iu khin ng c DC dng PID

TRNG I HC S PHM K THUT

KHOA C KH CH TO MY

B MN C IN T

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YU CU GII HN TI Thit k mch cu H iu khin tc ng c DC theo phng php iu chnh rng xung (PWM).

Tm hiu thut ton iu khin vng kn PID theo hai phng php: Ziegler Nichols (Z N).

Cohen Coon (C C).

Vit giao din chng trnh iu khin ng c DC theo thut ton iu khin vng kn PID bng c hai phng php Z N v C C.C S L THUYTGii thiu chung v thut ton iu khin vng kn PID

B iu khin PID l mt b iu khin vng kn c s dng rng ri trong cng nghip. S dng b iu khin PID iu chnh sai lch gia gi tr o c ca h thng (process variable) vi gi tr t (setpoint) bng cch tnh ton v iu chnh gi tr iu khin ng ra.

S h thng iu khin dng PIDMt b iu khin PID gm 3 thnh phn: Khu P (proportional) to tn hiu iu khin t l vi sai lch (error e) Khu I (integral) to tn hiu iu khin t l vi tch phn theo thi gian ca sai lch Khu D (derivative) to tn hiu iu khin t l vi vi phn theo thi gian ca sai lch.Bng sau cho ta thy r nh hng ca cc thng s KP, KI, KD n h thng iu khin bng PID:Effects of increasing parameters

ParameterRise TimeOvershootSettling TimeS.S. Error

KpDecreaseIncreaseSmall ChangeDecrease

KiDecreaseIncreaseIncreaseEliminate

KdSmall ChangeDecreaseDecreaseNone

Nhn xt:

KP dng gim thi gian tng trng. KD dng gim vt l v thi gian xc lp. KI dng loi tr sai s trng thi n nh.

Ri rc ha b iu khin PIDHm truyn ca b iu khin PID theo thi gian:

B iu khin s khng th ly mu lin tc theo thi gian nn cn phi ri rc ha cc khu ca b hiu chnh PID: Khu t l:

Khu tch phn:

Khu vi phn:

Suy ra hm truyn ca b iu khin PID s s l:

Vi:

;

B iu khin PID ri rc c sai s, tnh ton v xut ng ra iu khin theo mt khong thi gian xc nh (khng lin tc) gi l thi gian ly mu (T). Thi gian ly mu cn nh hn n v thi gian ca h thng.

Thit k b PID s theo phng php Ziegler Nichols (Z N)Z N l phng php thit k bng thc nghim, c s dng trong trng hp khng th xy dng m hnh ton hc cho i tng iu khin.iu kin: khi a trng thi lm vic ca h n bin gii n nh th mi gi tr ca tn hiu trong h thng iu phi nm trong gii hn cho php.Cc bc tin hnh: Trc tin, s dng b P lp vo h kn (hoc dng b PID v chnh cc thnh phn KI v KD v gi tr 0). Khi ng qu trnh vi h s khuch i KP thp, sau tng dn KP ti gi tr ti hn Kgh h kn ch gii hn n nh, tc l tn hiu ra h(t) c dng dao ng iu ha. Xc nh chu k ti hn Tgh ca dao ng.

Xc nh thng s ca b iu khin theo bng sau :

B iu khinKPTNTV

P0,5 Kgh__

PI0,45 Kgh0,83 Tgh_

PID0,6 Kgh0,5 Tgh0,125 Tgh

//Mt s ti liu ghi l TN = 0,85 Tgh (b iu khin PI). TV = 0,12 Tgh (b iu khin PID).

H s tch phn :

H s vi phn :

Thit k b PID s theo phng php Cohen Coon (C C)

C C l phng php thit k da trn mt s p ng in hnh ca h thng, cung cp kh nng c lng tnh ton cc thng s ca h thng.Cc bc thc hin: Khi ng h thng v i cho trng thi tng i n nh. To mt bc nhy ca tn hiu u vo.

Quan st p ng u ra sau mt khong thi gian.

Da vo p ng u ra, xc nh cc thng s (bng php o): t0: Thi im bt u thc hin bc nhy ca tn hiu u vo. t2: Thi im gi tr ca tn hiu u ra thc hin c na bc nhy (0.5B). t3: Thi im gi tr ca tn hiu u ra t c 63.2% bc nhy (0.632B).

A: ln bc nhy ca tn hiu u vo. B: ln bc nhy ca tn hiu u ra.

T suy ra cc thng s qu trnh: t1 = (t2 - ln(2) t3)/(1 - ln(2)) = t3 - t1

DEL = t1 - t0

K = B/A Da vo cc thng s K, v DEL ta c th tnh c KC, TI v TD da theo cc biu thc trong bng di y:

Cui cng thu c cc thng s c trng ca b PID s: KP = KC KI = KC/tI KD = KC*tDSo snh gia Z N v C C C 2 phng php ny u cung cp kh nng c lng cc thng s qu trnh ca b PID s. Phng php Z N i hi phi theo thc nghim l chnh.

Phng php C C yu cu tnh ton nhiu hn l thc nghim.THIT K V THI CNGS khi ca m hnh thi cng:

Mch nguyn l ca khi iu khin:

Mch nguyn l ca khi hi tip vn tc t Encorder:

Mch nguyn l ca khi cch ly:

Mch nguyn l ca khi cng sut:

Tnh ton Encorder:Tn s p ng:

ng c dng Encorder Incremental vi 2 knh xung A (dy trng) v B (dy vng) vi thng s l 100 xung/vng cho mi knh ( tng phn gii ln 4 ln (tc 400 xung/vng), ta thit k mch hi tip tc nh sau:

Dng 1 cng EX-OR (74HC86) bt tt c cc cnh ln v cnh xung ca 2 knh xung A v B.

Cho vi iu khin nhn ngt ngoi vi bt c s thay i no ca u vo l ng ra ca cng EX-OR trn.Tc ti a ca ng c khong 3800 vng/pht in p ngun 24VDC.Suy ra tn s ti a ca Encoder l:

y dng vi iu khin ATMEGA32 vi tn s thch anh ngoi l 16Mhz ( hon ton p ng c tn s ca Encorder.Chu k ly mu (Tmu):

Trong chng trnh Codevision dng bin m count kiu nguyn khng du unsigned long (16 bit) c gi tr ln nht l 65535.Ta c:

Nh vy chn Tmu = 10 ms l hon ton tha mn iu kin trn.Sai s khi ly mu vi Tmu = 10 ms:

C 10 ms ly mu s m c count xung ( 1 pht = 60000 ms ly mu s m c 6000*count xung. Mt khc, mch hi tip Encorder cho 400 xung/vng ( Tc p ng ca ng c s l:6000*count/400 = 15*count (vng/pht)Do s dng bin m count kiu nguyn nn sai s ca count s l 1 ( sai s ca tc p ng l 15 vng/pht.VN V GII PHP Trong phng php C C khi h p ng qu chm (t3 ln) th s xy ra trng hp t1