02_chuong 1_cac khai niem chung

8/12/2019 02_chuong 1_cac Khai Niem Chung.

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Chng 1: Cc Khi Nim Chung

Trang 2

Hnh 1.1 Cc loi cm bin thng dng hin nay

Cc b cm bin c s dng nhiu trong cc lnh vc cng nghip v dn

dng. Cc b cm bin c bit v rt nhy cm c s dng trong cc th nghim

cc lnh vc nghin cu khoa hc. Trong lnh vc t ng ho ngi ta s dng cc

sensor thng thng hay cc cm bin c chc nng c bit.

1.2 ng cong chun ca cm bin1.2.1Khi nim

ng cong chun cm bin l ng cong biu din s ph thuc ca i lng

in (s) u ra ca cm bin vo gi tr ca i lng o (m) u vo. ng cong

chun c th biu din bng biu thc i s di dng s=F(m), hoc bng th.

Hnh 1.2 (a) ng cong chun b) ng cong chun ca cm bin tuyn tnh ha.

Da vo ng cong chun ca cm bin, ta c th xc nh gi tr micha bit

ca mthng qua gi tr o csicas. d s dng, ngi ta thng ch to cm


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Trang 3

bin c s ph thuc tuyn tnh gia i lng u ra v i lng u vo, phng

trnhs= F(m)sau php bin i tuyn tnh s c dng:

s = am +b (1.2)

vi a, bl cc h s, khi ng cong chun l ng thng.

1.2.2 Phng php chun cm binChun cm bin l php o nhm mc ch xc lp mi quan h gia gi tr s o

c ca i lng in u ra v gi tr m ca i lng o c tnh n cc yu t

nh hng, trn c s xy dng ng cong chun di dng d nhn bit hn (

th hoc biu thc i s). Khi chun cm bin, vi mt lot gi tr bit chnh xc mi

ca m, o gi tr tng ng sica s v dng ng cong chun.

a. Chun n ginTrong trng hp i lng o ch c mt i lng vt l duy nht tc ng ln

mt i lng o xc nh v cm bin s dng khng nhy vi tc ng ca cc i

lng nh hng, ngi ta dng phng php chun n gin. Thc cht ca chun

n gin l o cc gi tr ca i lng u ra ng vi cc gi tr xc nh khng i

ca i lng o u vo. Vic chun tin hnh theo 2 cch:

o Chun trc tip: cc gi tr khc nhau ca i lng o ly t cc mu chunhoc cc phn t so snh c gi tr bit trc vi chnh xc cao.

o Chun gin tip: kt hp cm bin cn chun vi mt cm bin so snh c snng cong chun, c hai c t trong cng iu kin lm vic.

b. Chun nhiu lnKhi cm bin c phn t b tr (tr c hoc tr t), gi tr o c u ra ph

thuc khng nhng vo gi tr tc thi ca i lng cn o u vo m cn ph

thuc vo gi tr trc ca ca i lng ny. Trong trng hp nh vy, ngi ta

p dng phng php chun nhiu ln v tin hnh nh sau:

o t li im 0 ca cm bin: i lng cn o v i lng u ra c gi trtng ng vi im gc, m=0 v s=0.


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Trang 4

o o gi tr u ra theo mt lot gi tr tng dn n gi tr cc i ca i lngo u vo.

o Lp li qu trnh o vi cc gi tr gim dn t gi tr cc i.Khi chun nhiu ln cho php xc nh ng cong chun theo c hai hng o

tng dn v o gim dn.

1.3 Cc c trng c bn ca cm bin1.3.1 nhy ca cm bin

a. Khi nimi vi cm bin tuyn tnh, gia bin thin u ra s v bin thin u vo m

c s lin h tuyn tnh:

mSS = . (1.3)

m

SS

= : gi l nhy ca cm bin. (1.4)

Trng hp tng qut, biu thc xc nh nhy S ca cm bin xung quanh

gi tr mica i lng o xc nh bi t s gia bin thin s ca i lng u rav bin thin m tng ng ca i lng o u vo quanh gi tr :

immm

SS

=

= (1.5)

b. Lu php o t chnh xc cao, khi thit k v s dng cm bin cn thc hin

cho nhy S ca n khng i, ngha l t ph thuc nht vo cc yu t:

o Gi tr ca i lng cn o m v tn s thay i ca n.o Thi gian s dng.o nh hng ca cc i lng vt l khc (khng phi l i lng o) ca mi

trng xung quanh.

Thng thng nh sn xut cung cp gi tr ca nhy S tng ng vi nhng

iu kin lm vic nht nh ca cm bin.


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Trang 5

1.3.2 tuyn tnh ca cm bina. Khi nim

Mt cm bin c gi l tuyn tnh trong mt di o xc nh nu trong di ch

c nhy khng ph thuc vo i lng o. Nu cm bin khng tuyn tnh,

ngi ta a vo mch o cc thit b hiu chnh sao cho tn hiu in nhn c

u ra t l vi s thay i ca i lng o u vo. S hiu chnh c gi l s

tuyn tnh ho.

b. ng thng tt nhtKhi chun cm bin, t kt qu thc nghim ta nhn c mt lot im tng

ng (si,mi) ca i lng u ra v i lng u vo. V mt l thuyt, i vi cc

cm bin tuyn tnh, ng cong chun l mt ng thng. Tuy nhin, do sai s khi

o, cc im chun (mi, si) nhn c bng thc nghim thng khng nm trn cng

mt ng thng.

ng thng c xy dng trn c s cc s liu thc nghim sao cho sai s l

b nht, biu din s tuyn tnh ca cm bin c gi l ng thng tt nht.

Phng trnh biu din ng thng tt nht c lp bng phng php bnh

phng b nht. Gi s khi chun cm bin ta tin hnh vi N im o, phng trnh

c dng:

bmaS += . (1.6)

vi( )

= 22

...

ii

iiii

mmN

msmsNa ;

( )

= 22

2 ...

ii

iiiii

mmN

msmmsb (1.7)

c. lch tuyn tnhi vi cc cm bin khng hon ton tuyn tnh, ngi ta a ra khi nim

lch tuyn tnh, xc nh bi lch cc i gia ng cong chun v ng thng

tt nht, tnh bng % trong di o.

1.3.3 Sai s v chnh xc ca cm bina.

Sai s


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Trang 6

Cc b cm bin cng nh cc dng c o lng khc, ngoi i lng cn o

(cm nhn) cn chu tc ng ca nhiu i lng vt l khc gy nn sai s gia gi

tr o c v gi tr thc ca i lng cn o. Gi x l lch tuyt i gia gi tr

o v gi tr thc x (sai s tuyt i), sai s tng i ca b cm bin c tnh bng:

100x

x= [%] (1.8)

b. Sai s ca b cm bin mang tnh cht c tnhBi v khng th bit chnh xc gi tr thc ca i lng cn o. Khi nh gi

sai s ca cm bin, ngi ta thng phn chng thnh hai loi: Sai s h thng v sai

s ngu nhin.

Sai s h thng:L sai s khng ph thuc vo s ln o, c gi tr khng i

hoc thay i chm theo thi gian o v thm vo mt lch khng i gia gi tr

thc v gi tr o c. Sai s h thng thng do s thiu hiu bit v h o, do iu

kin s dng khng tt gy ra

Sai s ngu nhin:L sai s xut hin c ln v chiu khng xc nh. Ta c

th d on c mt s nguyn nhn gy ra sai s ngu nhin nhng khng th d

on c ln v du ca n. Nhng nguyn nhn gy ra sai s ngu nhin c thl:

o Do s thay i c tnh ca thit b.o Do tn hiu nhiu ngu nhin.o Do cc i lng nh hng khng c tnh n khi chun cm bin.

1.3.4 nhanh v thi gian p ng ca cm bina. nhanh ca cm bin

nhanh:L c trng ca cm bin cho php nh gi kh nng theo kp v

thi gian ca i lng u ra khi i lng u vo bin thin. Thi gian p ng l

i lng c s dng xc nh gi tr s ca nhanh.


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Trang 7

nhanh tr:L khong thi gian t khi i lng o thay i t ngt n khi

bin thin ca i lng u ra ch cn khc gi tr cui cng mt lng gii hn tnh

bng %.

b. Thi gian p ng tng ng ca cm binThi gian p ng tng ng vi % xc nh khong thi gian cn thit phi

ch i sau khi c s bin thin ca i lng o ly gi tr ca u ra vi chnh

xc nh trc. Thi gian p ng c trng cho ch qu ca cm bin v l

hm ca cc thng s thi gian xc nh ch ny.

o Trong trng hp s thay i ca i lng o: Trong trng hp s thay ica i lng o c dng bc thang, cc thng s thi gian gm thi gian tr khitng (tdm) v thi gian tng (tm) ng vi s tng t ngt ca i lng o hoc

thi gian tr khi gim (tdc) v thi gian gim (tc) ng vi s gim t ngt ca i

lng o. Hnh 1.3

o Khong thi gian tr khi tng tdm: L thi gian cn thit i lng u ra tngt gi tr ban u ca n n 10% ca bin thin tng cng ca i lng ny v

khong thi gian tng tm l thi gian cn thit i lng u ra tng t 10%n 90% bin thin bin thin tng cng ca n.

o Thi gian tr khi gim tdc: Thi gian tr khi gim tdcl thi gian cn thit ilng u ra gim t 10% n 90% bin thin bin thin tng cng ca n.

Hnh 1.3 Cc khong thi gian c trng cho ch qu .


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Trang 8

1.3.5 Gii hn s dng ca cm binTrong qu trnh s dng, cc cm bin lun chu tc ng ca ng lc c hc, tc

ng nhit... Khi cc tc ng ny vt qu ngng cho php, chng s lm thay i

c trng lm vic ca cm bin. Bi vy khi s dng cm bin, ngi s dng cn

phi bit r cc gii hn ny.

o Vng lm vic nh danh: Vng lm vic nh danh tng ng vi nhng iukin s dng bnh thng ca cm bin. Gii hn ca vng l cc gi tr ngng

m cc i lng o, cc i lng vt l c lin quan n i lng o hoc cc

i lng nh hng c th thng xuyn t ti m khng lm thay i cc c

trng lm vic nh danh ca cm bin.o Vng khng gy nn h hng: Vng khng gy nn h hng l vng m khi cc

i lng o hoc cc i lng vt l c lin quan nhau v cc i lng nh

hng vt qua ngng ca vng lm vic nh danh nhng vn cn nm trong

phm vi khng gy nn h hng.

Cc c trng ca cm bin c th b thay i nhng nhng thay i ny

mang tnh thun nghch, tc l khi tr v vng lm vic nh danh cc c trngca cm bin ly li gi tr ban u ca chng.

o Vng khng ph hy: Vng khng ph hy l vng khi m cc i lng o hoccc i lng vt l c lin quan v cc i lng nh hng vt qua ngng

ca vng khng gy nn h hng nhng vn cn nm trong phm vi khng b

ph hy, cc c trng ca cm bin b thay i v nhng thay i ny mang tnh

khng thun nghch, tc l khi tr v vng lm vic nh danh cc c trng ca

cm bin khng th ly li gi tr ban u ca chng. Trong trng hp ny cm

bin vn cn s dng c, nhng phi tin hnh chun li cm bin.

1.4 Nguyn l ch to ca cm binCc cm bin c ch to da trn c s cc hin tng vt l v c phn

lm hai loi:


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Trang 9

o Cm bin tch cc:L cc cm bin hot ng nh mt my pht, p ng (s) lin tch, in p hay dng.

o Cm bin th ng:L cc cm bin hot ng nh mt tr khng trong png (s) l in tr, t cm hoc in dung.

Cc cm bin tch cc c ch to da trn c s ng dng cc hiu ng

vt l bin i mt dng nng lng no (nhit, c hoc bc x) thnh nng

lng in. Sau y m t mt cch khi qut ng dng mt s hiu ng vt l

khi ch to cm bin

1.4.1 Hiu ng nhit inHiu ng nhit ng dng trong ch to cc cm bin nhit . Hai dy dn (M 1)

v (M2) c bn cht ho hc khc nhau c hn li vi nhau thnh mt mch in

kn, nu nhit hai mi hn l T1v T2khc nhau, khi trong mch xut hin

mt sut in ng e(T1, T2) m ln ca n ph thuc vo s chnh lch nhit

gia T1v T2.

Hnh 1.4 Hiu ng nhit in.

Hiu ng nhit in c ng dng o nhit T1khi bit trc nhit T2,

thng chn T2= 0oC.

1.4.2 Hiu ng ha nhitMt s tinh th gi l tinh th ho in (v d tinh th sulfate triglycine) c tnh

phn cc in t pht vi phn cc ph thuc vo nhit , lm xut hin trn cc

mt i din ca chng nhng in tch tri du. ln ca in p gia hai mt ph

thuc vo phn cc ca tinh th ho in.


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Trang 11

Hiu ng cm ng in t c ng dng xc nh tc dch chuyn ca vt

thng qua vic o sut in ng cm ng.

1.4.5 Hiu ng quang inHiu ng quang dn: (hay cn gi l hiu ng quang in ni) l hin tng gii

phng ra cc ht dn t do trong vt liu (thng l bn dn) khi chiu vo chng mt

bc x nh sng (hoc bc x in t ni chung) c bc sng nh hn mt ngng

nht nh.

Hiu ng quang pht x in t: (hay cn gi l hiu ng quang in ngoi) l

hin tng cc in t c gii phng v thot khi b mt vt liu to thnh dng c

th thu li nh tc dng ca in trng.

1.4.6 Hiu ng quang-in-tKhi tc dng mt t trng B vung gc vi bc x nh sng, trong vt liu bn

dn c chiu sng s xut hin mt hiu in th theo hng vung gc vi t

trng B v hng bc x nh sng.

Hnh 1.8 Hiu ng quang- in t.

1.4.7 Hiu ng HallHiu ng Hall c ng dng xc nh v tr ca mt vt chuyn ng. Vt

cn xc nh v tr lin kt c hc vi thanh nam chm, mi thi im, v tr thanh

nam chm xc nh gi tr ca t trng B v gc tng ng vi tm bn dn mng

lm vt trung gian. V vy, hiu in th VHo c gia hai cnh tm bn dn l hm

ph thuc vo v tr ca vt trong khng gian.


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Trang 12

Hnh 1.9 Hiu ng Hall.

oNguyn ch to cm bin th ng: Cm bin th ng thng c ch to tmt tr khng c cc thng s ch yu nhy vi i lng cn o. Gi tr ca tr

khng ph thuc kch thc hnh hc, tnh cht in ca vt liu ch to (nh

in tr sut , t thm , hng s in mi ). V vy tc ng ca i lng

o c th nh hng ring bit n kch thc hnh hc, tnh cht in hoc ng

thi c hai.

o S thay i thng s hnh hc: S thay i thng s hnh hc ca tr khng gyra do chuyn ng ca phn t chuyn ng hoc phn t bin dng ca cm

bin. Trong cc cm bin c phn t chuyn ng, mi v tr ca phn t ng s

ng vi mt gi tr xc nh ca tr khng, cho nn o tr khng c th xc nh

c v tr ca i tng.

o Phn t bin dng ca cm bin:Trong cm bin c phn t bin dng, s bindng ca phn t bin dng di tc ng ca i lng o (lc hoc cc i

lng gy ra lc) gy ra s thay i ca tr khng ca cm bin. S thay i trkhng do bin dng lin quan n lc tc ng, do lin quan n i lng

cn o. Xc nh tr khng ta c th xc nh c i lng cn o. Cc i

lng cn o c kh nng lm thay i tnh cht in ca vt liu s dng ch to

cm bin:

i lng cn o c trng nhy cm Loi vt liu s dng

Nhit Kim loi (Pt, Ni, Cu)


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Trang 13

Bn dn

Bc x nh sng Bn dn

Bin dng

T thm ()

Hp kim Ni, Si pha tp

Hp kim st t

V tr (nam chm) Vt liu t in tr: Bi,InSb

Hnh 1.10 S chuyn i gia cc i lng ca cm bin

1.5

Phn loi cm binCm bin c phn loi theo nhiu tiu ch khc nhau. Ty theo cch chn

m ngi ta c th phn loi cm bin theo cc cch sau:

a. Theo nguyn l chuyn i gia p ng v kch thch.Hin tng Chuyn i gia p ng v kch thch


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Trang 14

Vt l

Nhit in.

Quang in

Quang t.

in tT in

Ha hc

Bin i ha hc

Bin i in ha

Phn tch ph

Sinh hc

Bin i sinh ha

Bin i vt lHiu ng trn c th sng

b. Theo dng kch thch.Kch thch Cc c tnh ca kch thch

m thanh

Bin pha, phn cc

Ph

Tc truyn sng

in

in tch, dng in

in th, in p

in trng

in dn, hng s in mi

T

T trng

T thng, cng t trng.

t thm

C

V tr

Lc, p sut

Gia tc, vn tc, ng sut, cng

M men

Khi lng, t trng

nht


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Trang 15

Quang

Ph

Tc truyn

H s pht x, khc x

Nhit

Nhit

Thng lng

T nhit

Bc xKiu: Nng lng

Kiu: Cng

c. Theo tnh nng. nhy chnh xc phn gii tuyn tnh Cng sut tiu th

d. Theo phm vi s dng Cm bin cng nghip Cm bin nghin cu khoa hc Cm bin mi trng, kh tng Cm bin thng tin, vin thng

Cm bin nng nghip Cm bin dn dng Cm bin giao thng vn ti

e. Theo thng s ca m hnh mch in thay tho Cm bin tch cc (c ngun): u ra l ngun p hoc ngun dng.


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o Cm bin thcn c thm ng

tch cc th kh

trng bng cc

f. Cc loi cm bin tho Cm bin timo Cm bin quano Cm bin muo Cm bin thngo Cm bin si qo Cm bin lc co Cm bin nhit.o Cm bin v tro Cm bin lu lo Cm bin siu

g. K hiu cc loi c

Hnh 1

Chng 1: C

Trang 16

ng (khng c ngun): Cm bin gi l

un nng lng ph hon tt nhim

ng cn thm ngun nng lng. Cm bi

hng s: R, L, Ctuyn tnh hoc phi tu

ng s dng trong cng nghip v dn

n.

.

c.

minh.

ang.

ng p sut.

dch chuyn.

ng.

m.

bin

.11 K hiu cc loi cm bin thng d

c Khi Nim Chung

th ng khi chng

o kim, cn loi

th ng c c

n.

dng

g


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Trang 17

Cm bin c k hiu nh hnh 1.11, ty theo loi cm bin m trong k hiu s

km thm cc k hiu ring c trng cho loi cm bin nh cun dy cho cm

bin in cm, t cho cm bin in dung

V d: Hnh 1.12 l k hiu ca cm bin in dung, ng ra dng ON/OFF loi thng

h, chuyn mch dng.

Hnh 1.12 K hiu cm bin in dung

1.6 Mch o cm bin1.6.1S mch o

Mch o bao gm ton b thit b o (trong c cm bin) cho php xc

nh chnh xc gi tr ca i lng cn o trong nhng iu kin tt nht.

o u vo ca mch, cm bin chu tc ng ca i lng cn o gy nntn hiu in mang theo thng tin v i cn o.

o u ra ca mch, tn hiu in qua x l c chuyn i sang dng cth c c trc tip gi tr cn tm ca i lng o. Vic chun h o m

bo cho mi gi tr ca ch th u ra tng ng vi mt gi tr ca i lng

o tc ng u vo ca mch.

Hnh 1.13 S mch o nhit bng cp nhit


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Trang 18

Trn thc t, do cc yu cu khc nhau khi o, mch o thng gm nhiu

thnh phn trong c cc khi ti u ho vic thu thp v x l d liu,

chng hn mch tuyn tnh ho tn hiu nhn t cm bin, mch kh in dung

k sinh, cc b chuyn i nhiu knh, b khuch i, b so pha lc nhiu, bchuyn i tng t - s, b vi x l, cc thit b h tr...

Hnh 1.14 biu din s khi mt mch in o in th trn b mt mng

nhy quang c lp rp t nhiu phn t.

Hnh 1.14 Mch o in th b mt

1.6.2Mt s phn t c bn ca mch oa.B khuch i thut ton (KTT)

B khuch i thut ton mch tch hp l b khuch i dng mt chiu c

hai u vo v mt u ra chung, thng gm nhiu transistor v cc in tr, t

in ghp ni vi nhau. u vo ca mch, cm bin chu tc ng ca i

lng cn o gy nn tn hiu in mang theo thng tin v i cn o.

Hnh 1.15 S mch khuch i thut ton

1) My pht chc nng

2) Cm bin in tch

3) Tin khuch i

4) So pha lc nhiu

5) Khuch i

6) Chuyn i tng t s

7) My tnh


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Trang 19

b.B khuch i o lng IAB khuch i o lng IAc hai u vo v mt u ra. Tn hiu u ra t

l vi hiu ca hai in p u vo:

UAUUAUra == + )( (1.9)

Hnh 1.16 S mch khuch i o lng gm 3 b KTT ghp in tr

c.Kh in p lcho i vi mt b khuch KTT l tng khi h mch phi c in p ra bng

khng khi hai u vo ni mass.

o Thc t v cc in p bn trong nn to ra mt in p nh (in p phncc) u vo KTT c vi mV, nhng khi s dng mch kn in p ny

c khuch i v to nn in p kh ln u ra. kh in p lch c

th s dng s hnh nh sau, bng cch iu chnh bin tr R3.


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Trang 20

Hnh 1.17 S mch kh in p lch

d.Mch cuo Cu Wheatstone thng c s dng trong cc mch o nhit , lc, p

sut, t trng... Cu gm bn in tr R1, R2, R3 c nh v R4 thay i

(mc nh hnh 1.18) hot ng nh cu khng cn bng da trn vic pht

hin in p qua ng cho ca cu.

o u ra ca mch, tn hiu in qua x l c chuyn i sang dng cth c c trc tip gi tr cn tm ca i lng o. Vic chun h o m

bo cho mi gi tr ca ch th u ra tng ng vi mt gi tr ca i lng

o tc ng u vo ca mch.

Hnh 1.18 S mch cuWheatstone

02_chuong 1_cac khai niem chung

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