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Effectiveness of alternate operations in a flexiblemanufacturing systemW. E. WILHELM a & HYUN-MYUNG SHIN aa Department of Industrial and Systems Engineering , 1971 Neil Avenue, Columbus, Ohio,43210-1271, U.S.A.Published online: 24 Oct 2007.

To cite this article: W. E. WILHELM & HYUN-MYUNG SHIN (1985) Effectiveness of alternate operations in a flexiblemanufacturing system, International Journal of Production Research, 23:1, 65-79

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I N T . J . PRO!). H N S . , 1985, VOI.. 23, NO. 1 , 6.579

Effectiveness of alternate operations in a flexible manufacturing system

W. E. WILHELMt and HYUN-MYUNG SHIN7

This paper describes a study which investigated the influence that alternate operations might have on the performanca of flexible manufacturing systems (FMS). An alternate operation might be directed, for example, to avoid a long queue and would, perhaps, be performed by amachine of lesser capability so that a time penalty would be incurred. Three schemes for implementing alternate operations within the hierarchical structure of the FMS are devised and teat reaulta are compared with the performance achieved using no alternate oper- ations. Resulta show that alternate operationa can reduce flow (cycle) time (and therefore i n -p row inventory) while increasing machine utilization. The most effective of the thme schemes requires use of multiple levels of the control hierarchy, applying a linear programming model to prescribe production plans and adaptive control to implement the plans over time.

Introduction A flexible manufacturing system by definition, according to Buzacott and

Shantikumar (1980), consists of a set of machine tools, material handling equipment and in-process storage facilities which are all under the control of a computer system. Figure 1 depicts an FMS in which workpieces are affixed to pallets, perhaps by a robot, as they enter the FMS at the load/unload station: The material handling system may consist of an automatic conveyor line, rail, or an automatic guided vehicle system (AGVS) asshown in Fig. 1. Idle work-in-process is held in the common storage facility to promote handling efficiency. Some PMS designs may include a small buffer a t each machine tool to improve machine utilization.

Machine tools may be, for example, lathes, drills or machining centres. Each machine tool may have an automatic tool changer which can hold as many as 60 tools to perform operations on different parts. Since tool changers operate rapidly, setup time is eliminated, making the system flexible to produce economically in lots of any - size required by customers.

Other flexibilities offered by the FMS include

(a) machine centre versatility to perform a variety of operations on aworkpiece; (6) quick changeover to new products by changing software; (c) rerouting of workpieces in-process to avoid machines in repair or those with

relatively long queues.

These flexibilities are particularly important for manufacturing systems that produce a variety of small lots (i.e.; up to 200 workpieces). It has been estimated (Cook 1975) that 75% of all machined parts are produced in small lots; hence i t is expected that the FMS offers the potential for making dramatic improvements in the productivity of manufacturing operations.

Revision received Februam 1984 TDepartment'of Industrial and Systems Engineering, 1971 Neil Avenue, Columbus,

Ohio 43210-1271. U.S.A.

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W . h'. Wilhelm and I1:M. Shin

Figure I . Model of a flexible manufacturing system

The research described in this paper was directed toward an additional flexibility, the use of alternate operations, that could be implemented by the computer control system. An alternate operation could be used if one machine tool is temporarily overloaded while another is idle. The primary machine (in this example, the overloaded one) may offer certain features which allow i t to be more productive, 86 alternate operations are typically performed with some time penalty. Even if all machine tools are identical machining centres, one machine may be preferred to another for a particular operation due to the tooling assigned to each centre. For example, a two-inch milling cutter might be used instead of a four-inch cutter, but a longer processing time would be required.

Even though alternate operations incur time penalties, they may be used to ornoad bottleneck machines with the objective of balancing machine utilization and expediting the flow of workpieces. These objectives appear to be particularly important, since the substantial investment required,to install an FMS can be recovered most handsomely by the most efficient operation of the system.

Problem statement The primary purpose of this study was to investigate the influence that alternate

operations might have on the performance of an FMS. A problem intimately related to the use of alternate operations concerns determining the most effective method of implementation by the computer control system.

An FMS control system is arranged in a hierarchical structure in which a set of computers interact to direct and/or to carry out various instructions. At the lowest level in this hierarchy, computers direct the motions of robots, machine taols and material handling equipment. Progressively higher levels in the hierarchy deal with functions over longer time periods and integrate the sequence of operations with longer range production plans that are, in turn, prescribed by even higher levels in the network.

Overview of the paper Relevant literature is reviewed in the remaining subsection of this introduction.

The main body of this papei consists of three sections. The three schemes devised to implement alternate operations are described in the first section. An hypothetical example used to test the schemes is presented in the second subsection: Finally, conclusions are given.

Literature review Dupont-Coldmund (1982) and Ingersoll Engineers (1982) describe actual flexible

manufacLuring systems, and reviews of relevant research have been given by Huzacott and Yao (1982) and Wilhelm and Sarin (1983). Buzacott and

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Alternate ope~ations in ajezible manufacturing system 67

Shanthikumar (1980) studied the importance of balancing workloads in the FMS and stated that a linear programming model could be used to achieve balance by pre- production planning. They also indicated that when the total number of jobs in the system is restricted, for example by limited storage space and/or tooling (pallets), strict balance may not be optimum.

Strategies for controlling the FMS using the hierarchical network have been studied by Buzacott (1980). Wilhelm and Sarin (1983) describe aframework in which decision making in this environment occurs in four levels: strategic planning, facilities design, intermediate range planning and dynamic (time dependent) operations. The current study is connected mainly with the last two levels.

Studies of intermediate range planning include those by Stecke (1981), who addressed machine loading issues, and by Kimemia and Genhwin (1978,1980), who formulated nonlinear network flow models to determine optimal part routings using alternative operations. Tooling is also assigned to machines as part of the intermediate range planning process. Stecke (1978) studied heuristic procedures for assigning tooling and later (1981) investigated the efficacy of pooling the assignment of identical tooling to several machines so that they can be used interchangeably. Generally, pooling improves system performance, but can only be implemented in cases in which the required tools can be accommodated by tool changers. In other cases, alternate operations might be used to improve performance.

Detailed simulation models (e.g., Hutchinson 1977) have been developed to study dynamic operations; but apparently none have been used to study the usb of alternate operations. Simulation models have been used, for example, by Stecke (1978) and Fox (1983) to study scheduling and dispatching policies that are implemented by lower levels of the computer control hierarchy. Analytical approaches for scheduling have been' developed by Hitz (1979) and Hildebraut (1980). Optimal control theory has been applied by Hildebrant and Suri (1980) to prescribe part routing, for example, to compensate for short range problems such as machine downtime.

The current research is also related, in general, to prior studies of the conventional job shop. Irastorza and Deane (1974) devised algorithms for loading and releasing work into a job shop with the objective of balancing workloads. Their study included sequencing but they did not consider the use of alternate operations. Gere (1966) studied a variety of heuristics and found that the use of alternate operations could improve the performance of the traditional job shop.

Implementation schemes Once tooling is assigned to machining centres, alternate operations could be

directed by lower levels in the control hierarchy based on the state of the FMS system a t an instant in time. Such a control scheme may improve short-term performance but could fail to achieve longer term production plans efficiently.

Use of alternate operations could also be planned a t higher levels in the hierarchy, for example, by prescribing the number of specific operations that should be performed as alternate operations. This control scheme yields operating goals but does not specify releasing and dispatching instructions that would assure achieve- ment of the goals by lower levels in the control hierarchy.

Three schemes for executing alternate operations are therefore compared in this paper with performance that results from using no alternate operations. Each scheme is designed for rapid computation to facilitate implementation. None entails

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68 W. E. Wilhelm and H.-M. Shin

prescription of a complete sequence of operations, since such sequencing problems are known to be N P complete, and hence may require prohibitive computational time.

Control schemes are described in detail in the following subsections. Assumptions invoked to structure the FMS modelled are:

(a) no machine breakdowns occur; (b) the sequence of operations (and hence operation sets) for each part type is

known; (c) processing time for each operation a t primary and eligible alternate

machines are known deterministically; (d) guided vehicle travel time from station to station is known; (e) the system is not constrained by the number of guided vehicles, pallets, or

storage spaces; ( f ) raw materials (e.g., castings, forgings, plate stock) required to produce parts

required during the planning period (e.g., a week or a month) are available a t time zero.

No alternate operalions (NA) If no alternate operations are permitted, each part must be processed according

ta a fixed sequence of operations and the machining centre that performs an operation (or operation' sets) is determined by the assignment of tooling to the centres. Marked imbalances may occur among the relative workloads aasigned to each machining centre due to the mix of parts required during a planning period.

Workpieces are released to begin production in the FMS, when a machining centre becomes idle, according to the following priority:

(1) pert with the most imminent due time (2) part with largest ratio, R, defined as

number of parts of this type not released R =

total requirement for this part type a t time zero (1 )

(3) any part that can be loaded on the idle machine.

Due times are assigned on a random basis. Upon completing an operation, a workpiece is transported to the machining

centre required next by the sequence of operations if a 'look ahead' procedure confirms that the centre will be idle by the time the workpiece would amve. Otherwise, the workpiece is transported to the central storage facility, which dispatches workpieces of rtgiven part type according to a first-come-first-sewed rule. Upon completion, a workpiece is transported to the unloading station to make its egress from the FMS.

Allemate operations#irected dynamically (AD) The most straightforward acheme for implementing alternate operations can be

effected by lower levels of the control hierarchy that correspond to dispatching in traditional job shops. This approach includes no attempt to look ahead to balance workloads on machines or to meet part requirements.

An alternate operation is directed if the primary machine is busy and the alternate is idle. In addition, when a machine becomes idle and finds no primary

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Alternate operations in a jexible manufacturing system 69

operations waiting in the central storage facility, an alternate operation is directed if an appropriate workpiece is in the central storage facility. Failing these possibilities, a new workpiece may be released, perhaps to begin an alternate operation a t the idle machine.

Alternates are thus used onlv if orimaries are either busv or not available. " " Priorities among candidate alternatives are determined as described in the previous subsection. If more than two candidate alternatives exist, the same approach could be used; the number of combinations (i.e., operation sequences J,asdefined below) or ways of using alternates would increase correspondingly.

Alternate operatium planned (AP) A given set of parts required during a planning period establish workloads for

each of the machines in the FMS. Since these workloads are likely to be imbalanced, alternate operations may be planned to offload bottlenecks with the objective of improving machine utilization as well as workpiece flow (cycle) time.

Alternate operations may thus be planned a t a higher level of the control hierarchy using a model which holds the objective of balancing machine workloads:

minimize Z

subject to CXijk=N,, i= I , 2, . . . , I j k

Xijk80, integer (4)

with

i par t type( i=1,2 , ..., I) j operation sequence (i = 1,2,. . . , J ; ) k machine (k=1,2, . . . , K)

N , number of parts of type i required tjjk processing time of operation sequence j for part type i on machine k

This model requires enumeration of all possible combinations of the use of primary and alternate machines in distinct operation sequences, of which J, will be identified with part type i. Given part requirements Ni and processing times of each operation sequence, ti,x, the model will prescribe decision variables

Xilk=number of parts of type i to be produced in operation sequence j on the machine k

so that workload is balanced among all machines. Equation (2) assures that all required parts will be produced, and (3) relates the workloads assigned to individual machines. ~ ~

The planning modelwas implemented in this study by relaxing (4) to Xijk>O so that (rounded) linear pro~rarnming solutions are used to conserve run time. . - - Solutions prescribe the number of alternate operations for the entire planning period but do not instruct lower levels in the control hierarchy to direct alternates for specific parts.

The AP control scheme is implemented for specific parts by releasing planned operation sequences that are specified by Release ratio R, defined by (I), is

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70 W. E. Wilhelm and H . - M . Shin

therefore modified by substituting 'operation sequences' for 'part types'. Basically, this merely expands the number of part types considered to include all operation sequences.

Altemte operations planned and direcied dynamically (APD) The fourth control scheme extends the nrevious one to aliow certain disnatchine -

decisions to be made dynamically. Goals for production of operation sequences, Xijk . are still observed but the control system is permitted to adapt to the state of the FMS system as it evolves over time.

This adaptive scheme allows lower levels in the control hierarchy to prevent machine idleness by fedefining the type of operation sequence, if possible. Basically, this is a method of second guessing as to which operation sequence should be released. If an operation sequence is redefined in-process, the number of sequences of each type that have been released is updated. Still, Xijk is inviolate and imposes a constraint on the number of redefinitions that might be made.

As an example consider (see Table 2) operation sequence 1 for part type A. Upon completing the second operation a t machining centre 3, the workpiece would be transported to machining centre 2 if that machine is idle. However, if machining center 2 is not idle, the third operation could be performed on machining centre 1. If this machine is idle, the operation sequence could be redefined (from 1 to 3) to execute an alternate operation. In addition to observing the constraints imposed by Xijk values, redefinitions must also he limited to operation sequences that are identical up to the operation to be executed. Operation sequence 3 is therefore a candidate for exchange with 1 a t the third operation while 2 is not, since operation sequences 1 and 2 do not require the same machining centre for the first operation.

Examole r--

An hypothetical example was devised to compare performance that would result from the application of the four control schemes. Data elements are described in the following subsection. Results of the tests are then presented

Data The FMS modelled was assumed to consist of four CNC machining centres, a

load/unload station, and a set of automatic guided vehicles. It was further assumed that the vehicles did not interact due to congestion on their iaths of travel.

Three different types of parts were defined. Each required four operations and had to visit each of the machining centres, but in different sequences. It was assumed that all processing times were known deterministically, since operations are computer controlled.

Each part type was permitted to use one pair of machining centres (MC) as alternates. Operations for part type A planned for MC 1 were permitted to run alternately on MC2 and vice uersa. Similarly MCs 2 and 3 were alternatives for part type 8; and MCs 3 and 4, for part type C. This set of alternates allows workloads to be

'transferred among all machining centres to achieve the balance sought by the AP and APD control schemes using the linear programming model. If similar relation- ships for transferring work among machines do not exist in other examples, tooling assignments must be used to avoid potential bottlenecks.

Table 1 gives the sequence of operations for each part type along with processing times. Eligible alternate operations are noted in parentheses. Time penalties

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Allernnte q e r n t i o n ~ in nflezible mnnufacluring system 7 1

Part Type A Part Type B Part Type C Sequence Position MC . Time MC Time MC Time

MC: Machine Centre. ( )Alternative Machine Centre.

Table I. Machining timis.

Sequence positiont Part Operation

TY Ps sequences I 2 3 4

t MC numberware tabled.

Table 2. Operation sequences for alternative operations

assumed for each operation are also noted. According to the definition of eligible alternates, each part type may be produced in four operation sequences, which are itemized in Table 2.

Due time (date) for each part was selected randomly. Vehicle travel times between machining centres, the load/unload station and the common storage facility were defined to represent relative locations and travel/handling times.

A SIMSCRIPT model was developed for use in this test which compared the 4 control schemes a t 6 levels of output requiring, respectively, 26,52,104,195,260 and 325 parts to be produced. The number of parts of each type for each case is given in Table 3 (column 6). Results of the linear programming model which prescribed the number of each operation sequence for use in control schemes AP and APD are also

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7 2 W . E . Wilhelm and f 1 . - M . Shin

No. of parts routed according to operation sequence

Type I 2 3 4 Sum ~ o t d

Table 3. Number of part8 used for simuiation

given in this table. Since all da ta elements were assumed t o be known deterministi- cally, each of the 24 (6 output levels and 4 control schemes) tests consisted of a single replication.

It was assumed tha t the output level represented the demand during a planning period, so each test completed a run-out to determine whether the output could be produced in the alloted time or not. Thus, makespan is an important measure of performance. In acbual systems, requirements for subsequent time periods could begin production immediately after machines hove completed current requirements.

Resulls Performance measures for the four control schemes are compared in six figures in

this subsection:

Measure Figure Makespan 2 System ut.ilimtinn 3 Ut.ilization of individual MCs 4 Flowtime 5 Maximum spaces needed in common storage 6 Maximum number of vehicles needed . 7

General comments and observations concerning the control schemes are also given.

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Allernnle operalions in a flexible manufacluring system 73

..-.,,A

AP

- - A D - APD

lbo 2% do 4460

Total Number of Ports

Figure 2. Total system operation time

NA

APD

0 I& 200 m 400 Total Number ol Ports

Figure 3. System utilization.

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W . E. Wilhelm and !I.-M

100.1

Shin

N4

4P - 4D - 4PD

01 0 1 0 0 200 300 400

Totol Nurnbsr of Parts

Figure 4 (a). Utdization of machine centre 1 .

:: so. ..-- . -. - . - . - N4

I 4P

AD .E 60. .' . , . ,

& - 4PD

rJ ' '

o i a3 & 3ia 4~

Total Numbar of Parts

Figure 4 ( b ) . Utilization of machine centre 2.

o ! 0 lbo Et.2 Y h 4 b

Total Number 01 Ports

Figure 4 (c). Utilization of machine centre 3.

0 IW 2W 360 460 - Totol Numbsr of Ports

Figure 4 ( d ) . Utilization of machine centre 4.

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0.1 0 loo 2W 3ta 460

Tot01 Number of Parts

Figure 5 (a). Production cycle time for part type A,

Figure 5 (b) . Production cycle time for part type B.

i ,' , 2' /, ./.,/

/ , /' ,,' . .... NA

/' ,,. ./' : AP

./ ,,/' -- AD ./ ,, 1 ,; - APD

/. .: ,' :'

0 1 0 100 200 m 4M

Tot01 Number of Ports

Figure 5 (c). Production cycle time for part type C.

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W. E . Wilhelm and 1 l : M . Shin

90.1

80. - fj m. 0

r? 60. m g so- b z 40 0 P lo. .- > Z X . LL

0.1 0 I& 200 %% 400

Total Numbsr of Parlr

Figure 6. Maximum number of storage spaces

Figure 7. Maximum number of carts

Makespan As the number of parts produced increases, marked differences result in makespan values of the four control schemes, which can be ranked from most to least preferable, as APD, AD, AP, NA. As expected, the APD scheme, which achieves workload balancing goals using adaptive control, yields minimum makespan values. Directing use of alternates dynamically using scheme AD yields modest improve- ment compared to scheme A P but cannot achieve the efficiency of the adaptive APD approach. AD does, however, give slightly lower makespan values (than APD) in cases with few parts.

Uliliullion The same relative comparisons hold true with respect to the system utilization measure which is defined as

100 ' System utilization %= - 1 ( T k / M )

K t - 1

in which T, cumulated machining time of machine k during makespan, and dl makespan

Appropriate use of alternate operations increases workloads of alternate machines and tends to balance completion times of all machines, reducing makespan. Relative performance of the centrol schemes differ a t individual machine centres (see Fig. 4) - due to subleties such as the priority and relative volume of each part type. For example, toward the end of production, certain parts may remain in the FMS in

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disproportionate numbers, imbalancing utilization among certain machines. I t should be noted, however, that use of alternate operations creates 'artificial' workloads (due to the time penalty associated with alternates) a t certain machines but tends to improve overall system performance approximately five to ten per cent in the cases tested.

Flow time Average flow (cycle) time required to produce each part type (see Fig. 5) indicates that AP and APD actually increase flow times for low production volumes due to the time penalty associated with alternative operations.

However, a t high volumes, the scheme which does not use alternatives is dominated by all others, since time penalties are offset by the improved balance of machine utilization. I n all cases schemes AP and NA give comparable flow times while AD and APD givesimilar and significantly better performance (approximately 50% better in these tests). APD is best for part types A and B, but AD is best for part type C due to that part's relative release priority. I n fact, the AD scheme retains a large portion of part C to run a t the end of the makespan with no other parta in the FMS. This selectivity essentially converts the FMS to a flowshop toward the end of the run.

Sbrnge space The maximum number of workpieces held in the central storage facility is shown in Fig. 6. Performance tendencies with respect to this measure parallel those described in relationship to the average flow time.

Storage requirements increase with production volume, since available raw material is released to prevent a machine from becoming idle. However, the dynamic AD and adaptive APD schemes amelioriate this increase by performing alternate operations on work-in-process instead of releasing new workpieces.

In actual systems, storage space is fixed and acts aa a constraint on system operations. Results of this study suggest that NA and AP would require approxi- mately twice as much storage space as AD and ADP.

Vehicles The maximum number of vehicles needed to support machining operations varies little over all 24 test cases. This result corresponds to an observation made by Solberg (1977) in which he noted that increasing the number or speed of vehicles has little effect on steady state system performance.

The time t o transport workpieces is expected to be much less than the time required to perform a machining operation. For this reason, the number of vehicles required appears to be dependent primarily upon the number of machines in the FMS, rather than the number of workpieces.

Conclusions Three approaches for implementing the use of alternate operations in flexible

manufacturing systems have been tested in this study. These schemes represent application in various degrees of the capabilities offered by the hierarchical computer control structure by which the FMS is operated.

Limited test results reinforce the expectation that full use of the hierarchical control would give the best system performance. The control scheme which led to the most preferable performance of the system depended upon upper levels in the hierarchy to prescribe long range plans that were executed dynamically (over time) by adaptive controls exercised by lower levela.

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78 W . &. Wilhelm and I!.-M. Shin

Effectively integrated, multi-level control of th is type is n o t typically associated with the traditional job shop. Hence this s tudy demonstrates a productivity advantage which t h e computer control network affords t o t h e F M S in comparison to the conventional iob s h o ~ .

Conclusions are, of course, based on t h e specific example tested. More realistic examples including actual features such as machine breakdown, machine buffers, eligibility of several al ternate machines t o perform a n operation, a n d production of parts with more opportunities for using alternative operations need to be inves- tigated t o more completely understand the impact of al ternates on system performance.

Use of alternate operations is interrelated t o t h e rationale used to assign tooling t o machines. If pooling is used (i.e., identical tools a re assigned t o each machine centre) all machining centres can be used interchangeably with n o t ime penalty. As the number of par t types and/or complexity increases, i t appears t h a t pooling could no t be accomplished effectively, since more tools would be needed than tool changers can accommodate. I n these more complex caaes, t h e control system m a y be relied upon to improve productivity b y implementing alternative operations.

Cet article decrit une Ctude durant laquelle on a examid I'influence que pourraient avoir des o p h t i o n s alternatives sur lea performanoes de syatimea de fabrications flexibles. Une opiration alternative pourrait i t re requise par exemplepur Cviter une longue queueet serait peut+treetTectu& par une machine de moindre capaci6, ce qui entrainerait unepdnali8ation de temps. TroisachCmea Dour la r5aliaation d'o&rations alternatives dans la structure hihrchique de syatimesde fabncat~onsflex~blwsont exam~n&. pula lea &ultatasontcompar6a avec lee performances qul r h l t e r a ~ e n t des casuu on auralr ur#l~&deaop&at~ons non alternatives. Les &sultata montrent aue lea orhrations alternatives ~ e u v e n t r5duire la du& du dCbit (cycle) (et done lea stocks en coura de fabrication), tout en augmentant I'utilisation des machines. Le sch6ma le plus efficace des tmia sch6masexsminb requiertl'utiliaationde niveaux multiplesdans la h ihrchiedu contrdle en appliquant un modgle lineaire de proaramrnation pour prescrire lea .. . - - plans dens le futur.

Diese Arbeit beschreibt eine Untersuchung des EinfluBea, den alternierende Vorgiinge auf die Leistung 'Flexibler Herstellungssysteme (FMS)' hahen konnen. Ein allternierendeDurchfihrunn konnte zum Beisoiel voreeschrieben werden. um - - eine lsnge Wartezeit zu vermeiden, und wiirde vielleicht von einer weniger leistunesfihieen Maschine auseefihrt, wodurch eine Zeitatrafe aufiommt. Drei Plane Fur d& Emaatz altern;emndcr Vorgange lnnerhalh dcr hlerarchlnchen Struktur ded FMS werdcn aufaefuhrt, und d ~ e Testereebn~ase werden mlt der a~ch durch die Anwendung nichi alternierender vor&nge ergebender Leistung verglichen. Ergebnisse zeigen, daB alternierende Vorgange die FluB(zyklus)zeit (und daher das Inventar der aich in der Durchfuhmng befindenden Artikel) verrringern kann, wahrend die Maschinenauanutzung gesteigert wid . Der effezienteste der drei Plane setzt die Benutzung mehrerer Bereiche der Uberwa- chungshierarchievoraus bei Anwendungeineslinearen Programmiemngsmodells fur die Festlegung von Pmduktionspliinen aowie der adaptiven Uberwachung fur Einfihmng der Plane iiber eine gewisse Zeit.

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