sensors and actuators - computer science &...

Ioannis Rekleitis

SensorsandActuators

RobotSensors

•  Sensorsaredevicesthatcansenseandmeasurephysicalpropertiesoftheenvironment,•  e.g.temperature,luminance,resistancetotouch,weight,size,etc.

•  Thekeyphenomenonistransduction•  Transduction(engineering)isaprocessthatconvertsonetypeofenergytoanother

•  Theydeliverlow-levelinformationabouttheenvironmenttherobotisworkingin.–  Returnanincompletedescriptionoftheworld.

CSCE 574 Robotics 2

RobotSensors

•  Thisinformationisnoisy(imprecise).•  Cannotbemodelledcompletely:

–  Reading=f(env)wherefisthemodelofthesensor–  Findingtheinverse:

•  illposedproblem(solutionnotuniquelydeEined)•  collapsingofdimensionalityleadstoambiguity

CSCE 574 Robotics 3

Typesofsensor

•  GeneralclassiEication:– activeversuspassive

• Active:emitenergyinenvironment– Morerobust,lessefEicient

• Passive:passivelyreceiveenergyfromenv.– Lessintrusive,butdependsonenv.e.g.lightforcamera

• Example:stereovisionversusrangeEinder.– contactversusnon-contact

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Sensors

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•  ProprioceptiveSensors(monitorstateofrobot)

–  IMU(accels&gyros)– Wheelencoders–  Dopplerradar…

•  ExteroceptiveSensors(monitorenvironment)

–  Cameras(single,stereo,omni,FLIR…)

–  Laserscanner– MWradar–  Sonar–  Tactile…

Sensors

CSCE 574: Robotics - A. Quattrini Li 6

•  Rawdataprocessedatdifferentlevels,dependingonthesensor–  Low:electronics– Medium:signalprocessing–  High:computation

09/06/2017

Source: [Mataric, 2007, MIT Press]

SensorCharacteris4cs

•  Allsensorsarecharacterizedbyvariouspropertiesthatdescribetheircapabilities– Sensitivity:(changeofoutput)÷(changeofinput)

– Linearity:constancyof(output÷input)• Exception:logarithmicresponsecameras==widerdynamicrange.

– Measurement/Dynamicrange:differencebetweenmin.andmax.

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SensorCharacteris4cs

– ResponseTime:timerequiredforachangeininputtocauseachangeintheoutput

– Accuracy:differencebetweenmeasured&actual

– Repeatability:differencebetweenrepeatedmeasures

– Resolution:smallestobservableincrement– Bandwidth:resultofhighresolutionorcycletime

CSCE 574 Robotics 8

TypesofsensorSpeciEicexamples

–  tactile–  close-rangeproximity–  angularposition–  infrared–  Sonar–  laser(varioustypes)–  radar–  compasses,gyroscopes–  Force–  GPS–  vision

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•  Therearemanydifferenttechnologies–  e.g.contactclosure,magnetic,piezoelectric,etc.

•  FormobilerobotsthesecanbeclassiEiedas–  tactilefeelers(antennae)oftensomeformofmetalwirepassingthroughawireloop-canbeactive(poweredtomechanicallysearchforsurfaces)

§  tactilebumperssolidbar/plateactsonsomeformofcontactswitch

e.g.mirrordeElectinglightbeam,pressurebladder,wireloops,etc.

§  Pressure-sensitiverubberwithscanningarray

Tac4leSensors

“last line of defense” 10 CSCE 574 Robotics

Tac4leSensors(more)

•  Vibrassae/whiskersofrats– Surfacetextureinformation.– DistanceofdeElection.– Blindpeopleusingacane.

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ProximitySensors

•  Tactilesensorsallowobstacledetection– proximitysensorsneededfortrueobstacleavoidance

•  SeveraltechnologiescandetectthepresenceofparticularEieldswithoutmechanicalcontact– magneticreedswitches

•  twothinmagneticstripsofoppositepolaritynotquitetouching

•  anexternalmagneticEieldclosesthestrip&makescontact


ProximitySensors

– Halleffectsensors•  smallvoltagegeneratedacrossaconductorcarryingcurrent

–  inductivesensors,capacitivesensors•  inductivesensorscandetectpresenceofmetallicobjects

•  capacitivesensorscandetectmetallicordielectricmaterials

BIVH ×∝


InfraredSensors

•  Infraredsensorsareprobablythesimplesttypeofnon-contactsensor–  widelyusedinmobileroboticstoavoidobstacles

•  Theyworkby–  emittinginfraredlight

•  todifferentiateemittedIRfromambientIR(e.g.lights,sun,etc.),thesignalismodulatedwithalowfrequency(100Hz)

–  detectinganyreElectionsoffnearbysurfaces•  Incertainenvironments,withcarefulcalibration,IRsensorscanbeusedformeasuringthedistancetotheobject–  requiresuniformsurfacecoloursandstructures


InfraredSensors(Sharp)

•  Measuresthereturnangleoftheinfraredbeam.

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InfraredProblems•  IftheIRsignalisdetected,itissafetoassumethatanobjectispresent

•  However,theabsenceofreElectedIRdoesnotmeanthatnoobjectispresent!–  “Absenceofevidenceisnotevidenceofabsence.”C.Sagan

–  certaindarkcolours(black)arealmostinvisibletoIR–  IRsensorsarenotabsolutelysafeforobjectdetection

•  Inrealisticsituations(differentcolours&typesofobjects)thereisnoaccuratedistanceinformation–  itisbesttoavoidobjectsassoonaspossible

•  IRareshortrange–  typicalmaximumrangeis50to100cm


SonarSensors•  Thefundamentalprincipleofrobotsonarsensorsisthesameasthatusedbybats–  emitachirp(e.g.1.2milliseconds)

•  ashortpowerfulpulseofarangeoffrequenciesofsound–  itsreElectionoffnearbysurfacesisdetected

•  Asthespeedofsoundinairisknown(≈330m·s-1)thedistancetotheobjectcanbecomputedfromtheelapsedtimebetweenchirpandecho– minimumdistance =165tchirp(e.g.21cmat1.2ms)– maximumdistance =165twait(e.g.165mat1s)

•  Usuallyreferredtoasultrasonicsensors


SonarProblems•  Thereareanumberofproblemsanduncertaintiesassociatedwithreadingsfromsonarsensors–  itisdifEiculttobesureinwhichdirectionanobjectisbecausethe3Dsonarbeamspreadsoutasittravels

–  specularre5lectionsgiverisetoerroneousreadings•  thesonarbeamhitsasmoothsurfaceatashallowangleandsoreElectsawayfromthesensor

•  onlywhenanobjectfurtherawayreElectsthebeambackdoesthesensorobtainareading-butdistanceisincorrect

–  arraysofsonarsensorscanexperiencecrosstalk•  onesensordetectsthereElectedbeamofanothersensor

–  thespeedofsoundvarieswithairtemp.andpressure•  a16°Ctemp.changecancausea30cmerrorat10m!


LaserRangeFinders•  LaserrangeEinderscommonlyusedtomeasurethedistance,velocityandaccelerationofobjects– alsoknownaslaserradarorlidar

•  Theoperatingprincipleisthesameassonar– ashortpulseof(laser)lightisemitted–  thetimeelapsedbetweenemissionanddetectionisusedtodeterminedistance(usingthespeedoflight)

•  Duetotheshorterwavelengthsoflasers,thechanceofspecularreElectionsismuchless– accuraciesofmillimetres(16-50mm)over100m– 1Dbeamisusuallyswepttogivea2Dplanarbeam

•  Maynotdetecttransparentsurfaces(e.g.glass!)ordarkobjects


RADAR

•  Radarusuallyuseselectromagneticenergyinthe1-12.5GHzfrequencyrange–  thiscorrespondstowavelengthsof30cm-2cm

• microwaveenergy– unaffectedbyfog,rain,dust,hazeandsmoke

•  Itmayuseapulsedtime-of-Elightmethodologyofsonarandlidar,butmayalsouseothermethods– continuous-wavephasedetection– continuous-wavefrequencymodulation

•  Continuous-wavesystemsmakeuseofDopplereffecttomeasurerelativevelocityofthetarget


AngularPosi4on:RotaryEncoder•  Potentiometer

– UsedintheServoontheboebots•  OpticalDisks(Relative)

– Countingtheslots– Directionbyhavingparsofemitters/receiversoutofphase:Quadraturedecoding

– Canspinveryfast:500kHz


AngularPosi4on:RotaryEncoder

•  OpticalDisks(Absolute)– Greyencodingforabsolute:

•  0:0000,1:1000,2:1100,3:0100,4:0110,•  5:1110,6:1010,7:0010,8:0011•  9:1011,10:1111,11:0111,12:0101,13:1101,14:1001,15:0001


CompassSensors

•  Compasssensorsmeasurethehorizontalcomponentoftheearth’smagneticEield– somebirdsusetheverticalcomponenttoo

•  Theearth’smagneticEieldisveryweakandnon-uniform,andchangesovertime–  indoorstherearelikelytobemanyotherEieldsources•  steelgirders,reinforcedconcrete,powerlines,motors,etc.

– anaccurateabsolutereferenceisunlikely,buttheEieldisapprox.constant,socanbeusedforlocalreference


Gyroscopes

•  Agyroscopeisaspinningwheelwithmostofitsmassconcentratedintheouterperiphery–  e.g.abicyclewheel

•  Duetothelawofconservationofmomentum–  thespinningwheelwillstayinitsoriginalorientation–  aforceisrequiredtorotatethegyroscope

•  Agyro.canthusbeusedtomaintainorientationortomeasuretherateanddirectionofrotation

•  Infacttherearedifferenttypesofmechanicalgyro.–  andevenopticalgyro’swithnomovingparts!

•  thesecanbeusedine.g.spaceprobestomaintainorientation


Vibra4ngStructureGyroscopes

Halteres

MEMS


Ringgyro's

•  Usestandingwavessetup– betweenmirrors(laserringgyro)– withinaEiberopticcable(Eibreopticringgyro)

•  Measurerotationbyobservingbeatsinstandingwaveasthemirrors"rotatethroughit".


IMU's•  Gyro,accelerometercombination.•  Typicaldesigns(e.g.3DM-GX1™)usetri-axialgyrostotrackdynamicorientationandtri-axialDCaccelerometersalongwiththetri-axialmagnetometerstotrackstaticorientation.

•  TheembeddedmicroprocessorscontainsprogrammableEilteralgorithms,whichblendthesestaticanddynamicresponsesinreal-time.


GPS

•  GPSusesaconstellationofbetween24and32MediumEarthOrbitsatellites.

•  Satellitebroadcasttheirposition+time.•  Usetraveltimeof4satellitesandtrilateration.•  Suffersfrom“canyon”effectincities.


WiFi

•  UsingtheSSIDanddatabase.


OdorsensingSmellisubiquitousinnature…bothasaactiveandapassivesensor.Whyisitsoimportant?

Advantages:evanescent,controllable,multi-valued,useful.

References:[1]T.Hayes,A.Martinoli,andR.M.Goodman.“SwarmRoboticOdor

Localization:Off-LineOptimizationandValidationwithRealRobots”.SpecialissueonBiologicalRobotics,Robotica,Vol.21,Issue4,pp.427-441,2003.©CambridgeUniversityPress

[2]T.Yamanaka,R.Matsumoto,andT.Nakamoto,“Fundamentalstudyofodorrecorderformulti-componentodorusingrecipeexplorationmethodbasedonsingularvaluedecomposition”,IEEESensorsJournal,Vol.3,Issue4,2003,pp.468-474.


Interfacing

CSCE 574: Robotics - A. Quattrini Li 31 09/06/2017

•  Protocols–  RS-232–  I2C–  Ethernet–  USB–  …

•  Eachofthemhasdifferentcharacteristics,e.g.,bandwidth,powerconsumption,maximumcablelength

•  ReadthespeciEicationstointerpretcorrectlythedata

ROSdrivers


•  E.g.,laserhttps://github.com/ros-drivers/urg_node/blob/indigo-devel/src/urg_node_driver.cpp

Whatisanactuator?

•  Deviceformovingorcontrollingasystem.

•  “RobotMuscles”


HydraulicActuators

•  Pros:– Powerful– Fast– Stiff

•  Cons– Messy– Maintenance– ExternalPump


HydraulicActuatorApplica4on•  BigDogfromBostonDynamics


Pneuma4cActuators

•  Pros:– Powerful– Cheap

•  Cons– Soft/Compliant– ExternalCompressor


Pneuma4cActuators•  3DBiped(‘89-’95)fromMITLegLab


AirMuscle


AirMuscleApplica4on


PiezoelectricActuator

•  Pros– Verysmallmotionspossibleathighspeeds

– Low-powered•  Cons

– Shortactuation– Highvoltages– Expensive– Fragile


Source: robotics.eecs.berkeley.edu/~ronf

Folded millirobots (2008)

Source: robotplatform.com

PiezoelectricActuator


ShapeMemoryAlloyActuators

•  WorksbywarmingandcoolingNitinolwires.

•  Pros:– Light– Powerful

•  Cons:– Slow(cooling)


S4quito

Jonathan Mills, Indiana University CSCE 574 Robotics 44

ElectricActuators

•  Pros– Betterpositionprecision– Wellunderstood– Noseparatepowersource– Cheap

•  Cons– Heavy– Weaker/slowerthanhydraulics– Coolingissue


ElectricActuators

•  Steppermotors•  DCmotors

– Servos•  Continuous•  Position

•  Others(notdiscussed)– Linearactuators– ACmotors


S

N

Stator:madeoutofcoilsofwirecalled“winding”

Rotor:magnetrotatesonbearingsinsidethestator

•  Directcontrolofrotorposition(nosensingneeded)• Mayoscillatearoundadesiredorientation(resonanceatlowspeeds)• Lowresolution

printers computer drives

S N

Electromagnetstator

rotorN S

StepperMotorBasics


Half stepping

S

S

N

N

angle

torque

IncreasedResolu4on


More teeth on rotor or stator

Half stepping

S

S

N

N

IncreasedResolu4on


MoreTeethonRotor


•  Pros: – Direct position control – Precise positioning – Easy to control

•  Cons: – Oscillations – Low torque at high speeds

StepperMotors


DCmotors--exposed!


N

S

N S

stator

rotor

permanent magnets

commutator attached to shaft

V+

-

brush

DCmotorbasics


N

S

N S

stator

rotor

permanent magnets

N S S N

V+

-V

+

-

DCmotorbasics


N

S

N S

stator

rotor

permanent magnets

N S S N N S N S

V+

-V

+

-V

+

-

DCmotorbasics


DCmotortorqueτ

I∝τ

torque=τcurrent =I


Processing


Processing(Example)


Communica4on

•  Tethered•  Untethered

– Radiomodems– 802.11Wirelessnetworks–  Infrared– RC– Bluetooth– GSM/GPRS– Acoustic


Summary

•  Hardwarefor– Sensing– Locomotion– Reasoning– Communication


sensors and actuators - computer science &...

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