Scheduling Flexible Manufacturing Systems
Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus, Prof. drs. P.A. Schenck, in het openbaar te verdedigen ten overstaan van
een commissie aangewezen door het College van Dekanen op 14 november 1989 te 16.00 uur
door
Marcellinus Jozef Zeestrate geboren te Leidschendam,
werktuigkundig ingenieur.
TR d i s s 1 7 6 7
2
Dit proefschrift is goedgekeurd door de promotor prof. ir. L.N. Reijers
CIP-DATA K O N I N K L I J K E B I B L I O T H E E K , DEN HAAG Zeestraten, Marcellinus Jozef
Scheduling flexible manufacturing systems / Marcellinus Jozef Zeestraten. - [S.l. : s.n.]. - 111.
Thesis Delft. - W i t h ref. ISBN 90-9003006-9
SISO 366.3 U D C 658.5(043.3)
Abstract
In many Flexible Manufacturing Systems (FMS) in operation today, all but the most detailed scheduling decisions are made by the system operator. In a number of cases the system operator is supported by a simulation system which confronts him with the consequences of his decisions. Nevertheless, because of the complexity of the FMS scheduling problem (the large number of variables and constraints involved), it is to be expected t h a t his scheduling decisions are far from optimal. T h e goal of the research project presented in this thesis has been the design and implementation of an a u t o m a t i c sched uling system for F M S . With the aid of such a scheduling system, it should be possible to meet due dates more reliably, obtain shorter lead times, and reduce the a m o u n t of money t h a t must be invested in FMS equipment to meet certain production requirements.
During the design of this FMS scheduling system, a lot of emphasis was put on scheduling of random FMS: Flexible Manufacturing Systems designed to process a relatively large variety of parts in a r a n d o m sequence. The op posite, dedicated F M S , machines a fixed set of p a r t types with well-defined manufacturing requirements over a known time horizon [11]. In compari son with dedicated FMS, the scheduling of r a n d o m FMS poses additional scheduling problems. Not just machine tools have to be considered as scarce resources, but pallets, fixtures and some (expensive) cutting tools as well. T h e attention for scheduling constraints resulting from the limited availabil ity of pallets, fixtures and cutting tools is an i m p o r t a n t difference between the scheduling system presented in this thesis and most other FMS sched uling systems described in literature.
T h e main objective of this scheduling system is meeting the due dates of orders obtained from a higher level scheduling system (the shop controller). If it is possible to meet all due dates using only part of the processing ca pacity of the F M S , meeting due dates even becomes a scheduling constraint. Under those circumstances a second objective is activated: creating excess
4 A b s t r a c t
capacity during later planning periods by rilling the available processing ca pacity in the near future as densely as possible. In the first place this excess capacity is a safeguard against possible system failures. This makes it possi ble to meet due dates even after a temporary downtime. In the second place this excess capacity is an indication for the shop controller t h a t additional orders can be accepted. This way the utilization of the processing capacity of the FMS can be improved.
The largest category of scheduling constraints observed by this FMS scheduling system is the result of the limited availability of resources: work stations, pallets, fixtures, cutting tools, storage space in the local tool mag azines of the machine tools, and human operators. Another category of constraints is a consequence of the fact t h a t many FMS are operated in an unmanned night shift. During the night shift part clamping and unclamping operations cannot be performed.
An effective way to deal with complex control tasks such as the FMS scheduling problem is the use of a hierarchy of control levels. In this work the following three level structure was defined: the task selection function, the tool allocation function and the sequencing function. An important ad vantage of this hierarchy of interacting control levels is the ability to respond to unexpected events. Scheduling decisions are made in real time. When a scheduling system produces a schedule prior to actual production, handling unexpected events becomes much more difficult.
T h e task selection function is the top level in the hierarchical decision structure. It uses a mixed integer programming model to select a set of
tasks to be performed during the next planning period (e.g. a shift). The
mixed integer programming model represents all scheduling constraints and objectives mentioned above. Some constraints are described in an aggregate manner, because the output of the task selection function does not describe a schedule in full detail. Advantages of this mixed integer programming approach are close-to-optimal results and the ability t o point out bottleneck resources and critical orders.
The tool allocation function assigns operation types and the correspond ing tool sets to machine tools in the F M S . The operation types t h a t need to be assigned are those selected for a particular planning period by the task selection function. The objective of the tool allocation function is to obtain workload balance as well as routing flexibility. Due to the way in which the tool allocation problem is formulated, it is not a very critical problem. This is why a simple heuristic approach is sufficient.
A b s t r a c t 5
workstation in the F M S , such t h a t the time required to complete all tasks selected for a particular planning period (the makespan) is minimized. The sequencing problem resembles the minimum makespan job shop schedul ing problem, which has been given a lot of attention in literature. Two new approaches to this problem are presented in this thesis: the look ahead
dispatching procedure, and the simplified look ahead dispatching procedure.
Although the original version performs slightly b e t t e r than the simplified version, the latter is the most significant one. The makespan values ob tained with the simplified look ahead dispatching procedure are still con siderably shorter and more consistent than the makespan values obtained with priority dispatching rules. On top of t h a t , in comparison with priority dispatching rules the simplified look ahead dispatching procedure does not have any drawbacks with respect to ease-of-use and required c o m p u t a t i o n time.
A c k n o w l e d g e m e n t s
The research presented in this thesis was carried out at the Laboratory for Manufacturing Systems of the Delft University of Technology. T h e a u t h o r wishes to t h a n k the leader of the laboratory Prof. Ir. L.N. Reijers for his support throughout the course of the project. T h a n k s also to all staff mem bers and students of the laboratory who contributed to the project in some way. Martin Vinke deserves special mentioning for his implementation of an early version of the look ahead dispatching procedure.
The author wishes to thank Prof. Dr. W . H . M . Zijm for his advice and valuable comments on early design documents.
Dr. Ir. T. Storm is thankfully acknowledged for proofreading the manu script, and for his suggestions for improving the thesis.
Samenvatting
P r o d u k t i e p l a n n i n g in Flexibele Fabrikage Systemen
In veel van de huidige Flexibele Fabrikage Systemen (FFS-en) worden alle beslissingen met betrekking tot de produktieplanning genomen door de sys teembeheerder, met uitzondering van de meest gedetailleerde beslissingen. In een aantal gevallen wordt de systeembeheerder gesteund door een simula tiesysteem dat hem confronteert met de gevolgen van de door hem genomen beslissingen. Desondanks is het te verwachten dat zijn beslissingen verre van optimaal zullen zijn, o m d a t het grote a a n t a l variabelen en randvoorwaar den het F F S planningsprobleem dermate complex m a a k t dat een mens het niet volledig kan bevatten. Het doel van het onderzoeksproject dat in deze dissertatie wordt gepresenteerd is het ontwerp en de implementatie van een automatisch F F S planningssysteem. Met behulp van een dergelijk systeem, zou het mogelijk moeten zijn om met grotere zekerheid de uiterste gereed-heidsdatum van de orders te halen, om kortere doorlooptijden te realiseren, en om het bedrag dat geïnvesteerd moet worden in een F F S ten einde in een bepaalde produktiebehoefte te voorzien, te verlagen.
Tijdens het ontwerp van dit FFS planningssysteem, is veel aandacht besteed aan de produktieplanning in zgn. random1 FFS-en: Flexibele Fa
brikage Systemen die ontworpen zijn om een relatief grote variëteit aan on derdelen in een willekeurige volgorde te verwerken. Hier tegenover staan zgn.
dedicated FFS-en. Deze verwerken een vaste verzameling onderdeel-typen,
met een van te voren goed gedefinieerd produktievolume [11]. In vergelij king met dedicated FFS-en, is er bij r a n d o m FFS-en sprake van additionele planningsproblemen. Niet alleen moet men hier rekening houden met
bewer-1 Wanneer voor een engelstalige vakterm geen goede ncderlandse vertaling voor handen
is, wordt in deie samenvatting de engelstalige term gehanteerd.
10 S a m e n v a 11 i n g
kingsmachines als schaarse middelen, m a a r ook met paletten, spanmiddelen, en sommige (dure) snijgereedschappen. De aandacht voor randvoorwaarden in het F F S planningsprobleem die een gevolg zijn van de beperkte beschik baarheid van paletten, spanmiddelen en snijgereedschappen, is een belan grijk aspect waarin het F F S planningssysteem dat in deze dissertatie wordt gepresenteerd zich onderscheid van veel FFS planningssystemen die in de literatuur worden beschreven.
De belangrijkste doelstelling van dit F F S planningssysteem is het ha len van de uiterste gereedheidsdatum die wordt opgelegd door een hoger gelegen planningssysteem (de shop controller). Als het mogelijk is de uiter ste gereedheidsdatum van alle orders te halen, terwijl slechts een deel van de produktiecapaciteit van het F F S wordt gebruikt, dan wordt het halen van de uiterste gereedheidsdatum zelfs als een randvoorwaarde gezien. In die situatie wordt een nieuwe doelstelling geactiveerd: het creëren van een overschot aan produktiecapaciteit tijdens latere planningsperioden, door de beschikbare produktiecapaciteit in de nabije toekomst zo efficient mogelijk te b e n u t t e n . In de eerste plaats kan dit overschot aan produktiecapaciteit worden gebruikt als een veiligheidsmarge tegen mogelijke storingen. Dit m a a k t het mogelijk om zelfs na een tijdelijke storing de uiterste gereed h e i d s d a t u m van orders nog te halen. In de tweede plaats is dit overschot aan produktiecapaciteit een teken voor het hoger gelegen planningssysteem dat het F F S extra orders kan accepteren. Op deze manier kan de b e n u t t i n g van het F F S worden verbeterd.
De grootste groep van randvoorwaarden die in beschouwing worden geno men door dit planningssysteem, zijn het gevolg van de beperkte beschikbaar heid van schaarse middelen: bewerkingsmachines, paletten, spanmiddelei., snijgereedschappen, gereedschapposities in de lokale gereedschapmagazijnen van de bewerkingsmachines, en bedieningspersoneel. Een andere groep van randvoorwaarden is het gevolg van het feit dat een FFS vaak onbemand doorwerkt tijdens de nacht. Tijdens de nacht kunnen onderdelen niet wor den opgespannen, c.q. afgespannen van de paletten.
Een effectieve manier om complexe besturingstaken zoals het F F S plan ningsprobleem aan te pakken is het gebruik van een hiërarchische besturing sarchitectuur. In dit onderzoek is gekozen voor een structuur bestaande uit de volgende drie beslissingsniveau's: de task selection function, de tool allo
cation function, en de sequencing function. Een belangrijk voordeel van een
dergelijke hiërarchie van samenwerkende besturingsniveau's, is de mogelijk heid om op onverwachte gebeurtenissen te reageren. Beslissingen m.b.t. de planning worden in real time genomen. Wanneer een planningssysteem een
S a m e n v a t t i n g 11
schedule2 genereert voorafgaand aan de eigenlijke produktie, dan wordt het
aanzienlijk moeilijker om onverwachte gebeurtenissen goed te verwerken. De task selection function is het hoogste niveau in de beslissingshiërarchie. De funktie m a a k t gebruik van een mixed integer programmeringsmodel om een verzameling tasks te selecteren welke gedurende de volgende plan ningsperiode (ca. 8 uur) moeten worden uitgevoerd. Het mixed integer pro grammeringsmodel representeert alle hierboven beschreven randvoorwaar den en doelstellingen. Sommige randvoorwaarden worden op een globaal niveau beschreven, omdat de uitvoer van de task selection function niet uit een volledig gedetailleerd schedule b e s t a a t . Een voordeel van deze mixed integer programmeringsbenadering is dat bijna optimale resultaten kunnen worden bereikt. Bovendien maakt deze benadering het mogelijk op eenvou dige wijze te bepalen welke produktiemiddelen een bottleneck vormen, en welke orders de werklast op deze produktiemiddelen het sterkst beïnvloeden. De tool allocation function wijst bepaalde bewerkingstypen en de daar bij behorende gereedschapsets toe aan bewerkingsmachines in het F F S . De bewerkingstypen die moeten worden toegewezen zijn diegenen die door de task selection function voor een bepaalde periode zijn geselecteerd. Het doel dat wordt nagestreefd is een gelijkmatige werklast verdeling tussen de be werkingsmachines in het F F S te bewerkstelligen, en bovendien te zorgen dat zo veel mogelijk bewerkingen door meer dan één bewerkingsmachine kunnen worden uitgevoerd. Als een gevolg van de manier waarop het tool alloca tion probleem is geformuleerd, is het niet een bijzonder kritiek probleem. Hierdoor is een eenvoudige heuristische benadering voldoende.
De sequencing function bepaalt een volgorde van bewerkingen voor ie der werkstation in het F F S , zodanig dat de tasks die zijn geselecteerd voor een bepaalde planningsperiode in zo kort mogelijke tijd kunnen worden uit gevoerd. Het sequencing probleem lijkt sterk op het minimum makespan probleem voor job shops. Dit probleem wordt uitvoerig behandeld in de literatuur. De dissertatie behandelt twee nieuwe benaderingen voor dit pro bleem: de look ahead dispatching procedure, en de vereenvoudigde look ahead
dispatching procedure. Hoewel de resultaten die kunnen worden bereikt met
de originele versie iets beter zijn dan die welke met de vereenvoudigde versie worden bereikt, is de importantie van de laatste het grootst. De resultaten die met de vereenvoudigde versie kunnen worden bereikt zijn nog steeds be duidend beter dan die welke met priority dispatching rules worden bereikt. Bovendien heeft de vereenvoudigde look ahead dispatching procedure t.o.v.
12 S a m e n v a t t i n g
priority dispatching rules geen nadelen met betrekking tot het gebruiksge mak en de vereiste rekentijd.
Contents
A b s t r a c t 3 A c k n o w l e d g e m e n t s 7 S a m e n v a t t i n g ( A b s t r a c t in D u t c h ) 9 1 I n t r o d u c t i o n 19 2 W h a t is F M S ? 23 2.1 Field of application 23 2.2 Hardware 25 2.2.1 Machine Tools 25 2.2.2 Washing Stations 28 2.2.3 Measuring Machines 28 2.2.4 Fixture Building Stations 29 2.2.5 P a r t Load/Unload Stations 29 2.2.6 Pallet Transport System 312.2.7 Pallet BufTer 31 2.2.8 Tool Room 31 2.2.9 Central Tool Magazine 32
2.2.10 Tool Transport System 32 2.2.11 FMS Control Computer 33
2.3 Performance 33 2.3.1 Numerical control 33
2.3.2 Reduction in setup times 34 2.3.3 Integration of machining operations 34
2.3.4 The capability of unattended operation 34
2.3.5 A better organization 35 2.4 Characteristics of FMS operation 35
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2.4.1 R a n d o m P a r t Mix 35 2.4.2 Night Shifts 35 3 A n F M S r e f e r e n c e m o d e l 37
3.1 Why a reference model? 37 3.2 Material flow in an FMS 38 3.3 Opening t h e Machining/Washing/Measuring box 41
4 F M S as p a r t of a larger p r o d u c t i o n f a c i l i t y 45
4.1 The M P C S reference model 45
4.2 T h e shop controller 48 4.3 Consequences for the FMS controller 51
5 T h e F M S c o n t r o l s y s t e m 55 5.1 The FMS controller in its environment 55
5.1.1 Interface to the Shop Controller 55 5.1.2 Interface to the process planning d e p a r t m e n t 57
5.1.3 Interface to the external material transport system . . 59 5.1.4 Interface to the FMS workstations and t r a n s p o r t sys
tems 59 5.2 Scheduling and operational control 60
5.2.1 Interface between scheduling and operational control . 60
5.2.2 The function of the scheduling module 64 5.2.3 The functions of the operational control module . . . 64
6 S c h e d u l i n g c o n s t r a i n t s a n d o b j e c t i v e s 6 9 6.1 Testing FMS scheduling systems 69
6.1.1 Performance measures 71 6.1.2 The costs of FMS equipment 75
6.1.3 The ClaLT curve 78 6.1.4 Conclusion 83 6.2 Operational constraints and objectives 84
6.2.1 Constraints 84 6.2.2 Primary objective: meeting due dates 87
6.2.3 Secondary objective: minimize variable costs 89 6.3 Review of other scheduling constraints and objectives . . . . 90
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7 H i e r a r c h i c a l s t r u c t u r e of t h e s c h e d u l i n g m o d u l e 91
7.1 Characteristics of hierarchical control 91
7.2 Literature survey 92 7.2.1 The strategic level 93
7.2.2 The tactical level 93 7.2.3 The operational level 98
7.2.4 Discussion 100 7.3 T h e selected structure 104
7.3.1 T h e sequencing function 105 7.3.2 The task selection function 112 7.3.3 T h e tool allocation function 118
8 T h e t a s k s e l e c t i o n f u n c t i o n 123
8.1 Modelling concepts 123 8.1.1 Description of process plans 123
8.1.2 Makespan estimate 126 8.1.3 Detailed and aggregate models 128
8.1.4 Fixtures 131 8.1.5 Scarce cutting tools 132
8.2 The mixed integer programming model 134
8.2.1 Symbol definitions 134 8.2.2 Constraint formulations 138 8.2.3 Objective function 143 8.3 The task selection program 145
8.3.1 Input files 146 8.3.2 Main menu 150 8.3.3 Solve primal problem 153
8.3.4 Solve mixed integer problem 168
8.3.5 Print result 168 9 T h e t o o l a l l o c a t i o n f u n c t i o n 173
9.1 Problem s t a t e m e n t 173 9.2 The tool allocation algorithm 175
10 T h e s e q u e n c i n g f u n c t i o n 183 10.1 Problem statement 183
10.1.1 Minimize makespan 184 10.1.2 Taking advantage of the available routing flexibility . 186
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10.1.4 Additional precedence constraints 188 10.1.5 Applicability to real time control 189
10.2 Literature survey 190 10.2.1 Search 190 10.2.2 Optimal scheduling procedures 194
10.2.3 Heuristic scheduling procedures 198
10.2.4 Evaluation functions 201
10.2.5 Discussion 206 10.3 T h e look ahead dispatching procedure 207
10.3.1 Overall structure 207 10.3.2 State space representation 208
10.3.3 T h e evaluation function 215 10.4 Simplified look ahead dispatching 224
10.4.1 State space representation 225 10.4.2 The evaluation function 225
10.5 Experimental results 227 10.5.1 The priority dispatching rules 227
10.5.2 Tests comparing LAO, LAI and LA2 with priority dis
patching rules 228 10.5.3 More elaborate tests with L A I and LA2 231
10.5.4 Conclusion 235 11 E x c e p t i o n h a n d l i n g 2 3 7
11.1 A hierarchy of interacting control levels 237
11.1.1 Task selection function 23^ 11.1.2 The tool allocation function 23 11.1.3 The sequencing function 240 11.2 Examples of exception handling 240
11.2.1 Tool breakage 240 11.2.2 Machine failure 242
11.3 Implementation 242
12 O u t l o o k 2 4 5 12.1 Interface with operational control 245
12.2 Application to job shop control 246
1 3 C o n c l u s i o n 249 13.1 Main features of the FMS scheduling system 249
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B i b l i o g r a p h y 255 A Glossary 259 B Symbols used in t h e task selection function 265
Chapter 1
Introduction
Flexible Manufacturing Systems (FMS) are designed to process efficiently a family of similar part types in small batches. In this respect FMS closes the gap between the traditional job shop and the transfer line.
The t y p e of FMS considered in this theses are FMS for metal cutting operations. These FMS consist of a group of computer controlled machine tools, linked together by a u t o m a t e d material and tool handling systems. A central computer coordinates the activities of all system components.
T h e machine tools applied in these FMS are able to exchange cutting tools automatically between a local tool magazine and the main spindle of the machine. P a r t s are clamped on pallets instead of being clamped on the machine bed itself. These pallets can be exchanged quickly a t the machine tools. As a result, the setup times of machines in FMS are negligible. This is why FMS are capable of processing a mix of several different part types simultaneously.
In m a n y Flexible Manufacturing Systems (FMS) in operation today, all but the most detailed scheduling decisions are made by the system operator. In a number of cases the system operator is supported by a simulation system which confronts him with the consequences of his decisions. Nevertheless, because of the complexity of the FMS scheduling problem (the large number of variables and constraints involved), it is to be expected t h a t his scheduling decisions are far from optimal. T h e goal of the research project presented in this thesis has been the design of an automatic scheduling system which can be applied to the real time control of F M S . The resulting system should be general enough to be applicable to a large range of F M S . With the aid of this scheduling system, it should be possible to meet due dates more reliably, obtain shorter lead times, and reduce the a m o u n t of money t h a t must be
20 I n t r o d u c t i o n
invested in FMS equipment to meet certain production requirements. During the design of this FMS scheduling system, a lot of emphasis was put on scheduling of random F M S : Flexible Manufacturing Systems designed to process a relatively large variety of parts in a r a n d o m sequence. T h e opposite, dedicated FMS, machines a fixed (and relatively small) set of part types with well-defined manufacturing requirements over a known time horizon [11]. Because fixtures are usually dedicated to the clamping of only one part type or a small group of very similar part types, and because fixtures are expensive, a large portion of the capital invested in r a n d o m F M S is spent on fixtures. For this reason often only one or two fixtures are available of each type. T h e same can be true for some expensive cutting tools.
In comparison with dedicated F M S , random FMS pose additional sched uling problems because not only machine tools must be regarded as scarce resources, but fixtures and some cutting tools as well.
T h e main structure of this thesis will be described below. Chapters 2 through 6 can be viewed as an elaborate problem analysis, resulting in a problem definition in chapter 6:
C h a p t e r 2 defines the field of application for FMS and describes a typical FMS hardware configuration.
C h a p t e r 3 describes a logical model of the material flows ( p a r t s , pallets, fixtures, and cutting tools) in F M S . This model will be used as a reference model for the development of an FMS scheduling system. C h a p t e r 4 describes a reference model for Manufacturing Planning and
Control Systems ( M P C S ) . This model shows how a manufacturing facility is controlled by a hierarchy of control units. According to this model, an FMS controller receives its orders from a higher level production scheduling system called the shop controller. An analysis of the shop controller leads to conclusions with respect to the orders the FMS controller should expect to receive from the shop controller, and the way in which the shop controller expects the FMS controller to respond to these orders.
C h a p t e r 5 is dedicated to the FMS controller itself. It s t a r t s with a de scription of the d a t a exchange between the FMS controller and its environment. Next, it is shown how an FMS controller can be divided into a scheduling module and an operational control module. The d a t a
21
exchange between the scheduling and the operational control module is identified, and the functions of the operational control module are described. The scheduling module is the subject of the rest of the thesis.
C h a p t e r 6 defines the constraints t h a t should be observed by the schedul ing module, and the objective it should try to optimize.
Chapter 7 discusses the top level structure of the scheduling module. After a literature survey on this subject, the hierarchical decision structure selected in this study is presented and justified. The hierarchical decision structure consists of three levels: the task selection function, the tool alloca tion function and the sequencing function. A description of the functionality t h a t should be ofTered by these decision levels is described in chapter 7 as well.
T h e implementation of the task selection function, the tool allocation function, and the sequencing function is the subject of chapters 8, 9 and 10 respectively. Together with chapter 7 these chapters form the main body of the thesis.
Chapter 11 discusses how the scheduling module responds to failures in the F M S . Chapter 12, describes possible directions for further research and development extending the work presented in this thesis. Conclusions can be found in chapter 13.
N o t i c e
T h e terms scheduling and dispatching are not used very consistently in lit erature. For instance, some authors use the term scheduling only for off-line scheduling procedures that produce a schedule. In t h a t case implementing a schedule by sending commands to the machines is referred to as dispatching. Other authors recognize the existence of real time scheduling procedures. To avoid confusion, the reader is urged to look up our definition of these terms in the glossary in appendix A. These definitions will be used throughout this thesis.
Chapter 2
W h a t is FMS?
According to a recent united nations report [16] there is as yet no inter nationally agreed definition of the term "flexible manufacturing system". Several different definitions can be found in literature. After having studied an extensive list of definitions from other organizations, the united nations report suggests the following definition:
A flexible manufacturing system (FMS) is a n integrated com puter controlled complex of numerically controlled machine tools, a u t o m a t e d material and tool-handling devices and a u t o m a t e d measuring and testing equipment t h a t , with a minimum of man ual intervention a n d short change-over time, can process any product belonging to certain specified families of products within its stated capability a n d to a predetermined schedule.
This chapter will give a description of the kind of system the a u t h o r had in mind when developing a scheduling system for F M S . T h e description is not meant as a contribution to discussions in literature a b o u t what kind of systems deserve the label "Flexible Manufacturing System". We will focus our attention on FMS for prismatic p a r t s . These systems have enough characteristics in common to allow a common approach to the scheduling problem.
2.1 Field of application
The typical field of application for FMS is illustrated in figure 2.1. Flexible manufacturing systems are used for the medium volume, medium variety
24 W h a t is F M S 7 B a t c h s i z e 1 ()()()( 2000 500 25
Tra sfer lines
Sp ;cial Systems
F MS,
MTG cell
Conventional equipment 1 5 100 500
-< Number of parts per system
►-F i g u r e 2 . 1 : ►-Field of application for ►-FMS. Source: [16]
production. In this respect FMS fills the gap between the job shop and the flow line.
J o b shops are characterized by a high variety, low volume production. Because of the high part variety, a relatively large a m o u n t of machine time is lost in machine setup. As a consequence, machine utilization in job shops is relatively low. The machine utilization can be improved to a certain extend if batches of identical parts are sent from one workstation to another instead of individual parts (economical batch sizes). This way a machine only has to be set u p once for each batch of parts.
J o b shops are also characterized by a r a n d o m part flow. T h e sequence of machines visited by a batch of parts can differ from one part type to another. A random part flow results in a relatively high risk of workstations becoming idle due to a lack of parts to be machined. This risk can be reduced by a high level of work in progress. If the average number of batches waiting to
H a r d w a r e 25
be processed on a machine is high, the risk of this machine becoming idle due to a lack of parts to be processed will be low.
Because parts are produced in batches, and because of the high n u m b e r of batches waiting to be machined at each machine, j o b shops usually have a high level of work in progress. As a consequence lead times are often as high as one week for each machine a part has to visit, even though the operation times at each machine are only a few minutes.
Flow lines on the other hand, are used for low variety, high volume production. Because of the low variety, setup times do not play an i m p o r t a n t role. Because of this, and because of the linear part flow the a m o u n t of work in progress can be very low. When a flow line is designed such t h a t operation times are approximately the same for every machine in the line, a high machine utilization can be combined with short lead times.
The aim of FMS is to combine the flexibility of a job shop (the ability to produce m a n y different part types) with the high workstation utilization and short lead times of the flow line. In practice of course, an FMS is less flexible than a job shop and has a higher lead time than a flow line.
2.2 Hardware
Figure 2.2 identifies the major components of a typical flexible manufac turing system for prismatic p a r t s . T h e FMS in this example was built by Cincinnati Milacron in 1985 for its plastics machinery plant in M t . Crab [17]. The parts to be machined in this FMS may be up to 80 cube centimeter in size and may weigh up to 10000 kg. Eventually the manufacturer expects to find 3500 different part types within this class suitable for machining in t h e F M S . This kind of versatility however, is rather unusual for F M S .
Descriptions of ten different flexible manufacturing systems can be found in [37]
In the next few sections the major components of an FMS will be dis cussed in more detail. The emphasis will be on those features t h a t are relevant to the development of an FMS scheduling system.
2 . 2 . 1 M a c h i n e T o o l s
The main process in an FMS is preformed by machine tools. In figure 2.2 these are indicated as number 1. According to the UN report [16] approxi mately 50 % of all machine tools in operation in FMS are machining centers. In FMS for prismatic parts this must be even more. Other machine tools
26 What is FMS?
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H a r d w a r e 27
F i g u r e 2 . 3 : A Werner TC 800 machining center
t h a t can be used in an FMS for prismatic parts are milling machines, (multi-spindle) drilling machines, grinding machines, boring machines and vertical lathes.
Figure 2.3 shows an example of the kind of machining center t h a t can be part of an F M S .
A machining center is controlled by a local CNC (computer numerical control), controller running an NC-program. An NC-program contains all technological d a t a required to process a p a r t . It specifies the p a t h of a cutting tool, its speed and feed rate, the t y p e of cutting tool to be present in the spindle, when to turn the flow of coolant on and off etc . . .
28 W h a t is F M S ?
accurately position a pallet on the machine, pallets are provided with an accurate mechanical interface on the b o t t o m side.
In the example of figure 2.3 the machine is equipped with two pallet buffer spaces. Usually one of the buffer spaces is empty, while the other contains a part waiting to be machined. When the machine finishes an operation, it can move the pallet containing the finished part to the empty buffer space and move the next pallet to the machining position. While the next part is being machined, the pallet transport system can take away the finished part and replace it with yet another part to be machined. This way the activities of the pallet transport system are decoupled from the activities of the machine tool.
Usually several different cutting tools are needed to complete an opera tion on a certain part type. In addition it is often required that the machine can process several different part types without intermediate setup times. For this reason the machine is equipped with its own local tool magazine which is large enough to contain the required tools for a number of different part types. The exchange of tools between the main spindle of the macliining center and the local tool magazine can be done automatically.
2 . 2 . 2 W a s h i n g S t a t i o n s
Washing stations are used in an FMS to wash off chips left on the product after having been machined on one of the machine tools. From a control point of view washing stations can be treated as regular machine tools, except for the fact t h a t they do not have to be provided with cutting tools or NC-programs.
Not all FMS are equipped with a washing station. 2 . 2 . 3 M e a s u r i n g M a c h i n e s
Measuring machines are used to check if the dimensions of finished parts are within specified limits. If this is not the case, a warning will be issued to the system operator.
In some systems the cause of an inaccuracy can be traced automatically to a certain cutting tool. T h e system operator can use this information to replace the defective tool or to change its offsets.
From a control point of view, measuring machines can be regarded as regular machine tools.
Only few FMS in operation today are equipped with a measuring ma chine.
H a r d w a r e 29
2.2.4 Fixture Building Stations
N u m b e r 11 in figure 2.2 is a fixture building station. In this station fixtures are built and mounted on a pallet.
T h e top side of the pallets used in an FMS is usually flat with a regular p a t t e r n of grooves or (tapped) holes. On the b o t t o m side the pallets have a mechanical interface necessary t o position t h e m on t h e machine tools a n d to interface them with the pallet transport system. T h e pallets themselves are all identical.
Before a part can be clamped on a pallet, first a fixture must be mounted on t h a t pallet. The part in turn, is clamped in the fixture. Usually a fixture can only be used for one specific part type or a small family of very similar part types. A distinction can be made between two types of fixtures:
• Dedicate fixtures.
• Modular systems of fixture building blocks.
Dedicated fixtures are built once for a specific part type or small group of very similar part types. They are stored in a fixture magazine. Whenever there is a demand for a certain part type, the corresponding fixture is taken from the fixture magazine, and mounted on a pallet.
It should be clear t h a t when an FMS is capable of machining many different part types, it must have a considerable number of dedicated fixtures in store. For some systems the total a m o u n t of money invested in fixtures equals the investment cost of the rest of the FMS [25] !
Figure 2.4 shows an example of a modular system of fixture building blocks. Using these elements fixtures can be built for m a n y different part types. In comparison with dedicated fixtures they can mean a considerable reduction in capital investment. On the other hand, building a fixture from m o d u l a r fixture building blocks takes a lot more time t h a n simply mounting a dedicated fixture on a pallet.
Because of the large amount of time required to build a fixture from m o d u l a r fixture building blocks, some companies never take these fixtures apart until the end of the product life cycle. In such the cases, fixtures built from modular fixture building blocks must be regarded as dedicated fixtures.
2.2.5 P a r t L o a d / U n l o a d S t a t i o n s
In the part load/unload stations (number 10 in figure 2.2), parts to be machined are clamped on a pallet, and finished parts are taken from a pallet.
3 0
What is FMS?
Montageplalle
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H a r d w a r e 3 1
It is not unusual t h a t a pallet contains more than one p a r t . T h e p a r t s clamped together on the same pallet do not even have to be of the same type.
P a r t s can only be clamped if the right t y p e of fixture has already been mounted on the pallet. Therefore, when finished p a r t s are taken from a pallet, they are usually replaced with the same t y p e of parts to be machined next.
In some FMS there is no distinction between p a r t load/unload stations and fixture building stations. B o t h tasks are performed in the same stations. P a r t loading and unloading as well as fixture building are usually m a n u a l operations
2 . 2 . 6 P a l l e t T r a n s p o r t S y s t e m
An FMS m u s t always contain some kind of a u t o m a t e d material handling system to transport pallets from one workstation to another. The FMS in figure 2.2 employs a u t o m a t e d guided vehicles ( A G V ' s ) , indicated as number 3. AGV's find their way through the FMS by tracking a wire t h a t runs j u s t underneath the floor. These wires are also represented in figure 2.2.
According to the UN report [16] there is a trend towards the application of AGV's in newly installed F M S . Other types of material handling systems in use in FMS are rail guided vehicles, tow carts and conveyor systems. 2 . 2 . 7 P a l l e t B u f f e r
N u m b e r 6 in figure 2.2 is a pallet buffer. In addition to the pallet buffer positions on the machine tools, 10 extra pallets can be stored here. T h e number of pallets t h a t can be stored in this system may seem very small relative to the number of workstations in the system. However this is not unusual in F M S , because pallets and fixtures are very expensive.
2 . 2 . 8 T o o l R o o m
In the tool room the cutting tools for the machine tools are prepared. Tool preparation includes the following activities:
T o o l a s s e m b l y Tools must be placed in a tool holder that fits in the main spindle of a machine tool.
T o o l m a i n t e n a n c e When the cutting edges of a tool are worn, the tool must be ground, or the throw away cutting tips must be replaced.
32 W h a t is F M S ?
T o o l offset m e a s u r e m e n t When NC programs are developed, the pro grammer assumes nominal tool dimensions. T h e actual tool dimen sions will always differ slightly from the nominal ones. A tool offset is the difference between the actual and the nominal tool dimensions. Provided t h a t tool offsets are known to a CNC controller, they can be compensated automatically. This is why tool offsets must be measured in the tool room. Tool offset d a t a are uploaded from the tool room t o the FMS control computer. The FMS control computer downloads these d a t a to the CNC controller of a machine tool when the tool is placed in the local tool magazine of the machine.
The tools t h a t have been prepared in the tool room are placed in the central tool magazine.
T h e tool room is not depicted in figure 2.2. 2 . 2 . 9 C e n t r a l T o o l M a g a z i n e
T h e central tool magazine is a storage place for tools t h a t have already been prepared, but t h a t have not yet been placed in the local tool magazine of a machine tool. At least part of the central tool magazine is located at number 8 in figure 2.2. It may be t h a t another part is located in the tool room outside the picture.
2.2.10 T o o l T r a n s p o r t S y s t e m
T h e tool transport system transports tools between the central tool mag azine and the local tool magazines of the machine tools. From a contrt point of view the following distinction between tool transport systems is significant:
• Systems t h a t exchange individual tools in the tool magazine of a ma chine tool.
• Systems t h a t exchange entire tool magazines.
The second type of tool transport is the least flexible one. It forces the FMS control system to replace the entire tool set of a machine tool periodically. When individual tools can be exchanged the tool set of a machine tool can be adjusted gradually.
T h e second type of tool transport is also more expensive with respect to tool usage. It makes it very hard if not impossible to use all tools up to the limit of their nominal tool life.
P e r f o r m a n c e 33
If there is an advantage of exchanging entire tool magazines instead of individual tools it must be the fact t h a t a lot of tools can be exchanged in a relatively small a m o u n t of time. However, this is hardly an advantage know ing t h a t in most cases individual tools can be exchanged without stopping the machine tool.
Another i m p o r t a n t distinction can be made between a u t o m a t i c tool transport systems and manual tool t r a n s p o r t . When tool t r a n s p o r t is man ual, no tool transport can take place during an unmanned night shift. 2 . 2 . 1 1 F M S C o n t r o l C o m p u t e r
T h e FMS control computer coordinates all activities in the F M S . Its tasks are discussed in more detail in chapter 5.
2.3 Performance
An interesting question one might ask is: " W h y is it t h a t FMS can achieve so much better machine utilizations and shorter lead times t h a n conventional job shops?". T h e answers to this question are:
1. Numerical control.
2. A reduction in setup times.
3. Integration of machining operations. 4. The capability of unattended operation. 5. A b e t t e r organization of the production. 2 . 3 . 1 N u m e r i c a l c o n t r o l
T h e conventional type of machine tools applied in job shops are controlled manually. Therefore, a large portion of the available cutting time is lost in measuring the product and setting the machine for the next cut. Because the machines in an FMS are computer controlled, all this is done automatically, very fast and very precise. As a result a large portion of the time a part spends on the machine is actually used for cutting metal.
3 4 W h a t is F M S ?
2 . 3 . 2 R e d u c t i o n i n s e t u p t i m e s
W h e n using conventional machine tools, considerable setup times are re quired for the following activities:
• Replacing a cutting tool in the main spindle of a machine tool. • Building a fixture on the table of a machine tool.
• Clamping a part in a fixture.
In F M S these setup times are very small for the following reasons.
In the first place, the machine tools applied in FMS are equipped with a local tool magazine and an a u t o m a t i c tool changer. The exchange of a cutting tool between the main spindle of a machining center and the local tool magazine can be performed in j u s t a few seconds.
In the second place the machine tools applied in an FMS are equipped with a u t o m a t i c pallet changers. Building a fixture on a pallet, and clamping a part in a fixture is still a very time consuming activity. However, these activities can now take place while the machine is busy processing the p a r t s clamped on another pallet.
2 . 3 . 3 I n t e g r a t i o n o f m a c h i n i n g o p e r a t i o n s
In conventional job shops, machining operations such as drilling, milling, boring, grinding etc. are all performed on different machines. The machine tools used in FMS however, are often machining centers. These machines are capable of performing several different machining operations on one ma chine. This results in a further reduction of setup times and in reduced lead times.
2 . 3 . 4 T h e c a p a b i l i t y o f u n a t t e n d e d o p e r a t i o n
Conventional machine tools require continuous a t t e n d a n c e . Whenever the operator takes a break, the machine will be stopped. In F M S , unattended production is possible for at least a few hours. At the beginning of an unmanned period, all pallets are loaded by operators with parts to be ma chined. From t h a t moment on production can go on unattended until all these parts have been processed.
C h a r a c t e r i s t i c s o f F M S o p e r a t i o n 35
2.3.5 A b e t t e r organization
In FMS the whole production process is under rigid computer control. The status of all workstations and all parts in the system are known to the computer at all times. This creates the opportunity for detailed scheduling of the production process and of executing the production schedule with a high degree of accuracy. Scheduling will be discussed in ample detail in this thesis.
2.4 Characteristics of FMS operation
2 . 4 . 1 R a n d o m P a r t M i x
An FMS cannot simply process any part type within a specified part family. A part type which is to be machined in the FMS must be known to t h e system in advance. The FMS must have information regarding its process plan, the required NC-programs for the part type must have been provided, and the F M S must either have at least one fixture in stock which is suitable for clamping this part type, or it must have information on how to built a suitable fixture from modular fixture building blocks.
Of course the list of parts t h a t can be machined in the F M S can be extended by providing extra process plans, extra NC-programs and extra fixtures. However, this is usually done on a much longer time scale t h a n the ordering of parts to be produced.
Within the specified list of part types t h a t the FMS can process, the FMS can be ordered to process any part mix. In order to make efficient use of its scarce resources, an FMS must always process several different part types concurrently. T h e reason for this is twofold:
• T h e available number of fixtures suitable for clamping a particular part type is usually very small. Therefore, if parts were processed one batch after the other the a m o u n t of work in progress in the system would be too small to keep all workstations occupied.
• A balanced workload among the machines in the FMS can only be achieved with a combination of part types.
2 . 4 . 2 N i g h t S h i f t s
In order to justify the high capital investment of FMS it is often necessary to operate the FMS 24 hours a day. In m a n y cases the night shift is an
36 W h a t is F M S ?
unmanned shift. Because p a r t s loading and unloading are usually m a n u a l operations, the system must be scheduled such t h a t a t the beginning of the night shift all pallets are loaded with parts to be machined. These p a r t s are machined during the night, and are unloaded next morning by the operators.
Chapter 3
An F M S reference model
In this chapter an abstract model of flexible manufacturing systems will be presented on which the design of an F M S scheduling system can be based. The model consists of two diagrams representing the material flow in an F M S , accompanied by an explanatory text.
3.1 W h y a reference model?
T h e previous chapter described flexible manufacturing systems in general terms. Such a description is very useful to give a general understanding of what is meant by the term "Flexible Manufacturing System". As the basis for a design project aimed at the development of a scheduling system for a range of FMS however, such a description is inappropriate. For this application, the description has two important drawbacks.
In the first place the description in the previous chapter is not accurate enough. The description leaves too much room for misinterpretation.
In the second place the description in the previous chapter contains a lot of information which has no relevance to the development of a scheduling system. If this information ends up being used in the design, there is a risk t h a t the final result is less generally applicable than it could have been.
These kind of problems can be avoided by using an abstract model of an FMS as the basis for the design process. Such a model should have the following characteristics:
• T h e model should be general enough to represent most real life F M S . • T h e model should stress all those FMS characteristics which are thought
to be relevant to the design of an FMS scheduling system.
38 A n F M S r e f e r e n c e m o d e l
• T h e model should leave out all those F M S characteristics which are t h o u g h t not to be relevant to the design of an FMS scheduling system. A secondary advantage of such a model is the following. Since the model is a clear s t a t e m e n t of the FMS characteristics on which the design was based, it can help to answer questions about the applicability of the scheduling system to a particular F M S . If some of the characteristics in the model do not apply to a particular F M S , one should have serious doubts about the applicability of the FMS scheduling system designed in this project to t h a t system. In such cases one should take a closer look at the design to see which parts might still be applicable.
3.2 Material flow in an FMS
Figure 3.1 shows a schematic representation of the material flow in an F M S ' . Raw material enters the FMS in the form of blanks, castings or forgings. These p a r t s are stored temporarily in the part storage area until the first time they are clamped on a pallet. When parts are u n d a m p e d from a pallet, they are returned to the part storage area. Finished parts stay in the p a r t storage area until they are collected by the external material transport system.
T h e upper cycle in figure 3.1 represents the parts flow in an F M S . The actual transformations on the parts are performed inside the Machining/-Washing/Measuring box. Before a part can enter this box, it must be clamped on a pallet. For this purpose we need a pallet which is equipped with the right type of fixture. Conceptually, the pallet is obtained from the storage area for pallet/fixture combinations. In practice, such an area can seldom be recognized in an FMS, but the pallet must be present somewhere in the system.
In many cases not just one part will be clamped on a pallet. A pallet can carry several p a r t s . These parts do not even have to be of the same type. However, one cannot randomly select a combination of parts a n d decide to put these together on the same pallet. The combination must be mentioned on a list of possible combinations. For each of these combinations, a suitable fixture must have been defined.
When a pallet leaves the Machining/Washing/Measuring box, the p a r t s on t h a t pallet are u n d a m p e d , and are returned to the part storage area.
M a t e r i a l flow in a n F M S 39
r a w
m a t e r i a l
part
storage
(achined
" p a r t s
clamping
fixture
assembly
machining
washing
measuring
storage of
pallet / fixture
combinations
storage of
pallets
storage of
fixtures
unclamping
fixture
disassembly
40 A n F M S r e f e r e n c e m o d e l
To indicate the fact t h a t these p a r t s have undergone a transformation, the parts can be assigned a different part type indicator.
In future the flow of a pallet through the Clamping - Machining/-Washing/Measuring - Unclamping pipeline will be referred to as running a task. Running a task results in the transformation of a group of p a r t s .
In m a n y cases a part must go through the Clamping - Machining/-Washing/Measuring - Unclamping pipeline a number of times in order to go from raw material to finished p a r t . This is because there is always one side of a part t h a t cannot be machined when the part is clamped in a fixture. So, if a part must be machined from all sides, it must be clamped at least twice.
Note t h a t in an FMS two kinds of work in progress can be distinguished. A part s t a r t s contributing to the a m o u n t of work in progress of a manufac turing system the moment t h a t two conditions are m e t . In the first place the p a r t must be available to the manufacturing system. T h a t is, the manu facturing system must have the authority to control this p a r t . In the second place, an order must have been issued to the manufacturing system to per form one or more operations on this p a r t . A part stops contributing to the a m o u n t of work in progress of a manufacturing system the m o m e n t t h a t the last operation on this part has been completed. Using this definition of work in progress, two kinds of work in progress can be distinguished in an F M S :
• T h e parts on the pallets.
• T h e parts in the part storage area.
T h e first kind of work in progress is much more costly t h a n the second type of work in progress, because these parts are occupying an expensive pallet and fixture. Within the limited time horizon observed at FMS level (a few weeks), interest costs on work in process inventory do not play an important role. Therefore, having semi-finished parts in the part storage area is only slightly more expensive than having raw material in the part storage area.
While the the first type of work in progress is strongly restricted by the number of pallets and fixtures in the system, the second type of work in progress is restricted only by demands on the lead time of individual p a r t s . It can be deduced from figure 3.1 that the production rate of a certain part t y p e2 depends on the following factors:
Note that in this respect a part type doesn't always have to indicate a finished pait. It can also be a semi-finished part type which needs to be transformed by one or more other tasks before it can leave the FMS
O p e n i n g t h e M a c h i n i n g / W a s h i n g / M e a s u r i n g box 41
• The number of pallets available in the system t h a t are equipped with t h e right type of fixture to clamp this part type.
• T h e number of parts t h a t can be clamped in such a fixture.
• The flow time of such a pallet inside the Machining/ Washing/Measuring box. This factor in t u r n , depends on many other factors such as the available machine capacity and on scheduling decisions.
The number of pallets in the system t h a t are equipped with a certain type of fixture can be adjusted by executing fixture assembly and fixture disassembly operations.
Note t h a t fixture assembly operations are like machine setup operations in a traditional job shop. If the pallet/fixture combination resulting from a fixture assembly operation goes through the Clamping - Machining/-Washing/Measuring - Unclamping pipeline m a n y times, the costs of the fixture assembly operation can be divided between many p a r t s . A difference between a fixture assembly operation and the machine setup operation in a traditional j o b shop is the fact t h a t in FMS fixture assembly can be done while all machine tools are running. Therefore, the costs of fixture assembly does not include valuable machine time.
T h e storage area for fixtures indicated in figure 3.1 may contain dedi cated fixtures, fixture building blocks or a combination of both. For instance, it may be advantageous to have a few dedicate fixtures for those p a r t types t h a t need a complicated fixture and for which the annual demand is high. For the rest of the parts one may decide to build fixtures from modular fixture building blocks.
W i t h respect to the dedicate fixtures, the scheduling system predomi nantly has to reckon with the scarcity of these fixtures.
W i t h respect to the modular fixture building blocks the scheduling sys t e m predominantly has to reckon with a considerable fixture building time.
3.3 Opening t h e M a c h i n i n g / W a s h i n g / M e a s u r i n g
box
Figure 3.2 zooms in on the contents of the Machining/Washing/ Measuring box in figure 3.1. The units entering this box are pallets carrying one or more p a r t s .
A pallet must go through a specified sequence of operations. T h e se quence of operations being dependent upon the type of parts clamped on
4 2 A n F M S r e f e r e n c e m o d e l worn tools LTS Ws 1 Ws 2 tool preparation Ws 3 Ws 4 clamped parts storage new tools Ws 5 Ws 6 Ws = Workstation LTS = Local Tool Storage
F i g u r e 3.2: Schematic representation of the material flow in an FMS:
zoomed in on Machining/Washing/Measuring
the pallet. An operation can be defined as all activities t h a t are performed by one workstation to change some of the characteristics of the p a r t s on one pallet. So, saying "An operation is performed on the pallet" is the same as saying " T h e pallet visits a workstation". For machine tools performing an operation is usually the same as running an NC-program. Other example of operations are washing a pallet and measuring the parts on a pallet.
Each operation is characterized by a number of a t t r i b u t e s : • T h e duration of the operation.
• T h e type of workstation required for the operation. • The tool set required for the operation.
• The NC-program a machine tool must run to execute the operation. T h e last two items are only applicable to machining operations.
In many FMS a number of identical workstations can be found. The FMS in figure 2.2 for instance, contains 4 identical machining centers. In these cases any one of a set of identical workstations can be selected to perform a particular operation.
O p e n i n g t h e M a c h i n i n g / W a s h i n g / M e a s u r i n g b o x 43
When selecting a machine tool for a particular machining operation, the set of tools already available in the local tool magazine of the machine tool (LTS in figure 3.2) can be an important consideration. All tools required for the operation and not yet available in the local tool magazine m u s t be t r a n s p o r t e d to the machine tool before the operation can be started.
In figure 3.2 the tool transport system is represented by the arrows be tween the local tool storage and the central tool storage. Because of the limited transport capacity of the tool transport system, it must be avoided t h a t too m a n y tools must be transported at the same time. If the tool trans port system becomes a bottleneck, the machine tools will be kept waiting for tools. This results in a loss of efficiency.
As was mentioned earlier, in some FMS tool transport is performed manually. If this is the case, the transport capacity of the "tool transport system" is likely to be even smaller. When tool transport is m a n u a l , no tools can be transported at all during unmanned periods.
In some FMS the tool transport system exchanges entire tool magazines instead of individual tools. If this is the case, the local tool magazine must remain static during relatively long periods of time. During these periods, operations can only be assigned to a machine tool if all tools required for t h a t operation are available in the local tool magazine. Only once in a while an entirely new tool set can be assigned to a machine tool.
Apart from the tool transport activities the setup times on the work stations in an FMS are independent of the sequence of operations on these workstations. In many FMS the cutting tools in the local tool magazine of a machine tool can be replaced without stopping the machine. If this is the case, it can be assumed t h a t there are no sequence dependent setup times. This is an important assumption because all sequence independent setup times can be viewed as being part of the nominal operation times. This means t h a t a scheduling system can be allowed to ignore all setup times.
The contents of the central tool magazine must be kept in accordance with the tool requirements of the parts currently being processed by the F M S . This is a task of the tool preparation station. In addition, the tool preparation station takes care of tool maintenance.
Chapter 4
F M S as part of a larger
production facility
In this chapter the position of a flexible manufacturing system in a larger production facility will be discussed. The discussion will lead to a conclusion regarding the functionality t h a t should be offered by an FMS controller.
4.1 The M P C S reference model
As a reference for determining the position of flexible manufacturing sys tems in a larger production facility, t h e reference model for Manufacturing Planning and Control Systems ( M P C S ) described by Biemans and Vissers [9] will be used.
T h e M P C S model is part of the CAM reference model developed by NV Philips and Digital Equipment Corporation [12]. This model forms a framework for the development of architectures of Production Process Con trol Systems, and Production Process Development Systems. The model is a collection of ideas from m a n y sources including the American National Bureau of Standards, Philips, Digital Equipment Corporation, a n d the In ternational Organization for Standardization.
Biemans and Vissers define a Manufacturing Planning and Control sys t e m as a system t h a t :
• aims at earning a target a m o u n t of money by buying and selling certain types of products in certain target numbers.
• negotiates for exchange of products for money a n d exchanges products
4 6 F M S a s part of a larger p r o d u c t i o n f a c i l i t y
for money with customers and suppliers.
A manufacturing planning and control system is described as a hierarchical structure of components t h a t cooperate to achieve the goals described above. T h e model assigns a specific task to each of the components.
R a t h e r t h a n describing the actual internal structure of one particular company, t h e M P C S reference model describes how a Manufacturing Plan ning and Control System should be structured. The hierarchical decompo sition of the model is motivated by the following three principles:
S e p e r a t i o n o f C o n c e r n s : A decomposition of the MPCS should result in components t h a t have essentially different or relatively independent tasks.
G e n e r a l i t y : If independent tasks have been identified, they should be de fined in general terms.
P r o p r i e t y : Inessential tasks should not be introduced.
T h e MPCS reference model is summarized in figure 4.1. The hierarchical structure of the model consists of 9 levels:
• Company • Factory • Shop • Workcell • Workstation • Automation Module • Equipment • Device • Sensor or Actuator
At each level a command unit consists of a command unit controller and one or more command units from a lower level. A workcell for instance, consists of a workcell controller plus one or more workstations. Similarly, a workstation consists of a workstation controller plus one or more a u t o m a t i o n modules, and a company consists of a company controller and a factory. The controllers are indicated in the leftmost column of figure 4.1.
The M P C S reference model 47
CONTROL
LEVEL CONTROL SERVICE
TASK OF CONTROLLER EXAMPLES OF COMMANDS EXAMPLES OF REPORTS COMPANY CONTROLLER survive in changing economics, technology.
select target markets and profits.
FACTORY CONTROLLER
sell a radios with profil b in year c realise p r o f i t by selling p r o d u c l s . negotiate exchange o ' raw materials and pro ducts; predict d u e dates of products. SHOP CONTROLLER (prepare to) dispatch products at due dates
control decoupling stocks to be able to dispatch products at their actual or expected due date.
WORKCELL CONTROLLER process parts in allotted time slots. schedule when.where which operations are executed on parts, and parts are exchanged.
WORKSTATION CONTROLLER
execute operations o n parts
determine which physi cal modifications parts should undergo. AUTOMATION MODULE CONTROLLER
Z E
m o d i f y o b j e c t s .determine required paths of joints. EQUIPMENT CONTROLLER
35:
lollow j o i n t p a t h .select values of control variables that
determine energy flows.
DEVICE CONTROLLER
3 E
set control variable.
issue control signals so that control variable is servoed by physical p a r a m e t e r s . SENSOR OR ACTUATOR modulate physical o a r a m e t e r s . modulation of physical p a r a m e t e r s
sensing tasks, services omitted.
dispatch radios d al day e to f process boards g between day h and i pul components h.i.j on board k move object m to n, max pressure o. ncrease pressure lollow palh p. forces q.r.s at lomls, electrical c u r r e n l to motor. profit a realised b radios sold radios c dis patched to d.. e boards I made quality board I is ok. object with dimensions g,h at i 'me segment ai k with velocily i video Iramns
4 8 F M S as p a r t of a larger p r o d u c t i o n f a c i l i t y
Each layer in the control hierarchy offers a service to its s u p e r i o r ' . A service is a specification of the functionality a layer has to offer. Going from the b o t t o m to the top, each controller enhances the service of all its subordinates taken together.
As an example, let us look at the workcell controller. The subordinates of the workcell controller are workstations. T h e service offered by a workstation is "execute operations on p a r t s " . The model describes the task performed by the workcell controller as: "schedule when, where which operations are executed on parts, and parts are exchanged (between workstations)". T h e workcell controller coordinates the activities of the workstations in such a way t h a t all required operations for a particular part are completed within a certain time slot. Therefore, the service a workcell as a whole can offer the shop controller is "process parts in allotted time slots".
In this model a flexible manufacturing system coincides with a workcell. Examples of workstations in an FMS are: a machining center, a washing sta tion, a measuring machine, a clamping station and a fixture building station. A pallet transport system can be considered as a specialized instance of a workstation, executing transport operations. The FMS controller coincides with the workcell controller.
T h e levels below the workstation level in the MPCS model are irrelevant to the development of an FMS controller.
4.2 T h e shop controller
According to the M P C S model, the FMS controller receives its command., from the shop controller. This is why the shop controller deserves a closer look.
The shop controller receives its commands from the factory controller. The factory controller commands the shop to deliver products (finished goods) at certain due dates based on customer orders, or informs the shop t h a t it may have to deliver goods at certain due dates based on sales pre dictions.
T h e MPCS model describes the task of the shop controller as: "control decoupling stocks to be able to dispatch' products at their actual or expected due d a t e " .
For those who are familiar with the I S O / O S I m o d e l for d a t a c o m m u n i c a t i o n : the t e r m
tervice is used here in the s a m e sense as in this I S O / O S I m o d e l .
' T h i s usage of the word " d i s p a t c h " should n o t be confused with t h e i n t e r p r e t a t i o n defined in the glossary ( a p p e n d i x A). Dispatch here m e a n s : send to t h e c u s t o m e r
