03_chuong 2_cam Bien Tiem Can.

Chng 2: Cm Bin Tim Cn

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Chng 2

CM BIN TIM CN (Proximity Sensor) 2.1 Gii thiu

Cm bin tim cn bao gm tt c cc loi cm bin pht hin vt th khng cn

tip xc ging nh cng tc hnh trnh, m da trn nhng mi quan h vt l gia

cm bin v vt th cn pht hin. Cm bin tim cn chuyn i tn hiu v s

chuyn ng hoc xut hin ca vt th thnh tn hiu in. C 3 h thng pht hin

thc hin cng vic chuyn i ny:

o H thng s dng dng in xoy c pht ra trong vt th kim loi nh hin

tng cm ng in t

o H thng s dng s thay i in dung khi n gn vt th cn pht hin

o H thng s dng nam chm v h thng chuyn mch cng t.

Theo tiu chun cng nghip ca Nht (JIS) nh ngha cm bin tim cn (JIS C

8201-5-2) ph hp vi chun IEC 60947-5-2 l b chuyn mch pht hin v tr

khng tip xc. Cm bin tim cn dng pht hin vt th kim loi t tnh, kim loi

khng t tnh (nh Nhm, ng..) s dng cm bin loi in cm (Inductivity

Proximity Sensor) v pht hin vt phi kim s dng loi cm bin tim cn kiu in

dung (Capacitve Proximity Sensor). ng thi c sn cc loi p ng c hu ht

cc iu kin mi trng lp t: nhit cao, nhit thp, chng nc, chng ha

cht

2.2 c im cm bin tim cn

Theo hng ng dng, cm bin tim cn thng c cc c im sau:

o Pht hin vt khng cn tip xc

o Tc p ng cao

o u cm bin nh c th lp nhiu ni

o C th s dng trong mi trng khc nghit

Hnh 2.

Cm bin tim cn l

mt vt th vi cm bin in t khng cng tc (khng ng chm). Cm bin tim

cn c mt v tr rt quan trng trong thc t. Th d pht hin vt tr

robot bt gi ly; pht hin chai, lon nhm tr

cm bin thng dng logic

Cm bin tim cn c

o Loi cm ng t: c hai lo

v ch pht hin c nh

o Loi cm ng in dung: tm pht hin ngn pht hin

in mi ln hn 1, th

o Loi cm bin quang: c ba loi xuy

tm o tng i xa c th l

o Loi cm bin siu m: c th

2.3 Cm bin tim cn

2.3.1. Gii thiu cm tim cn cm ng t

a. Hnh dng cm bin

Cm bin cm ng t

dng hnh ch nht nh h

Chng 2: C

Trang 22

Hnh 2.1 Cm bin tim cn loi pht hin vt ri v v

m bin tim cn l mt k thut nhn bit s c mt hay khng c mt ca

t vt th vi cm bin in t khng cng tc (khng ng chm). Cm bin tim

n c mt v tr rt quan trng trong thc t. Th d pht hin vt tr

ly; pht hin chai, lon nhm trn bng chuyn. Tn hiu ng

ng dng logic (c hoc khng).

m bin tim cn c cc loi:

c hai loi in cm v t cm, loi ny c t

c nhng vt c t tnh

n dung: tm pht hin ngn pht hin c cc vt c hng s

, thng c b phn chnh nhy.

i cm bin quang: c ba loi xuyn sut, phn x v khuy

i xa c th ln n 30m, pht hin c hu ht cc vt

u m: c th pht hin tt c cc vt tm pht hin trung b

m cn cm ng t (Magnectic sensor)

i thiu cm tim cn cm ng t

m bin cm ng t

t c rt nhiu hnh dng khc nhau, nhng th

hnh 2.2

Hnh 2.2 Hnh dng cm bin t


i v vt cn.

t k thut nhn bit s c mt hay khng c mt ca

t vt th vi cm bin in t khng cng tc (khng ng chm). Cm bin tim

n c mt v tr rt quan trng trong thc t. Th d pht hin vt trn dy chuyn

n. Tn hiu ng ra ca

y c tm pht hin ngn

c cc vt c hng s

khuyt tn loi ny c

c hu ht cc vt.

pht hin tt c cc vt tm pht hin trung bnh

ng thng s dng l


Trang 23

b. Nguyn tc ch to cm bin cm ng t

Nguyn tc ch to cm bin t da theo hin tng cm ng t Hall

Hnh 2.3 Hin tng cm ng t Hall.

Khi t mt tm vt liu mng (thng l bn dn) khi c dng in I chy qua

s sinh ra mt t trng B c phng to vi dng in mt gc th s xut hin mt

in th Vh theo hng vung gc gia B v I nh hnh 2.4

sin... BIKV hh = (2.1)

Trong Kh l h s vt liu

Hnh 2.4 Hot ng cm ng t Hall.

Hiu ng Hall c d

c gn vo thanh nam chm t

to ra s thay i khi tng tc v

th l kim loi.

2.3.2. Cm bin t loi in cm

a. Hnh dng

Cm bin in cm

dng l dng hnh tr trn v b

Cm ng t loi in cm da v

quanh n nhn bit vt. Cm bin loi in cm rt a dng nh

bin tim cn in cm c nhiu kch th

ng dng khc nhau. Cm bin tim cn in cm

Chng 2: C

Trang 24

c dng ch to cm bin xc nh s di chuyn ca vt, vt

o thanh nam chm to t trng B. T trng do cun dy ca cm bin

ng tc vi vt th kim loi do ch pht hin

Hnh 2.5 Nguyn tc chung ca cm bin t.

loi in cm (Inductive sensor)

m cng c rt nhiu hnh dng khc nhau, nh

n v bc bng kim loi nh hnh 2.6

Hnh 2.6 Cc loi cm bin t loi in cm

loi in cm da vo in cm ca cun dy sinh ra t tr

nhn bit vt. Cm bin loi in cm rt a dng nh

n in cm c nhiu kch thc v hnh dng khc nhau t

m bin tim cn in cm c dng


ch to cm bin xc nh s di chuyn ca vt, vt

ng do cun dy ca cm bin

i vt th kim loi do ch pht hin c vi vt

a cm bin t.

ng khc nhau, nhng thng s

i in cm.

n cm ca cun dy sinh ra t trng xung

nhn bit vt. Cm bin loi in cm rt a dng nh hnh 2.6. Cm

ng khc nhau tng ng vi cc

ng pht hin cc i

tng l kim loi (nhng chng

phi l kim loi).

b. Cu to

Cm ng t lo

nh vo cun dy to cm ng t vi

ny thng a ra 2 dy cp k

ra s c led ch th bo hiu cm bin nhn thy vt

Hnh 2.

Cu trc ca cm bin g

o 1 - Cun dy v li ferit

t trn cun dy ln li ferit, vng t

bin, nn vng ny s

Chng 2: C

Trang 25

nhng chng khng th pht hin cc i tng c cu to khng

Hnh 2.7 Cu to cm bin t loi in cm

loi in cm bao gm cc phn: Vng t tr

n dy to cm ng t vi tn s lm vic t 300 n 800kHz, loi

a ra 2 dy cp kt ni ngun v mt dy tn hiu ra. Khi c tn hiu

c led ch th bo hiu cm bin nhn thy vt trong vng tc

Hnh 2.8 Cu trc ca cm bin t loi in cm.

u trc ca cm bin gm 4 phn chnh nh hnh 2.9

li ferit: Phn ny to ra vng t trng nh hin t

n li ferit, vng t trng tp trung pha tr

n vng ny s tc ng ln vt cn o


ng c cu to khng

i in cm.

trng c to ra

c t 300 n 800kHz, loi

t dy tn hiu ra. Khi c tn hiu

trong vng tc ng.

u trc ca cm bin t loi in cm.

ng nh hin tng cm ng

p trung pha trc u ca cm


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o 2 - Mch dao ng: Chc nng mch ny to ra mch dao ng vi dng tn hiu

sin tn s cao (c th ln n 1MHz) khi c s tng tc ln b mt cm bin th

bin dao ng ca tn hiu ny s gim

o 3 - Mch pht hin: Chc nng mch ny da vo in th tham chiu pht

hin gi tr bin ca mch dao ng khi c vt tc ng ln b mt cm bin,

lc bin ny gim n mt gi tr cho php no s xut ra tn hiu ng ra

o 4 - Mch u ra: Ng ra l kt qu ca mch so snh ngng nhn bit c vt

nm trong tm hot ng ca cm bin hay khng, do tn hin ra dng

ON/OFF

c. Nguyn l hot ng

Cm bin cm ng t hot ng da trn nguyn tc thay i tng tc t trn b

mt ca cm bin nh hnh 2.9

Hnh 2.9 Cc thnh phn ca cm bin t loi in cm.

Tn s mch dao ng LC thng t 300Khz n 800 Khz c tnh theo cng

thc:

LCF

2

10 = (2.2)

nh hng ca dng in xoy lm thay i bin b dao ng LC. Mch pht

hin ngng (trigger) gim st s thay i bin v kch mch ng ra (output switch)

mi khi c s thay i vt ngng. Ng ra cm bin l tn hiu ON/OFF.


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Hnh 2.10 Trng thi tc ng ca cm bin t loi in cm.

Khi c vt th bng kim loi i vo vng t trng. T trng ny cm ng ln

vt th mt dng in xoay trn gi l dng in xoy. Dng in xoy to ra mt t

trng ngc chiu vi t trng ban u. T trng ny lm gim bin mch dao

ng LC bn trong cm bin. Hnh 2.10

Hnh 2.11 Hin tng t trng xoy ca cm bin t loi in cm.

T trng to ra xung quanh cun dy khi c dng in xoay chiu chay qua.

Khi c vt th bng kim loi a vo t trng. Dng in xoy s c cm ng vo

kim loi. Dng in xoy trong kim loi to ra t trng th hai ngc vi t trng


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ban u nh hnh 2.11. Dng in s b ngt qung khi c vt nt trn vt liu dn

in.

Cm bin tim cn in cm c thit k to ra mt vng in t trng, khi

mt vt bng kim loi tin vo khu vc ny, xut hin dng in xoy (dng in cm

ng) trong vt th kim loi ny. Hnh 1.12

Hnh 2.12 Nguyn l lm vic ca cm bin in cm.

Dng in xoy gy nn s tiu hao nng lng (do in tr ca kim loi), lm

nh hng n bin sng dao ng. n mt tr s no tn hiu ny c ghi

nhn. Mch pht hin s pht hin s thay i tn hiu v tc ng mch ra ln mc

ON (hnh 2.13). Khi i tng ri khi khu vc in trng, s dao ng c lp li,

cm bin tr li trng thi bnh thng.

Hnh 2.13 Hot ng ca cm bin tim cn in cm.


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Khi vt cn tc ng ln b mt ca cm bin, bin tn hiu ca mch dao

ng s thay i, bin ny s gim dn khi vt cng tin gn b mt tc ng. Khi

khong cch ln ng ra ca cm bin s cho ra mt xung v n ch bo s sng

ln.

d. Phn loi

Cm bin tim cn in cm c th phn lm 2 loi: Shielded (c bo v) v

unshielded (khng c bo v) nh hnh 1.14. Loi khng c bo v thng c tm pht

hin ln hn loi c bo v.

Hnh 2.14 phn loi cm bin in cm.

o Cm bin tim cn in cm loi c bo v: c 1 vng kim loi bao quanh (hnh

2.15) gip hn ch vng din t trng vng bn, tuy nhin khi co lp bo v

ny th vng t trng cng b gii hn bt i lm cho cm bin c tm pht hin

ngn li. V tr lp t cm bin c th t ngang bng vi b mt lm vic .

o Cm bin tim cn in cm loi khng c bo v: khng c vng kim loi bao

quanh, khng th lp t cm bin ngang bng b mt lm vic (bng b mt kim

loi). Loi cm bin khng c bo v xung quanh phi c 1 vng khng c cha

kim loi phn u cm bin (hnh 2.16) v th tm pht hin c xa hn

nhng d b nh hng bi cc vt xung quanh lm bng kim loi.

Hnh 2.15 Cm bin tim cn in cm loi c bo v.


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Hnh 2.16 Cm bin tim cn in cm loi khng c bo v.

c 2 loi cm bin c bo v v khng c bo v, nu c 1 b mt kim loi v

tr i din cm bin, khng nh hng n hot ng ca cm bin th b mt kim

loi ny phi cch b mt cm bin 1 khong cch c ln t nht gp 3 ln tm pht

hin ca cm bin.

e. u nhc im

o u im

Khng chu nh hng ca m.

Khng c b phn chuyn ng.

Khng chu nh hng ca bi bm.

Khng ph thuc vo mu sc.

t ph thuc vo b mt i tng hn so vi cc k thut khc.

Khng c vng cht (blind zone): cm bin khng pht hin ra i tng

mc d i tng gn cm bin.

o Nhc im

Ch pht hin c i tng l kim loi.

C th chu nh hng bi cc vng in t mnh.

Phm vi hot ng ngn hn so vi cc k thut khc.

2.3.3. Cm bin t loi t cm: (Inductive Magnectic sensor)

a. Loi tc ng bng tip im: (Reed Switch: li g)

Cu to

Cm bin t loi tc ng bng tip im

o B phn cha tip im: L

kh tr.

o Tip im (li g): lm b

o Led ch bo v dy n

Nguyn l hot ng

Cm bin loi n

khong cch cho php

li g lm ng tip im.

cn bng kim loi, k

Khi t trng to ra gn v tr ca l

ng li nh hnh 2.

Chng 2: C

Trang 31

m bin t loi tc ng bng tip im (li g) cu to gm cc b phn sau:

phn cha tip im: L mt ng thy tinh bn trong ch

): lm bng cht c kh nng t tnh nh st hay niken.

dy ni ngun, tn hiu ra.

Hnh 2.17 Cu to cm bin t loi li g

t ng

m bin loi ny pht hin c tt c cc loi vt liu dn in trong

ng cch cho php. T trng c to ra t cun dy cm ng tc ng l

p im. Cm bin b nh hng bi 3 y

khong cch vt cn.

ng to ra gn v tr ca li g, vi lc t ln s ht l

2.18

Hnh 2.18 Hai trng thi ca li g.


u to gm cc b phn sau:

n trong cha chn khng hay

t hay niken.

i g.

c tt c cc loi vt liu dn in trong

c to ra t cun dy cm ng tc ng ln

yu t: Nhit , vt

i lc t ln s ht li g

.

Khi c t trng i qua

v ht nhau, hai tip im s

ng bng tip im

S ph thuc cc t tr

o V tr ca im tc ng ph thuc v

o Dng cc i cn

b. Loi tc ng bng

Cu to

Cm bin t loi tc ng bng mch t cu to gm cc b phn sau:

o B phn cha tip im: L

kh tr.

o Tip im (li g): lm b

o Led ch bo v dy n

Hnh 2.

Cm bin t loi tc ng bng mch t da v

du thu khi c vt ti gn. Cm bin c cu to gm cc phn ging nh

tc ng bng li g

bng mch t.

Nguyn l hot ng

Chng 2: C

Trang 32

ng i qua vng cng tt th s lm cho hai ti

p im s c ni li. y l c s ch to cm bin

ng bng tip im.

trng khc nhau cn c trnh:

tr ca im tc ng ph thuc vo khong cch dch chuy

c gii hn trnh chy tip im.

i tc ng bng t cm (Inductive Magnectic sensor)

m bin t loi tc ng bng mch t cu to gm cc b phn sau:

phn cha tip im: L mt ng thy tinh bn trong ch

): lm bng cht c kh nng t tnh nh st hay niken.

dy ni ngun, tn hiu ra.

Hnh 2.19 Cu to cm bin t tc ng bng t tr

m bin t loi tc ng bng mch t da vo s tc

du thu khi c vt ti gn. Cm bin c cu to gm cc phn ging nh

i g ch khc tip im trong cm bin t loi l

t ng


m cho hai tip im nhim t

ch to cm bin tc

ch chuyn.

m bin t loi tc ng bng mch t cu to gm cc b phn sau:

cha chn khng hay

t hay niken.

u to cm bin t tc ng bng t trng.

tc ng ca t trng

du thu khi c vt ti gn. Cm bin c cu to gm cc phn ging nh loi

khc tip im trong cm bin t loi li g c thay

Cu to gm m

ng thu cm bin, khi vt c t tr

trng u thu v lm cho m

snh pht hin cho ng

Hnh 2.20 Cc thnh ph

Cn ch khi thc hin tr

o Thit b lm vic trng thi ngm tn s l

o Trnh dng loi cm bin n

2.3.4. c tnh v cc thng s

a. c tnh

Cm bin tim cn t

thng c hai loi nh

o Cm bin tim cn loi c bo v:

t b nhiu bi kim loi xung quanh, tuy nhi

o Cm bin tim cn loi khng c bo v:

mt cm bin nn kho

xung quanh.

Chng 2: C

Trang 33

mt b to dao ng tn s cao, dao ng n

m bin, khi vt c t trng tin n gn th s lm

lm cho mch dao ng ca cm bin b nh h

n cho ng ra cm bin dng on/off.

Cc thnh phn chnh ca cm bin t tc ng bng t tr

n ch khi thc hin trn cc dng t khc nhau.

c trng thi ngm tn s lm vic cao - 1kHz

i cm bin ny nhng ni c nhiu nh hng ca t tr

v cc thng s s dng

m bin tim cn ty theo cu to s c cc c tnh s dng khc nhau,

ng c hai loi nh sau

n loi c bo v: t trng c tp trung tr

nhiu bi kim loi xung quanh, tuy nhin khong cch o s ngn i.

Hnh 2.21 Cm bin t loi c bo v.

n loi khng c bo v: khng c bo v t tr

nn khong cch o s di hn, tuy nhin d b nhiu bi kim loi


ng ny to t trng

m nh hng n t

ch dao ng ca cm bin b nh hng, mch so

n chnh ca cm bin t tc ng bng t trng.

ng ca t trng.

c cc c tnh s dng khc nhau,

c tp trung trc mt sensor nn

ng cch o s ngn i.

m bin t loi c bo v.

o v t trng xung quanh

b nhiu bi kim loi

b. Cc thut ng thng s dng

Vt chun (Standard Object)

Mt vt c xem l

hp pht huy c cc c tnh k thut ca

Khong cch tc ng

Khong cch tc ng l

tnh ton nh l sai s

cp. Trong mt vi tr

khong cch tc ng n

n khi n n gn h

xem xt khi c lng

2.24

Khong cch hot ng hiu qu (E

Khong cch hot ng hiu qu iu kin ngun in cung cp n nh

vi nhit dao ng trong khong t 23

sai s trong qu trnh s

Chng 2: C

Trang 34

Hnh 2.22 Cm bin t loi khng c bo v.

ng s dng

t chun (Standard Object)

c xem l chun nu: hnh dng, vt liu, kch c ca vt ph

c cc c tnh k thut ca cm bin.

Hnh 2.23 M t vt chun.

ng cch tc ng (Rated Distance): Sn

ng cch tc ng l mt gi tr l thuyt m n khng ph

tnh ton nh l sai s ca qu trnh sn xut, nhit hot ng v

i trng hp cm bin c th pht hin ra vt th nm ngo

ng cch tc ng ny. Hoc c trng hp vt th khng

n khi n n gn hn khong cch tc ng. C mt vi gi

ng khong cch tc ng trong mi trng hp c th.

ng cch hot ng hiu qu (Effective Operating Distance

ng cch hot ng hiu qu iu kin ngun in cung cp n nh

ng trong khong t 23oC 0.5oC. N da

nh sn xut. Khong cch hot ng hiu qu

Vt chun: vt liu, hnh dng, kch thc, tc


m bin t loi khng c bo v.

ng, vt liu, kch c ca vt ph

n khng ph thuc vo

n xut, nhit hot ng v in p cung

ng hp cm bin c th pht hin ra vt th nm ngoi

ng hp vt th khng c pht hin cho

i gii hn phi c

ng hp c th. Hnh

istance): Sr

ng cch hot ng hiu qu iu kin ngun in cung cp n nh

a vo vic tnh ton

n xut. Khong cch hot ng hiu qu khong 10oC

t chun: vt liu, ng, kch

c, tc


Trang 35

ca khong cch tc ng . iu ny c ngha l vt th s c pht hin trong

khong t 0 n 90% ca khong cch tc ng. Ty thuc vo mi thit b, tuy

nhin, khong cch tc ng hiu qu c th xa hn 110% ca khong cch tc

ng. Hnh 2.24

Khong cch chuyn mch hu ch (Useful Switching Distance): Su

Khong cch chuyn mch hu ch l khong cch chuyn mch c tnh

ton theo iu kin in p v nhit l thuyt. Khong cch chuyn mch hu

ch th bng khong 10% ca khong cch hot ng hiu qu. Hnh 2.24

Khong cch hot ng chc chn (Guaranteed Operating Distance): Sa

Khong cch hot ng chc chn l khong cch chuyn mch cho s hot

ng ca cm bin tim cn trong phm vi iu kin hot ng c c th chp

nhn c m bo. Khong cch hot ng chc chn nm trong khong 0

81% ca khong cch hot ng. Hnh 2.24

Hnh 2.24 Khong cch hot ng ca cm bin.

Cc chuyn mch ca cm bin tim cn ch p ng li vi vt th khi vt

th nm trong khu vc xc nh ng trc b mt ca cm bin. im m

cm bin tim cn nhn ra c mt vt th mang n gi l im tc ng. V

im m khi vt th i ra lm thit b chuyn li trng thi ban u gi l im

thot. Khong cch gia 2 im ny gi l vng tr (hysteretis zone). Hnh 2.25


Trang 36

Hnh 2.25 Cc im chuyn mch ca cm bin.

Khong cch pht hin (Detecting Distance)

Khong cch pht hin l khong cch t b mt cm bin u cm bin

ti v tr vt chun xa nht m cm bin c th pht hin. Hnh 2.26

Hnh 2.26 M t khong cch pht hin.

Khong cch ci t (Sitting Distance)

Khong cch ci t l khong cch t b mt cm bin u cm bin ti

v tr vt cm bin cm bin c th pht hin vt n nh (thng khong cch

ny bng 70% n 0% khong cch pht hin). Hnh 2.27

Khong cch pht hin

Vt cm bin

Khong cch Reset

Cm bin tim cn B mt

cm bin

ON OFF


Trang 37

Hnh 2.27 M t khong cch ci t.

Thi gian p ng (Response Time)

o T1 l khong thi gian t lc i tng chun chuyn ng i vo vng

pht hin ca cm bin ti khi u ra cm bin bt v ON.

o T2 l khong thi gian t lc i tng chun chuyn ng i ra khi vng

pht hin ca cm bin ti khi u ra cm bin tt v OFF. Hnh 2.28

Hnh 2.28 M t thi gian p ng.

Tn s p ng (Response Frequency)

S ln tc ng lp li khi vt cm bin i vo vng hot ng ca cm

bin. Hnh 2.29

Vt cm bin

Dy hot ng Cm bin tim cn

B mt cm bin

Trong di hot ng

Ngoi di hot ng

ON

OFF

T1 T2

Khong cch ci t

Vt cm bin

Khong cch pht hin c lng

Cm bin tim cn

B mt cm bin


Trang 38

Hnh 2.29 M t tn s p ng.

c. Khong cch o v cc yu t nh hng

Vt liu i tng (Material):

Vt liu i tng l khong cch pht hin ca cm bin ph thuc rt

nhiu vo vt liu ca vt cm bin. Hnh 2.30

Hnh 2.30 M t c tnh t ca vt liu.

Cc vt liu c t tnh hoc kim loi c cha st s c khong cch pht

hin xa hn cc vt liu khng c cha t tnh hoc cha st. Hnh 2.31

Kim loi cu to nn vt th cng nh hng n khong cch vng tc

ng. Nhng khi mt kim loi no khc thp th mt h s chnh sa c s

dng hiu chnh.

Khong cch pht hin u sensor u sensor

Vt Vt

Kim loi khng c t tnh Kim loi c t tnh

Cm bin tim cn

khong cch cm bin

M

M 2M

T1 T2 T3

Vt cm bin

f=1/(T2+T2)


Trang 39

Hnh 2.31 Biu m t dn theo c tnh t.

Kch c ca i tng (Size):

Nu vt cm bin nh hn vt th chun (test object) khong cch pht hin

ca cm bin s gim. Hnh 2.32

Hnh 2.32 M t kch thc tng i ca ca i tng.

B dy ca i tng (Size):

dy ca vt th l mt h s khc cn c xem xt. Khong cch vng

tc ng th khng i i vi vt chun. Tuy nhin i vi vt th khng c st

trong thnh phn nh ng thau (brass), nhm (aluminum), v ng (copper)

th s xy ra hin tng hiu ng b mt (skin effect). Khong cch vng cm

ng s gim khi dy ca vt th tng ln. Nu dy ca vt th khc so vi

vt chun th mt h s chnh sa s c dng iu chnh li.

Vi vt cm bin thuc nhm kim loi c t tnh (st, Niken, SUS), b

dy vt phi ln hn 1mm.

Khong cch pht hin u sensor

u sensor

Vt Vt

Kch thc vt ln Kch thc vt nh

dn ca vt

Khong cch cm bin

Iron

SUS

Brass Aluminum

Copper

Vi vt cm bin thuc nhm kim loi khng c t tnh

cng mng th khong cch

Lp m bn ngoi c

Nu vt b m th

d

Khng m

Zn (5 15m)

Cd (5 15m)

Ag (5 15m)

Cu (10 20m)

Cu (5 15m)

Cu (5 10m) + Ni(Cu 10

Cu (5 10m) + Ni(10

S giao thoa v nh h

Cn phi ch khi s dng nhiu cm, khi c hai hay nhiu h

t k nhau hay i din nhau c th xy ra nhiu hoc s nh h

gia chng, dn n sai tn hiu u ra.

Chng 2: C

Trang 40

i vt cm bin thuc nhm kim loi khng c t tnh

ng cch pht hin cng xa. Hnh 2.33

Hnh 2.33 M t b dy ca i tng.

n ngoi ca vt (Plating):

u vt b m th khong cch cm bin ca vt s b nh h

dy v vt liu m Thp

100

m) 90 120

m) 100 110

m) 60 90

m) 70 90

m) ---

m) + Ni(Cu 1020m) 75 95

m) + Ni(10m) Cr(0,3m) 75 95

nh hng bi kim loi xung quanh:

n phi ch khi s dng nhiu cm, khi c hai hay nhiu h

t k nhau hay i din nhau c th xy ra nhiu hoc s nh h

n n sai tn hiu u ra.


i vt cm bin thuc nhm kim loi khng c t tnh, b dy ca vt

ng cch cm bin ca vt s b nh hng

ng

100

120 95 105

110 95 105

90 85 100

90 95 105

95 105

95 ---

95 95 105

n phi ch khi s dng nhiu cm, khi c hai hay nhiu hn cm bin

t k nhau hay i din nhau c th xy ra nhiu hoc s nh hng ln nhau


Trang 41

Khi c nhiu cm bin tim cn c t st nhau, s lm vic sai ca cc

cm bin c th l nguyn nhn giao thoa. V th phi chc chn quy nh mt

khong cch nh nht gia 2 cm bin nh hnh 2.34

Hnh 2.34 Khong cch m t cho s giao thoa.

Khi cc cm bin c t trn bng kim loi, n i hi phi che chn cm

bin trnh nh hng bi cc vt bng kim loi ngoi tr vt cn pht hin.

V th phi chc chn quy nh mt khong cch nh nht gia cc cm bin nh

hnh 2.35

Hnh 2.35 Khong cch m t nh hng bi kim loi

2.3.5. Cc loi u ra v cch mc ti

a. Loi u ra DC-2 dy

Khi cm bin c 2 dy ra ngun cung cp DC loi ny thng ti c mc

vo l relay thc hin nh hnh 2.36 v hnh 2.37

o S tc ng ng ra

Hnh 2.36 S ng ra iu khin loi DC-2 dy.


Trang 42

o S u dy

Hnh 2.37 S u dy ra ti loi DC-2 dy.

Ti c th mc dy ngun hoc dy mass cng ng

b. Loi DC-3 dy

Khi cm bin c 3 dy ra ngun cung cp DC loi ny thng ti c mc

vo l relay thc hin nh hnh 2.38, hnh 2.39, hnh 2.40, hnh 2.41

o S tc ng ng ra

Hnh 2.38 S ng ra iu khin loi NPN.

Hnh 2.39 S ng ra iu khin loi PNP.


Trang 43

o S u ni:

Hnh 2.40 S u dy ra ti loi 3 u dy ra.

Hnh 2.41 S u dy ra ti loi 3 u dy ra.

c. Loi AC-2 dy

Khi cm bin c 2 dy ra ngun cung cp AC loi ny thng ti c mc

vo l relay thc hin nh hnh 2.42 v hnh 2.43

o S ng ra iu khin:

Hnh 2.42 S ng ra iu khin loi AC-2 dy.

o S u dy:

d. Kt ni cm bin vi PLC

Cm bin c th kt ni trc tip hay gin tip v

dng dy ra ca cm bin hai hay ba dy nh

hnh 2.47

o Loi 2 u dy ra:

Hnh 2.44

Hnh 2.

Chng 2: C

Trang 44

Hnh 2.43 S u dy cho ti loi DC

t ni cm bin vi PLC

m bin c th kt ni trc tip hay gin tip vo ng vo PLC, ty theo

ng dy ra ca cm bin hai hay ba dy nh cc hnh 2.44, hnh 2.45, hnh 2.46,

i 2 u dy ra:

44 S kt ni cm bin vi PLC loi AC 220V.

Hnh 2.45 S kt ni cm bin vi PLC loi DC 24V.


u dy cho ti loi DC-2 dy.

o ng vo PLC, ty theo

nh 2.44, hnh 2.45, hnh 2.46,

kt ni cm bin vi PLC loi AC 220V.

kt ni cm bin vi PLC loi DC 24V.

o Loi 3 u dy ra:

Hnh 2.46

Hnh 2.47 S

e. Mt vi ch khi s dng

o Khi s dng cm bin tim c

nu khng cc linh lin b

trong 2 dy nh h

Hnh 2.

Trng hp dng t

o Loi AC-2 dy:

mong mun bi in p d

d l nh hn i

song vi ti nh

Chng 2: C

Trang 45

ra:

S kt ni cm bin vi PLC loi NPN D

S kt ni cm bin vi PLC loi PNP DC

dng

dng cm bin tim cn loi DC hay AC-2 dy, ph

u khng cc linh lin bn trong s h hng. Ti c th

nh hnh 2.48

Hnh 2.48 Kt ni ti vi cm bin loi 2 dy ra.

ng ti nh:

2 dy: C th l nguyn nhn lm ti hot ng p li khng nh

n bi in p d. Nu dng ti di 5mA, ta phi m bo in p

n in p p li ca ti bng cch kt ni mt in tr s song

nh hnh 2.49


kt ni cm bin vi PLC loi NPN D -3 dy.

kt ni cm bin vi PLC loi PNP DC -3 dy.

2 dy, phi kt ni vi ti

i c th c kt ni vi 1

t ni ti vi cm bin loi 2 dy ra.

i hot ng p li khng nh

i 5mA, ta phi m bo in p

i bng cch kt ni mt in tr s song


Trang 46

( )=I

VR S ; ( )W

R

VP S

2

= (2.3)

I: dng lm vic ca ti

R: in tr s

P: cng sut gii hn

Hnh 2.49 Kt ni ti vi cm bin loi AC- 2 dy ra ti nh.

o Loi DC-2 dy: Hy to dng trn cm bin tin cn nh hn dng p li

ca ti bng cch kt ni thm mt in tr s song song vi ti nh hnh

2.50

( )

=offo

S

II

VR ; ( )W

R

VP S

2

= (2.4)

VS: ngun cung cp

Io : dng lm vic Min ca cm bin tim cn

Ioff: dng p li ca ti

R: in tr s

P: Tng s W ca in tr s

Hnh 2.50 Kt ni ti vi cm bin loi DC-2 dy ra ti nh.

2.3.6. Mt vi cm bin tim cn loi cm ng t

o Loi c b khuch i ri

Hnh

Hnh 2.52 Kim tra kp.

o Loi hnh ch nht

o Loi pht hin vt kim loi r

Hnh 2.

Chng 2: C

Trang 47

n tim cn loi cm ng t

i c b khuch i ri:

Hnh 2.51 Cm bin loi c b khuch i ri.

m tra kp. Hnh 2.53 Kim tra gy mi khoan.

nht

Hnh 2.54 Cm bin hnh ch nht.

i pht hin vt kim loi ri

Hnh 2.55 Cm bin pht hin vt kim loi r


m bin loi c b khuch i ri.

i khoan.

nht.

m bin pht hin vt kim loi ri.


Trang 48

Hnh 2.56 Cm bin pht hin m hay kim loi ri.

2.4 Cm bin in dung (Capacitive Sensor)

2.4.1. Gii thiu cm bin in dung

a. Hnh dng

Cm bin in dung thng c cu to dng tr trn bn ngoi c bao

bc bi lp nha do nh hnh 2.57

Hnh 2.57 Cm bin tim cn loi in dung.

Cm bin in dung dng pht hin cc loi vt th c hng s in mi ln

hn hng s in mi khng kh nh: Nha, thy tinh, g, kim loi,... Cm bin

tim cn in dung ging v kch thc, hnh dng, c s hot ng so vi cm

bin tim cn in cm. im khc bit cn bn gia chng l cm bin tim cn

in dung to ra vng i

in t trng. Cm bin tim cn in dung c th pht hin i t

liu kim loi cng nh

b. Cu to

Hnh 2.

Cm bin in dung c cu to bao g

ngun, in cc ni mass

Cng ging nh c

dung c 4 phn nh h

in cc, tip theo l

mch ng ra cng c hai dng NPN hay PNP

Chng 2: C

Trang 49

ng in trng cn cm bin tim cn in cm to ra v

ng. Cm bin tim cn in dung c th pht hin i t

i cng nh khng phi kim loi. Hnh 2.58

Hnh 2.58 Cm bin in dung pht hin vt

Hnh 2.59 Cu to cm bin tim cn loi in dung.

m bin in dung c cu to bao gm cc thnh ph

n, in cc ni mass, led ch bo v nt hiu chnh nhy nh

nh cm bin tim cn in cm, cm bin tim cn loi in

nh hnh 2.60 bao gm phn to in trng cu to gm hai

n cc, tip theo l mch dao ng, mch pht hin v m

ng c hai dng NPN hay PNP


m bin tim cn in cm to ra vng

ng. Cm bin tim cn in dung c th pht hin i tng c cht

m bin in dung pht hin vt.

m bin tim cn loi in dung.

m cc thnh phn: in cc ni

u chnh nhy nh hnh 2.59.

m bin tim cn in cm, cm bin tim cn loi in

ng cu to gm hai

mch ng ra thng

Hnh 2.

o B phn cm bin (cc bn cc

o Mch dao ng

o Mch ghi nhn tn hiu

o Mch in ng ra

c. Nguyn l hot ng

Cm bin tim cn in dung p

i in dung gia v

Hnh 2.62

Cm bin mc in dung hot ng da tr

gia cht lu v khng kh. i

Chng 2: C

Trang 50

Hnh 2.60 Cc thnh phn ca cm bin dung.

Hnh 2.61 Cc in cc cm bin dung.

phn cm bin (cc bn cc cch in)

ch ghi nhn tn hiu

ra

t ng

m bin tim cn in dung pht hin theo nguyn tc tnh in (s thay

vt cm bin v u cm bin), c th pht hin

Nguyn tc hot ng cm bin tim cn loi in dung.

m bin mc in dung hot ng da trn s khc bit hng s in mi

u v khng kh. iu kin cn thit p dng ph


n ca cm bin dung.

n cc cm bin dung.

c tnh in (s thay

pht hin tt c cc vt.

m bin tim cn loi in dung.

khc bit hng s in mi

u kin cn thit p dng phng php ny l


Trang 51

hng s in mi ca cht lu phi ln hn hng s in mi ca khng kh,

thng l gp i. Hng s in mi ca khng kh l 1.0; du c hng s in

mi t 1,8 n 5; nc c hng s in mi gia khong 50 n 80.

Khi mc cht lu thay i th hng s in mi cng thay i tng ng.

Mt loi t in c gi l cu in dung o ton b in dung v cho tn hiu

o lin tc.

T in gm hai bn cc bng kim loi t cch nhau qua vt liu in mi.

Cm bin dung l t in m: Gm hai bn cc t hng ra pha trc nh

hnh 2.63

Hnh 2.63 Hai loi cm bin tim cn loi in dung.

B mt cm bin l mt bn cc. Khi c in p xoay chiu t vo b mt

to ra mt in trng bin thin. in p ny ly im t nh mc chun

T in gm hai bn cc v cht in mi gia. Khong cch gia hai

in cc nh hng n kh nng tch tr in tch ca mt t in (in dung l

i lng c trng cho kh nng tch tr in tch ca mt t in). Nguyn tc

hot ng ca cm bin tim cn loi in dung da trn s thay i in dung

khi vt th xut hin trong vng in trng. T s thay i ny trng thi On

hay Off ca tn hiu m ng ra s c xc nh. Mt bn cc l thnh phn

ca cm bin, i tng cn pht hin l bn cc cn li. Mi quan h gia bin

sng dao ng v v tr i tng cm bin tim cn in dung tri ngc so

vi cm bin tim cn in cm. Hnh 2.64


Trang 52

Hnh 2.64 Hot ng ca cm bin tim cn in dung

Hnh 2.65 Sng dao ng mch dao ng cm bin in cm v in dung

Cm bin tim cn loi in dung c th pht hin bt c loi i tng no

c hng s in mi ln hn khng kh. Vt liu cng c hng s in mi cng

cao th cng d c cm bin pht hin. V d nc v khng kh, cm bin

tim cn in dung rt d dng pht hin ra nc (hng s in mi = 80) nhng

khng th nhn ra khng kh (hng s in mi = 1). i vi cc cht kim loi

khc nhau, kh nng pht hin ca cm bin l khng i. Nhng i vi cc

cht khc, th phm vi pht hin ca cm bin i vi tng cht l khc nhau. V

vy, cm bin tim cn in dung c th dng pht hin cc vt liu c hng

s in mi cao nh cht lng d n c cha trong hp kn (lm bng cht liu

c hng s in mi thp hn nh thy tinh, plastic). Cn chc chn rng i

tng cm bin pht hin l cht lng ch khng phi hp cha.


Trang 53

Hnh 2.66 Cm bin in dung pht hin cht lng trong bnh cha.

Khi cha c vt gia hai bn cc: Hnh 2.67

Hnh 2.67 Khi khng c vt th gia hai in cc

o in dung gia hai bn cc:

d

AC r

0= (2.5)

r :Hng s in mi gia hai bn cc.

0 : Hng s in mi chn khng. ( mF /10.854187.812

0= )

o Nu khng c vt th gia hai bn cc:

d

ACr

0;1

== (2.6)

Nu gia hai bn cc tn ti vt th khng dn in: Hnh 2.68


Trang 54

Hnh 2.2.68 Khi c vt th (phi kim loi) gia hai in cc


d

AC r

0= (2.7)

o Khi c vt th gia hai bn cc th: >1r C s tng

Do s tng gi tr ca C c s dng pht hin vt th.

Nu tn ti vt th kim loi gia hai bn cc dn in: Hnh 2.69

Hnh 2.69 Khi c vt th (kim loi) gia hai in cc


ed

AC r

=

0 (2.8)


Trang 55

o V mi trng gia hai bn cc l khng kh nn:

ed

ACr

== 01

(2.9)

S c mt ca vt dn in gia hai bn cc cng lm in dung tng

Trong trng hp in cc th hai cch ly vi mt t:

o Nguyn tc nh cc trng hp trn.

o Cn tm ng lin kt bn cc vi im ni t.

Hnh 2.70 Khng c in cc tip t

o Nu khng c ng ni t cho bn cc th 2.

o in cc t ni trc tip trn b mt cm bin.

o iu ny to ra mt in trng c lp vi im cc t.

d. Phn loi cm bin in dung

Cm bin tim cn in dung cng phn thnh 2 loi: shielded (c bo

v) v unshielded (khng c bo v).

o Loi c bo v c vng kim loi bao quanh gip hng vng in trng v

pha trc v c th t ngang bng vi b mt lm vic.

o Loi khng c bo v khng c vng kim loi bao quanh v khng th t

ngang bng vi b mt lm vic. Xung quanh cm bin phi c 1 vng trng

(ging cm bin tim cn in cm loi khng c bo v), kch thc vng

trng ty thuc vo tng loi cm bin.


Trang 56

e. c im cm bin in dung

o C th pht hin st, kim loi, nha, nc, si, g,

o Tui th di v tinh cy cao.

o C mch bo v chng ni ngc cc ngun.

o D dng iu chnh khong cch bng volume. iu chnh c nhy bn

trong.

o C th kim tra tnh trng hot ng bi ch th LED .

o D dng kim tra mc v v tr.

f. Nhng yu t nh hng n tm pht hin ca cm bin tim cn in dung

o Kch thc ca in cc ca cm bin.

o Vt liu v kch thc i tng.

o Nhit mi trng.

i tng tiu chun v hng s in mi

i tng tiu chun c ch nh ring vi tng loi cm bin tim cn

in dung. Thng thng cht liu ca i tng tiu chun c nh ngha l

kim loi hoc nc.

Hnh 2.71 i tng tiu chun v hng s in mi ca cm bin in dung.


Trang 57

Hnh 2.71 Biu din mi quan h gia kh nng pht hin i tng v

hng s in mi.

g. u im v nhc im ca cm bin tim cn in dung

u im:

o C th cm nhn vt dn in v khng dn in.

o Tnh cht tuyn tnh v nhy khng ty thuc vo vt liu kim loi.

o N c th cm nhn c vt th nh, nh.

o Tc hot ng nhanh.

o Tui th cao v n nh cng cao i vi nhit .

Nhc im:

o B nh hng bi m

o Dy ni vi sensor phi ngn in dung dy khng nh hng n b

cng hng ca b dao ng.

2.4.2. Cc i lng cn ch khi s dng

a. iu chnh nhy

Khi khng c mc tiu pht hin, hay khi c s dao ng th khong cch

gia cm bin v vt. Ty theo tng loi vt v loi cm bin m ta c th chnh

cho ph hp (xem ch dn ca nh sn xut)

b. S giao thoa v nh hng bi kim loi xung quanh

Khi c nhiu cm bin tin cn c t st nhau, s lm vic sai ca cc

cm bin c th do nguyn nhn ca s giao thao ln nhau. V th ta phi chc

chn quy nh mt khong cch nh gia 2 cm bin nh cc hnh 2.72

Hnh 2.72 Cc cch iu chnh nhy.


Trang 58

Khi cc cm bin c t trn cc bng kim loi, n i hi che chn cm

bin trnh nh hng bi cc vt bng kim loi ngoi tr vt cn pht hin.

V th ta phi chc chn quy nh mt khong cch nh gia 2 cm bin nh

hnh 2.73

Hnh 2.73 Cc cch che chn cm bin.

c. nh hng ca cc loi vt liu

o Vt liu ca vt cm bin: Khong cch cc cm bin c th khc nhau bi

cc c trng in ca vt cm bin (tnh cht dn in, hng s in mi) v

tnh trng ht nc, kch thc.

o in trng c tn s cao: N c th gy ra s c cho my mc m cc my

ny pht ra tn s cao ca in trng v d nh my git

o Mi trng xung quanh: C nc hoc du trn b mt cm bin, hay cc

chai pht hin mc b ph du n c th gy ra trc trc. c bit loi c

ng knh nh (15mm) c nhy cao vi cc vt c in cm ta phi ch

ng b thm nc.

o Du: khng cho du chy vo cm bin nht l i vi loi v lm bng

nha.

d. S tip t

Khong cch cm bin s c thay i bi tnh trng tip t ca cm bin

tim cn loi in dung v vt [50x50x1mm (st)]. Hy kim tra vt liu trc

khi lp n vo bng.


Trang 59

2.5 Cm bin siu m: (Ultrasonic sensor)

2.5.1. Khi nim dng siu m

Siu m l sng c hc c tn s ln hn tn s m nghe thy (trn 20kHz).

Thnh gic ca con ngi rt nhy cm vi di tn s t m trm (vi chc Hz)

n cc m thanh rt cao (gn 20kHz). Mt s loi vt nh di, ong c th cm

nhn c siu m.

Gii thiu v cm bin siu m: Cm bin siu m l thit b dng xc

nh v tr ca cc vt thng qua pht sng siu m. Cm bin siu m c th pht

hin ra hu ht cc i tng l kim loi, phi kim, lng hoc cht rn, vt trong

hoc m c (nhng vt c h s phn x sng m thanh ln).

2.5.2. Nguyn l ch to

Sng siu m c truyn i trong khng kh vi vn tc khong 343m/s.

Nu mt cm bin pht ra sng siu m v thu v cc sng phn x ng thi, o

c khong thi gian t lc pht i ti lc thu v, th my tnh c th xc nh

c qung ng m sng di chuyn trong khng gian. Qung ng di

chuyn ca sng s bng 2 ln khong cch t cm bin ti chng ngi vt, theo

hng pht ca sng siu m. Hay khong cch t cm bin ti chng ngi vt

s c tnh theo nguyn l TOF (Time Of Flight) hnh 2.74

Hnh 2.74 Nguyn l TOF.

Cm bin mc sng si

gian t lc truyn sng

m s dng sng di tn s

trong khng kh 15 C) ph thuc v

bnh cha.

Hnh 2.76 Mt vi lo

2.5.3. Gii thiu cm bin si

a. Cu to

Cm bin tim cn si

o B phn pht v nh

o B phn so snh (Comparator)

o Mch pht hin (Detector Circuit)

b phn so snh tnh ton khong cch bng cch so snh

nhn v vn tc m thanh.

o Mch in ng ra (Output):

hiu t cm bin digital bo c hay khng s xut hin i t

Chng 2: C

Trang 60

Hnh 2.75 Cc loi cm bin siu m.

m bin mc sng siu m c xc nh mc bng cch o khong thi

sng ti lc nhn c sng phn hi. Cm bin mc sng si

dng sng di tn s hn 10 KHz. Tc truyn ca sng (34

15 C) ph thuc vo loi kh v nhit ca kh b

i loi cm bin tim cn siu m do Siemens s

i thiu cm bin siu m

n siu m c 4 phn chnh: Hnh 2.77

nhn sng siu m (Transducer / Receiver)

phn so snh (Comparator)

n (Detector Circuit): Khi cm bin nhn

phn so snh tnh ton khong cch bng cch so snh

n tc m thanh.

ra (Output): Tn hiu ng ra c th l digital ho

u t cm bin digital bo c hay khng s xut hin i t


nh mc bng cch o khong thi

m bin mc sng siu

n ca sng (343m/giy

t ca kh bn trong

u m do Siemens sn xut.

u m (Transducer / Receiver)

m bin nhn c sng phn hi,

phn so snh tnh ton khong cch bng cch so snh thi gian pht,

digital hoc analog. Tn

u t cm bin digital bo c hay khng s xut hin i tng trong vng


Trang 61

cm nhn ca cm bin. Tn hiu t cm bin analog cha ng thng tin

khong cch ca i tng n cm bin.

Hnh 2.77 Cc thnh phn ca cm bin tim cn siu m.

b. Nguyn l hot ng

K thut cm bin siu m da trn c im vn tc m thanh l hng s.

Thi gian sng m thanh i t cm bin n i tng v quay tr li lin h trc

tip n di qung ng. V vy cm bin siu m thng c dng trong

cc ng dng o khong cch. Hnh 2.78

Hnh 2.78 Sng m thanh phn hi khi i tng l cht rn, cht lng.

Tn s hot ng: Nhn chung, cc cm bin cng nghip hot ng vi tn

s 25 khz n 500 Khz. Cc cm bin trong lnh vc y khoa th hot ng vi

khong tn s t 5MHz tr ln. Tn s hot ng ca cm bin t l nghch vi

khong cch pht hin cm bin. Vi tn s 50 kHz, phm vi hot ng ca cm

bin c th ln ti 10 m hoc hn, vi tn s 200 kHz th phm vi hot ng cm

bin b gii hn mc 1 m.

Vng hot ng: l khu vc gia 2 gii hn khong cch pht hin ln nht

v nh nht Cm bin tim cn siu m c mt vng nh khng th s dng gn

b mt cm bin gi l khu vc m hay vng cht (blind zone). Hnh 2.79


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Hnh 2.79 Vng hot ng ca cm bin tim cn siu m.

Cm bin siu m bao gm: mt b pht sng m lm bng a gm s s

pht sng lan tuyn trong khng gian n khi gp vt cn s c sng di li. Hnh

2.80

Hnh 2.80 S lan truyn sng m ca cm bin siu m.

Trong mt chu k lm vic cm bin s c lin tc sng di v cho n khi

xung di v c bin bng xung pht i ban u s kt thc mt chu k thu

nhn. Hnh 2.81

Hnh 2.81 Chu k c xung ca cm bin siu m.


Trang 63

Vng hot ng ca cm bin siu m bao gm: Vng cht, vng nhn thy

v vng ci t trc c (vng hot ng). Hnh 2.82

Hnh 2.82 Cc vng hot ng ca cm bin siu m.

Khi bt u pht xung l lc sng m s pht i n khi gp vt cn sng s

di li nhng vng hot ng ph hp ch cch u cm nhn ca cm bin

khong 6 n 8 cm vng ny gi l vng cht. Hnh 2.83

Hnh 2.83 Hot ng ca cm bin siu m

Gc qut ca cm bin siu m l vng mu xanh, cn li l vng m cm

bin khng o c gi l vng m. Hnh 2.84


Cm bin siu m c m hnh ha thnh mt hnh qut, trong cc

im gia dng nh khng c chng ngi vt, cn cc im trn bin th

dng nh c chng ngi vt hnh 2.85


Trang 64


2.5.4. Sai s nhiu ph bin vi cm bin siu m

a. Hin tng forecasting

Hin tng Forecasting l hin tng phn x gc sai lch ca cm bin.

Do nguyn l TOF, c khong cch ng, cm bin siu m phi hng

vung gc vi b mt chng ngi vt cn o. Tuy nhin, cc chng ngi vt

khng bao gi l phng, mn, nn tia phn x c th khng tng ng vi gc ti.

Cc chm tia phn x ny c nng lng phn x thp hn nh hnh 2.86.

Hnh 2.86 S phn x ca sng siu m trn b mt vt liu

Tuy vy, mt khong cch no , cm bin siu m vn c th ghi nhn

c nhng tn hiu phn x ny. Kt qu, thng s c v ca cm bin siu m

b lch do gc m ca cm bin siu m ln. Hnh nh v hin tng Forecasting

nh hnh 2.87.


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Hnh 2.87 Hin tng Forecasting

Ngoi ra, v gc m rng nn khng ch sai v nhn dng v tr chng ngi

vt, m khong cch ghi nhn cng b sai lch. Tuy vy, sai s ny khng ng

k nh sai s do hin tng c cho gy ra. V sai s ny l sai s c th qun l

c, trong khi , sai s do hin tng c cho khng th qun l c. Sau

y chng ta s tm hiu v hin tng c cho v phng php x l hin

tng c cho.

b. Hin tng crosstalk

Hin tng c cho (crosstalk) l hin tng m cm bin siu m ny ghi

nhn tn hiu phn x hoc tn hiu trc tip t cm bin siu m khc, hoc sau

qu trnh sng siu m truyn i v phn x qua cc b mt quay li cm bin

mt cch khng mong mun. Hnh 2.79 m t hin tng c cho ca cm bin

siu m

Hnh 2.79 Hin tng c cho ca cm bin siu m


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Hin tng c cho c th phn loi thnh hai loi chnh:

o Loi 1 l hin tng nhiu robot hot ng trong cng mt mi trng, v cm

bin siu m ny ghi nhn tn hiu ca cm bin siu m kia mt cch trc tip

hoc gin tip, hoc do sng siu m di chuyn trong khng gian sau mt thi

gian ngu nhin no quay tr li cm bin.

o Loi 2 l hin tng cm bin siu m ny ghi nhn tn hiu ca cm bin siu

m kia trn cng robot sau qu trnh phn x.

Cn phi phn bit hai qu trnh c cho ny, bi v ta thy rng, i vi

hin tng c cho loi mt, r rng, sng siu m sai lch l ngu nhin, khng

d on c. Tuy nhin, v tnh ngu nhin ny, vic x l nhiu (tn hiu sai t

cm bin khc) tr nn d dng hn thut ton EERUF cho php x l c hai loi

ca hin tng c cho.

tng c bn ca thut ton ny nm ch, xem cc tn hiu ghi nhn

c l nhiu, v s dng nhng quy nh pht v thu loi b c cc nhiu

khng hp l.

Phn tch x l hin tng crosstalk

Mc tiu ca vic x l hin tng c cho l nhm nng cao tc c

ca cm bin v on trc c ng c cho khi no xy ra. Ngoi vic xc

nh thi im xy ra hin tng c cho th ta c th pht thm mt cm bin

v ch c cm bin, i mt thi gian t cho nng lng sng m yu dn v

khng lm nh hng ln cc cm bin khc trong cng mt mi trng. hnh

2.80

Hnh 2.80 t lch thi gian pht sng


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Trong hin tng c cho loi 1, nu cc ng c cho hnh thnh ngu

nhin v khng th xc nh trc, ta nhn thy c th s dng phng php lp,

c nhiu ln nhn ra c tn hiu ln u ghi nhn c l tn hiu gy nhiu.

nh vy vic c cm bin nhiu ln cho php ta loi b c cc ng c

cho ngu nhin.

Trong hin tng c cho loi 2 khng phn bit c ng cho ln hai

v ln ba. Chng ta s dng phng php Borenstein pht hin ng c

cho khi n xut hin v thay i cc gi tr thi gian ch sau mi chu k c.

Hnh 2.81

Hnh 2.81 phng php x l hin tng c cho

c. Sai s do mi trng

Sai s do nhit i tng cn o, nhit v m mi trng c m

t nh hnh 2.82

Hnh 2.82 nh hng ca nhit i tng cn o


Trang 68

Khi nhit mi trng thay i cng nh hng n kt qu o, th m

t qu trnh thay i nh hnh 2.83

Hnh 2.83 nh hng ca nhit i tng cn o

Khi m t mi trng thay i cng nh hng n kt qu o, th

m t qu trnh thay i nh hnh 2.84

Hnh 2.84 nh hng ca m mi trng

Nhit b mt ca i tng nh hng n phm vi hot ng ca cm

bin. Hi nng ta ra t i tng c nhit cao lm mo dng sng, lm cho

khong cch pht hin ca i tng ngn li v gi tr khong cch khng chnh

xc.


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Hnh 2.85 nh hng ca nhit i vi sng phn hi.

2.5.5. Cc ch khi s dng cm bin siu m

a. Cch b tr cm bin

Kch thc v vt liu ca i tng cn pht hin quyt nh khong cch

pht hin ln nht (xem hnh 2.86)

Hnh 2.86 Khong cch hot ng ca cm bin tim cn siu m.

Khong cch gia cc cm bin khi b tr cc cm bin gn nhau cng cn ch

khi b tr cm bin siu m cn ch phi tha mn cc iu kin theo cc bng theo

tng trng hp nh cho trong cc bng nh sau:

o Khi b tr song song hai cm bin khong cch gia chng nh hnh 2.87

o Khi b tr i mt hai cm bin khong cch gia chng nh hnh 2.88


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o Khi b tr cm bin gn nhng vt c b mt phn x khc nhau khong cch

gia chng nh hnh 2.89

Hnh 2.87 B tr song song hai cm bin

Hnh 2.88 B tr mt i mt.

Hnh 2.89 nh hng ca vt so vi vng hot ng

B mt phng phn hi nng lng ca sng m thanh tt hn b mt g

gh. Tuy nhin b mt trn phng li c i hi khc khe v v tr gc to thnh

gia cm bin v mt phng i tng xem hnh 2.91


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Hnh 2.91 t cm bin siu m i vi i tng c b mt g gh v gc

nghing so vi phng ngang 3o.

b. u nhc im ca cm bin siu m

u im:

o Khong cch m cm bin c th pht hin vt th ln ti 15m.

o Sng phn hi ca cm bin khng ph thuc mu sc ca b mc i tng

hay tnh cht phn x nh sng ca i tng v d b mt knh trong sut, b

mt gm mu nu, b mt plastic mu trng, hay b mt cht liu nhm sng,

trng... l nh nhau.

o Tn hiu p ng ca cm bin tim cn siu m analog l t l tuyn tnh vi

khong cch. iu ny c bit l tng cho cc ng dng nh theo di cc

mc ca vt cht, mc chuyn ng ca i tng.

Nhc im:

o Cm bin tim cn siu m yu cu i tng c mt din tch b mt ti

thiu (gi tr ny ty thuc vo tng loi cm bin).

o Sng phn hi cm bin nhn c c th chu nh hng ca cc sng m

thanh tp m.

o Cm bin tim cn siu m yu cu mt khong thi gian sau mi ln sng

pht i sn sng nhn sng phn hi. Kt qu thi gian p ng ca cm

bin tim cn siu m nhn chung chm hn cc cm bin khc khong 0,1s.


Trang 72

o Vi cc i tng c mt vt cht thp nh bt hay vi (qun o) rt kh

pht hin vi khong cch ln.

o Cm bin tim cn siu m b gii hn khong cch pht hin nh nht.

o S thay i ca mi trng nh nhit (vn tc m thanh ph thuc vo

nhit ), p sut, s chuyn khng ng u ca khng kh, bi bn bay

trong khng kh gy nh hng n kt qu o.

03_chuong 2_cam Bien Tiem Can.

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Transcript of 03_chuong 2_cam Bien Tiem Can.