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CHNG V: NG DNG B IU KHIN M T CHNH VO
IU KHIN NHIT L IN TR.
5.1 Gii thiu chung v l in tr
L in tr l mt thit b bin in nng thnh nhit nng thng qua dy
t (dy nung). T dy t nh bc x, i lu, dn nhit, nhit nng c
a ti vt cn gia nhit
5.1.1 c im
- C kh nng to c nhit cao do nhit nng c tp trung trong
mt th tch nh.
- C tc nung ln, nng sut cao do nhit nng tp trung nhit
cao.
- Nung u v chnh xc nh vic d iu chnh v khng ch ch
in v nhit .
- L bo m c kn, c kh nng nung trong chn khng hoc
trong mi trng c kh bo v, v vy chy hao kim loi nh.
- C kh nng c kh ho, t ng ho cao.
- m bo iu kin v sinh: khng c bi, khng c khi, t ting n.
5.1.2 Nguyn l lm vic
Theo nh lut Joule Lence, khi dng in chy qua vt dn c in tr
R (vt rn hoc cht lng), n s to ra mt nhit lng. Nng lng nhit
ny
s t nng bn thn vt dn hoc gin tip t nng cc vt nung gn n:
Q = R.I2.t
Trong :
Q: Nhit lng to ra trn dy dn (J)
I: Cng dng in chy qua dy dn (A)
R: in tr ca dy dn ( )
t: Thi gian dng in chy qua (s)
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5.1.3 Phn loi l in tr
*) Theo phng php to nhit, chia thnh 2 nhm:
- L in tr tc dng trc tip: l l in tr m vt nung c dun
nng trc tip bng dng in chy qua n. Loi l ny t dng v ch
c dng nung cc vt dng c hnh dng n gin.
- L in tr tc dng gin tip: l l in tr m nhit c to ra
dy in tr, ri dy in tr s truyn nhit cho vt nung bng bc x,
i lu hoc dn nhit
*) Theo nhit ca l, chia lm 3 loi:
- L c nhit thp: Nhit lm vic nh hn 6000C
- L c nhit trung bnh: Nhit lm vic t 6000C 12000C
- L c nhit cao: Nhit trn 12000C
5.1.4 Cu to l in tr
*) Yu cu c bn i vi cu to l in tr
- Hp l v mt cng ngh: c ngha l cu to l khng nhng ph hp
vi qu trnh cng ngh yu cu m cn tnh n kh nng s dng n
i vi qu trnh cng ngh khc nu nh khng lm tng phc tp
trong thit k v gi c khng qu cao.
- Hiu qu v k thut: l kh nng biu th hiu sut cc i ca kt
cu khi cc thng s ca n xc nh (kch thc ngoi, cng sut,
trng lng)
- Chc chn khi lm vic: l mt trong nhng tiu ch quan trng nht
ca cht lng kt cu ca cc l in. Nu trong khi lm vic, 1 b
phn no khng lm vic s nh hng n qu trnh sn xut
chung, gy thit hi cho sn xut.
- Tin li khi s dng
- R v n gin khi ch to
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69
- Hnh dng b ngoi p
*) Cu to l in tr
L in tr thng thng gm 3 phn chnh: v l, lp lt, dy nung.
- V l: V l in tr l mt khung cng vng, ch yu chu ti trong
trong qu trnh lm vic ca l. Mt khc v l cng dng gi lp cch
nhit v m bo s kn hon ton hay tng i ca l. Khung v l cn
cng, vng chc chu c ti trng ca lp lt, ph ti l (vt nung) v
cc chi tit c kh gn trn v l.
V l c hai dng c bn:
+ V l ch nht: dng trong cc l ging, l lin tc, l y bc v l
y rung
+ V l trn: dng trong cc l ging, l chp.
V l trn chu c tc dng bn trong tt hn v l hnh ch nht khi
cng mt lng kim loi lm v l. Khi lm v l trn ngi ta thng chn
dy t tng ng ca tm thp so vi ng knh v l nh sau:
ng knh v l (mm) 1000-2000 2500-4000 4500-65000
dy tm thp (mm) 3-6 8-12 14-20
- Lp lt:
Lp lt l in tr thng gm 2 phn: vt liu chu la v cch nhit
+ Phn vt liu chu la c th xy dng bng gch tiu chun, gch hnh
v gch hnh c bit tu theo hnh dng v kch thc cho ca bung l.
Cng c khi ngi ta m bng cc loi bt chu la gi l khi m. Khi
m c th tin hnh ngay trong l hay ngoi nh cc khun. Phn vt liu
chu la phi t cc yu cu sau:
Chu c nhit cc i ca l
C bn nhit ln khi lm vic
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C bn c hc khi xp vt nung v t thit b vn chuyn
trong cc iu kin lm vic
m bo kh nng gn dy nung bn v chc chn.
C bn ho hc khi lm vic, chu c tc ng ca kh
quyn l v nh hng ca vt nung
m bo kh nng tch nhit cc tiu. iu ny c bit quan trng
trong l lm vic c chu k.
+ Phn cch nhit thng nm gia v l v vt liu chu la. Mc ch
ch yu ca phn ny l gim tn tht nhit. Cc yu cu c bn i vi phn
cch nhit l:
H s dn nhit cc tiu
Kh nng tch nhit cc tiu
n nh v tnh cht l, nhit trong iu kin lm vic xc nh.
- Dy nung: Dy nung l b phn pht nhit ca l, thng c t
trong bung l hoc bung pht nhit, chng lm vic nhit cao, v vy
vt liu lm dy nung cn c yu cu sau:
Kh nng chu nng tt, khng b oxi ho trong mi trng khng
kh nhit cao.
C bn nng cao, bn c hc tt
C in tr sut ln
C h s nhit in tr nh.
Cc kch thc phi n nh
Cc tnh cht in phi c nh
D gia cng: ko dy, d hn, i vi vt liu phi kim cn p khun
c.
Ngi ta chia dy nung thnh 2 loi:
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+ Dy nung kim loi: c ch to bng cc hp kim Cr Al, Cr Ni v
cc hp kim c in tr ln.
+ Dy nung phi kim loi: Dng b bin l SiC, graphit, than.
5.2 Kho st v m phng c tnh ca l in tr ti phng th
nghim o lng i Hc Bch Khoa H Ni
5.2.1 Nhn dng l in tr
Vi c tnh i tng cho trc nh hnh v:
- Gii hn thang o: 0 10V (tng ng vi 00C 1000 C)
- Tc my ghi: 10 mm/h (tng ng vi 1mm/6s)
- p ng u vo 3V 5V
- p ng u ra 47,50C 640C (tng ng vi 4,75V 6,4V)
T th a v dng so chun ta xc nh c tr vn chuyn 0 = 12(s)
5.2.2 Xc nh m hnh i tng ca h thng
)(1 t )(1 th )(th
Hnh 5.1: ng c tnh i tng l in tr ghi trn my Recorder
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Hnh 5.2: M hnh i tng
Hm truyn t: s
1DT0e).s(W)s(W
Ta c th xc nh )(W1 p t hm )(1 ty :
Xc nh hm so chun t :
Ta c h( ) = h 1( )
T im 0,7 k song song vi trc t ct ng c tnh ti im A.
- T im A k song song vi trc )(t ct trc t ti im 7t .
- Chia 7t lm 3 phn bng nhau v ly im 3t .
- T im 3t k song song vi trc )(t ct ng c tnh ti im B.
- T B k song song vi trc t ct trc )(t ti )( 3t
Hm so chun c xc nh :
33,0)(y)t(y)t( 33
Ta nhn thy rng 0,31< )( 3t .Vy i tng cho l khu qun tnh
bc nht c tr.
Hm truyn t ca i tng c dng :
1s.T
e.K)s(Ws.
Xc nh cc thng s ca hm truyn t:
Do h s khuch i: 82.0265.1
yxK dt
Xc nh tr dung lng v hng s thi gian T
)(1 pW pe 0
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i tng l khu bc nht c tr nn hm so chun c m t dng:
T)t( 1
e1)t(
Chn hai im A v B trn th:
A = 0,2 tA = 54(s)
B = 0,8 tB = 276(s)
Tt(
B
T)t(
A
)1B
1A
e1
e1
T)t()1ln(
T)t()1ln(
1BB
1AA
1B
1A
B
A
tt
)1ln()1ln(
)1ln(.)1ln(.t)1ln(.)1ln(.t B1BAA1AB
Tt
Tt)1ln()1ln( ABBA
Thay vo hm )(t ta c:
)s(160
2,0ln8,0ln54276
1ln1lnttT
BA
AB
s188,0ln2,0ln
8,0ln.2762,0ln.54)1ln()1ln(
)1ln(.t)1ln(.t
AB
ABBA1
Do hm truyn t ca khu qun tnh bc nht c tr l:
1s.160
e82,01s.T
e.K)s(Ws18s
dt1
1
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Hm truyn t ca i tng c xc nh bi:
1s.160
e.82,0e.sWWs.30
s.1DT
0
Vy hm truyn t ca h thng l:
1s160
e82.0Ws30
DT
5.3 Phng php Ziegler Nichols xc nh tham s cho b iu
khin PID truyn thng
5.3.1 Phng php Ziegler Nichols I
Lut Kp Ti TD P T/KL
PI 0.9T/KL 10L/3
PID 1.2T/KL 3L L/2
Bng 5.1: Tham s b iu khin theo phng php Ziegler Nichols I
i vi i tng l l in tr c hm truyn:
1s160
e82.0Ws30
DT
Th bng thng s cho cc b iu khin l:
Lut Kp Ti TD P 6.5
PI 5.8 100
PID 7.8 90 15
Bng 5.2: Tham s b iu khin theo phng php Ziegler Nichols I
cho i tng l in tr
5.3.2 Phng php Ziegler Nichols II
Phng php ny tnh cc tham s ca b iu khin da trn Kth v Tth
a) Xc nh Kth v Tth
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>> Gh=tf(0.82,[160 1])
Transfer function:
0.82
---------
160 s + 1
>> Gh.inputdelay=30
Transfer function:
0.82
exp(-30*s) * ---------
160 s + 1
>> Gh_app=pade(Gh,3)
Transfer function:
-0.82 s^3 + 0.328 s^2 - 0.05467 s + 0.003644
--------------------------------------------------------------
160 s^4 + 65 s^3 + 11.07 s^2 + 0.7778 s + 0.004444
>> rlocus(Gh_app)
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Hnh 5.1: Xc nh Kth
Vy Kth ca hm xp x l: 11,9
Sau khi xc nh c Kth ca hm xp x Gh_app, thay vo i tng
hiu chnh li tham s Kth h kn ca i tng ch bin gii n nh
tc l h(t) c dng dao ng iu ho:
Hnh 5.2: S m phng khi Kth = 11
Vi Kth = 11 th h kn c dng dao ng iu ho:
Hnh 5.3: c tnh hng s ti hn
T th tm c Tth = 110
Lut tnh ton b tham s theo phng php Ziegler Nichols II
Lut Kp Ti TD P 0.5Kth
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PI 0.45Kth 0.85Tth
PID 0.6Kth 0.5Tth 0.12Tth
Bng 5.3: Tham s b iu khin theo phng php Ziegler Nichols II
i vi i tng l l in tr c hm truyn:
1s160
e82.0Ws30
DT
Th bng thng s cho cc b iu khin l:
Lut Kp Ti TD P 5.5
PI 4.95 93
PID 6.6 55 13
Bng 5.4: Tham s b iu khin theo phng php Ziegler Nichols II
cho i tng l in tr
5.4 Kt qu m phng
+ Trong trng hp ny ta chn cc h s t l nh sau:
Ge = 0.35
eG = 20
Gu = 0.33 (i vi b iu khin m t chnh)
Gu = 0.11 (i vi b iu khin m truyn thng)
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Hnh 5.4: p ng ca h thng l in tr cho FPIC (--); STFPIC (___)
V b PID chn theo phng php Ziegler Nichols I
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Hnh 5.5: p ng ca h thng l in tr cho FPIC (--); STFPIC (___)
V b PID chn theo phng php Ziegler Nichols II
*) Nhn xt
Qua kt qu m phng ta thy p ng ca h thng m t chnh l tt
hn so vi cc b iu khin PID truyn thng chn theo phng php
Ziegler
Nichols
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