Ebook Cơ sở kỹ thuật đo lường điện tử (in lần thứ tư có sửa chữa, bổ sung): Phần 1

Phần 1 cuốn sách Cơ sở kỹ thuật đo lường điện tử giới thiệu tới người đọc các nội dung: Giới thiệu chung về kỹ thuật Đo lường điện tử, định giá sai số đo lường, quan sát và đo lường dạng tín hiệu, đo tần số, khoảng thời gian và đo độ đi pha. Mời các bạn cùng tham khảo.

VO QUY DIEM (Chu bien)

PHAM VAN TUAN

PGS Vu Quy Diem ( Chu bien)

Pham Van Tuan, Nguyen Thuy Anh

Do Le Phu, Nguyen Ngoc Van

In Ian thit tit co sua chita va bo sung

NHA X U A T BAN KHO A HOC VA K Y TH UA T

LCfl N O I D A U

Giao trinh “ Co so ky thuat do luong dien tur” duoc bien soan nhim phuc vu viec

hoc tap ciia sinh vien dai hoc thuoc cac nganh ky thuat Dien tir - Vi£n thong Cuon sach nay ciing co the dung lam tai lieu tham khao cho cac nganh ky thuat khac co sir dung ky thuat Do ludng dien tir nhu la mot phuong phap de nghien curu khoa hoc, sir dung khai thac ky thuat cua nganh minh

Do sir phat trien cua nganh Dien tir - Vi6n thong va su phat trien cua ky thuat Dien tir, ky thuat Do luong dien tir cung thay doi va phat trien rat nhieu Noi ve sir phat trien cua ky thuat Do luong dien tu' tru'cfc het phai noi den nhung thay d6i co ban cua cac thiet

bi do co su dung cac bo vi xir ly (microprocessors) Viec ap dung ky thuat vi xir ly va do

ludng da lam tang tinh nang va thong so cua thiet bi do len rat nhieu, da mo ra cach giai quyet cac van de ma trudc kia chua dat ra duoc Cac bo vi xir ly da tro thanh mot bo phan thiet yeu cua cac thiet bi do Co bo vi xu ly cac thiet bi do co the da chirc nang hon, don gian hoa viec dien khien, tir dong di6u chinh, tir dong lay chuan, tir dong kiem tra, lam tang them do tin cay cua cac thong so phep do, cho phep thuc hien tfnh toan, thong

ke ket qua cua phep do nhanh chong va da dang hon, nhu vay da tao ra duoc thiet bi do luong lap trinh tu dong Mot phan cua cuon sach nay dung de trinh bay nhung nguvcn tdc va kha nang ciia cac thiet bi do co bo vi xir ly

Tuy nhien, trong thuc te nhifiu khi chi can cac thiet bi do don gian hon, nen rat nhieu thiet bi do trong ky thuat hien nay van dang dung cac thiet bi do sir dung ky thuat tuong tu cung nhu ky thuat so Cuon sach nay con de cap den ca nhung thiet bi do nhu vay va cac nguyen tac truyen thong ciia ky thuat Do luong dien tir

Ky thuat Do luong dien tir la mot nganh ky thuat co pham vi rat rong ca v i doi

tuong do, moi truong va dieu kien do cung nhu dai tAn do, lirong trinh do va cau tao mach do Tham vong ciia cac tac gia la lam the nao de co the goi gon duoc ca pham vi rong Ion noi tren vao nhung van de rat co ban va cach trinh bay phai the hien duoc cac nguyen tile truygn thong cung nhu cap nhat duoc cac nguyen tac moi trong cuon sach ciia minh Mac dii da co ging de cuoii sach dat duoc y tuong noi tren, song chdc chdn khong tranh khoi con sai sot, cac tac gia mong duoc sir gop y va chi dftn ciia ban doc.Moi y kien xin giri ve: Khoa Dien tii-Vien thong, trmmg dai hoc Bach Khoa H a Noi, dien thoai 04.8692242.

L&n xuat ban nay co nhieu chinh ly, sira chua va bo sung so voi lan xuat ban truoc (thang 10 nam 2001) do Thac sy Nguyfin Thuy Anh, Thac sy D6 Le Phu, Thac sy

Nguyfin Ngoc Van, giang vien giang day mon hoc “ Co so kv thuat do luong dien tir”

ciia Khoa Dien tir - Vi<!n thong, truong Dai hoc Bach Khoa Ha Noi cung thuc hien duoi sir chi’ dao ciia tac gia chii bien

Ha noi, ngay 15 thdng 8 nam 2003

Tac gia chu bien

PG S Vu Quy Diem

Cu the!, noi dung nay bao gom cac van de ve cac phuong phap do luong cac thong

so cua tin hieu va mach dien, cac bien phap cau tao cac mach do cung nhir can true tinh nang cua may do, cach nang cao do chfnh xac cua phep do cung nhu cach xac dinh, han che sai so cua ket qua do

Do luong cac thong so dac tmli cua tin hieu nhu la do cac thong so cirong do cua tin hieu (V i du nhu cac thong so dong dien, dien ap, cong suat ), nhu quan sat dang cua tin hieu, do tan so, do di pha, phan tich pho cua tin hieu Do cac thong so cua mach dien nhu cac thong so cac linh kien ducfng thing, linh kien khong duong theing (cac linh kien

co so nhu dien tro, tu dien, den dien tu, den ban dan den cac linh kien nhir 1C, cac loai mach tich hop ), trong cac mach dien co phan tu tap trung, cac thong so cua cac linh kien duong thang trong mach sieu cao tan

Dac diem co ban cua^ ky thuat do luong dien tu la cac phep do duoc thuc hien trong mot dai ph6 rat rong, tir OHz (tin hieu khong bien doi) den 3.10" Hz (song quang)

Do vay cac phuong phap do, cau true cua may do va ca do cln'nh xac cua phep do cung deu tuy thuoc vao dai tin qua, doi tuong do luong V i du 6 tan so thap thi d6 dang do

diroc dong dien va dien ap, nhurng (V sieu cao tan thi cac thong so can xac dinh la dong dien, dien ap tra nen vd nghTa khi c^n djnh luong thong 6 tren mach, ma phai xac dinh

3

chung thong qua cong suat Hay vi du, cung la dai lirong cin do la tro khang cua mach,

ma d tan so thap thi co the dung cac thiet bi do la cac loai cau bon nhanh, o tan so cao hon thi thiet bi do la cau cong huong dien ap hay dong dien, va d tan so sieu cao tan thi thiet bi do phai dung la day do hoac do bang ong din song hay day d6ng true

Do chinh xac cua phep do thudng phu thuoc nhieu vao sir khu bo cac anh lnrdng ghep ky sinh cua cac thong so cua ban than may do toi mach can do, vi du nhu dien dung, dien cam, dien tro cua may do, anh hirong nay tang khi tan so cang tang cao Do vay khi do cung mot dai luong ma d tan so khac nhau thi khong nhung can co cac phuong phap khac nhau ma may do duoc dung de do cung phai co cau tao khac nhau Khi da chon dung phuong phap do va may do thfch hop roi thi cung can phai chu y toi thao tac can thiet, each mac do the nao der nang cao hon do chinh xac cua phep do V i du nhu can giam toi mire toi thieu dien ap tap tan, dien dung ky sinh cua day noi, cua may

do Cac anh huong tren thirong tro nen rat dang k6 trong ITnh vuc do lirong dien tii, vi phep do thirong duoc thuc hien d tan so cao, cong suat be va hay duoc tien hanh 6 trang thai cong huong

Luong trinh cua dai luong can do trong ky thuat dien tu cung kha rong va da dang Vi' du nhu vdi viec do tan so thi phai thuc hien phep do co luong trinh tir OHz den

10i5Hz Do cong suat thi tu cac thiet bi co cong suat Ion toi 10* W , dudi cac phuong thuc diSu che tin hieu khac nhau nhu: dieu bien, dieu tan, di£u pha va ca dieu xung, voi do rong xung toi 10 V

Sir can thiet cua do ludng trong ky thuat dien tu la rat Ion, hiu nhu chung ta phai sir dung d moi luc, moi chd Khi nghien ciru thiet ke, di£u chinh khai thac, lip dat van hanh cac he thong dien tii, vi£n thong, khong the khong co may do Cho mot he thong lam viec, hay di6u chinh mot thiet bi dien tur, la mot qua trinh do ludng cac che do cong tac, lay dac tinh cua tung khoi, tung khau rieng biet hay toan bo Do vay chi vdi cac may do co do chinh xac cdn thiet thi moi co the didu chinh duoc thiet bi dat duoc cac yeu cau mong muon Khi lip rap, che tao cac thiet bi dien tu, cac thiet bi vien thong, cung rat can do lirong Vi tinh toan thiet ke chi cho duoc cac so lieu so bo muon co duoc che do cong tac thuc te va thong so thich hop nhat thi chi tren co so thuc nghiem moi co Vdi cong tac nghien cuu thi viec xay dung phuong phap do va kien toan thiet bi

do lai cang quan trong hon Khong phai chi co so luong cac ket qua, ma sir phan tich chat luong cung co ich loi cho cac cong viec lien quan tdi 17nh vuc nghien ciru Khi khai thac cac he thong dien tu, can phai luon luon kiem tra phat hien cac hu hong, thuc hien cac qua trinh cong tac, gifr duoc cac chi tieu ky thuat cao trong qua trinh lam viec xac dinh nhanh chong cac nguyen nhan lam mat cac tieu chuan cong tac Tat ca nhfmg dieu

do dfiu khong thuc hien duoc neu khong co sir to hop phep do va may do

Cung vdi qua trinh phat trien cua khoa hoc cong nghe, ky thuat dien tu va ky thuat viln thong la nhffng qua trinh tien trien gin chat vdi kha nang thuc hien va hoan thien ky thuat do ludng V f du, nhung thanh tuu cua linh vuc nghien ciru khong gian vu tru nhu

ve tinh cua trai dat la nhung he thong rat phuc tap cua cac may moc do ludng dien tu Qua trinh dieu khien va tu dong bao ham mot so ldn cac phep do cac loai khac nhau vdi

do chinh xac cao Tren co sd phat hien nhung doan tan so song mdi, nhung phuong phap

do mdi cung xuat hien theo, tao ra them cac yeu cau mdi va dac biet ve che tao cho cac may do Ngay nay, vdi sir phat trie'll cua khoa hoc cong nghe ngirdi ta da tir dong hoa duoc cac qua trinh san xuat, nhung thanh tuu mdi nay da duoc thfch ung vdi cac phuong phap do va thiet bi do mdi cua ky thuat do ludng tir dong

Do chfnh xac cua phep do phu thudc nhieu vao chu quan cua ngudi do Muon ket qua do chfnh xac, phai chon duoc phep do diing vdi iiliiem vu dat ra, thfch hop vdi doi tuong can do Can phai n&m duoc cac phuong phap do khac nhau, biet duoc tinh nang cac may do, xir ly thfch dang duoc cac nguon goc sai sd do Khong can than trong qua trinh do, kh6ng biet day dii dac diem ciia doi tuong do, dac tinh ciia tin hieu can do, kha nang ciia may do thi khong the; co ket qua do chfnh xac Sir thong thao ciia ngudi lam ky thuat khi do ludng co the nang cao duoc do chfnh xac cua phep do, va co the thuc hien cac phep do mot cach linh hoat Sir thong thao khong nhung the hien d chd hieu ro cac phep do va sir dung thanh thao cac may do, ma con the hien d chd biet van dung de! hieu duoc cac nguyen ly do ludng d cac he thong dien tir hien dai Vf du nhu he thong Radar

la he thong co yeu cau do klioang thdi gian, co sd ciia mot sd he thong dieu khien la phep do di pha rdi rac hoa va chi' thi bang sd

De nghien ciru giao trinh “ Co sd ky thuat do ludng dien tir” nay, yeu cau ngudi doc da dupe nghien ciru cac giao trinh ky thuat co sd ciia nganh ky thuat dien tir Cac giao trinh ky thuat co sd co quan lie true tiep can phai ke den nhu giao trinh dien tir va ban din, ly thuyet co sd ve tin hieu va mach dien vo tuyen dien, co sd thiet bi khuech dai, ky thuat xung va sd, ly thuyet xac suat va thong ke Sd dT nhu vay, vi tat ca cac kien thire chung cua cac giao trinh tien la ly thuyet co sd can thiet de xay dirng cac phuong phap do ludng ve cac thong sd ciia tin hieu cung nhu ciia mach dien tir Do cung la cac

ky thuat co sd de xay dung cac bien phap thuc hien cac phuong phap do nay, tire la cau true cu the ciia cac mach do va ciia cac may do

Co sd phat trien ky thuat dien tir la tir ky thuat dien, nen do lirong trong dien tir cung xuat phat tir cac co sd ciia ky thuat do ludng dien Tuy co quan he gin bo nhu vay, nhung hai moil hoc nay co quan diem co ban khac nhau Nhiem vu cua ky thuat dien la tao ra, truyen dan va bien doi nang luong dien tir Con nhiem vu ciia ky thuat dien tir la truyen lan va gia cong tin tire nlid dao dong dien tir Do vay, hai nganh dien va dien tir phai dupe nghien ciru theo hai quan diem khac nhau, quan diem nang lupng va quan diem thong tin

5

Nhu vay, khi do ludng trong lTnh vuc dien tu ngudi ta thudng it quan tam tdi khfa canh nang luong cua qua trinh Dieu chu trong nhieu hon la cac thong sd va dac tfnh dac trung cho mach va tfn hieu ve mat thong tin, vf du nhu tan sd, pha, tro khang dac tfnli he

sd truyen dat va cac thong so dang cua tfn hieu

Cac phan sau day, chung ta se xem xet tdi cac doi tuong va phuong phap do luong dien tu mot cach chi tiet

1.1 DOI TUONG CUA DO LUONG DIEN TLf

1.1.1 Cac dac tinh va thong so ciia tin hieu

De: mo ta tfn hieu trong linh vuc dien tu, ngudi ta sir dung cac bieiu thuc toan hocsau day:

ft so vdi sd luong tfn hieu tren thuc te va dupe goi la nhung tfn hieu mau Sd tfn hieu mdu nay la tdi thieu nhung ve mat do ludng, chung da thoa man duoc yeu cau la bie’u dien dupe mo hlnh don gian cua cac tfn hieu tren thuc te

Khi do ludng cac thong sd va dac tinh cua cac m;ich dien, ngudi ta cung dun" cac tfn hieu mSu nay Biet dupe phan ung cua mach vdi cac dang cua tin hieu ay thi cuna co the suy ra phan ung cua mach vdi cac dang tfn hieu khac

Cac tfn hieu trong dien tu thudng dupe bieu dien theo ham ciia thdi gian hoac theo ham theo tan sd

Dang ciia cac tin hieu co ban duoc khao sat thong so, bao gdm:

-Tfn hieu dieu hoa

-Tfn hieu tiutn hoan

-Tfn hieu xung

-Tin hieu so

1 Cach bieu d iin tin hieu theo ham so cua thdi gian va theo ham so cua tan so

a Ham so theo thdi gian

Ham so theo thdi gian la ham sd dang:

s=f(t)

Ngoai tham sd la thdi gian ra, nhir da trinh bay d phan tren, con co cac tham sd khac trong bi£u thuc cua f(t), nen bidu thuc cua s co the dirpc bi£u di£n thanh dang nhusau:

S=f(t, a,, a2, , a„)

Cac thong sd a,, a2, , an cua tin hieu se xac dinh dang cua tin hieu, do do vdi mdi tin hieu khac nhau, ta se co nhung tham sd khac nhau va cac ham sd khac nhau Vdi nhung loai tin hieu khac nhau do, d£ do cac thong sd tin hieu ciia chung, ngudi ta phai

co nhung phuong phap phu hop nham dua ra ket qua gdn vdi thuc te nhat

Vi du khi sir dung O-xi-lo de hi£n thi mot tin hieu theo thdi gian, ta co the thay

dupe cac tham sd v6 dang cua tin hieu nhu cudng do, chu ky, do di pha

Cac thiet bi trong do ludng cung dupe thiet ke cho viec do dac mot vai thong sd nao do nen tuy theo thong so nao can do, ta phai chon cac loai may do thich hop, vf du

Hinh 1-1 Tin hieu hien thj tren O-xi-lo nhu de do cac thong sd v£cudng do ta co the dung von-met de do dien ap, ampe-met de do dong dien, oat-met de

do cong suat Ngoai ra vdi mdi giai lupng trinh khac nhau, ngirdi ta cung phai sir dung nhung phirong phap va thiet bi do phu hop, vi du nhu vdi cac giai tan sd khac nhau, ngudi ta phai co nhung phirong phap va thiet bi do khac nhau

h Ham so theo tan so

Ham theo tan so la ham co dang

s=cp(f)

7

Ham so theo tan so thudng dupe dung de bieu dien cho cac tin hieu tuan hoan hoac cho mot tin hieu trong mot khoang thdi gian huu han.

Khi bidu dien mot ham so theo tin sd, iru diem cua no la ngudi ta co the thay duoc dai tin cua tin hieu, tu do ngudi ta se co nhung phuong phap phu hop cho viec gia cong

Hinh 1-2 Bieu dien tin hieu

V f du khi can lay m&u cua mot tin hieu, ngudi ta phai biet dupe dai tan cua no va

se lay mlu trong cac khoang thdi gian phu hop vdi tin sd cua tin hieu theo dinh ly lay mlu:

u

a, Tin hieu tn/dc khi lay mau

Hinh 1-3 Lay mau tin hieu

b, Tin hieu sau khi lay mau

“ MAX

Vdi T,m la chu ky lay mau;

Fmax la tan so Ion nhat ciia tin hieu

2* Cac thong so cua cac dang tin hieu

a Tin hieu dieu hoa

Dao ddng dieu hoa dung de m6 phong tiS'ng noi, am nhac, , va co bi^u thuc toan hoc dU(ri dang hinh sin (hoac cos):

s (t)= A msin(27cf0t-Kp0)

Do thi ciia no nhu trong hinh 1-4

Ngoai thdi gian t con co cac thdng sd' Am, f0, va cp0 tham gia anh hudng vao tin hieunay

Am: BiSn do cua dao dong, co thir nguyen la von (V ) neu s(t) la dien ap, hoac co thu' nguyen la ampe (A ) neu s(t) la dong dien

f0: Tan sd cua dao dong, do bang hec ( Hz); Tii tan sd f0, con co cac thong sd dSn xuat sau day:

co0: Tan sd goc, do bang radian/s;

CD0=27Cf0

T„: Chu ky, do bang gi&y va ta co:

T„ = —

A.,,: Budc song, do bang met va

trong do c«3.108tn/s, la van toe anh sang

(p„: Goc pha dau cua dao dong, do bang do hoac radian Goc pha ddu tinh tCr mot thdi diCm bat ky dupe chon lam goc Vi goc thdi gian la tuy y nen khi ngudi ta noi do pha, thi khong phai la do pha dau cua mot dao dong ma la do sir dich pha giira hai dao dong di6u hoa cung tan so (hinh l -5)

Bien do A„, do bang vdn-met neu s(t) la dien ap, hoac bang ampe- met neu s(t) la dong dien Tren thang

t dt) ciia cac dung cu nay, ngudi ta

khong khac dp theo gia tri bien do cua

eo0

Hinh 1-5 Si/lech pha cua 2 tin hieu

9

dao dong ma khac do theo gia tri hieu dung Giua gia trj hieu dung A va gia trj bien do

A m co quan he sau:

A = * 0,707A m

Tan sd f0 hoac budc song X() do bang may do tan so (tan so-met) hay may do song

That ra, trong hai dai luong nay co the chi can do mot dai hrpng roi suy ra dai lucmg kia

*Djch pha giua hai dao ddng di£u hoa duoc do bang may do pha (pha-met)

Trong ky thuat do ludng dien tu, dao dong di6u hoa duoc tao ra bang cac bp tao song (am tan, cao tan va sieu cao tan) dac biet Nhd co nhung co cau dieu chinh va cac

bo hien thj kiem tra nen bien do va tan sd cua dao dong tao ra ta co the bien doi duoc trong mot pham vi nao do, thudng la kha rong Trong sd cac may phat tin hieu do ludng, thi cac may phat tin hieu dao ddng di6u hoa la pho bien nhat

b Tin hieu tudn hoan

Tin hieu loai nay co dang tuy y va vi vay v£ mat nao do, no la tong quat hon cac

trudng hop tren Do tinh tuan hoan nen co the bieu dien no dudi dang sau:

s(t)= s(t+nT) khi -oc < t < +0 0;

trong do T la chu ky lap lai cua tin hieu ( T = —, vdi F la tan sd lap lai)

F

Xet mot dang mau cua tin hieu nay d hinh l-6a

Vi dang la bat ky nen de dac trung cho loai nay, chung ta phai dung kha nhidu thdng sd, ta se lan lupt neu ra dudi day

Trong trudng hop tong quat, dao dong co the co thanh phan mot chieu (hinh 1 -6b)

va do do ta co the xem no nhu tong cua thanh phan mot chieu nay vdi thanh phan xoay chi£u (hinh l-6c):

s(t)=s_ + s _(t)

Trong do thanh phan mot chi6u:

1 ft" + t

s_ = - } s(t)dt

t* la thdi difem tuy chon, neu chon t -0 thi

1 fT

s_ = f j;s(,)d ,

Hinh 1-6 Phan tich tin hieu tuan hoan

Tu day ta tha> ring s_ chinh la chieu cao (bien do) cua mot xung vuong co do rong la T va dupe tmh bing dien tich phan mat phing gidi han bdi phan dudng cong s(t) nim trong khoang T va true thdi gian t Neu ke trong mot chu ky T thi phan dien tich nim giua thanh phan xoay chieu s_(t) va muc mot chieu s_ dupe phan bo deu tren va dudi muc nay

Do lech cuc dai cua s(t) tmh tir mire mot chieu ve hai phia A llin va A dlllJi co the khac , nhau nen d day khong dung khai niem bien do chung dupe Tong cua hai dai lirong nay xac dinh khoang bien thien cua thanh phan xoay chieu:

A =A -i-A

Cong suat tire thdi cua dien ap hoac ddng dien tieu thu tren mot dien trd R van tinh nhu thudng le:

Tuy nhien trong tinh toan va do ludng ngudi ta hay dung khai niem cong suat trung binh hem Neu dien trd tai R bang IQ thi cong suat trung binh se tinh nhu sau:

P = I f PCKIt = u;(l)dt = i £ i2(t)dt ( I »

Khai niem cong suat trung binh co lien quan den cac gia tri hieu dung cua daodong:

sd k6 tren, ngudi ta dung

nhieu dung cu do khac nhau Von-met (hoac ampe-met) mot chidu de; do s_ Thanh phan xoay chieu co the tach rieng ra de do cac thdng so ciia no bang cach cho tfn hieu s(t) di qua tu dien hoac bien

ap Cac thdng so A,,,,,, A (tU(I1 do bang von-met dinh (neu s(t) la dien ap) Thdng thudng de do cac gia tri tuc thdi va nghien cuu dang cua tin hieu dao dong, ngudi ta dung dao dong ky Cong suat trung binh P do bang oat-met

Cac gia tri dinh, trung binh, hieu dung, cong suat cua dao dong cung nhu cac gia tri tuc thdi thudng dupe goi chung la “ cac thdng sd cudng do"

Ngoai phirong phap do true tiep bang cac dung du ke tren, ngudi ta con co the do cac thong so va dac tinh ciia loai tin hieu nay dua vao nguyen ly dupe neu ra sau day

Moi dao ddng tuan hoan co dang bat ky ddu co the; phan tich thanh tong cua vo so dao ddng di6u hoa vdi thanh phan mot chieu:

s(t)=s + A,sin(Qt+(p, ) + A :sin(2Qt-Kp2)+ + A nsin(nQt+<pn) (4)

trong do, cac dao dong hinh sin thanh phan goi la cac song hai, Q =2nF goi la tan

so co ban Cac song hai co tan sd bang boi so nguyen Ian cua tSn so co ban: nQ; n=l,

2, goi la bAc cua song hai Bien do A n va pha ban dau (p„ phu thuoc bac cua song hai

a, Do thj pho bien do-tan so

b, Oo thi pho pha-tan so

c, Lap lai tin hieu tCr cac thanh phan hai

Tir day, chung ta thay rang s(t) khong nhung co the bieu diln theo thofi gian o dang hinh 1-6 ma con co the bi6u dien theo tan so nua Khi do, tach s(t) lam hai thanh phan: bien dp va pha Do thi bieu dien su phu thuoc ciia bien do cac hai vao tan so: A=A(io) goi la d6 thi pho bien dp- tan so (vi du nhu tren hinh 1.8a)

Con do thi bieu dien sir phu thuoc cua pha dau cac hai vao tan so (p=ip(o>) goi la

do thi pho pha-tan so (vf du nhu hinh 1 -8b)

Dung cong thuc (4) va tir hai do thi nay co the lap lai dupe dang cua s(t) ban dau (hinh l-8c) Dang va gia tri ciia pho bien do quan sat va do dupe bang may phan tich pho bieu thi bang ong tia dien tir

Con mot thong so nua lien quan den dao dong, la b£ rong pho Af, do bang Hz Do

la dai tan so tren do co phan bo cac vach ph6 cua dao dong Viec bo tri cac kenh thong tin trong mot dircmg thong tin duy nhat va tmh toan dai thong cua cac thiet bi trong he thong thong tin nhat thiet yeu cau phai biet b£ rpng ph6 ciia tin hieu chon dung Khi d6 thi pho bien do cua dao dong da hien tren man hien song cua may phan tich pho, ta dd dang do dupe be rong pho Af (xem chuong 7)

13

c Tin hieu xung

Tin hieu xung duoc sir dung dac biet rong rai trong ky thuat vA tuyen: Thong tin

xung, rada, di£u khien, truyen hinh vo tuyen v.v V i vay do ludng cac thong so cua xung chiem mot vi tri kha quan trong

Tin hieu xung co nhidu loai va voi m6i loai lai co mot nhom thong so dac trung Ngudi ta phan biet nhung loai tin hieu xung sau:

-Xung don

-Nhom xung

-Nhom xung cot

-Day xung tuan hoan

mot khoang thdi gian huu han, co the so sanh duoc voi thdi gian qua do cua mot mach dien nao do khi xung nay tac dong vao

Rieng xung don lai co nhieu loai phan biet nhau theo dang va m6i loai gan li6n vdi nhung thong so khac nhau

Thong dung nhat la xung vuong (dung ra phai goi la xung vuong goc), hinh l-9a

cua xung

Xung rang cua (hinh 1 -9b) duoc dac trung boi hanh trinh thuan (doan ab) va hanh trinh nguoc (doan be) Hinh chieu ciia cac doan nay tren true thdi gian, tuong ung la thdi han (do rong) hanh trinh thuan x lh va thdi gian (do rong) hanh trinh ngiroc x,lg Gia tri dinh cua xung goi la bien do, ky hieu la A m

Xung hinh thang (l-9c) co sudn trudc la doan ab, khi duoc chieu len true thdi gian thi dirge do long sudn trudc x sudn sau cd chieu len true thdi gian se duoc do rong sudn sau x2 Bien do A m tinh tir dinh xung Do rong xung x la khoang thdi gian giua hai

Adiem sudn trudc va sudn sau dat tdi - Do do:

Trong do, A m la bien do, con x la hang sd thdi gian cua xung

x tinh bang cach ke mot tiep tuyen vdi dudng cong d mot diem bat ky, hoanh do cua no ky hieu la t,, tiep tuyen nay cat true hoanh tai die!m t2, ta co :

x =t2 -1,

Ngoai nam dang tren, ngudi ta con dung xung don co cac dang khac nhau nua, nhu xung hinh chuong, xung hinh sin binh phuong Tuy nhien, cac dang xung nay khong diein hinh lam

*Nhom xung: la tap hop cua mot sd xac dinh cac xung don cung dang cach d£u

nhau Cac thong sd cua loai nay la: sd xung k, chu ky lap tai T, thdi han nhom xung T„ Nhom xung ve d hinh 1-10, vdi gia thiet cac xung thuoc nhom la xung vuong, co do rong x Tir hinh ve ta tha'y:

T„ =(k-l)T+ x

15

I ? 3 *

AfT)

Hinh 1-10 Nhom xung

*Nhdm xung cot khac voi nhom xung 6 cho cac xung trong nhom co the khong

cung mot dang va khoang each giua chung la tuy y Tuy nhien, so xung trong nhom, dang va gia tri cac thong so cua mdi xung da biet mot cach chinh xac Khi tmh khoang cach giua hai xung trong nhom ngudi ta tmh tu diem cac sudn trudc dat toi mot nua bien do (hinh l -11)

*Ddy xung tuan hoan: la su lap lai mot cach ddu dan (vdi chu ky T) xung don co

• Tdang nao do Doi vdi loai tin hieu nay, cac thdng sd la do rong xung t, ty sd — = q goi

T

la do trong (do rdng) cua day xung, con nghich dao cua no d = — = — goi la he sd lap

q Tday

Doi khi con dung ca nhung nhom xung tuan hoan hoac nhom cot xung tuan hoan

De' do fudng cac thdng sd xung, ngudi ta dung cac von-met dinh (von-met xung), may do thdi gian may do tan so lap lai Nhung trong do ludng xung thdng thudng hon

ca la dung cac dao dong ky xung co trang bi cac khoi chuan thdi gian chuan bien do theo ca hai true X va Y

Do yeu cAu tao ra tin hieu xung vdi gia tri cac thdng so cho trudc, ngudi ta thiet ke cac may phat xung do ludng cung co nhung co cau di6u chinh nhu cac may phat sons dieu hoa da noi d tren Mdi may phat xung thudng co nhi£u dau ra rieng biet cho phep lay ra nhung xung co dang khac nhau

cl Tin liieu so

Tin hieu sd co ban chat la tfn hieu mang thdng tin dang sd nhi phan bao gdm cac tfn hieu bieu thi cho so 0 va so 1 De djnh nghTa cac bit 0 va 1, ngudi ta co the sir dung rat nhidu each Vf du nhu ngudi ta co the' dung mure dien ap, cu the thdng thudng ngudi

ta thudng djnh nghTa bit 0 irng vdi mure dien ap OV (0-0,5V) va bit 1 urng vdi muc dien

ap 5V (4,5-5,5V) Ngoai ra ngudi ta cung co the quy dinh nguoc lai, mure dien ap thap urng vdi bit 1 va mire dien ap cao ung vdi bit 0 Ta se xet cu the theo dang djnh nghTa trudc

Tfn hieu sd thudng duoc bieu thj dudi dang chudi xung vuong Neu tin hieu la bit

1, ta se co mot xung vuong co mure dien ap cao, neu tin hieu la bit 0 thi dien ap d muc thap Do do, tfn hieu dien ap du la miic cao hay mure thap thi no deu co y nghTa mang tin tire Khoang cach giua cac bit cach deu nhau va do rong cua mot bit tin hieu thi hoan toan phu thuoc vao viec ngudi dung djnh nghTa tfn hieu do Hinh 1-12 la mot vf du ve tfn hieu so

u

Hinh 1-12 Tin hieu so

Khi tfn hieu sd duoc truyen di, ngudi ta co the dung cac phuong phap ma hoa khac nhau Tuy theo phuong phap ma hoa cua tfn hieu ma dang cua tfn hieu sd da duoc dieu che' nay se khac nhau

1.1.2 Cac tham so va dac tinh cua mach dien tii

Trong qua trinh thdng tin, dieu khien trong dien tu , tfn hieu luon luon duoc bien doi tu dang nay sang dang khac To hop cac linh kien theo mot each nao do nhim thuc hien viec bien doi noi tren goi la mot mach dien Cac linh kien cua mach dien bao gdm dien trd, tu dien, dien cam, cac loai den dien tu va ban dUn, IC va cac phu kien khac nua

Tuy thuoc vao tmh chat cac phan tu duoc su dung, mach dien co the chia thanh mach tuyen tfnh va mach phi tuyen V I ban chat hai loai mach nay khac nhau nen cac thdng sd va dac tfnh cua chung cung khac nhau

17

/ Mach tuyen tinh

Mach tuyen tinh tao thanh tu nhfrng phan tu co gia tri khong phu thuoc vao dong dien chay qua no Mot thuoc tinh quan trong cua mach tuyen tmh la doi vdi no co the

ap dung nguyen ly xep chong

Phuong phap do ludng cung nhu thiet ke cau tao cac may do thong so va dac tinh cua mach tuyen tfnh dua vao hai dac diem tren Viec do ludng cac thong so va dac tmh cua mach tuyen tinh la rat pho bien vi trong ky thuat dien tu, ly thuyet mach tuyen tinh duoc phat trien manh va duoc su dung dac biet rong rai

Cac phan tu cua mach tuyen tmh thudng la: dien tro, tu dien, cuon cam khong loi sat; den dien tir va ban din lam vice d tren doan dirdng thing cua dac tuyen Tuy theo tan so cua tin hieu can thong qua mach, ma cau tao cua cac phan tu cua mach co khac nhau va do do mach tuyen tmh lai con duoc chia thanh hai nhom: mach co phan tu tap trung va mach co phin tu phan bo

Mach co phan tu tap trung dung de bien doi tin hieu co tan sd khong ldn lam (nho hon vai chuc megaHz) Nhu da noi d tren, gia tri cua cac phin tu tuyen tfnh la khong phu thuoc vao dong chay qua no, va do do, co the xem do la hang sd Ngudi ta lay ngay cac gia tri nay lam thong sd dac trung cho cac phan tu cua mach Ban than mdi phan tu

R, C hoac L rieng biet co the: xem nhu mot mang hai cue, cho nen cac thdng sd cua mang hai cue la gia tri cua dien trd R, dien dung cua tu dien C va dien cam cua cuon day L

Neu nhu m ic noi tiep hoac song song cac phan tir R, C va L hoac h6n hop ca hai cach mic ay, ta se duoc cac mang bon cuc

Vi mang bon cuc co the gom ca dien tro R tieu thu nang lirong va ca cac thanh phan cam khang C va L nen de dac trung cho mang bon cue, ngudi ta dung thong sd trd khang toan phan Z Dung khang X c va cam khang X L phu thuoc tan sd tin hieu thong qua mach, nen trd khang toan phan Z cung la mot ham cua tan sd:

De clo cac thong so vita ke d tren cua mach co phan tu tap trung, ngudi ta dung: may do dien tro' (om-met), may do dien cam va dien dung, may do tro' khang toan phan

va may do he so pham chat (Q-met)

Ngoai cac thong so tren, de’ dac trung cho phan ung cua mach vdi nhung tac dong dpt bien ngudi ta con dung dac tmh thdi gian (dac tmh qua do) va cho phan ting cua mach doi vdi nhung tac dong dieu hoa (hinh sin), thi dung dac tinh tan so

Uvoo <*>

Khi tim dac tinh thdi gian, ngudi la cho tac dong vao mach ham don vi l(t) nhu hinh 1-13 Ham l(t) duoc bieu dien toan hoc nhu sau:

khikhi

t <0

t >0

Hinh 1-13 Ham don vi

Dac tmh nay co the: quan sat true tiep tren man ong tia dien tu ciia nhung thiet bi dac biet: nhung may do dac tmh thdi gian Vdi mot vai loai mang bon cue don gian, dang cua dac tmh qua do chi ra d bang l I Tir dac tmh qua do nay, co the xac dinh mot vai thong sd ciia mach nhu hiing sd thdi gian, he sd pham chat

Q va tan sd cong huong ciia mach

Khi tim dac tmh tan sd, or dau vao ciia mach, ngudi ta dat tin hieu dieu hoa

va — Khi thay doi tan sd ciia tin hieu vao, gia tri cua cac

ty sd nay cung thay doi theo Goi =A(f) la dac tmh bien do-tan sd, — =<p(f) la dac

tinh pha-tan so

D6 thi A (f) va (p(f) vdi f bien thien trong mot dai rong cho ta khai niem v6 dac tmh thdng sd ciia mang Thdng thudng, ca A (t) va (p(l) bieu dien chung trong lie toa do cuc (he toa do doc cue) De ve dac tuyen tan sd, ngudi ta dung mot bo tao dao dong dieu hoa

19

RCRb) a = —

2L

x=l/a

g(t)=Ae sino>,,t R

a = —

2L1

co,

v L C

g(t)= A (l-ae ulcoso)„t) R

2 Mach phi tuyen

Nguoc lai vdi mach tuyen tinh, d mach phi tuyen gia tri cac linh kien cua mach phu thuoc vao cudng do ddng dien chay qua no VI vay vdi cac linh kien phi tuyen nay, khong dung khai niem thdng so cua ban than no

Theo djnh luat Om, dien ap roi tren cac phiin tu phi tuyen se khong ty le thuan vdi dong dien chay qua no Tuy co kho khan v6 mat tfnh toan va trong nhidu trudng hop con

bat Ipi ca v6 mat sir dung nua nhung mach phi tuyen khong the thieu duoc trong ky thuat dien tu Dira vao tmh khong duong thang cua dac tuyen Von-Ampe cua mach phi tuyen, ngudi ta dung no de thuc hien mot loat cac bien doi: dieu che, tach song, nhan va chia tin sd, tao dao dong va han che, Tuy timg muc dfch bien doi, phai chon nhung doan thfch hop cua dac tuyen Von-Ampe, chon cang chfnh xac thi hieu qua bien doi cang cao

Trong ca hai trudng hop, ro rang phai biet dupe chinh xac dang dac tuyen Von- Ampe cua phan tu va cua mach Quan sat va nghien ciru dac tuyen Von-Ampe cua cac phan tu phi tuyen dupe thuc hien nhd nhung may ve dac tuyen, mot vf du thong dung cua loai nay la may ve dac tuyen den dien tu va ban din (d chuong 3, quan sat va do ludng dang tfn hieu, co d6 cap den van d£ nay)

1.2 CAC KHAI NIEM CO BAN VE DO LUONG DIEN TLf

1.2.1 Khai niem do ludng

Do luong la khoa hoc vd cac phep do, cac phuong phap va cac cong cu dam bao cho chung, cac phuong phap de dat dupe do chfnh xac mong muon

Cac hu'dng nghien ciru chfnh cua do ludng bao gdm:

-Cac ly thuyet chung v£ phep do

-Cac don vi vat ly va he thdng cua chung

-Cac phuong phap va cong cu do

-Phuong phap xac dinh do chfnh xac ciia phep do

-Co sd bao dam cho viec thdng nhat giua phep do va rat nhidu cong cu thuc hienno

-Cong cu do chuan va barem

-Cac phuong phap de' chuydn don vj do tir cong cu chuain hoac goc ra cong cu lamviec

21

Phep do la cong viec thuc hien chinh cua do lirong, do la viec tim ra gia tri vat ly bang cach thf nghiem voi sir tro giup ca cac cong cu ky thuat dac biet Gia tri tint duoc goi la ket qua cua phep do Hanh dong thuc hien trong qua trinh do de cho ta ket qua l;l mot dai luong vat ly goi la qua trinh ghi nhan ket qua Tuy thuoc vao doi tuong nghien ciru, vao tmh chat cua cong cu do va ngudi ta can thuc hien phep do ghi nhan mot Ian hay nhieu Ian Neu nhu co mot loat ghi nhan thi ket qua phep do nhan duoc la ket qua khi xu ly cac ket qua tu cac ghi nhan do.

Phep do co ban chat la qua trinh so sanh dai luong vat ly can do voi mot dai lirong vat ly duoc dung lam don vi Ket qua cua phep do duoc bieu dien bang mot so' la ty le cua dai luong can do voi don vi do Nhu vay de thuc hien phep do ta can thiet lap don vi

do so sanh gia tri cua dai luong can do voi don vi va ghi nhan ket qua so sanh dupe Thong thudng ngudi ta thudng bien doi tin hieu den dang thuan tien nhat cho viec so sanh

Nhu vay, ta co the tom tat lai thanh bon budc chfnh cua phep do la: thiet lap don vi vat ly bieu didn tin hieu do so sanh tfn hieu do vdi don vi dupe lay lam chuan va ghi nhan ket qua so sanh

1.2.2 Cac phirong phap va bien phap do ludng co ban

I Cac phuong phap do

Cac phuong phap co ban cua ky thuat do ludng thudng dupe chia thanh:

-Ph.rong phap do true tiep

-Phuong phap do gian tiep

-Phirong phap do tuong quan

Do true tiep la phuong phap dung cac may do hay cac mau do (cac chuan) de

danh gia sd luong cua dai luong do dupe Ket qua do duoc chfnh la tri sd cua dai luong ciin do, ma khong phai tfnh toan thong qua mot phirong trinh vat ly nao lien quan cifra cac dai luong Neu khong tfnh den sai so thi tri sd dung cua dai luong can do X se bang ket qua do dupe a:

Do gian tiep la phirong phap do ma ket qua do dupe khong phai la tri so cua dai

lupng can do, ma la cac so lieu co sd d6 tinh ra tri sd cua dai lupng nay NghTa la d day, X=F(a,, a-,, , an)

Cac vf du vd phuong phap do gian tiep nhu: do cong suat bang von-met va ampe- met; do he sd song chay bang day do

Trong ky thuat do ludng, thong thudng ngudi ta muon tranh phuong phap do gian tiep, v ' i trudc het no yeu cau tien hanh nhieu phep do (ft nhat la hai phep do) va thudng la khong nhan biet ngay dupe ket qua do Song trong mot sd trudng hop thi khong the tranh dupe phuong phap nay

Hien nay, ky thuat do ludng da phat trien nhieu ve phuong phap do tuong quan

No la mot phuong phap rieng, khong nam trong phuong phap do true tiep hay phuong phap do gian tiep Phirong phap tuong quan dung trong nhung trudng hop can do cac qua trinh phuc tap, ma d day khong the thiet lap mot quan he ham sd nao giua cac dai lupng

la cac thong sd cua mot qua trinh nghien ciru Vf du: tfn hieu dau vao va tfn hieu dau ra cua mot lie thdng nao do

Khi do mot thong sd cua tin hieu nao bang phuong phap do tuong quan, thi can ft nhat la hai phep do ma cac thdng sd tir ket qua do ciia chung khong phu thuoc lan nhau Phep do nay duoc thuc hien bdi cach xac dinh khoang thdi gian va ket qua cua mot sd thuat toan co kha nang dinh diroc tri sd ciia dai lupng thfch hop Do chfnh xac ciia phep

do tuong quan dupe xac dinh bang do dai khoang thdi gian ciia qua trinh xet Khi do true tiep, that ra la ngudi do da phai gia thiet he sd tuong quan giua dai luong do va ket qua rat gan 1, mac dii co sai sd do quy luat ngau nhien ciia qua trinh bien doi gay nen

Ngoai cac phirong phap do co ban noi tren, con mot so cac phuong phap do khac thudng dupe thuc hien trong qua trinh tien hanh do ludng nhu sau:

-Phuong phap do thay the: Phep do dupe tien hanh hai lan, mot lan vdi dai lupng

can do va mot lan vdi dai lupng do m5u Dieu chinh de hai trudng hop do co ket qua chi thi nhu nhau

-Phuong phap hieu so: Phep do dupe tien hanh bang cach danh gia hieu so tri sd

ciia dai lupng can do va dai lupng mau

-Phuong phap vi sai, phuong phap chi thi khong, phuong phap bii, cung la nhiing

trudng hop rieng ciia phuong phap hieu so Chung thudng duoc diing trong cac mach cau

do hay trong cac mach bii

-Phuong phap do thdng: ket qua do dupe dinh lupng true tiep tren thang do ciia

thiet bi chi thi Tat nhien sir khac do ciia cac thang do nay la da dupe lay chuan trudc vdi dai lupng mau cimg loai vdi dai lupng do

23

-Phuong phap rai rac hod (chi thi sd): Dai lupng din dupe do duoc bien doi I hanh

tin trie la cac xung rdi rac Tri sd cua dai lupng can do dupe tinh bang sd xung tuong ufn£ nay

2 Phan loai tong quat ciia cac may do

May do va cac phan tu mau la cac phuong tien de thuc hien cac yeu cau ve do ludng Ban than may do da la mot mach do dupe cau true theo mot phuong phap do, de

do ludng mot dai lupng nao do V i vay, dua vao cac doi tuong ma do ludng dien tu can giai quydt, thi cac may do co the phan loai tong quat thanh bon nhom may ldn vdi cac churc nang sau

a May do cac thong so va dac tinh cua tin hieu

Nhom nay bao gom nhiiu loai: Cac loai may do thuoc nhom nay nhu von-met dien tu, tan so-met, dao dong ky dien tu, may phan tich pho, may phan tich ham tuong quan So do khoi chung cua cac loai may nay nhu hinh 1-14 Tin hieu cdn do dupe dua tdi dau vao cua may M ach vao co nhiem vu truyen dan tin hieu tu ddu noi vao tdi bo

bien doi Thdng thudng mach vao la bo khuech dai tai eatot, vi mach khuech dai eatot co trd khang vao cao, co the thuc hien phoi hop trd khang dupe giua mach bien doi co trd khang thap vdi trd khang ra cua doi tuong do co tri sd trd khang cao Mach vao thudng

co nhu bo suy giam, ho day lam chain (vdi cac may do xung) bo phan mach dinh hudng Mach vao quyet dinh muc do anh huong cua may do vdi che do cong tac cua

Hinh 1-14 May do thong so va dac tinh cua tin hieu

doi tupng cdn do 0 pham vi tan sd thap va tan sd cao thi dac tinh nay dupe bieu thi bang trd khang vao cua may 0 sieu cao tan thi dac tfnh nay dupe bieu thi bang cong suat ma may do hap thu

Thiet hi bien doi, sc) sanh hay phan tfch la bo phan chu yeu cua cac may thuoc

nhom nay.Trong nhieu loai may thi bo bien doi dung de tao ra tin hieu can thiet de so sanh tin hieu can do nay vdi tin hieu mau Vf du nhu tan sd-met kieu deni, ban dau thi

bien doi song hai cua tfn hieu co chu ky T thanh xung co cung chu ky, sau do thi so sanh chu ky ciia xung voi khoang thdi gian choain Bien doi co the thirc hien trudc so sanh, sau so sanh hay dong thdi (vf du nhu d tdn so-met kieki cong hirdng) Vdi vai loai may khac nhau thi thiet bi nay co chuc nang phan tfch tfn hieu do ve bien do, ve tan sd hay chon loc theo thdi gian Thiet bi bien doi trong cac may nhu vay thirong la cac mach khuech dai, tach song, bien doi dang dien ap tfn hieu, chuyen doi dang nang luong Vdi vai loai may khac thi bo bien d6i xac dinh thuat toan cho tfn hieu do (lay tri sd logarit, lay tri so b'inh phirong ) de co dupe quan lie thuat toan mong muon giua dai lupng do va thang do chi thi cua thiet bi chi thi.

Thiet bi chi thi de bieu thi ket qua do dudi dang sao cho thfch hop vdi giac quan

giao tiep ciia sinh ly con ngudi, hay vdi tin tuc dua vao bo phan hieu chinh, tfnh toan Cac thiet bi chi thi thudng la cac dong ho do dien chi thi bing kim (ma thong dung nhat

la loai tir dien), ong tia dien tu, he thong den chi thi sd, ong nghe, bo ghi, bo phan ghi hinh, thiet bi nhd

Nguon cung cap de cung cap nang lupng cho may, va con lam nguon tao tfn hieu

chuan

Cac loai may thuoc nhom thu nhat nay thi thuc hien theo phuong phap do true tiep, ket qua do co the dupe doe thang hay thong qua phep so sanh vdi dai lupng m&u

h May do dac tinh va thdng so ciia mach dien

Mach dien can do thong so d day nhir mang bon cuc, mang hai cue, va cac phan tu

Hinh 1-15 May do thong so va dac tinh cua mach dien

25

cua mach dien tu So d6 khoi chung cua cac loai may nhom nay nhu hinh 1-15 Dac diem cua nhom nay la cau tao cua may g6m ca nguon tin hieu va thiet bi chi thi Cac loai may do thuoc nhom nay nhu: may do dac tinh tin so may do dac tinh qua do may

do he so pham chat, do dien cam, dien dung, dien trd, may thu den dien tu, ban dan va

IC

Nhom may nay cung bao gom nhieu loai, chung dung lam nguon tin hieu chuan khi can do ludng, de nghien curu va dieu chinh thiet bi V i du nhu khi can khac do may, khi do cac thong sd cua tin hieu bang cach so sanh, khi can ve dac tuyen thuc nghiein

So d6 khoi chung cua nhom may nay nhu hinh trang 1-16

Bo tao song chu la bo phan chu yeu, no xac dinh cac dac tinh chu yeu cua tin hieu

nhu dang va tan sd dao dong Thong thudng la tao song hinh sin hay xung cac loai

Bo bien doi de nang cao muc nang luong cua tin hieu hay tang them do xac lap

cua dang tin hieu No thudng la bp khuech dai dien ap, khuech dai cong suat, bo dieu che, thiet bi tao hinh xung Cac may phat tin hieu d sieu cao tdn thudng khong co bo bien doi dat giua bo tao song chu va dau ra, ma hay dung bo dieu che true tiep de khong che dao dong chu

Mach ra de dieu chinh muc tin hieu ra bien d6i trd khang ra cua may Thudng thi

mach ra la bo suy giam (bo phan ap), bien ap phoi hop trd khang, hay bo phu tai eatot

Thiet bi chi thi de kiem tra thong sd cua tin hieu dau ra Thudng la von-met dien

tu, thiet bi do cong suat, do he sd dieu che, do tan so

Hinh 1-16 Cau tao may phat tin hieu

Nguon de cung cap cho cac bo phan Thudng lam nhiem vu bien doi dien ap xoay

chieu ciia mang ludi dien thanh dien mot chieu co do on dinh cao

d Cac linh kien do lifang

Nhom nay bao g6m cac linh kien le, phu them vdi cac may do de tao n6n cac mach do can thiet Chung la cac linh kien co tieu chuan cao de lam vat mau (nhu dien

trd, dien cam, dien dung mau) hay cac linh kien de ghep giua cac bo phan ciia mach do Cac linh kien nay chu yeu hay dung 6 sieu cao tan nhu bo suy giam, bo dich pha, bo phan mach dinh lurdng

27

2.1 NGUYEN NHAN VA PHAN LOAI SAI SO TRONG DO LUONG

2.1.1 Nguyen nhan gay sai so

Khong co phep do nao la khong co sai sd Van dd la khi do can phai lira chon sir dung phuong phap thfch hop, cung nhu can chu dao, thanh thao khi thao tac , da han che sai sd cac ket qua do sao cho den muc ft nhat

Cac nguyen nhan gay sai sd thi co nhieu, ngudi ta phan loai nguyen nhan gay sai

sd la do cac yeu to khach quan va chu quan gay nen

Cac nguyen nhan khach quan vf du nhu: dung cu do ludng khong hoan hao, dai lupng do dupe bi can nhidu nen khong hoan toan dupe on dinh

Cac nguyen nhan chu quan, vf du nhu: do thieu thanh thao trong thao tac, phuong phap tien hanh do khong hop ly

Vi co cac nguyen nhan do va ta khong the tuyet doi loai tru hoan toan duoc nhu vay, nen ket qua cua phep do nao cung chi cho gia tri gan dung Ngoai viec co gjfng han che sai sd do den muc ft nhat, ta con can danh gia duoc xem ket qua do co sai so den mure do nao

2.1.2 Phan loai sai so

Cac sai sd mac phai trong phep do co nhieu cach phan loai Co the phan loai theo nguon goc sinh ra sai so, theo t|uy luat xuat hien sai sd hay phan loai theo bieu thuc dien dat sai sd

Phan loai theo quy luat xuat hien sai sd dupe chia lam hai loai: sai sd he thdng va sai sd ngdu nhien

1 Sai so he thong

Sai sd nay do nhung yeu to thudng xuyen hay cac yeu to co quy luat tac dong No khien cho ket qua do co sai sd cua lan do nao cung nhu nhau, nghTa la ket qua cua cac lan do d6u hoac la Idn hon hay be hon gia tri thuc cua dai lupng can do

Tuy theo nguyen nhan tac dung, ma sai sd he thdng co the phan thanh cac nhom sau day:

-Do dung cu, may moc do che tao khong hoan hao

V i du: kim chi thi cua thiet bi chi thi khong chi dung vi tri ban dau, may moc khong dupe chuan lai thang do vdi cac may chuain

-Do phuong phap do, hoac la do cach chon dung phuong phap do khong hop ly; hoac khi xir ly ket qua do, khi tmh toan de cho don gian hon da tu y bo qua mot sd yeu

to nao day V i du nhu bo qua cac anh huong ghep ky sinh cua mach do

-Do khi hau, vi du nhiet do, do aim khi tien hanh do khac vdi dieu kien khi hau tieu chuain da quy dinh trong quy trinh sir dung may do

Sai sd ngdu nhien la sai sd do cac yeu to bien doi bat thudng, khong co quy luat tac dong Tuy ta da co gang thuc hien do lirong trong cung mot dieu kien va chu dao nhu nhau, nhung vi do nhieu yeu to khong biet, khong khong che dupe, nen da sinh ra mot loat ket qua do khac nhau V i du: do dien ap cung cap cua mach do khong on dinh, do bien thien khi hau ciia moi trudng chung quanh xay ra trong qua trinh do ludng

Ngoai hai loai sai sd tren, eon nhung loai sai sd hoac khi do ta con nhah dupe nhung ket qua cac lan do co cac gia tri sai khac qua dang No thudng do nhung yen to chu quan cua ngudi do gay ra, nhu thieu chu dao; hay do cac tac dong dot ngot cua ben ngoai Cac ket qua do nay qua suy xet chu quan, ta co the biet dupe no la cac gia tri vo nglna va ta co the loai bo ngay dupe Thudng ngudi ta goi cac ket qua do nay la cac tri

sd do sai

29

Sau khi do de hieu chinh va danh gia ket qua do, ta co the loai bo cac sai so he thong duoc Su xu ly nay dupe thuc hien don gian bang phep cong dai so (co ke ca dau) khi ma da dinh lupng dupe gia tri cua sai sd he thdng Du la sai so he thdng cua mot hay nhidu nguyen nhan thi ta co the hieu chinh duoc vf du nhu bang cach chuain lai may moc thiet bi do vdi may mau.

Vdi sai sd ngau nhien, ta khong the xir ly dupe V i khong biet gia tri sai sd la bao nhieu va theo chieu hudng nao, ldn hon hay be hon gia tri thuc te De co the “ dinh luong" duoc gia tri sai sd ngdu nhien tire la danh gia duoc dp chfnh xac cua ket qua do thi ngudi ta dung cong cu toan hoc la ly thuyet xac suat va thdng ke Vi vay noi dung cach xu ly ket qua do se xet tdi o cac tiet sau, cung chi chu yeu la de giai quyet van de nay

2.1.3 Cac bieu thuc dien dat sai sd

Thong thudng cac sai sd hay dupe phan loai theo bieu thuc dien dat Theo cach phan loai nay thi co hai loai sau: sai sd tuyet doi va sai so tuong doi

1 Sai so tuyet doi

Ngudi ta dinh nghTa sai sd tuyet doi la tri tuyet doi cua hieu sd giua hai gia tri do duoc va gia tri thuc cua dai luong can do

Neu goi a la gia tri do dupe, X la tri thuc cua dai lupng can do thi:

la sai sd tuyet doi

Tren thuc te vl chua biet duoc X nen khong dinh lupng cu the duoc Ax" Nhung can cu vao dung cu do va kha nang dat duoc chfnh xac cua phep do, cung nhu thuc hien cach do nhieu lan ta co the’ tim dupe gioi han cuc dai cua Ax’: Ax'<Ax va lay Ax la sai

gia tri trong thuc tien tfnh toan vl chua biet duoc X

Trong trudng hop Ax « X va Ax « a (tuc la a va X coi nhu xap xi nhau)

5 x =Ax (3)

Sai so tuong doi nhu- bieu thufc (3) la sai so tifcfng doi danh dinh.

Con co loai bieu thirc sai sd tuong doi khac hay dirge dung de danh gia pham chat cua cac dong ho do Do la sai so tuong doi chiet hop:

Ax

O day, A la gidi han cue dai cua lirong trinh thang do cua dong ho de do Sai sd tuong doi chiet hop la cap chinh xac cua dong ho No dirge ghi trirc tiep bang chu sd len

tren mat d6ng h6 do, cung cac ky hieu khac Vf du nhu d hinh 2-1, chu sd 1,5 ghi d goc

la hieu thi cap chfnh xac cua dong ho do bang 1,5

Sai sd tuyet doi la mot dai lugng co thu- nguyen Sai so tuong doi la dai lugng khong co thu nguyen Khi danh gia pham chat cua phep do thi sai sd tuong doi bieu thi

day du hon va no con co the’ dung dS’ so sanh do chfnh xac giua cac phep do cac dai lugng khac nhau

Vf du, khi do hai tan sd f, = 100Hz, f2 = 1000Hz; ca hai deu co sai sd tuyet doi la f=±lHz Neu nhu chi so sanh bang sai sd tuyet doi ihi hai phep do la nhu nhau Nhung hai phep do co do chfnh xac khac nhau; do chfnh xac nay duoc bieu thi bane sai so tuone doi:

■ o So

Nhu vay phep do tan sd f2 co do chfnh xac cao hon phep do f,

2.2 CfNG DUNG PHUONG PHAP PHAN BO CHUAN DE 0|NH GlA SAI SO

De danh gia ket qua cua phep do, ta phai giai han, dinh lugng dugc sai sd ng&u nhien Muon lam dugc dieu nay, thi can tim dugc quy luat phan bo cua no De tim dugc, ngudi ta dung cong cu toan hoc can thiet cho viec nghien cUu su phan bo la ly thuyet xac suat va thong ke

Vdi sai sd cua moi kin do rieng biet, sau khi ta da loai bo sai sd he thong roi thi no hoan toan co tfnh chat cua mot sir kien ngau nhien Ke cjua cua lan do nay hoan toan

31

khong phu thuoc gi voi ket qua cua lan do khac, vi cac lan do deu rieng biet va deu chin nhung yeu to anh huong tdi ket qua do mot cach ng&u nhien khac nhau Vdi mdi lan do chi cho ta mot ket qua nao do Nhu vay, dung phep tfnh xac suat de1 nghien ciru, tinh toan cac sai so ngiu nhien, thi can thuc hien cac ditSu kien sau:

-Tat ca cac lan do deu phai tien hanh vdi do chfnh xac nhu nhau NghTa la khong nhung cung do o mot may, trong cung mot dieu kien ma vdi ca sir than trong chu dao nhu nhau

-Phai do nhi6u lan Phep tfnh xac suat chi dung khi co mot sd nhieu cac su kien

2.2.1 Ham mat do phan bd sai sd

De xay dung va hieu duoc quy luat phan bd, ma tu do ap dung duoc vao phep tfnh toan sai sd Ta cung can phai xet tdi dac tfnh cau tao ciia ham sd phan bd sai sd

De de trinh bay, ta gia sir la khi tien hanh do mot dai luong nao do, ta do nhieu lan, va dupe mot loat sd lieu ket qua do co cac sai sd lan lupt la x,, x:, xn

So luong lan do la n, cung dong thdi la sd lupng cua cac sai sd Ta sap xep cac sai

sd theo gia tri do Ion ciia no thanh timg nhom rieng biet Vf du, co n, sai so co tri sd tir

0h-0,01; co n: sai sd x co trj gia cung vf du nhu tir 0,01-^0,02, cung tien hanh sap xep

ca ve phfa co trj gia am: tu0-^-0.01, tir-0,01^-0,02 nhir tren

Ta co cac ty sd:

n, n->

— = V| ; — = v 2 ., o day, v, va v , goi la tan xuat (hay tin sd xuat hien) cac

lan do co cac sai sd ngau nhien nam trong khoang co gia tri gidi han do

Hinh 2-2 Bieu do phan bo tan xuat Hinh 2-3 Bieu do phan bo xac suat

Lap cac sd lieu tren thanh bieu do phan bd tan xuat nhu hinh 2-2 True hoanh la gia trj cua cac sai sd x; true tung la tan xuat v; dien tfch cua mbi hinh chu nhat nho bieu thi

so lupng xuat hien cac sai sd ngdu nhien co tri gia nam trong khoang khac do tuang ting tren true hoanh theo mot ty le nao do.

Gian do nay cho ta hinh anh don gian ve sy phan bd sai sd, nghTa la quan he giua

sd lupng xuat hien cac sai sd theo gia tri do ldn cua sai sd

Neu tien hanh do nhieu lan, rat nhieu lan, tuc sd lan do la n—>0 0, thi theo quy luat phan bd tieu chua’ 11 cua ly thuyet xac suat, gian do cua v theo x se tien den mot dudng cong irung binh p( \) nhir hinh ve 2-3:

1

limn_>aov(x)=p(x)

Ham sd p(x) la ham sd phan bd tieu chuan cac sai sd, (con goi la ham sd chinh tac) Goi la ham sd phan bd tieu chuan vi no bieu thi theo quy luat phan bd tieu chuan Trong phan ldn cac trudng hap sai sd trong do ludng dien tu, tren thuc te, phan bd xac suat ddu thfch hap vdi quy luat nay Rat ft khi co trudng hop sir dung quy luat phan bd ddng deu, quy luat phan bd cung nhau sin hay quy luat phan bd tam giac, , nen takhong de cap den cac quy luat nay

Ham sd p(x) con goi la ham sd “ Got” (Gauss) No co bieu thuc sau:

0 day chi co mot thong sd h, ung vdi cac trj sd h khac nhau thi dudng cong co dang khac nhau Hinh 2-4 bieu thi cac dudng cong phan bd sai sd ung vdi thdng sd h khac nhau Uiig vdi dudng co h ldn thi dudng dudng cong hep va nhon, co nghTa la xac suat cac sai sd co tri sd be thi ldn han Thiet bi do ludng nao U n g vdi dudng cong co h ldn thi co do chfnh xac cao; khi dung thiet bi nay de do, thi sai sd hay gap phai la sai sd

co tri so be Vdi y nghTa nhu vay ngudi ta goi h la thong so do chinh xac.

2.2.2 He qua cua su nghien cun ham mat dp phan bd sai so

Tir ham phan bd cua sai sd, ta rut ra hai nhan xet ve quy tac phan bd:

cac sai so co tri so Ion Difcfng bieu dien trong tnfdng hop nay co dang hinh chudng.

33

b Xac suat xuat hieu sai so thi khong phu thuoc vao clan ngliTa Id cc'u sai so co tri

so bang nhau ve tri so tuyet doi nhifng khac dan nhau, thi c o xac suat unit hien nliu' nhau Difdng bieu dien trong tnfcnig hcf]i nay doi xi'tng qua true tung.

Voi ham so phan bo p(x), ta co the tmh dugc so lugng sai so nam trong mot khoang dx giua hai tri so x va x+dx nao do Ta biet ring lugng nay phai ty le vdi p(x), vi p(x) la mat do phan bo sai so; phiii ty le voi n la t6ng so cac sai so (hay cua cac ldn do);

va phai ty le vdi dx la khoang tri so do ldn sai sd can tmh:

h^

Ti\

P(X| < x < x2) = —j= j£‘ e h x dx (9)

Tri sd nay chinh la dien tich gidi han bdi dudng cong va true hoanh vdi hai dudng

co hoanh do la x, va x2 (nhu da gach cheo trong hinh 2-5)

Xac suat ciia cac sai so co trj sd khong vugt qua mot trj sd x, nao do cho trudc, dugc bieu thj bang dien tfch gach cheo trong hinh 2-6:

Phan tfch phan ddu cua ve phai ( I I ) chfnh la tri so xac suat sai so trong khoang tir -oo;den +co No chfnh la su kien tat yeu, va co tri sd bang 1 Phdn tfch phan thir hai chfnh

la bieu thire (10) Do vay co the viet:

Hinh 2-5 Xac suat vdi x,<x<x2 Hinh 2-6 Xac suat vdi ! x! < x

Nhu vay, biet dirge sir phan bd sai so ta co thd tfnh dugc xac suat xuat hien nhung lan do co sai sd ma tri sd cua no Ion hon hay be hon mot gia tri sai sd nao do cho trudc Di6u nay dira tdi mot y nghTa thuc te, d ket qua do ta can lay gidi han cua tri sd sai sd phai bang bao nhieu thi dam bao chfnh xac vdi mot do tin cay nao do

35

2.2.3 Six dung cac dac so phan bd de djnh gia ket qua do va sai sd do

Hinh 2-7 Xac djnh h ung vdi xac suat

Tuy theo tri gia cua h ma xac suat cua chung khac nhau Tren hinh 2-7 ta co xac suat cue dai urng vdi h:, h- duoc goi la tri gia cuc dai cua h

Vdi mot loai tri sd do thi coi h la khong doi Khi do xac suat sai so xuat hien tai tri gia x, va lan can cua x, la:

Xac suat cua ca n lan do co the1 coi nhu xac suat cua mot sir kien phurc hop Theo

ly thuyet xac suat, thi xac suat cua mot sir kien phuc hop bing tfch sd cua xac suat cua cac sir kien doc lap rieng re:

Nhir vay, co nghTa la neu do 1000 lan mot dai luong nao do thi trong mot 1000 lan do do, chi co 3 lan do co sai so vuot qua gia tri sai so M=3a.

2 T ri so trung binh cong

Khi do mot dai luong X ta co mot loat n ket qua do co tri so la a, a;, a„ Cac sai

so cua m6i ldn do rieng biet lan luol la:

Vi chua biet cac x, (i= l, 2 n), nen X can do la cung chua biet VI vay tren thuc

te ta chi co kha nang xac dinh duoc tri so gan dung nhat vdi gia tri thuc te can do, tire phai chon sao cho tri so ay co xac suat Ion nhat Ta ky hieu tri so nay la ath va dung no cho bieu thuc cua ket qua do

DT nhien, de a,h co duoc tri so co xac suat ldn nhat thi tat ca cac sai so x, x:, x„ cung phai co xac suat Ion nhat:

Tri sd alb tuong ung vdi ^ x " cuc tieu tun duoc bAng each dao ham ^ x ~ theo

aIh ioi cho bang 0:

- 2( a ; a !b ) + 2( a 2 - a tb ) + + 2( a n - a tb )

do do

nn