sổ tay cdt chuong 19(1,2,6,7,8)- sensor

8/6/2019 s tay cdt Chuong 19(1,2,6,7,8)- Sensor

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19Cm bin

Kevin M. LynchNorthwestern University

Micheal A. PeshkinNorthwestern University

Helit ErenCurtin University of Technoloty

Pamela M. Norris vBouvard Hostickai hc Virginia

Jorge Fernando Figueroav H. R. (Bart) Everett

Stanley S. Ipson

19.1 Cm bin tuyn tnh v cm bin gc ............. ......... ........1

19.2 Cm bin gia tc ...............................................................9

19.6 o nhit ............................................................. ........26

19.7 Cm bin o khong cch v cm bin tim cn ............37

19.8 Cc h thng nhn bit nh sng, hnh nh v nhn dng 60

19.1 Cm bin tuyn tnh v cm bin gc

Kevin M. Lynch and Michael A. PeshkinHu nh tt c cc chuyn ng ph bin nht trong cc h thng c kh l dch chuyn thng dc theo mt trc c nhv quay mt gc quanh trc c nh. Nhng chuyn ng phc tp hn c th c to ra bi vic kt hp nhng chuynng n gin . Chng ny s gii thiu tm tt mt s cng ngh sn c o cc chuyn ng thng v chuyn ngquay dc theo mt trc n. Cc phng thc o c sp xp theo cc hiu ng vt l c khai thc a ra cc phngphp o lng.

Tip xc

Loi n gin nht ca cm bin dch chuyn l mt cng tc c kh, n gi tr mt bit thng tin: chm hoc khng chm.Mt cng tc in hnh bao gm mt n by, khi n c nhn s to ra mt lin kt c kh bn trong n, t to ra mtmch in kn( hnh 19.1). C th s dng cc vi cng tc ny nh l nhng cm bin va chm dng cho cc mobile robot,thng l gn mt vt liu d ch to vo mt n by (nh mt ci ru) bo v robot khi va chm vi cc vt cn cng.

Mt ng dng thng dng khc ca vi cng tc trong robot l cng tc hnh trnh, dng xc nh mt khp t ti imgii hn trong hnh trnh cho php ca n.

HNH 19.1 Mt vi cng tc c bn HNH 19.2 Tn hiu to thnh t mt cng tc ng ngt

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S tay C in t

Hnh 19.2 m t cu hnh c bn ca mt vi cng tc. Ci in tr treo gi tn hiu trng thi +V ti khi cng tc ng,a tn hiu ti t. Tuy nhin khi cng tc ng, mt lot cc nh hng nh c th dn n tnh trng nhiu ca tn hiu.Mt mch gim nhiu c to ra lm sch tn hiu ra.

Cc cng tc c th c hai loi: thng m NO, hoc thng ng - NC. Mt cng tc c th c nhiu chuyn mch (P -poles) v 1 hoc 2 tip im (T - throw) cho mi chuyn mch. Mt chuyn mch dch chuyn nh mt cng tc c kchhot, v ni vi tip im. V vy mt cng tc SPDT (mt chuyn mch hai tip im) s chuyn mt chuyn mch t tipxc vi tip im ny sang tip sang tip xc vi tip im kia, v mt cng tc (double pole single throw) chuyn hai

chuyn mch t m sang ng (Hnh 19.3).

HNH 19.3 Cu to ca cc cng tc SPDT v DPST HNH 19.4 Cc k hiu mch quang in t v cu hnhmt b pht/thu in hnh

Tia hng ngoi

Ngi ta c th s dng tia hng ngoi o cc dch chuyn thng hoc dch chuyn quay. in hnh nh diode pht tiahng ngoi (LED), hoc l n pht quang (photoemitter), c dng nh mt ngun pht sng, v mt thit b cm nghng ngoi c dng pht hin tia sng. B thu c th l mt quang in trhoc t bo quang in, mt bin tr c intr thay i theo cng ca tia ti (tia hng ngoi hoc tia khng nhn thy); mt diode quang, n cho php dng inchy theo mt hng khi c tia hng ngoi, nu khng n hot ng nh mt phn t h mch; hoc transistor quang. Trong

mt transistor quang, tia hng ngoi ti hot ng nh dng in base ca transistor, dng in collector t l vi cng ca tia hng ngoi nhn c (cho ti trng thi bo ho ca transistor). Cc k hiu mch cho cc phn t bin i c chra trong hnh 19.4.

Nu thit b pht v thit b d t hng v nhau th chng c th c s dng nh mt b ngt tia pht hin ra btk vt g i qua chng. Thit b ny c gi l thit b ngt quang(hnh 19.5). Nu b phn pht v thu di chuyn t do theong thng ni chng, th cng ca tn hiu nhn c c th c s dng o khong cch gia chng. Tuy nhin,cc b pht hin tia hng ngoi cng c th nhy cm vi c nh sng xung quanh. phn bit tia sng ca b pht vi nhsng nn, c th iu bin ngun pht (c ngha l ng hoc m tn s cao), v cc thit b thu c thit k ch png vi tia hng ngoi c iu bin.

Mt b pht sng v b d t cng hng v mt pha c th o th khong cch ti mt b mt gn nh cng ca tia phn x khi va vo b mt. Thit b gi l thit b phn x quang(hnh 19.6). Mt cm bin nh vy c th cdng cho cc robot di ng pht hin cc b mt hp th hay phn x nh sng mt khong cch khng i. Cc b lc

phn cc tia sng c th dng vi b d v pht b d ch pht hin ra nh sng c phn x bi mt b mt c bit.

HNH 19.5 B ngt quang bn dn QVA11234 HNH 19.6 Cm bin phn x quang bn dn QRB1114

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Cm bin

HNH 19.7 B d v tr (PSD) (UDT cm bin Inc)

Cc thit b ngt quang v cc cc thit b phn x quang c th c ng gi sn hoc ch to ring bit t mt nLED hng ngoi v mt diode quang hoc mt transistor quang, sau khi chc chn rng thit b d l nhy cm vi bc sngsinh ra bi n LED.

Cc khi thit b phn x quang cng c sn cc b d v tr (PSD) hin i hn, n s bo v tr ca tia sng hng ngoitrn b mt o (hnh 19.7). V tr c nh ca n LED c quan h vi PSD, cng nh v tr nh ca tia hng ngoi trn PSD,cho php s dng php tam gic ho xc nh khong cch ti ch. Cc cm bin o khong cch nh vy c sn xutbi Sharp v Hamamatsu.

HNH 19.8 Encoder tng i

Cc Encoder quang hc

Mt encoder quang s dng mt b ngt quang bin chuyn ng thnh mt chui xung in. Nhng chui xung nym ho chuyn ng, v cc xung c m hoc c gii m bi mt mch in a ra s o dch chuyn. Chuynng c th l thng hoc quay, nhng chng ta tp trung nhiu hn vo cc encoder quang quay thng thng.

C hai encoder quang quay c bn l encoder tng i (incremental encoder) v encoder tuyt i (absolute encoder).Trong mt encoder tng i, mt a (a m) c gn vo mt trc quay trn gia hai b ngt quang hc (hnh 19.8). ac th l nha trong hoc knh vi c t cc vch hng tm, hoc nha m vi cc khe nh, v vy khi a quay th ccvch ny s ngn hoc cho qua tia sng hng ngoi ti thit b d quang hc. (Cn c mt mt n ng yn ging nh a mt gia b pht v b thu) Cc dy xung thu c t cc b d ny c tn s tng ng vi vn tc gc ca a. Nhng tn

hiu ny c k hiu l A v B, v chng lch pha nhau chu k. Cc tn hiu c th n t cc b ngt quang hc thnghng vi hai rnh tch bit ti cc im ng vi cc gc khc nhau ca a, hoc c th c sinh ra bi cng mt rnh, vicc b ngt quang hc c t tng i vi b ngt khc a ra cc chui xung pha.

Thng qua vic m s lng cc xung v s lng ca cc ng xuyn tm trong a bit, ta c th o c tc quay ca trc. Hng quay c xc nh da trn quan h pha ca cc chui xung A v B. V d, A tng khi B = 1 c thch ra l chuyn ng quay ngc chiu kim ng h, trong khi A tng khi B = 0 ch ra chuyn ng quay thun chiu kimng h. Hai tn hiu pha ny c bit nh l cc tn hiu cu phng.

Cc encoder tng i thng c mt u ra tn hiu th ba gi l tn hiu ch s, k hiu l I hoc Z. Tn hiu ch s cly t mt rnh ring bit sinh ra mt xung n trn mt vng quay ca a, nh hng tuyt i. Trong thc t, c ththay th nhiu b ngt quang hc bng mt ngun n v mt thit b d tn hiu n.

Cc chp gii m c dng gii m cc chui xung. u vo ca cc chip l cc tn hiu A v B, v u ra l mt hocnhiu chui xung c a vo mt b m. V d nh b LS7083 s ca M cho ra hai chui xung, mt cho chuyn ngquay theo chiu kim ng h v mt cho chuyn ng quay ngc chiu kim ng h, chng s c gi ti cc u vo cab m 74193 (Hnh 19.9). Cc phng php gii m chun cho u vo cu phng l cc phng php vi phn gii1X, 2X v 4X. Trong phng php 1X, gi tr ca b m s tng ln mt n v khi c mt sn ln hoc xung ca ch mtchui xung, do tng s m ca encoder cho mt vng quay ca a bng tng s vch trn a. Trong phng php 4X,

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S tay C in t

gi tr ca b m c tng ln mt n v khi c mt sn ln hoc xung ca mt trong hai dy xung, v vy phn giigc c gim 4 ln. Mt encoder vi 1000 vch trn a m gii m c theo phng php 4X vi phn gii gc l

0 0360 /(4 1000) 0,09 = .

HNH 19.9 Encoder quang, chip gii m cu phng s LS7083 ca M, v b m (US Digital Inc.)

HNH19.10 a ca encoder tuyt i m Gray 8 bit, (BEI Technogies Industrial Encoder Division)

Trong khi mt encoderu ra n (single-ended output encoder) a ra tn hiu A, B v c th l Z, th mt encoder ura vi phn (differential output encoder) cung cp thm cc u ra b A, B v Z. Cc u ra vi phn, khi c dng vi thitb thu vi phn, c th lm tng tnh chng nhiu in ca encoder.

Mt hn ch ca encoder tng i l khng c cch no bit c v tr tuyt i ca trc khi cp ngun m khngquay n n tn khi nhn c xung ch s. Gii php cho nhng vn ny l encoder tuyt i. Mt encoder tuyt idng k b ngt quang hc v k vch m sinh ra mt t nh phn k bit biu din duy nht 2k v tr khc nhau ca a, v a

ra phn gii gc l 0360 / 2k(hnh 19.10). Khng ging nh encoder tng i, mt encoder tuyt i lun a ra gcchnh xc.

Cc vch trn cc rnh c sp xp sao cho khi encoder quay theo mt hng, mt t nh phn tng hoc gim tu thucvo mt m nh phn. Mc du c th dng m nh phn t nhin tuy nhin m Gray l mt gii php thng dng hn. Vim nh phn t nhin, vic tng ln 1 bit c th thay i nhiu hoc tt c c cc bit, v d nh 7 thnh 8 trong h thp phnl 0111 thnh 1000 trong h nh phn t nhin. Vi m Gray, ch mt bit thay i tng hoc gim, v d nh 7 thnh 8 trongh thp phn l 0100 thnh 1100 trong m Gray. chnh xc trong sut qu trnh chuyn dch m Gray ch l mt bc m

hoc 0360 / 2k . Vi m nh phn thng thng, mt s sp xp sai lch d rt nh gia cc ng v v cc ngt quang hc

c th gy nn vic c li t 0111(7) thnh 1111(15) trong khi gi tr cn chuyn ti l 1000(8).Nhn chung, encoder tng i cho php phn gii tt hn mc gi thp v l s la chn thng dng nht cho nhiung dng trong cng nghip v robot.

Tr khng

Mt trong nhng cch n gin nht v t tn km nht o cc chuyn ng quay hoc chuyn ng thng l vic sdng bin tr hay cn gi l ci phn th. Chng ta tp trung vo cc phn th quay, nhng nguyn l iu khin cng tngt nh trong trng hp thng.

Bin tr quay gm mt ci cc cha 3 cc( hnh 19.11(a,b)). Hai cc cui, gi l cc 1 v cc 3, ni vi mi im cuica phn vt liu c tnh tr khng, ging nh dy dn cc b bng nha, gm, hoc mt dy di v mng. ( tng tnh rnchc, si dy di c cun li thnh nhiu vng to thnh cun dy, nn n c gi l ci phn th cun dy).

Cc s 2, c ni vi mt cn gt, trt qua vt liu khi trc ca cc quay. Tr khng ton phn R13 ca cc l bng vi

tng tr khng R12 gia cc 1 v cn gt, v tr khng R23 gia cn gt v cc 3. Thng thng, cn gt c th quay t mtu ca vt liu c tr khng ( R13 = R12) ti u kia ( R13 = R23). Trc ca cc quay mt hoc t hn mt vng c th to nnchuyn ng y ca cn gt, lc ci cc c gi l cc mt vng quay. Nu chuyn ng y c to ra binhiu vng quay ca trc th gi l cc nhiu vng quay.4


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Cm bin s dng bin tr, ta ni cc 1 vi mt ngun V, cc 3 ni t, v s dng in th lm mt php o gc quay. in

th c o ti cn gt l V(R23/R13) v l mt hm tuyn tnh theo gc quay ca trc.

HNH 19.11 (a)Khi trc ca mt b phn th quay, ci qut s dch chuyn t u ny ti u cn li ca vt liu in tr. (b) Bn trong mt

b phn th, biu din mi lin h gia ci qut v di in tr

HNH 19.12 S nguyn l (Space Age Control, Inc)

Mt cm bin tuyt i n gin dng cho mt di rng cc khong cch l mt chui cc cc hoc cm bin dy ko( hnh 19.12). N bao gm mt si dy c qun trn mt ng, vi mt phn th kim sot cc gc quay ca ng. Mt lx n hi gi cho dy lun cng. Bng vic s dng cm bin kt hp vi nhiu cc a vng c th o c di ln ti

hng mt. Vi khong cch ngn (vi cm) ta cng dng k thut tng t s dng cc cc 1 vng v mt ng nh. C haitrng hp u b qua vic khng thng hng hoc s di ng ging mt cung trn. Chui cc cc d dng ph hu cc dytrong cc ng dng nu trn, nhng phn t cm bin l nh v kn. Cc nh sn xut bao gm RDP Electronics, SpaceAgeControl, v UniMeasure.

Mt loi khc ca cm bin tr khng l cm bin un linh ng. Mt loi mc c tnh dn in gia hai cng tc intrn mt vt liu c tr khng thay i in tr khi vt liu b b cong hoc ko di. c s dng trong b phn in vimt in tr c nh, in p tng t c th c s dng nh mt php o un. Ging nh vic s dng cm bin pht hin mi lin h hoc ging nh mt php o th mo m ca b mt m n c gn vo.

Cm bin o nghing (trng trng)

Ta c th dng mt cng tc thu ngn cung cp mt bit thng tin v hng so vi vc t trng trng. Mt git nhthu ngn t trong mt bng in thu tinh m hoc ng in gia hai u ph thuc vo hng ca cm bin. C th s

dng mt vi cng tc thu ngn theo cc hng khc nhau dng xc nh th nghing. Tn hiu t mt cng tc thungn c th b ny ln ging nh tn hiu t mt cng tc c kh (hnh 19.2).

Thit b o nghing c th c dng o mt gc nghing. V d nh cm bin gc nghing s dng tnh in phn.Cc nh sn xut gm c The Fredericks Company v Spectron Glass. M hnh hai trc c 5 in cc que song song trongmt v bc c bt kn, v c in y mt phn bi mt cht lng dn in. Bn in cc t bn gc mt hnh vung,v mt ci c t gia. nghing ca cm bin thay i s phn b ca dng in thng qua in cc trung tm s utin cc in cc ngm su hn trong cht lng.

Cm bin nghing c th c cha vi cht lng c tnh do bin i, lm gim thiu ting n. Bi dng mt chiukhi chy qua cht lng s gy nn hin tng in phn v c th gy hng cm bin, do vy phng php o tnh dn cadng xoay chiu c s dng. Theo , cc h tr in t l ng k.

Tnh dn in ca cht lng ph thuc nhiu vo nhit . Cc h tr in t cho cm bin o nghing phi da vo t lca tnh dn in gia cc cp cc. Mc du cc cm bin o nghing in phn iu chnh qua mt vng nhit rng,

nhng n b nhiu lon ln bi tnh khng u ca nhit qua cc t bo.C th to ra mt loi khc ca thit b o nghing n gin t mt bin tr quay c gn vo mt qu lc. C mt vn

ny sinh trong gii php ny l s sai lch c th lm ngng chuyn ng ca con lc khi m n khng theo phng thngng. Mt tng lin quan n vic ny l s dng mt encoder quang hc tuyt i vi mt con lc. Cc cm bin hon

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S tay C in t

chnh iu khin theo mt nguyn tc c th c ci tin vi cc la chn tt hn, nh l mt ci lm gim cng ttrng gim s qu v s dao ng. V d nh thit b o nghing tuyt i US Digitals 12-bit A2I.

D nhin, mt thit b hot ng theo trng trng khng c kh nng phn bit vi gia tc khc. Nu nghing ca thitb l n nh th php o phi dng cc iu kin tn hiu tt, n gin chc chn rng cc php c c thc hin.

Cc cm bin nghing tinh vi hn bao gm con quay hi chuyn v cc thit b vi in t c kh (MEMS), khng cp y.

Cm bin in dungC th s dng in dung o cc chuyn ng gn hoc cc chuyn ng thng theo th t tng millimet. in dung C

ca mt cp hai bn cc t song song l 0 /rC A dee= , trong re l hng s in mi tng i ca cht in mi giacc bn cc, 0e l hng s in mi ca khng kh, A l din tch b mt ca hai bn cc, v d l khong cch gia hai bncc. Khi cc bn cc dch chuyn theo 1 hng vung gc vi cc mt phng cha chng, th C l hm khng tuyn tnh vid. Khi mt bn cc dch chuyn tng i vi bn cc kia trong mt phng ca chng, th C l hm tuyn tnh vi din tchb mt A. S dng iu ny to nn cm bin in dung pht hin ra cc vt, cht lng c tnh kim loi hoc phi kimhay bt k vt no c hng s in mi ln hn khng kh.

Mt m hnh o c thng thng c mt bn cc ca mt t in trong mt my d, t trong mt cht cch in. Vtquan tm bn ngoi to nn mt bn cc khc ca t in, v n phi c ni t qua mt cm bin gn (proximity) nit. Khi cm bin t ti mc tiu, in dung ca n tng, n thay i dao ng ca mch my d bao gm c t in. Sbin i dao ng ny c s dng xp x tn hiu hoc dng o khong cch.

Cc nh sn xut t in c Cutler-Hammer v RDP Electronics.

HNH 19.13 Nguyn tc hot ng ca mt LVDT

Cm bin cm ng xoay chiu

LVDT

Cm bin cm ng xoay chiu c bit nhiu nht l bin th vi sai bin i tuyn tnh, hay LVDT. LVDT l mt ngvi li st t, dch chuyn ca li l mt bin c th o c (Hnh 19.13). Ci ng c qun quanh vi t nht l haicun dy, mt cun kch thch v mt cun cm ng. Mt dng xoay chiu (khong 1kHz) chy qua cun kch thch, v mttn hiu xoay chiu c ly t cun cm ng v c so snh v cng v pha ( 0 hoc 180 0) vi dng in kch thch.Cc thit b in t ph tr l cn thit trnh qu trnh iu bin, c gi l s nhn dng ng b. li mang mt thanhkim loi c st t, lm tng t tnh gia cun kch thch v cun cm ng. Tu thuc vo v tr ca thanh st t trong cuncm ng m tn hiu ly ra c th bng khng (khi m thanh st t gia cun cm ng), hoc l lm tng cng hay pha,ph thuc vo v tr thanh st t.

LVDT l mt ng dng cng ngh cao v c th rt chnh xc, trong mt vi trng hp c th t mc micron (10-8).Chng c mt lot cc bc dch chuyn t vi mm n m. Chng khng b qua cc sai lch v tr v dch chuyn phi tuyn,ging nh dy cc bin tr.

Cm bin o gc tuyt i

Cm bin o gc thc hin php o gc quay trc, nh cc tn hiu hnh sin, cosin. N cng s dng k thut t trngca dng in xoay chiu tng t nh LVDT, v cc thit b in t ph tr cng c s dng cung cp s nhn dngng b. Cm bin o gc tuyt i cho gi tr rt th, bi vy n c u tin hn encoder quang hc trn cc trc ng c,mc d chng khng chnh xc v chng yu cu thit b in t ph tr hn. Mt vi cm bin c thm cc tn hiu ra gingnh l cc encoder tng i, dng so snh. Thm vo , cc cm bin loi ny cn a ra mt php o tuyt i gcquay ca trc. Ging nh LVDT, cc cm bin o gc tuyt i cng c thit k v ch to vi trnh cng ngh cao.

Cm bin t tnh mt chiuMt trng in t hot ng da vo s di chuyn ca cc in tch (v d nh dng in trong cht bn dn) sinh ra mt

lc ht cc in t theo phng ngang, v lc ny c th sinh ra mt hiu in th vung gc vi hng dng in. Hiu ng

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Cm binny l nh, thm ch c trong cht bn dn, nhng n tr thnh c s cho vic hnh thnh mt lp cc cm bin th, r va nng.

Cc cng tc da trn hiu ng Hall

Cc cng tc da trn hiu ng Hall l cc thit b sinh ra tn hiu nh phn, ph thuc vo vic cng t trng cvt qu ngng hay khng. Trong hnh dng cu thnh ca chng, nhng cng tc ny c th c kt hp nh l cc thitb c 3 cc vi kch c nh mt transistor (TO-92), ch c mt chn ngun (3-24V), mt dy ni t, v mt u ra. Bnhthng, u ra c th t ni t hoc khng, ph thuc vo trng thi ca t trng. Cc loi cng tc da trn hiu ng Hall

ny lun sn c trn th trng.Cc khong ngng kch thch t mt vi Gauss (t trng tri t l 0,5 G) n hng trm Gauss, nh l trong nam chm

vnh cu. Thng th c mt vi cp trong vic i n v trong ngng. Cc cng tc da trn hiu ng Hall l c tr:ngng m ca chng c th khc ngng ng. Thnh thong hiu ng tr c s dng to nn mt cng tc cht, n c th ko di trng thi ca n (ng hoc m) n tn khi t trng trong n b o ngc. Cc cng tc khng chiu ng cht c th l n cc (ch phn ng vi mt hng ca t trng) hoc hai cc (phn ng vi c trng). Thigian ng hoc m ch l vi micro giy.

HNH 19.14 o mt rng ca bnh rng trong mt vt liu t s dng mt cng tc Hall v mt t trng nghing (AllegroMicrosystems, Inc)

HNH 19.15 u ra ca mt cm bin Hall tng t vi v tr tng i vi mt t trng. (Allegro Microsystem, Inc)

Cc cng tc Hall c di nhit hot ng rng v thng c s dng trong cc b phn ca ng c t. Chng cnhng u im khc l khng nhy vi nhng sai lch c kh ca cc cng tc quang hc hoc c kh, nh l cc cht lnghoc bi bn. Trong khi thng thng phn chuyn ng c xc nh l mt nam chm, n c th c t gn mt namchm c nh, nghing v b kch thch khi cng tc Hall chm vo phn st t, v d nh mt rng ca bnh rng, theo ncho php pht hin i tng khng nhim t (hnh 19.14).

Nhng ng dng c bn l o mt thnh phn quay, thay th mt cng tc gii hn c kh. Cng tc Hall khng cnhng phn di ng hoc h v n c treo t do. Mt ng dng thng thng khc l trong vic lp mi lin h gia ccchuyn ng tnh tin v chuyn ng quay. Cng tc Hall c dng pht hin mt v tr, v xung u ra ca n cdng nh l mt ch dn cho mt encoder tng i ci m c th tnh khong cch t im ch dn . Cc cng tc Hallthng nh v khng t, do vy trong khong vi mm c th t nhiu ci, to nn mt encoder quay hoc tnh tin vi phn gii thp hoc cng tc a v tr. Nh th mt encoder hay mt cng tc c nhng u im th ca cc cng tc Hall.

Cc cm bin Hall tng t

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S tay C in t

Trong mt ci b c kch thc tng t nh cc cng tc Hall, v gi cng khng cao hn, cng c th c cc thit bHall ci m c mt u ra analog t l thun vi cng t trng (hnh 19.15). in hnh nh chng c nhiu mc nhycm t trng trong dy 100 G. Chng khng c ng dng trong vic o cc t trng ph trn tri t.

Cc cm bin Hall u c tc dng nh cc encoder tnh tin hoc quay. C th t hai cm bin Hall hai cnh gcvung tnh c sin hoc cosin ca mt gc ca mt nam chm quay, do to thnh mt cm bin quay tuyt i. Ccthit b mang tnh thng mi ca thuc tnh ny gi l in th Hall v c cc u ra bin i (v d nh sin v cosin, hocmt s lp li bc nht vi mi thay i). Tng phn vi cc in th l cc thanh trt v cc di in tr, cc cm bin

Hall cho php quay 3600

khng ngng v khng b hao mn. Tuy nhin, tt c cc thit b da theo hiu ng Hall u nhycm vi t trng ngoi.

Cc cm bin Hall cng l nhng b chuyn i tuyt vi ca cc chuyn ng thng nh hoc chuyn ng theo cungtrn. Chuyn ng ca mt thanh nam chm qua mt cm bin Hall to ra cho cm bin mt t trng n c th c spxp lm bin i tuyn tnh vi cc dch chuyn qua mt dy t vi mm ti vi cm. (Thanh nam chm cha i ht chiudi ca n). S b xung mang tnh thng mi l cc cm bin iu khin v tr.

Cc cm bin dy

Nhiu loi cm bin thng v cm bin quay, k c tng i v tuyt i, l tng t nh cc encoder quang hc nhngchng s dng tnh cht t nhiu hn quang hc. Cc ng dng thng yu cu di h. Trong cc ng dng nu, cm bint c u im trong vic chng li bi bn, mc du cc di t trng cn c bo v trnh b ph hng.

Cm bin sng siu mCm bin sng siu m (US) o thi gian truyn ca mt xung ca sng siu m xuyn qua khng kh hoc cht lng

xc nh khong cch. C cc loi cm bin o c cc di t vi cm ti 10 m. u im ln nht ca cm bin siu m lcc thit b in v cc b phn chuyn i ca n u nm trn cng mt v tr. Nhng cng c nhc im tng ng ln c xu hng khng phn bit c: chng c th pht hin sai mc tiu, thm ch mt ci rt nh, c bit khi chng qugn b chuyn i. Cc cm bin lun ph bin vi cc cnh sng nh hng cn thn (gim xung 7 ) ti gin s phthin sai cc mc tiu. Mt s thc hin thm vic b cho s thay i ca nhit khng kh, v n nh hng ti vn tc mthanh. Loi cm bin ny c th s dng trong vic b cc kch thc hnh hc. V d nh, chng c th c dng phthin mc cht lng trong mt ng thng ng; s phn x ca xung sng m t cc thnh ca ng l nh nht.

Cc cm bin siu m in hnh c mt u ra analog t l thun vi khong cch ti vt ch. Trong mt mi trngthun li, th mc chnh xc c th l 1%.

Magnetostrictive thi gian truynMt k thut c chnh xc cao hn vi vic s dng thi gian truyn xc nh khong cch l b chuyn i dy

magnetostrictive (MTS). Mt nam chm di ng to nn mt mc tiu tng ng vi mc tiu m thanh trong cc cmbin siu m, v khng cn chm vo dy magnetostrictive, n chnh l trung tm ca thit b. T trng hot ng trong dymadnetostrictive to ra mt xung sng siu m trong dy khi mt xung in chy qua dy. Khong thi gian gia sng xungin ti s pht hin ca xung sng siu m ti im cui ca dy dng quyt nh v tr ca nam chm dc theo dy (hnh19.16).

HNH 19.16 Nguyn tc hot ng ca mt cm bin v tr tuyn tnh t gio (Temposonics, Inc.)

B chuyn i t gio khng c tnh chc chn vn c v th ca sng siu m truyn qua khng kh, nhng n tc php o chiu di c ln tng t, ln ti vi m. S chnh xc v s n nh l tt hn rt nhiu so vi cc cm binsng siu m. Mt vi chuyn ng khng thng c b qua, bi v nam chm ch khng cn t ti v tr xp x gn vidy t gio.

Cm bin ng dng hin tng giao thoa ca Laser

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Cm binCc giao thoa k Laser c kh nng o c cc chuyn ng thng tng i vi chnh xc mc nanomet. Trong

mt giao thoa k, cc tia sng laser chun trc truyn qua mt b tch nh sng, a nng lng nh sng theo hai ngkhc nhau. Mt phn x trc tip vo b thu, ging nh dy cc cm bin quang, v ra mt ng i vi di c nh.ng cn li phn x ngc tr li b thu t mt b phn x ngc (gng) c gn vo vt cn o. Hai sng tng cnghoc trit tiu ln nhau khi giao thoa vi nhau ti b thu, to ra cc vn sng v ti. Cc vn giao thoa c phn tch tmra mi quan h v pha gia hai sng, n ph thuc vo di tng i ca ng i, bi vy c th o c khong cchti vt ang dch chuyn. Khi vt di chuyn, hin tng lp li khi di ca ng i tia sng thay i bi di bc sngtia laser. V th giao thoa k laser chnh l mt thit b o tng i.

Giao thoa k laser l mt thit b t tin, chng ta s tho lun v n trong chng ny. Chng cng l loi c phngii cao nht. Cc giao thoa k rt nhy cm vi cc sai lch c kh v cc rung ng.

Cc bn c th tm thm nhiu thng tin v cm bin cc tp ch v cc loi cm bin hoc trong trang web: http://www.sensorsmag.com/

Ti liu tham kho:

[1] Histand, M. B. and Alciatore, D. G.,Introduction to Mechatronics and Measurement Systems, McGraw-Hill, Boston,MA, 1999.

[2] Bolton, W., Mechatronics, 2nd edition, Addison Wesley Longman, New York, NY, 1999.

[3] Horowitz, P. and Hill, W., The Art of Electronics, 2nd edition, Cambridge University Press, Cambridge, UK, 1998.

[4] Auslander, D. M. and Kempf, C. J., Mechatronics: Mechanical System Interfacing, Prentice-Hall, Upper Saddle River,

NJ, 1996.[5] Jones, J. L., Flynn, A. M., and Seiger, B. A., Mobile Robots: Inspiration to Implementation, 2nd edition, A. K. Peters,Boston, MA, 1999.

19.2 Cm bin gia tc

HalitErenGia tc ca chuyn ng l mt thnh phn rt quan trng ca cc thng s ng hc nh: v tr, vn tc, gia tc, v tc

bin thin gia tc. Mi mt thng s trn li c lin h tuyn tnh vi thng s cnh n. Bi th, tt c cc thng s nghc trn u c th c ly ra t mt thng s. V d nh, gia tc c th ly c qua o hm ca vn tc tng ng hocl tch phn ca tc bin thin gia tc. Tng t nh th, vn tc c th ly t o hm v tr tng ng hoc tch phn giatc. Trong thc t, ch s dng rng ri tch phn v n em n kt qu tt hn khi c nhiu v c s tt dn.

C hai k thut o gia tc: phng php o trc tip qua gia tc k c bit v phng php gin tip o hm vn tc.Kh nng ng dng ca cc k thut trn ph thuc vo dng chuyn ng (chuyn ng thng, trn, chuyn ng cong)hoc trng thi cn bng gia cc dao ng. Vi cc chuyn ng thng v chuyn ng cong, ngi ta u tin dngphng php o gia tc k trc tip. Tuy nhin, ngi ta thng o gia tc gc bng cc phng php o gin tip.

Gia tc l mt thng s quan trng cho cc mc ch o dch chuyn tuyt i, dao ng, v o shock. Vi nhngmc ch nh vy, c nhiu loi gia tc k vi cc tnh v di o rng nhm p ng cc yu cu ng dng a dng, ch yul trong 3 lnh vc: (1) cc ng dng thng mi - t, tu, dng c, th thao, (2) cc ng dng cng nghip - robot, iukhin my mc, kim tra dao ng, v o c; v (3) cc ng dng c tin cy cao - qun s, khng gian, hng khng, ong t, chn ng, dao ng, v cc php o shock.

Gia tc k c s dng trong nhiu nm, nhng loi u tin l loi c kh vi cc tn hiu in tng t. Mc du,trong cc ng dng hin nay vn c th tm thy cc loi gia tc c nhng cc gia tc hin i rt cn thit s dng cc thitb bn dn trong cc chip in t c tch hp vi cc mch x l tn hiu. Cc gia tc k c kh pht hin ra lc da trnmt khi lng khi c gia tc. Mt loi gia tc k mi, dng nhit, c th o v tr nh ng dng hin tng truyn nhit.

Tng quan v cc loi gia tc k

Mt gia tc k c bn gm mt khi lng c th di chuyn t do dc trc o bn trong mt ci v. Cng ngh ch togia tc k da nhiu vo c s ny v c th phn chia chng theo nhiu cch, nh l loi c kh hay in, ch ng hay bng, lch hay v tr cn bng khng,... Loi thng c dng trong o dao ng hay shock thng l dng lch, khim dng chng trong cc php o chuyn ng ca cc phng tin, my bay, v hn c l mc ch nh hng th ta c thdng loi lch hay v tr cn bng khng.

Bi ny s tp trung vo cc php o trc tip vi cc dng gia tc k sau:

Qun tnh v c kh

in c

p in p tr

o theo sc cng

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S tay C in t

in dung hoc cn bng lc tnh in

Gia tc k nh v siu nh

Da trn cc nguyn tc hot ng ngi ta c th phn thnh cc nhm nh hn trong mi nhm chnh.

ng lc hc v cc c tnh ca cc gia tc k

Gia tc c quan h vi chuyn ng, mt vc t lng, biu din mt hng cng nh ln. Hng ca chuyn ng

c m t theo ngha ca mt s h trc to cc v trc giao. Hnh 19.17 minh ha ln lt cc chuyn ng thng,gc, v cong.

HNH 19.17 Cc dng chuyn ng m gia tc k thng ng dng

HNH 19.18 Mt gia tc k ng in hnh

HNH 19.19 S sp xp ca cc gia tc k trong cc h thng nh v v ch dn

Ta c cc phng trnh cho mi loi chuyn ng:

Gia tc thng2

20

( / )lim

D t

dds dt Dv dv d sa

Dt dt dt dt = = = = =

(19.1)

Gia tc gcq q

2

2/

( / )lim

D t

d d dt Dw dw d a

Dt dt dt dt = = = =

(19.2)

Gia tc cong2 2 2

2 2 2

dv d x d y d z a i j k

dt dt dt dt = = + + (19.3)

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Cm bin a v a l cc gia tc; nv wl tc ; s l khong cch; q l gc; i, j, v k l cc vc t n v tng ng theo cchng x, y, v z.

Vi ng dng cn chnh xc ca gia tc k, th s hiu bit ng n v cc c im ca chuyn ng l rt quan trng.Cc lnh vc p dng c th l chuyn ng thng v cc dao ng, chuyn ng gc, s kim sot nghing ca mt itng, hoc s bin i dng chuyn ng ca qu trnh tng hp. Trong mi trng hp, vic chn la ng n cc gia tck l cn thit.

Phn chnh ca cc gia tc k cn c xem xt v phn tch ging nh mt thit b o a chn mt bc t do, n c thc m t bi mt khi lng, mt l xo, mt dm c b tr ging nh hnh 19.18. Trong trng hp cc h thng c

nhiu bc t do, c th p dng cc nguyn tc ca dch chuyn cong nh trong phng trnh 19.3 v phi s dng cc bchuyn i a nng to ra phng php o vi 1, 2 hoc 3 im o. Mt v d in hnh l nhng thit b dn hng vnh v bng qun tnh nh minh ho trong hnh 19.19. Trong cc ng dng nh th, cc cm bin ng vai tr quan trngtrong vic nh hng v tm ng. Thng thng, h cc cm bin 3 trc thu nh c th pht hin ra s thay i trongtrc, bc rng, v gc phng v theo cc hng x, y, v z. Nu mt h thng mt bc t do tuyn tnh trong mt khongthi gian bt bin, phng trnh vi phn bc hai c bn m t chuyn ng ca h thng lc l xo - khi lng c vit nhsau:

2

2( )

d x dxf t m c kx

dtdt= + + ( 19.4)

f(t) l lc, m l khi lng, c l hng s vn tc, v k l hng s ca l xo.

Tuy nhin, c s ca cc gia tc k cng l chuyn ng. Khi , lc c truyn qua l xo ti khi lng treo, ph thuc

vo kh nng chuyn i ca lc ti khi lng. Phng trnh (19.4) c hnh thnh qua vic a ra cc nh hng cachuyn ng da trn phng trnh sau:

221

2 2

dosq - m

xd z dzm c kz mgc

dtdt dt + + = (19.5)

2 1z x x= l chuyn ng tng i gia vt v gc, 1x l dch chuyn ca gc, 2x l dch chuyn ca vt, v lgc gia trc cm bin v trng trng.

Phng php gii hon chnh cho phng trnh 19.5 c th thu c qua vic p dng nguyn tc xp chng. Nguyn tcxp chng ni rng nu c ng thi nhiu hnh ng cng tc ng vo mt vt, th nh hng chung ca n l tng cc nhhng thnh phn tc ng vo vt. S dng nguyn l xp chng v php chuyn i Laplace cho ta:

2

( ) 1

( )

X scs k

F s ms= + +

(19.6)

hoc

2 2n n

( )

( ) / 2 / 1

X s K

F s s zs =

+ +(19.7)

s l ton t Laplace, K = 1/k l nhy tnh, n /k m = l tn s dao ng ring (rad/s), v ( / 2)c km = l h ssuy gim.

Tt c u l cc hm ca khi lng, vn tc, v cc h s l xo. Cc gia tc k c thit k v ch to vi nhng cim khc nhau theo s la chn hp l ca nhng thng s trn. Danh sch cc nh sn xut c a ra trong bng 19.1.

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S tay C in t

Dao ng

Chng ny cp n cc ng dng ca gia tc k o cc c tnh vt l nh l gia tc, dao ng hay s rung ng,v cc chuyn ng chung. Mc du c nhiu khc nhau c bn trong cc dng chuyn ng, vic hiu r v cc nguyn tcc bn ca dao ng s a cc ng dng ca cc gia tc k vo cc tnh hung khc nhau bi phng php xp x gn ng.

Dao ng l mt chuyn ng qua li c c t s tc dng ca cc lc thay i vo mt cu trc. Cc dao ng c thl tun hon, ngu nhin tnh, ngu nhin ng, hoc tm thi.

Cc dao ng tun hon

Trong cc dao ng tun hon, mt i tng lp li chuyn ng ca chnh n trong mt khong thi gian no . Cth biu din n theo dng sng hnh sin:

( ) sin( t)px t X = (19.8)

y x(t) l dch chuyn ph thuc thi gian, 2 ft = l tn s gc, v Xp l bin ca dao ng ( dch chuyn lnnht tnh t gc).

Vn tc ca vt cng l mt hm theo thi gian v c tnh nh sau:

p( ) os( t)=V sin( / 2)pdx

v t X c t pdt

= = + (19.9)

trong v(t) l vn tc ph thuc vo thi gian, v p pV X= l vn tc ln nht.Gia tc ca vt l o hm ca vn tc theo thi gian,

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Cm bin2

2

2( ) sin( ) sin( )

p p

dv d xa t w X wt A wt p

dt dt = = = = + (19.10)

trong a(t) l gia tc ph thuc thi gian, v 2p p pA X V = = l gia tc ln nht.

T cc phng trnh c trc ta c th thy rng cc dng c bn v chu k dao ng tn ti trong cng gia tc, vn tc,v dch chuyn. Nhng vn tc sm pha hn so vi dch chuyn mt gc 90 0 v gia tc cng sm pha hn vn tc mt gc900.

Trong t nhin, dao ng c th d on c, nhng khng hn l hm sin. Nu ta bit trc chng nhng chngkhng dng sin, c th biu din chng qua tng ca cc hm sin, theo phn tch Fourier:

0 1 1 1 2 2 2( ) sin( ) sin( ) ... sin( )n n nx t X X t X t X t = + + + + + + + (19.11)

1 2, ,..., n l cc tn s (rad/s), X0, X1, ,Xn l bin tng ng vi cc tn s, v 1 2, ,..., n l cc gc pha.

S cc s hng c th l gii hn, v s cc phn t cng cao th xp x s c kt qu tt hn. Nhng phn t ny tothnh mt chui rng vi cc tn s khc nhau. Chng ta c th biu din dao ng theo phm vi thi gian hay phm vikhng gian, c hai phng php u rt hu ch trong khi phn tch.

Dao ng ngu nhin tnh

Cc dao ng ngu nhin thng gp trong t nhin, chng to thnh cc chuyn ng vi chu k bt quy tc vchng khng bao gi lp li chnh chng mt cch chnh xc. V mt l thuyt m ni, ta cn quan st vi thi gian di vtn thu c mt qu trnh hon thin ca nhng dao ng ny. Tuy nhin, c th s dng cc phng php thng k hayl thuyt xp x phn tch qua vic cc mu i din. Cc cng c ton hc nh cc hm phn b xc sut, mt xc sut,ph tn xut, cc hm tng quan cho, hm t tng quan, cc chuyn i Fourier s (DFTs), cc chuyn i Fourier nhanh(FFTs), phn tch ph t ng, cc gi tr cn qun phng, v php phn tch lc s l mt vi k thut c th c p dng.

Cc dao ng ngu nhin bin i

Trong trng hp ny, cc tnh cht thng k ca cc dao ng bin i theo thi gian. C th p dng cc phng phpnh trung bnh thi gian hay cc k thut thng k.

Qu v shock

Bnh thng ta cn o c cc dao ng xy ra t ngt v trong thi gian ngn. C th m t shock v qu trnh qu ca dao ng thng qua cc i lng nh lc, gia tc, vn tc, di chuyn. Nh trong trng hp qu v va p ngunhin, th trong phn tch ngi ta s dng cc phng php thng k v bin i Fourier.

Cc ngun sai s in hnh v li m hnh

Cc li trong phng php o gia tc xy ra do bn nguyn nhn chnh: cc cm bin, s thu nhn in t, x l tn hiu,v cc li ng dng c bit khc. Trong cc phng php o gia tc trc tip, cc ngun gy ra sai s chnh l t cc cmbin v t s thu nhn d liu bng in t. Nhng li ny s c tho lun trong phn khc v trong mt vi trng hp,cc li cm bin v li thu nhn in t c th cao mc 5%. Vic ly mu v cc chuyn i A/D thng em li mt phnca nhng li ny v n l c hu cng nh n tn ti trong tt c cc h thng thu thp v tnh ton d liu. Tuy nhin c thgim thiu nhng li ny qua vic la chn cn thn b dn knh, cc mch gi v ly mu, v cc chuyn i A/D.

Khi dng phng php o trc tip, phi kim tra cn thn la chn ng gia tc k ph hp vi cc yu cu ca mt

ng dng c th. gim thiu cc li, cc c im ca chuyn ng cn c nghin cu, v cn phi xem xt cc c tnhsau ca cc gia tc k: p ng qu hoc nhy cho trc; di tn s; nhy, dy ng hc v khi lng; phn ving hc; v cc iu kin mi trng nh nhit , s n nh in p, v s sai lch,

nhy ca gia tc k

Trong qu trnh o c, cc dch chuyn ngang ca h thng nh hng hu ht gia tc k v nhy ca ca nhngchuyn ng ny chim mt vai tr chnh trong chnh xc ca cc php o. nhy theo phng ngang, cng nh cho,ca gia tc k l phn ng ca n vi gia tc trong mt phng thng ng vi trc chnh ca gia tc k nh trong hnh 19.20. nhy cho trc bnh thng c th hin l phn trm ca nhy cm trc chnh v nn thp mc c th. Cc nh snxut thng cung cp hng c nhy ti thiu.

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S tay C in t

HNH 19.20 Minh ho v nhy cho trc.

Vic o hng nhy cho trc ln nht l mt phn ca qu trnh hiu chnh tng phn v nn nh hn 3 hoc 4%.Nu dao ng ngang qu ln, n c th gy nn sai lch rt ln v kt qu. Trong trng hp ny, ta nn sp xp ring ccmc dao ng v tn s cc ni dung ca cc chuyn ng cho trc. nhy cho trc ca cc gia tc k in hnh c mt trong cc phn lin quan l khong 2-3% cc loi p in v 1% cho hu ht cc loi khc.

Di tn s

Cc php o gia tc bnh thng gn ging nh vic s dng php chia tuyn tnh ng cong nhy ca gia tc k. nhy b gii hn cc vng tn s thp cng nh cc vng tn s cao quanh tn s cng hng t nhin. C mt quy tc kinhnghim, c th t gii hn tn s cao cho php o bng 1/3 tn s gy cng hng ca gia tc k nh vy, cc dao ngc o trong tuyn tnh di 1dB. Ta nn ch rng mt di tn s hu dng cho gia tc k c th cao hn 1/2 hoc 2/3 tns cng hng ca n. Tn s o c th c t cao hn gi tr trong cc ng dng m m tuyn tnh thp hn (vd 3dB) l c th chp nhn. C hai nguyn nhn quyt nh gii hn tn s o thp. u tin l ngng tn s thp cacc b tin khuch i lin i. Th hai l nh hng ca cc s thay i ca nhit xung quanh trong khi gia tc k rtnhy cm vi s thay i ny.

Khi lng ca gia tc k v di ng lc hc

Theo l thuyt, nhy ca b chuyn i cng cao th cng tt. C th phi a ra nhng tho hip cho nhy i vitn s, dy, qu ti, kch c,

Trong mt vi trng hp, nhiu li gy ra do vic la chn sai cm bin, khng ph hp vi ng dng. V d nh khilng ca gia tc k tr nn quan trng khi s dng kim tra cc vt nh v nh. Gia tc k khng nn mang thnh phnc cu trc, do khi lng thm vo c th dn n s thay i ng k ti mc v tn s ti cc im o v lm sai lch cckt qu. C mt quy tc chung l khi lng gia tc k khng nn ln hn 1/10 khi lng hiu dng ca phn hoc cu trcc gn ln o.

Di ng lc hc ca gia tc k nn lm cho hp vi cc mc gia tc cao hay thp ca cc i tng c o. Vi giatc k dng cho mc ch chung th di nm trong khong tuyn tnh t 5000g n 10.000g, y l di ph hp nht vi ccthay i t ngt c kh. Cc gia tc k c bit c th o trn 100.000g.

p ng qu

Cc thay i t ngt c m t nh cc s gii phng nng lng t ngt di dng cc xung ngn biu din bi hnhdng thay i v tng theo thi gian. Chng c cc cng cao v cc di tn rng. Trong cc ng dng m cc php o

qu trnh qu v thay i t ngt c mi lin quan, tnh cht tuyn tnh ton cc ca h thng o c th b gii hn cctn s cao hoc thp bi mt hin tng gi l bm v lch khng.Lch khnggy nn bi s khng tuyn tnh v pha trongc cc b tin khuch i v gia tc k khng tr v trng thi n nh sau khi phi chu nhng thay i t ngt ln. Bmgy nn bi cc thnh phn c tn s cao ca s kch thch ca gn vi tn s cng hng, ngn cn gia tc k tr li trngthi n nh ca n. trnh sai s php o v nhng hiu ng ny, di tn dng cho iu khin nn c t trong vngtuyn tnh.

Di thang o v qu ti

Hu ht cc gia tc k u c kh nng o theo c hai chiu: dng v m. Chng c thit k c th s dng khi quti. Cc nh sn xut cng cung cp thng tin cho c hai c im ny.

iu kin mi trng

Khi chn la v lm vic vi cc gia tc k, cn phi xem xt cc iu kin mi trng nh cc di nhit , s thay inhit , cc nh hng ca t trng, m, v tp m. Cc nh sn xut cng cung cp cc thng tin v cc iu kin mitrng.

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Cm binGia tc k qun tnh

Cc gia tc k qun tnh l cc gia tc k c kh s dng mt khi lng ng c treo bi 1 l xo hoc mt n by bntrong mt khung cng nh trong hnh 19.17. Khung mang khi lng ng c ni cng vi ngun dao ng. S dng cctnh cht ca h thng dng thc hin php o. Khi h dao ng, khi lng c xu hng tr v v tr cn bng ca n chuyn ng c th c ghi li ging nh mt dch chuyn tng i gia khi lng v khung. Mt b chuyn i xcnh dch chuyn ny v a ra tn hiu n b x l. C th ch to cc phn t chuyn i bng cc vt liu c tnh in tr,in dung, cm ng, p in, p tr, v quang hc. Trong thc t, khi lng ng khng em n mt trng thi n nhtuyt i, nhng n c th t c mt v tr gn vi vi cc tn s xc nh.

Qua vic la chn hp l gia khi lng, l xo, v gim chn, c th s dng thit b ny cho c mc ch o gia tc vo s dch chuyn. Ni chung, vt c khi lng ln cng vi mt l xo mm thch hp cho vic o dao ng v chuyndch, trong khi mt khi lng tng i nh v l xo cng c s dng trong cc gia tc k. Tuy nhin, thut ng ngc s dng thng xuyn trong cc thit b, chng o cc mc dao ng rt chm ca nn v ca cc kt cu.

m t nhy ca gia tc khi lng ng trong hnh 19.17, phng trnh chuyn ng theo nh lut 2 Newton cth c vit nh sau:

22 2 1

2 12os( )

d x dx dxm c kx c kx mgc

dt dt dt+ + = + + (19.12)

x1 l dch chuyn ca khung dao ng, x2 l dch chuyn ca khi lng ng, c l hng s vn tc, l gc lch giatrc o v phng trng lc, v k l cng l xo.

Tr c hai v phng trnh cho2 2

1 /md x dt v sp xp li ta c22

12 2

dos( ) - m

xd z dzm c kz mgc

dt dt dt + + = (19.13)

z = x2 x1 l chuyn ng tng i gia khi lng v vt c s.

Trong phng trnh (19.12), ta gi s rng lc ca l xo ln khi lng ng ch t l thun vi vn tc. Nu ta a vothit b mt dao ng iu ho nh sau:

1 0 1( ) sin( )x t X t = (19.14)

1 l tn s ca dao ng (rad/s), ta c

21

12 sind xm mX t dt =

bin i phng trnh (19.13) thnh

2

1 12cos( ) sin

d z dzm c kz mg ma t

dtdt + + = + (19.15)

21 0 1a mX =

Phng trnh (19.15) c th suy ra trng thi qu v trng thi n nh ca h thng. Trng thi n nh ca phngtrnh vi phn (19.15) c th c xc nh:

1 1

21 1

sincos( )

( )

ma tmgz

k k m jc

= +

+(19.16)

Sp xp li phng trnh (19.16) ta c

1 1

2 2 2 2n

sin( )os( )

(1 ) (2 )n

a tmgcz

r zr

= +

+(19.17)

Trong ( / )n k m = l tn s dao ng ring ca khi lng ng, ( / 2 )V c km= l h s suy gim. C th vit h s

suy gim theo dng sau / ,cV c c= vi 2cc km= . Trong gc pha l1 2

1 1( tan /( ) )c k m = v 1( / )nr = l t l

tn s.

Mt th ca phng trnh (19.17), 1 2 0 0( ) /x x x vi t l tn s 1 / n , minh ho trong hnh (19.21). Hnh ny ch ra

rng bin u ra bng vi bin u vo khi / 0.7cc c = v 1 / 2n > . u ra gn tr thnh mt hm tuyn tnh theou vo tn s cao. tho mn h thng, cc hng s thit b / cc c v n nn c tnh ton k hoc thu c qua vi lnkim tra.

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S tay C in t

HNH 19.21 Tn svi t l bin ca gia tc k ng.

Bng cch ny, chnh xc c d on ca php o c th c on trc qua cc tn s quan tm. Mt php sosnh phn tch c thc hin bi McConnell [7]; cc c gi quan tm xem phn ny c c cc thng tin c th.

Cc thit b dao ng c xy dng theo nhiu cch khc nhau. Trong mt thit b o in th, mt b phn th c sdng o cc dch chuyn tng i gia khung v khi lng ng. Trong phn ln cc gia tc k o in th, thit bc in y mt loi cht nht, n lin tc tng tc vi khung v khi lng ng gim chn. Nhng gia tc k ny c

tn s hot ng thp (di 100 Hz) v ch yu c dnh cho cc gia tc c thay i chm v tn s dao ng thp. Mth in hnh ca cc thit b ny c cc m hnh khc nhau, gm cc khong 1g ti 50g . Cc di tn s t nhin t 12n 89 Hz, v h s suy gim c th c gi trong khong t 0,5 n 0,8 bng vic b nhit v b bng gim chn.in tr ca b phn th c th c chn trong di t 1000- 10000 W, vi phn gii tng ng 0.45- 0.25% ca ton dio. nhy ngang trc nh hn 1% . chnh xc ton b l 1% ca ton di o hoc thp hn nhit phng. Kchthc c vo khong 350mm vi tng khi lng khong 1/2 kg.

Cc b chuyn i vi phn tuyn tnh c th thay i (LVDTs) s dng cch o khc ca dch chuyn tng i ca khilng ng v v ca gia tc k. Cc thit b ny c cc tn s t nhin cao hn cc gia tc k, ti khong 300 Hz. Do ccLVDT c mc cn tr chuyn ng thp hn, n t c phn gi tt hn. Mt h in hnh ca cc gia tc k chuyni vi phn vi cht nht th hin cc c tnh sau. Mt di o t 2g ti 700g , tn s ring t 35 ti 650 Hz, khngtuyn tnh l 1% ca mc ti a. Mc ti a ca u ra l khong 1V vi kch thch LVDT l 10V ti 2000 Hz, di h s suygim t 0,6 ti 0,7 , vi in th d ti v tr khng l nh hn 1%, v tr nh hn 1% mc cc i. Kch thc vo

khong 350mm vi khi lng vo khong 120g.B o bin dng dng in tr strain gages c s dng o dch chuyn ca khi lng ng. Trong trng hp ny,

khi lng ng c trn mt dm cng xn tt hn l dng l xo. in tr strain gages c gn cht trn mi mt can cn bng o bin dng trong n xc nh dch chuyn dao ng ca khi lng. Mt cht lng nht c y gim chn cho h thng. u ra ca cc thit b bin dng c ni vi mt mch cu thch hp. Tn s ring cah thng ny vo khong 300 Hz. Tn s ring thp l do cn mt dm cng xn rng ph hp vi vic treo cc intr strain gages.

Mt iu tr ngi ln ca cc thit b dao ng l cn thm cc mch b gim cc nh hng ca nhit . gimchn ca thit b c th cng b nh hng bi s thay i v nht ca cht lng theo nhit . V d nh chng ta thngdng du silicone trong cc thit b v tnh nht ca n ph thuc nhiu vo nhit .

Gia tc k khi lng treo, cng xn, v gia tc k qun tnh dng qu lc

C nhiu kiu gia tc k qun tnh khc nhau, chng hu ht u ang trong giai on pht trin v c s dng ph hpcho nhiu trng hp khc nhau, v nh qu lc hi chuyn, rotor phn ng, dy dao ng, v cn bng lc li tm.

Thit b dy dao ng, hnh 19.22, li dng mt khi lng th c h tr theo chiu dc bi mt cp dy cng, daong theo chiu ngang vi hnh dng tit din ngang v cc di bng nhau v cc khi lng. Tn s dao ng ca cc dyc t mc vi nghn vng trn giy. Khi lng th c qua tm nn theo gia tc theo phng vung gc vicc si dy khng nh hng n bin dng si dy. Vi s hin din ca gia tc dc theo trc o, mt bin dng tnti trn hai dy, theo lm thay i tn s ca dao ng. Theo nh lut th hai v chuyn ng, cc tn s c vit nhsau:

2 1

1

1

Tf

m l=

v

2 2

2

2

Tf

m l=

(19.18)

T l bin dng, ms l khi lng, v l l chiu di cc si

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Cm bin

HNH 19.22 Mt gia tc k dy khi lng dao ng treo in hnh

Hiu T1 T2 t l thun vi ma, trong a l gia tc dc theo trc ca cc si dy. Mt cch biu din sai lch ca bnhphng tn s nh sau:

2 2 1 21 2 4 4s s

T T maf f

m l m l

= = (19.19)

T :

1 21 24 ( )s

maf f

m l f f ==

+(19.20)

Tng ca cc tn s 1 2( )f f+ c th c gi c nh bng cch lin h sc cng ca dy so vi tn s ca mt b daong chun. T , s sai khc gia cc tn s tr thnh t l thun vi gia tc. Trong mt s phin bn, tnh cht ging cidm ca cc phn t dao ng c dng bng cch kp cht chng ti cc im nt khng tng ng vi kiu c bn cadao ng ca n by, v ti cc tm tng ng ca s va chm ca khi lng th chung. Tn s ra t l vi gia tc v vntc th t l vi pha, y l mt u im quan trng. Nhng phin bn ci tin mi hn ca cc thit b ny a n cc giatc k dng cng xn, v chng c tho lun trong phn ti.

Trong mt gia tc k dng cng xn, mt dm cng xn nh c gn trn mt khi v c t ngc li vi b mt daong, v mt c cu c kh thch hp c dng thay i ca chiu di dm. Chiu di dm c ngm nh sao cho tns t nhin ca n cn bng vi tn s ca b mt dao ng, v t thu c iu kin cng hng. Gn y cc thay inh ca cc s sp xp dm cng xn ang tm ra nhng ng dng mi trong cc vi gia tc k.

Mt dng khc ca cu hnh gia tc k khi lng treo, khi mt con lc quay quanh trc thng ng. H thng l iu

khin servo duy tr n v tr khng. Gia tc trng trng c cn bng bi gia tc ly tm. Tc trc t l vi bnhphng gi tr a phng ca gia tc.

Gia tc k c in

Cc gia tc k c in c thnh phn c bn l iu khin servo v cn bng v tr khng hot ng da trn nguyn lphn hi. Trong nhng thit b ny, mt vt c khi lng nhy gia tc c gi rt gn vi mt v tr trung tm hoc imkhng bng cch xc nh dch chuyn v phn hi cc nh hng ca dch chuyn. Mt lc t trng tng ng c tora chng li chuyn ng ca khi lng dch chuyn t v tr khng, do qu trnh tr li gi tr ny ging nh l mt lxo c kh trong mt gia tc k thng vn lm. Cc u im ca vic tip cn ny l tuyn tnh tt hn v hn ch cc hintng tr, ging nh cc l xo c kh. Cng nh th, trong mt s trng hp, c th to thm b gim chn in, n c nhy nh hn vi s thay i nhit .

Mt c im rt quan trng ca cc gia tc k c in l kh nng kim tra v ng hc v tnh hc ca cc thit b bngcch a kch thch in vo cc h thng. Vic kim tra ny kh thun tin trong cc trng hp phc tp v tn km viyu cu chnh xc cao. Cc thit b ny cng hu ch cho cc h thng iu khin gia tc, v gi tr gia tc c th c a ra

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S tay C in t

bi mt dng in tng ng t mt ngun ngoi. Chng c s dng cho mc ch o c cc chuyn ng thng thngv iu khin cc dao ng tn s thp.

C nhiu loi gia tc k c in nh: Dng t trng v cun dy, cc dng cm ng,

Gia tc cun dy - t trng

Cc gia tc ny da trn nh lut Ampare: Mt cun cm mang dng in t trong mt t trng sinh ra mt lc t lvi dng in, di cun cm trong t trng, mt t trng, v sin ca gc lch gia cun cm v t trng. Cc cun

dy trong cc gia tc k ny c t trong min hnh ng tr nm gia mt nam chm vnh cu v mt ng st non hnh tr.Chng c gn bng cch mt ci cn c t trn mt bi c ma st rt nh hoc ch un cong to thnh mt khilng ng nhy vi gia tc. C cu nh (pickoff) o dch chuyn ca cun dy c gia tc v khin cun dy c cungcp bi mt dng trc tip thng qua mt b iu khin servo thch hp tr li hoc duy tr iu kin khng. Cc dngin trong mch tr li t l tuyn tnh vi gia tc, cung cp tng tc phn ng l khng ng k v b trung ho hon tonbi mt cun dy b v tr i din cun dy chuyn ng, v h s khuch i ca h thng servo l ln ngn chns dch chuyn ca cun dy ni m t trng khng i.

Trong nhng gia tc k ny, cu trc ca t trng phi c che chn thch ng lm cho nhy ca h thng vicc nhiu lon hoc t trng ngoi l khng ng k. Cng nh th, khi xut hin gia tc th nhit cng tng do s tonhit theo 2i R . Cc nh hng ca s to nhit trn trong vic biu din c quyt nh qua s phn b nhit v cc tnhcht v truyn nhit ca cc gia tc k.

Gia tc k cm ngMi quan h gia dng in, t trng, v lc chnh l c s cho cc gia tc k in t trng dng cm ng. Cc gia tc

k ny c cc my pht in nhiu hn cc mt. Mt dng ca thit b ny, dng mn che t ( cup and magnet design),bao gm mt phn t dao ng vi mt c cu nhv mt b iu khin servo dn ng mt kt ni my o tc . Mt ttrng vnh cu v mt vnh tr v thng lng, phn khng gian hp tng ng vi mt hnh tr dn in, c gn vophn t dao ng. Mt lc ko t l vi tc thu c bi s cm ng ca in t trng gia t trng v vt dn. C cunho vng ca dao ng di gia tc v gy cho b iu khin servo quay rotor ko phn t dao ng hng ti v trkhng. Di iu kin n nh, tc motor l mt php o ca gia tc nh tc ng ln thit b. Hot ng n nh caservo c thc hin bi s dng mt mng tin nh b vo thi gian tr qun tnh ca motor v s tng hp t trng. chnh xc ca cc gia tc k dng servo l hn ch c bn bi s n nh v chc chn ca cc h s t l ca cc thit bghp ni v thit b chn t cng nh tnh cht ca thi gian v nhit .

Mt gia tc k khc da trn thit k v cm ng s dng s hnh thnh momen cm ng dng in xoy. C cu to lc

ca mt gia tc k cm ng bao gm t trng khng i, thng c cung cp bi mt nam chm vnh cu, xuyn thngvo mt tm dn in. S dch chuyn ca tm dn trong t trng thay i to thnh mt sc in ng trong mi mch cain tr. Kt qu ny c c l theo nh lut Faraday. Trong cc gia tc k dng cm ng, cc dng in cm ng xoy tipgip vi tm in tr, cn thit to cho h thng mt khp ni ko. V tc gc t l vi gia tc, v tr gc i din chos thay i vn tc. y l tnh nng c bit hu ch trong cc ng dng dng nh hng.

Mt thit b in hnh cho tnh kinh t da trn nguyn tc gia tc k iu khin servo c th c mt b to dao ngchun thch anh cc nh, mt mang nn khng kh cng vi s b u tr ng cho h thng gim chn. Mt s m hnhbin i, v du nh mt khi nng 30g c ngng v phn gii l 1 g , mt p ng tn s 10 Hz l 0,05% v 100Hzl 2%, mt tn s t nhin l 1500Hz, h s gim chn l t 0,3 - 0,8, v nhy chuyn i hoc ngang trc l 0,1%. V dnh nu u ra dng in l 1,3 mA/g, mt in tr ch th 250 W s c mt in th cc i 10V cho 30g. Cc gia tc k nyl ph hp cho cng vic yu cu chnh xc v c dng trong nhiu ng dng nh l hng khng v cc h thng iukhin tn la, php o cc gc nghing dng cho vic nh hng cc ng dng vi cc l khoan trong lng t, v gc lntrc trong cc h thng cn bng v trng lng my bay.

Gia tc k p in

Cc gia tc k p in c s dng rng ri vi mc ch o gia tc, shock, v dao ng. Chng l cc b chuyn itruyn ng c bn vi cc tn hiu ra ln v kch thc tng i nh v chng l cc b ngun t to khng cn cp ngunt bn ngoi. Chng c th hot ng tn s t nhin rt cao v do thch hp cho cc ng dng tn s cao v cc php oxung ng.

Cc thit b ny s dng mt khi lng gn trc tip vi thnh phn p in hoc tinh th nh hnh 19.23. Khi p mtchuyn ng bin i vo gia tc k, tinh th chu mt lc kch thch bin i (F = ma), gy nn mt in tch q t l chnh thnh ngang qua n. Nn,

ij ijq d F d ma= = (19.21)

Trong q l in tch hnh thnh v d ij l h s p in ca vt liu.Phng trnh trn ch ra rng, in th u ra ca vt liu p in ph thuc vo thuc tnh c ca n d ij. Hai loi tinh

th p in thng c s dng l gm lead-zirconate titanate (PZT) v tinh th thch anh. Chng l cc vt liu t sinhin v sn sinh ra in tch ln tu theo kch thc ca chng. H s bin dng p in ca PZT gp khong 150 ln so vi

18


19/72

Cm binthch anh. Theo , cc PZT l nhy v nh hn nhiu so vi thch anh. Cc gia tc k ny c dng trong cc ng dngtn s cao. Tn s ban u vo khong 100 Hz. Cc thit b ny khng c p ng 1 chiu. V cc gia tc k p in c cntr c kh tng i thp, nh hng ca chng ln chuyn ng ca cc thnh phn cu trc nht l khng ng k.

HNH 19.23 S b tr ca mt gia tc k p in dng nn.

Trn phng din ton hc, hm truyn ca chng ging mt h thng bc 3 c vit nh sau:

0

2 2 2

( )

( ) ( 1)( / 2 / 1)q

n n n

K se s

a s C s s s

=

+ + +(19.22)

trong Kq l hng s p in lin quan ti in tch (C cm), l hng s thi gian ca thch anh, v s l bin Laplace. Cmt lu cn quan tm l thch anh khng c hng s thi gian , nhng hng s thi gian c quan st khi cc gia tc kc ni vi mt mch in, v d nh, mt mch RC.

p ng tn s thp c gii hn bi c tnh p in /( 1)s s + , trong khi p ng tn s cao lin quan ti p ngc kh. H s gim chn l rt nh, thng nh hn 0,01 hoc gn 0. chnh xc p ng tn s thp yu cu rng, nthng c thc hin bi vic s dng bin p in tr cao. cc tn s rt thp, cc hiu ng nhit c th nh hngthng xuyn vo cc c tnh hot ng.

Trong cc gia tc k p in, hai cu hnh thit k c bn c s dng: cc dng ng sut nn v ng sut trt. Trongcc gia tc k dng nn, tinh th c gi dng nn bi mt phn t c ti trng t trc; do dao ng thay i ngsut trong dng nn. Trong gia tc dng ng sut trt, dao ng n gin lm bin dng tinh th dng trt. Cc gia tc k

dng nn c mt khi lng tng i tt cho t l nhy v t n hot ng tt hn. Nhng, v v lm vic nh l mtphn ca h thng lo xo - khi lng, n c th sinh ra mt mt phn cch gi to trong u ra ca gia tc k nu c kchthch xung quanh tn s t nhin ca n.

Cc gia tc k p in c di rng cc c tnh v c nhiu nh sn xut. V d, cc c tnh ca mt gia tc k shock cth c nhy mc 0.004 pC/g v mt tn s t nhin ti 250.000 Hz, trong khi mt b c thit k cho cc dao ng mc thp c th c nhy mc 1000 pC/g v tn s t nhin ch vo khong 7000 Hz. Chng c sn xut theo kch c3 x 3 vi khi lng 0.5 g, bao gm c cc dy ni. Chng c khong nhit tuyt vi v mt s c thit k thch ngvi mi trng bc x ca cc phn ng ht nhn. Tuy nhin, cc gia tc k p in theo xu hng c nhy cm ngangtrc ln hn cc loi khc, khong 2-4%. Trong mt vi trng hp, nhy cm ngang trc ln c th c lm ti ginnh ta chn ra hng ng ca thit b trong sut qu trnh thit lp. Cc gia tc k ny c th c gn vi nt ren, viximng hoc cht dnh, hoc mm cp t trng.

Gia tc k p tr

Cc gia tc k p tr ch yu l cc bn dn hiu chun bin dng vi cc h s hiu chun ln. Cc h s hiu chunln t tnh khng in ca vt liu l ph thuc ch yu vo bin dng, khng ch ph thuc vo kch thc. nh hngny c th c lm tng ng k bi vic cho thm vo bn dn cc cht ph gia v d nh silicon. Tt c cc gia tc k ptr u s dng hai hoc bn hiu chun hot ng c sp xp thnh mt cu Wheatstone. S dng thm cc in tr chnhxc, nh l mt phn mch in, trong chui vi u vo iu khin nhy, cn bng, v hn ch nhng nh hngca nhit . nhy ca mt cm bin p tr c c t phn ng mm do trong cu trc ca n v in tr ca vt liu.Dy v in tr dy hoc mng c h s hiu chun thp, nh vy, s thay i in tr da vo bin dng l nh. Hnh19.24 m t cu trc c kh ca gia tc k p tr.

Cc gia tc k p tr l hu ch trong vic thu thp thng tin dao ng cc mc tn s thp, v d nh di 1 Hz. Trongthc t, chng vn l cc cm bin gia tc khng dao ng. Nhn chung chng c rng bng thng ln, khng tuyntnh nh v dch khng, v cc c tnh tr tt hn khi so snh chng vi cc c im ca gia tc k p in. Chng thchhp o cc bin i t ngt trn 100.000 g.

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S tay C in t

HNH 19.24 Gn cc gia tc k p in v p tr vo cc h thng qun tnh

Cc c trng in hnh ca cc gia tc k p tr nh sau: nhy 100 mV/g, di tn s t 0 750 Hz, tn s cnghng 2500 Hz, di bin 25g, t l shock 2000g, v di nhit 0 95 0C, vi tng khi lng khong 25g.

Hu ht cc cm bin p tr hin ti c sn xut t mt ming silicon n. N mang li s n nh hn v gim nhithn gia cc phn. Trong mt phn t o dng khi in hnh c mt chip silicon 1 mm kt hp vi 1 l xo, khi lng, vmt cu gm 4 nhnh. Cc phn t ny c to thnh bi mt c im ca cht ph gia trong khi silicon phng nguynbn. Tip sau s khc mn ca cc knh gii phng s hiu chun v ng thi nh ngha cc khi lng nh cc phn casilicon trn bn gc.

Gia tc k hiu chun ng sut

Cc gia tc k hiu chun ng sut da trn cc thuc tnh in tr ca cc dy dn in. Nu mt dy dn c ko dihoc b nn li, in tr ca n vn ph thuc (a) vo chiu di dy dn, v (b) thay i theo tnh cht c bn ca vt liu vc gi l p tr. iu ny ch ra rng in tr sut ca dy dn ph thuc vo ng sut c kh t ln n. S ph thucny c biu din theo h s hiu chun:

/ /1 2

/ /

dR R d v

dL L dL L

= + + (19.23)

trong 1 xc nh in tr thay i theo chiu di, 2 ch ra in tr thay i theo din tch, v ( / ) /( / )d dL L nirng in tr thay i theo tnh cht p tr.

C nhiu dng hiu chun ng sut: Cc hiu chun dy kim loi khng tip xc, cc hiu chun dy kim loi lin kt, cchiu chun l kim loi lin kt, cc hiu chun mng kim loi mng t trong chn khng, cc hiu chun bn dn lin kt, vcc hiu chun bn dn khuch tn. Tuy nhin, y cc hiu chun dy kim loi tip xc hoc khng tip xc thng cs dng rng ri hn. Mt phn ca cc gia tc k hiu chun ng sut c s dng, c bit l dng bn dn lin kt, gingnh cc chuyn i p tr, nhng chng chu cc nhy cm vi nhit cao, s khng tuyn tnh, v mt s kh khn khc.Tuy nhin, cng vi s pht trin ca cng ngh vi gia cng gn y, cc cm bin ny c ci thin ng k, t ta cth tm ra c nhiu ng dng mi.

Cc gia tc k hiu chun ng sut khng tip xc s dng cc si dy ng sut ging nh cc phn t l xo v nh lchuyn i chuyn ng, b tr tng t nh trong hnh 19.25. Chng hu ch vi cc chuyn ng thng thng v cc phpo dao ng t cc tn s t thp n trung bnh. Vi nhiu chn loi c cc c tnh sau: thang o t 5g ti 200g , ditn t nhin t 17 800 Hz, in th kch thch 10 V DC hay AC, thang u ra 20mV ti 50mV , phn di nh hn0,1%, khng chnh xc thp hn 1% di o, v nhy cho trc nh hn 2%. T l suy gim (s dng du nhn silicone)l 0,6 0,8 nhit phng. Cc thit b ny nh v nh, thng di 25g.

Cc gia tc k hiu chun ng sut lin kt thng thng s dng mt khi lng c bi mt dm un cong mng. t c nhy ln nht, s b nhit, v nhy cho c gia tc cho trc ln gia tc gc ngi ta gn cc hiu chunng sut vo dm ny. Chng c cc c im tng t vi cc gia tc k hiu chun ng sut khng tip xc nhng chngc kch c v trng lng nng hn. V thng s dng du silicone gim chn. Cc hiu chun ng sut bn dn c sdng rng ri nh cc cm bin ng sut trong cc dm cng xn v cc dng khi lng ca cc gia tc k. Chng cho phpcc mc u ra cao (0,2 - 0,5 V). Thng thng mt my 25g c p ng phng t 0 n 750 Hz, h s suy gim 0,7, khilng l 28g, v nhit hot ng t -180 C ti +930 C. Mt m hnh 20.000g 3 trc c p ng phng t 0 ti 15 kHz,h s suy gim l 0.01, v di b nhit t 0 45 0 C, v kch c l 13 x 10 x 13 mm3 v khi lng l 10g.

Cc gia tc k tnh in

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Cm binCc gia tc k tnh in hot ng da trn nh lut Coulomb gia hai vt mang in; do , chng l dng in dung.

Ph thuc vo nguyn tc hot ng v cc mch ngoi chng c th c phn loi chung thnh (a) gia tc k phn hi lctnh in, v (b) cc gia tc k in dung vi phn.

Gia tc k phn hi lc tnh in

Mt gia tc k phn hi lc tnh in bao gm 1 in cc, vi khi lng m v din tch S, treo trn mt cn trc nh cth di chuyn tng i vi mt vi in cc c nh. Khong trng nh h gia trc v cc in cc c nh c duy tr bimt h thng servo cn bng lc, n c kh nng bin i in th ca cc in cc trong tng ng ti cc tn hiu t c cu

o s thay i tng i trong khong trng. Biu din ton hc ca in trng gia cc in ccQ

EkS

= (19.24)

trong E l mt hoc gradient in th (dV/ dx), Q l in lng, S l din tch ca vt dn, v k l hng s cht inmi ca khong trng b mt vt dn.

T cng thc ny, ta c th biu din lc trn mt n v din tch in cc (N/m 2) nh sau:

2 2

2 22

F Q kE

S kS

= = (19.25)

Xem mt in cc l c nh v cc kia c th di chuyn v gi s rng in cc c th di ng c in th V 1 v cc kiac in th V2. Cng in trng trong khong trng E c tnh nh sau:

1 2

1

V VE

h

= (19.26)

T ta tm ra lc tng tc nh sau:

22

1 2

1 2 2

( )

2 2

k V V S kE SF

h h

= = (19.27)

Trong s hin din ca gia tc, nu V2 c hiu chnh ngn tr cc c th di ng ti v tr khng, biu thc quan hvi gia tc v in th c vit nh sau:

2

1 1 2

2

( )

2

F k V V S a

m h m

= = (19.28)

Thit b ny nh m t ch c th o gia tc theo mt hng v u ra c c tnh dng qun tnh bc hai

1 2( )V V D a = (19.29)

D l h s t l.

C th tuyn tnh ho u ra theo nhiu cch, mt trong s l phng php gc phn t. Nu b iu khin servo to rain th -V2 so vi cc in cc khc, lc tng tc gia in cc ny v in cc di chuyn tr thnh

2

2 1 2

2

( )

2

F k V V S a

m h m

+= = ( 19.30)

v phng trnh cn bng lc ca in cc di chuyn khi thit b c gia tc a l

2 21 2 1 2

2 1 2[( ) ( ) ]

2kS V V V V ma F F

h + = =

hay

1 2

2 1 2

2 kSVVma F F

h

= = (19.31)

T , nu in th V1 c gi c nh v h s khuch i thu c t vng lp iu khin (loop) ln cc daong trong khong trng l khng ng k, gia tc tr thnh hm tuyn tnh ca in th ra ca b iu khin.

S kh khn c bn trong vic c kh ho gia tc k lc tnh in l cng in trng tng i cao thu c mtlc duy nht. suy gim c th c a ra do in hoc bi tnh nht ca mi trng kh trong khong khng gia haiin cc nu khong trng h l nh. Hin tng nh thng in th phi tuyn cho php gradient ln hn trong nhng

khong trng rt nh.Mt gia tc k tnh in in hnh c nhng c im sau: di 50g , phn gii 310 g , nhy 100 mV/g, phi

tuyn


22/72

S tay C in t

900C, ngun nui 5VDC, v trng lng 45g. u im chnh ca gia tc k tnh in l kt cu c kh rt n gin, yu cungun thp, khng c cc nguyn nhn dn n hin tng tr, h s nhit 0, v d che chn trnh cc ngun nhiu.

Gia tc k in dung vi phn

Nguyn tc thit k cc gia tc k ny l s thay i in dung tng tng vi gia tc tc dng. Trong dng ny, ngita to ra mt khi lng ng ca gia tc k nh phn t di ng ca mt b dao ng in. Khi lng ng c bimt dm n hi di chuyn song song t chn . H thng ny c t c mt tn s danh ngha xc nh c th khi

khng b lm nhiu. Nu thit b l c gia tc, tn s s bin i cao hn hoc thp hn gi tr tn s danh ngha ph thucvo hng ca gia tc.

Khi lng ng mang theo mt in cc c t i din vi cc in cc gc c nh m n xc nh cc t in bini. Cc in cc gc c nh xc nh trc m bo duy tr dao ng trng mt khong thay i ca in dung xc nhbi gia tc tc ng. Gi tr in dung ca t C bin thin c tnh theo:

kSC

h

= (19.32)

trong k l hng s ca cht in mi, hng s in mi ca khng gian t do chuyn ng ca in cc, S l din tchin cc, v h l khong cch bin thin.

Thay th ln ca khong khng gian cho gia tc khng ti h0, gi tr ca h tng ng vi gia tc a c th c vit nhsau:

0

mah h

K= + (19.33)

trong m l khi lng vt, K l hng s l xo, do vy

6

2f

RC= (19.34)

V d nh, tn s ca dao ng ca mch dng tr khng c cho bi biu thc:

0( / )

kSC

h ma K

=

+(19.35)

Thay gi tri C 19.34 ta c

06[ ( / )

2

h ma K f

R kS

+= (19.36)

t B = 6 /(2 )R kS biu thc c vit li nh sau

0

Bmaf Bh

K= + (19.37)

S hng u tin ca biu thc bn phi biu din tn s lch khng c nh f0 v s hng th hai th hin s thay i catn s ly ra t gia tc, do biu thc c vit li nh sau

0f f f = + (19.38)

Nu so snh tn s u ra vi mt ngun c lp ca mt tn s khng i f0, do fa c th c tnh d dng.

Thng thng kiu gia tc k loi ny da trn mng chn mng un hnh xon c to ra l xo l xo, khi lng th, vmt a di ng cn cho t in vi phn. Chuyn ng ca a gia cc in cc y khng kh song song vi b mt a vqua cc l trong a to mng n hi nn. Do nht ca khng kh c nhy thp hn ca du, hng s suy gimmong mun l 0,7 v rt kh thay i n 15%. Cc thit b ny rt d tm vi phm vi t 0.2g (4Hz phn ng hon ton)ti 1000g (3000Hz), nhy cho trc


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Cm binb mt. Trong khi gia cng siu nh, cong, cc dm cng xn, v cc thnh phn c bn khc ca gia tc k c snxut t cc khi silicon s dng cc tnh cht c kh ca cc tinh th silicon. Vi thit k v quy trnh phim hp l, cckhi gia cng siu nh em li cc gia tc k hon ton n nh v bn vng.

Cng ngh khc c chn la nhiu lp ca cc tm phim mng, hay vi gia cng b mt, cho php cc vi cu trc di ng c sn xut trn cc mng silicon siu mng. Vi b mt vi gia cng, cc lp ca cu trc vt liu uc b tr v ly lmmu nh hnh 19.25. Cc cu trc ny c to thnh bi polysilicon v cc vt liu ph hp khc nh cc oxit silic. Cc vtliu hot ng nh mt lp trc tip c khc n mn i sinh ra mt cu trc ng t do. Cng ngh gia cng b mtcng cho php cc cu trc nh hn v phc tp hn c ch to theo nhiu lp trn mt nn n. Hnh 19.26 m t mt v

d c bn ca vi gia cng gia tc k hi i. Ngi ta gn nhiu gia tc k ln mt chip n, o gia tc theo cc hng x, y,v z. Cng chnh chip ny a ra quy nh v tn hiu chnh. Tn hiu ra t chip thng c c di dng s.

HNH 19.25 Cc bc vi gia cng sn xut cc gia tc k nh v siu nh

HNH 19.26Nhiu gia tc k trn mt chip n

Tt c cc gia tc k dng micro hoc nano u pht hin gia tc qua vic o chuyn ng tng i gia khi lng mu

v nn treo. Khi lng mu c treo trn nn qua mt l xo c kh. Khi cm bin chu gia tc, khi lng mu c xuhng tr v v tr cn bng v do n dch chuyn i vi nn chuyn ng. Vic o cc dch chuyn ny nh kh nngca cc phng php p tr hoc p in s dng cng ngh CMOS. Mch in t c nhim v trit tiu lch khng, do vyn nh lch khng, n nh h s khuch i, n nh lch khng gia tc, b nhit, tin lc, gim nhiu u ra

Nguyn tc hot ng ca mt s gia tc k khad tng t nh cn bng lc in dung hay gia tc k dm dao ng c nu phn trc. K thut sn xut c th thay i tu theo cc nh sn xut khc nhau. Tuy nhin, gia tc k dm giaong thng c quan tm hn v n c thuc tnh khe h khng kh tt hn v ci thin tt hn c tnh lch khng.

Cc gia tc k dm dao ng, cn gi l cc chuyn i lc dm cng hng, sinh ra mt gia tc dc theo trc dng tnhiu u vo t phn cng ca khung dao ng. Do , tn s cng hng ca dm dao ng tng hoc gim khi c giatc.

Trong DETF, mt b to dao ng in kt hp nng lng vo trong hai khung dao ng gi chng dao ng ti tns cng hng ca chng. Cc dm dao ng 180 0 v pha hu cc lc hot ng ti im cui. nh hng hu ng hc

ca thit k DETF ngn nng lng b mt mt qua cc im cui ca dm. T b cng hng cn bng ng DETF cmt nhn t Q cao, dn n mt mch dao ng n nh. Cc tn hiu gia tc c sinh ra t mt b to dao ng nh l mtsng vung c iu bin tn s c s dng cho giao din s.

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S tay C in t

Tn s cng hng ca h thng phi ln hn nhiu bt k mt gia tc u vo no, v iu ny gii hn di o. Trongmt gia tc k vi gia cng, s dng trong cc ng dng qun s, c cc c tnh sau: phm vi 1200g , nhy 1,11 Hz/g ,bng thng 2500 Hz, tn s DETF khng ti 9952 Hz. Tn s ti +1200g l 11.221 Hz, tn s ti -1200g l 8544 Hz, v nhy nhit l 5 mg/0C. Kch thc ca gia tc k: ng knh 6 mm, chiu di 4,3 mm, khi lng khong 9g. Thi gianbt nh hn 60 s, ngun nui t +9 n +16V DC, tn hiu ra trung bnh l mt xung vung 9000Hz.

B mt vi gia cng c s dng to ra cc gia tc k dng cho cc ng dng c bit, nh cc ng dng v ti khtrong cng nghip t. Mt t in vi ba lp khc nhau c to bi cc lp xen k ca polysilicon v thu tinh

phosphosilicate (PSG) trn mt tm di 100mm, dy 0,38mm. Mt ming silicon dng lm nn cho kt cu c kh. Mt lpm l xo gia c treo bi bn thanh tr gip. Cu trc di ng ny chnh l khi lng ng cho gia tc. Cc lppolysilicon trn hoc di l cc a c nh cho cc t in vi phn. Tm knh c n mn dng axt hydrofluoric (HF).

X l tn hiu v lch khng

Cc b x l tn hiu thng thng thch hp cho qu trnh giao tip t gia tc k ti cc my tnh hoc cc thit b khccho qu trnh x l tip theo. Nhn chung, cc tn hiu th mi to ra c khuch i v lc ph hp bi cc mch lc trongcc gia tc k. Tuy nhin, cc b chuyn i p tr v p in yu cu cc tn hiu c x l c bit vi cc c tnh c ths c tho lun sau.

Gia tc k p in

Cc gia tc k p in cung cp mt nng lng nh ti cc b bin i tn hiu v chng c ngun tr khng cao. C thxem mch tng ng ca mt gia tc k p in nh mt t in hot ng v t tch in cho chnh n khi chu ti c kh.S chn la cc phn t ca mch bin i tn hiu ngoi ph thuc vo cc c tnh ca mch tng ng. Mt cch tipcn ph thng nht l khuch i in tch v h s khuch i ca h thng v p ng tn s thp ca b khuch i lhon ton xc inh. Hot ng ca mch cng ph thuc vo di dy ni v in dung ca gia tc k. Trong nhiu ngdng, cc dy ni chng nhiu l cn thit trnh cc in tch xut hin do cc chuyn ng ca cc dy ni.

HNH 19.27 Mch khuch i in tch in hnh.

Khuch i in tch (c tr khng u vo khong 1000 M ) trc tin chuyn ho in tch u vo thnh in p,sau khuch i in p ny. N bao gm mt chuyn i in tch c u ra in p, n l kt qu ca tn hiu in tch

u vo thng qua mt t in phn hi duy tr in tch u vo mc u vo gn gi tr khng, nh trong hnh 19.27.Nh c loi khuch i thut ton phn hi, u vo b khuch i c gi mc 0V; do , n trng ging nh hin tngngn mch u vo. Do vy, in tch u vo c lu tr trong t in phn hi, sinh ra mt in p qua n v in pny l bng vi gi tr ca in tch u vo c chia bi in dung ca t in phn hi. Hm truyn ca mch m t quanh gia in p u ra v ln gia tc u vo c th c xc nh bi chuyn i phc sau:

0

0

1 11a c

a f f f f f

E C C S jR C jR C C

a G

+ = + + + + (19.39)

Trong , E0 l chuyn i in tch u ra (V), a 0 l ln gia tc (m/s2), Sa l nhy gia tc k (mV/g), Ca l indung gia tc k (F), Cc l in dung dy dn (F), Cf l in dung phn hi (F), Rf l in tr vng phn hi, v G l h skhuch i vng h ca b khuch i.

Trong hu ht cc ng, Cf c chn ln so vi (Ca + Cc)/(1+G), h s khuch i h thng tr nn c lp vi di ca dy dn. Trong trng hp ny, ngi ta n gin ho mu s ca phng trnh a ra mt h thng bc nht vi(roll off) ti

24


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Cm bin

3

1

2dB e ff

R C= (19.40)

vi dc l 10dB trn decade. Trong thc t, p ng tn s thp ca h thng l mt hm ca cc thnh phn inhon ton xc nh v khng thay i theo di ca cable. y l mt c tnh quan trng khi o cc dao ng tn s thp.

Nhiu gia tc k p in c sn xut vi b tin khuch i v cc mch bin i tn hiu c t trong cng mt v.Mt s b tin khuch i gia tc k bao gm nhiu b tch phn chuyn i cc u ra t l vi gia tc thnh tn hiu vntc hoc dch chuyn. lm tt dn nhiu v cc tn hiu dao ng nm ngoi di tn s quan tm, ngi ta s dng cc b

tin khuch i v cc b lc thng thp hoc thng cao. iu ny trnh s giao thoa ca cc nhiu in vi cc tn hiu bntrong di tn s tuyn tnh ca gia tc k. Tuy nhin, c thun li l nhng thit b ny thng c hai hng s thi gian, niv ngoi (Nevertheless, it is worth mentioning that these devices usually have two time constants,external and internal). S ln ln ca hai hng s thi gian ny c th dn n mt s vn , c bit vng tn s thp.Cc nh sn xut thng qua cc thit k hoc thnh lp thng c nh cc hng s thi gian ni. Tuy nhin, cn phi tnhton cn thn cc nh hng ca cc hng s thi gian ngoi thng in khng ph hp.

HNH 19.28 S mt mch iu bin tn hiu in hnh cho cc gia tc k mt chip n.

Chuyn i p tr

Cc chuyn i p tr nhn chung c tn hiu u ra ln, in tr u ra thp, v cc nhiu ni thp. Tt c nhng thit b

ny c thit k cho cc in p kch thch khng i. Chng thng c chun ho cho cc dng in kch thch khngi trnh cc nhiu bn ngoi. Chng c ghp vi mt mch cu in tr. Mt s c nhnh tr hot ng, cng vi haiin tr chnh xc c nh cho php chun ho cc nhnh.

Vi gia tc k

Trong vi gia tc k, cc mch in t bin i tn hiu c tch hp trn ch mt chp bm bin. S khi trong hnh19.28 a ra mt v d in hnh ca mch bin i tn hiu. Trong dng ny ca gia tc k, h thng in t c iu khinbng mt mch dao ng thch anh v tn hiu ra ca mch to dao ng l tn hiu gia tc c iu bin tn s. Mt smch in cung cp u ra dng xung vung c th lm giao din s trc tip. Trong nhng trng hp ny khng yu cucho b chuyn i A/D do hn ch mt trong nhng nguyn nhn chnh gy li. cc loi gia tc k khc, cc mch bini tn hiu nh cc b chuyn i A/D c gn bn trong chip.

Gia tc k phn hi lc

Cc tn hiu t cc gia tc k phn hi lc thng c s ho dng trong cc h thng s. Mt gii php thngthng l dng cc b chuyn i in th thnh tn s hoc dng in thnh tn s chuyn i cc tn hiu tng t thnhcc chui xung. Cc b chuyn i ny l rt t, thng t hn nhiu so vi cc gia tc k, v gy ra thm sai lch kt qu.

y, cc h thng GPS ngy cng tr nn c ngha trong cc h thng o v tr trong c hc. Bi v cc ng lc hcantenna, vic d, cc hiu ng a ng, v cung cp thm cho cc h thng ti hn nh l my bay, nhiu trong s nhngh thng ny cn s tr gip qun tnh, b hp trong cc gia tc k v cc con quay hi chuyn. Vi s pht trin ca cngngh vi gia cng, trong tng lai chng ta s c cc h thng tr gip qun tnh GPS nh v mang li nhiu thun li. Nhngs pht trin trn s yu cu qu trnh x l tn hiu vi khi lng ln v c chnh xc cao. Cn phi x l cc di nghc theo th t t 1000.000 ti 1( v d, 30-32 bit). t c cc yu cu chnh xc, mt thch thc ln ang ch i ccnh lm v x l tn hiu.

Ti liu tham kho

[1] Bentley, J. P.,Principles of Measurement Systems, 2nd ed., Burnt Mill, UK: Longman Scientific and Technical, 1988.

[2] Doebelin, E. O., Measurement Systems: Application and Design, 4th ed., Singapore: McGraw-Hill, 1990.

[3] Frank, R ., Understanding Smart Sensors, Boston: Artech House, 1996.

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S tay C in t

[4] Harris, C., Shock and Vibration Handbook, 4th ed., McGraw-Hill, 1995.

[5] Holman, J. P.,Experimental Methods for Engineers, 5th ed., Singapore: McGraw-Hill, 1989. FIGURE 19.28 A typicalsignal conditioning arrangement for single chip microaccelerometers. 2002 CRC Press LLC

[6] Lawrance, A., Modern Inertial Technology-Navigation, Guidance, and Control, Springer-Verlag, New York, 1993.

[7] McConnell, K. G., Vibration Testing: Theory and Practice, New York: Wiley, 1995.

[8] Machine Vibration: Dynamics and Control, London: Springler, 19921996.

[9] Measuring Vibration, Bruel & Kjaer, 1982.[10] Sydenham, P. H., Hancock, N. H., and Thorn, R., Introduction to Measurement Science and Engineering, New York:

Wiley, 1989.

[11] Tompkins, W. J. and Webster, J. G., Interfacing Sensors to the IBM PC, Englewood Cliffs, NJ: Prentice-Hall, 1988.

19.3

19.4

19.5

19.6 o nhit

Pamela M. Norris v Bouvard Hostickai hc Virginia

Gii thiu

Trong cng ngh cng nh cng nghip, nhit l mt trong nhng thng s rt c coi trng, tuy nhin cho n nay,mt nh ngha chnh xc cho nhit vn cn ang b b ng. nh ngha n gin nht l gn nhit vi ng nng trung bnhca tng phn t cu thnh nn h thng. Khi nhit tng th hot tnh ca cc phn t tng do ng nng trung bnh

tng. l mt nh ngha y cho cc k thut o nhit c trnh by y. nh ngha ny gip chng ta hiu ckhi nim v nhit tuy nhin n li khng gip ta a ra c mt gi tr s cho nhit hay cung cp cho ta mt phngphp thun tin trong vic o nhit. Lut th khng ca nhit ng hc c a ra vo nm 1931 sau hn na th k so vilut th nht v th hai to c s cho vic o nhit . Lut ny cho rng nu hai vt cng cn bng nhit vi vt th ba thchng cn bng nhit tng i mt. Bng vic thay vt th 3 bng mt ci nhit k, ta c th ch ra hai vt l cn bng nhitkhi chng c cng nhit cho d chng khng tip xc vi nhau.

BNG 19.4 Cc im xc nh c x dng trong ITS90*

im cn bng ca hyr 13.8033K

Triple point ca nn 24.5561 K

Triple point ca argon 54.3584 K

Triple point ca thu ngn 234.3156 K Triple point ca nc 273.16 K

im tan chy ca gali 302.9146 K

im ng bng ca Indi 429.7485 K

im ng bng ca thic 505.078 K

im ng bng ca km 692.677 K

im ng bng ca nhm 933.573 K

im ng bng ca bc 1234.93 K

im ng bng ca vng 1337.33 K

im ng bng ca ng 1357.77 K

* Magnum (1990) cha ton b nh ngha ca cc im ny

26


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Cm binLut th khng khng c kh nng a ra mt gi tr s cho nhit . Do , ta phi tham chiu nhit theo mt thang

o chun. Hai thang o nhit tuyt i uc nh ngha sao cho nhit ti im 0 tng ng vi mt trng thi gi nhm ti khng mt phn t no chuyn ng bn trong vt cht. y chnh l c s ca thang o Kelvin trong h chun (SI)v thang o Rankine trong h Anh. Ngoi ra cn c thang o hai im da trn vic nh ra hai im nhit bt k. Ccim ny thng c chn bng vi nhit m ti xut hin s chuyn pha ca vt cht trng thi tinh khit. Quenthuc nht chnh l im ng bng v si ca nc 0C v 100C. S chuyn pha xy ra trong mt khong rng, khnhiu trng hp c im cn bng, ti c ba pha u cn bng, im ny c nh ngha trong Thang o nhit quc t(International Practical Temperature Scale of 1990 - ITS90) - bng 19.4. Chng c th c s dng trc tip nh lim chun gim st nhit vi gi thit cht l nguyn cht cng mt s cc iu kin khc nh p sut n nh tiim t. Tiu chun ITS90 cng gii thiu c vic ni suy nhit gia hai im cho trc. V d, in tr bch kim cdng trong gii hn nhit t 13.8 n 1235 K. Thng qua mt a thc bc cao c n gin ha ta ni suy ra nhiugi tr nhit gia hai mc nhit nh ngha t in tr s c nhit ph hp hn. S khc nhau gia ni suy tuyntnh ca in tr gia hai im nh ngha v ni suy vi a thc bc cao l khng bao gi vt qua 2mK (Magnum andFurukawa, 1990).

Mt s phc tp khc na l thc t th nhit th hin l mt thuc tnh bn trong nhiu hn bn ngoi. V vy, nhit khng th cng, tr v chia theo cch thng thng nh cc gi tr chiu di hay in p.

Bt k thuc tnh no thay i p ng theo nhit m d bo uc u c th c s dng trong cm bin nhit. Ccthit b o nhit trong cp trong ti liu ny c phn loi theo phng php o. Phng php u tin da trn s ginn nhit khc nhau ca cc loi vt liu rn, lng hoc kh. Nhit k thu ngn thuc loi nhit k ny l mt trong nhngnhit k thng dng nht. Tip n phi k n cc thit b da trn s thay i pha. Cc cm bin v b chuyn i nhitin. Bao gm cp nhit in, RTD, v nhit in tr, cng nh cc mch tch hp cm bin nhit. Loi cui cng ca cm

bin nhit l cm bin khng tip xc. Mt tho lun ring v o lng nhit vi c micro c a ra phn cui. Nhiuk thut c m t trong phn ny bt ngun t nhng g c tho lun trc t th yu n quan trng phi cthc hin cho php cc o lng nhit kch thc nh v/hoc p ng nhanh.

Cc loi nhit k da trn h s gin n khc nhau

Nhng nhit k da trn h s gin n khc nhau thng dng nht l nhit k c nhit trc tip. Nhng nhit k nyc th phn loi theo trang thi vt liu s dng. Ta s nghin cu chi tit tng loi mt.

Kh - Rn

Nhit k bu kh dng xc nh im khng tuyt i bng cch ngoi suy s thay i p sut ca mt n cht khtrong khi cu kim loi khi nhit thay i c th coi l mt v d ca loi nhit k kh - rn. Nu bu kim loi c cng hs gin n nhit vi cht kh th p sut bn trong s l hng s v khng th l mt nhit k. Thay vo , cht kh theonh lut kh l tng ch ra rng, ti mt th tch khng i, p sut quan h tuyn tnh vi nhit v bnh cha kh thayi tuyn tnh vi h s gin n theo th tch ca kim loi lm bu cha. H s gin n nhit ca kim loi thng c bqua tr trong trng hp c yu cu c bit v s chnh xc ca im khng tuyt i.

Trong khi mt khi cu kim loi ln c bm y khng kh khng phi l mt thit b thun tin nhiu cho o lngnhit, tr vic dng nh l mt dng c nghin cu hoc minh ho, th bu kh c th lm tng i nh v ni vi ng maodn nh iu khin p sut bm vo. Trong dng cu hnh thu nh, nhit k dng bu kh tr thnh thit b thc t cho olng nhit. Ch cn cc thit b lm vic trong vng kh l tng, t p sut bnh ta c th c nhit mt cch trc tip dop sut t l tuyn tnh vi nhit .

Mt s hn ch chnh ca nhit k bu kh l thit b o ny nn c cn chun ring cho mt ci t c th do chiu dica mao dn b t nng, cng nh p sut v nhit xung quanh ti u o p sut, s nh hng n chnh xc cathit b. Nhng hn ch ny c th khc phc bng cch s dng cc phn t lng kim trong u o p sut b nhit tiim ny hoc bng mt mao dn song song khng c bu kh theo mao dn chnh ln ti im o lng v mao dn song

song ny gn vi u o p sut loi b cc hiu ng t u o chnh. Ngoi ra, bt k s nguy him no v th tch cabu kh, nh mt vt lm, s lm x dch s cn chun. Loi dng c ny khng nn ln ln vi nhit k p sut hi nc cth c hnh dng bn ngoi ging nh vy, nhng thay v cha kh l tng, chng cha cht lng hai pha v o p sut boha ca cht lng . Kiu cm bin nhit ny s c tho lun phn sau.

Lng - rn

Nhit k thy ngn v thy tinh ph bin l mt v d v cm bin nhit dng lng rn. H s gin n nhit ca chtlng, mc d khng c ln nh kh, nhng ni chung cng ln hn cht rn v trong nhiu trng hp h s gin n nhitca thu tinh c th b qua. Tuy nhin, trong cc thit b o chnh xc, h s gin n nhit ca thu tinh c coi l sai sng k. C hai cch ph bin lin quan h s gin n nhit ca thu tinh. Cc nhit k dng cho c nhit ca mt bnhcht lng c th c mt su ph kn ring c ch r bi mt du hiu trn ng. Gi s phn cn li ca nhit k nmtrong iu kin chun ca phng th nghim. Tuy nhin, iu ny khng phi l mt gi thit tt, v c mt cch chnh xchn nm bt h s gin n nhit ca thy tinh so vi h s gin n ca thy ngn l dng mt cp ng nhit k. Mt dng o nhit ca cht lng trong khi ci cn li o nhit trong vng ln cn trung gian ca ng. Mt cng thc hiu chnhng dn n gin c cung cp bi nh ch to nhit k c th c ng dng xc nh nhit ca bnh cha.

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S tay C in t

T l dung tch bu vi l ca ng mao dn xc nh phn gii ca nhit k. Tng lng cht lng ban u trong nhitk s xc nh gii hn ca nhit k. chnh xc ca l v vch chia quyt nh chnh xc ca nhit k. Nhit c c t nhit k cht lng-thu tinh ch c gi tr khi m cht lng trong l ca ng mao dn l lin tc t bu ti imc. Phn chia cch c th loi tr bng cch lm co ton b cht lng v bu hay trong mt s trng hp bng cch mrng mt bu bn trn ca nhit k. Thu ngn l rt ph hp cho nhit ng bng khong -40 0C n quanh 5000C.Nhit si ca thu ngn ch l 3570C p sut tiu chun do vy nhit k thu ngn thit k cho loi nhit rt cao phic nn p bng mt cht kh trc khi hn kn nhit k. Cn hay cht lng khc c th c s dng thay cho thu ngn,tuy nhin chnh xc thng thng l khng cao. Khong nhit ca cho nhit k dng cn l -200 0C n 2500C.

Mt phng php thay th dng s sai khc trong h s gin n ca cht lng v cht rn o nhit l dng mt hthng c in y vi mt cht lng v o s thay i dung tch ca cht lng bng v tr ca ng (a bourdon tube orbellows). Nu dung tch ny c hng s gin n cao, t s nn c th c xem xt n (Doebelin, 1990). S kt hp ny chn ch ging nh nhit k bu kh nu trn, cng nh cng mt cch b gim nhng hn ch ny.

Kh, lng v rn

Mt vi nhit k s dng trc y bao gm mt bu kh ni vi ng dng ch U ng kn cha thu ngn hoc mt loicht lng khc. y l mt dng nhit k p sut s dng p k thu ngn hin th p sut. m bo chnh xc, cnphi xt n h s gin n nhit khc nhau ca c ba pha kh, lng v rn. Tuy vy, nhng thit b o ny t khi c s dngkhi dng c o yu cu c chnh xc cao. Nhit k dng ny c s dng rng ri o di nhit nh nht-ln nhtca khng kh mi trng c mt si kim loi nh ni vi chn ca p k thu ngn. Si kim loi ny phi c ma st ln bn trong ng v khng b dnh thu ngn n c th duy tr trng thi t do trong ng thu tinh sau khi mc thu ngn

thay i. ng h o trn cu kim loi s hin th nhit nh nht cn ng h lp trn cc chn s hin th nhit lnnht khi thu ngn rt xung. Khi quan st c nhit ln nht v nh nht, cc si kim loi c th c t trn nh cahai ct thu ngn bng lc li tm hoc t trng nu cc si c dng lm t st.

Rn - rn

Nhit in k lng kim bao gm hai kim loi c h s gin n nhit khc nhau c gn vi nhau. Khi nhit thay it nhit lin kt ban u th cc thanh kim loi ny s gin n khc nhau v gy ra lc trt gia hai thanh kim loi. Tthit b o trt ca hai thanh kim loi s cho php chng ta xc nh c s thay i nhit. Cu trc ca nhit k loiny rt phong ph t dng l n gin c nh trn mt u vi mt kim ch trn u cn li n ng xon c c c nhtrn mt u v mt trc quay ti u cn li c lin kt vi kim ch, c th thng qua h thng bnh rng. Trc ny cgi trn cc trc trn vi kim ch nh mt ng h t ng xon c lng kim. Nhit k dng gy vi mt s mt u lmt v d phn tip theo.

Do nhng thay i nhit s to ra lc, nn lun c mt s lc hi phc phn cp tc dng ln di lng kim. ng dngph bin nht l dng ngay di lng kim nh mt l xo hi phc. V tr cui cng ca di l im cn bng gia s trt docc h s gin n v nhit khc nhau v hng s n hi ca di. Trong mt s tnh hung, nhit k lng kim c cn kch hot mt cng tc. Trong cc trng hp ny, lc trt trn di phi thng ti do cng tc v nh thit k phi coi n lti trng ngoi. Di nhit ca nhit k lng kim b gii hn bi nhit ti hoc nhit chuyn pha ca kim loi. Do, cc nhit k loi ny lm vic tt vi nhit di 7000C, v chng c th b ph hy vnh vin nu cc kim loi thay itnh cht hoc lin kt gia cc kim loi khc nhau b hng. Mt cp kim loi ph bin l thp niken, nh hp kim inva vi hs gin n v nhit rt thp, lin kt vi hp kim ng thau vi h s gin n v nhit cao.

Nhit k lm vic da trn s thay i pha

S thay i pha ca cc vt liu nguyn cht ti cc p sut ring c s dng trong bng thng s ITS90 xc nhmt vi im nhit . Khi nim thay i pha l mt hm ca nhit cng nh p xut v dng kim loi ny c th dng

trong mt s dng nh mt phng tin xc nh nhit ca mt h thng bng cch quan st chnh s thay i pha hoccc iu kin ti hai pha cn bng. Mt s ng dng hu ch ca n s c tho lun sau.

Chuyn t trng thi lng sang kh

Nhit k t xa thng thng bao gm mt bu nhit k c cha cht lu hai pha lng-kh c ni vi u o p sutthng qua ng mao dn. Khi xut hin c hai pha, p sut c trn u o cho ta gi tr p sut bo ho ca cht lu. Vickhc phc nhng khuyt im ca nhit k dng ny c thc hin tng t vi vi nhit k dng kh v cht lng trn.Bng cch gim st p sut bo ho, nhit biu th ph thuc vo nhit phn cn li ca h thng v khng nhy voth tch thc ca bu nhit k cng nh ng mao dn. Cht lng thng l dung mi hu c nh ethane c chn cho dinhit ring. gi cht lng 2 pha hon ton trong bu nhit k, p sut c truyn qua ng mao dn bng cch dngcht lng n pha nh du. Mt vi loi cht lng c ng cong bo ho tuyn tnh. Do vy, phn ln cc p k c t l phituyn ng k khi chng c phn chia tng ng vi n v nhit . khc phc c iu ny, ngi ta c th s dngl xo b c bit c t bn trong p k to ra thang o nhit tuyn tnh. Tuy nhin, i khi vic khc phc ny cth gy ra nhng rc ri khng lng trc.

Di nhit ca cht lng ti nhit k kh b gii hn bi 2 pha ca lu cht v chng thng c s dng trong dinhit t -400C n 3000C, mc d mt thit b o n l him khi lm vic vi di nhit vt qu 1500C. Nu p sut

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Cm binbo ho rt ln hn 100kPa, p sut kh quyn k c dng thay cho p k. Mt khc, chnh xc nht c th t c khis dng p k tuyt i. Th tch ca bu nhit k phi tng i ln so vi s thay i th tch ca ng mao dn trong p k c hai pha c th tn ti bn trong bu nhit k.

B iu chnh nhit thay i pha nghch

B iu chnh nhit im c nh c th c xy dng da trn s thay i pha ca cc phn t cm nhn c bit. Mtv d trong cc tham chiu im ng bng c hc s gin n t ngt ca nc khi ng bng c hiu l chu k hthng lm lnh duy tr trng thi hai pha ca lu cht. S tan chy v ng lnh thc t ca bng duy tr nhit tham chiu.

Cc cht sp (mm, dnh, d chy) vi nhiu im tan chy c th c s dng cho nhng mc ch tng t.Mt v d khc l kha in t c gi bng mt nam chm vnh cu cho n khi t ti nhit Curie m ti nam

chm mt i tnh cht t ha ca n, hay thay i tnh cht t st t sang thun t do kha s m. Khi vt liu lnh i, nly li tnh cht st t v do vy ng kha nhit. Nhng vt liu t tnh c nhit Curie khc nhau s c chn cho ccng dng thch hp.

B ch th nhit c nh

Bt k cht no thay i pha ti mt nhit c nh c th c s dng nh mt b ch th nhit . V d phn mulm bng sp vi im tan chy xc nh. Mt du hiu c to ra trn vt cn c quan st nhit , v nu sp chy, thnhit ca vt ln hn nhit ca phn mu. Cc b ch th nhit c nh thng khng thun nghch v c th dngvi nhiu cht khc nhau khng ch l sp.

Mt bin th c th l thun nghch hay khng thun nghch l sn cha th rn l lng ca sp hoc vt liu tng t c

nhit tan chy xc nh. Ch cn mt lng nh vt liu c sn ln vt to mt lp m c, khi chng tan ra v chuynthnh dng lng c ch s khc x gn ti ch s khc x ca vt liu c sn, n s tr nn trong sut. Ch th ny c thc lm thnh chui im thay i theo nhit gip quan st nhit thc t tt hn. Cc im c th c lm thunnghch ch th lin tc hay bt thun nghch ch th nhit cc i t c trong chu k gim st. Thc t, ch th btthun nghch thng dng hn ch th thun nghch.

Mc d sp c s dng rng ri, bt k vt liu no c thay i pha ring ti mt nhit xc nh c th c dngnh mt b gim st. Cc k thut hnh nh cng c th c dng xc nh thay i pha. Mt ng dng nh th l dnggadoli hoc cc phn t khc hp thu d dng ntron dng thch hp v quan st s tan chy bn trong bng cc phngtin soi tia X ntron.

Cm bin v b chuyn i nhit in

Cm bin trong ng cnh ny l mt phn t thay i mt tham s in nh l mt hm s ca nhit . Sau , tham s

in ny c bin i thnh mt hm in hu dng, nh in p tuyn tnh vi nhit , vi cc thnh phn in t bsung. Cm bin v cc thnh phn in t b sung to thnh b chuyn i. S bin i ca c tnh in theo nhit l skhi ngun ca cc kh nng o lng cng nh s sp ca ton b h thng o in do nhng thay i khng mongmun ca ca cc yu t nh h s khuch i ca mt b khuch i dn n nhng sai s nhit. N lc nhm loi trnhng bin i in do nhit nhiu hn l khai thc chng cho vic o lng nhit .

Cp nhit in

C s lin h gia nhit ca mt vt dn v ng nng ca cc electron t do. V vy, khi mt kim loi c t trongkhu vc c nhit , cc in t t do s khuch tn t khu vc c nhit cao (ng nng ln) n khu vc c nhit thp(ng nng thp hn). Graien tp trung electron to mt graien in p do cc nguyn t mt li to thnh in tchdng khng t do di chuyn. Graien in p ny s chng li s khuch tn sau ny ca cc electron trong dy dn vthit lp trng thi cn bng n nh khi khng c dng in.

Cng sut nhit ca vt liu lin h s cn bng khuch tn nhit ca electron vi tnh dn in ca kim loi v l duynht cho mi vt dn cng nh thng thay i theo nhit . Tnh dn in ca vt liu c nh hng mnh n cng sutnhit v n xc nh kh nng ca vt liu chu ng graien in p. Mt vt dn n l c nhit khc nhau hai u sc s sai khc in th gia hai u . B quyt l kh nng o in p ti c hai u vt dn v nh vy xc nh sai khcnhit gia hai u ny. Nu chng ta s dng cng mt kiu dy ni o in p dy dn ban u, th dy dn th hais sinh ra chnh xc sai khc in p khi cc u ca n c t vo cng nhit nh dy dn ban u. Do graien in pl mt hm ca cng sut nhit l khc nhau i vi mi dng kim loi, nn c th dng dy dn th hai c dng dy dnkhc o graien nhit ban u. Ch mt vt dn khng c chuyn ng electron hoc dn v hn c th dng ograien in p tuyt i lin quan n graien nhit ca vt dn ban u. khng phi mt nh l thc t, nn ch ssai khc trong graien nhit-electron cm gia hai vt dn c th o. chnh l c s ca cp nhit.

Trong thc t, khi c hai kim loi c ni vi nhau v mi ni t ti nhit khc vi nhit hai u t do ca haidy dn, th hai u t do ny s xut hin s sai lch in p, l mt hm ca nhit tuyt i ti mi ni v nhit . Milin h gia sai khc in p v sai khc nhit s l c tnh ca cp dy dn c chn. Trong mch in, hai u dy t

do ca cp nhit in chnh l im ni vi mch. iu ny nhc chng ta rng lun c mt mi ni th han cn xem xt nuhai dy dn t cp nhit ni vi mt mch o. Trong mt s v tr ca b o, mch ng v mi ni th hai hnh thnh.

T cc gii thch trn, tt c cc nh lut cp nhit c in c th c xem xt. Cc nh lut ny c tng kt v tmkim ng dng nh sau:

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S tay C in t

nh lut 1. Mt kim loi th ba c a vo mch vi c hai u ca kim loi th ba ti im ng nhit khng nhhng ti in p cm ng nhit ca cp kim loi ban u.

C hai gi quan trng lin quan ti nh lut ny. Gi th nht l bn cht tip xc in gia cc dy dn ti mi nikhng quan trng, v bn thn cp nhit c th hnh thnh t hai dy c hn, hn bng ng thau, hn li hoc dp p vinhau. Trong tt c cc trng hp, kim loi th ba c mt ti mi ni nn n l cht n trong lin kt hn, hp km trung gianc sinh ra bng vic hn cc kim loi khng ging nhau, hoc p kim loi gi cc u dy vi nhau. iu khng cngha rng khng c s quan tm khc vi cch hnh thnh ca mi ni. R rng khng th hn cc dy vi nhau v sau

dng mi ni trn im tan chy ca cht hn. Hu ht cc cp nhit trn th trng u c hn do nguyn nhn ny. nhlut ny cho php o nhit ca mt vt kim loi bng cch hn trc tip mt dy dn vi n v tr cn o nhit . Cchny trnh thi gian tr khi truyn nhit gia vt v b cp nhit c lp. Gi th hai ca nh lut ny cho php mt mcho lm t cc dy dn khc so vi nhng th c dng trong cp nhit c chn vo mch ngay c khi cc kt ni giamch o v hai dy cp nhit cng mt nhit .

nh lut 2. Nhit dc theo dy dn khng nh hng n c tnh in p cm nhit ca nhit hai mi ni.

iu c ngha l cc dy dn chnh ca cp nhit c th c dn thun tin qua cc vng nhit khc nhau, v chnhng nhit ti mi ni v

sổ tay cdt chuong 19(1,2,6,7,8)- sensor

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