sổ tay cdt Chuong 45-Kt Do(Cb Va Bo Convert at Ion)

8/6/2019 s tay cdt Chuong 45-Kt Do(Cb Va Bo Convert at Ion)

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45K thut o: Cm binv b chuyn i

Cecil HarrisonUniversity of Southern Mississippi

45.1Gii thiu ...................................................................1

45.2B chuyn i lc v chuyn ng ..........................3

45.3B chuyn i qu trnh ................................. .........1145.4Kh nng hot ng ca b chuyn i ........ ........ ..22

45.5S ph hp gia ti v b chuyn i .................... .22

45.1 Gii thiu

Mt h thng iu khin t ng c cho l d sai sbi cc thnh phn hng thun (b sosnh, b iu khin, c cu chp hnh, v thit b hoc qu trnh) phn ng li tn hiu sai s(hnh 45.1). Tn hiu sai s c nhn ra bng cch so snh gi tr o c ca u ra c iukhin vi mt s u vo tham chiu, v vy chnh xc v trung thc ca u ra c iu

khin ph thuc phn ln vo chnh xc v trung thc ca thit b o. Sau kt qu o cca u ra iu khin, c a qua mt h thng ph c gi l b chuyn i, n c coi lmt chc nng c lp quan trng nht trong mt h thng iu khin t ng.

Mt b chuyn i c chc nng nhn bit bin hoc cng ca u ra iu khin vsinh ra mt tn hiu tng ng di dng nng lng thch hp cho vic truyn dn theo ngphn hi n b so snh. [S tng ng thut ng y c s dng mang tnh cht tng ibi v u ra ca b chuyn i khng phi lc no cng tng ng vi u ra iu khin; cngha l, b chuyn i c th khng phi l mt thnh phn tuyn tnh. Trong cc h tuyn tnh,nu u ra ca b chuyn i (thit b o) khng tuyn tnh th n s c tuyn tnh ha bng biu ha tn hiu.] Phn t ca b chuyn i nhn bit u ra iu khin c gi l cm bin,cc phn t cn li ca b chuyn i c chc nng bin i u ra ca cm bin sang dng nnglng cn cho ng phn hi. Cc cu hnh c th ca ng phn hi bao gm:

Lin kt c hc

Cng sut lu cht (Cht kh hoc Cht lng)

in, bao gm ghp quang, truyn RF, ghp t, hoc truyn m

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S tay C in t

Hnh 45.1 S chc nng ca h iu khin t ng tiu chun

Nhng tn hiu in ph hp miu t kt qu o bao gm:

Bin in p hoc dng in mt chiu.

Hiu in th hoc cng dng in AC, tn s hoc pha (iu bin CW)

Tn hiu in p hoc dng xung

Trong mt s trng hp, s m t c th thay i (v d, t cng DC sang dng xung sho) theo ng phn hi.

Phn tip theo s trnh by v cc h thng iu khin t ng trong tn hiu phn hi l tnhiu in v ng phn hi gm c dy dn hoc dy cp lin kt cc thnh phn ca ng phnhi. Nhng b chuyn i c xem xt di y nhn bit u ra iu khin v to ra s biudin tn hiu in ca bin , cng hoc hng ca u ra iu khin.

B iu ha tn hiu nhn u ra dng in ca b chuyn i v truyn tn hiu n b sosnh di mt dng tng thch vi u vo tham chiu. Chc nng ca b iu ha tn hiu baogm:

Khuch i/suy gim (nh c)

Cch ly

Ly mu

Kh nhiu

Tuyn tnh ha

Dch nhp v tham chiu Thao tc ton hc (v d, vi phn, chia, tch phn, nhn, tm nghim, bnh phng, tr,

hoc cng)

Chuyn i tn hiu (v d, DCAC, ACDC, tn sin p, in ptn s, stngt, tng ts, v.v)

B m

B s ha

B lc

ng b tr khng

Dng sng

Dch phaTrong trng hp mt phn hoc ton b trng thi tn hiu yu cu c nhn dng trong

phm vi ca b chuyn i, th u ra ca b chuyn i c th c kt ni trc tip vi b sosnh. [Kt ni trc tip u ra ca b chuyn i trc tip n b so snh khng nn nhm ln viphn hi ng nht. Phn hi ng nht xy ra khi cc thnh phn phn lp ca ng phn hi(b chuyn i v b iu ha tn hiu) c hm truyn t hp bng 1 (ng nht).] Trong h thng2


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S tay C in t

)()()(2

2

txdt

dtv

dt

dta == 2

211

)(

2

t

xxx

t

vva

iiiii

i

+=

=

Gia tc gc:

)()()(2

2

tdt

dt

dt

dt ==

2

211

)(

2

tt

iiiii

i

+=

=

V trn gin l mt v tr trong mt h qui chiu; do bt k o lng dch chuyn no linquan n h qui chiu ny u l php o v tr v bt k b chuyn i dch chuyn no c uvo c tham chiu ti h qui chiu ny u c th c dng nh mt b chuyn i v tr.

B chuyn i v tr (dch chuyn)

Cc b chuyn i v tr c th phn loi theo tnh nng nh b chuyn i v tr tng th(ln)hoc b chuyn i nhy (nh). Ranh gii gia b chuyn i tng th hoc nhy i khi l ty ,nhng thun tin c th ly xp x 1 mm i vi dch chuyn thng v xp x 1 cung10 (1 /6 ) i vi dch chuyn gc. Cc dng in hnh ca b chuyn i tng th (hnh 45.2) l:

Chit p [Hnh 45.2(a)] B chuyn i vi phn bin thin (VDT) [Hnh 45.2(b)]

B ng tc [Hnh. 45.2(c)]

Dng c phn tch [Hnh. 45.2(d)]

B m ha v tr [Hnh. 45.2(e)]

B chuyn i da trn chit p rt d s dng v yu cu iu ha tn hiu ti thiu nhng ccchit p li d b hao mn do c s tip xc trt gia u d vi phn t in tr v c sinh ranhiu do s ny ln ca u d (hnh 45.2 (a)). Hin nay cc chit p sn c vi hnh trnh bini t nh hn 1cm n ln hn 50 cm (ng thng) hoc t vi n ln hn 50 vng (quay).Cc VDT khng b hao mn nh chit p nhng di ti a ca hnh trnh rt nh, xp x 25cmhoc thp hn i vi VDT tuyn tnh (LVDT) v xp x 60o hoc nh hn i vi VDT quay(RVDT). Cc VDT cn iu ha tn hiu m rng di dng gii iu ch nhy pha ca tn hiu

xoay chiu; tuy nhin, tnh sn c ca b gii iu ch VDT chuyn dng trong mch tch hp (IC) gp phn lm gim bt nhng nhc im ca VDT.

B ng tc l thit b xoay chiu ba pha phc tp v t , chng c sn xut c chnhxc cao v khe. B ng tc c kh nng o s sai khc v gc (c th ln ti 180 o) gia haitrc quay lin tc. Thm vo , b ng tc c th thc hin ng thi nhiu chc nng nh: uvo tham chiu, thit b o lng u ra, ng phn hi v b so snh [Hnh 45.2 (c)].

Dng c phn tch n gin hn v r hn b ng tc v chng c nhiu u im hn so viRVDT vi kh nng o dch chuyn gc trong phm vi 360o. Hnh 45.2(d) th hin mt trong mtvi kh nng ng dng ca dng c phn tch, bin tn hiu t l vi cosin ca gc o c timt cun dy u vo v sin ca gc o c ti u cn li. Cc IC chuyn dng c sn cho iuha tn hiu v cho bin i u ra ca dng c phn tch sang dng s. IC tng t, khi cdng vi b chuyn i Scott-T, c th c dng bin i u ra ca b ng tc sang dng s.

B m ha v tr c kh nng tng thch cao vi s iu khin s do chng gim thiu ccyu cu bin i s-tng t ca tn hiu phn hi. Rnh ghi m ha c c bi cm bin dngrnh ghi, thng thng l u c hoc cc thit b quang in (nh l hng ngoi hoc laser). Bm ha v tr sn c cho c cc ng dng thng ln ng dng quay, tuy nhin chng thng hayc dng nh b m ha dng trc trong cc ng dng quay. iu ha tn hiu d thc hin vicc b m ha tuyt i [Hnh 45.2(e)] ch yu cu mt b gii m, nhng phn gii v tr ph

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K thut o: Cm bin v b chuyn i

thuc vo s lng rnh ghi v s lng rnh ghi cng nhiu th cng lm tng phc tp ca bgii m. B gii m tng i cn iu ha tn hiu phc tp hn, di dng b m v b x lcho vic v tr tnh ton. Tuy nhin, s lng rnh c c nh l ba rnh [Hnh 45.2(f)]. phngii v tr ch b gii hn bi kh nng phn chia mn ca rnh m ha trn b mt chuyn ng.

Hnh 45.2 B chuyn i v tr tng th: (a) Chit p, (b) B chuyn i vi phn bin thin (VDT),(c) B ng tc, (d) Dng c phn tch, (e) B m ha v tr tuyt i, (f) Rnh m ha cho b m

ha v tr tng i

Mc d b chuyn i dch chuyn tng th c thit k ring cho c chuyn ng thng hocchuyn ng quay, nhng mt bnh rngv rng chuyn, hoc mt b bin i chuyn ngtng t, thng c dng lp vo cc b chuyn i c thit k cho chuyn ng thngnhm o chuyn ng quay v ngc li.

Cc dng in hnh ca b chuyn i dch chuyn nhy (nh)[hnh 45.3] l:

T vi phn

B in tr bin dng k

Tinh th p in

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Hnh 45.3 B chuyn i dch chuyn nh: (a) T vi phn, (b) in tr bin dng k, (c) tinh thp in

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S tay C in t

Hnh 45.4 B chuyn i vn tc: (a) Nam chm v cun dy, (b) cm bin tip cn

B chuyn i vn tc

Nh c trnh by trn, k thut iu ha tn hiu cho ta bit tt c cc thng s vchuyn ng-dch chuyn, vn tc, hoc gia tct mt php o bt k mt trong ba thng strn. Tuy nhin, i khi thun li khi o trc tip vn tc, c bit trong cc chuyn ng thng chnh trnh ngn hoc trc quay c vn tc cao. Cc b chuyn i tng t thng c dng

p ng hai yu cu ny l: B chuyn i vn tc nam chm-cun dy [Hnh 45.4(a)]

My o tc

Loi th ba ca b chuyn i vn tc, b chuyn i vn tc kiu b m [Hnh 45.4(b)] ds dng v tng thch vi b iu khin s.

Hat ng ca b chuyn i vn tc nam chm v cun dy c da trn nh lut Faraday.i vi li slnit c t l di/ng knh cao c to ra bi s quay vng ca cun dy thin p cm ng trong li c quan h t l vi vn tc ca nam chm. B chuyn i vn tc namchm-cun dy sn c vi hnh trnh nm trong khon t nh hn 10mm ti xp x 0,5m.

Mt my o vn tc gc, nh l tn ca n ni ln, l mt my pht in AC hoc DC nhm in p u vo ca n t l trc tip vi vn tc gc ca trc quay. iu c quyt nh

bi trc u ra c iu khin. My o vn tc gc c th c tc trc ln ti 5000 vng/phthoc ln hn nhng u ra c th l phi tuyn v in p u ra nhp nh khng th s dng c tc thp.

Cc my o vn tc gc AC c gi r hn v d bo dng hn cc my o vn tc gc DC,nhng my o vn tc gc DC li c kh nng tng thch trc tip vi cc b iu khin tng tv chiu phn cc ca u ra biu th hng quay vng. u ra ca my o vn tc gc ACphic demodulated v demodulator phi nhy cm vi pha biu th s quay vng.

B chuyn i vn tc kiu b m hot ng da trn nguyn tc m cc xung in trongmt khong thi gian c nh, sau chuyn i s xung m c trong mt n v thi giansang vn tc. B chuyn i vn tc kiu b m ph thuc vo vic s dng b cm bin tip cn(pickup) hoc b m ha tng i [Hnh 45.2(f)]. Cc b cm bin tip cn c th c cc dngsau:

Quang in T tr bin thin

Hiu ng Hall

T cm

in dung

Hai ng dng ph bin ca b chuyn i vn tc kiu b m c thy trn hnh 45.4.

Do b iu khin s nht thit phi c ng h in t rt chnh xc nn vic m v bin ixung sang vn tc c th c thc hin bng phn mm (tc l, thnh mt phn trong chngtrnh iu khin). Vic dng phn cng thc hin m xung c th cn thit nu vic m thigian tp trung nh hng ti nhng chc nng iu khin cn thit khc ca b iu khin. Mt ICc bit, gi l giao din m/gii m cu phng(quadrature decoder/counter interface), c th

thc hin cc chc nng gii m v m ng thi truyn kt qu ti b iu khin di dngword d liu.

B chuyn i gia tc

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Ging nh o vn tc, i khi ngi ta hay o gia tc mt cch trc tip hn l nhn gia tc to lng dch chuyn hoc vn tc. Phn ln b chuyn i gia tc c gi l gia tc k dng achn bi v vic o gia tc da trn vic o s dch chuyn ca khi lng c gi l phn t achn (Hnh 45.5). Cc cu hnh trong cc Hnh 45.5 (a,b) yu cu phi c s b tr cc l xo khchnh xc cho h thng treo v nh tm cho khi lng a chn. Mt trong nhng nhc imca gia tc k dng a chn l khi lng a chn b dch chuyn trong qu trnh o gia tc vvic dch chuyn ny gy ra s phi tuyn v sai lch cho o lng. Loi cn bng lc trong hnh45.5 (c) s dng li nam chm in nh l phn t a chn. Mt cm bin dch chuyn nhy phthin ra s dch chuyn ca li v s dng tn hiu dch chuyn di dng phn hi m iukhin cun dy a li tr v v tr trung tm ca n. u ra ca gia tc k lc cn bng l s phnhi cn cn tr s dch chuyn hn l nguyn nhn gy ra s dch chuyn.

Mt dng gia tc k dng a chn n gin hn dng in cc ca tinh th p in nh phnt a chn [Hnh 45.5(d)]. Tng t, mt dng gia tc k n gin khc li coi bn mt chung cat vi phn [Hnh 45.3(a)] l phn t a chn.

B chuyn i lc

Vic o lc thng thng c da trn vic o chuyn ng, n chnh l kt qu ca ngoitc tc dng ln. Nu ngoi lc gy ra chuyn ng tng th ca u ra iu khin v khi lngca phn t u ra bit th bt k gia tc k no c gn vi u ra iu khin u sinh ra mt

u ra t l vi ngoi lc (F=Ma).Mt thang chia cn bng l xo n gin [hnh 45.6(a)] ph thuc vo vic o dch chuyn, nl kt qu ca vic ngoi lc (trng lng) ko dn l xo. Cc thng s o lc c chnh xc caotrong cc c cu ph nh nhng thng s c s dng trong vic ch ra v nhn dng thit b,thng ph thuc vo con quay hi chuyn ng vai tr nhn bit ngoi lc tc ng.

S trnh by trn hnh 45.6(b) l mt con quay hi chuyn c khp cc ng v phn tquay trn. B chuyn i chuyn ng (dch chuyn hoc vn tc) trn trc quay cung cp u ratng ng n ngoi lc tc dng. Dng khc ca con quay hi chuyn v cm bin trc quaycng c s dng nh l mt cng c trong vic o lc.

o lc tnh (loi m khng thy r s chuyn ng) thng c da vo vic o s cng dongoi lc tc dng gy ra. Hnh 45.6(c) minh ha mt cu trc c th ca b chuyn i lc phbin gi l t bo ti. Lc tc dng to ra bin dng t l trong thnh phn cu trc hnh S, c

o bng b chuyn i dch chuyn nhy, thng l mt in tr bin dng k hoc tinh th pin.

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S tay C in t

Hnh 45.5 Gia tc k dng a chn: (a) B chuyn i gia tc thng, (b) Gia tc k quay, (c) Giatc k cn bng lc, (d) Gia tc k p in

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Hnh 45.6 Chuyn i vn tc: (a) Thang o l xo, (b) Con quay hi chuyn, (c) T bo ti

45.3 B chuyn i qu trnh

Phn ny tho lun v cc b chuyn i s dng trong o lng v iu khin cc tham squ trnh thng gp trong cc qu trnh cng nghip, l:

p sut lu cht

Dng lu cht

nht

Nhit

B chuyn i p sut lu cht

Phn ln b chuyn i p sut lu cht thuc dng n hi, trong lu cht c gi trongmt khoang c t nht mt vch n hi v s bin dng ca vch n hi biu th p sut. ng

Bourdon v cc ng xp l nhng v d v b chuyn i p sut dng n hi c s dng trongnhng b chuyn i dng trong phng th nghim v trong mt s ng dng iu khin qu trnhcng nghip. B chuyn i p sut lu cht m t trong Hnh 45.7 s dng mng ngn n hi phn tch hai khoang l dng thng thy c s dng trong iu khin qu trnh cng nghip.Cc mng ngn c to ra t mt trong nhiu vt liu n hi t kim loi mng n vi polymeha.

Vi o lng p sut th, dch chuyn ca mng ngn c o bng chit p hoc LVDT; vicc o lng p sut nhy, bt c cm bin no trong ba cm bin dch chuyn nhy m t bntrn c dng. Trong hu ht cc dng thng thng ca cm bin p sut nhy, in tr bindng k hnh hoa th thng c gn vo mng ngn. Trong mt dng khc, cc vch ngoi cacm bin p sut ng vai tr l bn t v mng ngn ng vai tr l bn mt chung ca t vi phn.Trong mt cu hnh rt nhy v tch hp cao, mng ngn l mng silicon vi mt bin dng k ptr v cc mch iu ha tn hiu c tch hp trong silicon.

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S tay C in t

Hnh 45.7 B chuyn i p sut dng mng ngn

Hnh 45.8 B chuyn i piston-l xo

Cc o lng c chn khng cao (p sut rt thp), thng da trn cc quan st v nht,tnh dn nhit, thuc tnh m thanh, hoc in th ion ha ca cht lng, s khng c gii thiutrong phn ny. Cc b chuyn i c dng trong cc h thy lc p sut cao [70 MPa (10,000psi) hoc cao hn] thng c dng piston v l xo [Hnh 45.8]

Trong c hai b chuyn i p sut ny, u ra thc t l o lng v s sai khc p sut gia

khoang lm vic v khoang tham chiu cu b chuyn i (tc l, OUT REF p p p=

). o lngny c gi l:

p sut tuyt i nu khoang tham chiu c ng kn v to chn khng (tc l,

0REFp=

v OUTp p=

) p sut tiu chun nu khoang tham chiu c m thng vi khng kh (tc l

OUT ATMp p p=

)

p sut vi sai nu bt k p sut no khc c a vo khoang tham chiu

B chuyn i dng lu cht (Lu tc k)

Do s lng bin s lin quan, vic o lu tc c s dng rng ri trong nhiu ng dng vcng ngh o. Trong cc qu trnh cng nghip, thut ng lu chtkhng ch c p dng chocht kh v cht lng m cn cho cc hn hp dng dng (thng thng gi l cc cht sn sthoc bn c) nh b tng, nc thi hoc bt giy. Vic iu khin dng cht lng, ty theo dng

o lng cn thit, c th lin quan n lu lng th tch, lu lng khi lngv hng dngchy. Dng kh c th c kh nng nn gy tc ng n k thut o. Ngoi ra, trng thi ca dngchy - d ng nht v sch hay khng (s t do ca cc phn t) - cng c nh hng n cngngh lu tc k. Mt nhn t khc cn quan tm l vn tc dng chy; dng cht nht dng lpchuyn ng cn c cc k thut o khc so vi k thut o dng chy ri tc cao. Mt vn khc cn lu l s giam hm dng chy. Trong khi hu ht cc o lng dng cht lng lin

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quan n dng chy y qua cc knh kn nh l ng , th mt s ng dng li yu cu cc olng dng chy tng phn qua cc knh mnh mng. Ch c nhng lu tc k c s dngrng ri c quan tm y.

Lu tc k c chia thnh cc loi chnh sau:

Dng p sut vi phn, dng tht (venturi, vi phun, ng phun o lu lng, ng khuu(ng ni cong), v ng pitot tnh) (Hnh. 45.9)

Cng sut cht lng (hp s, turbine, v gung quay) (Hnh 45.10)

Siu m (Hnh. 45.11)

Th xoy (Hnh 45.12)

Phong tc k nhit (Hnh 45.13)

in t (Hnh. 45.14)

Lu tc k kiu phao (lu tc k c tit din thay i trn ng i) (Hnh. 45.15)

Hnh 45.9 Lu tc k p sut vi phn: (a) Lu tc k Venturi, (b) Lu tc k dng vi phun, (c)Lu tc k ng phun , (d) Lu tc k dng ng ni cong (ng khuu), (e) Lu tc k ng pitot tnh

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S tay C in t

Hnh 45.10 Lu tc k cng sut cht lng: (a) Lu tc k turbine, (b) Lu tc k gung quay,(c) Lu tc k hp s

Lu tc k p sut vi phn ph hp vi dng chy kh v cht lng sch, nht thp c vntc cao v trung bnh. Lu tc k Venturi [Hnh 45.9(a)] c chnh xc cao nht nhng chng ckch thc ln v t. Lu tc k dng vi phun [Hnh 45.9(b)] c kch thc nh hn, gi c rhn nhng li khng chnh xc bng lu tc k Venturi. Lu tc k ng phun [Hnh 45.9(c)] l skt hp gia lu tc k Venturi v dng vi phun. Lu tc k ng khuu [Hnh 45.9(d)], c thc lp t bt k ch cong no trong h thng ng dn, c s dng trong cc o lng lulng th. Lu tc k pitot tnh [Hnh 45.9(e)] c dng vi nhng dng chy c tit din lnnh trong hm gi. Lu tc k pitot tnh cng c s dng trong cc ng dng lung t do nhb ch vn tc gi trong my bay.

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Hnh 45.11 Lu tc k siu m: (a) Lu lng k siu m dng truyn, (b) Lu lng k siu mdoppler

Hnh 45.12 Lu lng k th xoy

Lu tc k cng sut cht lng c dng trong nhng dng chy tc thp, nht trungbnh. Ngoi cc ng dng iu khin trong cng nghip, i khi cc lu tc k turbine [Hnh45.10(a)] cn c dng nh l ng h ch tc cho tu thuyn. Lu tc k gung quay [Hnh45.10(b)] c s dng trong cc ng dng dng chy kn v m nh dng cht lng trong mng.Do ng c bnh rng cng sut cht lng [Hnh. 45.10(c)] l thit b th tch khng i, nn tc trc quay lun lun biu th trc tip tc dng cht lng.

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S tay C in t

Hnh 45.13 Phong tc k nhit

Hnh 45.14 Lu tc k in t

Hnh 45.15 Lu tc k c tit din thay i trn ng i (lu lng k kiu phao)

Lu tc k siu m dng truyn [Hnh 45.11(a)], da trn nguyn l tc m thanh s tngbi tc dng cht lng, c dng cho tt c cc dng dng chy sch v di tc m thanh.

Lu tc k Doppler [Hnh 45.11(b)] li ph thuc vo s di li bn trong dng chy v do chhu ch vi dng chy bn mang theo phn t l lng hoc dng chy ri to ra bong bng. Lutc k siu m l thit b khng xm nhp thng c trang b thm vo cc h thng ng dn csn.

Lu tc k dng th xoy (Hnh 45.12) a vt th vo dng chy sinh ra dng chy ri.m thanh km vi s sinh ra v mt i ca dng chy ri c gim st v phn tch. Tn s mthanh tri biu th tc sinh ra v mt i ca dng chy ri, v do n cng s biu th cho tc dng chy. Lu tc k dng th xoy rt hu ch i vi nhng dng chy tc thp v khnghn lon.

Phong tc k nhit (Hnh 45.13) c dng trong cc dng kh tc thp vi tit din ln nhtrong cc ng dn nhit, ng thng hi, v iu ho khng kh (HVAC). S lm lnh bng i luca phn t nhit c lin quan n tc dng chy. Vic o tc dng chy da trn dng kh

cn thit duy tr nhit n nh trong phn t nhit hoc da trn s thay i nhit khidng kh c duy tr khng i.

Lu tc k in t (Hnh 45.14) hu ch i vi nhng dng chy chm ca cht lng, bn choc huyn ph. Cht liu dng chy phi h tr cho vic dn din gia cc in cc v do trong mt s trng hp s dng c lu tc k in t th nht thit phi ion ho dng chyt pha ngc li so vi im o.16


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Lu tc k c tit din thay i trn ng i (Hnh 45.15) hay cn gi l lu lng k kiuphao i khi c coi nh cc thc ngm do chng cung cp s ch c th quan st c ca tc dng chy. Nhng thit b ny nu c gn thm cm bin tip cn (cm bin in dung) cchc nng nhn bit s hin din ca phao th c th c dng trong cc ng dng iu khinng-ngt.

B chuyn i mc cht lng

Vic o mc cht lng l kh d dng, v b chuyn i c phn thnh dng tip xc hockhng tip xc. Cc o lng c th lin tc, trong mc cht lng c gim st lin tc trongpham vi hot ng ca n, hoc theo tng im, trong mc cht lng c xc nh nm trnhoc di mt s mc nh trc.

B chuyn i tip xc thng gp l:

Dng phao

p sut thy tnh

in dung

Siu m

B chuyn i khng tip xc thng gp l:

Cm bin tip cn dng in dung Siu m

Tn s radio

Quang in

B chuyn i mc cht lng dng phao c rt nhiu hnh dng c dng trong cc o lnglin tc hoc theo tng im. Mt dng c m t trong Hnh 45.16 dng cho o lng lin tcv cho c o lng n im ln lng im.

B chuyn i mc cht lng p sut thu tnh c th c dng trong cc ng dng thng hihoc to p sut (Hnh 45.17). Trong c hai trng hp ny, p sut vi phn t l trc tip vitrng lng ct cht lng, do b chuyn i p sut vi phn gy nn p sut b mt.

Cc u d in dung [Hnh 45.18(a)] c s dng rng ri trong vic o mc cht lng. Khi

vch ca thng cha l kim loi th c th coi mt thanh kim loi cch in hoc khng cch inn nh bn mt t in cn vch ca thng cha nh bn mt khc ca t in. Nhn chung, ud in dung thng gm c mt thanh kim loi nm trong mt hnh tr ng tm h hai u gipcho b chuyn i khng ph thuc vo cu trc thng cha. Mt ng dng kh th v ca u din dung l ng h ch lng nhin liu trong my bay. B chuyn mch in dung c th cs dng nh m t trong Hnh 45.18(b) thc hin cc o lng mc cht lng theo tng imkhng tip xc.

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S tay C in t

HNH 45.16 B chuyn i mc cht lng dng phao

HNH 45.17 B chuyn i mc cht lng p sut thu tnh

HNH 45.18 B chuyn i mc cht lng dng in dung: (a) u d in dung, (b) B chuynmch in dung

HNH 45.19 B chuyn i mc cht lng siu m: (a) B chuyn i mc cht lng dng phmvi phn hi, (b) B chuyn mch siu m

B chuyn i siu m dng phm vi phn hi c th c dng di dng nhng t(tipxc) hoc khng nhng t(khng tip xc) thc hin php o mc cht lng [Hnh 45.19(a)].Mt ng dng kh th v ca b chuyn i dng nhng t l my d su v my d c chotu thuyn. B chuyn i khng nhng t cng c th c s dng cho cc vt liu ri nh ht

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ng cc v bt m. B chuyn i mc cht lng dng tn s radio v quang in thng l nhngthit b dng khng tip xc v phm vi phn hi c nguyn l v ng dng tng t vi bchuyn i siu m khng nhng t.

B chuyn i siu m c th thch hp cho cc o lng theo tng im bng cch t btruyn v b nhn i din nhau v cch nhau mt khong nht nh [Hnh 45.19(b)]. Khi chtlng lp y khong ny th s suy gim nng lng siu m r rng s thp hn so vi khi khngkh lp y khong ny. Mch iu ha tn hiu s dng s tng mnh mc nng lng siu m

ny c tch bi b nhn kch hot b chuyn mch.

B chuyn i nhit

o nhit da trn mt trong cc nguyn l vt l sau:

Gin n nhit

Hin tng nhit in

Tc ng nhit ln in tr

Tc ng nhit ln dn ca cc mi ni bn dn

Bc x nhit

(Ni ng ra, bt k thit b no c dng o nhit u c th gi l nhit k, tuy nhin

ngi ta thng dng nhng thut ng m t mang tnh chnh xc hn ni v nhng thit bdng kim sot nhit .)

B chuyn mch lng kim (Hnh 45.20) c s dng rng ri trong cc h thng kim sotnhit dng ng-m. Nu hai tm kim loi c h s n v nhitkhc nhau c ni li vi nhau cng nhit , th cu trc lng kim ny s b un cong khi nhit thay i. Mc d nhngthit b ny thng c gi l cu ch nhit, ng l chng c s dng trong b chuyn mchkn thng thng, nhng chng c th c sn xut theo dng ng hoc m. Cc phn t lngkim cng c th c ch to dng cun hoc xon c m rng phm vi chuyn ng do nv nhit.

Cp nhit l nhng cm bin nhit khe v linh hot thng thy trong cc h thng iukhin cng nghip. Mt cp nhit gm mt cp dy kim loi khc nhau c xon li vi nhauhoc c ni vi nhau ti im cui. Hiu ng Seebeckl hin tng vt l gii thch hot ngca cp nhit , do cp nhit c th c gi l mi ni Seebeck. S chnh lch in th

(in p Seebeck) gia cc u t do ca dy in t l vi s chnh lch gia nhit ti mi niv nhit ti cc u t do. Cp nhit c th o nhit t -270 C n 2300 C mc d khng mt cp nhit n no ph ton b phm vi ny. Cp nhit c phn loi thnh dngB, C, D, E, G, J, K, N, R, S, hoc T ty thuc vo kim loi dng trong dy dn.

HNH 45.20 B chuyn mch nhit lng kim

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S tay C in t

HNH 45.21 Cp nhit : (a) S cp nhit in hnh (b) Mt s ph tng v hnh dng cacp nhit

S iu ha v khuych i tn hiu ca in p Seebeck kh nh ch ra rng dy dn cp nhit phi c ni vi cc cc ca mch iu ha tn hiu. Nhng lin kt ny to ra thm hai mini Seebeck, mi mi ni ny li to ra mt in p Seebeck ca ring n cn loi b trong mchiu ha tn hiu. thc hin vic loi b cn cc hiu chnh sau [Hnh 45.21(a)]:

Cc cc u vo ca mch iu ha tn hiu phi c lm t cng mt loi kim loi.

Hai cc phi nm trn mt khi cc ng nhitsao cho mi mi ni Seebeck c tora t lin kt ny c nhit ging nhau.

Nhit ca khi cc phi c bit trc.

HNH 45.22 Mt ng dng b d nhit in tr (RTD) tiu biu

Hai yu cu u tin c th c tho mn bng vic xy dng hp l mch iu ha tn hiu.Yu cu th ba c th c p ng bng cch s dng mt cm bin nhit tham chiu, c thl loi b chuyn i nhit IC c trnh by di y.

Cp nhit v cc ph kin ca n c ch to cho rt nhiu ng dng [Hnh 45.21(b)].Tm chn bo v (lng nhit) c dng bo v cc mi ni ca cp nhit trong mi trngn mn hoc ni cht lng dn in lm ngn mch in p ca cp nhit ; tuy nhin cc mini trn (khng cch ly) c th c s dng khi c th, c bit khi cn c p ng nhanh.

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B d nhit in tr (RTD) c da trn nguyn l sutin tr ca hu ht cc kim lois tng ln theo nhit . i vi RTDs th bch kim l loi kim loi c a chung hn c, mcd nhiu loi kim khc loi r tin hn vn c dng trong mt s ng dng. Sut in tr cabch kim l mt trong nhng sut in tr tiu chun dng o nhit . tuyn tnh tng itt ca sut in tr bch kim trong mt phm vi nhit rng (t -200 n 800 C) lm cho RTD bch kim ph hp vi cc b chuyn i nhit chnh xc, n nh, c th d dng thchnghi vi cc ng dng h thng iu khin.

Nhc im ca RTD l cc phn t cm nhn nhit l mt si kim loi kh mng manhc cun quanh mt cun dy bng gm hoc mt mnh kim loi mng c gn trn mt chtnn bng gm. Cc phn t RTD thng c gi gn li v t khi c s dng nh nhng phnt khng cch in. Cc ph tng v gi ng dng dng vi RTD tng t nh nhng g c sdng vi cp nhit [Hnh 45.21(b)].

Hu ht cc RTD bch kim u c ch to sao cho chng c in tr danh ngha l 100 W 0 C. H s nhit in trca bch kim xp x trong khong 34 mW/W/ C, do phn gii nhit trong vng 1oC i vi mt phn t RTD 100-W danh ngha i hi phn gii ca intr tuyt i trong khong 0.30.4 W. Nhng yu cu v phn gii nhit ny cho thy cnphi s dng cc k thut iu ha tn hiu c bit hn ch in tr dy dn v in tr tipxc ca phn t RTD (Hnh 45.22). Mch c m t trong hnh 45.22 l dng bin th ca m k4 dy. Hu ht cc RTD u c sn xut vi 4 dy dn c th tng thch c vi nhngmch nh vy.

Nhit in trl cht bn dn oxit kim loi c bit c h s nhit m mnh, hon ton ilp vi h s nhit dng yu ca RTDs. in tr danh ngha ca nhit in tr, thng thng 25 C, nm trong khong t di 1000 W n hn 1 MW, vi nhy ln hn 100 W/oC. V vy, nhit in tr l c s cho cc cm bin nhit nhy hn v cn t iu ha tn hiu c bithn nhng cp nhit k hoc RTD. S tha hip c to ra nh tnh phi tuyn r rt ca c tnhin tr-nhit . gim thiu vn ny, nh sn xut cung cp cc gi trong in tr nhitc ni trong mng in tr c la chn cung cp cc c tnh tuyn tnh tng i trongkhong nhit danh ngha.

S pht trin ca cng ngh in tr nhit cho ra i cm bin nhit IC trong cc mini nhy nhit v mch iu ha tn hiu c cung cp trong mt gi nguyn khi. Yu cu t rai vi ngi s dng l phi cung cp in p ngun (thng thng 5V DC) cho IC nhm cc in p u ra dng tng t t l vi nhit . Nhit in tr v cm bin nhit IC c th

c sn xut vi hnh dng nh gn cho php o nhit cao ti mt khu vc. Mt s nhit intr c thit k cho nghin cu sinh hc c gn trn u kim tim.Nhc im ca c nhitin tr v cm bin nhit IC l chng khng mnh, khng th s dng trong mi trng hacht c hi v b gii hn mc nhit xp x 200 oC.

Hnh 45.23 Gin ca lc nhit in k bc x

Nhit in k bc x c s dng nhn bit nhit t xa (khng tip xc) trong nhngtnh hung cm bin tip xc khng th dng c. Qu trnh hot ng ca n da trn nguyn ltruyn nhit thng qua bc x nhit. Nhit in k bc x tp trung nng lng hng ngoi t mt

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S tay C in t

ngun nhit vo mt vt en (mc tiu) trong phm vi ca nhit in k bc x (Hnh 45.23). Mttrong nhng cm bin nhit tip xc c m t trn c gn vo mc tiu o nhit ca mc tiu. S tng nhit ti mc tiu c lin quan n nhit ngun. Nhit in k bc xin hnh c phm vi cn bng ( di tiu c) trong khong 0.5-1.5m, nhng cc cng c olng hin nay c di tiu c t 1 cm n 10 m. Nhit in k bc x c phm vi rng, n sc,hoc nhit biu hc hai mu.

45.4 Kh nng hot ng ca b chuyn iKh nng lm vic ca b chuyn i trong mt h thng iu khin c miu t di dng

c tnh tnh v c tnh ngca n. c tnh tnh hc c quan tm nhiu nht l:

H s qui i (hoc nhy)

chnh xc, sai s, tnh ng n, v sai s h thng (hoc lch)

Ngng, phn gii, di cht v hin tng tr.

S tuyn tnh

tri tng t

c tnh ng c quan tm nht l:

Hng s thi gian, thi gian p ng v thi gian tng trng

Qu mc, thi gian dng v tn s gim chn.

p ng tn s

c tnh tnh c ti liu ho thng qua s cn chun bao gm vic p dng mt u vo bit (s lng hoc hin tng c th o c) cng nh quan st v lu tr u ra ca b chuyni. Trong mt quy trnh cn chun in hnh, u ra c tng ln di dng s gia t gii hndi n gii hn trn ca b chuyn i, sau li gim xung gii hn di. Phm vi ca mtb phn cu thnh bao gm cc gi tr u vo cho php. S chnh lch gia gii hn trn v giihn di l khong u vo ca b phn cu thnh; s chnh lch gia u ra gii hn trn viu ra gii hn di l khong u ra.

c tnh ng c ti liu ho bng cch p dng mt s thay i bit, thng l mtbc, u vo, ri quan st v ghi li u ra ca b chuyn i, thng vi mt my ghi biu hoc mt my hin sng c lu tr.

45.5 S ph hp gia ti v b chuyn i

Mt yu cu thit yu i vi mt b chuyn i thch hp l n phiph hp vi u vo can. S ph hp c ngha l nng lng u vo cn thit cho qu trnh hot ng bnh thngca b chuyn i, v sau l mt o lng chnh xc ca u ra iu khin, khng lm thay ing k u ra c iu khin . Mt b chuyn i khng c c s ph hp ny c gi lti u ra c iu khin. V d, mt vn k phi c u ra tr khng cao sao cho vic o hiuin th khng thay i ng k dng in trong mch v lm thay i hiu in th ang c o.

nh ngha thut ng

u ra iu khin: Sn phm quan trng ca h thng iu khin t ng; s lng hay tnhhot ng vt l c o phc v iu khin t ng.

ng phn hi:S lin kt phn lp cc thnh phn ca b chuyn i viu ha tn hiu trong mt h thng iu khin t ng.

ng tin: S lin kt phn lp ca b iu khin, c cu chp hnh vthit b hoc qu trnh trong h thng iu khin t ng.

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B chuyn i chuyn ng:Mt b chuyn i c dng o lng u raiu khin ca c cu servo; thng c hiu bao gm cc b chuyn i choo lng lc tnh.

Thit b hoc qu trnh: Thit b iu khin to ra u ra chnh trong mt h thngiu khin t ng.

iu khin qu trnh: l mt thut ng dng ch vic iu khin ccqu trnh cng nghip; phn ln c dng ch vic iu khin nhit , p

sut lu cht, dng lu cht v mc cht lng.B chuyn i qu trnh: Mt b chuyn i c s dng o u ra iu khin

ca mt h thng iu khin t ng dng trong iu khin qu trnh.

u vo tham chiu: Tn hiu c cung cp cho mt h thng iukhin t ng thit lp u ra iu khin cn thit; v cng c gi lim t.

C cu servo: Mt h thng trong mt s dng ca chuyn ng l u ra iu khin.

iu ha tn hiu: Trong trng hp ny, n l thut ng c s dng cp nvic iu chnh tn hiu trn ng phn hi ca mt h thng iu khin t ng; iu ha tnhiu bin i u ra cm bin thnh tn hiu in ph hp so snh vi u vo tham chiu (imt); thut ng ny cng c th p dng cho vic iu chnh tn hiu ng tin.

B chuyn i: Thit b c s dng o u ra iu khin trong mt h thng iukhin t ng, thng bao gm cm bin v cc thnh phn iu ha tn hiu.

Ti liu tham kho

[1] Bateson, R.N. 1993.Introduction to Control System Technology, 4th ed. Merrill, Columbus,OH. Berlin, H.M. and Getz, F.C., Jr. 1988. Principles of Electronic Instrumentation andMeasurement. Merrill, Columbus, OH.

[2] Buchla, D. and McLachlan, W. 1992. Applied Electronic Instrumentation andMeasurement. Macmillan, New York.

[3] Chaplin, J.W. 1992. Instrumentation and Automation for Manufacturing. Delmar, Albany,NY.

[4] Doeblin, E.O. 1990. Measurement Systems Application and Design, 4th ed. McGraw-Hill,

New York.[5] Dorf, R.C. and Bishop, R.H. 1995. Modern Control Systems, 7th ed. Addison-Wesley,

Reading, MA.

[6] ODell, T.H. 1991. Circuits for Electronic Instrumentation. Cambridge Univ. Press,Cambridge, England, UK.

[7] Seippel, R.G. 1983. Transducers, Sensors, and Detectors. Reston Pub., Reston, VA.

[8] Webb, J. and Greshock, K. 1993. Industrial Control Electronics, 2nd ed. Macmillan, NewYork.

Thng tin b sung.

Omega Engineering, Inc l ni ch to v cung cp catolog, ti liu, s tay v ghi ch p dng,

c th l chui s tay (nm nay):S tay v dng v mc

S tay p sut, ng sut v lc

S tay v nhit

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S tay C in t

Cc tp san, tp ch v t tin tc thng mi, c th:

Tin tc o lng (o lng quc gia ca M)

Tin tc o lng v k thut c nhn

Tin tc o lng v kim tra (Hewlett Packard)

Th gii o lng v kim tra

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