Chng 1CC PHNG PHP IU CH C BN Trongcc bbinibndn,cckhointlmnhimvnigiauraviuvotrong nhngkhong thi giannht nh, lp litheo mtchuk nhtnh.inp hocdng in to ras bng gi tr trung bnh ca cc gi tr tc thi theo chu k lp li . Yu cu chnh y l chu k lp li phi nh hn nhiu ln so vi chu k sng hi c bn ca cc i lng cn quan tm, nh m to nn in p, dng in u ra v u vo c dng nh mong mun. Hnh dng mong mun y c th l dng hnhsintheo snghic bn,nul mtchiuth cdng pmchvi pmchchophp. Quy lut ng ct cc phn t bn dn nh vy gi lcc phng php bin iu, hay cc phng php iu ch rng xung (Pulse Width Modulation PWM). C nhiu phng php bin iu khc nhau, tu thuc vo s b bin i v cng ph thuc vo mc tiu t ra. Tuy vy c th k ra ba phng php c bn nht sau y: -Phng php iu ch nh b so snh c ngng. -Phng php bin iu b rng xung tuyn tnh. -Phng php bin iu vect khng gian. Cc phng php iu ch c p dng trong nhiu loi b bin i khc nhau. Di y, nu ln nhng c tnh c bn ca cc phng php, ta s a ra cc v d ng dng ch yu cho cc s b bin i DC-AC ngun p, trn c s cu mt pha, cu ba pha. 1.1.Phng php iu ch nh b so snh c ngng Theo phng phpnytnhiu cn iuchnh,lyvqua php o, c sosnhvi mttn hiu t. Tn hiu u ra b so snh s c gi tr, v d l mc lgic 1, nu tn hiu o c ln hn tn hiu tmtgitr ngng nht nh,lmclgic 0 nutn hiuo nhhnlng tgi tr ngng.Tn hiu lgic 1 c a n iu khin ng ct cc van trong b bin i tn hiu iu chnh phi gim i. Tn hiu lgic 0 s lm cho lng iu chnh phi tng ln. Trn hnh 3.1.a th hin tc ng ca b so snh hai ngng, u ra tn hiu c gi tr (0, 1) vi c tnh vo ra nh trnhnh 3.1.b. C th s dng nhng b so snh phc tp hn, gm ba ngng, vi u ra tn hiu c gi tr (-1, 0, 1). I+Ai Ai AIA Hnh 1.1. (a) Tc ng ca b so snh c ngng; (b) c tnh vo ra ca b so snh. V d v tc ng ca phng php ny c m t qua s b bin i DC-AC cu mt pha, lm vicnhmtb nghchlupngun p,ti tr cmRL,chotrn hnh 3.2. Gi sta mun iuchnh dng u ra pha xoay chiu theo mt lng t hnh sin, theo th biu din trn hnh 3.3. Trn hnh 3.2, iref l lng t dng hnh sin mong mun. Tn hiu dng xoay chiu u ra nghch lu o cin cso snh vi lng t qua khu sosnh ctr,tonntnhiuiukhinchoccvan V1, .., V4. Xt chiu dng in ang dng. Khi V1, V4 m s lm cho dng tng ln, cn V2, V3 m s lm cho dng gim i.CRLuninV1V2V3V4o inV1V2V3V4irefD1 D3D2 D4Etin Hnh 1.2. S cu trc h thng iu khin. I+AIAHnh 1.3. Tc ng ca b iu chnh dng in c ngng. Khi V2, V3 m, dng ch chy qua it D2, D3. iu tng t xy ra vi chiu dng in m, khi vai tr ca cc van i cho nhau. Nu mun iu chnh in p ra trn ti R, phi dng mch lc LC to ra in p ti c dng gn sin. Khi in p trn ti un o c s a n b iu chnh in p, u ra b iu chnh in p c th l tn hiu thay i bin ca lng t dng in. Qua hnh 3.3 c th thy rng tn s ng ct ca cc van khng c nh. y lnhc im chnh ca phng php ny.Khi mc ngng nh, tn s ng ct c th rt ln. Nu mc ngng ln, dng dng in s xu i vi nhiu thnh phn sng hi bc cao.u im chnh ca phng php ny l n gin, d p dng, c bit l tc ng nhanh rt tt. C th thy r u im th ba ny qua hnh 3.3, trong lng cn iu chnh chnh l dng in in, c o mt cch trc tip, do s phn nh mt cch tc thi nhng thay i c th xy ra. Trong s trn y tn hiu iu khin l tn hiu xoay chiu. Trong cc s b bin i khc th lng t c th c dng tn hiu mt chiu, v d nh trong cc b bin i xung p DC-DC. 1.2.Phng php bin iu b rng xung (PWM - Pulse Width Modulation).1.2.1.Nguyn l bin iu b rng xung Hnh 1.4. th m t phng php PWM. Phng php bin iu b rng xung c pht trin trong k thut iu ch sng radio, sau c p dng cho k thut bin i in nng. Ni dung ca phng php bin iu b rng xung l so snh mt tn hiu chun, l tn hiu ra mong mun,vimt inp rngcacchuk cnh,Ts, thng gi lchuk ly mu.u raca bso snh s l dy xung c rng thay i, tu theo gi tr ca tn hiu t, tx:0x st T s s . Trong khong thi gian tx cc kho in t s ni u ra Uo vi u vo Ui ca b bin i. Nh vy trong mi chu k ct mu, gi tr u ra Uo s c gi tr trung bnh ph thuc gi tr u vo Ui v thay i theo dng iu ca tn hiu chun. y cng gi l phng php bin iu c sng mang (Carrier based PWM). v d,ta s xtmttrng hp ngin,mt qua thtrn hnh 3.4. Gis Ui= E,khng i, cn tn hiu t thay i theo hm sin tn s thp, c bin bng Ur,m: ( ),sinref r mu U t e =(3.1) 2 f e t = , f l tn s sng sin chun. Tn hiu rng ca c dng tuyn tnh, bin bng Us,m: ( ),, 0,1, 2,...s s m su U t kT k = = (3.2) Ti chu k th k ca tn hiu rng ca, tx,k c xc nh bng:tx,k = tk* - kTs , trong : ( ) ( ),,sins mk s r m ksUt kT U tTe- - = , ngha l tk* l thi im tn hiu chun bng tn hiu rng ca chu k ct mu th k. Do ta c: ( ) ( ), ,,, ,sin sin ,r m r mx k s k s ks m s mU Ut T t T tU Ue e - -= = = (3.3) gi l h s bin iu,0 1 s s . +E-E00ttUs,mUr,mT/2 T+E-E00ttUs,mUr,mT/2 T-Us,m(a)(b)Hnh 1.5. Phng php bin iu b rng xung PWM. (a) Mt cc tnh; (b) Hai cc tnh. Vy gi tr trung bnh ca tn hiu ra trong chu k ct mu th k l: ( )( 1),,sinssk Tx ko k o i i ks kTtU u dt U U tT e+-= = =}(3.4) C th thy rng gi tr trung bnh ca tn hiu ra sau mi chu k ct mu bm theo dng ca tn hiu sin chun mong mun. Biu thc dng (3.4) gi l gi tr trung bnh trt. Nu phn tch ra chui Furie c th thy rng dng in p trung bnh trt thc s cha thnh phn sng hi c bn nh sng sin chun v cc sng hi bc cao vi tn s lfs +/- f , l = 1, 2, Nu tn s ct mu fs ln hn nhiu ln tn s f ca sng sin ra mong mun th cc tn s sng hi bc cao c th d dng loi b nh cc mch lc n gin hoc di tc dng ca in cm ti.Phng phpbiniubrng xungcdngmtcctnhvdnghaicctnh. Theodngpra mt cc tnh, trong nhng khong tn hiu chun cao hn tn hiu rng ca, van c m a in p ra ti, trong nhng khong tn hiu chun thp hn tn hiu rng ca, van kho li in p ra ti bng khng. in p ra s c to thnh ring cho na chu k dng v na chu k m. Theo dng p ra hai cc tnh in p ra s l +E khi tn hiu chun cao hn rng ca v l -E khi tn hiu chun thp hn. Hnh 3.5 m t nguyn l hot ng PWM cho hai trng hp trn cho s nghch lu p cu mt pha. Phng php hai cc tnh c dng tn hiu iu khin ra n gin hn nhng in p ra s c bc nhy inp ln hn,2E. Bc nhyin plns lmchodngdng inp mch nhiu hn,ni cch khc l dng dng in s xu hn so vi phng php iu ch mt cc tnh. Tn hiu rng ca c th c dng khng i xng, nh trn hnh 3.4, hoc c dng i xng, nh trn hnh3.5.Trongdngkhngixngrnglicthcsniln,nhtrnhnh3.4,hocsni xung. Dng rng ca i xng l kt hp ca c hai hai dng khng i xng trn. Trong dng rng ca i xng chu k ct mu c chia lm hai na Ts/2, v vy trn ng in p ra sng hi bc cao s c tn s l bi ca 2fs, ngha l cht lng in p ra s tt hn. m bo in p ra c cha t nht thnh phn sng hi bc cao, s phi lm vic trong ch tuyn tnh, ngha l phi m bo0 1 s s . Hnh 1.6. (a) S xut hin dng m xuyn; (b) Tn hiu iu khin IGBT. Trong s cc b bin i p dng PWM, in p ra s cha cc thnh phn sng bc cao vi tn s bng cc bi s ca tn s xung rng ca. Do tn s xung rng ca rt cao so vi tn s sin chun nn d dng loi b c cc sng hi bc cao ny. y l u im c bn ca phng php bin iu b rng xung. u im th hai l tc ng nhanh cao nh bn cht ca cc h thng iu khin tng t. Nu PWM c xydngtrn b x l tn hius thng qua qutrnhri rc ha tnhiuth u imny cng mt i. Nhc im ca phng php ny l cc van phi lm vic vi tn s ng ct cao nn tn hao cng sut do ng ct ln, h thng iu khin cng phc tp hn. CV1V4D1D4ZtEidin t t tV1V4tt(a)(b)Hai van trn cng mt nhnh cu phi p dng phng php chuyn mch c thi gian cht trnh dng mxuyn.Hnh3.6m ts xuthindng mxuyn vyucuivitnhiuiukhin trongs nghchlu nacu. Thigian trtgiatn hiumV1v V4 phitnht bng thi gian kho ca van. i vi IGBT gi tr tiu biu1, 5 2 s t = . 1.2.2.B bin i cu ba pha PWM B bin i cu ba pha, c s cho trn hnh 2.10.c, cn gi l b bin i hai mc, l mt s c bn, c s dng rng ri trong cc b bin i DC-AC v AC-DC.Trc ht ta s xem xt s hot ng ca s cu ba pha trong s mt nghch lu ngun p ba pha, nh c biu din trn hnh 3.7. Mch lc cu to t 6 van iu khin hon ton V1, V2, , V6 v 6 it ngc D1, D2, , D6. Cc it ngc gip cho qu trnh trao i cng sut phn khng gia ti vingun. uvomt chiu lmtngun pvictrng ctC,gi trln.Ph ti3pha i xng, ZA = ZB = ZC, c th u Y hoc. Hnh 1.7. S nghch lu c lp ngun p ba pha. Phng php c bn to ra h thng in p xoay chiu ba phatrn ti l iu khin cc van theo th t nh c k hiu trn s , mi van vo dn cch nhau 60o. Khong iu khin dn ca mi van = 180o. Vi cch iu khin nh vy in p dy trn ti s ch c hai mc, hoc l +UDC hoc l UDC. V vy y cn gi l b bin i hai mc. 0 60 u s s60 120 u s s 120 180 u s s Hnh 1.8. S tng ng mch ti ng vi cc khong dn ca van.(a) V1, V6, V5 dn; (b) V1, V6, V2 dn; (c) V1, V2, V3 dn. AStothnhcainpuratrnccphacgiithchquaccsmchtitng ng trong cc khong dn ca van nh trn hnh 3.8. Xt cc khong dn ca van trong mi 60o na chu k th nht nh sau: - Trong khong0 60 u s cc in p ra uAn, uBn,uCn s b gii hn bi +/-UDC/2 v vect khng gian in p ra b gii hn trong hnh lc gic c nh l cc vect bin. Hnh 1.21. Dng in p bin iu uAn, uBn, uCn, uZn v in p trn cc pha ti uA, uB, uCvi UDC = 300 V, Urm = 150 V. Hnh 1.22. Dng in p bin iu uAn, uBn, uCn, uZn v in p trn cc pha ti uA, uB, uCvi UDC = 300 V, Urm = 173 V. Trnhnh3.21,3.22,3.23minhhaccdnggitrtrungbnhtrt,xcnhbiccbiuthc (3.44),(3.45),(3.46),cainpracaqutrnhiuchchobatrnghp: 1 1; ;2 3rm DC rm DC rm DCU U U U U U = = > . th th hin qu trnh qua su sect ca mt chu k in p ra. Trn hnh 3.21, 3.22 cho thy dng trung bnh trt in p ra trn ti, UA, UB, UC, c dng sin hon ton. Hnh 1.23. Dng in p bin iu uAn, uBn, uCn, uZn v in p trn cc pha ti uA, uB, uCvi UDC = 300 V, Urm = 200 V. th trn hnh 3.23 cho thy cc in p uAn, uBn, uCn u t n gi tr gii hn v in p ra trn ti khng cn l hnh sin khi 13rm DCU U > . Hnh 1.24. Vect khng gian in p ra vi UDC = 300 V, Urm = 150 V. Hnh 1.25. Vect khng gian in p ra viUDC = 300 V, Urm = 173 V . Hnh 1.26. Vect khng gian in p ra viUDC = 300 V, Urm = 200 V. Trn cc hnh 3.24, 3.25, 3.26 th hin dng ca vect khng gian in p ra trn tr khng ti tng ngvibatrnghp iuchtrn y.Trnhnh 3.24,3.25vectkhnggianquaytheomtng trn. Hnh 3.26 cho thy vect in p ra t n ht gi tr gii hn l hnh lc gic c nh l cc vect bin chun. 1.3.4.6.S mch tng ng S mch in tng ng cho php thit lp nn h phng trnh m t qu trnh in t, t c th kho st h thng, xy dng cc cu trc iu chnh v xc nh cc tham s ca b iu chnh theo cc phng php bit. S thay th tng ng ca nghch lu p ba pha cho trn hnh 3.27. Mi pha in p ra c thay th bng mt ngun in p c dng xung gia +UDC/2 v UDC/2, nh u ra ca mt s na cu. .UDCiAiBiCZuAuBuCiDCiDCuAnuBnuAn Hnh 1.27. S mch in tng ng ca nghch lu ba pha. Phn mt chiu c th hin bng mt ngun p UDC. Dng mt chiu id to bi cc mnh ca dng xoay chiu iA, iB,iC do tc dng ng ct ca cc kha S1, S2, , S6. iu ny c th gii thch qua hm ng ct ca cc kha. nh ngha hm ng ct ca mi pha SA,SB, SC nh sau: SA = 1 nu S1 ng; SA = -1 nu S2 ng; SB = 1 nu S3 ng; SB = -1 nu S4 ng; SC = 1 nu S5 ng; SC = -1 nu S6 ng. Khi c th thy rng: | | | |2DCA B C A B CUu u u S S S = (3.47) Dng in biu din qua cc hm ng ct nh sau: | | | | .TDC A A B B C C A B C A B Ci S i S i S i S S S i i i = + + = (3.48) Trong phng php iu ch vect khng gian, ti mi chu k ct mu Ts php iu ch s la chn bn trng thi van ng vi cc khong thi gian t0, t1, t2, t7 to nn vect in p ra. Cc trng thi van sxc nhhon tongi tr gi trcavecthm ng ct[SA SB SC], t m cc biuthc (3.47), (3.48) c xc nh. Bng phng php iu chvectkhnggian,b quanhhng catn sctmu vccthnh phn in cm, t k sinh cng nh qu trnh chuyn mch gia cc van bn dn, gi tr trung bnh trt ca in p ra c xc nh nh (3.44). in p ra trn cc tr khng ti xc nh bi (3.45). Thay th cc in p dng xung ca s iu ch hnh 3.26 bng cc gi tr trung bnh trt ca chng, UAf, UBf, UCf, ta thu c m hnh trn gi tr trung bnh. Hnh 3.27 th hin m hnh cho nghch lu p ba pha vi mch lc LC trn u ra. M hnh cho chnh lu ba pha cho trn hnh 3.28. Cc i lng in p, dng in k hiu bng ch in ln cho gi tr trung bnh. Hnh 1.28. M hnh tn hiu trung bnh cho nghch lu p ba pha. Hnh 1.29.M hnh tn hiu trung bnh cho chnh lu ba pha. 1.3.5.M hnh trn h ta ba pha 1.3.5.1.M hnh nghch lu S dng m hnh tn hiu trung bnh trn hnh 3.28, gi thit ngun mt chiu UDC l l tng, cc tn hiu in p iu khin UAf, UBf, UCf c th biu din di dng: Af AfBf DC BfCf CfU dU U dU d (( ((= (( (( (3.49) dAf,dBf, dCfl cc h s lp y xung ca tn hiu iu ch pha-trung tnh. So vi (3.47) thy rng cc h s ny l dng trung bnh ca cc hm ng ct SA, SB, SC. Phng trnh vi phn i vi dng in qua cc cun cm v in p trn cc t uc vit di dng: 1AfA ADCB Bf BC CCfdI UU dI d Udt L LI Ud ( (( ( ((= ( (( ( (( (3.50) 1A A LAB B LBC C LCU I IdU I Idt CU I I ((( (((= ` ((( ((( )(3.51) ILA, ILB, ILC : dng ba pha ti. Dng mt chiu biu din di dng: | |TDC Af Bf Cf A B CI d d d I I I( = (3.52) Cc phng trnh t (3.49) n (3.52) cho thy y l mt h thng nhiu u vo, nhiu u ra. C su bin trng thi l dng trn cc cun cm v in p trn cc t. C ba u vo iu khin l cc h s lp y xung dAf,dBf, dCfv ba u ra l cc in p trn ti UA, UB, UC. Trong ch xc lp, in p trn ti l h thng in p ba pha: cos2cos32cos3AB rmCUU UUututu ( ( ( ( (| |( = (| (\ . ( ( (| |+ ( |\ . (3.53) Thay (3.53) vo (3.50), (3.51), vi iu kin bit c cc thng s ca ti s tnh ton c in p iu khin t (3.49). C th thy rng tt c cc i lng in p, dng in xoay chiu u c dng sin. 1.3.5.2.M hnh chnh lu tch cc ba pha Da trn s m hnh tn hiu trung bnh trn hnh 3.29, gi thit rng in p lui u vo chnh lu UA, UB, UCl ngun in p ba pha l tng, ta c h phng trnh vi phn sau: 1AfA ADCB Bf BC CCfdI UU dI d Udt L LI Ud ( (( ( ((= ( (( ( (( (3.54) | |( )1 TDCAf Bf Cf A B C LdUd d d I I I Idt C( = (3.55) IL l dng ti pha mt chiu. M hnh c bn bin trng thi l dng qua cun cm IA, IB, IC v in p trn t mt chiu UDC. Ba bin vo iu khin l dAf, dBf, dCf v mt bin ra in p UDC. 1.3.6.M hnh trn h ta 0dq 1.3.6.1.M hnh nghch lu ng dng ma trn bin i T4 nh trong (3.26): 42 2cos cos cos3 32 2 2sin sin sin3 3 31 1 12 2 2Tt tu u ut tu u u ( | | | | + ||(\ . \ . ( (| | | |= + (||\ . \ . ( ( ( cho cc php bin i t h ta 0abc sang 0dq, vi lu cc php o hm c thc hin nh trong (3.56), (3.57): 11 44dqabcdqdXdX dTX Tdt dt dt= + (3.56) 1440 00 00 0 0dTTdtee( (=( ( (3.57) Cc biuthc trong hta ba pha t (3.49) n(3.52) c bin ithnhtng ng t(3.58) n (3.62). 4 40 0df Af Af dfqf Bf DC Bf DC qff Cf Cf fU U d dU T U U T d U dU U d d (((( ((((= = = (((( (((( (3.58) 0 0 00qd d dq DC q q dII d UdI U G d G U IdtI d Ue (((( ((((= + (((( (((( (3.59) 0 0 010qd d Ldq d q LqLUU I IdU U I Idt CU I Ie (((( ((((= + ` (((( (((( )(3.60) 10 010 00 0 0LGL ( ( ( (=( ( ( ( (3.61) Dng mt chiu bng: 0 03 332 2TDC d q d qI d d d I I I ( ( = ( (3.62) 1L1C} }1L1C} }IdILqL eL eIqILqC eC eUdUqXXddUDCdq-+++--++-+-- Hnh 1.30. M hnh nghch lu trong h ta d 0dq. Mhnhnghchlutronghta0dq,xydngtheo(3.59),(3.60),khngknthnhphn khng, cho trn hnh 3.30. Trn m hnh c th thy c bn bin trng thi l Id, Iq, Ud, Uq, hai bin iu khin l cc h s bin iu dd, dq, hai thnh phn nhiu l cc dng ti ILd, ILq. Hai knh theo hai trc d,q clinhchovinhauthngquaccthnh phninptrncuncmeLId,eLIq,ccthnhphn dng in eCUd, eCUq. Cc lin h cho ny ny khng d phn ly to thnh hai knh iu khin c lp. u vo l in p mt chiu UDC, u ra l in p Ud, Uq. Khi cc thnh phn ph ti l i xng, tt c cc thnh phn U0, d0, I0 u bng 0, cc biu thc trn s n gin i ng k. Trong ch xc lp cc o hm trong (3.59), (3.60) cho bng khng, in p ra s c dng ng nh tn hiu t mong mun: | |00 0TTd q rmU U U U(= (3.63) Dng in ra trn in cm c dng: 0000d Ldq Lq rmI II I U CIe (( ( (( (= + (( ( (( ( (3.64) Cc h s bin iu bng: 21100 00 0Lqd Ldrmq Lq LdDC DC DCId I LCU G d Ld I IU dt U Uee(((( ((((= + + (((( (((( (3.65) Trong ch xc lp o hm cc thnh phn dng in trong h ta 0dq s bng khng, v vy cc h s bin iu s bng: 2100 00Lqdrmq LdDC DCId LCU Ld IU Uee((( ((( = + ((( ((( (3.66) Tuyn tnh ha quanh im lm vic xc lp bng cch cho cc bin trng thi v cc bin iu khin nhng bin ng nh nh (3.67).; ; ; ;; ;d d d q q q d d d q q qd d d q q qDC DC DCd d d d d d I I i I I iU U u U U uU U u= + = + = + = += + = += + (3.67) Trong cc k hiu c m ng trn ch cc bin ng nh. Tuyn tnh ha c tin hnh theo cch thng thng bng cch b qua cc bin ng bc hai, v d vi tch hai bin sau: ( )( )DC d DC DC d d DC d DC d DC dU d U u d d U d U d u d = + + ~ + + (3.68) p dng cc php gn ng tng t nh (3.68) cho cc phn t trn s hnh 3.30, b qua phn cn bng gia cc gi tr xc lp, c th thu c m hnh trn cc tn hiu bin ng nh. Nu m phng s trn SIMULINK qu trnh tuyn tnh ha c th thc hin mt cch t ng. 1.3.6.2.M hnh chnh lu pdngmatrnbiniT4chocc phngtrnh(3.54),(3.55)thu cmhnhchochnhlu trong h ta 0dq nh sau: 0 0 00qd d dq DC q q dII d UdI U G d G U IdtI d Ue (((( ((((= + + (((( (((( (3.69) 0 01 3 332 2TDCd q d q LdUd d d I I I Idt C ( ( = ` ( )(3.70) Mhnhchnhlutronghta0dq,xydngtheo(3.69),(3.70),khngknthnhphn khng, cho trn hnh 3.30. Trn m hnh c th thy c ba bin trng thi l Id, Iq, UDC, hai bin iu khin l cc h s bin iu dd, dq, thnh phn nhiu l dng ti IL. Hai knh theo hai trc d,q c lin h cho vi nhau thng qua cc thnh phn in p trn cun cm eLId, eLIq. u vo l cc thnh phn in p li Ud, Uq, u ra l in p mt chiu UDC. 1L}}1L}L eL e32C Hnh 1.31 M hnh chnh lu trong h ta 0dq. Gi thit rng s c iu khin cho h s cng sut bng mt. Trong ch xc lp Ud = Ulm, Uq = 0, Ulm l gi tr bin in p pha. Dng in c gi tr Id = Ilm, Iq = 0, vi Ilm l bin dng in pha. Cho v phi cc phng trnh (3.66), (3.67) bng khng, c th xc nh cc h s bin iu nh sau: 03230dLqMdIdMd ( ( (( ((= (( (( ( ( (3.71) Trong M l h s bin iu, IL l dng ti. Thc hin th tc tuyn tnh ha, tng t nh lm cho m hnh nghch lu, ta s thu c m hnh cho cc tn hiu bin ng nh, lm c s cho vic thit k cc b iu chnh trong h thng iu khin.