Convertibility Measures for Manufacturing Systems

Convert1bl llty Measures for Manufacturl ng Systems V. Maler-Speredelonl, Y. Koren (1). S.J. Hu NSF Englneerlng Research Center for Reconflgurabla Manufacturlng Systems, Unherslty of Mlchlgan, Ann Arbor, Mlchlgan, USA

Abstract W t h Increased consumer demands for a Ader varlety of products In changeable, unpredlcted quantltles, manufadurlng system responsheness has become Increaslngly Important for Industry competltkeness. Manufacturers need systems that can be rapldly adjusted wlth regard to both fundlonallty and throughput capaclty over the IKetlme of the system. Convertlblllty Is defined as the capablllty of a system to adjust produdlon fundlonallty, or change from one product to another. Endusers of manufadurlng systems are struggllng M h the Iswe of hrm to measure and quantlfy convertlblllty. Mdrlcs for convertlblllty are proposed In thls paper so that dlllerent manufadurlng systems can be compared wlth respect to thls area of performance. These metrlcs are based on assessmerts of the conflguratlon ltseK. and the system components such as machlnes and materlal handllng devlces. Metrlcs for quantlfylng convertlblllty are useful for comparlng system conflguratlons durlng the early phases of deslgn, wlthout requlrlng detalled produd or process plan Informatlon.

Keywords: Manufacturlng system, Performance analysis, Convertlblltty

1 INTRODUCTION Consumers of manufactured goods today deslre a greater varlety of products In unpredlctable quantltles. Tradltbnal dedlcated manufadurlng llnes (DML) were not deslgned to handle these types of demands. Although nexlble manufadurlng systems (FMS) can handle product varlety [I], they can be justltled only for very small volumes. Researchers are lnvestlgatlng more costerecthe technologleal solutlons that All allow manufacturers to be more responshe to the market, such as reconflgurable manufadurlng systems [2]. These systems are approprlate for both medlum and hlgh volume Industrles, particularly when frequent product changes are expected. In large manufadurlng systems the productlon Is done In stages, where product Is partlally processed In one stage and then transfamed to the next. These manufadurlng systems may hwve dlllerent coflguratlons, defined by the way that the machlnes are arranged In the stages and the way that they are connected. It has been shown that the conflguratlon of a system can hwve slgnltlcant erects on performance [3], [4]. Performance can be assessed In many areas lncludlng quallty, productlvtty, and responsheness. Responshgness Includes both convertlblllty and capaclty scalablllty. Convertlblllty Is defined as the capablllty of a system to rapldly adjust productlon fundlonallty, or change from one produd to another. Better responsheness usually makes a system more expenshe. The manufadurlng Industry has been struggllng wlth Issues related to quartlfylng system responsheness. A key research questlon asks what features enable certdn @ems to be rapldly adjustable and how can those features be measured so that deslgners can compare multlple system alternathes and justify the hlgher cost of more convertlble systems. Responsheness Issues and varlous performance metrlcs lncludlng nexlblllty and reconflgurablllty were Introduced In [5], [6], [O], and [9], but more well-defined metrlcs

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for responsheness are needed, partlcuhrly In the area of convertlblllty. Durlng the early phases of manufacturlng system deslgn, convertlblllty metrlcs can be defined uslng the lntrlnslc characterlstlcs of the components and conflgurdlon that make one system Inherently more convertlble than another. Thls approach Is useful when detalled Informatlon about produds and process plans Is not yet wallable. In addtlon, there are sltuatlons M e r e convertlblllty assessments of manufacturlng systems are deslred MhoUf conslderatlon of the produds that they All manufacture, such as when capital Investment dBClSlOM regardlng equlpment purchases must be justltled. Accordlngly, In thls paper such meMcs for convertlblsty are proposed. 2 SYSTEM CONYERTIBILTTY -em convertlblllty Includes contrlbutlons due to machlnes, thelr arrangements or conflguratlon, and materlal handllng devlces. These factors we comblned In Equatlon (1) for an overall lntrlnslc assessment of system convertlblllty.

c, = w l c , + w z c , + w sc, M e r e Cc. CM, and Cy are convertlblllty metrlcs assoclated wlth the mnflguratlon, machlne, and materlal handllng, respecthely, M l c h are further defined In subsequent sectlons such that each metrlc has a scale of 1-10. The welghts, wl,w2, and & can be adjusted. Manufadurm hwve the optlon of selectlng dlllerent types of systems such as dedlcated, nexlble, or reconflgurable, as well as the level of convertlblllty. The conflguratlon, machlne, and m a t e h l Rndllng components that comprlse these systems provlde varylng levels of convertlblllty to the system whlch greatly Heds adaptabllltyfor M u r e alternate uses of the same system, such as when produd mbt or produd deslgns change over tlm e.

R,=Zn+fCi I-1

If connedlons between machlnes unldlredlonal flow P I , but for bldlredlonal any mnflguratlon of 6 machlnes wlth only connedlons, such as those In Flgure 1 , R-

allow only flw P2.For unldlrectlonal equals 27.

3.3 Replicated Machlnes

Flgure 1: Sample Conflguratlons of Six Machlnes. 3 CONFIGURATION CONMRTIBILITY Conflguridlon refers to the arrangement and connedlons of machlnes In a manufadurlng system, examples of W l c h are shDrm In Flgure 1. Pure serlal conflguratlons such as the six-stage llne In (a) h m only me part f l q a t h through the system. Pure parallel conflguratlons such as (h) have as many flowpaths through the system as there are machlnes. In other mrrds, each machlne can process the workplece from start to finish. klybrld conflguatlons are comblnatlons of serlal and parallel Instances. Of partlcular Interest Is a comparlson between @) and (c) or a eomparlson of [d) and (e). These palrs of conflgurdlons have the same arrangement of machlnes, but dlllerent materlal handllng connedlons. Asymmetric corflgumtlons have flowpaths W h dlllerenl numbers of machlnes. and thus, dlllerent process plans. These conflgumtlons are not consldered here due to less frequent Industry use. Conflguratlon convertlblllty, Cc,Is depended upon the mlnlmum Increment of converslon, the routlng connedlons, and the number of repllcated machlnes. 3.1 Increment of C o n m l o n The mlnlmum Increment of converslon (r) was brlelly Introduced In M e r e it was one of many h d o r s used to seled preferred manufadurlng system conflguratlons. It Is an Important lndlcatlon of how qulckly new or dlllerent produds can be Introduced. For example, conflguatlon (a) In Flgure 1 has a mlnlmum Increment of converslon of 1.00,or loo%, that Is, In order to Introduce a new produd, the entlre llne mud be shut down, changed over, and restarted, Conflguratbn (b), however, can be partlaliy converted to a new product M e r only 50% of the machlnes have been shut dDrm and reconflgured. Thls Is valuable M e n a company wants to Introduce a new produd to the market as qulckiy as posslble. and then later ramp up to full produdlon.

Certaln conflguratlons allow for easler schedullng of the produdlon of more t h m one produd at a t h e . Thls Is Important M e n a company expects part m k demands to vary over t h e . The mlnlmum number of repllcated machlnes at a partlcular stage In the process plan ( x ) dldates the number of part types that can be produced wlthwt requlrlng changeovers. Thls number Is slmllar to conflguratlon M t h . as deRned In [lo]. For example. a serlal manufadurlng Ilne, or trmsfer Ilne, such as conflguratlon (a) h Flgure 1 typlcally has only one flowpath by M l c h parts progress through the system. At each stage of the process plan, there Is only one machlne present. Thus, if more than one part Is to be produced, the llne must be stopped, purged of old products, converted, and ramped 4 agaln, all of M c h consumes valuable produdlon tlme. By contrast, a nexlble manufadurlng system Wh six CNC machhes In parallel, each capable of completely manufadurlng a part, could produce up to six dlllerent part types, as conflguratlon (b). Conflguratlons (t) and (4) howaver, could only slmuitaneousiy produce h p o d u d s emclently slnce the Rrst processlng stage has just two repllcated machlnes, not four. If more t h n two produds are produced on the system slmukaneousiy, the system produdMty Is substantlally reduced. Belng able to produce more than one part type at a tlme Is also valuable when manufadurers want to produce prototypes of M u r e produds whlle current produds are stlll belng manufactured at a reduced rate. 3.4 Measure of Conflguratlon Convertlblllty The three h d o r s dlscussed above, mlnlmum Increment of converslon, mutlng connedlons, and number of repllcated machlnes are used In Equatlon (3) to provlde a prellmlnary assessment of mnflguratlon convertlblllty.

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Routlng Conneetlons In a mmufadurlng system, a greater number of routlng connectlons lndlcates a hlgher degree of convertlblllty. The number of routlng connedlons In each conflguratlon (R) Is counted by lncludlng connedlons between machlms m well as connedlons to an Input and output statlon. For example, conflguratlon (a) In Flgure 1 has seven routlng connedlons whereas conflguratlon (b) has elght. Conflguratlon (c) has tweke routlng connedlons due to the f a d that crossover Is allowed badween processlng stages. F# conflguratlons of n machlnes, the mwlmum number of mnnedlons Is gken In Equatlon (2). 3.2

where I . R , and X were below Is used to normalhe wlth the same number of scale so that a11 systems range of one to ten.

deRned above. Equatlon (4) Cc'relative to a serlal system machlnes, and to ad]ust the belng compared fall W h l n a

(4)

If K Is the mwlmum number of machlnes In any system that Is belng consldered, then the pure prallel conflguratlon of K machlnes Is defined to have a Ccvalue of 10. All serlal conflgurdlons have a Cc value of 1. Thls Iogartthmlc transformatlon converts Cc to a 1-10 scale. Conflguratlon mnvertlblllty can then be comblned Wh machlne and materlal handllng convertlblllty metrlcs to Rnd system convertlblllty, as In Equatlon (1).

Y -fu

Table 1: Conflguratlon Convertlbllltyfor Sample Conflguratlons Shprm In Flgure 1.

Y

N

Y

N

Y

Ql

I\ Y N

N

(-1

Y N

4

/I

JI

10 (CNC wlth 5 (standard Flex. Fixtures) CNC)

1 (dedkat staUon)

Flgure 3: Machlne Convertlblllty, C'M the nature of the materlal handllng devlces that are used. Thls metrlc, CH, In Equatlon (6) Is developed In a manner analogous to the machhe convertlblllty. H

Flgure 2: Conflguratlon Convertlblllty for Symmetric Conflguratlons of up to Six Machlnes. The conflgurdlon cmertlblltty metrlc was used to compare the sample conflguratlons from Flgure 1. and the results are given In Table 1. Conflguratlon convertlblllty was also assessed for symmetric conflguratlons of up to six machlnes, shprm h F l g v e 2. 4 MACHINE CONMRTIBILTTY system convertlblllty Is dependent not only on the conflguratlon thmt Is selected, but also on machlne convertlblllty, CM,whlch Is b u n d uslng Equatlon (5). N

The machlne convertlblllty for each of the N IndMdual machlnes In the system, CM,Is based on the premlse that some machlnes have features and characterlstlcs that make them Inherently more convertlble. These features Include whether the machlne Is: Q I . equlpped wlth an automatlc tml changer or multC head splndle; Q2. easlly reprogrammed, wlth nexlble s o w r e ; Q3. modular, wlth nexlble hardware components; 04. equlpped wlth nexlble M u r l n g capablllty; Q5. equlpped wlth a large capacttytool magmlne. As shown In Flgure 3 , these questlons help determlne a rough estlmate of machlne convertlblllty, CM. 5 MATERIAL HANDLING CONMRTIBILTTY One lmprtant factor In system performance that has not yet been Included In nexlblllty or convertlbilty metrlcs Is

The C'" metrlc for each materlal handllng devlce that connects machlnes Is found by assesslng If ll Is: Q I . f o l l W n g a free route or not; Q2. multldlrectlonal; Q3. reprogrammable; 04. asynchronous motlon; Q5. automatlc. It must be noted that the most nexlble or convertlble solutlons mmy also Incur larger Investment costs. For materlal handllng, h d n g people carry wrkpleces from sttalon to sttalon Is very nexlble. F # example, an Intel plant that produces 50 products slmultaneously has people carry wafer cartrldges between statlons. Thls soluflon, howaver, may be very expensive and Is not always the best utllhatlon of human resources. 6 APPLICATIONS

An Industry case M l c h was studled earller wlth regard to productMty Is now used to compare the convertlblllty of h dlferenl conflguratlons shown In Flgure 4 [3]. Both conflguratbns (a) and @) have 18 CNC mmhlnes Wh relatively small tool magmlnes and manual materlal handllng. Thus, CM, CH, and Cc are found and then Cs Is calculated uslng equal wlghtlng factors, as shown In Table 2, cases (a) and @).

Flgure 4: Industry Case Conflguratlons.

a b

N 18 18

CM 5 6

CH 9 9

x 1 3

I 1

33

R cc 1 9 1 21 4.22

cs 5 6.4

markets or h w e products that requlre frequent deslgn changes. Another method for assesshg convertlblllty could expand on the Intrlnslc measures presented here to Include product Informdon. When detalled Infomatlon Is k n m about the products that are belng manufactured and thelr respective process plans, product-based metrlcs can be used to assess conwrtlblllty requlrements. These metrlcs can Include the tlme requted to make converslons or the cost of a changeover. A further situation whlch has not yet been accounted for Is the case where a system has many nexlble machlnes mixed wlth a few dedlcated statlons that happen to Rt the process plan for that famlly of products.

Flgure 5: Future Industry Conflguratlon for a Reconflgurable Wnufacturlng System.

8 ACKNOWLEDGMENTS The authon gratefully acknowledge the nnanclal support of the Englneerlng Research Center I# Reconllgurable Manufacturlng Systems (NSF Grant EEC95-92125) at the Unkerslty of Mlchlgan and the valuable Input from the center9 Industrlal sponsors. REFERENCES 111 Koren, Y., g

121 Table 3: Convertlblllty Calculatlons for Industry Cases. 131 A second example studled here Is a planned Industry appllcdlon of a reconflgurable manufacturlng system s h m In Flgure 5. Many varlatlons of thls system can be compared, as reported In Table 3 and descrlbed below: Case 1 - elgM CNC machlnes (CM=5),materlal handllng by three gantrles (C'p4) llnked wtth a slngle bell conveyor that allows f o M r d motlon only (C'p2) Case 2 - extra Iwge tool magmlnes Case 3 - AGV (C'fl 0) Instead of f o M r d conveyor Case 4 - a reverse conwyor (c'H=3) Case 5 - Incorporates cases 2 and 4 wtth large capaclty tool magmhes and bldlrectlonal conveyor capablllty. Thus, the system In case 5 shows a 12% Improvement In system convertblllty over the basellne system In case 1 (from 4.81 to 5.39).

7 CONCLUSIONS When companles deslgn and Install new systems, they must be concerned not only wtth the products belng manufactured today, but also those that wlll be made throughout the IKetlme of the system. Thus, the ablllty to respond to M u r e market conditlons Is Important. By measurhg the convertlblllty of the conflguratlon, machlnes, and materlal handllng elements, the convertlbllEy metrlcs denned here provlde a quantitative assessment for characterlstlcs of manufacturlng systems that make certaln deslgn aiternattves Inherently better than others In terms of responsivenes. Intrlnslc metrlcs of convertlblllty are partlcularty useful durlng the early phases of deslgn, M e n detalled product and process plan Infomatlon m y not be known. These assessment technlques can be used to compare candldate systems and conflguratlons. It Is oRen the case that more nexlble and cmvertlble systems requlre a hlgher lnitlal Investment. Intrlnslc convertlblllty metrlcs can be used to justify the purchase of these systems, partlcuhrty for manufacturers who deal In hlghly volatlle

141

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m 181

191

1101

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