Sensors and Actuators Introduction to sensors

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1 Sensos an ctuatos Intouction to sensos Sane Stuijk Depatment of Electical Engineeing Electonic Systems

2 PITIE SENSORS (hapte 3., 7., 9.,.6, 3., 3.)

3 3 Senso classification type / quantity measue Quantity Position, istance, isplacement Flow ate / Point velocity Foce Tempeatue Resistive Magnetoesisto Themisto Stain gage RTD S e n s o Potentiomete apacitive Diffeential capacito apacitive stain gage Inuctive an electomagnetic Ey cuents LDT Loa cell + LDT Hall effect Magnetostiction Themisto t y p e Selfgeneating LDT Magnetostiction Themal tanspot + themocouple Pieoelectic senso Pyoelectic senso Themocouple PN junction Photoelectic senso Dioe Bipola tansisto eactance vaiation sensos (capacitive an inuctive sensos) typically equies no physical contact exets minimal mechanical loaing

4 4 apacitive sensos

5 5 apacitive senso chage on two plates sepaate by a ielectic Q cuent I is the change in chage Q pe time unit I Q t t evice possibly usable as senso (why?) evice pouces electic signal epens on physical evice popeties altenating cuent () signal equie to pouce output signal esistive sensos opeate using iect cuent signal capacito equies potentially moe complex inteface cicuit

85 pf/m) ε elative ielectic constant evice usable as senso (why?

6 6 apacitive senso capacitance efine by Q an capacitance epens on physical popeties ε ielectic constant fo vacuum (8.85 pf/m) ε elative ielectic constant evice usable as senso (why?) I changing,, ε changes capacitance change in capacitance can be sense t capacitance of a cylinical capacito l ln( b / a)

7 7 apacitive level senso example capacitive level senso senso base on two concentic cylines ( = 8mm, = 4mm) cylinical stoage tank (L = 5cm, H =.m) stoe liqui has ε =. what is the sensitivity of the senso (pf/l) when use in the stoage tank? v o hint: capacitance of two cylinical concentic electoes is equal to h ε h ln( / ) H h ε L

8 8 apacitive level senso example capacitive level senso capacitance of two cylinical concentic electoes h ln( / ) h v o ε elative pemittivity vaies with height h h ln( / ) H h ε use: h = h (height of liqui), h = H h capacitance is equal to H h h H ln( / ln( / ) ) h L min, max capacitance H 8.85pF / m. m min pf ln( / ) ln(4mm/8mm) H max 4.46 pf pf ln( / )

9 9 apacitive level senso example capacitive level senso volume of the stoage tank.5m. m 35. L L H h v o ε sensitivity max min 87.7 pf 4.46 pf S L pf L H h ε L

10 Dielectic mateial example capacitive level senso

11 apacitive senso capacitance epens on physical popeties ε ielectic constant fo vacuum (8.85 pf/m) ε elative ielectic constant evice usable as senso changing,, ε changes capacitance change in capacitance can be sense v o capacitive level senso changes ε senso assumes that ε is a (mateial) constant H h ε x ln( / ) H h x h ε L

(ielectic polaiation) ipoles fom electic fiel opposite to extenal fiel capacitance epens on

12 Dielectic mateial ielectic mateials ae electical insulatos (shae pai of electons) with a high polaiability ielectic mateials have a pemanent ipole moment extenal fiel aligns molecules (ielectic polaiation) ipoles fom electic fiel opposite to extenal fiel capacitance epens on popeties of ielectic mateial ε wate elative ielectic constant epens on mateial (ai, wate,...) tempeatue

13 3 Dielectic mateial example tempeatue senso base on baium titanate (BaTiO 3 ) elative pemittivity of feomagnetic mateials k T T c T c uie tempeatue k mateial epen constant ε (BaTiO 3 ) = 5 at ε (BaTiO 3 ) = at (+) sensitive senso (TR = ) (+) simple to integate in silicon pocess

14 4 Dielectic mateial example elative humiity (RH) senso (H) elative pemittivity ε (ai) = ε (wate) = 88 at, ε (wate) = at use ielectic that absobs an exues wate without hysteesis capacitance of the senso RH = 5pF α 76 = (9) x -6 /(%RH) (-) sensitivity.4pf/%rh (+) lineaity eo <.5%RH (+) tempeatue epenency ΔRH = -.3 RH (T )

15 5 apacitive senso capacitance epens on physical popeties ε ielectic constant fo vacuum (8.85 pf/m) ε elative ielectic constant evice usable as senso changing,, ε changes capacitance change in capacitance can be sense capacitance efine by Q an I I I t I R I IR

16 6 apacito impeance an amittance impeance (eluctance to chage flow) Z R jx R esistance, X eactance impeance of a capacito Z j jf amittance (conuctance) Y Z R jx amittance of a capacito Y j inteface cicuit measues impeance (though voltage) o amittance (though cuent) ZI j I I Z Y j

17 7 apacitive senso sensitivity measue impeance (voltage op) o amittance (measue cuent) j senso is linea when changing ε o while measuing amittance linea with espect to ε an sensitivity oes not epen on changing paamete I I j senso is non-linea when changing

18 8 apacitive senso senso is non-linea when changing solution: allow only small isplacement of plates total istance between plates ( + ) small isplacement () allowe capacitance equal to sensitivity x x x x x x x non-linea sensitivity (sensitivity epens on though x) sensitivity inceases when an ae small (choose small ) istance limite by ielectic beakown (3k/cm fo ai)

19 9 apacitive senso senso is non-linea when changing altenative: non-lineaity impove by aing ielectic two capacitos an in seies sensitivity of seies capacito fist tem inepenent of senso is moe linea then senso without ielectic mateial... 3

20 Diffeential capacito iffeential senso with two capacitos voltage op acoss capacitos substitute capacito values use iffeential amplifie to subtact voltages linea elation between isplacement () an output voltage j j j

21 output voltage () output voltage () Diffeential capacito vesus single capacito

22 Diffeential capacito iffeential capacito with changing aea voltage iffeence linea elation between isplacement () an output voltage w w w w

23 3 Diffeential capacito example capacitive otation senso two equal sie paallel cicula plates sepaate by an insulato one pai of plates acts as oto, othe pai as stato senso is place in bige cicuit show that the output voltage v o is popotional to the angle of otation Θ oto plates R 4 insulato gap 4 o 3 3 stato plates

24 4 Diffeential capacito example capacitive otation senso ¼ ovelap when Θ = a R 4 capacitance is then equal to maximal ovelap when Θ = π/ a capacitance an 3 ae maximal an 3 have ½ ovelap, hence R 3 4 capacitance an 4 have no ovelap at Θ = π/ a, hence 4 R 4 R 4 oto plates 3 R 4 stato plates insulato gap

25 5 Diffeential capacito example capacitive otation senso capacitance popotional to angle capacitos in bige cicuit output popotional to angle 4 3 R 4 4 R o 4 3 R insulato gap oto plates stato plates 3 4

26 6 Finge effect eo souce capacitance of a flat plate capacito is equal to only when << eo souce (finge effect) electic fiel oes not en at ege fiel bens outsie the plates eal capacitance lage than fomula suggests euce finge effect with oute gua ing at same voltage w x w x

27 7 Stay capacitance eo souce only one of two capacito plates can be goune othe plate can fom capacito with any neaby conucto stay capacitance exists between each pai of conuctos stay capacitance euces sensitivity of the senso stay capacitance can be euce using shieling shieling ceates anothe capacito in paallel with the senso conucto s3 goun plane x stay capacitos s s

28 8 Stay capacitance eo souce capacito has high output impeance Z Z j equies pocessing cicuit with high input impeance senso use at high fequency ω [log] fequency is limite by stay capacitances s3 solution: place pocessing cicuit close to senso x s s

29 9 Signal pocessing vaiable eactance sensos single vaying capacitance ( ± Δ) iffeential capacitance ( + Δ, - Δ) voltage / cuent elation fo capacito I t I j voltage o cuent souce neee typically ~ pf excitation fequency typically between kh to MH to get easonable impeance

30 3 Signal pocessing output voltage of senso vo v v v vo v sint sin t infomation pesent in amplitue (magnitue) phase shift (iection) v v o (Z=) v o (Z=-) v o (Z=)

31 3 Signal pocessing paallel plate capacito x x x non-linea elation between capacitance an istance cicuit fo linea impeance changes R povies bias cuent path allows x to ischage impeance R >> impeance x at excitation fequency output voltage v Z x o v e Z linea elation between output voltage an isplacement offset voltage pesent in output x v e e + R x - x v o

32 3 Signal pocessing v o capacitive level senso h ε x ln( / ) H h x H h ε cicuit fo linea amittance changes R povies bias cuent path output voltage L R v x o v e linea elation between output voltage an input signal x offset voltage pesent in output cicuit known as chage amplifie x v e e x - + v o esponse lage compae to measuing voltage op ove x

33 33 Signal pocessing stay capacitance influences output signal output voltage (ignoing stay capacitance) v o x v e stay capacitance s an s o not affect output voltage s in paallel with v e both ens of s at same voltage output voltage (consieing stay capacitance) x s3 vo v e s3 R shieling euces s3 e s x s - + v o

Sensing, Computing, Actuating

Sensing, Computing, Actuating Sande Stuij (s.stuij@tue.nl) Depatment of Electical Engineeing Electonic Systems SENSING TEMPEATUE, SELF-HEATING (Chapte.,., 5.) 3 Engine coolant tempeatue senso https://www.youtube.com/watch?=q5637fsca