Ứng dụng biến đổi wavelet chẩn đoán dầm côngxon nhiều vết nứt

NGHIEN cuu-TRAO001

U'ng dung bi4n ddi wavelet ch^n doan

ddm congxon nhi^u v4t nut

(*) LE THANH PHONG

TOMTAT:

Bai bao trinh bay iing dung phep bien doi

wavelet rdi rac (Discrete Wavelet Transform-DWT)

chan doan dam congxon cd nhieu vet nut vdi tai

trpng tac dpng tinh. Diia vao tfn hieu chan doan

gdc xoay de xac dinh vi trf va chieu sau vet nut. Tfn

hieu chan doan gdc xoay sau khi diigc do tren dam

cd nhieu vet nut se diigc xii If bang wavelet biorG.S

miic 1. Ket qua la, tai vi trf cd vet niit gia trj he so chi

tiet wavelet (detail coefficient) Idn dot bien va khi

chieu sau vet niit tang thi he so chi tiet tang theo.

1. Gioi thieu

Trong nhieu linh vuc, chan doan ddng vai trd bet

sire quan trpng. Dac biet, trong ky thuat, lien quan true

tiep den nhiing ung dimg ky thuat thue te, chan doan cd

the xae dinh, du bao dugc nhiing tac bai cd the xay xa.

Cu the la, nhirng tae hai do xuat hien vet nirt tren cau

tnic. Nam 1993, Rytte [1] de xuat 4 muc do hu hdng;

Miic 1: Nhan dien cau tnic cd khuyet tat hay khdng;

Muc 2: xac dinh vi tri hu bdng; Mirc 3: xac dinh muc

do hu hdng; Mirc 4: du bao kha nang sir dung tiep cua

cau tnic.Vi vay, trong chan doan ky thuat, da cd nhieu

bai bao trinh bay viec chan doan vet mit tren dam, tam

va khung phang khac nhau, vdi quan niem vet nirt trong

ket eau gay ra mot vai nhieu loan nhd trong dap img

tai vi tri cd vet nirt. Neu ehi sir dung cac phuong phap

truyen thdng de kiem tra thi khd nhan thay rd rang duge

cac nhieu loai ay, nhung neu dap img dugc ket hgp vdi

bien ddi wavelet tbi hoan toan cd the xac dinh duge.

Dap irng d day chinh la cac tin hieu chan doan nhu

chuyen vi, tan sd rieng, dang dao ddng...

Mdt trong nhiing uu diem chinh cua Wavelet la

kha nang thuc hien phan tich cue bg ma nhd vay, cd

the phan tich mdt vimg tin hieu trong ca tin hieu ldn.

Do dd, wavelet da dugc van dung vao trong co hpc de

xir Ii tin hieu va dem lai nhiing ket qua kha quan ban

eho nhirng phuong phap dua vao dao ddng hien nay.

A.V.Ovanesova, L.E.Suarez [2] su dung tin hieu chan

doan chuyen vi va chpn wavelet bior6.8 de xir Ii tin

hieu tren dam don va khung phang, vi bior6.8 thda man

day du nhiing dac tinh cua wavelet ve dp dn dinh, mien

gia tri, tinh ddi xung va moment triet tieu. S.Zhong,

S.Oyadiji [3] su dung tin hieu chan doan la hieu giiia

hai bd he sd chi tiet cua dang dao ddng dam va tin hieu

dugc xu Ii bang wavelet Sym4. W.X.Rena, Z.S.Sunb

[4] su dung tin hieu chan doan la chuyen vi eua dao

ddng tit dan. Tin bieu nay dugc xir Ii bang wavelet

packet, he sd wavelet dugc lay nang lupng va dugc

so sanh trudc va sau khi nirt. Gia tri ciia he so nay la

he sd nang lupng wavelet gdi (wavelet packet energy

rate - WPERI). Thuc chat WPERI chinh la ti so nang

lupng tuong ddi. Salgado, R.Cmz[5] khao sat 3 dang

dao ddng dau eua dam, su dung CWT de phan tieh va

wavelet co sd la db4. Tin hieu chan doan la ti sd giira he

sd wavelet trudc va sau khi dam nirt.

2. Co* so* ly thuyet phep bien doi wavelet

Wavelet duge biet den vao nam 1909 bdi Alfred

Harr, ma co sd toan hpc tu Joseph Fourier. Wavelet

dung de phan tieh tin hieu f(t) thanh cac thanh phan

tan sd khac nhau theo thdi gian. Ket qua cua bien ddi

wavelet la rat nhieu be sd wavelet, C (a,b) la ham eiia ti

le a va dich muc b

(2.1)C(fl,6)= \f{t^,j,{t)di

(*) Khoa Khoa hpc U'ng dung, Tnrcmg Dai hpc Bach khoa -

Dai hpc Qu6c gia TP. HCM

Ham t|/ (Oggi la wavelet me hay wavelet ca sd; ham

nay ed the la cac bam Harr, Daubeehies, Biorthogonal,

Coiflets, Symlets, Morlet, Mexican Hat, Meyer... Bang

each lay ti le va dich chuyen ham \J/ (0 theo thdi gian,

ta duac mdt ho wavelet:

TAP CHi c a KHi VIET NAM V So 3-Thang 3 nam 2011