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8/17/2019 Mechatronics_Session 3_ Position Sensors
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MECHATRONICS
Unit 1
Classification of Sensors & Transducers(Session-5
Mec!anical En"ineerin" #e$art%ent
' Uniersit)
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'earnin" O*+ecties
In t!is lecture students ,ill learn• Classification of sensors• Classification of transducers
• #is$lace%ent transducers• Resistance transducers• Ca$acitie transducers• Inductance *ased transducers
• O$tical encoders
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Classification of Sensors
1. Actie and .assie sensors
/0 Contact and non-contact sensors0 A*solute and relatie sensors
20 Ot!er sc!e%es
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10 Actie and $assie sensors
.assie Sensor a sensor t!at re3uires e4ternal $o,er too$erate0
E4a%$les t!e car*on %icro$!one t!er%istors strain"au"es ca$acitie and inductie sensors etc0
Ot!er na%e $ara%etric sensors (out$ut is a function of a $ara%eter - li6e resistance
Actie Sensor "enerates its o,n electric si"nal and does
not re3uire a $o,er source0E4a%$les t!er%ocou$les %a"netic %icro$!ones $ie7oelectric sensors0
Ot!er na%e self-"eneratin" sensors
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/0 Contact and noncontact sensors
Contact sensor a sensor t!at re3uires $!)sical
contact ,it! t!e sti%ulus0
E4a%$les strain "au"es %ost te%$erature sensors
Non-contact sensor re3uires no $!)sical contact0E4a%$les %ost o$tical and %a"netic sensors
infrared t!er%o%eters etc0
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0 A*solute and relatie sensors
A*solute sensor a sensor t!at reacts to a sti%ulus on
an a*solute scale
E4a%$les T!er%istors strain "au"es etc0
(t!er%istor ,ill al,a)s read t!e a*solute te%$erature
Relatie sensor T!e sti%ulus is sensed relatie to a
fi4ed or aria*le reference0
E4a%$les T!er%ocou$le %easures t!e te%$erature
difference $ressure is often %easured relatie to
at%os$!eric $ressure0
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20 Ot!er sc!e%es
Classification *) *road area of detection10Electric sensors
/0Ma"netic
0Electro%a"netic
20Acoustic50C!e%ical
80O$tical
90Heat Te%$erature
:0Mec!anical
;0Radiation
1
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20 Ot!er sc!e%es (cont0
Classification *) $!)sical la,
10.!otoelectric/0Ma"neto electric
0T!er%oelectric
20.!otoconductie
50Ma"nitostrictie80Electrostrictie
90.!oto%a"netic
:0T!er%oelastic
;0T!er%o%a"netic1
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20 Ot!er sc!e%es (cont0Classification *) s$ecifications
10Accurac)
/0Sensitiit)0Sta*ilit)
20Res$onse ti%e
50H)steresis
80>re3uenc) res$onse90In$ut (sti%ulus ran"e
:0Resolution
;0'inearit)
1
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20 Ot!er sc!e%es (cont0Classification *) area of a$$lication
10Consu%er $roducts 1/0 Scientific
/0Militar) a$$lications
0Infrastructure
20Ener")
50Heat
80Manufacturin"
90Trans$ortation
:0Auto%otie
;0Aionic
1
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Classification of transducers10 =ased on $rinci$le of transduction
(a T!er%o electric
(* Ma"neto resistie
(c Electro 6inetic
(d O$tical
/0 Actie & $assie
0 Analo" & di"ital20 Inerse transducer
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.assie transducer #eice ,!ic! derie $o,er re3uired for
transduction fro% au4iliar) $o,er source - e4ternall) $o,ered
E4a%$le resistie inductie ca$acitie
?it!out $o,er t!e) ,ill not ,or6 Actie transducer
No e4tra $o,er re3uired to $roduce I@$
Self "eneratin" #ra, $o,er fro% in$ut a$$lied
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Analo" transducer conert I@$ 3uantit) into an analo" o@$
Analo" o@$- a continuous fn0 Of ti%eE"0 Strain "au"e ' #T t!er%ocou$le
#i"ital transducer
Conerts I@$ into an electrical O@$ in t!e for% of $ulses
Inerse transducer
?!ic! conerts electrical si"nal to $!)sical 3uantit)
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Inductie Transducers
T!ere is a ariet) of transducers ,!ose o$eration isinductie0
B >or %otion %easure%ent a$$lications a c!an"e in t!e
3uantit) to *e %easured %ust *e conerted into a c!an"e in
inductance (and t!en to olta"e usin" a circuit0
• ?e first reie, inductanceDD *riefl)0
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Inductance
• A %a"netic field can *e isuali7ed *) lines of %a"netic
force0 T!e entire "rou$ of %a"netic field lines is called
%a"netic flu40
B Ma"netic field lines are $racticall) unaffected *)
non%a"netic %aterial suc! as air acuu% $a$er "lass,ood and $lastics0 Ho,eer ,!en a %a"netic su*stance
li6e iron is $laced in t!e field t!e %a"netic field lines
*eco%e concentrated0
B T!is a*ilit) of a su*stance to concentrate %a"netic flu4 iscalled $er%ea*ilit)0 A %a"netic field can *e created *) a
$er%anent %a"net or an electro%a"net ,!ic! is a loo$ed
coil of conductin" %aterial ,ound on a core %an) ti%es0
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Inductance
T!e (self inductance ' is an indication of t!e a*ilit) of a
coil to o$$ose an) c!an"e in current flo,in" t!rou"! t!e
coil0 T!e inductance ' of a c)lindrical coil is "ien *)
?!ere ' is t!e inductance Henr) HF
G is t!e $er%ea*ilit) of t!e core H@%F
n is t!e nu%*er of turns of coil A is t!e cross sectional area of core %/F
l is t!e len"t! of core %F
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Inductie transducers
Inductie transducers are t!ose in ,!ic! SE'>
IN#UCTANCE of a coil or t!e MUTUA' IN#UCTANCE
of a $air of coil is altered due to ariation in t!e
%easurand0 C!an"e in inductance ' is %easured0 T!e self inductance of a coil refers to t!e flu4 lin6a"e
,it!in t!e coil due to current in t!e sa%e coil0
Mutual inductance refers to t!e flu4 lin6a"es in a coil
due to current in ad+acent coil0
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Inductie transducers
T!e self inductance of a coil refers to t!e flu4 lin6a"e
,it!in t!e coil due to current in t!e sa%e coil0
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Inductie transducers
Mutual inductance refers to t!e flu4 lin6a"es in a coil
due to current in ad+acent coil0
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Inductie transducers
• Consider a set of t,o coils s!arin" t!e sa%e core i0e0 a
transfor%er *asicall)0 Note t!at one of t!e coil ($ri%ar)coil is drien *) a $o,er source ,!ile t!e ot!er
(secondar) coil $roduces a olta"e induced fro% t!e
$ri%ar) coil0
B T!e olta"e on t!e secondar) coil de$ends on %utual
inductance *et,een t!e t,o coils0 If t!e core is %ade
%oa*le in res$onse to t!e dis$lace%ent of a tar"et o*+ect
t!e %utual inductance ,ill c!an"e resultin" in a olta"e
c!an"e in t!e secondar) coil0
• Usin" t!is $rinci$le t!e '#T and R#T ,or6s0
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Inductie transducers
• In t!e %a"netic circuits reluctance is e3uialent to resistance
and de$ends on
Nu%*er of turns
Area of cross section of ,ire .er%ea*ilit) of t!e %ediu%
• Air !as %uc! lo,er $er%ea*ilit) t!an a ferro%a"netic
%aterial0
• Introducin" air "a$ is li6e introducin" !i"! resistance0
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Inductie transducers
• In t!e %a"netic circuits reluctance is e3uialent to resistance
and de$ends on
Nu%*er of turns
Area of cross section of ,ire
.er%ea*ilit) of t!e %ediu%
• Air !as %uc! lo,er $er%ea*ilit) t!an a ferro%a"netic
%aterial0
• Introducin" air "a$ is li6e introducin" !i"! resistance0
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Inductie transducers
• Air Cored Coils
Can *e o$erated at !i"!er
fre3uencies *ecause t!ereare no edd) current losses
in air core0
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Inductie transducers
• Iron cored coils
Inductance de$ends on
t!e alue of current0
At !i"! fre3uencies
edd) current losses are
!i"!0
Si7e %uc! s%aller t!an
air cored0
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Linear Variable-Differential Transformer(LVDT)
• T!e t,o secondar) coils are connected in t!e o$$osite
$!ase0• ?!en t!e core is in t!e %iddle t!ere is no out$ut olta"e0
• Moin" t!e core fro% t!e central $osition un*alances t!esecondar)Js deelo$in" an out$ut0
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Linear Variable-Differential Transformer(LVDT)
Applications: To %easure linear dis$lace%ent e0"0 for%easurin" tu*e len"t!s in a steel $lant a$$lied in linearsero%ec!anis%s etc0
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'inear aria*le-#ifferential
Transfor%er ('#T
-x
Vo=V1-V2
Vi
V1 V2
V1 > V2 Vi
Vo
'#Ts are deices to %easure
dis$lace%ent *) %odif)in" s$atialdistri*ution of an alternatin"
%a"netic field0
Oscillating excitation voltage-5 !" to 25 #!"
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'inear aria*le-#ifferential Transfor%er
('#T
$=
Vo=V1-V2
Vi
V1 V2
V2 = V1
Vi
Vo
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'inear aria*le-#ifferential Transfor%er
('#T
%x
Vo=V1-V2
Vi
V1 V2
V2 > V1
Vi
Vo
&o' te irection of is*lacement can be etermine from te relative *ase of te
signal+
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Linear Variable Diferential
Trans!r"er #LVDT$
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• T!e s$an ran"es fro%
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A rotar) aria*le differential transfor%er(R#T
A rotar) aria*le differential transfor%er (R#T isfor %easurin" an"ular dis$lace%ents and
o$erates in t!e sa%e %anner as a '#T0
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, ca*acitor is an electrical com*onent ic
essentiall. consists of to *lates se*arate
b. an ins/lator+Te *ro*ert. of a ca*acitor to store an
electric carge en its *lates are at ifferent
*otential is referre to as ca*acitance+
CA.ACITIE TRANS#UCERS
05/03/16 09:05 AM
0f te ca*acitance is large more carge is
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d
AC
r <εε
=
+
0f te ca*acitance is large' more carge is
neee to establis a given voltage ifference+
Te ca*acitance beteen to *arallel metallic
*lates of area
×= −
m
F 1/<
1
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10 Circular %otion %easure%ent• Aaila*le in arious si7es and resolutions
• Used in co%$uter %ouse rollers
/0 Co%$onents• Circular dis6 ,it! slots
• #iode and $!oto sensor
• #i"ital circuitr) to $rocess $ulses
Incre%ental O$tical Encoders
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%!r&in' (rin)i(le
Ligt so/rce' ioe
*oto sensor
•1 f/ll *inc is e/ivalent to one f/ll c.clemae b. te trans*arent an o*a/e
inos
05/03/16 09:05 AM &al*an
t,
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+)!nt,.
oto sensor
ioe
oto sensor
ioe
•3otating is#
blocking ligt
•oto sensor as a
low logic o/t*/t
•3otating is#
allowing ligt tro/g
•oto sensor as a
high logic o/t*/t
•4ontin/o/s train of */lses forme as te *oto sensor traverses
beteen logic ig an logic lo in res*onse to te rotation of te is#
Logical Logical 1
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10 O$tical incre%ental encoder is used to %easure
10 .osition
/0 elocit)
0 #irection of rotation/0 No need for ot!er A@# circuits
10 Out$uts can *e directl) fed to
%icro$rocessor@%icrocontroller
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Absolute Optical Encoder working
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Absolute Optical Encoder working
• A disc or a $late containin"o$a3ue and trans$arent se"%ents $asses *et,een a li"!t source(suc! an 'E# and detector tointerru$t a li"!t *ea%0
• T!e electronic si"nals t!at are"enerated are t!en fed into t!econtroller ,!ere $osition and
elocit) infor%ation iscalculated *ased u$on t!e si"nalsreceied
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A*solute O$tical Encoder
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A*solute O$tical Encoder
• In t!e fi" ,ord si7e of data is 2*its0
• Outer%ost Ele%ent is 'S=0• Inner%ost Ele%ent is MS=0• T!e An"ular $osition is "ien as8
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'inear Encoder Codes
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Reca$
To$ics dealt ,it! in t!is class• Classification of transducers• #is$lace%ent transducers
• resistance *ased transducers $otentio%eter
• Inductie transducers '#T• Ca$acitie transducers• Non contact $osition sensors O$tical Encoders