6-Linear & Rotational Sensors – II


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Sensors & Actuators
Dr. Hassan SHARABATY
BY
Aleppo – April 2012
Department of Mechatronics Engineering
EEM304 Mechatronics
6- Linear & Rotational Sensors – II
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
2/27
Linear & Rotational Sensors
Themostcommonmotionsinmechanicalsystemsarelineartranslationalongafixedaxisandangularrotationaboutafixedaxis.
Morecomplexmotionsareusuallyaccomplishedbycomposingthesesimplermotions.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
3/27
Resistive Sensors
OneofthesimplestandleastexpensivewaystomeasurerotationalorlinearmotionisusingavariableresistorcalledaPotentiometerorrheostat.
The Rheostat circuit is used to control currents (Iv)
The Potentiometer circuit is used to control voltages (Uv)
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
4/27
Resistive Sensors
Wefocusonrotarypotentiometers,or“pots”forshort,buttheprincipleofoperationisthesameinthelinearcase.Apotconsistsofthreeterminals.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
5/27
Resistive Sensors
Twoendterminals,callthemterminals1and3,connecttoeitherendofalengthofresistivematerial,suchaspartiallyconductiveplastic,ceramic,oralongthinwire.
Theotherterminal,terminal2,isconnectedtoawiper,whichslidesoverthematerialasthepotshaftrotates.
ThetotalresistanceofthepotR13isequaltothesumoftheresistanceR12betweenterminal1andthewiper,andtheresistanceR23betweenthewiperandterminal3.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
6/27
Resistive Sensors
Typicallythewipercanrotatefromoneendoftheresistivematerial(R13=R12)totheother(R13=R23).
Ifthefullmotionofthewiperiscausedbyonerevolutionoftheshaftorless,thepotiscalledasingle-turnpot.
Ifthefullmotioniscausedbymultiplerevolutions,itiscalledamulti-turnpot
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
7/27
Resistive Sensors
Typicallyapotisusedbyconnectingterminal1toavoltageV,terminal3toground,andusingthevoltageatthewiperasameasureoftherotation.
ThevoltageobservedatthewiperisV(R23/R13)andisalinearfunctionoftherotationoftheshaft.
Typicalpotentiometer
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
8/27
Resistive Sensor Application
Inmostpracticalcircuits,theresistancemeasurementisreplacedbyameasurementofvoltagedrop.
Thevoltageacrossthewiperofalinearpotisproportionaltothedisplacementd,whereDisthefull-scaledisplacementandEisthevoltageacrossthepot(excitationsignal).
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
9/27
Resistive Sensor Application
Thisassumesthatthereisnoloadingeffectfromtheinterfacecircuit.Ifthereisanappreciableload,thelinearrelationshipbetweenthewiperpositionandtheoutputvoltagewillnothold.Inaddition,theoutputsignalisproportionaltotheexcitationvoltageappliedacrossthesensor.
Thisvoltage,ifnotmaintainedconstant,maybeasourceoferror.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
10/27
Capacitive Sensors
Theabilityofcapacitivedetectorstosensevirtuallyallmaterialsmakesthemanattractivechoiceformanyapplications.
Thecapacitivedisplacementsensorshaveverybroadapplications,theyareemployeddirectlytogaugedisplacementandpositionandalsoasbuildingblocksinothersensorswheredisplacementsareproducedbyforce,pressure,temperature,andsoforth.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
11/27
Capacitancecanbeusedtomeasureproximityorlinearmotionsontheorderofmillimeters.Usedasproximitysensors,capacitivesensorscandetectmetallicornon-metallicobjects,liquids,oranyobjectwithadielectricconstantgreaterthanair.
Capacitive Sensors
Changesinthedistancebetweenthesurfaceschangesthecapacitance.Thischangeofcapacitanceisusedtoindicatechangesinpositionofatarget.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – II 12/27
where
k is the relative permittivity or called the dielectric constant,
εo is the permittivity of vacuum; εo =8.85 x 10-12 F/m
A is the area of overlap of the two plates,
and d is the plate separation.
d
A
C K 0 The capacitance C of a parallel plate capacitor is given by  
Capacitive Sensors
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – II 13/27
Flat Capacitive Sensor
d
A
C K 0  
( ) ( ) ( )
Capacitive Sensors
 





   f A
d
C K 0 
variable
measure rotational or linear motion
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – II 14/27
 





   f K
d
A
C 0 
Flat Capacitive Sensor
d
A
C K 0  
Capacitive Sensors
variable
Ex: Capacitive Humidity Sensors
Capacitive RH sensors are produced in a wide
range of specifications, sizes, and shapes. The
sensors shown here are from various
manufacturers.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – II 15/27












   
d
1
C K A f 0 
Flat Capacitive Sensor
d
A
C K 0  
Capacitive Sensors
variable
C is a nonlinear function of the distance d.
 we use Differential Capacitive Sensor
Ex. Pressure Capacitive Sensor
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – II 16/27
Cylinder Capacitive Sensor
log
2
1
2
0 l
r
r
K
C  
 
Capacitive Sensors
Position, Force, …
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – II 17/27
Capacitive Sensors
Ex.
Pressure Capacitive Sensor
  1 2
2 0
1 0
C S P P
C C C
C C C
   
  
  
+
Differential Capacitive Sensor
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – II 18/27
Capacitive Sensors
;
2 1 2
1 2
0 C C
V C C
E in


 
2 0
0 d
V X
E in  
+
= ( )
= ( )






  
  
C C C
C C
1 0
2 0 C
Sauty Bridge
+
Differential Capacitive Sensor
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
19/27
Capacitive Sensors
Inmanypracticalapplications,whenmeasuringdistancestoanelectricallyconductiveobject,theobject’ssurfaceitselfmayserveasthecapacitor’splate.Thedesignofamono-polarcapacitivesensorisshowninFig.,whereoneplateofacapacitorisconnectedtothecentralconductorofacoaxialcableandtheotherplateisformedbyatarget(object).Asthesensorapproachesthetarget,thecapacitanceincreases,modifyingtheoscillationofadetectorcircuitincludingthecapacitor.
a mono-polar capacitive sensor
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
20/27
Capacitive Sensors
Acapacitiveproximitysensorcanbehighlyefficientwhenusedwiththeelectricallyconductiveobjects.Thesensormeasuresacapacitancebetweentheelectrodeandtheobject.
Nevertheless,evenforthenonconductiveobjects,thesesensorscanbeemployedquiteefficiently,althoughwithaloweraccuracy.
Anyobject,conductiveornonconductive,thatisbroughtinthesurroundingareaoftheelectrode,hasitsowndielectricpropertiesthatwillalterthecapacitancebetweentheelectrodeandthesensorhousingand,inturn,willproducethemeasurableresponse.
Themeasurementrangedependsonthesizeoftheactivesensorareaaswellasontheelectronicsused.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
21/27
Capacitivesensorsarebasicallypositionmeasuringdevices.Theiroutputsalwaysindicatethesizeofthegapbetweenthesensor’ssensingsurfaceandthetarget.Whentheprobeisstationary,anychangesintheoutputaredirectlyinterpretedaschangesinpositionofthetarget.Thisisusefulin:
Position Measurement/Sensing
•Automationrequiringpreciselocation
•Finalassemblyofprecisionequipmentsuchasdiskdrives
Capacitive Sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
22/27
DynamicMotion
Measuringthedynamicsofacontinuouslymovingtarget,suchasarotatingspindleorvibratingelement,requiressomeformofnoncontactmeasurement.Capacitivesensorsareidealwhentheenvironmentiscleanandthemotionsaresmall,requiringhigh-resolutionmeasurements.Somecapacitivesensorshavehighfrequencyresponse(15kHz)toaccommodatehigh-speedmotion
•Vibrationmeasurements
Capacitive Sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
23/27
ThicknessMeasurement
Measuringmaterialthicknessinanoncontactfashionisacommonapplicationforcapacitivesensors.Themostusefulapplicationisatwo-channeldifferentialsysteminwhichaseparatesensorisusedforeachsideofthepiecebeingmeasured.
Capacitive Sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
24/27
NonconductiveThickness
Capacitivesensorsaresensitivetononconductivematerialswhichareplacedbetweentheprobe’ssensingareaandagroundedbacktarget.Ifthegapbetweenthesensorandthebacktargetisstable,changesinthesensoroutputareindicativeofchangesinthickness,density,orcompositionofthematerialinthegap.
Capacitive Sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
25/27
Assemblytesting
Capacitivesensorshaveamuchhighersensitivitytoconductorsthantononconductors.Therefore,theycanbeusedtodetectthepresence/absenceofmetallicsubassembliesincompletedassemblies.Onlinecapacitivesensingcandetectthedefectivepartandsignalthesystemtoremoveitfromtheline.
Capacitive Sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
26/27
Using pressure capacitive sensor, design an altitude meter, if you know that:
-Standard atmosphere pressure equal [760 mm/Hg].
-Distance between the two fixed plates of sensor [d=8mm].
-The movable plate move 1μm for a change in pressure of 1 mm/Hg.
-The atmosphere pressure decrease 1 mm/Hg for an increase of altitude of 10 meters.
1.Draw the pressure capacitive sensor showing the conditioning circuit.
2.Derive the equation of output voltage with relation to changes of movable plate.
3.Complete the last design to give an alarm when we enter a city whose altitude is more than 800m.
Problem:
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -II
27/27
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