hierarchical specification and validation of operating sequences in the context of fmss

Hierarchical specification and validation of operating

sequences in the context of FMSs

D CRUETTE, J P BOUREY* and J C GENTINA*

Abstract: This paper presents an original methodology for the step-by-step hierarchical specification and validation of the operating sequences in the context of Flexible Manufacturing Systems. It is based on a progressive refinement of the description of the operations applied on each manufactured part and describes the techniques that must be applied, using a cell consisting of robots, conveyor and machine tools to provide an example.

Keywords: object Petri nets, flexible manufacturing systems, access protocols.

T he development of flexible manufacturing systems often leads to the use of modular and polyvalent production methods. It is also the

case that the degree of success of the control of such processes can vary. The need for parallel and synchronized operations can make it difficult to validate the behaviour of the process.

This paper illustrates an original method of generation of the operating sequences used in the CASPAIM plan (Conception Assist6e de Syst~mes de Production Automatis6s dans I'Industrie Manufacturi~re) 1-5. The CASPAIM plan is first explained, then the structural description of the process is disscused. Afterwards, the description of the operations applied on each produced part, and the aided generation of the operating sequences, are detailed. Finally, the specification of the access protocols is explained.

C A S P A I M plan

The aim of this plan is to build up a methodology for the specification, validation and evaluation of flexible manufacturing systems. The main points are:

• the management of an hierachical description of the process and of the operating sequences: allows the processing of batches, of batches of batches, and the use of complex means such as conveyors of trays;

• an homogeneous description of both the process and the produced parts;

• the generation of the operating sequences step-by- step, according to the hierarchical description of the process, and validation of the behaviour at each step;

• the process is described in terms of maximum flexibility, hence we may identify all the management functions which have to be defined;

• a consistent description of the process from the specification phase up to the implementation phase.

The complex processes taken into account by the methodology are, for instance:

• conveyors with single place or multiple place trays • multi-handed robots • rotating storage areas • travelling gantry crane • lifts

The function of all these means is the carrying of an object from one place to another. The entry points can be multiple (conveyors, rotating storage areas), as well as the exit points. The differences between them are:

Boursier CNRS/REgion Nord Pas de Calais, Laboratoire d'Automa- tique et d'Informatique Industrielle de Lille (LAII) (URA CNRS D1440), Institut Industriel du Nord, BP 48, 59651 Villeneuve d'Ascq, France Paper received: 4 September 1990

(i) their ability to carry several objects at the same time,

(ii) the time delay for a basic movement, (iii) the delay for an object to be loaded on the

transportation means.

140

0951-5240/91/030140-17 © 1991 Butterworth-Heinemann Ltd

Computer-Integrated Manufacturing Systems

In the CASPAIM plan, the assignment of a limit storage area is managed in the Command Part by a module called 'Storage Allocator'. It uses a decisional function which is parametrized by the hierarchical level. The general structure of the produced modules is shown in Figure 1.

Since the process is widely shared by many sub-systems of command (for example, a lift can simultanously carry several parts which individually go to different floors), the command part sends macro- commands (e.g. 'send part i to floor j') to a module called INTERFACE. This module sends elementary commands to the real process (e.g. rotate engine number k of l steps), and is in charge of answering all the macro-commands of the Command Part. The process of managing depends on a functioning decision function which is parametrized by the hierarchical level.

The phases of a complete design of the command of a FMS are shown in Figure 2.

Phase 1: is related to the description of the operations which have to be executed on each produced part (functional description) and the validation of the description (e.g. milling then turning then assembly). Phase 1': is concerned with a top-down hierarchical specification of the process used in the studied FMS: means of production, means of transport, capacity of the storage areas. This phase is independent from phase 1. Phase 2: aided generation of the operating sequences from both phase 1 and 1'. The generation is developed

Hierarchical Level

allocat~ion \ strategy functioning

strategy Storage Allocator

Command Part

macro- commands commands

Figure 1. FMS general structure

Operative Part

I Description of the operatlons~ (Top-down description oq

=o.o,.o.,,o.o, .Ooo.coJ I ) .'

' f Specification of the access

Lprotocotes to ~he physical placesJ

--...... I Generation of the implementable~

command:, simulation J

Figure 2. Phases of the complete FMS design

and validated step-by-step and hierarchically, according to the depth of the hierarchical decomposition of the process. Phase 2': generation of the INTERFACE module, using the specification of the strategy of functioning of the process. Phase 3: for each produced part, specification of the access protocols to all the physical places used by the operating sequences: storage areas, means of transport, means of production. Phase 4: link between the operating sequences and the INTERFACE module. Simulation and implementation.

Hence, in this paper we propose a method to generate the operating sequences step-by-step and hierarchically by a progressive refinement of the process specification.

Structural decomposition of the process

In this section we discuss a method for identifying sub-objects in a process.

The aim of this decomposition is to obtain a description of the process in terms of mutually included objects. So 'inclusion relationships' are built between the identified objects. We point out that each object must be significant towards the studied process: such an object will have several states (stable or not) on which we can apply commands. In fact, the objects we isolate are material objects (rail, waggon, loading place, shelf).

We describe the 'inclusion relationship' with a tree-net. The root of the net is the object which contains the whole system (for example, the cell, the ground). Then we point out all the material objects of the process which are significant from the command point of view. For example, a rail must be pointed out because it can contain moving objects (a waggon, for example).

A top-down approach o f description is quite powerful if a multiple level model is necessary. In a design analysis it can greatly improve the validation phase, for we may validate one level and choose among several possibilities for a more detailed level. For example, a conveyor can be seen first as a huge storage area (level 1), containing 3 or 4 sections (level 2), and finally each section can be composed of some rails (level 3), consisting of basic storage areas (level 4).

Actually, any object which can occasionally contain one or more moving objects, or not, must be pointed out.

Let us consider an example of structural decomposition in Figure 3. The process is composed of two rails, one of them containing a waggon with three loading places (LP). The tree description of the process is then:

Ground Rail- 1 LP waggon LP- 1 0 LP-2 0 LP-3 basic part-1

Rail-2 LP O

Vol 4 No 3 Augus t 1991 141

Figure 3. Structural &composition example

Let us consider ah the isolated objects:

rails: are made of successive elementary storage areas. We consider in our example that the whole rail is composed of a unique loading place, which is loaded or not. Such an object has different states (empty, loaded), and can be commanded. waggon or Automated Guided Vehicles (AGV): are moving objects on several possible rests. They have different states and contain several loading places (also considered as objects). loading places: motionless on the waggon and can be loaded or not. They have at least two states.

The decomposition is done in order to isolate monofunctional material objects. In our example we have:

function of the waggon = positioning on the rail function of the loading place = storage of 1 object (basic storage area)

This restriction can be applied in all the manufacturing systems we have studied up to now.

This method of structural analysis is used to describe the process (machine tools, means of transport of any kind. . . ), and also to describe the produced parts. The main idea is that the same formalism is used to model the means of the process and the produced parts in order to have an homogeneous description. This description is used in several places of the CASPAIM project: in the design phase and in the exploitation phase.

In the design phase, the specification of the process is used to generate the operating sequences, and in the exploitation phase as a model of the process in order to manage its failures and its degraded modes of production.

Let us consider a turning-lathe (Figure 4), which is a complex process from the analysis point of view. The decomposition method allows all the parts to be isolated which:

l occasionally contain other objects l are moving from one object to another

The above method allows the step-by-step identification of all the objects which are significant on this process. It leads to a height level tree description:

Loading place

JawS

Centre stock

$4

- _

Turret Tao1 store

I / Loading plac*l

Longitudinal slide

\

Crbn slide

I

Tail stock Bed

Figure 4. Lathe decomposition

turning-lathe centrestock jaws LP 0 bed longitudinal-slide

cross-slide turret tool-store LP-1 0 LP-2 0 LP-n 0

tail stock

The same description is used to have a dynamic image of the process, related with the exploitation phases of the real process. For example, the turning of the basic part-2 by the turning-lathe with tool-l can be modelled as below:

turning-lathe centrestock jaws LP atomic part-2 bed longitudinal-slide

cross-slide turret tool-store LP-1 0 LP-2 tool-l LP-n 0

tail stock

Description of the produced purts Any part which is produced by the means of production moves through several elements of the process. Since only discrete operations are considered in our study, it is plain that all the produced parts at any stage of their production can be assimilated to one object in the manner previously described. Therefore, it is neces- sarily located on another object and moves from one object to another. Hence an homogeneous description of both the process and the produced parts is possible, since both discrete/continuous and continuous production systems are not taken into account.

142 Computer-Integrated Manufacturing Systems

P3:q P?:q ~ r37:q Part-1 Part-2 Part-1 Part-2 ..__.- Loading place

I I,:-:,-,I I I i,:-:,:,l I I I-:-:,:I I I ~ r . , : , : , . . : , : , : , . , : , . . . , . , . , . , . , . , . , . , . , . , . , . , . , . , : - , . ; , .~ Tray

" " " L o a d i n g place

J,:.:<,l b-~z3 Part-12 Part-12 Loading place I r . ; ; .1 I I r - ' ; - I I I _ _ 1 I I - + ~ - ~ - ' ' - -

I l-',','-'-'-'-'-'-:-:-:-:-'-'-','-'-'-'-'-','-','-','-'-'~ ~ Tray

L o a d i n g place

Figure 5. Process and produced parts description

Using this method of description, complex parts such as trays can be efficiently described. For example, let us consider shelves contining trays of parts which must be assembled together to make a final part (see Figure 5). The method leads to the following description of this process:

before the assembly operation: shelf LP tray LP-1 part-1

LP-2 part-2 LP-3 part-I LP-4 part-2

after the assembly operation: sheff LP tray LP-1 part-12

LP-2 part-12 LP-3 LP-4 ~)

The tray is considered as a produced part because the aim of the production cell is to manage all the parts it contains. It is considered as finished when all of its parts have been processed.

It is also considered as a means of transportation because it is used to bring the parts from a storage area to the production cell, then to carry the parts through the cell during the production, and finally to evacuate the assembled parts.

This example illustrates the necessity of considering the tray to fulfil two functions: as a produced part, and as a transportation means at the same time. Let us consider the example of production cell of Figure 6:

STORAGE LP-I LP-2 LP-a

CONVEYOR RAIL-I LP-1 LP-n

RAIL-2 LP-1 LP-n

TURIqnqG-OUT-I LP TURNIHG-OUT-n LP

ROBOT-I TRUNK ARM ROBOT-2 TRUNK GRIP-I LP

GRIP-2 LP GREASING ROBOT ARM GRIP TURNING LATHE CENTRE STOCX ASSEMBLY AREA LP-I

LP-2 NUT TAPPER LP

]FOREARM GRIP

G R E A S E R JAWS

I Storage Area (3000

01300

Robot-

Turninq LaUte

I--0- o TO~ =.

Greasing Robot

RobOt-2 TPJnk

Grip- I Grip-2

NN Assembly area

Figure 6. Flexible cell layout

A basic part can move in the production cell either from one loading place to another, or on a waggon in the conveyor. The w a g o n can contain one or more loading places. The conveyor is then considered as a complex means of transport. It can be used to load up an object somewhere and unload it elsewhere. The object can be elementary (basic part) or complex (tray of basic parts).

Accessibility relationship between objects

The aim of any production means is to carry parts from one place to another. This possiblility of movement is to be expressed in detail during the design phase. Therefore, 'accessibility relationships' are specified between the identified objects.

The accessibility relationship between Objectl and Object2 means that the object which may be included in Objectl can be carried to Object2. A simple arrow represents this relation in Figure 7.

Objectl, a tray for example, contains three elementary loading places. Each loading place (LP) of Objectl can be in relation with the LP of Object2 or not.

I Object 1 ] Object 2

LP-2 [ ]

I kP-n I - I ~ ~ Accessibility r e l a t i o n s h i p

Figure 7. Accessibility relationship

Vol 4 No 3 August 1991 143

In most of the real cases, all the LP of a storage ~rea has the same accessibility relations with other objects. For example, the grip of a robot can load and unload parts of any LP of a tray. So aggregated relations are expressed with a dotted arrow, as shown in Figure 8.

All the accessibility relations are specified for the considered process as shown in Figure 9.

Considering these relations, we can verify that at least one circuit from the input point to the output point exists. All the possible routings from one object to another can be deduced from these relations. In the next section, produced parts will be taken into account.

Tray ~ ~R°b°t is for

Figure 8. Aggregated accessibility relation

Description of operations applied on each produced part

This section is about the design of the sequences of operations which have to be achieved for each part to be produced. No assumptions of the means of production are made in this section.

The model used in a first approach is a Petri net. A finite state automation is built for each kind of produced part where:

one place is associated to only one state of the produced part in its operation sequence, one transition is associated to only action applied to the part. This transition models a state transfer from one physical state to the next one.

Let us consider for example part P1 in Figure 10. Part P1 is being milled then washed, so has three successive states.

Turning lathe Centre stock

Jaws I LP [ ]

Cell

Nut tapper [ ]

Storage area

r-I D D []

D r-ID []

Rail 1

Conveyor

i~ ~ . ~ Rail 2 TO1 I"I DDDD

I Rail 9 I DDDD

Rail 10

Rail 3

Rail 4 DDnD

Rail 7 J ~ Rail 6 TO4 [] DODD

Rail 5 D D D

Robot-1 Trunk

LP []

Greasing robot Arm

G r i p

Greaser R o bot L LP,OII I

Assembly area

Figure 9. Accessibility relationships on the cell


The assembly or disassembly operation is equivalent to a 'rendez-vous' synchronization between parts. The assembly of P1 and P2 leads to the disappearance of the original parts P1 and P2, and the appearance of a new part, P12. This new part can be treated like any other for milling or assembly operations (for example, like in Figure 11).

Let us consider a flexible sequence of operations. If the order of operations can be swapped in some cases, it leads to an indeterministic Petri net. The example of Figure 12 is about a part which has to be milled and turned, in any order, then to be painted. It is clear that the initial and final states of this part are the same, but an alternative choice appears for the first operation to be done: milling or turning.

PI

Rough Mil l Milled Wash Milled+Washed

I = - 0 I ~ 0 0 I I

Figure 10. Successive states in a sequence of operations

Rough Mil led Milled+Washed Mill Wash

Rough

p2 ~

PIP2

AssemDle

gh Dried

iY=o

Figure 11. Assembly sequence example

Case 1

Milled+ Rough Mil led Turned Painted

Mi l l Turn Paint

Turn e d Rough Turned +Milled Painted

Turn Mil l Paint

Case 2 ~ - I

Figure 12. Flexible sequence

Mi l led M i l l Tu rn

~ Turned ÷

Rough Mi l led Painted

C ~ T u r n Turned M i l l ~

Figure 13. Aggregation of a flexible sequence

Figure 12 shows the two possible sequences of operations for this part. Since the two states milled + turned and turned + milled are considered as being equivalent, an aggregated automaton is being built for the part (see Figure 13).

Processing of virtually linked parts

A number of parts which have to be treated together are called a set of Virtually Linked Parts (VLP). Let us consider a simple example. An electronic card and its separate components are considered as logically linked together, and form a set of VLP. It means that each component is associated to one particular card. The electronic card assembly operation consists of carrying all the components of the card on the card itself.

This simple example illustrates the limits of the Petri net model used previously. Actually, the logical link between objects cannot be modelled. This is why the Object Petri Net (OPN) model is necessary each time a set of VLP has to be managed. An object token carries a data structure which can be tested in the firing conditions of the transitions of the net.

In the previous example, both the set of VLP and each basic physical object have to be treated simultaneously. The set of VLP is considered as an object because the separate parts are logically linked first, then processed, and finally finished. The Petri net which can model this treatment is shown in Figure 14.

However, the logical links between the objects cannot be modelled, so the OPN is used. One class per used object is defined. The fields of the token are:

class field class' instance a-set-of-LVP my-card a-card

my-comp 1 a-kind- 1 -component my-comp-2 a-kind-2-component

a-card my-set-of-VLP a-set-of-VLP a-kind-l-component my-set-of-VLP a-set-of-VLP a-kind-2-component my-set-of-VLP a-set-of-VLP

(The OPN of this example is shown in Figure 15. )

This example shows how the fields of the token have to be chosen. If we consider a set of VLP (VLP), it is

I Getting I Rough processed Finished

I R ~ h I - " ~ ed

',@ =l

~ As~mble

Set of V LP

Electronic board

Component 1

Component 2

I

I

L

1

I

I

I

Figure 14. VLP processing


Set of VLP (VLP)

Electronic card (ca)

Component(d)

Component 2 (c2)

Getting Rough processed Finished

V LP disassemble V LP assemble

0 ---' =0 ..

- ' ° _ _ .

] ~Rough ca.drill Drilled ca.assemble Finished

t - - - - . , ~ - - ~ - - - I cl.my-set-of-VLP= I I Rough / ~" I c2.my-set--of-VLP I

,

Figure 15. OPN of the example (see text)

clear that, since it is fictitiously separated into several parts, each part it is composed of must be refered to on the token of the set of VLP. So, a field per separate part is defined (my-card, my-compl, my-comp2). At the same time, the only link between the separate parts being the set of VLP they come from, a field is defined to memorize the origin of the part (my-set-of-VLP).

So the preveious configuration of fields is absolutely necessary to guarantee the consistence of all the logical links between the objects. The minimum configuration of fields is shown in Figure 16.

This is why the assembly condition of compl and comp2 with the electronic card is the coincidence of the set of VLP they come from.

I Class a-card Fields Class of fields k my-set-0r-VLP a-set-of-VL~J "%

, v% N.

~ fClass a-set-of-VLP "~ fc , - c,asso,,,ol,,s / I l.l c=so,.o, i " c r o,I ~my--set-of-VLP a-set-0f-VLP-- - - y - . ~ ..~my-comp, a-kind-l-component

~¢. my-c0m p2 a-klnd-2-c0mpone

(Class a-ktnd-2-component ~, " : " ~' j ,," I Flel~ Classof f ie lds . ' k Lmy-set-of-VLP a-set-of-Vl.P I )

Figure 16. Minimum configuration of fields

However, the designer may wish to add a few fields for his own use. Up to now, the electronic card is seen as a produced part like any other. According to the process point of view, the electronic card is seen as an object containing two loading places. These places are empty first, then receive the components during the assembly. So, two fields can be added on the a-card class:

class field class' instance

a-card my-set-of-VLP a-set-of-VLP my-LP1 ~ or a-kind-l-component my-LP2 ~ or a-kind-2-component

The assemble method of class a-card is now:

assemble (ca) : ca.my-LP1 := cl ca.my-LP2 := c2

At last, the assembly operation of the three parts is equivalent to the loading of the two components on the electronic card, which is seen both as a produced part, and as a support of other parts.

Set of VL P of several identical par ts

Let us consider a set of VLP of n identical parts coming from a tray. These parts have to be mil led, for example, then to be reassembled on the original tray. The OPN graph is shown in Figure 17.


Set of VLP (VLP)

Rough VLP.dlsassemble

Part(p)

C ~ m Getting VLP.assemble Finished

Processed

~ AND(o.my-set-of-VLP-VLP) with p In VLP.nb-of-parts

VLP.nb-of-parts VLP.nb-of-parts times tlmes

VLP.part p Rough p.mill HI~led

Figure 17. OPN as a set o f VLP

The class of objects are:

class field class' instance a-set-of-VLP nb-of-parts integer

my-part a-part a - p a r t my-set-of-VLP a-set-of-VLP

Therefore, the management of included sets of VLP is also possible. This power of description seems enough to model the management of all the real cases which have been encountered up to now.

Let us examine the example, which will be fully explained in the following sections. It deals with the processing of screw-bolts. The screw and the screw-nut are logically linked together and are processed separately. So each couple screw + screw-nut is a set of VLP. These couples do not belong to sets of VLP themselves, so there is no logical link between two couples. The four treated objects are: the set of VLP, the screw, the screw-nut, and the screw-bolt.

The automata of the objects are shown in Figure 18.

This graph describes the sequence and the synchronization of the operations which have to be applied on all the objects, without taking into account the place where these operations happen.

Aided generation of operating sequences

The location of the operations is now introduced. The aim of this section is to reach a complete description of the operating sequences, that is to give a description where both the location of the part and its achievement degree appear. So the previous Object Petri Net is being enriched, since the state of an object is now the couple (location, achievment degree). A location place being part of the process, the structural tree description of the process is used for the design of the operating sequences. However, it seems difficult to make the entire design using the whole description at first.

The operating sequences method of generation is based on a step-by-step approach. The idea is to design

Rough VLP.disassemble Gettlngprocessed VLP.assemble Finished o,

VLP.screw - ~ v b ; y - s e t ~ _~sb.my-set-of-VLP Rough s.wlre-draw Wire-drawn

Screw(s) s ~

. . . . . . . VLP. _ . . . . . . .

~ Rough sb.grease Greased Screw-bolt(sb) I s.my-set-of-VLP= 1

[ sn-my-set~°f-VLP I ~ =

Figure 18. Automata o f the objects


the sequences by a progressive refinement of the process. In the first step, the process is assimilated to the first level of the structural description. In our example, the cell is abstracted to its first level objects. In addition, each object is considered as containing all the loading places which are the lowest levels of the subtree it is the root of. So, in our example, the process used for the first step of the generation is:

CELL STORAGE LP-1 LP-2 LP-n

CONVEYOR LP-1 LP-n

ROBOT-1 LP LP

GREASING ROBOT TURNING LATHE LP ASSEMBLY AREA LP-1

LP-2 NUT TAPPER LP

Then the operating sequences are built according to this simple specification and the behaviour, dead-lock and performance studied in order to validate the first step of the generation.

Cell

//!, Turn,og Lathe I I "°~ta00er I D_____R_.__] I [] I

/I oo

•[]DrlOrl

Greasing Robot ] Robot2 ~ Assembly area

DD I

Figure 19. Accessibility relations

The process then becomes more detailed and the second level of the description of the process is taken into account, where as in the first step, the terminal objects are considered as containing as many LPs as the lowest level nodes of the subtree they are the root of. Here again, behaviour, dead-locks and performances are studied.

This step-by-step generation ends at the last level of the description of the process. Moreover, in a full design phase, considering that some components such as conveyor architecture and robot configurations are not yet established, this step-by-step generation allows the basic evaluation of several possible variants to be achieved.

Let us examine the first two steps in our example.

First step of the generation

As has been explained, the process is assimilated to the first level of the tree description. From the accessibility relations described in Figure 9, the accessibility relations can be built as in Figure 19.

The operating sequences are developed according to the following designer specification:

• a location of operation is associated to each state which is figured out in the graph of the previous section;

• the necessary condition this specification has to verify is that, considering one produced part, at least one path must exist in the accessibility relations between two consecutive location places the part has to go to.

In the example, the specification of Figure 20 satisfies all the above conditions.

This OPN is being developed in order to have, for each produced part, a finite state automaton for which

Rough VLP.dlsassemble Getting processed VLP.assemble Finished

Set of V, P @ -- I " ~ ~ o ~ g e Iv,p, " ~ = Q = Storage Area Area Storage Are

VLP.screw_ ~ eT-or- V~LP \\.oo,o s w , r e o aw .C;:-. ,..ew,., - -

w_.',~°~vey°r c~e~r x J _ _ _ v L p , c r e -o~, . . . . . . . - k - - 1

Rouhg sntap Tapped ~ I

Screw-nut(sn) ~ ~assemble /

. . . . . . . . _ /_

screw-oo.,~> I~ ~y-~et-of-wP°l i s n . m y - s e t - o f - V L P l v , v

k~ Conveyor Conveyor

Figure 20. Operation sequence


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the following requirement is achieved:

ei ther the part is processed from one physical state to the next one, or the part moves from one location place to the next one.

It is never possible to simultaneously have a process and a transport of a part.

This leads to e lementary transitions from the produced part point of view, which are associated to only one action (movement or manufacturing). So additional states are introduced in the OPN. The resulting graph is shown in Figure 21.

This operating sequences OPN is complete, according to the first level description. The next section discusses on the access protocols. Since any part moves from place to place, access conditions (access protocols) are being specified. Adding the operation durations, an evaluation of the whole system is possible, even with approximate values.

Moving from the first to the second step leads the designer to detail the process in sub-objects.

Second step of the generation In the second step, the process is assimilated to the second level of its t ree description. In our example the second level is:


CONVEYOR RAIL-1 LP-1 LP-n RAIL-2 LP-1

LP-n TURNING-OUT-1 LP TURNING-OUT-n LP

ROBOT-1 TRUNK LP ROBOT-2 TRUNK LP-1

LP-2 GREASING ROBOT ARM TURNING LATHE CENTRE STOCK LP ASSEMBLY AREA LP-1

LP-2 NUT TAPPER LP

The accessibility relations between these objects are shown in Figure 22.

0

1

1

Turning lathe

Centre stock

[]

Cell

Greasing robot

Arm

Conveyor

OOOO

0000

[] []

Nut tapper

[]

Rail 3

Rail 10 Rail 4

OOOO

Rail 5

0 0 0

I LP-,oll LP-2o I Assembly area

Figure 22. Accessibility relations


The conveyor Let us focus on the conveyor. From the functional point of view, the conveyor is a complex means of transport. In our example, it has four input and output points. However, as it shown in Figure 22, any object can move from any input point to any output point. So, from the produced part point of view, the conveyor is seen as a complex means of transport, composed of several mutually reachable input and output points. So, new indirect accessibility relations are added on Figure 22, as shown on Figure 23.

From the produced part point of view, the trip inside the conveyor does not matter. The only important point is to know which output point it has to go to.

As was described in the first step of the generation, according to the second level specification of the process the designer has to specify the places each part has to go to. It leads, for example, to the OPN of Figure 24. We have to verify that two successive places of one part are reachable.

Then, as in step one, the graph is developed in order to move from one place to the next, or from one achievement goal to the next one. This complete graph is shown in Figure 25.

Third step of the generation In step three, the process is being refined once more. As far as the conveyor is concerned, the transport of parts can be achieved according to several alternatives. They can either be transported on simple pallets, or on trays with several location places. This last solution allows the carrying of several sets of VLP at the same time, and the increase of the global flows of sets of VLP to be managed. It is the solution which is considered now, and trays, composed of several loading places will move on the conveyor. The third level decomposition of the process is shown as follows:


CONVEYOR RAIL-1

RAIL-2

TURNING-OUT-1 TURNING-OUT-n

ROBOT-1 TRUNK ROBOT-2 TRUNK

GREASING ROBOT ARM TURNING LATHE CENTRE STOCK ASSEMBLY A R E A LP-1

LP-2 NUT TAPPER LP

TRAY-1 LP-1 LP-n

LP-1 LP-n LP LP ARM LP GRIP-1 LP GRIP-2 LP GRIP JAWS LP

Considering Robot-2, the two grips are equivalent which means that when a part is to be loaded on the robot either of the two grips can be used. This means that, on the graph which is generated, an alternative will be expressed each time Robot-2 is to be used.

Let us consider a part P which has to be loaded onto Robot-2. The state graph of part P is shown in Figure 26. Since the two grips are equivalent, an aggregated

Conveyor

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Figure 23. Indirect accessibility relations

graph is used where the two grips are grouped together in one virtual grip, Grip-i, as shown in Figure 27.

This aggregation is used in the graph of Figure 28 which is Figure 25 modified by the third step in the generation.

This method of generation leads up to the last level of the structural description of the process, that is the 8 th level in our example. It allows the specification of the operating sequences to be expressed step-by-step, and potential alternatives in the design of the architecture to be managed.

The last section discusses the specifications of the access protocols to the physical places.

Specification of the access protocols

Each part moves from one loading place to another. So the access protocols to all the LPs must be defined. The occupation of the LP, being loaded or not, is modelled by an object token. On the operating sequences graph, one place per loading place is added.

Let us consider a simple example. It leads with the transport of a part P which has to travel from place X to Y, then from Y to Storage, from Storage to Y, and from Y to Z. Another part P' travels through places X, Y and Z. The state graph of part P and P' is shown in Figure 29. The places X, Y and Z are elementary LP, and storage is composed of three LPs.

This graph is being expounded by adding one place per LP. One token at this place models the fact that the LP is free to be loaded. The transfer of a part to LP X requires the allocation of the LP to the part. So the token is taken away from its associated place before the transfer, and is given back afterwards. It leads, for example, to the graph of Figure 30.

However, the designer may wish to define other protocols. In our example, LP Y can stay allocated to part P while part P is located on the Storage. This means that the token which models LP Y is not given back when part P is loaded on the Storage.

Moreover, the LP can be allocated long before the part has to be loaded on it, as shown on Figure 31 with part P'.

So, the access protocols to the loading places are defined for each produced part. These protocols can be defined in any manner according to the designer specification.


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Figure 30. Protocols (see text)

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Figure 31. Another protocol in the sequence (see text)

An example of the access protocol to Robot-1 is shown on the first step of the generation of operating sequences in Figure 32 (see page 156).

This graph enables an evaluation of the behaviour and performances of the cell, and the identification of all the access conflicts to physical places. Deadlocks can also be found on this graph. This work is done on each level of the operating sequences generation in order to validate the step-by-step specification.

Conclusion

In this paper, an original method for the generation of the operating sequences has been explained. It is based on a step-by-step specification and validation, which follows the tree description of the process. It leads to a complete graph where both the sequence, parallelism and concurrency of elementary operations appear. This graph is detailed automatically into simple commands which are sent to the real process.

References

1 Bourey, J P 'Structuration de la partie proc6durale du syst~me de commande de cellules de production flexibles dans l'industrie manufacturi~re', Th~se de Doctorat d'Universit~. Universit6 de Lille Flandres- Artois, France (March 1988)

2 Bourey, J P, Castelain, E, Gentina, J C and Apusta, K M 'CASPAIM: A computer aided design of the control system of FMS' IMA CS Ann. Comput. Appl. Math. (1989) pp 131-135

3 Castelain, E 'Mod61isation et simulation interactive de cellules de production flexibles dans l'industrie manufacturi~re', Th~se de Doctorat d'Universit6. Universit6 de Lille - Flandres-Artois, France (Febru- ary 1987)

4 Craye, E 'De la mod61isation ~ l'implantation automatisre de la commande hirrarchis~e de cellules de production flexibles dans l'industrie manufactur- irre', Thrse de Doctorat d'Universitr. Universit6 de Lille - Flandres-Artois, France (January 1989)

5 Kapusta, M 'Une premirre 6tape de conception assistre du modrle de la partie commande de cellules flexibles de production dans l'industrie manufactur- irre', Thbse de Doctorat d'Universitr. Universit6 de Lille - Flandres-Artois, France (December 1988).


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