TEMPERATURE COMPENSATION OF A THERMAL FLOW SENSOR BY USING TEMPERATURE COMPENSATION NETWORK. *Corresponding author: [email protected].

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1 National Confrnc on Postgraduat sarch (NCON-PG) 2009 st Octobr 2009, UMP Confrnc Hall, Malaysia Cntr for Graduat Studis, Univrsiti Malaysia Pahang Editors: M.M. Noor; M.M. ahman and K. Kadirgama EMPEAUE COMPENSAION OF A HEMAL FLOW SENSO Y USING EMPEAUE COMPENSAION NEWOK Noraznafulsima Khamshah *, Ahmd N. Abdalla and adaruddin Muhammad Faculty of Elctrical & Elctronic Enginring, Univrsiti Malaysia Pahang,26300 UMP, Kuantan, Pahang, Malaysia; Phon: , Fax: [email protected], [email protected], [email protected] *Corrsponding author: [email protected] ASAC hr has bn a growing dmand for flow-tmpratur snsors for industrial, automotiv, domstic and mdical applications, a quick rspons and low powr consumption. h rror of flow tmpratur masurmnt is on of th most important problms of snsors. In this papr, th mpratur Compnsation Ntwork has bn dvlopd for tmpratur compnsation of a hrmal flow Snsor in th Whatston bridg circuit. h mthod allows for a broad rang of tmpratur compnsation from -20 to 70 o C. h proposd schm lads to an rror rduction of approximatly 96.45% from tmpratur uncompnsatd valu. Ky words: mpratur compnsation; thrmal flow snsor; tmpratur compnsation ntwork INODUCION hrmal flow masuring tchnology has com a long way sinc th introduction of thrmocoupl tchnology and arly hot wir anmomtrs. hrmal tchnologis dpnd on hat transfr and traditionally oprat on diffrntial tmpratur masurmnts btwn two tmpratur snsitiv matrials to gnrat a signal dirctly proportional to th tmpratur diffrntial and mass flow rat. Ovr th yars, hrmal typ flow snsors hav bn utilizd across a vry wid spctrum of applications and fatur an assortmnt of prformanc limits. Modrn hrmal flow snsor dsigns hav gratly volvd from laboratory dvics to ruggd procss instrumnts with ach nw gnration rprsnting a brakthrough in snsing prformanc. h Hot wir mthod of bio-chmical snsing dos not rquir any fluorscnc tagging, thrfor gts many attntions (ISO, 99 and Göpl t al., 989). As rcnt rsarch fforts advanc in svral convrging aras of scinc and tchnology, Hotwir basd snsors hav bn provd to b quit vrsatil and snsitiv dvics and hav bn usd mainly in th trac dtction of bio-chmical matrials. In considration of th possibl flow snsing, thr ar two principls, which sm to b th most promising: i.. diffrntial prssur dtction and thrmal flow masurmnt (Prira t al., 998 and alts t al., 993) Although diffrntial prssur dtction is vry suitabl for liquid flow masurmnt with high throughput, it is lss applicabl for low volum liquid flow and for gas flow. In this cas, thrmal masurmnt is suprior. hrmal flow snsors ar basically thrmally isolatd structurs, which carry hatrs and thrmomtrs. h thrmal flow snsor carry th hat transfr from a hatd snsor, 0

2 Noraznafulsima Khamshah, Ahmd N. Abdalla and adaruddin Muhammad usually a small wir, masurs th air vlocity. h mpirical rlationship proposd by Park (2000) has provd popular and indd forms th basis of many tmpratur compnsation schms. Analogu compnsation tchniqus mploying lctronic circuits hav bn usd, for xampl by Collis t al. (959) and Nam (2004). In this papr w addrssd th problm of hot-wir thrmal flow tmpratur compnsation using a tmpratur compnsation ntwork in th Whatston bridg. As th hat transfr procss is snsitiv both to th vlocity and tmpratur of th air, any changs in tmpratur must b compnsatd for in ordr to achiv accurat vlocity masurmnts. A hardwar implmntation of th proposd schm is also dscribd. A tmpratur snsor is usd as a part of compnsation ntwork. hory of Opration HEOEICAL ACKGOUND h hot-wir thrmal flow snsor using Whatston bridg quilibrium is shown in Figur. Figur : h hot-wir flow snsor basd on Whatston bridg circuit. h connction of four rsistiv componnts in a Whatston bridg configuration is usd quit xtnsivly in masurmnts of tmpratur. In this application, snsing rsistors xhibit tmpratur snsitivity. For tmpratur masurmnts, this tmpratur dpndnc has highly undsirabl ffct, which must b compnsatd for. W considr all four arms in th bridg bing rsponsiv to a stimulus with th snsitivity cofficint, α. An output voltag from th bridg is Eq.(). out s 0 = α () whr 0 is th offst voltag rsulting from th initial bridg imbalanc. If th bridg is not proprly balancd, th offst voltag may b a sourc of rror. Howvr, an appropriat trimming of th bridg snsor during ithr its fabrication or in application apparatus may rduc this rror to an accptabl lvl. If th offst voltag is not proprly compnsatd for, its tmpratur variations usually ar svral ordrs of magnitud smallr than that of th transfr function.

3 mpratur compnsation of a thrmal flow snsor by using tmpratur compnsation ntwork Snsitivity α is gnrally tmpratur dpndnt for hot-wir snsor and is major sourc of inaccuracy. hus, α may vary if is tmpratur dpndnt. If th bridg has a positiv tmpratur cofficint of rsistivity, th cofficint α dcrass with tmpratur. aking partial drivativ with rspct to tmpratur as in Eq.(2), out α = s α (2) If th output signal dos not vary with tmpratur, out = 0, thn, th following Eq.(3) holds; α α = α = = β α (3) Whr β is th CS (tmpratur cofficint snsitivity) of th bridg arm. h abov is a condition for an idal tmpratur compnsation of a fully symmtrical Whatston bridg. o compnsat for tmpratur variations in α, th xcitation voltag, must chang with tmpratur at th sam rat and with opposit sign. im spons to Flow & mpratur Changs Figur 2 shows th rspons of a CN thrmal snsor to a stp chang in vlocity, whil Figur 3 shows a typical rspons to a stp chang in tmpratur. his lvl of prformanc nabls th CN to b usd for combustion air flow masurmnt and control in coal-fird powr. Figur 2: Snsor flow rspons 2

4 Noraznafulsima Khamshah, Ahmd N. Abdalla and adaruddin Muhammad Figur 3: Snsor tmpratur rspons HE POPOSED MEHOD h thrmal snsor uss a modifid Whatston bridg in which th voltag diffrnc across th bridg is amplifid and fd back to th top of th bridg to maintain a constant tmpratur diffrnc btwn th hatd snsor and th tmpratur compnsation snsor. h hatd snsor is th activ lmnt in th control circuit. mpratur compnsation ntwork (CN) ar vry rpatabl, ssntially linar with tmpratur, and can b slf-hatd to provid a known ovrhat tmpratur basd on thir standard rsistanc vrsus tmpratur tabl. h CN is constructd using platinum bcaus of its narly linar charactristics. hus, th CN has th uniqu proprty of bing a hatr, as wll as a vry accurat tmpratur snsor. h tmpratur compnsation ntwork mthod usd to control. Figur 4 shows a gnral circuit which incorporats a tmpratur compnsation ntwork to control voltag across th bridg according to a prdtrmind function of tmpratur. Figur 4: Gnral circuit of bridg tmpratur compnsation his mthod usd of a tmpratur snsor as a part of th compnsation ntwork. Such a ntwork may b rprsnting by an quivalnt impdanc t, and an ntir bridg can b rprsntd by its quivalnt rsistanc. h voltag across th bridg is Eq. (4). 3

5 mpratur compnsation of a thrmal flow snsor by using tmpratur compnsation ntwork 4 E = (4) aking th drivativ with rspct to tmpratur as Eq.(5), ( ) = E 2 (5) and with compnsation condition Eq.(6); = = γ β (6) h hating lmnt is built in a tub whr air flows. his mthod is capabl of masuring any flow rat by adopting tubs with suitabl innr diamtrs. his stats that such compnsation is usful ovr a broad rang of xcitation voltag bcaus E is not a part of th quation. EXPEIMENAL SEUP AND SANDAD ESING POCEDUE Exprimntal Stup h tst stup is shown in Figur 5. h proposd mthod allowd for a broad rang of tmpratur compnsation from -20 to 70 C. his mthod rquirs a trimming of th compnsating ntwork to compnsat not only for CS and C but for as wll. h vlocity of air flow was changd from 0 to 35m/s. Figur 5: Exprimntal stup of hot-wir air flow snsor Standard sting Procdur Also on of th primary focuss of th currnt rsarch was to introduc a standard tsting procdur for analyzing thrmal flow snsor. Whn considring flow snsors prformanc,

6 Noraznafulsima Khamshah, Ahmd N. Abdalla and adaruddin Muhammad thr ar two variabls that hav th most substantial influnc: mpratur and prssur. his is du to th fact that as tmpratur and prssur chang, th dnsity of th fluid passing through a snsor will vary a standard procdur that liminatd th variabl of tmpratur was spcially dsignd and constructd. his procdur, which is schmatically shown in Figur 6, is outlind blow: - St a blowr to a spcific prssur and kp th prssur constant at all tims whn conducting th xprimnt. 2- Us nough tubing btwn th blowr and th snsor to mak sur th flow is laminar bfor it hits th snsor. 3- Us nough tubing aftr th snsor and bfor th valv to liminat th ffct of backprssur. 4- Us a calibratd flow snsor to link its rading, th flow output, to th voltag output rading of th voltmtr. 5- Mak sur th flow snsor, th voltmtr, and th powr supply hav bn rcntly calibratd. 6- st all snsors in an nvironmntal chambr at diffrnt tmpraturs, in our cas 0, 20, 50, and 70 C. 7- Mak sur all th quipmnt usd and th snsor subjct to tsting ar kpt in th nvironmntal chambr at all tims. Wait at last half an hour aftr th chambr rachs th targtd tmpratur and thn start th tsting of th snsors. 8- Evry tim you opn th chambr, rpat procdur It is rcommndd to hav on prson tst all th snsors. o conduct a tst on must chck that th tub is sald and th nvironmntal room is at th right tmpratur. Aftr th blowr and th snsor circuit board ar on, th flow rat can b rad on th thrmal flow snsor and th voltag output of th snsor can b rad on th voltmtr. Figur 6: Diagram for Standard sting Procdur 5

7 mpratur compnsation of a thrmal flow snsor by using tmpratur compnsation ntwork ESULS AND DISCUSSION his papr pointd out th rror of tmpratur compnsation that rsultd from th Whatston-bridg quilibrium, and prsntd tmpratur-compnsation mthod. In ths flow snsors, th hating tmpratur of th lmnt is compnsatd by a rfrnc lmnt, th rsistanc of which changs dpnding on th variations of th flow tmpratur. Figur 7(a) shows th output of th uncompnsatd circuit. Figur 7(b) shows th output of th compnsatd circuit using th tmpratur compnsation ntwork. h xprimnt rsult has a maximum rror prcntag of ovr th ntir vlocity rang (0 35 m/s). h max rror is 3.55% at 25m/s. (a) (b) Figur. 7: (a) h uncompnsatd output voltags (b) h compnsatd output voltags CONCLUSION hrmal flow masurmnt tchnologis inhrntly utiliz tmpratur snsing. Most hrmal flow instrumnt manufacturrs hav a rfrnc snsor containd in th flow lmnt that is ithr intgratd with th dlta tmpratur masurmnt or is indpndntly placd to dtct ral tim changs in procss tmpratur. hrmal dvics ar dsignd for automatic corrction of procss tmpratur changs. Equal mass flow snsor dsigns, nsur thos changs ar fr of lag ffcts and thus offr ral tim 6

8 Noraznafulsima Khamshah, Ahmd N. Abdalla and adaruddin Muhammad tmpratur compnsation. As a rsult, most hrmal flow mtrs ar inhrntly multivariabl and also provid th procss tmpratur as an output. h proposd mthod allows a broad rang of tmpratur compnsation from -20 to 70 o C. h rsult shows that by using this mthod th rror can b rducd to 3.55% ACKNOWLEDGEMEN h laboratory and quipmnt support by Ai Univrsity Collg (AiUC) is gratful acknowldgd. EFEENCES Intrnational Organization for Standardization, ISO 567-, Masurmnt of Fluid Flow by Mans of Prssur Diffrntial Dvic Part. Orific Plats, Nozzls ntura ubs Insrtd in Circular Cross-Sction Conduits unning Full, Gnva, Switzrland, 99. Göpl, W.; Hss, J.; Zml, J.N hrmal Mass Flow Mtrs, Snsors, 4: Prira, J.M.D.; Postolach, O.; Silvia Girao, P.M A tmpratur compnsatd systm for magntic fild masurmnts basd on artificial nural ntworks, IEEE rans. Instrum. Mas, 47(2): alts, H.; Mosr, D CMOS vacuum snsors and othr applications of CMOS thrmopils. Procding of ransducrs 93, Park, K mpratur Compnsation Using oltag Dividr for Hot-film Air Flow Snsor for Automobils. Mastr s hsis, Kyungpook National Univrsity. Collis, D.C.; Williams, M.J wo-dimnsional convction from hatd wirs at low ynolds numbrs, J.Fluid Mch, 6 : Nam,.; Kim, S.; Park, S h tmpratur compnsation of a thrmal flow snsor by changing th slop and th ratio of rsistancs, Snsors and Actuators- A, 4 : Nomnclatur Grk symbols rsistanc Ω α snsitivity cofficint quivalnt rsistanc Ω β tmpratur cofficint snsitivity t quivalnt impdanc Ω S stimulus tmpratur o C out output voltag xcitation voltag offst voltag o 7