Application of an Expert System to Hot Strip Mill Load Balance Control

Copyright © IFAC 12th Triennial World Congress, Sydney, Australia, 1993

APPLICATION OF AN EXPERT SYSTEM TO HOT STRIP MILL LOAD BALANCE CONTROL K. Kurihara*, S. Murakami*, H. Umeda* and G. Kameyama** ·Technology-Rolling Mills, Process Control Department, FukuyalTUJ Works, NKK Co rporation, 1 Kokan -cho, FukuyaITUJ City, HiroshilTUJ-Pre!ecture, 721, Japan ··Hot Strip Mill, Fuku yaITUJ Works, NKK Co rporation, 1 Kokan -cho, Fuku yalTUJ City, HiroshilTUJ-Pre!ecture, 721, Japan

Abstract. Draft-schedule in oot strip finisber lill is deterlined by C
distriootioo

backwa.rd -stands --

forward -stands -

L INOODOCTION

F1

In tile past, the optimum relationship of thickness between hot strip tandem finisher mill was decided by experiential metho~ such as to equalize the load-ratio (~) of each stand's electric motor power by manual design. In recent years, rapid progress of cauputer technology made it possible to calculate more cauplicated theoretical model in a very short time. But not every accurate data can be got, tilere is an inevitable limi t for model accuracy, and manual adjustment should be taken especially in case of material with high deformation resistance or thin gauge material. Operator adjusts to avoid the strip snaking movement in the direction of strip "idth or the waving movement between the stands. And these manual operation can only be done by aill experts, so it is eagerly needed to decide tile welH~ lanced thickness autooJatically.

Fig . l

F2

F3

F4

FS

F6

F7

Configuration of the hot strip mi 11

expert system. Fig. 1 shows the equipment of No. 2 hot strip finisher mill, which is composed of 7 stands, "forward stands" and "backward". 2.1 Expert System approach to draft-schedule Hard material (e. g. high carbon steel, stainless steel etc.) or thin gauge material (typical thickness is L 2sn) have the following character. (l)they are apt to snake when their tail-end pass through the mi 11. (2) they are apt to wave between the stands. Fig. 2 shows the severe allowance of transversal movement at tail of strip. 'hen rolling these materials, tile index of mi lling load is evaluated by the each stand's main motor current. Fig.3 shows the variation of load distribution over stands. In some cases higher load in forward stands, and another higher in back!Jar~ By this know-how, better rolling condition can be reappeared. I t is ratiler qualitative than theoretical, so these operation will be realized by means of expert system ; that is the theme of this paper.

Therefore the expert system is introduced in the finisher Bli 11 set-up system. In this paper, adjustIIeIlt of each mi 11 stand's well-balanced thickness, i. e. well-balanced roll gap is called as "Draftschedule", or "to schedule the draft".

2. A BACKGOOUND OF INOODOC ING AN EXPERT SYSIDI Before mentioning ahead, it should be explained the reason why the choice was not made to improve the theoretical model accuracy but to introduce an 603

Entry guide

~

s.. " .....

~-[It

Gap allowance

r! . .~ m 20-30,.,.

~HHH C+~.s.~

Flg.2

AII~ce

of transversal _ _ t at tall of strip

IOOOO~~~~~~~~~~~~~~~,

~

i

I IbIlfy target . . and cr-. for each .tMd 1 . - - - - - - - - , eo .. to reduce the ooat funct Ion.

8000 ,""

a 8000 i~ 12000

"'....

Averaga curr.nt of F5. Ft ..... " ..... " .... ~/

......'........

4

-, ' .. ,...,..

~

-

i 3

~

2

J~

0::

Moto,iol with hi."" •... .1 , ....\ ._... _._ ........_ Thin _ defol1Mtlon , .... ttnee GIge ...... ......

..to,iol ... . .

..

O~~~~~~~~~~~~~~~I

~

Fi~

........rofrollil"G

3 L..o.s dlatribution over ,trda In orw evel.

NO

.. I

";Co

Fig.S

I ~ u

...• -

.!

the material character to snake, nor wave. (2)there aren't sufficient sensor located between each stand, such as thickness gauge, profi lemeter, shape-meter. (3)as the thickness becomes thin, the absolute control range becaIes SEller. (e.g. nearly lua;micron meter accuracy of strip profile is ordered for lID thickness sheet.)

...

I!o

o.~ -30;;

Ibdling mi 11

Fl

F2

F3 F4 Stand 00

Fig.4lbJting probln in draft of finishing mi 11

F5

Flow of draft-schedul ing

F6

~Iing

2. 3 The advantage of Expert Systell\

2. 2 Accuracy of Conventional Finisher Hi 11 Set-Up Model

operator uses the conventional set-up systea with manual adjus1:Jlent. Because, ski llful operator can predict the IIkXieI error. This operator's ability is (l)to notice the visible inforlll!ltion such as the strip shape seen between stands, (2)to find the better rolling condition and improve his operation gradually, (3)to recognize habit or weakness of u1e1; what the way computer has, and to cope wi th it. If such lIodel weakness could be taken into the decision procedure of well-balanced gap, it would realize the fully automated stable rolling. So the expert systell is introduced for Draft Schedule.

Anyway,

This

explains that there is difficulty to keep mutual mill stand's well-balanced gap by means of conventional finisher mi 11 set-up system. As is shofm in Fig.4, to schedule the draft is to decide optimua route about strip thickness and profile (profile is thickness deviation in the direction of strip width) . The start point is rougher mill's delivery strip thickness or profile, and the final point is finisher mill's delivery ODeS. The detail calculation is shofm in Fig. 5. paragraph

This set-up systell\ involves two probleas; (1)1Iode1 accuracy is insufficient, and it is compensated by studying lIkXiel error. (2)The solutions are various because the opti_ route is not uniquely decided. So Draft Schedule is conventionally decided from previous strip's load distributiorL Conventional Draft Schedule operates well for mild steel rolling, but it doesn't in case of hard asterial or thin gauge. The reasons are (l)the theoretical IIkXieI doesn't consider ahoot

3. lfu1 EXPERT SYSIDI 3.1 Configuration Total configuration uf finisher .ill set up Systea (See Fig.6) Conventional Draft Schedule is calculated from previous strip' s one and is used to 604

Fig.7 Hardware configuration of expert-system

Strategy In cycle

Products data of all eolls In a cycle

JudgMll!ftt of specific _terlal Turning point of strated}'

. General I nference for each coil

Fig.6 Block diagram of finishing mill setup system

Real data of previous eoll ROll9h I ng .. Ill data of CIIrrent coil

actual mi 11 preset New Draft Schedule is inferred in a expert system and is also displayed in a guidance CRT. Le~ s explain an example of Draft Schedule.

Process

-8-

CASE A rougher finisher mi 11 aill No.1 No.2 No.3 No.4 No.5 No.6 No. 7 thickness l). ~ 18l1li 13.5aa 1O.8am 9aD 7. 2m 6l1li X. 66

x. 75 x. 75

x. 80

x. 83 x. 80

Guidance of schedule modlflcat Ion

+

Adjustlftent of output quanti ty

~

Min"1ll&x value adjustment Average val ue adjustment Transfonnat ion In specifiC ease

Fig.8 Inference flow

x. 83

strategy is inferred for a next-coming cycle each product data of cycle materials.

CASE B thickness l3.. :n. z.4. x. 83

H

Valuation of motor CIIrrent difference between nelghborIng stands Valuat Ion of ro !I-gap Valuat Ion of roll-force

I

ro. 16.. 12m 9!a S. x. 80 x.83 x. 80 x. 75 x. 75 x. 66

usi~

Inference for each coi l' s Draft schedule This block infers how IIIlch load ratio should be adjusted for every coil. The input datua are next coil's conventional Draft schedule and previous coil's actual rolli~ data. The major evaluation index is current difference between neighbori~ stands. roll force, and roll gap range. They are exaained by general aill exper~ s knowledge, then lIOdified.

"CASE A" is higher load in forward stands, "CASE B" is higher in reckward. The target thickness is also s.. and the ratio of target by rougher mill is equally 6/l). Operator can JOOdify the gap balance by E81lS of underlined 8 digital switches connected to COIIPUter.

Inference for actuating value For the finisher aill, two method of operation can cope with UIlbllanced strip condition. For exaaple, if aaterialflow between F1 and F2 becomes excessively loose, operator adjusts to open FZ s gap, or to narrow Fl's gap. So the final block decides which aill to be actuated and how IlUCh to be adjusted. The nlliler of each inference rules are 34, 49, 20, total 1m.

Hardware configuration (See Fig. 7) Expert shell operates in a workstation and the data for inference is received real time frOll the process COIIPUter. The 8Jk)U(lt of data for inference is 24 ite.s/strip x 100tol50strips :at roll c_e 56 i tells :at each strip rolli~. It needs 8 sec-.onds for a strip to infer and display the output The software is TDfS-Toshire supplies, an
3.2.2 Rules 3.2 The Inference Flow

The typical rules are written as follows.

3.2.1 The Block Component

Scheduling lIi 11 load distribution Operator usually varies the load distribution strategically in a cycle in forward or backward. (See Fig.9) The detail is as follows.

The inference flow is composed of following three blocks which are like operator's consideration pattern. Fig. 8 shows them.

i) when

rolling the first Eterial in a cycle or

the bard material, the load distribution is

Inference for a cycle strategy "A cycle" is a series of mterials which can be rolled in one roll chance. After a cycle rolling, the rolls are ~ed to new ones. In roll changing timing, rough

lkXiified to be higher in forward stands to aake them easy to be passed through. But to avoid the excessively rapid change, the distribution 605

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t

_

, -
.tn.

thin pge

_ ted.1

\

...

. ..

~'"T;;; ~ " ,.: , , 0

JiUlbetofrolll re

_ _-./---,:~--:::'::I

'-...\-

. .!

'i

~

F2 F3 F4

Fig. 9 Str.tegy of loed d i str i but i on in Fig, 3 cycle

""... A IlBterial with hipr recilx:tion 10000 " . in forward stands

~ _~

!

•

!: I-.~-"'-"-"-"'-"";':':""~< ~

c: 2000

Fig. " A eXlIIl>le of the _y to cl_lty . . In IIIDtor

z

00

". ".

1.5E+6

....::::\ ::::::---.....

A meterial with hipr reduction in bac"-rd stands

1.
current diatribution over F2.

~

F4 atrda

8

~

Table 1 Ev&lu&tion of the expert system

"\ ........:::. 5.lE+7 Roll force .

Enl U&t ion

j Stand no. Fi~

~

10 Carparison b e _ oortor current and roll force as the index of load distribution

Case2

C&.se3

Good

Cue 1 35~

51~

35~

Difficult to judge

55~

~

Em

Not good

1~

~

~

system is more than 90~ applicable to the real process. During the tested cycle, the expert systell fortunately predicted the actual rolling trouble; operator gradually made a mistake for Draft schedule adjustment and at last the strip snaked, moved aroun~ and made collision with entry-guide and crashed. If he adopted the expert system's guidance, the crash wouldn't occurred. So this expert system will surely be able to take the place of operator's attention surprisingly.

should be changed gradually before the special material comes. ii)When rolling thin gauge material, nearly full load must be used; higher in all stands, so the guide mark is intentionally made to roll surface (which is drilled as roll wears out by strip) for stable rolling; thin gauge material follows the ro 11 mark. (2)Motor Current Pattern (see Fig. 10) Motor current is easy to understand for operator as the index of load distributio~ The balance is observed IOOre easily in current pattern than roll force. It can be explained theoretically; torque = roll force x torque arm cons~~ x (reduction x roll diameter)l (1) Motor current is in proportion to IOOtor torque, so the current ~ttern is larger as the magnitude of (reduction) 1 than the roll force pattem

5. OONCL15ION

The new expert system was introduced in the hot strip finisher mi 11 set-up systea and I18de an excellent effect on strip rolling process. Though it is only operated in a guidance system 1lOIJ, it will be included in the conventional on-line set-up system if it proves to be fully applicable. Through the process to develop this expert system, we have got the perspective to schedule the well-balanced gap full-automatically. But from the stand point of free schedule rolling, it should be expected to improve the theoretical JOOdel accuracy furthe11lOre and to develop the new sensors. And this expert system in future would be IOOre important because new special products will be on process line.

Though the motor current pattern is sui table for the index, it is difficult to translate these obscur~ conception into numerical character, so various methods were sought to make rules as trial and error. Fig.ll shows the way to classify main IJK)tor current distribution over F2, F3, F4 stands.

4. RffiULT OF APPLICATION 6. REFEREN(}s Table.1 shows the score for the expert system by operators COOJpared with their actual adjustment in rolling. Some inferences differ from operator's know-how; the percentage of "difficuI t to judge", but operator cannot judge whether it is applicable or not because the on-line test hasn' t been tried. As it can be improved further IOOre, the prototype

Beadle, R. G. (1004). Iron Steel Eng., 4, P.71 Kobayashi,S. (1986). Knowledge Engineering, Shokodo Tsunozaki, Y., Takekoshi, A. and Hashiroto, K. (1987). NKK Technical Report, 119, PI

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