Sensor is an electrical/mechanical device

Sensor is an electrical/mechanical device

1. Robotics sensors
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Sensors ????
•Collectinformationabouttheworld
•Sensor-anelectrical/mechanical/chemicaldevicethat
mapsanenvironmentalattributetoaquantitative
measurement.
•Eachsensorisbasedonatransductionprinciple-
conversionofenergyfromoneformtoanother.

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Conti..
Definition.1.(Oxforddictionary)
•Adevicegivingasignalforthedetectionor
measurementofaphysicalpropertytowhichit
responds.
Definition.2.
•Asensorisadevicethatreceivesasignalorstimulus
andresponsewithanelectricalsignal.

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Human sensing and organs
.
•Vision:eyes(optics,light)
•Hearing:ears(acoustics,sound)
•Touch:skin(mechanics,heat)
•Odor:nose(vapor-phasechemistry)
•Taste:tongue(liquid-phasechemistry)

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Categorization of Sensor
Classificationbasedonphysicalphenomena
•Mechanical:straingage,displacement(LVDT),velocity
(laservibrometer),accelerometer,tiltmeter,viscometer,
pressure,etc.
•Thermal:thermalcouple
•Optical:camera,infraredsensor

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Categorization of Sensor
Classificationbasedonmeasuringmechanism
•Resistancesensing,capacitancesensing,inductance
sensing,piezoelectricity,etc.
•Materialscapableofconvertingofoneformofenergy
toanotherareattheheartofmanysensors.
•Inventionofnewmaterials,e.g.,“smart”materials,
wouldpermitthedesignofnewtypesofsensors..

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Classification of Sensors
1.Proprioception(Internalstate)v.s.Exteroceptive(externalstate)
•measurevaluesinternallytothesystem(robot),e.g.batterylevel,wheel
position,jointangle,etc,
•observationofenvironments,objects
2.Activev.s.Passive
•emittingenergyintotheenvironment,e.g.,radar,sonar
•passivelyreceiveenergytomakeobservation,e.g.,camera
3.Contactv.s.non-contact
4.Visualv.s.non-visual
•vision-basedsensing,imageprocessing,videocamera

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ProprioceptiveSensors
1.Encoders,Potentiometers
•measureangleofturnviachangeinresistanceorbycountingopticalpulses
2.Gyroscopes
•measurerateofchangeofangles
•fiber-optic(newer,better),magnetic(older)
3.Compass
•measurewhichwayisnorth
4.GPS:measurelocationrelativetoglobe
Touch Sensors
Whiskers,bumpersetc.
mechanicalcontactleadsto
•closing/openingofaswitch
•changeinresistanceofsomeelement
•changeincapacitanceofsomeelement
•changeinspringtension

9. •Accuracy:errorbetweentheresultofameasurement
andthetruevaluebeingmeasured.
•Resolution:thesmallestincrementofmeasurethata
devicecanmake.
•Sensitivity:theratiobetweenthechangeinthe
outputsignaltoasmallchangeininputphysical
signal.Slopeoftheinput-outputfitline.
•Repeatability/Precision:theabilityofthesensorto
outputthesamevalueforthesameinputovera
numberoftrials
Specifications of Sensor
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10. Accuracy vs. Resolution
True value
measurement
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11. Accuracy vs. Precision
Precision without
accuracy
Accuracy without
precision
Precision and
accuracy
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13. Continue…..
•Measurements:”Themomentumandpositionofaparticlecan
notbothbepreciselydeterminedatthesametime.”Measuring
activitydisturbsthephysicalprocess(loadingeffect).
•Measurementerror:Thatisthedifferencebetweenthe
measuredvalueandthetruevalue.
error=measuredvalue-truevalue
•Deterministicerrors:Theyarerepeatedateverymeasurement,
e.g.readingoffsetorbias.Sucherrorscanbecorrectedby
calibration.
•Randomerrors:Theyarecausedbyseveralparametersand
changeintimeinanunpredictablefashion.Theycanbe
quantifiedbymeanerrors,standarddeviation.
•Precision:Measurementswithsmalldeviation
•Accuracy:Measurementswithsmallerrors,i.e.smallbiasand
highprecision 13

14. Sensor properties
•Asensorshouldrepresentaphysicalvariableasfastandas
accuratelyaspossible.
•Asensorisrepresentedbyitscharacteristic.
•Ideally,thesensorcharacteristicisastraightline
input
output
ideal
factual
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Force Sensors

17. Strain Gauges
•Foil strain gauge
–Least expensive
–Widely used
–Not suitable for long distance
–Electromagnetic Interference
–Sensitive to moisture & humidity
•Vibration wire strain gauge
–Determine strain from freq. of AC signal
–Bulky
•Fiber optic gauge
–Immune to EM and electrostatic noise
–Compact size
–High cost
–Fragile
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18. Strain gauge
•Whenexternalforces
areappliedtoa
stationaryobject,stress
andstrainaretheresult.
•Stressisdefinedas
Strainisdefinedastheamountofdeformationperunitlengthof
anobjectwhenaloadisapplied.
Strain(ε)=ΔL/L
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(68) What are Strain Gauge -Passive
Transducer-Bonded Strain Gauge-Transducers -
Electronics Engineering -YouTube

19. •PiezoelectricStrainSensor
–Piezoelectricceramic-basedorPiezoelectricpolymer-based
(e.g.,PVDF)
–Veryhighresolution(abletomeasurenanostrain)
–Excellentperformanceinultrasonicfrequencyrange,very
highfrequencybandwidth;thereforeverypopularin
ultrasonicapplications,suchasmeasuringsignalsdueto
surfacewavepropagation
–Whenusedformeasuringplanestrain,cannotdistinguish
thestraininX,Ydirection
–Piezoelectricceramicisabrittlematerial(cannotmeasure
largedeformation)
Strain Sensing
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25. Displacement Sensing
•LVDT (Linear Variable Differential
Transformer):
–Inductance-based sensor
–“Infinite” resolution
•limited by external electronics
–Limited frequency bandwidth (250 Hz typical
for DC-LVDT, 500 Hz for AC-LVDT)
–No contact between the moving core and coil
structure
•no friction, no wear, very long operating
lifetime
–Accuracy limited mostly by linearity
•0.1%-1% typical
–Models with strokes from mm’s to 1 m
available
Photo courtesy of MSI
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(68) Construction & Working LVDT | Electrical
Engineering -YouTube

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29. LVDT
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(68) Construction & Working LVDT | Electrical
Engineering -YouTube

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Tactile Sensors

31. Atactilesensorisadevicethatmeasuresinformation
arisingfromphysicalinteractionwithitsenvironment.
What does it sense ?
Deformation of bodies (strain) or fields (electric or
magnetic).
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32. Typesofhumantouch
•CutaneousSensations(internalsensors)-Cutaneoussense
receivessensoryinputsfromthereceptorsembeddedinthe
skin.
Senses:temperature,pressure,pain
•KinestheticSensations(externalsensors)-Kinestheticsense
receivessensoryinputsfromthereceptorslocatedwithin
muscles,tendonsandjoints.
Senses:bodyposition,movement,equilibrium
TactileSensor CutaneousSensoryReceptors
Strengths Weakness
Linearity No Anticipation
Low Hysteresis
High Frequency of Signals
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33. TypesofSignalinHumanTouchSensing
Basis of Classification :
•Type of Signal Frequency of Signal
FA=FastAdapting,SA=slowadapting.
Type-1tellsusabouttheeffectiveareathatcanbesensedbythereceptor.Type1issmall
area,type2islargerarea.HenceFA1meansfastadapting,lowareasensor.
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34. Tactile sensing: Methods of transduction
Usually an array of discrete sensing
elements.
Sensing elements can be many types:
•Resistive: strain gauge,
Piezoresistive
•Capacitive
•Piezoelectric
•Others like (magnetic, optical,
conductive rubber, ultrasonic
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35. Tactile sensing: Methods of transduction
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36. Resistive Sensing Elements
Mechanicaldeformationchangesthe
capacitanceofparallelconductingplates
Straingauge:athinfilmhavingametalpatternthatchangesresistance
whenstrained.
Piezoresistiveelement:Pressureontheelement
causesthematerialtocompress,changingit’s
resistance
Advantages:verysimpleconstruction,durable,
gooddynamicrange,easyreadout
Disadvantages:non-linearity,hysteresis,lowsensitivity
Capacitive Sensing Elements
Applicationarea:Touchscreens.
Advantages:gooddynamicrange,linearity
Disadvantages:noise,measuringcapacitanceishard!
(comparedtomeasuringresistance)
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Proximity and range sensors

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41. Range Imaging Sensors
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RangeimagingSensor

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Velocity Sensors

43. Velocity Sensing
•Scanning Laser Vibrometry
–No physical contact with the test object; facilitate remote, mass-
loading-free vibration measurements on targets
–measuring velocity (translational or angular)
–automated scanning measurements with fast scanning speed
–However, very expensive (> $120K)
Photo courtesy of Bruel & Kjaer
Photo courtesy of Polytec
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