07_chuong 6_cam bien nhiet do
TRANSCRIPT
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Chng 6: Cm bin Nhit
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Chng 6
CM BIN NHIT (Temperature Sensors)
6.1 Khi nim
Trong tt c cc i lng vt l, nhit l mt trong cc i lng c
quan tm nhiu nht v nhit ng vai tr quyt nh n nhiu tnh cht ca
vt cht. Nhit c th lm nh hng n cc i lng chu tc dng ca n.
Th d nh p sut, th tch ca cht kh. Bi vy trong cng nghip cng nh
i sng hng ngy phi o nhit . Dng c o nhit n gin nht l nhit
k s dng hin tng gin n nhit. ch to cc b cm bin nhit ngi
ta s dng nhiu nguyn l cm bin khc nhau nh:
o Phng php quang da trn s phn b ph bc x nhit do dao ng nhit
(hiu ng Doppler).
o Phng php da trn s gin n ca vt rn, cht lng hoc cht kh (vi p
sut khng i) hoc da trn tc m.
o Phng php in da trn s ph thuc ca cc in tr vo nhit .
o c tr s chnh xc ca nhit l vn khng n gin. Nhit
l i lng ch c th o gin tip trn c s tnh cht ca vt ph thuc vo
nhit . Trc khi o nhit ta cn cp n cc c tnh ca cm bin nhit
.
6.2 Cc c tnh ca cm bin nhit
a. Thang o nhit
Vic xc nh thang nhit xut pht t cc nh lut nhit ng hc.
Thang o nhit tuyt i c xc nh da trn tnh cht ca kh l tng.
nh lut Carnot nu r: Hiu sut ca mt ng c nhit thun nghch hot
ng gia 2 ngun c nhit 1 v 2 trong mt thang o bt k ch ph thuc
vo 1 v 2:
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Chng 6: Cm bin Nhit
Trang 180
)(F)(F
2
1
= (6.1)
Dng ca hm F ch ph thuc vo thang o nhit . Ngc li, vic la
chn hm F s quyt nh thang o nhit . t F() = T chng ta s xc nh T
nh l nhit nhit ng hc tuyt i v hiu sut ca ng c nhit thun
nghch s c vit nh sau:
2
1
TT
1= (6.2)
Trong : T1 v T2 l nhit nhit ng hc tuyt i ca hai ngun.
b. Thang Kelvin
Nm 1664 Robert Hook thit lp im khng l im ng ca nc ct.
Thomson (Kelvin) nh vt l Anh, nm 1852 xc nh thang nhit . Thang
Kelvin n v l 0K, ngi ta gn cho nhit ca im cn bng ca 3 trng thi
nc nc hi mt tr s bng 273,15 0K.
c. Thang Celsius
Nm 1742 Andreas Celsius l nh vt l Thy in a ra thang nhit
bch phn. Trong thang ny n v o nhit l 0C, mt Celsius bng mt
Kelvin. Quan h gia nhit Celsius v nhit Kelvin c xc nh bng
biu thc:
T(0C) = T(0K) 273,15 (6.3)
d. Thang Fahrenheit
Nm 1706 Fahrenheit nh vt l H Lan a ra thang nhit c im
nc tan l 320 v si 2120. n v nhit l Fahrenheit (0F). Quan h gia
nhit Celsius v Fahrenheit c cho theo biu thc:
{ }95
32)F(T)C(T 00 = (6.4)
32)C(T59
)F(T 00 += (6.5)
Bng thng s c trng ca mt s thang o nhit khc nhau:
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Chng 6: Cm bin Nhit
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Nhit Kelvin (0K) Celsius (0C) Fahrenheit (0F)
im 0 tuyt i 0 -273,15 -459,67
Hn hp nc nc 273,15 0 32
Cn bng nc nc hi nc 273,16 0,01 32,018
Nc si 373,15 100 212
Nhit o c
Nhit o c nh mt in tr hay mt cp nhit, chnh bng nhit
ca cm bin v k hiu l TC. N ph thuc vo nhit mi trng k hiu l
TX v vo s trao i nhit trong . Nhim v ca ngi thc nghim l lm
th no gim hiu s TX TC xung nh nht. C hai bin php gim s
khc bit gia TX v TC:
o Tng trao i nhit gia cm bin v mi trng o.
o Gim trao i nhit gia cm bin v mi trng bn ngoi.
o nhit trong lng vt rn
Thng thng cm bin c trang b mt lp v bc bn ngoi. o
nhit ca mt vt rn bng cm bin nhit , t b mt ca vt ngi ta
khoan mt l nh ng knh bng r v su bng L. L ny dng a cm
bin vo su trong cht rn. tng chnh xc ca kt qu phi m bo hai
iu kin:
o Chiu su ca l khoan phi bng hoc ln hn gp 10 ln ng knh ca
n (L 10r).
o Gim tr khng nhit gia vt rn v cm bin bng cch gim khong cch
gia v cm bin v thnh l khoan. Khong cch gia v cm bin v thnh
l khoan phi c lp y bng mt vt liu dn nhit tt.
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6.3 Nhit in tr vi
in tr ca kim loi thay i theo nhit .
dao ng nhit quanh cc v tr cn bng ca chng, c
ng t do trong khong khng gian gia cc ion b
Khi nhit cng cao
cc ion cng tng dn ti
nng va chm gia ion d
vi cc ion dng nm mt trt t trong mng tinh th l
lm cn tr s di chuyn ca io
Vy in tr kim lo
6.3.1 in tr kim loi thay i theo nhit
S chuyn ng ca cc ht mang in tch theo mt h
dng in trong kim loi. S chuyn ng n
trng gy nn v i
nhau. dn in c
kim loi c h s nhit d
Trong hnh 6.2
Nh th in tr kim loi c h s nhit in tr d
Chng 6: C
Trang 182
t in tr vi Platin v Nickel
n tr ca kim loi thay i theo nhit . nhit bnh th
ng nhit quanh cc v tr cn bng ca chng, cn cc electron th c th
ng t do trong khong khng gian gia cc ion bn trong vt th kim loi.
Hnh 6.1 Cu trc mng tinh th ca kim loi
ng cao lm cho cc ion dao ng cng mnh
n ti vn tc chuyn ng nhit ca electron tng
m gia ion dng vi electron cng ln. Chnh s va chm gia electron
m mt trt t trong mng tinh th l nguyn nhn gy ra i
n tr s di chuyn ca ion. Hnh 6.1
kim loi s tng ln khi nhit tng.
n tr kim loi thay i theo nhit
chuyn ng ca cc ht mang in tch theo mt h
n trong kim loi. S chuyn ng ny c th do mt lc c
n v in tch c th l m hay dng dch chuyn vi chiu ng
ca kim loi thun t l nghch vi nhit hay in tr ca
i c h s nhit dng.
2 ta c cc c tuyn in tr ca cc kim loi theo nhit .
in tr kim loi c h s nhit in tr dng PTC (Positive
: Cm bin Nhit
nh thng, cc ion ch
n cc electron th c th chuyn
t th kim loi.
u trc mng tinh th ca kim loi.
nh, nn s mt trt t
n tc chuyn ng nhit ca electron tng lm tng kh
n. Chnh s va chm gia electron
nguyn nhn gy ra in tr
ng hnh thnh mt
do mt lc c hc hay in
ch chuyn vi chiu ngc
l nghch vi nhit hay in tr ca
c tuyn in tr ca cc kim loi theo nhit .
ng PTC (Positive
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Chng 6: Cm bin Nhit
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Temperature Coefficient): in tr kim loi tng khi nhit tng. hiu ng
ny c th s dng c trong vic o nhit , h s nhit cn phi ln. iu
c ngha l c s thay i in tr kh ln i vi nhit . Ngoi ra cc tnh
cht ca kim loi khng c thay i nhiu sau mt thi gian di. H s nhit
khng ph thuc vo nhit , p sut v khng b nh hng bi cc ha
cht. Gia nhit v in tr thng khng c s tuyn tnh, n c din t
bi mt biu thc a cp cao:
R(t) = R0 (1 + A.t + B.t2 + C.t3 +) (6.6)
Hnh 6.2 Cc c tuyn in tr ca cc kim loi theo nhit .
o R0: in tr c xc nh mt nhit nht nh.
o t2, t3: Cc phn t c ch nhiu hay t ty theo yu cu chnh xc ca
php o.
o A, B, C: Cc h s ty theo vt liu kim loi v din t s lin h gia nhit
v in tr mt cch r rng.
Thng thng c tnh ca nhit in tr c th hin ch mt h s , n
thay th cho h s nhit trung bnh trong thang o (v d t 00C n 1000C.)
0 200 400 600 800 Nhit
in tr St
ng
Than
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Chng 6: Cm bin Nhit
Trang 184
= (R100 - R0) / 100. R0 (C-1) (6.7)
6.3.2 Nhit in tr Platin
Platin l vt liu cho nhit in tr c dng rng ri trong cng nghip.
C 2 tiu chun i vi nhit in tr platin, s khc nhau gia chng nm
mc tinh khit ca vt liu. Hu ht cc quc gia s dng tiu chun quc t
DIN IEC751-1983 (c sa i ln th nht vo nm 1986, ln th 2 vo nm
1995), USA vn tip tc s dng tiu chun ring.
c 2 tiu chun u s dng phng trnh Callendar - Van Dusen:
R(t) = R0 (1 + A.t + B.t2 + C[t - 1000C].t3) (6.8)
R0 l tr s in tr nh mc 00C.
Standard [//C] R0 [] H s Nc sn sut
IEC751
(Pt100) 0.00385055 100
200C < t < 0C
A = 3.90830x10-3
B = -5.77500x10-7
C = -4.18301x10-12
0C < t < 850C
A &B nh trn,
ring
C = 0.0
c, o, B, Brazil,
Bulgaria, Canada, Cng
ha Czech, an mch, Ai
Cp, Phn Lan, Php, c,
Israel, , Nht, Ba Lan,
Rumania, Nam phi, Th
Nh K, Nga, Anh, USA
SAMA
RC-4 0.0039200 98.129
A= 3.97869x10-3
B = -5.86863x10-7
C = -4.16696x10-12
USA
R0 ca nhit in tr Pt 100 l 100, ca Pt 500 l 500, ca Pt 1000 l
1000. Cc loi Pt 500, Pt 1000 c h s nhit ln hn, do nhy ln
hn: in tr thay i mnh hn theo nhit . Ngoi ra cn c loi Pt 10 c
nhy km dng o nhit trn 6000C.
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Chng 6: Cm bin Nhit
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Tiu chun IEC751 ch nh ngha 2 Cp dung sai A, B. Trn thc t xut
hin thm loi C v D xem bng pha di. Cc tiu chun ny cng p dng cho
cc loi nhit in tr khc.
Cp dung sai Dung sai (C)
A t = (0.15 + 0.002.| t |)
B t = (0.30 + 0.005. | t |)
C t = (0.40 + 0.009. | t |)
D t = (0.60 + 0.0018. | t |)
Theo tiu chun DIN vt liu platin dng lm nhit in tr c pha tp. Do
khi b cc tp cht khc thm thu trong qu trnh s dng s thay i tr s
in ca n t hn so vi cc platin thun. Nh th c s n nh lu di theo thi
gian, thch hp hn trong cng nghip. Trong cng nghip nhit in tr platin
thng dng c ng knh 30m (so snh vi ng knh si tc khong
100m).
6.3.3 Nhit in tr nickel
Nhit in tr nickel so vi platin r tin hn v c h s nhit ln gn
gp hai ln (6,18.10-3 0C-1). Tuy nhin di o ch t -600C n +2500C, v trn
3500C nickel c s thay i v pha. Cm bin nickel 100 thng dng trong cng
nghip iu ha nhit phng.
R(t) = R0 (1 + A.t +B.t2 +D.t4 +F.t6) (6.9)
A = 5.485x10-3 B = 6.650x10-6 D = 2.805x10-11 F = -2.000x10-17.
Vi cc trng hp khng i hi s chnh xc cao ta s dng phng trnh
sau:
R(t) = R0 (1 + .t) (6.10)
= 0.00672 0C-1
T d dng chuyn i thnh gi tr nhit :
t = (Rt / R0 - 1) / = (Rt / R0 - 1) / 0.00672 (6.11)
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Chng 6: Cm bin Nhit
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Hnh 6.3 ng c tnh k thut ca cm bin ZNI1000
Cm bin nhit ZNI1000 do hng ZETEX Semiconductors sn xut s
dng nhit in tr Ni, c thit k c gi tr 1000 ti 00C.
6.4 Cm bin nhit vi vt liu bn dn silic
Cm bin nhit vi vt liu silic ang ngy cng ng vai tr quan trng trong
cc h thng in t. Vi cm bin silic, bn cnh c im tuyn tnh, s chnh xc,
ph tn thp, v c th c tch hp trong 1 IC cng vi b phn khuch i v cc
yu cu x l tn hiu khc. H thng tr nn nh gn, mc phc tp cao hn v
chy nhanh hn. K thut cm bin nhit truyn thng nh cp nhit in, nhit in
tr c c tuyn khng tuyn tnh v yu cu s iu chnh c th chuyn i chnh
xc t gi tr nhit sang i lng in (dng hay p) ang c thay th dn bi
cm bin silic vi li im l s nh gn ca mch in tch hp v d s dng.
6.4.1 Nguyn tc
Hnh 6.4 th hin cu trc c bn ca mt cm bin. kch thc ca cm bin l
500 x 500 x 200 m. Mt trn ca cm bin l mt lp SiO2 c mt vng hnh trn
c m kim loi c ng knh khong 20m, ton b mt y c m kim loi.
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Chng 6: Cm bin Nhit
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Hnh 6.4 Cu trc c bn ca mt cm bin.
Hnh 6.5 biu din mch in tng ng tng trng thay th cho cm bin
silic (sn xut theo nguyn tc in tr phn ri (spreading resistance)). S sp xp ny
dn n s phn b dng qua tinh th c dng hnh nn, y l ngun gc ca tn gi
in tr phn ri.
Hnh 6.5 Mch in tng ng cho cm bin silic.
o in tr cm bin nhit R c xc nh nh sau:
dR ./= (6.12)
R: in tr cm bin nhit.
: in tr sut ca vt liu silic ( l thuc vo nhit ).
d: ng knh ca hnh trn vng m kim loi mt trn.
Hnh 6.6 th hin loi kt cu th hai ca cm bin. Li im ca kiu kt cu
ny l in tr cm bin khng ph thuc vo chiu dng in. Tri li kiu kt cu
th nht, dnh cho dng in ln hn v nhit trn 1000C, s thay i in tr ca
cm bin nh.
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Chng 6: Cm bin Nhit
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Cm bin nhit silic vi nguyn tc in tr phn ri c h s nhit dng nh
trng hp cm bin nhit vi vt liu platin hay nickel. Hnh 6.6
Hnh 6.6 Kt cu gm hai cm bin mc ni tip nhng ngc cc tnh.
6.4.2 c trng k thut c bn ca dng cm bin KTY (hng Philips sn xut)
Vi s chnh xc v n nh lu di ca cm bin vi vt liu silic KTY s dng
cng ngh in tr phn ri l mt s thay th tt cho cc loi cm bin nhit
truyn thng. Cm bin KTY c cc u im chnh sau
o S n nh:
Gi thit cm bin lm vic nhit c gi tr bng mt na gi tr nhit
hot ng cc i, sau thi gian lm vic t nht l 450000 h (khong 51 nm), hoc
sau 1000 h (1,14 nm) hot ng lin tc vi dng nh mc ti gi tr nhit hot
ng cc i cm bin silic s cho kt qu o vi sai s nh bng.
Bng Sai s ca cm bin silic (do thi gian s dng)
TYPE Sai s tiu biu (K) Sai s ln nht (K)
KTY81-1
KTY82-1 0.20 0.50
KTY81-2
KTY82-2 0.20 0.80
KTY83 0.15 0.40
o S dng cng ngh silic:
Do cm bin c sn xut da trn nn tng cng ngh silic nn gin tip chng
ta s hng c li ch t nhng tin b trong lnh vc cng ngh ny, ng thi iu
ny cng gin tip mang li nhng nh hng ch cc cho cng ngh ng gi, ni
m lun c khuynh hng thu nh.
o S tuyn tnh:
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Chng 6: Cm bin Nhit
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Cm bin vi vt liu silic c h s gn nh l hng s trn ton b thang o.
c tnh ny l mt iu l tng khai thc, s dng. Xem hnh c trng k thut
ca KTY81 hnh 6.7.
Nhit hot ng ca cc cm bin silic thng thng b gii hn 1500C.
KTY 84 vi v bc SOD68 v cng ngh ni c bit gia dy dn v chip c th hot
ng n nhit 300 0C.
Hnh 6.7 c trng k thut ca KTY81.
6.4.3 Mch in tiu biu vi KTY81 hoc KTY82
Hnh 6.8 cho ta mt mch in in hnh c thit k cho cm bin KTY 81-
110 hoc KTY 82-110 (nhit o t 00C n 1000C). in tr R1, R2, cm bin v
cc nhnh in tr R3, chit p P1 v R4 to thnh mt mch cu.
Hnh 6.8 Mch o nhit s dng KTY81-110.
Gi tr R1 v R2 c chn sao cho gi tr dng in qua cm bin gn bng
1mA v tuyn tnh ha cm bin trong di nhit cn o. in p ng ra thay i
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Chng 6: Cm bin Nhit
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tuyn tnh t 0,2 VS n 0,6 VS (VS= 5 V th Vout thay i t 1V n 3V). Ta iu
chnh P1 Vout =1V ti 00C, ti 1000C iu chnh P2 Vout= 3V. Vi mch in ny
vic iu chnh P2 khng nh hng n vic chnh zero.
6.4.4 IC cm bin nhit
Nhiu cng ty trn th gii ch to IC bn dn o v hiu chnh nhit .
IC cm bin nhit l mch tch hp nhn tn hiu nhit chuyn thnh tn hiu
di dng in p hoc tn hiu dng in. Da vo cc c tnh rt nhy cm ca cc
bn dn vi nhit , to ra in p hoc dng in t l thun vi nhit tuyt i
C, F, K hay ty loi. o tn hiu in ta bit c nhit cn o. Tm o nhit
gii hn t -550C n 1500C, chnh xc t 10C n 20C ty theo tng loi.
S tch cc ca nhit s to ra in tch t do v cc l trng trong cht bn
dn bng s ph v cc phn t, bt cc electron thnh dng t do di chuyn qua cc
vng cu trc mng tinh th, to s xut hin cc l trng nhit lm cho t l in t t
do v cc l trng tng ln theo qui lut hm s m vi nhit . Kt qu ca hin
tng ny l di mc in p thun, dng thun ca mi ni p n trong diode hay
transistor s tng theo hm s m theo nhit .
Trong mch t hp, cm bin nhit thng l in p ca lp chuyn tip pn
trong mt transitor loi bipolar. Texinstruments c STP35 A/B/C; National
Semiconductor LM 35/4.5/50
Cm bin nhit LM 35/ 34 ca National Semiconductor
Hu ht cc cm bin nhit ph bin u kh s dng. Chng hn cp nhit
ngu c mc ng ra thp v yu cu b nhit, thermistor th khng tuyn tnh. Thm
vo ng ra ca cc loi cm bin ny khng tuyn tnh tng ng vi bt k thang
chia nhit no. Cc khi cm bin tch hp c ch to khc phc c nhng
nhc im . Nhng ng ra ca chng quan h vi thang chia Kelvin hn l
Celsius v Fahrenheit.
Loi LM35: Precision Centigrade Temperature Sensor.
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Chng 6: Cm bin Nhit
Trang 191
Vi loi LM35 ta c in p ng ra t l trc tip vi thang nhit Celsius
(thang bch phn). Nh th mt mch in b tr im zero ca thang Kelvin (thang
nhit tuyt i) khng cn cn thit nh mt s IC cm bin nhit khc.
c im:
in p hot ng: VS= 4V ti 30V
in p ng ra tuyn tnh: 10mV/0C
Thang o: -550C n1500C vi LM 35/35A,
-400C n1100C vi LM 35C/35CA
00C n1000C vi LM 35D
S t nung nng rt nh: 0,08 0C (trong mi trng khng kh)
Mc khng tuyn tnh ch 1/40C
Cm bin nhit AD 590 ca Analog Devices:
Cm bin AD 590 (Analog Devices) c thit k lm cm bin nhit c tng tr
ng ra kh ln (10 M ). Vi mch c cn bng bi nh sn xut, khin cho dng
mA ra tng ng vi chun nhit tuyt i K. in p lm vic cng nh cng tt
trnh hin tng t gia nhit. Khi cp in p thay i, dng in thay i rt t.
Thang o: -550C ti 1500C
in p hot ng: 4 ti 30 VDC
Dng in ra t l: 1 A/0K
6.4.5 Cc mch o nhit
Mch o nhit bng LM35:
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Chng 6: Cm bin Nhit
Trang 192
Hnh 6.9 Mch o nhit bng LM35.
Mch ng dng LM35 vi thit b khuch i m thanh:
Trong mch ng dng ny, nhit IC khuch i m thanh (IC1) l i
lng c quan tm. LM35 v IC1 c s gn kt v nhit. Tn hiu ng ra ca
b so snh s xung mc thp nu nhit vt qu gii hn (thng s ny c
chn bng R1, R2 v in p tham chiu). H thng c thit k qut hot
ng khi nhit vt qu khong gi tr 800C v tt qut khi nhit h xung
di 600C.
Hnh 6.10 LM35 vi b khuch i m thanh cng sut 60W.
6.5 Cch ni dy o
Nhit in tr thay i in tr theo nhit . Vi mt dng in khng
thay i qua nhit in tr, ta c in th o c U = R.I. cm bin khng b
nng ln qua php o, dng in cn phi nh khong 1mA. Vi Pt 100 0C ta
R2
V
5V
LM35
5V 12V
12V
R1
R3
R3 R4
R4
R5
R6 R7
VADJ
1k
1k
1k
1k 10k
10k
8,2k
1,8k
2,2k
-
c in th khong 0,1V. in th n
c cc k thut ni dy o.
Hnh
Tiu chun IEC 751 y
mu ging nhau ( hoc trng) v
6.5.1 K thut hai dy
Gia nhit in tr v
in no u c in tr, in tr n
tr ca hai dy o, mch in tr s nhn
o. Kt qu ta c ch th nhit k cao h
xa, in tr dy o c th l
V d vi dy ng:
Din tch mt ct dy o:
in tr sut: 0,0017
Chiu di: 100m
Chng 6: C
Trang 193
n th khong 0,1V. in th ny cn c a n my o qua dy o. Ta
thut ni dy o.
Hnh 6.11 Cch ni dy nhit in tr.
n IEC 751 yu cu dy ni n cng u nhit in tr phi c
ng nhau ( hoc trng) v dy ni n 2 u phi khc m
thut hai dy
Hnh 6.12 K thut 2 dy.
a nhit in tr v mch in t c ni bi hai dy. Bt c dy dn
u c in tr, in tr ny ni ni tip vi nhit in tr. Vi hai in
ca hai dy o, mch in tr s nhn c mt in th cao h
t qu ta c ch th nhit k cao hn nhit cn o. Nu khong cch qu
dy o c th ln n vi Ohm
vi dy ng:
n tch mt ct dy o: 0,5mm2
0,0017mm2m-1
100m
: Cm bin Nhit
n my o qua dy o. Ta
u nhit in tr phi c
i n 2 u phi khc mu.
c ni bi hai dy. Bt c dy dn
i ni tip vi nhit in tr. Vi hai in
c mt in th cao hn in th cn
t cn o. Nu khong cch qu
-
R = 6,8, vi 6,8
l 170C. trnh sai s ca php o do in tr ca dy o gy ra, ng
b tr in tr ca dy o bng mt mch in nh
ni vo mt trong hai dy o v
100. Mch in t
chnh chit p sao cho c ch
6.5.2 K thut 3 dy
T nhit in tr ca dy o
ny ta c hai mch o
chun. Vi k thut 3 dy, sai s cho php o do in tr dy o v
ca n do nhit khng c
v c cng mt nhit . K
6.5.3 K thut 4 dy
Vi k thut 4 dy ta
dng cho dng i
Chng 6: C
Trang 194
i 6,8 tng ng cho nhit in tr Pt 100 mt thay i nhit
trnh sai s ca php o do in tr ca dy o gy ra, ng
in tr ca dy o bng mt mch in nh sau: Mt
t trong hai dy o v nhit in tr c thay th bng mt in tr
ch in t c thit k vi in tr d phng ca dy o l
sao cho c ch th 00C: Chit p v in tr ca dy o l
Hnh 6.13 K thut 3 dy.
nhit in tr ca dy o c ni thm (hnh 6.13
ch o c hnh thnh, mt trong hai mch
n. Vi k thut 3 dy, sai s cho php o do in tr dy o v
a n do nhit khng cn na. Tuy nhin 3 dy o cn c c
t nhit . K thut 3 dy rt ph bin.
Hnh 6.14 K thut 4 dy.
i k thut 4 dy ta thu c kt qu o tt nht. Hai dy
in khng i qua nhit in tr. Hai dy
: Cm bin Nhit
ng cho nhit in tr Pt 100 mt thay i nhit
trnh sai s ca php o do in tr ca dy o gy ra, ngi ta
t chit p b tr c
c thay th bng mt in tr
a dy o l 10. Ta
n tr ca dy o l 10.
6.13). Vi cch ni dy
t trong hai mch c dng lm mch
n. Vi k thut 3 dy, sai s cho php o do in tr dy o v s thay i
n c cng tr s k thut
t qu o tt nht. Hai dy dng 1 c
n khng i qua nhit in tr. Hai dy dng 2 c dng
-
Chng 6: Cm bin Nhit
Trang 195
lm dy o in th trn nhit in tr. Trng hp tng tr ng vo ca mch
o rt ln so vi in tr dy o, in tr dy o coi nh khng ng k. in
th o c khng b nh hng bi in tr dy o v s thay i ca n do
nhit.
6.5.4 K thut 2 dy vi b bin i tn hiu o
Ngi ta vn c th dng hai dy o m khng b sai s cho php o vi b
bin i tn hiu o. B bin i tn hiu o bin i tn hiu ca cm bin thnh
mt dng in chun, tuyn tnh so vi nhit c cng t 4mA 20mA.
Dng in nui cho b bin i c ti qua hai dy o vi cng khong
4mA. Vi k thut ny tn hiu c khuch i trc khi truyn ti do khng
b nhiu nhiu.
o Nhit in tr vi k thut dy qun:
Nhit in tr vi v gm: Si platin c gi cht trong ng gm s vi
bt oxit nhm. Di o t -2000C n 8000C.
Nhit in tr vi v thy tinh: loi ny c bn c hc v nhy cao.
Di o t - 2000C n 4000C, c dng trong mi trng ha cht c n mn
ha hc cao.
Nhit in tr vi v nha: Gia 2 lp nha polyamid dy platin c ng
knh khong 30mm c dn kn. Vi cu trc mng, cm bin ny c dng
o nhit b mt cc ng hay cun dy bin th. Di o t -800C n 2300C.
Hnh 6.15 Cu trc nhit in tr kim loi dy qun (v ceramic).
o Nhit in tr vi k thut mng mng:
-
Chng 6: Cm bin Nhit
Trang 196
Cu trc cm bin gm mt lp mng mng (platin) t trn nn ceramic
hoc thy tinh. Tia lazer c s dng chun ha gi tr in tr ca nhit
in tr.
Hnh 6.16 Cu trc nhit in tr kim loi dng mng mng ( ceramic).
6.6 Phng php o nhit
C hai phng pht o nhit :
Phng php trc tip:
o Cp nhit in (Thermocouple)
o Nhit in tr kim loi (RTD)
o Nhit in tr bn dn.
o nhit gin tip:
o Phng php quang (Optical Pyrmeter)
o Phng php bc x in t (Photon Detector)
6.6.1 o trc tip
o Trng hp chn la o trc tip
Khi tm nhit cn o khng qu cao
Thng dng o cc vt tnh (khng di chuyn)
Vt cn o khng c yu cu cao v vic thay i cu trc, khi b gy nh
hng nhiu khi thc hin php o.
Phng php o khng qu phc tp d thc hin.
Gi thnh cho cc linh kin (nh cm bin, khi gia cng,) l khng cao
o Tm nhit o trong phng php o trc tip
c s dng tm o t: -190oC n 1063oC
-
Chng 6: Cm bin Nhit
Trang 197
Vi:
-190oC: l nhit m Oxy chuyn t th LNG th HI (Kh)
1063oC: l nhit nng chy ca vng (Au)
o Cu to:
Hnh 6.17 Cu to b o nhit phng php o trc tip.
Phng php o trc tip thng dng loi cm bin nhit platin v nickel
o Nguyn l hot ng:
in tr vt thay i theo nhit :
)1( TA
lR
o
+=
= (6.13)
L : chiu di dy dn
A: din tch tit din dy dn
: in tr sut
Khi nhit tng in tr tng
S ph thuc ny gn nh tuyn tnh:
( )TRRT += *10 (6.14)
( ) ( )[ ]000 1 TTbTTaRR += (6.15)
a,b,: hng s ph thuc kim loai
-
RT: in tr ti nhit cn o
Ro: in tr ti nhit T
a. Cp nhit in (Thermocouple)
Thermocouple (c
loi khc nhau c tip
, u ra in p.
Thermocouple ho
khm ph nm 1821)
Cu to Thermocouple
Chng 6: C
Trang 198
n tr ti nhit cn o
n tr ti nhit To
p nhit in (Thermocouple)
Thermocouple (cp nhit ) l cm bin nhit cu to bi 2 thanh kim
c tip xc nhit vi nhau. Cm bin tc ng: u v
Hnh 6.18 Nguyn l vn hnh cp nhit in
Thermocouple hot ng da trn hiu ng Seebeck. (Do Thomas Seebeck
khm ph nm 1821)
u to Thermocouple
Hnh 6.19 Cu to cp nhit in.
: Cm bin Nhit
m bin nhit cu to bi 2 thanh kim
m bin tc ng: u vo nhit
p nhit in.
(Do Thomas Seebeck
-
Nguyn l o:
o Thermocouple ch
chun so snh.
( 21 TCV =
o Quan h gia V vi T l
1 eeV +=
(/
/
TeV
ee
AB
BBA
=
=
o Hai u ni T1, T2. u nng l
li l u lnh hay u chun.
o o nhit chnh xc th
nh nhit chun n
c tuyn
Chng 6: C
Trang 199
Hnh 6.20 Nguyn l o cp nhit in
Thermocouple ch o c s sai bit nhit . Do cn phi c nhit
n so snh.
)(2) 212212 TTCT +
gia V vi T l phi tuyn. Nhng ta chp nhn cng thc
1/2/2 TeTee BAAB +=
)12
/
TTAB
u ni T1, T2. u nng l u tip xc vi nhit cn o. u c
u lnh hay u chun.
o nhit chnh xc th phi o c nhit u chun v
chun ny
Hnh 6.21 c tuyn cp nhit in.
: Cm bin Nhit
n.
c s sai bit nhit . Do cn phi c nhit
(6.16)
p nhn cng thc sau:
(6.17)
(6.18)
u tip xc vi nhit cn o. u cn
c nhit u chun v phi gi c
.
-
Chng 6: Cm bin Nhit
Trang 200
Cc phng php xc nh v gi c nh nhit chun
o Cch 1: C nh nhit chun
Hnh 6.22 Cch c nh nhit chun dng 1.
Nhit o c tnh nh sau
))(( Re fTipBAout TTSSV = )(Re BAout
fTip SS
VTT
= (6.19)
SA, SB l h s Seebeck ph thuc vt liu ch to
BABA SSe =/ (6.20)
o Cch 2: Dng mch in t to nhit chun.
Hnh 6.23 Cch c nh nhit chun dng 2.
o RT xc nh Tref, Tref s c chuyn sang Vref
V1= V-Vref ; T1=V1/e1 (6.21)
S v nguyn tc o
-
o in p o c ch ph thuc v
m khng ph thuc v
o Nu c mt vt liu kim loi th
nguyn l ny b
(/
1/
TeE
TeE
AB
AB
=
=
Hnh 6.25 Nguyn t
Ch : Tt c cc dy kt ni v
dy ng ni! Dy ni cho T/C l
o Nu ta ni 2 kim loi A v
khng nh hng
Hnh 6.26
Chng 6: C
Trang 201
Hnh 6.24 Nguyn tc o cp nhit in
c ch ph thuc vo s chnh lch nhit gia hai u ni
thuc vo nhng ni khc trong mch
u c mt vt liu kim loi th 2 chn gia th p ra khng b
nguyn l ny bo m cho vic i dy di
)21
2///1
TT
TeTeTeT BAjACiCA
+++
Hnh 6.25 Nguyn tc o cp nhit in khi c vt th 2 ch
t c cc dy kt ni vo TC phi ng loi dy, Khng s dng
ng ni! Dy ni cho T/C l loi dy cng chu nhit cao t tin!
u ta ni 2 kim loi A v B bi mt kim loi th 3 C th
ng
Hnh 6.26 Nguyn tc o cp nhit in khi ta ni kim loi A v
: Cm bin Nhit
c o cp nhit in.
ch nhit gia hai u ni
p ra khng b nh hng
(6.22)
c o cp nhit in khi c vt th 2 chn gia.
i ng loi dy, Khng s dng
i dy cng chu nhit cao t tin!
i mt kim loi th 3 C th in p o cng
c o cp nhit in khi ta ni kim loi A v B.
-
(/1/
TeE
TeE
AB
CB
=
=
V: CBAB ee // =
o Khi o nhit m
Hnh 6.27 Nguyn t
1213 EE =
o Hoc ta c th o gin tip nh
Hnh 6.28 Nguyn t
acab EE =
Vt liu ch to
Thermocouple
chn theo 4 tiu ch sau:
o nhy
o n nh
o Tng thch vi h thng o
o Chi ph hp l
o nhy: nhy nhit
Chng 6: C
Trang 202
)212/1/1
TT
TeTeT BAAC
++
ACe /
t m cha bit nhit chun ta c th o gin tip nh
Hnh 6.27 Nguyn tc o cp nhit in khi cha bit
23E+
c ta c th o gin tip nh sau:
Hnh 6.28 Nguyn tc o cp nhit in gin tip
cbE+
Thermocouple c ch to bi cc kim loi khc nhau. Cc kim loi
u ch sau:
i h thng o
nhy: nhy nhit c xc nh bi cng thc
: Cm bin Nhit
(6.23)
(6.24)
t nhit chun ta c th o gin tip nh sau:
t nhit chun.
(6.25)
c o cp nhit in gin tip
(6.26)
c ch to bi cc kim loi khc nhau. Cc kim loi c
-
Chng 6: Cm bin Nhit
Trang 203
dT
dEs BA /= CV o/ (6.27)
b. Nhit in tr kim loi Resistance Temperature Detector (RTD)
o RTD c ch to t nhng dy dn rt nhy vi nhit , vt liu ch to
thng l Niken, ng, Niken-ng, Platin
Hnh 6.28 Cu to cm bin nhit trc tip (RTD)
o o c nhit , RTD c mc theo cu in tr
+
+=
43
3
21
2
RR
R
RR
RVV inout (6.28)
Hnh 6.30 Cu in tr.
-
Chng 6: Cm bin Nhit
Trang 204
R1=R2=R4=R3=Rg (6.29)
Khi R3=Rg dR
dRR
dRVV
g
inout
=22
(6.30)
o RTD l mt in tr nhit kim loi, gi tr in tr thay i ph thuc vo s
thay i ca nhit . Khi nhit tng th in tr tng, khi nhit gim
th in tr gim.
o RTD c chnh xc cao: RTD trong cng nghip c chnh xc l
0.1C. Standard Platinum RTD c chnh xc l 0.0001C, thng dung
trong phng th nghim.
Hnh 6.31 Hnh dng RTD
Vt liu ch to:
o RTD thng lm bng cc loi kim loi c in tr thay i theo nhit
nh platin, ng, nikel
o Platin c tm nhit ln nht 184.44C (300F) n 648.88C (1200F),
v mt ha hc v s n nh trong cu trc tinh th, c th t chnh xc
cao 99.999%.
o Nikel c nhy nhit cao nht, tm nhit thp khong 250C, hot ng
ha hc cao d b oxi ha khi nhit cao.
-
Chng 6: Cm bin Nhit
Trang 205
o ng c tuyn tnh cao, tuy nhin do hot ng ha hc cao nn ch hot
ng trong tm < 180C.
Hnh 6.32 c tuyn ca cc loi kim loi thay i in tr theo nhit
RTD c 2 dng: RTD dy qun v RTD b mt.
o Cun dy Platin lm cho R c gi tr ln nn cho php chng do vi nhy
tng i tt
o Gm, mica t trong v bc chng rung ng v va chm mnh.
Cu to ca RTD b mt: RTD b mt thng dng o nhit b mt ca
vt rn.
Hnh 6.33 Cu to RDT loi b mt
Quan h gia in tr v nhit RTD
)......1( 2210N
N TTTRR ++++= (6.31)
Trong : R0 l in tr ca RTD 0C
-
Chng 6: Cm bin Nhit
Trang 206
o Cc l cc h s nhit in tr, hay nhy nhit T.
o H s nhit in tr c trng cho kim loi, ph thuc vo kim loi v nhit
.
o Trong tm o nhit ca kim loi, th R thay i tuyn tnh vi nhit
nn:
).(1 00
TTR
R=
(6.32)
o Khi nhit ngoi tm o th quan h R v nhit l khng tuyn tnh. C
th biu din theo quan h bc hai:
200
0
).(2).(1 TTTTR
R+=
(6.33)
Mch o RDT:
o Trng hp R tuyn tnh vi nhit
Hnh 6.34 o RDT trng hp R tuyn tnh vi nhit .
o Trng hp R khng tuyn tnh vi nhit
Hnh 6.35 o RDT trng hp R khng tuyn tnh vi nhit .
RTD Rshunt DVM
Dy 1 Dy 2
Dy 3 Dy 4
Vi
Ngun dng
RTD
Dy 1
Dy 2
Dy 3
Vi Ngun p in tr
ni tip
Chnh offset
R1
R2
+
_
-
Chng 6: Cm bin Nhit
Trang 207
o S dng thm Rshunt=3.RTD, lm tng tnh tuyn tnh (nhng khng hon
ton).
o Mch RTD c Rshunt hot ng trong tm -196C 260C
ng dng RDT
o Dng o nhit trong cc ng ng, kho cha, o nhit khng kh.
o Dng trong cc ngnh cng nghip thc phm, dc phm cn chnh
xc v nhit cao, v an ton (RTD khng gy hi vi mi trng).
c. Nhit in tr bn dn Thermistor
Nguyn tc: Ging nh RTD nhng c cu to t cc loi oxit ca nikel,
mangan, cobal
Tnh cht: C nhy nhit m v rt cao, khong 10 ln so vi cc in tr
kim loi.
o n nh ca nhit in tr ph thuc vo vic ch to v iu kin s
dng.
o Tm hot ng ca thermistor c th t 1 vi tuyt i n khong 300C.
o Thermistor c ch to t nhng vt liu bn dn, s thay i in tr ca
vt liu t l vi nhit trong gii o.
o Khi nhit tng th in tr gim, do vy Thermistor c h s nhit m.
Mc d vy cng c mt s Thermistor c h s nhit dng.
o Thch hp dng trong cc ng dng c gii nhit nh.
Quan h gia in tr v nhit :
))11
.((
00. TT
B
eRR
= (6.34)
o R0 l in tr nhit 25C
o Gi tr B nm trong khong 300o-500oK. Khi nhy nhit:
2T
B= (6.35)
-
Chng 6: Cm bin Nhit
Trang 208
o Quan h gia in tr v nhit ca thermistor l khng tuyn tnh.
Hnh 6.36 c tuyn ca RDT.
Mch o ca thermistor:
Hnh 6.37 Mch o RDT.
o Thermistor c ni vo mch cu H nh hnh v.
o Khi cu cn bng:
)).((
.
432
30
RRRRR
RR
E
E
TT
T
I +++
=
(6.36)
o Thng thng ngi ta chn R2=R3, Rt=R4. Khi :
TTTTT
TT
I RRRRRRRR
RR
E
E
////2
/
222
0
++++
=
(6.37)
Vo
Vi
R1
R3
RT RTD
R2
-
Chng 6: Cm bin Nhit
Trang 209
Hn ch ca phng php o trc tip:
Ngoi mt s u im c nu trn, phng php o trc tip ny cn
mt s hn ch nh sau:
o Khng th o nhit trn din tch ln
o Khng o c nhng vt ang chuyn ng
o Ch dng trong tm t -190oC n 1063oC
o C th nh hng n cu trc h thng cn o nhit v cm bin c th
tch ln
6.6.2 o gin tip (cch ly)
a. Cc trng hp cn o cch ly
o Xc nh nhit ca mt vt ang di chuyn.
o Xc nh nhit ca mt vt rt nng khng th o c bng dng c
thng thng.
o Xc nh nhit ca mt vt nm trong khu vc nguy him nh l phn
ng ht nhn, l nung, l luyn kim
o Xc nh nhit ca mt khu vc c din tch rng.
o Xc nh nhit m khng gy nh hng n i tng cn o.
o Xc nh nhit m khng lm thay i cu trc ca vt cn o.
b. Nguyn l o nhit cch ly
o Cc vt th nhit ln hn 0oK pht ra mt bc x in t trn b mt t
l vi nhit ca vt.
o Ta o nhit ca vt da trn bc x in t vt pht ra
o Lin h gia cng v bc sng bc x vi nhit
)1(
2/5
2
=
TkhcA e
hcW
(6.38)
W:cng quang ph bc x ca vt en
-
Chng 6: Cm bin Nhit
Trang 210
: bc sng bc x
h : hng s Planck
c : vn t nh sng
k : hng s Boltmann
o Ph pht x ca vt en theo bc sng
Hnh 6.38 Ph pht x ca vt en theo bc sng
c. Cc cch o nhit cch ly
o S dng phng php quang (optical pyrometer).
o S dng phng php bc x in t (photon detector).
d. o nhit cach ly dng phng php quang (optical pyrometer)
o Dng o nhit trong khong t 973 - 4273oK.
o Pyrometer hot ng da trn nguyn tc so snh sng ca dy tc bng
n vi sng ca vt cn o
o sng ca bng n c chun ha vi mt ngun en tuyt i pht x
mt nhit bit trc.
-
Chng 6: Cm bin Nhit
Trang 211
Hnh 6.39 o nhit cch ly bng phng php quang
o Khi cng sng ca nn v dy tc bng nhau th ta c phng trnh:
1
1
1 // 22 =
frr TCTC
(6.39)
l bc sng ca nh sng khong 0.63m
l h s pht x ca vt
Tf l nhit ca dy tc bng n
T l nhit ca vt
Khi T < 4273oK ta c:
1/2 TC v fr TC
T/1/)(ln
1
2 +=
(6.40)
e. o nhit cach ly dng phng php bc x in t (photon detector)
o C rt nhiu ng dng i hi phi xc nh nhit khng tip xc nh: o
lng nhit ta nh, h thng ng dn, khu vc c din tch rng, hoc
nhng l luyn kim
o Thit b o nhit s dng Photon Detector l gii php cho vn trn.
o Photon Detector l mt loi sensor m p ng ca n l in p t l vi mt
dng photon tp trung trn b mt cm bin.
-
Chng 6: Cm bin Nhit
Trang 212
Hnh 6.41 Hnh dng cm bin o nhit bc x in t
Nguyn l hot ng:
Hnh 6.42 Cm bin o nhit dng cch ly
Cng thc:
)(4 2
2
0 Tgf
DkE t = (6.41)
o kt : nhy ca h thng (bao gm h s truyn, li in p v nhy
ca cm bin)
o Do c nhiu ng dng thng mi v cng nghip s dng photon detector
nn tm o ca n rt a dng
o Vi 1 mt my o nhit quang c bn c th s dng o nhit trong
khong 253 n 1873oK
-
Chng 6: Cm bin Nhit
Trang 213
o nhy ca scanner trn 30oC l 0.1o
o H thng o lng nhit bng hng ngoi.
o Cc vt th nhit ln hn 0oK pht ra mt bc x in t trn b mt t
l vi nhit ca vt.
Hnh 6.43 H thng o lng nhit bng hng ngoi.
Hnh 6.44 S khi h thng o lng nhit bng hng ngoi.
Hnh 6.45 B iu khin nhit cm tay dng nh sng hng ngoi
o Ba bc chnh o nhit ca vt:
Bc 1: Chuyn tn hiu hng ngoi pht x ra t vt nhn c ra tn
hiu in
Bc 2: B nhit nn ca mi trng cn o.
-
Bc 3: Tuyn tnh ha kt qu nhn
cn o.
6.7 Mch o
6.7.1 Gii thiu
o IC (LM135/235/335) h
l tuyn tnh vi nhit v
o Khi mun c tm o ln h
cc sensor khc nh Thermocouple, RTD ho
o Vi sensor l RTD, Thermistor th
l in tr, cn v
i l khng tuy
o C thm cc m
n tm mong mun.
6.7.2 o dng mch c
Hnh 6.46 M
Vd: vi RTD 00C, R = 100
R
RVVout =
o R in p ra thay i rt nh
(RIV +=
Chng 6: C
Trang 214
c 3: Tuyn tnh ha kt qu nhn c v xut ra thng tin v
IC (LM135/235/335) hn ch v tm o t -55oC n +150
n tnh vi nhit v d s dng
n c tm o ln hn hoc mun c nhy cao h
cc sensor khc nh Thermocouple, RTD hoc Thermistor.
RTD, Thermistor th i lng thay i tng
n vi Thermocouple l in p, nhng rt nh (V). S thay
khng tuyn tnh vi nhit
C thm cc mch gia cng tn hiu bin i thnh i
n tm mong mun.
cu Wheatstone
Hnh 6.46 Mch o dng cu Wheatstone
C, R = 100 , TC = 0.385/0C
RR
RVV
RR
RR
+
=++
42
R in p ra thay i rt nh
) RIVR =+ *
: Cm bin Nhit
ra thng tin v nhit
n +1500C, tuy u ra (p)
c mun c nhy cao hn th phi s dng
c Thermistor.
ng ng vi nhit
t nh (V). S thay
nh in p, khuch i
(6.42)
(6.43)
-
Chng 6: Cm bin Nhit
Trang 215
o in p ra thay i khng tuyn tnh vi R.
o p ra ln khi I ln
o I khng th qu ln
o Gii hn cng sut
o To nhit trn phn t o.
o C mt phng php tt hn l s dng cu Wheatstone s dng ngun
dng/p
C 4 loi cu Wheatstone ngun p thng dng
Hnh 6.47 Cc dng cu Wheatstone ngun p
o Cu Wheatstone vi 1 phn t thay i
p ra khng tuyn tnh (c th gii quyt bng phn mm hoc k thut
mch)
nhy 0.5%/%
o Cu Wheatstone vi 2 phn t thay i (dng 1)
Khng tuyn tnh (0.5%/%)
nhy tng gp i so vi trng hp u
o Cu Wheatstone vi 2 phn t thay i (dng 2)
Vo:
Vo
R+R
VB
R+R
R-R
R-R
Vo
R
VB
R+R
R-R
R
Vo
R+R
VB
R+R
R
R
R
Vo
VB
R+R
R
R
VB
4
R
R+R/2
VB
2
R
R+R/2
VB
2
R R
VB R R
Sai s TT: 0,5%/%
0,5%/% 0 0
-
Chng 6: Cm bin Nhit
Trang 216
Tuyn tnh
nhy tng gp i so vi trng hp u
Cu ny thng dng cho cm bin p sut hoc lu lng.
o Cu Wheatstone vi 4 phn t thay i
Tuyn tnh.
nhy tng gp 4 so vi trng hp u
Cu dng ny thng dng vi Loadcell
Bn dng cu Wheatstone vi ngun dng
Hnh 6.48 Cc dng cu Wheatstone ngun dng
o u im:
in tr dy, tip xc khng gy sai lch kt qu
C th dng dy dn n gin, r hn.
6.7.3 Tuyn tnh ha khuch i tn hiu sau cu
o Tn hiu a ra cn nh (vi chc vi trm mV), do cn phi c
khuch i trc khi a vo b ADC hoc x l tip.
Khuch i n gin c th dng b OpAmp
Vo:
Vo
R+R
R+R
R-R
R-R
Vo
R
R+R
R-R
R
Vo
R+R
R+R
R
R
R
Vo
IB
R+R
R
R
IBR
4
R
R+R/4
IB
2 R
IB
2 R IB R
Sai s TT: 0,25%/%
0 0 0
IB IB IB
-
Chng 6: Cm bin Nhit
Trang 217
Hnh 6.49 Mch khuch i n gin dng Opamp
o Mch ny cho chnh xc khng cao
khuch i ph thuc vo in tr cu, RF
Tng tr khng cn bng ng vo.
p ra khng tuyn tnh do ch n thun l b khuch i
Hnh 6.50 Mch khuch i tuyn tnh c chnh li.
o Mch ny cho chnh xc cao
o u im:
chnh xc ln hn do khuch i ch ph thuc vo Rg.
Tng tr vo ln nn khng nh hng n cu o v in tr cu o
khng nh hng n mch s cn bng ca b khuch i.
o Khuyt im:
p ra khng tuyn tnh, khc phc bng software
R
REF
VB
R
R+R
R VOUT
+VS
-VS VOUT=
VB
4
R
R+R/2 GAIN
VREF=VB
RG
VS
R RF
VB
R
R+R R
2
+VS
-
Chng 6: Cm bin Nhit
Trang 218
Mch ny va khuch i, va tuyn tnh ho Vout so vi R
Hnh 6.51 Mch khuch i tuyn tnh dng 2 Opamp.
c tuyn ca cc loi cm bin
Hnh 6.52 c tuyn ca cc loi cm bin
6.7.4 Nguyn l x l tn hiu t RTD v Thermistor
Dng mch Opamp
Hnh 6.53 Mch x l tn hiu Thermistor dng opamp
R
VB
R
R+R R
VOUT
+VS
-VS
VOUT= VB
2
R R
R1 R2
1+
+VS
-VS
-
Dng mch cu cn bng
Hnh 6.54
Dng IC vi x l
Hnh 6.54
Chng 6: C
Trang 219
ch cu cn bng
Hnh 6.54 Mch x l tn hiu RTD dng cu cn bng
l
Hnh 6.54 Mch x l tn hiu Thermistor dng IC
: Cm bin Nhit
u cn bng
ng IC