremotecomputergroundstationwiththecapabilityofanalyzingthe

acquireddataforequivalentmilespergallon(MPG)perShellEcoMarathonmethods Evaluatevehiclealignment,horsepower,torqueandgearratio

Thesystemshallmeettheaboverequirementswithoutviolatingthefollowingconstraints:

Nottoexceed1%ofthevehicleweight(~8lbs),inlessthan1cubicfootofphysicalspace

Telemetryshallhavearangeofatleast1.5miles

OperateoverArizonaseasonaltemperature(20120degreesFahrenheit)

Willnotinterferewithsafevehicleoperation

Bedesignedandimplementedfor$2000orless


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3 PreliminaryDesignsandSelectingaFinalDesign3.1 Developingafinaldesign

Developmentofthefinaldesignbeganwithensuringeachdesignmetthefunctional

requirements. Theteamreviewedtherequirementsandensuredthateachdesignsatisfiedthe

functionalrequirements.

Inordertodecideonafinaldesign,theteamwentthroughaselectionprocessinvolving

bothcustomersandmentor. Tostart,wecameupwithanidealfinalresult(IFR)comprisedof

theelectricalandmechanicalnecessitiesaswellassomenicetohavefeatures. Wereviewed

theIFRwiththecustomerandrecordedtheirrecommendationsandtradeoffideas. Wealso

consultedourmentorforhisinputfromanelectricaldesignaspect. Afterreviewingwithboth

customerandmentor,theIFRwasdevelopedasshowninFigure4.

3.2 IdealFinalResult

Figure 4: IdealFinalResult


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3.3 PreliminaryDesignsPreliminarydesignsweredevelopedtomeettherequirementsandwerepresentedat

thePreliminaryDesignReview(PDR). AsummaryofthosedesignsisshowninFigure5. For

additionaldetailsofallaspectsofeachpreliminarydesign,pleaseseethePDRpresentationin

theEngineeringNotebookontheSeniorDesignWebsite:http://proj498.web.arizona.edu/sites/default/files/498PDR%20Final.pptx

Figure 5: PreliminaryDesigns
http://proj498.web.arizona.edu/sites/default/files/498PDR%20Final.pptxhttp://proj498.web.arizona.edu/sites/default/files/498PDR%20Final.pptx


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3.4 PreliminaryDesignScoringandSelectionAPughchartwasdevelopedtoassistinselectingthefinaldesign. Uponreviewingthe

designs,complexityandinnovationofthedesigns,andbudget,thescoreswererecordedand

areshowninFigure6:

Weight Design1 Design2 Design3SensorDesign 10 0 +

Telemetry 8 + + 0

Software 7 0 + 0

Alignment 6 0 + +

Total+ 8 31 6

Total0 23 0 15

Total 0 0 10

OverallScore 8 31 4

Figure 6: PughChart

ThePughchartwasdevelopedusingaworsethan/equalto/betterthanapproach,

resultinginweightedscoresacrossasimple /0/+scorewhichwastotaledtodeterminethe

bestattributesofeachdesign. Ratherthanselectingonedesigninitsentirety,eachaspectof

thedesignswereevaluatedindividuallytoallowthepotentialforahybridsystemusingpartsof

eachindividualdesign. Uponfinalreviewofthebestdesignattributescombinedwithavailable

budget,wearrivedatafinaldesign.


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3.5 FinalDesign

Figure 7: FinalDesign

Thefinaldesignmeetsorexceedsallofthecustomerrequirementshoweveritdoesnotinclude

theinvehicledisplay.


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4 FinalDesign4.1 SystemArchitecture4.1.1 Overview

Dataisfirstgatheredbysensors(somepreexistingwithinthevehicle,somesuppliedbytheproject)andsenttothesensorhub. Thesensorhubaggregatesthissensordataand

outputstheaggregateddatatoagroundstationcomputer,viaeitherwired(fortestbench

testing)orwireless(forinvehicletesting)telemetry. Thegroundstationcomputerrecordsthe

informationandprovidesarealtimevisualizationofthedata. Datapostanalysiscanthenbe

performeddirectlyonthegroundstationoronaseparatecomputer.

Theactualdatacollectedbythesensorswerechosentofacilitatetheanalysisand

optimizationofthesolarcarvehicle. Thesesensorsaredescribed,atahighlevel,below.

AgraphicalrepresentationofthesystemarchitectureisshowninFigure8:


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Figure8: Data LoggingArchitecture


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4.1.2 SensorsDataiscollectedfromavarietyofsensorswithdifferentinterfaces. Someofthese

sensorsarebuiltintothevehicle;othersareprovidedbyourproject.

4.1.2.1 V/ISensorsVoltage/Current(V/I)sensorssampleinstantaneousvoltageandcurrentandarea

customdesignbyourteam. Fromthevoltageandcurrentmeasurements,instantaneous

powercanbedetermined( IV=P ),aswellastotalenergyusage( Pdt=W ). Thesesensors

communicatewiththesensorhubviatheCAN(controllerareanetwork)2.0Abus.

Fourofthesedeviceswerecreated:

1. Motorpowerlinesensor

2. 12Vsubsystemsensor

3. 24Vsubsystemsensor

4. Sparesensor

4.1.2.2 SpeedSensorThespeedsensorsamplestheinstantaneousspeedofthevehicle. Thissensoris

providedbyourteam. ThespeedsensorcommunicateswiththesensorhubviatheCANbus.

4.1.2.3 TiltSensorThetiltsensorsamplesthetiltofthevehicleinorderto;forexample,provide

informationonthegradeofaninclinethevehicleistraversing. Thissensorisprovidedbyour

teamandcommunicatesovertheCANbus.

4.1.2.4 MPPTsThemaximumpowerpointtrackers(MPPTs)arealreadyexistentwithinthevehicleand

areusedtoprovidethemaximumpoweroutputfromsolarpanelsubarrays. TheMPPTs

providevoltageandcurrentmeasurementsfromthesolarsubarraysandcommunicatethis

informationtothesensorhubviatheCANbus.

4.1.2.5 BPSThebatteryprotectionsystem(BPS)isalsoalreadyexistentwithinthevehicleandis

usedtoprotectthevehiclesbatterycellsfromadverseconditionssuchasoverheating,short

circuiting,andundervoltage. TheBPSmeasurestotalbatterycurrent,individualcellvoltages,

andtemperatureatseverallocationswithinthebatterycompartment. Thisinformationis

communicatedtothesensorhubviaanRS232seriallink.


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4.1.3 SensorHubThesensorhubprovidesaninterfacetothevarioussensorsandaggregatestheirdata.

Thisaggregateddataisthensentouttothegroundstationcomputerinaserialstream. The

serialstreammustefficientlypackdataintothefewestbitspossibletobecompatiblewithslow

serialbaudrates(suchas9600Baud/s)thatmayberequiredbythetelemetryhardware.

Forwiredtelemetry,thesensorhubmaybedirectlyconnectedtothegroundstation

computerviaaserialcable. Forwirelesstelemetry,aradiomodemmaybeconnectedtothe

sensorhubandanotherradiomodemconnectedtothegroundstationcomputer. These

modemsactto"tunnel"serialdatabetweenthesensorhubandgroundstationcomputer

nodes.

ThesensorhubhastheabilitytointerfacewithoneRS232serialdevice(e.g.theBPS)

andupto13CANbusequippeddevices(thisdevicelimitationisduetothenumberofphysical

connectors). Thesensorhubwillalsoprovidepowertoallsensorsdesignedorsuppliedbythe

project.

4.1.4 GroundStationComputerThegroundstationcomputer(GSC)isageneralpurposecomputer(e.g.aPC)running

datacaptureandvisualizationsoftware. TheGSCcollectstheserialdatastreamsentbythe

sensorhub(eitherviawiredorwirelesstelemetry),parsesthestreamforsensordata,records

thedata,anddisplaystheinformationontheuserinterfaceinrealtime.

TherecordeddatacanbesavedtobeanalyzedlaterontheGSCoronadifferent

computer.Theseanalysescanthenbeusedtoperformoptimizationsontheactualsolarcar

vehicle.

4.2 SensorsandProcessing4.2.1 V/ISensorCircuitryDescriptionSensorblockdiagramisshowninFigure9:

SensorschematicisshowninFigure10:


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Figure9: Voltage/CurrentSensorHighSideShunt


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Figure10 :V/ ISensorSchematic


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4.2.1.1 I/ORatingsTable 3: V/ISensorInput/OutputRatings

LineIn Positivecurrentinputfromhighsideline( V1500 , A100 )

LineOut Positivecurrentoutputfromhighsideline( V1500 , A100 )

CANH,CANL CANbuscommunicationslines( V50 nominal, V36 maximum

continuous)

VBAT Powersource( V305.8 ,reversebiasprotectedupto 30V)

4.2.1.2 OverviewTheVoltage/Current(V/I)sensorisneededtomeasureinstantaneousvoltageand

currentvaluesfrompowerlineswithinavehicle. Fromthesevoltageandcurrentsamples,

instantaneouspowerdeliveredbythelinecanbecalculated. Thecalculatedpower

measurementtotalerrormustbelessthan5%. Toachievethis,thevoltageandcurrent

measurementsthatmakeupthisfiguremusthaveanevenlowerpercentageerror.

Additionally,theV/Isensormustbeportableacrossmanydifferentvehicletypesso

assumptionsaboutthephysicallayoutofthevehicleandvehiclewiringmustbeminimized.

4.2.1.3 ConnectorsThepoweranddataconnector( 1JP)suppliespowertotheV/Isensorandalsoprovides

accesstothetwoCANcommunicationlines(CANHandCANL). TheconnectorisanRJ45

connectorastheintendedcablingisCAT5UTPpatchcable.Thecablingwaschosento

accommodatethesuggested 120 characteristicimpedanceoftheCANbuslinesandasthe

twistedpairdesignofCAT5cablingmaximizesthenoisecancelingcapabilityoftheCANbus

differentialsignalcommunications.CAT5UTPcableisreadilyavailableduetoitsuseas

Ethernetpatchcabling. TheISPconnector( 2JP)allowsforinsystemprogrammingofthe

ATtiny261Amicrocontroller(MCU).

4.2.1.4 GeneralVoltageRegulationArawbatteryinputvoltage(5.8 to V30 )isregulatedto V5 withthe 6U linear

regulator.2

D ispresenttoprotectthecircuitryagainstnegativevoltagesofupto V30 . This

negativevoltagecouldbecaused,forexample,byabatterybeingconnectedwithareversed

polarity.

Ashuntregulator(toproduceafloatingground)andaninverterarealsousedwithinthe

circuitbutaredirectlyrelatedtothecurrentsensingcircuitryandarethusdescribedinthe

CurrentSensingCircuitrysection.


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4.2.1.5 V/ISensorComponentSizing4.2.1.5.1CurrentMeasureShuntResistor

shuntR wassizedassmallaspossibletominimizepowerdissipationfromthelinebeing

measured. Fromreadilyavailablesuppliers, m=Rshunt

0.2 wasthesmallestresistoravailable.

Thisresultsinarespectivefullscalevoltageof mV20 ( iR=v )acrosstheresistoratthepeak

currentof A100 .Atthispeakcurrent, shuntR willdissipate W2 ( Ri=P2 ).

Table 4: V/ISensorShuntResistor

Component Value ToleranceR

s h u n t 0 .2 m 1%

4.2.1.5.2 1U Pre AmplifierGainDuetocomponenttolerances(mainlythe5%toleranceofzener 1D )andasafety

margin,thecurrentsensingcircuitryisdesignedtooperatewithafloatingground(FGND)with

aminimumvoltageof V3 belowtheLINE_INvoltage. Afullscaleinputof A100 throughthe

m0.2 shuntR willproduce mV20 acrosstheresistor. Astheoutputissplitbyanoffset

betweentherails(toallowforbidirectionalcurrentsensing),the mV20 signalmustbe

amplifiedto V1.5 tospanacrosstheminimum( V3 )voltagerangeoftheFGNDcircuitry. In

ordertodothis,thedesiredgainof 1U wascalculated: VV=mV

V=G 7520

1.5.

ThegainoftheINA326/7amplifier( 1U )isgivenby:

1

22I

I

R

R=G

Equation 1: Pre AmplifierGain

TheINA326/7datasheetsuggeststhat 1IR bevaluednolessthan k2 andbesizedbasedon

theequation(personalizedtotheschematic):

A

V=R

maxRshunt

I12.5

_

1

Equation 2Pre AmplifierResistorSelection


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With VR s h u n t _ma x= 20 m V (thefullscalevoltageacross shuntR at A100 ),Equation2produces

k=RI 1.61 . Asthisvalueislessthantheminimumsuggestedvalueof k2 ,thefinalvalueof

1IR issetto k=RI 21 .

RearrangingEquation1,itcanbedeterminedthat k=RI 752 producesthedesiredgainof

VV=G 75 .

Table5: Pre AmplifierResistorSelection


I1 2 k 0.1%

RI2

75 k 0.1%

4.2.1.5.3Solid StateRelayControlLineResistors7R and 8R limitthecurrentsuppliedtothe 12U and 13U AQW610EHsolidstaterelays

(SSRs). TheAQW610EHdatasheetspecifiesaminimumof mA3 isrequiredforthecontrolLEDs

(ontheIC)toensuretheswitchingoftherelay. TheseLEDshaveamaximumvoltagedropof

V1.5 . AlthoughtheLEDwilldroplessthanthisvoltageatlowercurrents( V1.14 typicalwitha

diodecurrentof mA5 ),this V1.5 valueisusedtoprovideasafetymargin.

ThecontrollinesaredesignedsothatasingleresistorisinserieswithtwocontrolLEDs.

PuttingtwoLEDsinseriessavesoncurrentconsumptionfromthecontrolline(whichisdriven

byanMCUthatcandriveamaximumof mA20 fromaGPIOpin).

Thecurrentthroughthediodescanbemodeledbythefollowingequation( 8R canbe

substitutedfor 7R ):

7

_ 1.52

R

V)(V=i

INVI

d

Equation 3: CurrentthroughDiodes

RearrangingEquation3:

d

INVI

i

V)(V=R=R

1.52_87

Equation 4: Currentthrough DiodesinTermsofR


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and3dBcutoffangularfrequency:

11

1

1

CR=o

Equation 6: Passive FilterAngularFrequency

4.2.1.5.4.1.2PassiveFilter2

Similarly, 2IR and 2IC formapassive,singlepoleLPFwithtransferfunction:

sCR+=(s)H

II 22

21

1

Equation 7: PassiveFilterTransferFunction

and3dBcutoffangularfrequency:

22

2

1

II

oCR

=

Equation 8: Passive FilterAngularFrequency

4.2.1.5.4.1.3CombinedEffects

The design of the instrumentation amplifier (INA326/7) combines these two LPFs into an

equivalentLPFwithtransferfunction:

s)CR+s)(CR+(=(s)(s)HH=H(s)

II 2211

2111

1

Equation 9: TransferFunctionCombinedEffects

ThetwopolesofH(s resideinthelefthalfplane,sowecansafelyset j=s . Theresulting

magnitudeequationis:

2

1122

22

21211

1=)(

)CR+C(R+)CCRR(

|jH|

IIII

Equation 10: MagnitudeofCombinedEffects

Tofindthe3dBcutofffrequency( o ),set2

1=|)H(j| o andsolvetoget:


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2

2121

2

22

2

11

2

2121

4

22

4

11

2

6

)CCR(R

)C(R)C(R)CCR(R+)C(R+)C(R=

II

IIIIII

o

Equation 11 :CutoffFrequency

4.2.1.5.4.2 Component Value Determination

The12bitADC( 2U)willsampletheoutputofthe 1U INAat250SPS. Toprevent

aliasing,thelowpassfilterontheoutputoftheINAshouldfilteroutanyfrequenciesaboveat

least Hz=1252

250(seetheNyquistShannonsamplingtheorem). Asasafetyfactor,aslightly

lowercutofffrequencyof Hz=fo 100 ( s)rad(=f= oo 10022 )isdesiredforthefilter.

Thevalueof 2IR isdeterminedbythedesiredgainoftheamplifier,andistherefore

knownpriortothesizingoftheothercomponentspresentinthefilter.

Todeterminethevaluesof 1R , 1C ,and 2IC :

1. Makeaneducatedguess(basedontheINA326/7datasheet)andlet F=C 11

2. Assume 21 oo = andassume(fornow)that 21 IR=R (pretendingthevalueof 2IR is

unknown)andthat 21 IC=C

3. With s)rad(=o 1002 (i.e.100Hz),thedesiredcutofffrequency,and F=C=C I 121 ,plugintoEquation11andsolvefor 1R :

4. UsingEquation6,wecandetermine s)rad(o 159.154921

5. Since k=RI 752 (chosentoobtaindesiredgainoftheamplifier)and 21 oo = (ideally),

wecandetermine 2IC byusingEquation8:

6. Usingthesefinalvaluesfor 1R , 2IR , 1C ,and 2IC (seeTable7)andEquation11,the

k=RRvalueNearest

11024.3 15%

1

nF=CnFC IvalueavailableNearest

I 1313.3 2

2


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actual3dBcutofffrequencyoftheoverallLPFcanbeestimated(assumingideal

components)at:

Table 7: ResistorandCapacitorSelectionComponent Value ToleranceR

1 1 k 5%

RI2 75 k 0.1%

C1 1 F 10%

CI2 13 n F 10%

4.2.1.5.5FloatingGroundRegulatorZenerDiodeandBiasResistorThetwocomponentsofthefloatingground(FGND)regulatorthatcanbesizedare:the

zenerdiode, 1D ,andthebiasresistor, biasR .

4.2.1.5.5.1 Zener Diode

Thezenerdiode, 1D ,wassizedtohaveazenervoltageof V3.9 ;whenreversebiased,

thediodewillhaveanominalvoltageof V3.9 acrosstheterminals. Thisvoltagewaschosenso

that,whenconsideredwiththeestimated VV QBE 0.63_ ,FGNDwillsitapproximately V3.3

belowthevoltageofLINE_IN.AllICsonFGNDarecompatiblewiththis V3.3 supply.

AnadditionalconcernwiththesizingofthiszenerdiodeisuncoveredwhentheLINE_IN

voltageislow. Currentmeasurementmustfunctionproperlyonlinevoltagesdownto V0 so

the V5 supplywasintroduced. 2Q sitsbetweenthediodeandthe V5 supplyandis

desiredtomaintainoperationintheactiveregion. ThevoltagedropcausedbytheseBJTscan

beaminimumof VV SatQCE 0.32__ . The V0.3 minimumcollectoremittervoltageof 2Q plus

the V3.9 dropacrossthezenerdioderequireatleast V4.2 ,leavinganextra V0.8 between

the V0 LINE_INandthe V5 supplyforcomponenttolerancesandasasafetymargin.

4.2.1.5.5.2 Bias Resistor

Thezenerdiodeproducesa V3.9 zenervoltagewhenreversebiasedat A250 . To

accountforcurrentpulledinthroughthebaseof 3Q (approximately A50 peak)andto

provideasafetymargin,thecurrentmirrorbiasingthezenerdiodewasdesiredtopullabout

Hz=s)rad(o 102.43102.432


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A350 . Thevalueof biasR controlstheamountofcurrentdrawnthroughthemirrorandthis

currentisapproximatedby:

bias

QBE

bias

i

VV)(V=R

1_50

Equation 12: BiasResistor

Thisequationignoresthe 1Q and 2Q basecurrents.

Assuming V=V QBE 0.71_ (inreality,thisvaluewilllikelybeslightlysmaller),

k=Rbias 12 producesan Aibias 358.3 andisacceptableasaresistorwith5%tolerance(

biasi willdroptonolowerthan A341.2 ).

Table 8: ResistorandDiodeSelection

Component Value ToleranceD

1 3 .9 V 5%

Rbi as 12 k 5%

4.2.1.5.6VoltageSensingInputResistorsTheinputofthevoltagesensingcircuitrycontainsthreeresistors: 1VR , 2VR ,and 2R .

4.2.1.5.6.1 Voltage Divider

1VR and 2VR formavoltagedividertodividethe V1500 inputlinevoltage(LINE_IN)

downtothe V50 levelofthevoltagebuffer( 5U )andMCUwithADC( 7U ). Duetopower

supplytolerances,the V5 powersupplymayhaveaminimumvoltageof V4.9 (2%error).

Additionally,theOPA340buffer( 5U )canbeexpectedtooutputamaximumof mV50 below

thesupplyrail.Thus,wemustdividea V150 signaldownto V4.85 . Thisvoltagedivider

equationis:

V

V=

R+R

R

VV

V

150

4.85

21

1

Equation 13:VoltageDivider

k=RkA

VV=R bias

valueNearest

bias 1212.286350

0.75 5%


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Itisalsodesiredtohaveatotalvoltagedividerresistanceofabout 1 . Thisvalueis

largeenoughtolimitcurrentdrawfromLINE_INto A150 andsmallenoughtolimitelectrical

noiseproducedthebydividerresistors.

M=R+R VV 121 Equation 14:VoltageDividerResistance

SolvingEquation13andEquation14simultaneously,wefind:

However,pluggingtheresultsin:

wefindthesizingtoproduceanoutputvoltagegreaterthanmaximum V4.85 withaninputof

V150 . Tofixthis, 2VR isbumpeduptothenext0.1%resistorvalue: k=RV2 976 .Tryingagain:

Thisislessthanthe V4.85 maximum(evenwhen0.1%tolerancesareincluded),sothevalueof

k=RV2 976 isthefinalvaluefortheresistor.

Table 9: ResistorSelection


V1 32 .4 k 0.1%

RV2 976 k 0.1%

k=RkR VvalueNearest

V 32.432.333 10.1%

1

k=RkR VvalueNearest

V 965967.667 20.1%

2

Vk+k

k=

R+R

R

VV

V 4.8796532.4

32.4V150V150

21

1

Vk+k

k=

R+R

R

VV

V 4.8297632.4

32.4V150V150

21

1


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4.2.1.5.6.2 Buffer Input Protection

2R protectsthe(noninverting)inputofthe 5U bufferfromhighvoltagetransients.

k=R 5.12 waschosenbasedupontherecommendationoftheOPA340datasheet. Whilethe

datasheetspecifiesa k5 value, k5.1 isthenearest5%resistorvalue.

The 5U (OPA340)inputsarediodeclampedtothepowersupplyrailsandanyvoltageof

500 mV abovetherail(i.e. V5.5 )mustbecurrentlimitedto mA10 .

V+)mA)(R(=Vmax 5.510 2

Equation 15: BufferInputProtection

FromEquation15, k=R 5.12 protectstheinputofthe 5U bufferfromvoltagesupto

V=Vmax 56.5 .Dividingthisresultbytheattenuationfactorprovidedbythevoltagedivider 1VR

and RV2 ,themaximumLINE_INvoltagethe 5U buffercanhandleiscalculated:

Table 10:ResistorSelection


2 5.1 k 5%

4.2.1.5.7One PoleLow PassFilteron 5U (OPA340)OutputFollowingthereasoninginsection4.2.1.5.4.2,thelowpassfilterontheoutputofthe

5U voltagebufferhasadesiredcutofffrequencyof s)rad(=o 1002 .

ThefilterisasimplepassiveRClowpassfilterwithcutofffrequencygivenby:

63

1

CR=o

Equation 16:LowPassFilterCutoffFrequency

Duetothemediuminputresistance(tensof k )oftheADCinput,thevalueof 3R is

desiredtobesmalltomitigatethevoltagedividereffectof 3R inserieswiththeADCsinput

kVk

k+kV=V maxmaxINLINE 1.7585

32.4

97632.4__


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resistance. AsshownbyEquation16,alargervalued 6C resultsinalowervalued 3R forthe

same o . Thesizeof 6C issomewhatarbitrarilylimitedtoamaximumof F10 inorderto

limitthephysicalcapacitorsizeandtolimitcapacitorparasiticsthatlimitthecapacitors

responseathigherfrequencies(e.g.equivalentseriesinductance,ESL).

RearrangingEquation16:

oC=R

6

3

1

Equation 17: LowPass FilterCutoffFrequencyin termsofR

UsingEquation17with s)rad(=o 1002 andC

6= 10 F :

Table 11:ResistorandCapacitorSelection


3 160 5%

C1 10 F 10%

4.2.1.5.8 V5 VoltageRegulatorExternalComponents4.2.1.5.8.1 Decoupling Capacitors

RIC smoothestheinputvoltagerippleandissetto F=CRI 1 ,thevalue

recommendedbytheLP2950datasheet. ROC smoothestheoutput(regulated)voltageripple

andisrecommendedtobeaminimumof F1 forloadsdrawing mA100 . Theloadofthe

circuitisaexpectedtodrawmuchlesscurrent,sotheminimumvalueofthecapacitorcanbe

reduced. Still,tolimitthenumberofcapacitorvaluesused, F=CRO 1 aswell.

Table 12:CapacitorSelection

Component Value ToleranceC

R I 1 F 10%CRO 1 F 10%

4.2.1.5.8.2 LED Resistor

PWRR limitsthecurrentrunningthroughthepoweronLED, PWRD . About mA10 of

current(estimatedasagoodbalancebetweenluminanceandpowerconsumption)isdesired


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throughtheLED. TheLEDshoulddropabout V1.4 whenforwardbiased.Thevalueof PWRR is

calculatedusing:

However,as =R=R 39087 ,thenumberofdifferentvaluedresistorsusedinthe

designcanbereducedbyincreasing PWRR to 390 . Withthisvalue,thecurrentthroughthe

LEDcanbecalculated:

Witha5%resistortolerance, mAiDPWR 8.791 istheminimumLEDcurrentthatcanbe

expected.Thesecurrentsareacceptableso =RPWR 390 .

Table 13:ResistorandCapacitorSelection


PW R 390 5%

CR I 1 F 10%

CRO 1 F 10%

4.2.1.5.9 4U InverterCapacitorsThecapacitors NIC (inputvoltageripple), NOC (outputvoltageripple),and NFLYC

(flyingcapacitor)forthe 4U inverterareallsizedto F1 baseduponthegeneraluse

recommendationsoftheTPS60400datasheet.



I 1 F 10%

CO 1 F 10%

CN F LY 1 F 10%

4.2.1.5.10 GeneralPull upResistors4.2.1.5.10.1 CI2 Resistors

The CI2 linesarepulledhigh(asisspecifiedbytheprotocol)by ASDAR , ASCLR , BSDAR ,

and BSCLR ,allvaluedat k10 . The CI2 communicationwillbeconductedatlowbusspeeds

sothemaximumrecommendedvalueof k10 isused(athigherbusspeeds,theRCtime

=

mA

VV=RPWR 360

10

1.45

mA

VV=iDPWR 9.231

390

1.45


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constantofthelinemaylimitdatarates). Theuseofthislargevalueforthepullupresistors

reducespowerconsumptionofthe CI2 busversussmallervaluedresistors.



A S D A 10 k 5%

RAS C L 10 k 5%

RBS D A 10 k 5%

R B S C L 10 k 5%

4.2.1.5.10.2 Reset Resistor

TheRESETline(activelow)isheldhighby 6R . TheRESETlineisusedbothtoresetthe

7U microcontrollerandtodebugtheMCUviadebugWIRE. FromtheATtiny261Adatasheet,

therecommendedpullupresistorontheRESETlinewhenusingdebugWIREisbetween k10

and k20 . Asthe CI2 linesalreadyuse k10 pullups, k=R 106 waschosentolimitthe

numberofresistorvaluesusedinthedesign.



6 10 k 5%

4.2.1.5.10.3 PreAmplifier Enable Resistor

4R pullsthe 1U preamplifierenablelinehighduringnormaloperations. Thislineis

pulledlowbythealertpinofthe 2U ADCtodisable 1U inordertoconservepowerbetween

ADCsampling. Asallotherpullupresistorsarevaluedat k10 , k=R 104 wasalsochosento

limitthenumberofresistorvaluesusedinthedesign. 4R willdrawabout A330 whenthe

enablelineispulledlow.



4 10 k 5%

4.2.1.5.11 ICSupplyDecouplingCapacitorsDecouplingcapacitorsareplacedacrossthepowernetsofmostintegratedcircuits(ICs)

presentinthedesigninordertosmoothoutpowerlinenoiseandensureproperoperationof

theICs. ThesecapacitorsmustbeplacednearpowerpinsofeachICtobeeffective;non


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idealitiesoftracesonaphysicalcircuitboardnegatetheeffectsofthesecapacitorsifplacedfar

away. Asaruleofthumb,thesecapacitorsarevaluedat F0.1 . TheICswithsuggested

decouplingcapacitorsarenodifferent;thedatasheetsspecify F0.1 capacitors. Hence,

F=C=C=C=C=C=C=C=C 0.1109875432 .



2 0 .1 F 10%

C3 0 .1 F 10%

C4 0 .1 F 10%

C5 0 .1 F 10%

C7 0 .1 F 10%

C8 0 .1 F 10%

C9 0 .1 F 10%C

10 0 .1 F 10%

4.2.1.5.12 CrystalOscillatorCapacitorsTheMCP2510datasheetindicatestheplacementof 11C and 12C betweenthecrystal

pinsandground. However,thisdatasheetdoesnotrecommendanysizesforthesecapacitors.

SizinginformationisprovidedwithintheATtiny261Adatasheet,however,recommendsthat

thesecapacitorbebetween pF12 and pF22 . pF=C=C 201211 waschosensincethecrystal

usedinthedesigniscompatiblewiththisvalueforthecapacitors.



11 20 p F 10%

C12 20 p F 10%

4.2.1.6 SensorDesign4.2.1.6.1CurrentSensingCircuitry4.2.1.6.1.1

Current Sensing MethodologiesFourgeneralconceptswereconsideredforthecurrentmeasuringaspectofthesensor.

Theseconceptsconsistedof:highsideshunt,lowsideshunt,highsideHalleffect,andlowside

Halleffect. Highsidereferstothemeasurementtakingplaceonthelinebetweenthepower

sourceandtheload;lowsidereferstothemeasurementtakingplaceonthelinebetweenthe

loadandground. Shuntcurrentmeasurementworksbypassingcurrentthroughalowvalue

resistorandbydeterminingthecurrentflowingthroughtheresistorbasedonthevoltage


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acrosstheresistor,usingOhmsLaw( Rv=i ). Halleffectcurrentmeasurementutilizesthe

effectsofapermanentmagnetinteractingwiththemagneticfieldofcurrentflowingthrougha

conductortocreateavoltagewhichisproportionaltotheamountofcurrentflowingthrough

theconductor.

Tomeasurevoltageontheline,thesensormustbeconnectedonthehighsideoftheload(theloadvalueisnotknown). Thislowerstheattractivenessofthelowsidecurrent

measurementoptionasthelowsideoptionwouldrequireonelowsidelinein,onelowside

lineout,andalinetappingintothehighsidecabling. Thisextralinefromthehighsidecabling

tothesensordecreasestheeaseofinstallationofthesensorandmaynotbepracticalifthe

lowsideandhighsidelinesarenotincloseproximity. Thehighsidecurrentmeasuringoption

doesnothavethisproblemandrequiresonlyonehighsidelineinandonehighsidelineout.

Fewerassumptionsaboutthewiringofthevehicleareneededthanwiththelowsidecurrent

measuringoption.

Halleffectsensorsthatwereresearchedandthatmettherequirementsofthe

100 A rangeoflinecurrentwerelimitedtoanerrorof 5 %overoperatingconditions,

meaningthevoltagemeasurementwouldhavetobe100%accurateforthedevicetomeetthe

functionalrequirementsofthedesign. Shuntcurrentmeasurement,however,canmeasure

currentwithatotalerroroflessthan1%.

Fromtheseobservations,thehighsideshuntcurrentmeasurementconceptwaschosenfor

theV/Isensordesign.

4.2.1.6.1.2 Current Sensing Design

Thecurrentmeasurementcircuitryresidesonafloatingground(FGND),whichis

approximately VV 3.33.2 belowthevoltageofLINE_INatalltimes,duetolimitationsofthe

commonmodevoltagerangethatcanbesafelyhandledbyavailableinstrumentation

amplifiers(INAs). NoreadilyavailableINAcouldhandlethe V150 commonmodelinevoltage

foundintheapplication.

shuntR isa m0.2 currentshuntresistorwithafullscalevoltage(at A100 )of mV20

( W2 ). Thevoltageacross shuntR isdifferentiallyamplifiedbyafactorof VV75 bytheINA

preamplifier, 1U . Thegainof VV75 allowsforafloatinggroundtobeused(explained

below)thatisaminimumof V3.0 belowlinelevel( V3.3 belowlinelevelisdesiredbutthis

V3.0 takesintoaccountcomponenttolerances). Thisamplifiedsignalisaddedtoanoffset

voltageof FGND)IN(LINE=Voff _2

1fromthe 11U railsplitter. Thisbiasvoltageallows

bidirectionalcurrentsensingfromtheINA. TheINAwaschosenasitcanhandleaninputof

mV100 overthepositivesupplyvoltage,allowingittooperateonthefloatinggroundandstill

handle"negative"current. Theamplifiedsignalsuperimposedonthebiasisthenfedintoa12


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bitADCwithPGA( 2U ). ThePGAof 2U canamplifytheincomingsignalupto VV 16 ,

allowingforagreaterdynamicresolutionofthecurrentsensor(i.e.thevoltagesproducedby

smallercurrentscanbeamplifiedevenmore,resultinginagreaterresolutionatsmaller

currents). AfterthePGA,thevoltagesignalisdigitizedby 2U s12bitADC.Asthemain

microcontroller(thatcollectsthecurrentdata)ofthesensorisrunningatthe V50 level,the

ADCs CI2 outputsignal(FGNDlevel)mustbepassedthroughanisolateddigitalcoupler,

whichisolatestheFGNDcircuitryfromthe V50 circuitrywhilestillallowingthedigitaldata

tobetransferred.

Thecircuitryonthisfloatinggroundwilldrawanestimatedmaximumof mA10.5 from

thelinewhichitismeasuring(LINE_IN). Amajorityofthiscurrent(about mA5.5 )isdrawnby

the CI2 busandthe CI2 isolationcircuitry. Toconservepoweronthefloatinggroundline,

theADCsalertpinispulledlow(drawingabout A330 )andtheINAisputintoa"sleep"

mode,whereitdrawsatypicalcurrentof A2 versusthetypicalINAoperatingcurrentofmA2.4 .

TodealwiththefactthatFGNDisnotastablereferenceandtoremovethebiaserrors

associatedwithallofthecurrentmeasuringanalogfrontendcircuitry,solidstaterelays(SSRs),

12U and 13U ,areusedtoallowtheinputsofthe 1U INAtobe"switched". Whenswitched,

thenoninvertinginput(whichisnormallyconnectedtothe"top"of shuntR )isconnectedtothe

"bottom"of shuntR andtheinvertinginput(whichisnormallyconnectedtothe"bottom"of

shuntR )isconnectedtothe"top"of shuntR . Ameasurementoftheamplifiedvoltagewillbe

takenwiththeinputs"normal"andthen,immediatelyafter,theinputstotheINAwillbe

switchedandanothermeasurementwillbetaken. Fromthesetwomeasurements,thesignal

offset(neededforthebidirectionalsensingcapability)thatispresentattheADCinputcanbe

determinedandtheactualmeasurementvoltagecanbeproperlycalculated. Thecontrol

signalsfortheseSSRscanbedirectlydrivenfromadigitaloutputpinoftheMCUduetothe

electricalisolationbetweenthecontrolandswitchcircuitrywithintherelays. Thepinmust

onlysource mA10.3 totherelaycontrolswhenthelineisraisedhighand A0 whenthe

controllineislow.

FGNDisgeneratedbyashuntregulatorcomprisedof1

Q 3

Q ,1

D ,andbias

R . The

V3.9 zenerdiode, 1D ,"hangs"offoftheLINE_INline,providingareferencevoltageof V3.9

belowthelinevoltage. Thebasiccurrentmirrorformedby 1Q 2Q and biasR generatesa

currentofabout uA360 thatbiases 1D overtherangeoflinevoltages. Thegroundcurrentof

thedevicesonthefloatinggroundisroutedthroughthecollectoremitterof 3Q sothatthe


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majorityofthebiascurrentgoesthrough 1D (andnottheload). The BEV voltageof

approximately V0.6 to V0.7 raisestheactualFGNDvaluetoabout V3.2 to V3.3 below

theLINE_INlinevoltage.

Tobias 1D ,asimplecurrentmirrorwaschosenoveramoreadvancedcurrentsource,

suchasafullWilsoncurrentsource,asthe 12U and 13U SSRsolutionremovesanyoffset

voltageerrorswithinthecurrentsensingcircuitryfromthefinaldata. Alargesourceofthis

offseterroristhechangingofthevoltageacross 1D duetobiascurrentfluctuations,causedin

partbychanginglinevoltagesandsecondorderbehaviorsoftheBJTs. Thesefluctuations

impactthevalueofFGNDand,therefore,changethelinesplitoffsetvoltageoftheINA.Amore

advancedcurrentsourcewouldreducethesefluctuationsbutnoteliminatethem,astheSSR

solutiondoes. Therefore,asimplecurrentsourceisallthatisneededtobias 1D (withtheSSR

solutioninuse).

TheFGNDshuntregulatorisreferencedto V5 rathertheGNDtoaccommodatean

FGNDbelow V0 . ThisscenariooccurswhentheLINE_INlinevoltagedropsbelow V3.2 or

V3.3 andthepresenceofthe V5 referenceallowsthecurrentsensingcircuitrytooperate

onLINE_INlinevoltagesdownto V0 . The V5 isgeneratedbytheinverter 4U .

4.2.1.6.2VoltageSensingCircuitryThevoltagesensingcircuitryisbaseduponasimplevoltagedivider( 1VR and 2VR )and

unitygainvoltagebuffer( 5U ). Thevoltagedividerdividestheinputlinevoltage( V1500 )to

therangeofapproximately V50 ,a 301 ratio. Theactualratioisabitlower(i.e.32.131 )tocompensateforcomponentnonidealities,suchasa2%errorinthe V5 voltage

regulatorthatsuppliespowertothebufferandmicrocontroller. Currentdrawnthroughthe

voltagedivideratthemaximumlinevoltageof V150 is uA149 . Asboth 1VR and 2VR will

beconstructedusingthesametechnologiesandshouldhavesimilardevicecharacteristics

(exceptforresistance),thetworesistorsshouldexperienceroughlythesametemperatureand

anyeffectsoftemperatureshouldaffectbothresistorsinapproximatelythesameway. If,for

example,temperaturechangescausethevalueof 1VR toincreaseby0.05%,thesame

temperatureeffectsshouldalsocausethevalueof2V

R toincreasebyabout0.05%. These

changescanceleachotheroutwithinthevoltagedividerratio,causingtheratiotoremainthe

same.

2R protectsthe 5U opampfromvoltagespikesontheline.

Thebufferedoutputofthedividedvoltagesignalisdigitizedbythe10bitADConboard

theMCU( 7U ).


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4.2.1.7 DigitalCircuitryThe CI2 currentdataandtheanalogvoltagedataistakenbytheMCU( 7U ),any

calibrationdataisappliedtotherawvoltageandcurrentreadings,andthereadingsaresent

outtotheCANbus. 8U istheCANcontrollerthattakestheMCUdataandformsaproperCAN

message,whichissenttotheCANtransceiver( 10U ). TheCANtransceiverinterfaceswiththe

physicalCANbusandconvertstheCANmessageintothepropervoltagesignalstosendover

thebus.

4.2.1.8 V/ISensorFirmwareTheV/ISensorcodeworksbyqueryingthevoltageandcurrentmeasurementanalogto

digitalconverters(ADCs)forashortperiodoftime(about200 s),invertingtheinputsofthe

currentsensecircuitry,collectingmorevoltageandcurrentsamples,convertingtheADC

samplestovoltageandcurrentreadings,andthensendingthedataoutover5VlogicallevelRS

232(ifthesensorisbeingusedindependentlyoftherestofthesystem)orovertheCANbustoanexternalcomputerfordataloggingandvisualization.Theinversionoftheinputstothe

currentsensecircuitryallowsthecodetodeterminethe0Abiasvoltagethatispresentedtothe

ADCateverymeasurement,eliminatingallbiasingerrors(e.g.temperaturedependentbiasing

changes)intheanalogcurrentsensecircuitry. Specificscanbefoundintheheavily

documentedcode,ontheenclosedCDintheFirmwarefolder.


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4.2.2 SpeedSensorAHallEffectspeedsensorwasdeterminedtobethebestfitforthisproject. TheHall

Effectsensorisplacedbetweenthegearteethoftheshaftandamagnet. Thesensormeasures

thevariationinmagneticfieldbetweenthemagnetandthepassinggearteeth. Thechallenge

thatwearefacedwithonthesolarcaristhegearteethontheshaftaremadeoutofaluminum

soaregularHallEffectsensorwouldnotwork. ThatiswhyanIS1101SingleChannelSensor

withDigitalOutputwasselected. TheIS1101isasinglechanneldifferentialinductivesensor

suitableforcountingonperiodictargetslikegears,cogwheels,racks,slotteddiscs,PCBtargets

andforedgedetectionofmetallicobjectswithlargerdimensions. Thetargetmaterialcanbe

eitherferromagnetic(steel)orelectricallyconducting(aluminum,brass,andcopper).

GeneralSpecifications:

Countingfrequency 040kHz

Airgap upto1.5mm

Supply 5.00.5V,15mAtyp

Temperature 0100C

Outputformat digital

Figure 11:IS1101Single ChannelSensorwithDigitalOutput


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4.2.3 TiltSensor/AccelerometerThetripleaxisaccelerometerbreakout SCA3000waschosenforthefinaldesignforthe

tiltsensorandaccelerationsensor. ItisalowpoweraccelerometerwithdigitalSPIinterface.

Itssupplyvoltageneedstobebetween2.35V3.6VandthedigitalI/Ovoltageisbetween1.7

V3.6V. Thisdevicealsohasa2gmeasurementrangewhichisallthatisneedbasedonthe

informationthesponsorgavewhichwasthatthecarwillexperienceabout1ginacceleration

and 2gwhenstopping.

Anaccelerometermeasurestheamountofforceitexperienceswhenitismovedfrom

itsoriginalpositionthereforethetiltcanbedeterminedfromtheforcethroughsome

calculations,justlikehowtheaccelerationisfoundthroughcalculationsusingthe

measurementsofforceaswell.

TheaccelerometerconnectstothesensorhubviaCANbusshowninFigure12:

Figure 12:OverallConnection forAccelerometer


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4.2.4 SensorHub4.2.4.1 SensorHubCircuitryDescription

Figure13 :SensorHubSchematic


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4.2.4.1.1I/ORatingsTable 20: Input/OutputRatings

VBAT Powersource( V305.8 ,reversebiasprotectedupto 30V)

CANH,CANL CANbuscommunicationslines( V50 nominal, V36 maximum

continuous)

Seriallines RS232levels( V30 maximum)

4.2.4.1.2OverviewThesensorhubprovidesaninterfacetothevarioussensorsandaggregatestheirdata.

Thisaggregateddataisthensentouttothegroundstationcomputerinaserialstream. The

serialstreammustefficientlypackdataintothefewestbitspossibletobecompatiblewithslow

serialbaudrates(suchas9600Baud/s)thatmayberequiredbythetelemetryhardware.Forwiredtelemetry,thesensorhubmaybedirectlyconnectedtothegroundstation

computerviaaserialcable. Forwirelesstelemetry,aradiomodemmaybeconnectedtothe

sensorhubandanotherradiomodemconnectedtothegroundstationcomputer. These

modemsacttotunnelserialdatabetweenthesensorhubandgroundstationcomputer

nodes.

ThesensorhubhastheabilitytointerfacewithoneRS232serialdevice(e.g.theBPS)

andupto13CANbusequippeddevices(thisdevicelimitationisduetothenumberofphysical

connectors). Thesensorhubwillalsoprovidepowertoallsensorsdesignedorsuppliedbythe

project.

4.2.4.1.3ConnectorsThepowersourceconnector( 1JP)suppliespowertothesensorhub(andtoexternal

sensors). TheconnectorisaDeansUltramaleplug,aconnectorthatiscompatiblewithmany

batteriesusedforR/Capplications.

Theoutputpoweranddataconnector( 2JP )suppliesunregulatedpowertoexternal

customsensorsandalsoprovidesaccesstothetwoCANcommunicationlines(CANHand

CANL). TheconnectorisanRJ45connectorastheintendedcablingisCAT5UTPpatchcable.

Thecablingwaschosentoaccommodatethesuggested 120 characteristicimpedanceoftheCANbuslinesandasthetwistedpairdesignofCAT5cablingmaximizesthenoisecanceling

capabilityoftheCANbusdifferentialsignalcommunications. CAT5UTPcableisreadily

availableduetoitsuseasEthernetpatchcabling.

TheISPconnector( 3JP )allowsforinsystemprogrammingoftheATmega162

microcontroller(MCU).


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ThetwoRS232seriallines(telemetryI/OandperipheralI/O)useDB9connectors( 3JP

and 4JP )asthesearethemostcommonconnectorsusedforRS232communicationsandare

abletointerfacewiththecurrenttelemetryandperipheralhardware.

4.2.4.1.4DigitalCircuitry4.2.4.1.4.1 Microcontroller

Thesensorhubmicrocontroller, 1U ,istaskedwithaggregatingdatafromvarious

sensorsandoutputtingthisaggregateddataastelemetry. Thetelemetryispackedefficiently

sothataminimumnumberofbitsareneededforthedata. Thisisimportantwhenusing

slowertelemetrydevices,suchasa9600Baud/sradiomodem,sothatinformationcanbesent

inatimelymannerandwithoutbacklog. The 1U isanAVRATmega162withenoughprocessing

powerandmemorystoragetoaccomplishthistask(andmore,ifneeded). Additionally,the

MCUfeaturesdualUARTs(universalasynchronousreceiver/transmitters). TheseUARTsarehardwarefeaturesthatallowforeasydualfullduplexserialcommunications. Thisallowsfor

simultaneouscommunicationsbetweenthesensorhubandBPSandthesensorhubandground

stationcomputer(viatelemetry).

4.2.4.1.4.2 Serial Interface

TwoRS232serialinterfacesexist:onefortelemetrycommunicationsandonefor

peripheralsensorcommunications. Bothoftheseinterfacesarefullduplex,meaning

informationcanbesentandreceivedsimultaneously. Althoughthisfeaturewillmostlikelynot

beusedwiththecurrentsystemsetup,theabilitytoperformfullduplexserialcommunications

isimportanttoavoidlimitingfutureapplicationsofthesensorhub.

Whilethe 1U microcontrolleroperatesatthe V50 level,theRS232lineoperatesat

V12 . Inordertointerfacethesetwovoltageslevels,the 2U TRS232ERS232driver/receiver

isused. TheTRS232Esupportsdualfullduplexcommunicationsandispoweredbythe V5

supply. ChargepumpsareusedtogeneratetheneededRS232voltageslevels.

4.2.4.1.4.3 CAN Bus Interface

MuchofthesensordataisretrievedfromtheCANbus(whichsupportsupto13

sensors/devices).

8U istheCANcontrollerthattakestheMCUdataandformsaproperCANmessage,

whichissenttotheCANtransceiver( 10U ). TheCANtransceiverinterfaceswiththephysical

CANbusandconvertstheCANmessageintothepropervoltagesignalstosendoverthebus.

TheprocessworksinreverseforCANmessagesbeingreceived.


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4.2.5 CommunicationsHubTheschematicforthecommunicationshubisshowninFigure14:

Figure 14:Communication Hub Schematic

4.2.5.1 CommunicationsHubDescriptionThepurposeofthecommunicationshubistolimittheconnectorstoexternalsensors

onthesensorhubbyprovidinganauxiliarydedicatedconnectorboardforthesensorpower

andCANcommunicationscabling. Thismakesiteasytoupdatethesystemtohandleadditional

sensorsontheCANbus(eachofwhichrequirestheirownconnectortointerfacewiththebus):

simplydaisychainanothercommunicationshubboardforadditionalconnections. The

communicationshubcanalsolimittheamountofcablingusedtoconnectthesensorhubtothe


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sensors. Forexample,ifhalfofthesensorsarephysicallynearthesensorhubandtheother

halfareontheothersideofthevehicle,onecommunicationshubcouldbeusedtohandlethe

nearsensorsandanotherthefarsensors. Thefarsensorscablingwouldonlyhavetoreachthe

farcommunicationshub(whichisclosetothesefarsensors)andasinglecablefromthis

communicationshub

could

be

run

to

the

other

hub,

which

connects

to

the

sensor

hub,

rather

thanhavingallofthefarsensorcablesrunningbacktothesensorhub.

ThecommunicationshubisasimpleboardconsistingofeightRJ45connectorswithall

pin1sconnectedtogether,allpin2sconnectedtogether,etc.,uptothefourthpin. Pin1is

VBAT(batterypositivevoltage);pin2isGND(systemground),pin3isCANH(CANbushigh

line);andpin4isCANL(CANbuslowline). TheVBATandGNDwiressupplypowertocustom

sensorsandtheCANHandCANLlinesprovideaCANbusconnectionusedbythemajorityofthe

sensors(eventothosenotacquiringpowerfromthiscabling).

Aspreviouslymentioned,acommunicationshubboardmaybedaisychainedwith

another,expandingthenumberofsensorsthatcancommunicateovertheCANbus. Asthe

designisrequiredtosupport13devicesontheCANbus,atotaloftwocommunicationshubs

willbeused(daisychainedtogether)toprovideall13connectors.


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4.3 TelemetryThetelemetrycomponentselectedforthefinaldesignistheXStreamOEMRFModule.

Accordingtothedatasheet,thesedevicesarecapableofanoutdoorlineofsightrangeof7

mileswhichexceedsourFRforthetelemetryrange. Thisdevicehasafrequencyof900MHz

andreceiversensitivityof110dBm. Theonlyconcernwehaveisthatthethroughputdatarateof9,600bpsmaynotbeenoughforallofthedatawehavetotransmitbetweenthecarand

computeoverthetelemetrysystem.

ThesensorhubandthecomputerwillinterfacewiththeXStreamModulesasshowin

thefollowingfigures. InFigure15thesensorhubishostAandthecomputerishostBbecause

hostAisonthetransmissionsideandhostBisonthereceiverside. Figure16showsthecables

thatwillbeusedtomaketheconnectionsshowninFigure15.

Figure15:SystemBlockDiagramforTelemetry

Figure16:ModuleAssemblyfromthe ProductManual


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4.4 Software4.4.1 CaptureandVisualizationSoftwareDescription

Figure 17:Visualization Software

4.4.1.1 OverviewThecaptureandvisualizationsoftwareisusedtocapturetelemetrysentoutfromthe

sensorhubandtoplotthecaptureddatainrealtimeorafteradatacapture. Thesoftwareis

runonthegroundstationcomputer(GSC)andiswrittenusingLabView. TheLabViewenvironmentwaschosenforitseaseofuseincreatinggraphicaluserinterfaces(GUIs)andfor

itsabilitytoeasilyinterfacewithexternaldevices.

4.4.1.2 CaptureThesoftwarecapturesatelemetrystreamsentfromthesensorhubandparsesthis

streamintomeasurementsfromdifferentsensors(e.g.voltage,current,tiltreadings). Thedata

streamisaccessedfromasoftwareserialdevice. Anydevicewithasoftwaredevicedriver

replicatingaserialport(e.g.aUSBtoserialadaptor)maybeusedtosendthetelemetry

informationtothedevice. ThecurrentsetupinterfacesaradiomodemwithRS232outputto

theGSCviaaUSBtoserialadapter.

Oncethetelemetrydatastreamisparsed,thedataissavedtotexttabdelimitedfile. A

tabdelimitedfileseparatesfields(columns)witha"tab"delimiterandseparatesentries(rows)

bynewlines. Eachsensordatarecordisplacedinitsownfieldandeachcompletedataquery

(i.e.datafromallsensorstakenatonetime)isplacedinitsownentry. Thistextformatwas

chosenbecausetabdelimitedfilescanbereadbymanycommonapplications(e.g.Notepad,


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Excel)andthedatacanbestoredinahumanreadableway. Thishelpstoensurethedatawill

beabletobeaccessedinthefuture.

4.4.1.3 VisualizationDataisplottedonaconfigurablecanvas. Alldatacanbedisplayedorthedisplaycanbe

limitedtothelastnsecondsofdata(wherenisanarbitraryrealnumber)ifthedisplayeddata

isfromarealtimesource.

Thecanvasconsistsofoneormoreplots,whichcanbeaddedorremovedfromtheuser

interface(UI). Eachplotcandisplaydatafromanysensororanycombinationofsensors,all

configurablethroughLabViewutilities. Theplotscanberearrangedonthecanvasviatheup

arrowanddownarrowbuttons. Fordatafromstored(nonrealtimesources),theplotswill

havetheabilitytozoominondifferentportionsofthedata.


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4.5 Mechanical4.5.1 WheelAlignmentTool

Theteamchoseadigitalalignmenttooloutofthetwodesignsthatwerebeing

considered. ThefinaldesignisasshowninFigure18:

Figure 18:FinalDigitalAlignmentToolDesign

4.5.1.1 TheArmsThetoolhasthreearmswhichareusedtoattachtothetiresofanycar

4.5.1.1.1MaterialTheAluminumis6063T53metal.Thismetalwasthecheapestmetalwecouldfindtofit

thedesignhopedfor,whichissquaretubing1/2thick. Althoughitwasthecheapest,itwas

finefortheamountofworkwerequired. Themetalhasyieldstrengthof16,000psi. Thefactor

ofsafetywasinthenumberofhundredsduetotheonlyweightbeingthedigitalprotractor. To

findthissafetyfactor,thefollowingequationwasused:

Equation 18 Safetyfactordueto static loading

N=safetyfactorS=materialstrengthA=areaofloadingP=loadingforce


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4.5.1.1.2DesignThetwobottomarmsconsistoffootlongaluminumsquaretubinghaving10holesfor

configurationandabottomholetoattachtothedigitaltool. Theholesareinplaceevery.

Thisnumberwaschosenbasedonthewheelsizesoftrailertiresthatthesolarcarteamwas

using. Thedigitalprotractorwasgoingtobemostaestheticwhileplacedinthecenterofthe

tire. Thusthearmsshouldbeapproximatelytheradiusofthetire. Usingthatasareference

thearmswerepunchedalongthemostcommonradiioftrailertires. Theholeswereasizeof

3/8touseahandscrewofsuchadiametertoattachtothetires. Theholesweretappedfor

thehandscrewtoensuremostaccurateresults. Thefinalbottomholewasselectedtobe

diametertobeusedwiththeotherhandscrewswhichattachedittothedigitalprotractor.

Belowisaschematicofthisarm:

Figure 19 Wheelalignmentbottomarms


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Thetoparmspurposeistosolidifythetoolintoplace. Whilethetwobottomarms

simplyrestonthebottomrim,thetopsecuresthetoolsoastonothaveitfalloffthetire.

Thereforeitwaschosenthatthetoparmwouldhaveaventedcutalongthesamedistanceof

theholes. Whilethetooldoeslookmostaestheticwhilebeingheldinthecenterofthetire,

themeasurement

will

be

the

same

wherever

it

is

placed.

As

a

result

the

tool

was

best

held

on

byaslidehandscrewonthetoparm. Thisslidehandscrewwouldslidealongtotheverytopof

therim,andscrewedintoplacethereforecreatingenoughfrictionalforcetonotallowthetool

tofalloffthetire. Thethicknessofthisholeisthesameastheotherholestoallowformultiple

ordersofthesamehandscrew. Thebottomholeonthisarmhoweverhadtomeetthe

diameterofthescrewattachingtheprotractortothearms,andthereforewas3/8. Belowisa

schematicofthearm:

Figure 20:Top armforDigitalTool


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4.5.1.2 HandScrewsThehandscrewswereselectedbasedonappropriatecompatiblesizing.

4.5.1.2.110x1.5screwsTherearethree10x1.5handscrewsusedonthetool. Thesethreescrewsareusedto

attachthetooltothetire. Theselectionwasbasedonthesideofthethreads. Usingathread

countsmallerthan1.5mmallowedfor4ofthethreadstobeconnectedtothetirerimofthe

currentcar. Whileonlyonethreadisrequiredtocreatethefrictionalforcerequiredahigher

safetyfactorwasgoodtohavebasedonthefactthatsmallertirerimscouldbeusedinthe

future. Theequationusedforfrictionalforceisbelow:

Equation 19 FrictionalForce

Ff=frictionalforce

Thisscrewalsoworkedduetothelengthofthescrew. Itwasoneofthefewhand

screwswecouldfindthatwasatleast2inchesinlength.

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