sensors and actuators - computer science &...
TRANSCRIPT
Ioannis Rekleitis
SensorsandActuators
RobotSensors
• Sensorsaredevicesthatcansenseandmeasurephysicalpropertiesoftheenvironment,• e.g.temperature,luminance,resistancetotouch,weight,size,etc.
• Thekeyphenomenonistransduction• Transduction(engineering)isaprocessthatconvertsonetypeofenergytoanother
• Theydeliverlow-levelinformationabouttheenvironmenttherobotisworkingin.– Returnanincompletedescriptionoftheworld.
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RobotSensors
• Thisinformationisnoisy(imprecise).• Cannotbemodelledcompletely:
– Reading=f(env)wherefisthemodelofthesensor– Findingtheinverse:
• illposedproblem(solutionnotuniquelydeEined)• collapsingofdimensionalityleadstoambiguity
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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
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• Rawdataprocessedatdifferentlevels,dependingonthesensor– Low:electronics– Medium:signalprocessing– High:computation
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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
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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
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ProximitySensors
– Halleffectsensors• smallvoltagegeneratedacrossaconductorcarryingcurrent
– inductivesensors,capacitivesensors• inductivesensorscandetectpresenceofmetallicobjects
• capacitivesensorscandetectmetallicordielectricmaterials
BIVH ×∝
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InfraredSensors
• Infraredsensorsareprobablythesimplesttypeofnon-contactsensor– widelyusedinmobileroboticstoavoidobstacles
• Theyworkby– emittinginfraredlight
• todifferentiateemittedIRfromambientIR(e.g.lights,sun,etc.),thesignalismodulatedwithalowfrequency(100Hz)
– detectinganyreElectionsoffnearbysurfaces• Incertainenvironments,withcarefulcalibration,IRsensorscanbeusedformeasuringthedistancetotheobject– requiresuniformsurfacecoloursandstructures
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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
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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
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SonarProblems• Thereareanumberofproblemsanduncertaintiesassociatedwithreadingsfromsonarsensors– itisdifEiculttobesureinwhichdirectionanobjectisbecausethe3Dsonarbeamspreadsoutasittravels
– specularre5lectionsgiverisetoerroneousreadings• thesonarbeamhitsasmoothsurfaceatashallowangleandsoreElectsawayfromthesensor
• onlywhenanobjectfurtherawayreElectsthebeambackdoesthesensorobtainareading-butdistanceisincorrect
– arraysofsonarsensorscanexperiencecrosstalk• onesensordetectsthereElectedbeamofanothersensor
– thespeedofsoundvarieswithairtemp.andpressure• a16°Ctemp.changecancausea30cmerrorat10m!
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LaserRangeFinders• LaserrangeEinderscommonlyusedtomeasurethedistance,velocityandaccelerationofobjects– alsoknownaslaserradarorlidar
• Theoperatingprincipleisthesameassonar– ashortpulseof(laser)lightisemitted– thetimeelapsedbetweenemissionanddetectionisusedtodeterminedistance(usingthespeedoflight)
• Duetotheshorterwavelengthsoflasers,thechanceofspecularreElectionsismuchless– accuraciesofmillimetres(16-50mm)over100m– 1Dbeamisusuallyswepttogivea2Dplanarbeam
• Maynotdetecttransparentsurfaces(e.g.glass!)ordarkobjects
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RADAR
• Radarusuallyuseselectromagneticenergyinthe1-12.5GHzfrequencyrange– thiscorrespondstowavelengthsof30cm-2cm
• microwaveenergy– unaffectedbyfog,rain,dust,hazeandsmoke
• Itmayuseapulsedtime-of-Elightmethodologyofsonarandlidar,butmayalsouseothermethods– continuous-wavephasedetection– continuous-wavefrequencymodulation
• Continuous-wavesystemsmakeuseofDopplereffecttomeasurerelativevelocityofthetarget
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AngularPosi4on:RotaryEncoder• Potentiometer
– UsedintheServoontheboebots• OpticalDisks(Relative)
– Countingtheslots– Directionbyhavingparsofemitters/receiversoutofphase:Quadraturedecoding
– Canspinveryfast:500kHz
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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
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CompassSensors
• Compasssensorsmeasurethehorizontalcomponentoftheearth’smagneticEield– somebirdsusetheverticalcomponenttoo
• Theearth’smagneticEieldisveryweakandnon-uniform,andchangesovertime– indoorstherearelikelytobemanyotherEieldsources• steelgirders,reinforcedconcrete,powerlines,motors,etc.
– anaccurateabsolutereferenceisunlikely,buttheEieldisapprox.constant,socanbeusedforlocalreference
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Gyroscopes
• Agyroscopeisaspinningwheelwithmostofitsmassconcentratedintheouterperiphery– e.g.abicyclewheel
• Duetothelawofconservationofmomentum– thespinningwheelwillstayinitsoriginalorientation– aforceisrequiredtorotatethegyroscope
• Agyro.canthusbeusedtomaintainorientationortomeasuretherateanddirectionofrotation
• Infacttherearedifferenttypesofmechanicalgyro.– andevenopticalgyro’swithnomovingparts!
• thesecanbeusedine.g.spaceprobestomaintainorientation
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Vibra4ngStructureGyroscopes
Halteres
MEMS
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Ringgyro's
• Usestandingwavessetup– betweenmirrors(laserringgyro)– withinaEiberopticcable(Eibreopticringgyro)
• Measurerotationbyobservingbeatsinstandingwaveasthemirrors"rotatethroughit".
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IMU's• Gyro,accelerometercombination.• Typicaldesigns(e.g.3DM-GX1™)usetri-axialgyrostotrackdynamicorientationandtri-axialDCaccelerometersalongwiththetri-axialmagnetometerstotrackstaticorientation.
• TheembeddedmicroprocessorscontainsprogrammableEilteralgorithms,whichblendthesestaticanddynamicresponsesinreal-time.
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GPS
• GPSusesaconstellationofbetween24and32MediumEarthOrbitsatellites.
• Satellitebroadcasttheirposition+time.• Usetraveltimeof4satellitesandtrilateration.• Suffersfrom“canyon”effectincities.
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WiFi
• UsingtheSSIDanddatabase.
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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.
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Interfacing
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• Protocols– RS-232– I2C– Ethernet– USB– …
• Eachofthemhasdifferentcharacteristics,e.g.,bandwidth,powerconsumption,maximumcablelength
• ReadthespeciEicationstointerpretcorrectlythedata
ROSdrivers
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• E.g.,laserhttps://github.com/ros-drivers/urg_node/blob/indigo-devel/src/urg_node_driver.cpp
Whatisanactuator?
• Deviceformovingorcontrollingasystem.
• “RobotMuscles”
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HydraulicActuators
• Pros:– Powerful– Fast– Stiff
• Cons– Messy– Maintenance– ExternalPump
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HydraulicActuatorApplica4on• BigDogfromBostonDynamics
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Pneuma4cActuators
• Pros:– Powerful– Cheap
• Cons– Soft/Compliant– ExternalCompressor
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Pneuma4cActuators• 3DBiped(‘89-’95)fromMITLegLab
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AirMuscle
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AirMuscleApplica4on
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AirMuscleApplica4on
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PiezoelectricActuator
• Pros– Verysmallmotionspossibleathighspeeds
– Low-powered• Cons
– Shortactuation– Highvoltages– Expensive– Fragile
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Source: robotics.eecs.berkeley.edu/~ronf
Folded millirobots (2008)
Source: robotplatform.com
PiezoelectricActuator
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ShapeMemoryAlloyActuators
• WorksbywarmingandcoolingNitinolwires.
• Pros:– Light– Powerful
• Cons:– Slow(cooling)
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S4quito
Jonathan Mills, Indiana University CSCE 574 Robotics 44
ElectricActuators
• Pros– Betterpositionprecision– Wellunderstood– Noseparatepowersource– Cheap
• Cons– Heavy– Weaker/slowerthanhydraulics– Coolingissue
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ElectricActuators
• Steppermotors• DCmotors
– Servos• Continuous• Position
• Others(notdiscussed)– Linearactuators– ACmotors
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S
N
Stator:madeoutofcoilsofwirecalled“winding”
Rotor:magnetrotatesonbearingsinsidethestator
• Directcontrolofrotorposition(nosensingneeded)• Mayoscillatearoundadesiredorientation(resonanceatlowspeeds)• Lowresolution
printers computer drives
S N
Electromagnetstator
rotorN S
StepperMotorBasics
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Half stepping
S
S
N
N
angle
torque
IncreasedResolu4on
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More teeth on rotor or stator
Half stepping
S
S
N
N
IncreasedResolu4on
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More teeth on rotor or stator
Half stepping
S
S
N
N
IncreasedResolu4on
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MoreTeethonRotor
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• Pros: – Direct position control – Precise positioning – Easy to control
• Cons: – Oscillations – Low torque at high speeds
StepperMotors
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DCmotors--exposed!
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N
S
N S
stator
rotor
permanent magnets
commutator attached to shaft
V+
-
brush
DCmotorbasics
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N
S
N S
stator
rotor
permanent magnets
N S S N
V+
-V
+
-
DCmotorbasics
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N
S
N S
stator
rotor
permanent magnets
N S S N N S N S
V+
-V
+
-V
+
-
DCmotorbasics
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DCmotortorqueτ
I∝τ
torque=τcurrent =I
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Processing
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Processing(Example)
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Processing(Example)
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Communica4on
• Tethered• Untethered
– Radiomodems– 802.11Wirelessnetworks– Infrared– RC– Bluetooth– GSM/GPRS– Acoustic
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Summary
• Hardwarefor– Sensing– Locomotion– Reasoning– Communication
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