05_chuong 4_cam bien vi tri va khoang cach

8/12/2019 05_chuong 4_cam Bien Vi Tri Va Khoang Cach.

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Chng 4: Cm Bin V Tr V Khong Cch

Trang 101

Chng 4

CM BIN V TR V KHONG CCH

(Sensors for distance and displacement)

4.1Khi nimNguyn l o v tr v dch chuyn v vic xc nh v tr v dch chuyn ng

vai tr rt quan trng trong k thut. Hin nay c hai phng php c bn xc nh

v tr v dch chuyn:

o Phng php th nht: b cm bin cung cp tn hiu l hm ph thuc vo v trca mt trong cc phn t ca cm bin, ng thi phn t ny c lin quan n

vt cn xc nh dch chuyn.

o Phng php th hai: ng vi mt dch chuyn c bn, cm bin pht ra mtxung.

o Vic xc nh v tr v dch chuyn Yu cu o v tr v dch chuyn ln Dng o cc i lng khc

o Phng php o: Phn t cm bin gn vi vt di ng, tn hiu o l mt hm ph thuc v tr

trong phn t cm bin, thng thng l tr khng.

Khng i hi lin kt c hc gia cm bin v vt o: tn hiu o cthng qua cc i lng trung gian nh in trng.

4.2Cm bin o khong cch ng ra dng analog4.2.1 Loi in cm

Nguyn l:da trn hin tng cm ng in t, thay i h s t cm L v h s

h cm M.


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Vt cn o gn vo mt phn t mch t, vt dch chuyn gy nn s bin thin

t thng trong cundy o dn n thay i in p trn hai u ca n. Ngun cp l

tn hiu xoay chiu v c cc loi thng dng sau:

a. Mch t c khe t bin thino Nguyn tc hot ng

Hnh 4.1 Hot ng cm bin in cm analog

1) Li st t 2) Cun dy 3) phn ng

Trn hnh 4.1 trnh by s nguyn l cu to ca mt s loi cm bin t

cm n. Cm bin t cm n gm mt cun dy qun trn li thp c nh

(phn tnh) v mt li thp c th di ng di tc ng ca i lng o (phn

ng), gia phn tnh v phn ng c khe h khng kh to nn mt mch t h.

hnh 4.1a: Di tc ng ca i lng o XV, phn ng ca cm bin

di chuyn, khe h khng kh trong mch t thay i, lm cho t tr ca mch

t bin thin, do h s t cm v tng tr ca cun dy thay i theo.

hnh 4.1b: Khi phn ng quay, tit din khe h khng kh thay i,

lm cho t tr ca mch t bin thin, do h s t cm v tng tr ca cun

dy thay i theo.

H s t cm ca cun dy cng c th thay i do thay i tn hao sinh

ra bi dng in xoy khi tm st t dch chuyn di tc ng ca i lng o

XV(hnh 4.1c)

Nu b qua in tr ca cun dy v t tr ca li thp ta c:


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sW

R

WL o

22

== (4.1)

Trong :

W: S vng dy

0

Rs

= : T tr ca khe h khng kh.

: Chiu di khe h khng kh. s: Tit din thc ca khe h khng kh.

Trng hp W = const ta c:

dL

ds

s

LdL

+

= (4.2)

Vi lng thay i hu hn v s ta c:

( )s

sWs

WL

o

o

o

o +

=2

022

(4.3)

nhy ca cm bin t cm khi khe h khng kh thay i ( s = const):

2

0

1

+

=

=

o

oLLS (4.4)

nhy ca cm bin t cm khi thay i tit din khng kh ( = const):

0

0

s

L

s

LS =

= (4.4)

Tng tr ca cm bin:

sWLZ 0

2

== (4.5)

T cng thc tng tr ca cm bin: Ta thy tng tr Z ca cm bin lhm tuyn tnh vi tit din khe h khng kh s v phi tuyn vi chiu di

khe h khng kh .

o c tnh lm vic


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Hnh 4.2 S thay i gia L, Z vi chiu di khe h khng kh

c tnh ca cm bin t cm n ( )Z f = l hm phi tuyn v ph thuc tn

s ngun kch thch, tn s ngun kch thch cng cao th nhy ca cm bin

cng cao (hnh 4.2).

o Kiu mch t hnh ch U

Hnh 4.3 Cm bin cm ng t mch t hnh ch U

0

0

20

21

.2

l

xx

l

sNL

+

=

(4.6)

N: s vng dy L0: chiu di khe h khng kh S: tit din mH/10.4 70 =

o Kiu mch t hnh xuyn


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Hnh 4.3 Cm bin cm ng t mch t hnh xuyn

Loi ny gm ba phn: Mt li st t hnh xuyn, mt khe t bin thin v

mt b mt lm bng li st t. in cm ny s thay i ph thuc v khong

cch khe khng kh v gi tr in cm thay i phi tuyn.

)1()(0

0

2

d

L

kd

nL

+=

+= (4.7)

Vi:

[ ]trrR

AC /1/1010 += ,

20 .2

rk

= (4.8)

b. Loi c 2 mch tLoi ny c nhy cao hn loi mt mch t v tuyn tnh hn khi kt

hp vi mch cu o

Hnh 4.3 Cm bin cm ng t loi 2 mch t

[ ])(101

1 xd

LL

+=

;

[ ])(102

2 xd

LL

++=

(4.9)


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Trang 106

c trng cm bin in cm bin thin

Ng ra thay i phi tuyn, ph thuc tn s ngun kch thch, tn s cng

cao th nhy cng ln

LZL= (4.10)

Hnh 4.4 c tuyn cm bin in cm analog

c. Cm bin t cm c li t di ng

Hnh 4.5 S nguyn l cm bin t cm c li t

1) Cun dy 2) Li t

Di tc ng ca i lng o XV, li t dch chuyn lm cho di l fca

li t nm trong cun dy thay i, ko theo s thay i h s t cm L ca cun

dy. S ph thuc ca L vo lf l hm khng tuyn tnh, tuy nhin c th cithin bng cch ghp hai cung dy ng dng vo hai nhnh k st nhau ca

mt cu in tr c chung mt li st.

4.2.2Loi h cma. Cm bin h cm

Cu to ca cm bin h cm tng t cm bin t cm ch khc ch c

thm mt cun dy o. (hnh 4.6)


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Hnh 4.6 Cm bin h cm

1) Cun s cp 2) Gng t 3) li t di ng 4) Cun th cp (Cun o)

Trong cc cm bin n, khi chiu di khe h khng kh (Hnh 4.6a) hoc

tit din khe khng kh thay i (Hnh 4.6b) hoc tn hao do dng in xoy thay

i (Hnh 4.6c) s lm cho t thng ca mch t bin thin ko theo sut in

ng e trong cun o thay i.

o Cm bin c khe h khng kh: nhy ca cm bin vi s thay i ca chiu di khe h khng kh

(s=const):

2

00

02

0

20 11

+

=

+

=

=

EksES (4.11)

nhay khi tit din khe h khng kh s thay i ( const)= :

o

os s

Ek

s

ES ==

=

0 (4.12)

Trong : 021WWk=


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I :gi tr W2, W1: : Chi 00

0

ksE

=

o Cm bin vi sai tng nh

bin theo s vi s

gp i v phm vi

b. B bin i saiTransformer)

o Cu toLVDT l mt

trong lng cun dy

o Nguyn l hot Gm 4 cun

trong c li t di

nhau sao cho sut

Chng 4: Cm Bin V

Trang 108

iu dng ca dng in

s vng dy ca cun dy o.

di khe h khng kh.

: Sc in ng h cm ban u trong cu

y v tuyn tnh ca c tnh cm bi

i (hnh 4.6 d, e, f).Khi mc vi sai nh

lm vic tuyn tnh m rng ng k.

phn tuyn tnh LVDT (Linear V

c cu c - in to ra in p t l vi

Hnh 4.7 Cu to LVDT.

g

y ghp ng trc to thnh hai cm bing c (hnh 4.7). Cc cun th cp

n ng trong chng trit tiu ln nhau.

r V Khong Cch

n o W2khi XV=0

n ngi ta mc cm

y ca cm bin tng

riable Differential

tr ca li bin p

n i xng, bnc ni ngc vi


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V nguyn tc

th cp bng khng

s cp vi cc cu

vi cc cun th ctuyn tnh vi hiu

in p AC

cp. Khi li chuyn

Cun th cp

cc tnh khi li dch

H

L mt bin in p ng ra

Ng ra bng k

Chng 4: Cm Bin V

Trang 109

Hnh 4.8 Cm bin h cm vi sai.

, khi li t v tr trung gian, in p o

. Khi li t dch chuyn, lm thay i m

th cp, tc l lm thay i h s h c

. Khi in tr ca thit b o ln, i cc h s h cm ca hai cun th cp.

a vo cun s cp, to ra in cm ng

ng lm cho in p u ra th cp thay

qun theo hai chiu ngc nhau, nn i

chuyn.

nh 4.9 M t nguyn l hot ng LVD

gm mt cun s cp v ai cun t

hiu hai in p cm ng trn cun th

ng khi li st t nm trung tm thit b.

r V Khong Cch

m u ra hai cun

i quan h gia cun

m gia cun s cp

p o Vm gn nh

trn 2 u cun th

i.

n p ra s thay i

.

cp ging nhau.

cp.


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Khi li ti v tr

Ho Bin in p n Khi li ti v tr Khi di chuyn l

nhau.

Bin t l cdch chuyn.

Vng hot tuy

Chng 4: Cm Bin V

Trang 110

Hnh 4.10 Tn hiu ra LVDT.

trung tm th L1=L2, mch cu cn bng

h 4.11 Tn hiu ra ca LVDT thay theo l ra

trung tm, p ng ra bng khng.

ch v tr trung tm, in p cm ng h

hiu di dch chuyn ca li v pha c

nh 4.12 Bin tn hiu ra ca LVDT.

tnh ca LVDT

r V Khong Cch

nn Vout= 0.

i

i cun th cp khc

n th hin hng


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tuyn tnh

4.13).Khi li dch

ng ra cng tng.

Bin p ra

mt khong gii hcn tuyn tnh.

Phm vi tuyn

n v thng

LVDT cung c

Tn s ny chnh l

ca li t nht 10 l

Tn hiu in

li.

Khng c s

ny khng to ra s

hn cc phng ph

Chng 4: Cm Bin V

Trang 111

Hnh 4.13 Vng hot ng ca LVDT.

ca phng php LVDT c biu di

chuyn cng xa v tr cn bng, bin

ch tuyn tnh vi dch chuyn ca l

. Khi li cng tin gn cc v tr gii h

tnh LVDT t 1mm n 50cm (sai s

c s dng l mV/mm hay V/mm ha

p tn hiu sin cho cun s cp vi tn s

tn s sng mang v phi ln hn tn

.

p ng ra chnh l tn hiu iu ch c

ip xc no ca li vi cc b phn kh

hao mn ca trn thit b, dn ti thit

p khc.

r V Khong Cch

n trn th (Hnh

in p o c ti

i khi li nm trong

n th php o khng

tuyn tnh 0.25%)

mV/in hay V/in.

t 50hz n 25khz.

s ca chuyn ng

t chuyn ng ca

c nn phng php

ny c bn cao


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Trang 112

Khng c gii hn cho phn gii ca phng php ny do s dng t

trng lm trung gian phn gii ch ph thuc vo phn gii ca dng

c o in p

o Mch giao tip ca LVDT

Hnh 4.14 Mch giao tip ca LVDT.

o Mt s lu khi chn LVDT chn b bin i sai phn tuyn tnh ta cn quan tm n cc vn sau:

o v tr khng tn hao do ma st phn gii khng xc nh Bn vng Kh nng lp li cao Ng vo v ra cch ly

4.2.3 Loi in dungCm bin in dung c s dng rt ph bin trong cng nghip v trong cc

lnh vc khoa hc. Nguyn l ca chng l da trn s thay i ca in dung khi cs dch chuyn. Cm bin in dung c tuyn tnh ln v phm vi rng. Phn t

cm ng c bn ca cm bin in dung bao gm 2 cc ca mt t in c in dung

C. in dung l mt hm ca khong cch d gia 2 cc ca t in, din tch bn cc

A v hng s in mi.

),,( AdfC= (4.13)

a.Loi c khong cch bin thin


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Hnh 4.15 Cm bin dung loi khong cch bin thin

Loi cm bin ny to ra t 2 bn cc phng cch nhau mt khong cch x

c th thay i c. Do , in dung ca t in l:

x

A

x

AxC r 0)(

== (4.14)

y:

: Hng s in mi

r: Hng s in mi lin quan (hng s r=1 ca khng kh so vi chn

khng)

r: Hng s in mi chn khng (hng s r=8.854188x10-12F/m-1)

x: khong cch gia hai b mt tnh bng m

A: Din tch ca b mt tnh bng m2

in dung ca loi cm bin ny bin thin phi tuyn theo dch chuyn x.

nhy:20A

dx

dCS r

== (4.15)

Kiu cm bin ny thng s dng o dch chuyn c tng nh m

khng cn tip xc vi i tng cn o.

b.Loi din tch bn cc bin thin dch chuyn c th o bi cm bin in dung c din tch bn cc bin

thin.

d

wxAAC r

)(0 ==

(4.16)

Vi w: b rng, wx: din tch vng b gim khi di chuyn b mt


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Hnh 4.16 Cm bin dung loi in tch bn cc bin thin

Cm bin in dung loi in tch bn cc bin thin c c tnh sau:

in dung tuyn tnh vi dch chuyn. Dng o dch chuyn gc.

c. Loi in mi bin thin dch chuyn c th o dng cm bin in dung da trn s dch chuyn

tng i ca vt liu in mi gia cc bn cc.

Hnh 4.17 Cm bin dung in mi bin thin

[ ]xlwC )( 1220 = (4.17)

Vi:

1: Hng s in mi lin quan cht liu lm cht in mi

2: Hng s in mi ti v tr khi cht in mi b dch chuyn

Ng ra loi cm bin ny cng t l tuyn tnh vi dch chuyn x.

thng c s dng o mc ca cht lng trong thng. Vi iu kin cht

lng khng dn in dng in mi.


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d. Loi vi saiTrong mt s trng hp, ng ra ca cm bin in dung bin thin phi

tuyn vi dch chuyn. iu ny c th kh c bng cch s dng cm

bin in dung dng vi sai.

Cm bin in dung loi ny thng c 3 bn cc. Ty theo tng ng dng

c th m cm bin loi ny c th c cu to khc nhau.

Hnh 4.18 Cm bin dung loi vi sai

)(

2

)()(2 22000

21 dd

dlw

dd

lw

dd

lw

CCCrrr

+=+== (4.18)

)(

2

)()(2

22000

21dd

lwd

dd

lw

dd

lwCCC rrr

+=

++

=+= (4.19)

d

d

C

C = (4.20)

Cm bin in dung loi ny tuyn tnh hn nhiu so vi cm bin in

dung 2 bn cc. Tuy nhin, trong thc t vn tn ti mt s thnh phn phi tuyn

do nhng khuyt im trong cu trc. Do , ng ra cm bin loi ny cn phi

c x l cn thn nhm thu c ng ra ti u.

e. Mch oMch o thng l cc mch cu khng cn bng cung cp bng ngun p

xoay chiu vi tn s ln (MHz)


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Hnh 4.19 Mch o cm bin in dung analog

Mch cu khng cn bng, cp ngun xoay chiu. i vi loi vi sai, tn s

tn hiu phi ln

Hnh 4.20 Mch o cm bin analog

o Cu hnh 4 dy: dch chuyn = )( + CHCH VVGx (G l nhy) (4.21)

Cu hnh ny i hi t dy hn v kt ni n gin hn cu hnh 5 dy

nhng tn hiu o b nh hng nhiu bi nhit mi trng t bit khi dy

ni di v ngun c bin thp

oCu hnh 5 dy:

dch chuyn:)(

)(

+

+

+

CHCH

CHCH

VV

VVGx (4.22)

Cu hnh ny nhiu dy hn cc cu hnh khc nhng t b nh hng bi

nhit

4.2.4 Cm bin quang ina. Cu to v nguyn l hot ng


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Cm bin quang in gm b pht v b thu nh sng, ngun sng c s

dng l diode pht quang hay diode lazer, nh sng dng nh sng hng ngoi

hay mu ( hay xanh).

Hnh 4.21 Cm bin quang in

b. Phn loiTy theo cch b tr cm bin thu pht m ta c th chia cm bin quang

in ra nhiu loi khc nhau:

o Loi phn x

Hnh 4.22 Phn loi cm bin quang in

o Loi khe hnh mng nga


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Hnh 4.23 Cm bin quang in loi khe hnh mng nga

c. u nhc im ca cm bin quang ino u im

Cm bin quang in l loi cm bin pht hin vt khng cn tip xc v

c th pht hin vt khong cch xa. Cm bin quang in c th thit k n

gin r tin

o Nhc imS dng nh sng hng ngoi nn d b nhiu khi hot ng mi trng. B

tc ng bi nhim ca mi trng, khi cm bin quang in b bm bi th

cng sng ti b thu s gim.

4.2.5 Cm bin Lazer v siu ma. Cm bin Lazer

Cm bin lazer dng o dch chuyn khng cn tip xc, khong cch

t vt n cm bin tng i ln thng khong 4 10cm.

Cm bin lazer hot ng theo nguyn tc: mt tia lazer chiu ln vt c

phn x li v sau khi qua h thng thu knh s tc ng ln cm bin v tr. Vi

x l s da trn php tnh tam gic (so snh vi mt v tr chun) tnh ra

khong cch t vt n cm bin, tn hiu xut ra l dng hay p theo t l

khong cch.

b. Cm bin siu mCm bin siu m o khong cch da trn thi gian T t khi pht ra chm

sng siu m n khi nhn c chm sng phn x


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Trang 119

v

dT

2= (4.23)

d: khong cch t vt n cm bin.

v: Vn tc m thanh

phn gii ca cm bin siu m khong 0,2mm nn km chnh xc hn

so cm bin lazer. Sai s o ca cm bin ny cn ph thuc vo hng ca mt

tia phn x.

Hnh 4.24 Cc loi cm bin o mc nc

c im ca cm bin siu m:

o u ra analog: dng 4 20mA, in p 010Volto phn gii cao: khong 0,172mmo Ngun vo: 10 30VDCo

Khong cch pht hin trn 6mo Cm bin siu m c th iu chnh khong cch pht hin (Background

Suppression)

Mt s dng cm bin ng ra analog cho php iu chnh khong cch pht

hin, chng c th t chi vic pht hin cc i tng sau mt khong cch xc

nh. Khong cch pht hin c th iu chnh bi ngi s dng.


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Ngoi ra cm bin khng pht hin i tng d chng di chuyn vo

vng hot ng ca cm bin, ngi ta c th to 1 lp v bng cht liu c kh

nng khng phn x li sng m thanh.

4.3Chit p (B phn th): (Potentiometer)4.3.1Cu to hnh dng

Gm mt in tr c nh Rn v mt tip xc in c th di chuyn gn vi

chuyn ng cn o gi l con chy. V tr con chy t l vi gi tr in tr ti u ra

ca tip xc im. Hnh 4.25 v hnh dng hnh 4.26

Cn c vo hnh dng ca Rn v dng chuyn ng ca con chy ngi ta chia ra

2 loi:

Dch chuyn thng:L

lRlR n=)( (4.24)

Dch chuyn gc quay:0

nRR = (4.25)

Hnh 4.25 B chit p loi dch thng v dch chuyn gc quay

Hnh 4.26 Hnh dng chit p.

4.3.2c tnh


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Trang 121

Chit p c phn gii thng thng t c 10 m. Tui th ca con chy

106ln vi dng xoay v 107 108vi dng dch chuyn. tuyn tnh l gi tr

ca t s R(x)/Rn hai u ca in tr khng n nh, do u ng chy

hoc cui ng chy th tuyn tnh km. Hnh 4.27

Hnh 4.27 S thay i ca t s R(x)/Rn ph thuc vo v tr con chy.

Hnh 4.28 Cnh tay robot hn

Kho st chit p nh mt cm bin v tr. Hnh 4.28 s nh hng ca ti

ln chit p c th iu khin cnh tay robot hn.

Chit p dng chuyn i gc quay hoc khong cch tuyn tnh thnh

in p. Hnh 4.29, chit p c in tr thay i tuyn tnh so vi trc quay.


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Trang 123

R : Sai s ti ln nht

potR : Tng in tr ca chit p

in tr s dng nh cm bin v tr nn in p ng ra t l vi v tr gc

ca trc quay.

o Do sai s tuyn tnh c th biu din theo gc quay:

100xett

= (4.28)

Hnh 4.30 C cu truyn ng bnh rng.

4.3.3Cc loi chit pGi tr in tr Rn t 1k n 100k, i khi ln n hng M. Ty tng

trng hp c th chnh xc ca in tr l 20%, 10% v i khi t ti 5%.

Trn thc t khng cn i hi chnh xc cao v tn hiu o ch ph thuc vo

t s R(x)/Rn. C nhiu loi chit p, ty theo tng ng dng m ngi ta chn

loi chit p thch hp.

a. Chit p dy kim loiLoi chit p ny c th dng vi dng in kh cao. N c h s nhit

thp, t ting n cho mch in t v tr s in tr khng b tri theo thi gian v

do nh hng ca kh hu. Nhc im l phn gii thp.


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Trang 124

b. Chit p vi lp polymerLp in tr c cu to bi mt loi sn hu c trn vi mui than v

bt graphit. Lp in t loi ny rt trn phng, kh b mi mn. Tuy nhin n c

h s nhit kh ln (3001000ppm/K). Do n nh ca n so vi chit p

dy kim loi nhit cao km.

c. Chit p vi oxit kim loi thy tinh (cermet)Loi in tr ny c ti luyn nhit kh cao 800 900 0C, cho nn

rt cng. Tuy nhin b mt nhm, khng thch hp cho s dch chuyn con chy

nhiu ln. Chit p loi ny thch hp cho vic chnh in p trong cc mch in

t.

d. Chit p vi mng mng kim loiVi phng php phun ph hay phun bi catot ngi ta c th to mt lp

kim loi thc phng trn mt nn thy tinh. Vt liu c th l hp kim Ni/Cr hay

oxit kim loi Ta2O5. Mng mng kim loi c b dy khong 1 m.

Trong cc loi chit p trn ch c loi chit p vi lp polymer l thch hp

hn c cho cng vic nh v v tnh khong cch. Vi b mt trn lng v t b

mi mn, loi chit p ny c th chu ng c rt nhiu ln dch chuyn ca

con chy m c tnh k thut khng b thay i. Loi cm bin ny c dng

nhiu trong cc lnh vc nh: K thut xe hi, nh v trong mt h thng o c

v ngay c tnh h s gc cho vic iu chnh ng c bc, v vi cch ny s

thay i gc c chuyn i nhiu nc. dy ca lp polymer khong t 10 -

20 m v chu ng c nhit n 150

0

C. Vt liu ch to cc con chy lhp kim ca cc kim loi qu nh: Pd, Pt, Au v Ag. Vi cc hp kim ny s

hnh thnh cc lp dn in km do nh hng ca kh hu khng xy ra. Do vy

gia con chy v mng polymer lun lun c s tip xc in rt tt. Cc lp

polymer ny c ch to vi phng php in la tht thn trng trong iu kin

sch khng c bi. Sau c lm cng li qua s nung nng nhit 1500C

n 2500C. Lp polymer c th chu ng c 107 ln dch chuyn ca con

chy m in tr gia con chy v lp polymer th khng gia tng.


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Trang 125

4.4Cm bin o vn tc vng quay v gc quay: (Tachometter)Trong cng nghip c rt nhiu trng hp cn o vn tc quay ca my. Ngi

ta thng theo di tc quay ca my v l do an ton hoc khng ch cc iu

kin t trc cho hot ng ca my mc, thit b. Trong chuyn ng thng vic o

vn tc di cng thng c chuyn sang o vn tc quay. Bi vy cc cm bin o

vn tc gc chim v tr u th trong lnh vc o tc .

4.4.1 Mt s phng php c bnCm bin vn tc gc quay cung cp cho ta tn hiu o l tn s. Thng

thng trn trc quay c nh mt hay nhiu du v mt cm bin phn

khng chuyn ng s ghi nhn s chuyn ng ca cc du ny. Tn s o c

t l vi vng quay n v s du k:

f = n.k (4.29)

o tc quay ca rotor ta c th s dng cc phng php sau:

o S dng my pht tc mt chiu hoc xoay chiu, thc cht l cc my phtin cng sut nh c sc in ng t l vi tc cn o. c s dng rng

ri trong cc h chuyn ng kinh in.

o S dng b cm bin quang tc vi b m ha.o S dng my o gc tuyt i.o Xc nh tc gin tip qua php o dng in v in p stator m khng cn

dng b cm bin tc .

4.4.2 o vn tc vng quay bng phng php analoga. Tc k mt chiu (my pht tc)

My pht tc l my pht in mt chiu, cc t l nam chm vnh cu.

in p trn cc my pht t l vi tc quay ca n. My pht tc ni cng

trc vi phanh hm in t v cng trc vi ng c do tc quay ca n

chnh l tc quay ca ng c. Tc ny t l vi in p ca my pht tc


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Trang 126

, dng V mt in t hoc ng h o tc ni vi n c th o c tc

ca ng c. Gi tr in p m hay dng ph thuc vo chiu quay.

Er= ( n0)/ 2 = Nn0 (4.30)

N: s vng quay trong 1 s.

: vn tc gc ca rotor.

n: l tng s vng dy chnh trn rotor.

0: l t thng xut pht t cc nam chm

Cc phn t cu to c bn ca mt tc k dng mt chiu biu din trn

hnh 4.31.

Stator (phn cm) l mt nam chm in hoc mt nam chm vnh cu c

hai cc nam v bc nm ngoi cng. Rotor (phn ng) gm c li thp phn ng,

trn c x rnh, trong rnh c t dy qun

Hnh 4.31 Cu to ca mt my pht dng mt chiu.

b. Tc k dng xoay chiuTc k dng xoay chiu c u im l khng c c gp in v chi

than nn c tui th, khng c tng, gim in p trn chi than. Nhc im l

mch in phc tp hn, ngoi ra xc nh bin cn phi chnh lu v lc

tn hiu.

c. My pht ng bL mt loi my pht in xoay chiu loi nh. Rotor ca my pht c

gn ng trc vi thit b cn o tc . Rotor l mt nam chm hoc nhiu nam


27/47

chm nh hnh 4.3

cp sut in ng

e = E0si

E0= K1.

K1v K2l c

u ra i

khng ph thuc v

quay c th xc n

ny rt quan trng

truyn i xa v s s

xc ca php o. (v

Hnh 4.32 Cu

d. My pht khng Cu to ca m

hai pha (hnh 4.33)

Chng 4: Cm Bin V

Trang 127

. Stator l phn cm, c th 1 pha hoc

nh sin c bin t l vi tc quay c

t

, =K2.

thng s c trng cho my pht.

p c chnh lu thnh in p mt

o chiu quay v hiu sut lc gim i kh

h c bng cch o tn s ca sc in

hi khong cch o ln. Tn hiu t my

y gim tn hiu trn ng i khng nh

o tn s).

to ca mt my pht ng b. (a: 1 pha

ng b

y pht khng ng b tng t nh

r V Khong Cch

ba pha, l ni cung

a rotor.

(4.31)

(4.32)

chiu. in p ny

tn s thp. Tc

ng. Phng php

pht ng b c th

hng n chnh

, b: 3 pha)

g c khng ng b


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Hnh

Rotor l mt

lng v qun tnh

trn c t hai cuc cung cp mt

ve=Veco

Cun dy th

in ng c bin

em=Em

Do Em= k

k l hng s p

: lch ph

Khi o Ems

4.5o vn tc vng q4.5.1Cm bin quang t

Encoder l c

ti, Encoder c h

loi tuyt i (absol

Chng 4: Cm Bin V

Trang 128

4.33 Cu to ca mt my pht ng b.

nh tr kim loi mng c quay vi

ca n khng ng k. Stator lm bng t

dy c b tr nh hnh v. Cun thin p nh mc c bin v tn s kh

e t

ai l cun dy o, gia hai u ca cun

t l vi vn tc gc cn o.

os(et + ) = kVecos(et + )

e= k

thuc vo cu trc ca my.

.

c nh c

ay bng phng php quang in t

c vi a m ha

bin o v tr gc ca trc ng c

ai loi chnh: loi tng i (loi gia t

ute).

r V Khong Cch

n tc cn o, khi

p l k thut in,

nht l cun kch tng i e.

(4.33)

ny s sut hin sc

(4.34)

(4.35)

(Encoder)

my cng c, bng

g incremental) v


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Trang 129

Hnh 4.34 Cu to cc thnh phn bn trong ca Encoder.

Cu to ca encoder tng i: gm hai a, mt a ng yn v mt a

quay. a quay gm ba ng, hai ng ngoi chia lm n khong gc bng

nhau lin tip cc tit din m v trong sut, ng th ba ch c mt rnh nh.

C ba cm bin quang hc vi cc ngun sng tng ng v mt b iu khin.

Khi trc quay mt vng th tia sng b ngt n ln v gi tn hiu dng xung

(A v B) vung gc nhau.

B iu khin phi xc nh c chiu quay ca trc bng cch xc nh

s lch pha in gia hai tn hiu A v B:

o A vt trc B: quay thun.

Hnh 4.35 Xung quay thun ca encoder.


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Trang 130

o B vt trc A: quay nghch.

Hnh 4.36 Xung quay nghch ca ncoder.

ng trong (pha Z) ch c mt khe trong sut v to ra mt xung trong

mi vng quay. Z c dng xc nh v tr gc (Z: u khng) hoc bo hiu

quay ht mt vng.

4.5.2Cu to v nguyn tc hot ng

Hnh 4.37 M t nguyn tc hot ng ca encoder.

Bao gm mt a plastic hoc thy tinh quay gia mt/nhiu ngun pht

sng (LED) v mt/ nhiu photo-transistor nhn quang.

a c m ha vi cc phn hnh qut sng v ti xen k nhau xung c

th c to ra khi a quay.C hai loi encoder: encoder tng i v encoder tuyt i.

4.5.3Encoder tng i (Incremental Encoder)a. Cu to v nguyn l hot ng

Tn hiu s xut ra

Cc b nhn quang(photo-transistor)

1 hoc nhiu b pht quang(LED)

Track a code

Trc ca encoder


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Trang 131

Hnh 4.38 Cu to ca mt encoder tng i

Cu to encoder tng i gm ngun pht thng l LED pht sng qua

knh quang ph hi t chiu qua mng li lc sau s xuyn qua a c vch

chia trc khi n c t bo quang in.

Hnh 4.39 a quay ca encoder.

Cu to gm: Mt b thu pht quang, mt a vi cc rnh, mt trc quay,

cp ngun v tn hiu.

Encoder tng i: loi ny c xung xut ra c m cung cp v tr

quay ca encoder. Mi bc encoder tng i s xut ra mt xung tng ng.


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Hnh

Loi ny dn

xung A ln 1 t

Xung Z ln 1 khi

b. Encoder tng i

H

V tr c x

S rnh trn

Phototransthu

Trc

Code track

Chng 4: Cm Bin V

Trang 132

.40 Xung ra ca encoder Incremental E

2 xung A v B nhn bit chiu qu

c xung B, encoder ang quay chiu

encoder quay gip 1 vng.

loi mt knh

nh 4.41 Encoder tng i mt knh.

nh da vo s xung n c.

i vng xc nh s gc ng vi tng

LED phtistor

a quay

r V Khong Cch

coder.

y ca encoder. Nu

thun v ngc li.

i rnh.


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Trang 133

Hnh 4.42 V tr ca xung trn a encoder mt knh.

Hnh 4.43 Mt n encoder mt knh.

Cm bin loi mt knh khng cho thng tin v chiu quay.

V d:Mt encoder quang gm 360 rnh, bt u t v tr chun (reference).Cm bin quang m 100 rnh cng chiu kim ng h v 30 rnh theo

chiu ngc li ri 45 rnh theo cung chiu kim ng h. Tm v tr hin ti?

Gii

Encoder 360 th mi rnh c gi tr gc l mt . Vy v tr hin ti l:

100 30 + 45 = 115otheo chiu kim ng h.

c. Encoder tng i loi hai knh

Hnh 4.44 Encoder tng i hai knh.

Tn hiu s xut ra

Cc b nhn quang(photo-transistor)

1 hoc nhiu b pht quang(LED)

Track a code

Trc ca encoder


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Trang 134

Encoder mt knh khng xc nh c chiu quay ca trc. Loi hai knh

khc phc khuyt im ca encoder mt knh l cho bit chiu quay ca trc.

Hnh 4.45 Chiu quany ca encoder tng i hai knh.

o a quay encoder 2 knh.o V1 v V2: v tr hai cm bin quang.o CCW v CW: chiu quay ngc v quay thun theo chiu kim ng ho Dng sng ng ra hai cm bin khi quay CCW v CW nh hnh 4.22

Hnh 4.46 Dng sng ng ra hai cm bin khi quay CCW v CW.

d. Gii m encoder

Hnh 4.47 B gii m encoder.


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Trang 135

o Mch m tng dn mi khi a quay cng chiu kim ng h (CW).o Mch m gim dn mi khi a quay theo chiu ngc chiu kim ng h

(CCW).

o Cch n gin nht thc hin mch gii m l dng flip-lop. Mch gii mv dng sng gii m ca V1 v V2.

Hnh 4.48 B gii m encoder dng Flip-Flop.

Vd:Cho h thng cm bin v tr nh trong hnh trn. S dng encoderloi 250 xung/vng. Gi tr hin ti ca b m l 00100110. Tm v tr gc

hin ti?

Gii

Encoder gm 250 xung. Vy 1 xung ng vi 360/250 = 1.44o

. Gi tr nhphn ca b m l: 00100110 tng ng s thp phn l 38. Vy v tr hin

ti l: 38x1.44 = 54.72o.

4.5.4Encoder tuyt i (Absolute Encoder)a. Cu to cm bin tuyt i

Hnh 4.49 Cu to ca encoder tuyt i.


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Trang 136

Cu to ca encoder tuyt i cng ging nh loi encoder tng i ch

khc cu to ca a v s lng t bo quang in nhn. Mi encoder u c

board mch iu khin v gii m nh hnh 4.50

Hnh 4.50 Bng mch v hnh dng.

Encoder tuyt i gm nhiu b nhn quang (c th ti a ti gn 20 track).

Hnh 4.51 B nhn quangca encoder tuyt i.ng vi mi v tr ca encoder c mt tn hiu ra nh phn vi sai xc

nh chnh xc v tr trc ca encoder.

Vi encoder tuyt i, thng tin v v tr vn khng thay i ngay c khi tt

v bt ngun encoder tr li.


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Trang 137

Decimal Binary Gray Code Decimal Binary Gray Code

1 0000001 0000001 11 0001011 0001110

2 0000010 0000011 12 0001100 0001010

3 0000011 0000010 13 0001101 0001011

4 0000100 0000110 14 0001110 0001001

5 0000101 0000111 15 0001111 0001000

6 0000110 0000101 16 0010000 0001100

7 0000111 0000100 17 0010001 0001101

8 0001000 0001100 18 0010010 0011011

9 0001001 0001101 19 0010011 0011010

10 0001010 0001111 20 0010100 0011110

V d: vi Encoder dng m nh phn (binary) t c phn gii 1024 P/R, encoder tuyt i x l 10 bit (10 dy ra)

t c phn gii 4096 P/R, encoder tuyt i x l 12 bit (12 dy ra)

b. a encoder tuyt i

Hnh 4.52 Cu trc a gii m trn a gii m ca encoder.


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Trang 138

c. a encoder tuyt i 2 bit

Hnh 4.53 a gii m 2 bit.

Loi n gin nht gm hai bit ng ra tng ng bn trng thi nh hnh

4.53

d. Loi m ha 4 bit nh phn t 0000 n 1111

Hnh 4.54 a gii m 4 bit.

Loi m ha 4 bit nh phn t 0000 n 1111 tng ng 16 trng thi nh

hnh 4.54o Mi tn ch chiu quay ca a s cho ng ra m xung. Trng s thp nht

nm pha ngoi ca a, trng s ln nm pha trong ca a.

o chnh xc hn, ngi ta thm mt track hiu chnh nh hnh 4.55o Thm mt cm bin c xung chnh xc hn.


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Trang 139

Hnh 4.55 a gii m 4 bit thm track hiu chnh.

o a to m encoder truyt i

A a to m Gray, B a to m BCD v bng m BCD hay m Gray tng

ng cho loi a

4.5.5Mt s lu khi chn Encodera. Loi Encoder cn s dng

Hnh 4.56 Cc loi encoder.

Ty theo ng dng c th m ta nn chn loi quay trn hay tnh tin thng

Rotary encoder Linear encoder


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Trang 141

Shock, rung, Nhit hot ng

4.6 o vn tc vng quay vi nguyn tc in tr t4.6.1Cc n v t trng v nh ngha

a. T trngT trng l mt dng vt cht tn ti xung quanh dng in, hay ni chnh

xc l xung quanh cc ht mang in chuyn ng. Tnh cht c bn ca t

trng l tc dng lc t ln dng in, ln nam chm.

b. Cm ng t BV mt gy ra lc t, t trng c c trng bng vector cm ng t B.

Trong h thng n v SI n v cm ng t B l T (Tesla).

1 T = 1Wb/m2= 1V.s/m2

c. T thng T thng gi qua din tch dS l i lng c gi tr bng

SdBd

.= (4.36)

Trong :

B

l vector cm ng t ti 1 im bt k trn din tch y.

Sd

l vector c phng ca vector php tuyn n

vi din tch ang xt,

chiu l chiu dng ca php tuyn, ln bng ln din tch .

Trong h thng n v SI n v t thng l Weber (Wb). Nu t thng thay

i 1 n v trong thi gian 1 s, in p cm ng sinh ra trong cun dy l 1V.

1Wb = 1Vs (4.37)

d. Cng t trng HCng t trng H c trng cho t trng do ring dng in sinh ra

v khng ph thuc vo tnh cht mi trng trong t dng in.


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Trang 142

Trong h thng n v SI n v ca cng t trng H l A/m

e. Cm bin in tr tCm bin in tr t l 1 linh kin bn dn c hai cc, in tr ca n gia

tng di tc ng ca t trng. Trong trng hp t trng tc dng thng gc

mt phng ca cm bin ta c nhy ln nht. Chiu ca t trng khng nh

hng g n hiu ng in tr t trong trng hp ny.

ln ca tn hiu ra ca cm bin in tr t khng ph thuc vo tc

quay. Khc vi trng hp cm bin in cm, ln tn hiu ra quan h trc

tip vi tc quay, v vy i hi cc thit b in t phc tp c th thu

nhn c cc tn hiu trn 1 di in p rng.

Ngc li vi cm bin in tr t, tn hiu ra c hnh thnh bi s i

hng ca ng cm ng t thay i theo v tr ca bnh rng. Tn hiu ra ca

cm bin vn c hnh thnh d i tng khng di chuyn rt chm.

Hnh 4.57 Tn hiu to ra bi cm bin in tr t.

4.6.2Cm bin in tr t vi vt liu InSb / NiSbHiu ng in tr t vi vt liu InSb / NiSb

Vt liu bn dn InSb vi lin kt III V c linh ng rt ln. Trong vt

liu bn dn, di tc dng ca t trng hng dch chuyn ca cc in tch b


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Trang 143

lch i 1 gc tg= . B. Do s lch ny on ng dch chuyn ca electron

di hn. Kt qu l in tr cm bin gia tng di tc dng ca t trng.

hiu ng ny c th s dng trong thc t, gc cn phi ln. Trong kim loi, gc

ny rt b. Vi germanium gc lch ny khong 200, trong Indiumantimon do linh ng ca electron rt cao nn gc lch = 800vi B=1T.

to con ng dch chuyn ca electron cng di cng tt di tc dng

ca t trng, nh vy ng ra s c s thay i in tr ln hn, cm bin c

kt cu nh hnh 4.58. Nhiu phin InSb (b rng vi m ) c ghp ni tip

nhau. Gia cc phin ny l cc mng kim loi.

Hnh 4.58 Cm bin in tr t vi vt liu InSb / NiSb.

Trong thc t vi k thut luyn kim ngi ta to ra cc cy kim bng

Nickelantimon nm bn trong InSb c chiu song song vi hai cc in. Cho

mc ch ny, mt t NiSb c cho vo trong InSb chy lng v qua cc cng

on lm ngui v s cy kim NiSb c hnh thnh bn trong InSb. Cc cy

kim ny c ng knh khong 1 m v di 50 m . Cc cy kim ny dn in rt

tt v hu nh khng c in p ni trn n.

Mt in tch phn b khng u trong InSb do tc dng ca t trng,

s phn b c phn b u li trn cc cy kim. Nh th ta c s phn b

in tch ni khi u vng 1 ging nh ni khi u vng 2.

in tr t c th coi nh 1 hm ca cm ng t theo cch tnh gn ng

RB=R0(1+k22

.B ) (4.38)


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Trang 144

k l hng s vt liu c tr s khong 0,85.

in tr cm bin nm trong khong 10-500 . Din tch ct ngang ca bn

dn cng nh cng tt, tuy nhin chiu rng khng th nh hn 80 .m

4.6.3Cm bin in tr t vi vt liu permalloya. Hiu ng in tr t vi vt liu permalloy

Mt mng mng vt liu st t gi l permalloy (20% Fe, 80% Ni). Khi

khng c s hin din ca t trng, vector t ha bn trong vt liu nm song

song vi dng in. Vi 1 t trng nm song song vi mt phng mng mng

nhng thng gc vi dng in, vector t ha s quay i 1 gc . Kt qu l

in tr ca permalloy thay i theo . Hnh 4.59

Hnh 4.59 Hiu ng in tr t trn permalloy

R = R0 + 2

0 cosR (4.39)

max00 R=

min090 R=

R0v 0R l cc thng s ph thuc vo cht liu permalloy.

0R khong 2 n 3% ca R0.

Nguyn tc ny c ng dng o tc quay v gc quay.

b. Tuyn tnh ha c tnh ca cm binTheo phng trnh bc 2: R = R0 +

20 cosR th in tr cm bin in tr

t khng tuyn tnh (xem c tuyn a hnh 4.61). 1 cm bin tin li trong s

dng th tt nht l c tuyn ca n tuyn tnh, v vy bin php thit k tt hn

l iu cn thit.


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Trang 145

Hiu ng in tr t c th c tuyn tnh ha bng cch t 1 mng

mng nhm gi l (barber poles) ln trn mng mng permalloy vi gc 450so

vi trc ca mng mng (nh hnh 4.60). Nhm c tnh cht dn in tt hn so

vi permalloy, barber poles lm thay i gc ca dng in 450

. Nh vy gcgia dng in v vector s t ha t thnh(- 450). Hnh 4.60 biu din nh

hng ca barber poles ln c tnh ca cm bin in tr t.

Hnh 4.60 c tuyn ha dng mng mng nhm (barber poles)

to nn mt cm bin hon chnh cu Wheatstone vi 4 cm bin in

tr t c s dng. Trong cp cm bin i din nhau qua ng cho s c

cng s nh hng. iu ny c ngha l 1 cp cm bin c barber poles to

vi trc mt phng mng 1 gc + 450v 1 cp c barber poles to vi trc mt

phng mng 1 gc - 450.

iu ny lm cho bin tn hiu ra tng ln 2 ln v vn m bo s tuyn

tnh. Bn cnh nh hng ca nhit trong cu in tr s c b qua li.

Hnh 4.61 c tuyn R-H ca cm bin

a: c tuyn R-H ca cm bin loi tiu chun

b: c tuyn R-H ca cm bin loi c barber poles


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Trang 146

c. c im ca vic o tc vi cm bin in tr tCm bin in tr t khng th o trc tip tc quay m ch pht hin s

chuyn ng ca cc bnh rng lm t vt liu cha st (i tng th ng) v

hoc i tng quay c cc cc nam chm thay i tun t (i tng tch cc)

o i tng th ngc im hot ng ca cm bin vi i tng th ng c m t

hnh 4.62

Cm bin cn c gn vi 1 nam chm vnh cu.

Hnh 4.62 Cu trc i tng (hnh trn)

Cc thng s c trng ca i tng (theo tiu chun DIN)

K hiu Miu t n v

z S lng rng

d ng knh mm

m m =d/z mm

p (bc bnh rng) p = .m mm

Hnh 4.63 Cc thnh phn chi tit ca cm bin KMI 15/1 ca hng Philips

Semiconductors vi i tng th ng.


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