browne- clasification of fms

7/27/2019 Browne- Clasification of FMS

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Classification of flexible

manufacturing systemsBy Jim Browne, University College, Galway; Didier Dubois, Centre d'Etudes et

de Recherches de Toulouse: Keith Rathmill, Cranfield Institute of Technology:Suresh P. Sethi, Universityof Toronto; and Kathryn E. Stecke, The Universityof

Michigan.

There has been some uncertainty concerning the

conditions under which a manufacturing system may be

termed 'flexible'. To clarify this confusion eight types of

flexibilities are defined and described.

A FLEXIBLE Manufacturing System(FMS) is an integrated, computer-controlled complex of automatedmaterialhandling devices andnumerically controlled (NC) machinetools that can simultaneously process

medium-sized volumes of a variety ofpart types.

tslThis new productiontechnology has been designed to attaintheefficiency of well-balanced,machine-paced transfer lines, while

utilizing the flexibility that job shops

have to simultaneously machinemultiple part types.

Recently, many new manufacturingfacilities have been labelled FMS. Thishas caused some confusion about whatconstitutes an FMS. Flexibility andautomation are the key conceptualrequirements. However, it is the extent

of automation and thediversity of theparts that are important; some systemsare termed FMS just because they

contain automated material handling.

For example, dedicated, fixed, transferlines or systems containing onlyautomated storage and retrieval are

not FMSs. Other systems only containseveral (unintegrated) NC or CNCmachines. Still other systems use a

computer to control the machines, butoften require long set-ups or have no

automated parts transfer.

Some systems are called flexiblebecause they produce a variety of parts

(of very similar type, using fixedautomation). In most of theseexamples, the operating mode is either

transfer line-like or based on produc-

ing batches of si mi lar part types.

To help clarify the situation, eighttypes of flexibilities will be defined and

described. Examples or explanations

are provided when needed to illustratea particular flexibility type. Measure-ment and attainability of each are alsodiscussed.

qMachine Flexibility: the ease ofmaking the changes required toproducea givenset of part types.

Measurementof these changesinclude, for example, the time toreplace worn-out or broken cutting

tools, the time to change tools in a toolmagazine to produce a different subsetof the given part types, and the time toassemble or mount the new fixturesrequired. The set-up time required for

a machine tool to switch from one parttype to another includes: cutting tool

preparation time; part positioning andreleasing time; and NC programchangeover time. This flexibility can

beattained by:(a) technological progress, such as

sophisticatedtool-loading andpart-loading devices;

(b) Proper operation assignment, sothat there is no need to change thecutting tools that are in the toolmagazines, or they are changed lessoften;

(c) having the technological capabilityof bringing both the part andrequired cutting tools to themachine tool together. -

qProcess Flexibility: the ability to

produce a given set of part types, eachpossibly using different materials, in

several ways. Buzacott [ 1982] calls this

job flexibility', which relates to the

mix of jobs which the system canprocess.' Gerwin [1982] calls this mixflexibility'.

Processflexibilityincreases as machine set-up costsdecrease. Each part can be machinedindividually, and not necessarily inbatches. This flexibility can bemeasured by the number of part typesthat can simultaneously be processed

without using batches. This flexibilitycan be attained by having:

(a) machineflexibility; and(b) multi-purpose, adaptable, CNC

machining centres.ElProduct Flexibility: the ability

changeover to produce a new (set o

product(s) very economically aquickly. Mandelbaum [ 1978] calls taction flexibility, the capacity

taking new action to meet new circustances.' Included in this concep

Gerwin's [ 1982] 'design-change flebility'. This flexibility heighten

company's potential responsivenescompetitive and/or market chang

Product flexibility can be measuredthe time required to switch from opart mix to another, not necessar

of the same part types. This flexibilcan be attained by having:

(a) an efficient and automated prodution planning and control systecontaining:

(i) automatic operation assigment procedures; and

(ii) automatic pallet distributi

calculation capability.(b) machineflexibility.ElRouting Flexibility:the abilityhandle breakdowns and to contin

producing the given set of part typ

This ability exists if either a part ty

can be processed via several routes, equivalently, each operation can

performed on more than one machinNote that this flexibility can be:

Potential:part routes are fixed, b

parts are automatically reroutwhen a breakdown occurs;

Actual:identical parts are actua

processed through different rout

independent of breakdown situ

tions_The main, applicable circumstanc

occurs when a system componesuch as a machine tool, breaks dow

This flexibility can be measured by robustness of the FMS when brea

downs occur the production rate do

not decrease dramatically and pa


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Relationships Among Flexibility Types

Product FlexibilityMachine Flexibility Process Flexibility

Operation Flexibility

continue to be processed. This flexi-

bility can beattained by allowing forautomated and automatic rerouting ofparts (potential routing flexibility), bypooling machines into machinegroups,

1 6 1which also allows machine

tool redundancy; and also by duplicat-ing operation assignments."' These

latter policies provide actual routingflexibility. The FMS would then bestate-driven by a feedback control

Policy.qVolume Flexibility: the ability tooperate an FMS profitably at differentproduction volumes. A higher level ofautomation increases this flexibility,

partly as a result of both lowermachine set-up costs and lowervariable costs such as direct labour

costs. If it is not economical to run a

particular system at its usual volume,say during a decrease in marketdemand or a recession, then there areless personnel problems concerningthe idling of labour. Perhaps alterna-

tive uses of the FMS could befound. Also, production volumes can

vary from week to week, resulting invariable machine and system utilisa-

tions. This flexibility can be measuredby how small the volumes can be for

all part types with the system stillbeing run profitably. The lower the

volume is, the more volume-flexiblethe system must be. This flexibility

can be attained by having:(a) multipurpose machines; and(b) a layout that is not dedicated to a

particular process; and(c)asophisticated,automated

materials handling system, such as(possibly intelligent) carts, and notfixed-route conveyors;and

(d)routingflexibility.qExpansion Flexibility: thecapability of building a system, and

expanding it as needed, easily andmodularly. This is not possible withmost assembly and transfer lines. Thisflexibility can be measured accordingto how large the FMS can become.

This flexibility is attained by having:(a)anon-dedicated, non-process-

driven layout; and(b)aflexible materials handling

system consisting of, say, wire-guided carts; and

(c) modular, flexible machining cellswith pallet changers; and

(d)routingflex ibility.q Operation Flexibility: the ability tointerchange the ordering of severaloperations for each part type. There isusually some required partial pre-cedence structure for a particular

part type. However, for someoperations, their respective ordering isarbitrary. Some process planner has

usually determined afixed ordering ofall operations, each on a particularmachine (type). However, keeping

the routing options open and not pre-

determining either the 'next' opera-

tion or the 'next' machine increasesthe flexibility to make these decisionsin real-time. These decisions shoulddepend on the current system state

(which machine tools are currently

idle, busy, or bottleneck).O Production Flexibility: the universeof part types that the FMS canproduce. This flexibility is measured

by the level of existing technology. It is

attained by increasing the level oftechnology and the versatility of the

machine tools. The capabilities of all

. the previous flexibilities are required.

Not all of these flexibility types areindependent. The Figure displays the

relationships between the different

flexibilities.The arrows signifynecessary for'. An ideal FMS would

possess all of the defined flexibilities.However, the cost of the latest in hard-

ware and the most sophisticated (and

at present non-existent!) software to

plan and control adequately would bequite high on some of these measuresand low on others. For instance,

processing a particular group ofproducts may be made possiblethrough the use of head indexershaving multiple-spindle heads. How-

ever, they hinder both adding new parttypes to the mix and introducing new

part numbers, since retooling costs arehigh and changeover time can be aday. Also, some flexible systems (such

as the SCAMP system in Colchester,

UK) include special-purpose, non-

CNC machines, such as hobbing and

broaching, which also require

(relatively) huge set-up times.This classification of flexibilities

can help categorize different types of

FMS.

Relationship among types of flex ibility.

The level of automation helpdetermine the amount of availaflexibility. Because of the diffechoices of various flexibility lev

there are different types of FMSs. Itherefore,- useful to classify thsystems in terms of their oveflexibility.

Towards a classification of flexmanufacturing systems, Groo[ 1 980) divided FMSs into two disttypes:

(i) Dedicated FMS;

(ii) Random FMS.A dedicated system machines a fi

set of part types with well-defimanufacturing requirements ov

known time horizon. The 'randFMS', on the other hand, machine

greater variety of parts in randsequence.

In addition to these basic, extre

types of FMSs, all FMSs are differin terms of the amounts of the flebilities that they utilize. In this sectia classification of FMSs accordin

their inherent, overall flexibilityprovided. Four general types of Fwill be defined.

The following standards are pvided based on FMS componenwhich will be used to describe aclassify the different types ofFMSs:1. Machine tools:

General-purpose or specialize Automatic tool changing capa

lities (increase flexibility)

Regarding tool magazines, th

capacity, removability, and tochanging needs (affect the fle

bility).2. Materials handling system: Types include: conveyor or on

way carousel; tow-line with ca

network of wire-guided carstand-alone robot carts

Partmovement equipmepalletized and/or fixtu red

Tool transportation systemmanual; or, automatically, w

parts.

ProductionFlexibility

Routing Flexibility Volume FlexibilityExpansion Flexibility

The FMS Maqazine April 1984


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Storage areas for in-process inven- machine tools, and the finished parts

tory:

Central buffer storage

Decentralised buffer at each

machine tool

Local storage.

4. Computer control:

Distribution of decisions

Architecture of the information

system

Types of decisions: input

sequence; priority rules; part to

cart assignment; cart traffic

regulation

Control of part mix: through

periodic input; through a feed-

back-based priority rule_

These `flexibility' standards for the

physical FMS components are used to

clarify differences and similarities

between the FMS types.

Although not typically considered

FMS, this classification scheme will

include the flexible assembly system

(FAS).

The simplest possible component ofan FMS or FAS is a flexible assembly

cell (FAC). It consists of one or more

robots and peripheral equipment,

such as an input/output buffer and

automated material handling. To date,

only about 6% of robot applications

are in assembly.

A flexible assembly system (FAS)

consists of two or more FACs. In the

future, as the technology develops to

allow the interface between manufac-

turing and assembly, an FAS could

also be a component of a flexible

system.The types of FMS described, are

categorized according to the extent of

use of their flexibilities. The classi-

fication of a particular FMS usually

results basically from its mode of

operation as well as the properties of

the four components described above.

Type I FMS: Flexible Machining

Cell

The simplest, hence most flexible

(especially with respect to five of the

flexibilities) type of FMS is a flexible

machining cell (FMC). It consists ofone general-purpose CNC machine

tool,interfaced with automated

material handling which provides raw

castings or semi-finished parts from an

input buffer for machining, loads and

unloads the machine tool, and trans-

ports the finished workpiece to an out-

put buffer for eventual removal to its

next destination. An articulated arm,

robot, or pallet changer is sometimes

used to load and unload. Storage

includes the raw castings area, the

input and output buffers of the

area.

Since an FMC contains only one

metal-cutting machine tool, one might

question its being called a system.

However, it has all of the components

of an FMS. Also, it is actually an

FMS component itself. With one

machine tool, it is the smallest, most

trivial FMS.

Type II FMS: Flexible MachiningSystem

The second type of FMS can have

the following features: It can have real-

time, on-line control of part produc-

tion. It should allow several routes for

parts, with small volume production

of each, and consists of FMCs of

different types of general-purpose,

metal-removing machine tools. Real-

time control capabilities can auto-

matically allowmultiple routes for

parts, which complicate scheduling

software. Because of real-time control,

however, the actual scheduling might

be easier. For example, the scheduling

rule might be to route randomly, or

route to the nearest free machine tool

of the correct machine type. The

scheduling rule could be some appro-

priate, system-dependent, dynamic

priority rule with feedback.

Sometimes, dedicated, special-

purpose machines tools, such as multi-

ple-spindle head changers, are used in

an FMS to increase production. The

machine tools are unordered in aprocess-independent layout. It is the

part types that are to be processed by

an FMS which define the necessary,

required machine tools.

A Type II FMS is highlymachine-

flexible,

process flexible, andproduct-flexible. It is also highlyrouting-flexible, since it can easily andauto-

matically cope with machine tool or

other breakdowns if machines are

grouped or operation assignments are

duplicated.

Within the Type II category, the

various kinds of material handling

provide a sub-range of flexibility. Inorder of increasing flexibility, various

material handling systems include:

power roller conveyors, overhead

conveyors, shuttle conveyors, in-floor

tow line conveyors, and wire-guided

carts. Some examples include:

(i) a network of carts and decentral-

ized storage areas, for shorter pro-

cessing times (Renault Machines

Outils, in Boutheon, France);

(ii) a tow line with carts and central-

ized storage areas, for longer

processing times (Sundstrand/

Caterpillar DNC Line, in Peo

Illinois, USA).

Type III FMS: Flexible TransferLine

The third type of FMS has t

following features. For all part typ

each operation is assigned to, a

performed on, only one machine. T

results ina fixed route for each pthrough the system. The layou

process-driven and hence ordere

The material handling systemusually a carousel or conveyor. T

storage area is local, usually betwe

each machine. In addition to gener

purpose machines, it can conta

special-purpose machines, robots, a

some dedicated equipment. Sched

ing, to balance machine workloads

easier. In fact, a Type III FMS is ea

tomanage because it operat

similarly to a dedicated transfer li

The computer control is more sim

and a periodic input of parts

realistic. Once set up, it is easy to r

and to be efficient. The difference

that it is set up often and relativ

quickly.

A Type III FMS is less Proceflexibleand less capable of aumatically handling breakdowns. Ho

ever, the system can adapt by

tooling and manually inputting

appropriate command to the com

puter, to re-route parts to the capa

machine tool. This takes more tim

than the automatic re-routing ava

able to a Type II FMS.

Type I V FMS: Flexible TransferMulti-LineThe fourth FMS type consists

multiple Type III FMSs that are int

connected. This duplication does

increaseprocess flexibility. Similar t

Type III FMS, scheduling and cont

are relatively easy, once the system

set up. The main advantage is t

redundancy that it provides in

breakdown situation, to increase

routing flexibility.Itattempts achieve the best of both FMS Type

and III.

Flexibility rangeAll things being equal, a Type

FMS is operated flexibly', whil

Type III FMS is operated in a mu

more fixed' manner. These typ

provide the extremes, say, the bounon flexibility. There is, of course

whole range of flexibilities betwe

the two general types. However, the

smaller variations in flexibility

defined by the versatilities a

capabilities of the machine too

which are dictated by the particu


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FMS application, i.e., the part types tobe machined. The types of materialhandling system also provides sub-

groups of flexibility. The overall flexi-bility, however, is defined by an FMS's

mode ofoperation.In general, the FMSs of the United

States and the Federal Republic ofGermany tend to be more like theType II FMS, while those of Japan are

more similar to Type III. The second

floor of Fanuc's Fuji complex, consist-

ing of four flexible transfer lines, is anexample of an operating Type IVFMS. It consists of several identical

FACs, which are not all identicallytooled. Parts do have fixed routes, but

if an assembly cell is down, the parts

requiring it are automatically able to

be routed to another assembly cell,

which contains the correct tooling.

The first floor of this Fanuc plant, theMotor Manufacturing Division, is a

good example of Type II.

All FMSs consist of similar com-ponents. The numbers and typesof machine tool may differ. Whatreally defines the flexibility of aninstallation is how it is run. The levelof desired flexibility is an importantstrategic decision in the developmentand implementation of an FMS. This

paper has provided a framework for

such strategic decisions.

Acknowledgements

Kathryn E. Stecke's research was supported inpart by a summer research grant from theGraduate School of Business Administration atThe University of Michigan as well as by a grant

by the Ford Motor Company, Dearborn,

Michigan.

ReferencesI. J. A. Buzacott,'The Fundamental Principles

of Flexibility in Manufacturing Systems',

Proceedings of the 1st International Con-ference on Flexible Manufacturing Systems,Brighton, UK. (20-22 October 1982).

t. Donald Gerwin, 'Do's and Don'ts of Com-

puterized Manufacturing', Harvard Business

Review, Vol. 60, No. 2, pp. 107-1( March-April 1982).

3. Mikell P. Groover, Automation, ProductiSystems, and Computer-Aided Manufactuing.Prentice-Hall, Englewood Cliffs N(1980).

4. MarvinMandelbaum, 'Flexibility Decision-Making: An Exploration and Unfication.' Ph.D. dissertation, Department

IndustrialEngineering, University Toronto, Ontario, Canada (1978).

5. Kathryn E. Stecke, 'Formulation anSolution of Nonlinear Integer Productio

Planning Problems for Flexible Manufactuing Systems,' Management Science, Vol. 2

No. 3, pp. 273-288 (March 1983).6. Kathryn E. Stecke and James J. Solberg, 'T

Optimality of Unbalanced Workloads an

Machine Group Sizes for Flexible Manufaturing Systems,' Working Paper No. 29

Division of Research, Graduate School

Business Administration, The University

Michigan, Ann Arbor, MI (January 1982).

7. D. M. Zelenovic, 'Flexibility A Conditifor Effective Production Systems,' Inte

national Journal of Production ResearcVol. 20, No. 3, pp. 319-337 (May-Ju

1982).

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browne- clasification of fms

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