USING THE OAG TO BUILD A MODEL DEDICATED TO MODE

HANDLING OF FMS

Nadia Hamani

Departement of Informatics, Paris X University, 200 avenue de la République, 92001 Nanterre, France

Nathalie Dangoumau, Etienne Craye

LAGIS, Ecole Centrale de Lille, BP48 cité scientifique, Villeneuve d’Ascq, France

Keywords: Flexible Manufacturing Systems, control system, supervision, mode handling, functional modeling.

Abstract: This paper deals with a modeling approach for mode handling of Flexible Manufacturing Systems (FMS).

We show that using the plant model enables to establish aggregate operations. These are generic entities

which depend only on the plant and do not depend on production goals. Aggregate operations are then used

to build the model dedicated to mode handling. This study is illustrated through an example of a flexible

manufacturing cell.

1 INTRODUCTION

We are interested in problems of monitoring and

supervision in a fault tolerant control system

dedicated to Flexible Manufacturing Systems

(FMS) (Ranky, 1990). According to our approach,

the supervision is made up of three functions:

decision, piloting, and mode handling. The

monitoring function (Elkhattabi et al., 1995;

Toguyeni et al., 1996) detects and localizes the

failures at the plant level. The decision function

(Berruet et al., 2000) determines the new

configuration of the FMS. The functions of mode

handling and piloting (Tawegoum et al., 1994)

implement the decisions about the new

configuration of the FMS.

In order to achieve the role of mode handling

within the control system, one should provide

models representing the operating modes of the

production system and its subsystems. The existing

modeling approaches of operating modes of

Automated Production Systems (APS) are

compared in (Hamani et al., 2006). The advantages

of functional modeling approaches are showed.

Such approaches are concerned with the services

delivered by the FMS rather than production means.

Our approach (Hamani et al., 2006) is based on a

functional modeling method. This approach is well

adapted to FMSs because it is based on the mission

concept (a production goal) which represents the

flexibility which characterizes the FMS production.

The obtained model is generic. For a given FMS,

the predefined functional subsystems (called

entities) are instantiated to generate the model. An

aggregate operation is a generic entity depending

only on the plant and not on production goals.

The purpose of this paper is to present a method

to calculate aggregate operations from the plant

model. The paper is organized as follows. Section 2

reminds the basic concepts of our modeling method

and the steps of building the FMS functional model.

Section 3 presents a method to determine aggregate

operations from the plant model. An example of a

flexible manufacturing cell is used to illustrate this

study.

2 THE FMS MODEL

2.1 Basic Concepts

An FMS produces simultaneously a set of parts.

Usually we desire to change production goals. That

is why the mission concept is introduced in

(Hamani et al., 2006). A mission (M) is the subset

of Logical Operating Sequences (LOS) which are

282

Hamani N., Dangoumau N. and Craye E. (2008).

USING THE OAG TO BUILD A MODEL DEDICATED TO MODE HANDLING OF FMS.

In Proceedings of the Fifth International Conference on Informatics in Control, Automation and Robotics - RA, pages 282-287

DOI: 10.5220/0001503302820287

Copyright

c

SciTePress

produced simultaneously. A LOS is a set of ordered

machining functions performed on some parts. A

LOS is noted LOS f

1

...f

n

or LOS f

i

(i = 1, n).

With each function of a Logical Operating

Sequence is associated its possible achievements.

They are aggregate operations for which the

machining operation is defined. An aggregate

operation is a generic entity which depends only on

the FMS plant and not on production goals. An

aggregate operation corresponding to a machining

Major Characteristic Area (MCA) noted

Op

MCA_machining

is a set of the corresponding

elementary machining operations and Access

Transfers. MCA concept is defined in (Hamani et

al., 2006).

In an FMS, an operation (Op) is defined as a

function carried out by a resource (Berruet et al.,

2000; Toguyeni et al., 2003). An operation is noted

Op

Ri, fi

where f

i

is the performed function and R

i

the

resource which implements it. An elementary

operation is an operation carried out only once,

continuously, i.e. without the possibility to choose

another alternative during the normal execution of

the operation.

Access Transfers (TrA) associated with a

machining area (or a MCA), noted TrA

machining_MCA

,

correspond to the set of elementary transfer

operations that connect this area to the other MCA

of the FMS. An elementary transfer (TrE) is

performed by one resource between two MCA. An

elementary transfer is noted

DS

i

R

TrE

→

with S a source

CA, D a destination CA, and R

i

the transfer

resource.

2.2 The Specification Steps

The specification steps (Figure 1) of the FMS

functional model are described in the following.

1

st

Step: Identification of the entities of the model

- list the missions that the FMS should carry out

- list for each mission its corresponding Logical

Operation Sequences

- for each Logical Operating Sequence identify the

corresponding machining functions

A machining function is implemented by one or

several elementary machining operations. Each one

is belonging to an aggregate operation.

- identify the aggregate operations of the FMS (see

the 2

nd

step)

- for each aggregate operation, identify the

resources which perform it (see the 3

rd

step)

2

nd

Step: Determination of aggregate operations

(Figure 2). For each machining area of the FMS:

- identify elementary machining operations which

are performed in this area

- identify the Access Transfers related to this area

- gather elementary machining operations together

with Access Transfers identified previously to

obtain aggregate operations

3

rd

Step: Determination of the resources that

perform elementary operations

For each aggregate operation:

- associate with each elementary machining

operation the resource or the configuration of the

resource (in the case of a polyvalent resource)

which performs it

- associate also with each elementary transfer

operation the resource (or the resources) which

performs it, redundant resources are linked with a

logical OR.

The aggregate

operations

of the FMS

The FMS

resources

Set of missions

Determination o

f

FMS mission

s

Determination of logical sequences

Set of lo

g

ical sequences

Determination of a

gg

re

g

ate

operations

Set of a

gg

re

g

ate operations

Determination of machinin

g

and

transfer resources

Set of resources

Determination of machinin

g

functions

Set of functions

Figure 1: Specification steps of the FMS entities.

Set of machining

operations

Determination of

machining operations

Set of access

transfers

Machin ing area

Determination of access

transfers

Aggregate operation

Gathering

Figure 2: Aggregate operations specification.

USING THE OAG TO BUILD A MODEL DEDICATED TO MODE HANDLING OF FMS

283

The functional model of the machining cell is

represented using the following entities:

• The missions

• The Logical Operating Sequences

• The machining functions

• The aggregate operations

- elementary machining operations

- Access Transfers (set of transfer operations)

• Transfer resources, machining resources

2.3 Illustration Example

Consider an example of a flexible manufacturing

cell (Figure 3) with two machines M

1

and M

2

and

INPUT/OUTPUT buffers. The machines are loaded

with a transport system using three robots R

1

, R

2

and R

3

and a conveyer (CV). Moving directions of

CV are Z

1

→ (Z

2

or Z

5

), (Z

2

or Z

5

)Æ Z

3

, Z

3

→ (Z

4

or

Z

6

), (Z

4

or Z

6

)Æ Z

1

. It is assumed that M

1

is loaded

with R

1

and M

2

is loaded with R

2

. The parts are

loaded on the conveyor using the robot R

3

. The

machining functions performed by the system are

turning (t) and milling (m). Turning is carried out

by M

1

, milling by M

1

and M

2

.

According to the functional requirements of this

illustration example, three missions can be required

by the operator: M

1

, M

2

and M

3

. The corresponding

Logical Operating Sequences are the following:

M

1

: LOS

1

and LOS

2

, M

2

: LOS

1

, LOS

2

and LOS

12

and M

3

: LOS

1

, LOS

12

and LOS

21

The machining functions which compose each

Logical Operating Sequence are the following:

LOS

1

: turning; LOS

2

: milling; LOS

12

: turning then

milling; LOS

21

: milling then turning.

Turning function is performed by the

elementary machining operation Op

M1,t

belonging to

the aggregate operation Op

M1

. Milling function is

performed by the elementary machining operation

Op

M1,m

belonging to the aggregate operation Op

M1

or by the elementary machining operation Op

M2,m

belonging to the aggregate operation Op

M2

.

For the machining area M

1

: the elementary

machining operations performed by M

1

are Op

M1,t

and Op

M1,m

. Access Transfers related to M

1

are

1

M

TrA = AND (

1

Msource_MCA

Tr

→

,

ndestinatio_MCA

1

M

Tr

→

).

This notation is using the logical AND and OR

and also three distinct levels: ‘{’ for the first level,

‘[’ for the second level and ‘(’ for the third level.

Section 3 presents a method to determine TrA

using the plant model.

The aggregate operation related to the

machining area M

1

is Op

M1

= AND [OR (Op

M1,t

,

Op

M1,m

)

,

1

M

TrA ]. The aggregate operation related to

the machining area M

2

is obtained in the same

manner.

Op

M1

is performed by the following resources:

the polyvalent machining resource M

1

performs the

elementary operations Op

M1,t

et Op

M1,m

. For transfer

resources: R

1

performs the elementary transfer

operations

1

M

2

Z

1

R

TrE

→

and

2

Z

1

M

1

R

TrE

→

; R

2

performs the

elementary transfer operations

4

Z

2

M

2

R

TrE

→

and

2

M

4

Z

2

R

TrE

→

; R

3

performs the elementary transfer

operations

1

ZIN

3

R

TrE

→

and

OUT

1

Z

3

R

TrE

→

; CV performs

the elementary transfer operations

2

Z

1

Z

CV

TrE

→

,

3

Z

2

Z

CV

TrE

→

,

5

Z

1

Z

CV

TrE

→

,

3

Z

5

Z

CV

TrE

→

,

4

Z

3

Z

CV

TrE

→

1

Z

4

Z

CV

TrE

→

,

6

Z

3

Z

CV

TrE

→

and

1

Z

6

Z

CV

TrE

→

.

The obtained model (AND/OR graph) for the

machining cell is represented in Figure 4. The

underlined entities are not developed. AND nodes

do not have any notation, however OR nodes are

denoted using +. These nodes correspond to an

M

1

FIFO INPUT

FIFO_OUTPUT

M

2

R

1

R

2

R

3

Z2

Conve

y

er

CV

Z1

Z5

Z4 Z6

Z3

R

robot

M machining resource

Z zone

Accessibilit

y

relation

Figure 3: An example of a flexible cell.

ICINCO 2008 - International Conference on Informatics in Control, Automation and Robotics

284

inclusive OR or an exclusive OR according to the

constraints given in the functional requirements.

For example, an exclusive logical OR is necessary

for safety reasons, like two machining operations

which are performed by the same resource for

instance.

LOS

1

LOS

2

LOS

12

The cell

turning

milling

Op

M1

Op

M2

+

+

LOS

21

TrA

M2

M

1

M

2

M

3

+

Op

M2,m

TrA

M1

Op

M1,m

Op

M1,t

+

Figure 4: An extract of the functional model of the

machining cell.

3 DETERMINATION OF ACCESS

TRANSFERS

In order to determine TrA, a first step consists in

listing symmetrical transfers between MCA

representing both source and destination areas.

Then it is necessary to refine these transfers until

obtaining elementary transfer operations.

Once Access Transfers are determined, it is

necessary to identify elementary transfers which

compose them. If there is a direct accessibility

between two MCA then Tr

MCA_source→MCA_destination

corresponds to an elementary transfer. If not, it is

necessary to refine the transfers between the

Characteristic Areas until obtaining elementary

transfers. The possible paths are then established

and those which are redundant are linked together

with a logical OR. For example:

1

MIN

Tr

→

= AND (

1

ZIN

3

R

TrE

→

,

2

Z

1

Z

CV

TrE

→

,

1

M

2

Z

1

R

TrE

→

).

Due to increasing complexity of FMS, it could

be difficult to identify all the elementary transfers

which compose Access Transfers. That is why we

propose to determine them from the Operational

Accessibility Graph (OAG) (Berruet et al., 2000), a

graph which represents the FMS plant. The OAG

formalizes all the accessibilities between the

characteristic areas more precisely than informal

specifications provided in the functional

requirements.

To build an OAG, a partition of all elementary

operations is carried out and the concept of node is

introduced to simplify the modeling process. This

concept is defined in the following.

A node consists of an elementary operation or

some elementary operations. This regrouping is

governed by rules about the operations taxonomy

(Berruet et al., 2000; Toguyéni et al., 2003). The

nodes form OAG entities and allow relating the

operations using accessibility relations.

Based on this definition, several nodes are

defined: storage, machining, assembly, link, and

transfer nodes. The nodes are then linked together

using accessibility relations in order to build the

OAG.

3.1 The Operational Accessibility

Graph

The Operational Accessibility Graph (OAG) is a

directed graph where nodes are subsets of

operations performed by the resources of the

system and the arcs represent the accessibility

relations between operations (Toguyéni et al.,

2003). The OAG represents all the flexibilities of an

existing plant or a plant being designed. It is

obtained following these steps:

1

st

step- Identification of elementary operations

of the FMS: in this step elementary operations of

machining, storage (passive, active), and transfer

are identified.

2

nd

step- Regrouping the elementary operations:

the elementary operations carried out on the same

area and the equivalent elementary transfer

operations are gathered. A partition of all the

operations is thus obtained.

3

rd

step- Building the graph: a node is

associated with each operations subset established

in the previous step. The nodes of the OAG are thus

obtained. Then these nodes are connected with

respect to the accessibility between operations. The

OAG structure is then determined.

The method is applied to the illustration

example (Figure 3).

1) The elementary machining operations are

already identified (Op

M1,t

, Op

M1,m

, Op

M2,m

) and the

elementary transfer operations (

2

Z

1

M

1

R

TrE

→

,

1

M

2

Z

1

R

TrE

→

,

4

Z

2

M

2

R

TrE

→

,

2

M

4

Z

2

R

TrE

→

,

1

ZIN

3

R

TrE

→

,

OUT

1

Z

3

R

TrE

→

,

2

Z

1

Z

CV

TrE

→

,

3

Z

2

Z

CV

TrE

→

,

5

Z

1

Z

CV

TrE

→

3

Z

5

Z

CV

TrE

→

,

4

Z

3

Z

CV

TrE

→

,

1

Z

4

Z

CV

TrE

→

,

6

Z

3

Z

CV

TrE

→

,

1

Z

6

Z

CV

TrE

→

).

USING THE OAG TO BUILD A MODEL DEDICATED TO MODE HANDLING OF FMS

285

It is necessary to add the following storage

operations:

- Storage IN and storage OUT which are passive;

- Storage Z

1

, storage Z

2

, storage Z

3

, storage Z

4

,

storage Z

5

and storage Z

6

which are active.

2) Concerning the regroupings:

- One gathers Op

M1,t

and Op

M1,m

in a complex

operation on M

1

.

- Linking operations are: Link Z

1

, Link Z

2

, Link Z

3

,

Link Z

4

, Link Z

5

and Link Z

6

.

- The functions fulfilled by the elementary transfer

operations are all distinct. There is no regrouping

of transfers.

3) Table 1 summarizes the correspondence

between the nodes and the operations which

compose them. The resulting OAG is represented in

Figure 5.

Note: on Figure 5, storage nodes IN and OUT, link

nodes as well as machining nodes correspond to

characteristic areas of the cell. The subset formed

only by storage nodes and machining nodes

corresponds to main characteristic areas.

The obtained model is used to calculate the

elementary transfers as shown in the following.

3.2 A Procedure for Determination

Elementary Transfers

Based on the OAG, the following procedure is

proposed in order to calculate Access Transfers.

Beginning of the procedure:

1st Step: determination of Access Transfers associated

with machining nodes

For each machining node of the OAG:

- determine the paths which connect it with the others

machining nodes and the input of the cell;

- determine the paths which enable unloading parts

onto other machining nodes and the output of the cell;

The obtained paths are linked with a logical OR;

End For;

2

nd

Step: determination of elementary transfers which

compose the Access Transfers:

Do again for each identified path in the previous step

If the path relates two successive nodes of the OAG

Then the path is an elementary transfer

If not determine the paths which compose it

The redundant transfers are linked with a logical

OR; do not consider the paths which go over a

transfer node twice and those that contain

intermediary machining nodes;

Until all the obtained paths are elementary.

End of the procedure.

Table 1: The correspondence between nodes and operations.

N1 N2 N3 N4 N5 N6 N7 N8

Storage IN

1

ZIN

3

R

TrE

→

Link Z

1

2

Z

1

Z

CV

TrE

→

Link Z

2

1

M

2

Z

1

R

TrE

→

Machining M

1

Op

M1,t

Op

M1,m

2

Z

1

M

1

R

TrE

→

N9 N10 N11 N12 N13 N14 N15 N16

5

Z

1

Z

CV

TrE

→

Link Z

5

3

Z

2

Z

CV

TrE

→

3

Z

5

Z

CV

TrE

→

Link Z

3

4

Z

3

Z

CV

TrE

→

Link Z

4

2

M

4

Z

2

R

TrE

→

N17 N18 N19 N20 N21 N22 N23 N24

Machining M

2

Op

M2,m

4

Z

2

M

2

R

TrE

→

6

Z

3

Z

CV

TrE

→

Link Z

6

1

Z

6

Z

CV

TrE

→

1

Z

4

Z

CV

TrE

→

OUT

1

Z

3

R

TrE

→

Storage OUT

17

OUT

1

Z

3

R

TrE

→

1

ZIN

3

R

TrE

→

Turning

Milling

on M

1

2

Z

1

M

1

R

TrE

→

1

M

2

Z

1

R

TrE

→

12

8

3

6

Link Z

2

Storage OUT Storage IN

23

24

20

2

21

5

19

10

Link Z

4

4

Z

2

M

2

R

TrE

→

2

M

4

Z

2

R

TrE

→

18

15

16

13

7

Millin

g

on M

2

14

1

Z

6

Z

CV

TrE

→

6

Z

3

Z

CV

TrE

→

11

5

Z

1

Z

CV

TrE

→

3

Z

5

Z

CV

TrE

→

9

Link Z

6

2

Z

1

Z

CV

TrE

→

3

Z

2

Z

CV

TrE

→

Link Z

5

4

Z

3

Z

CV

TrE

→

1

22

4

1

Z

4

Z

CV

TrE

→

Link Z

3

Link Z

1

Figure 5: The OAG of the illustration example.

ICINCO 2008 - International Conference on Informatics in Control, Automation and Robotics

286

For each machining node, the access paths

which are associated with it, added with machining

operations carried out on this node, are linked with

logical AND. This regrouping is an aggregate

operation.

For the illustration example, the access paths

calculated in the first step of the procedure for the

machining area M

1

are the following: OR

(

1

MIN

Tr

→

,

12

MM

Tr

→

) and OR (

2

M

1

M

Tr

→

,

OUT

1

M

Tr

→

). The

following elementary transfers are then obtained

using the second step of the procedure.

1

MIN

Tr

→

= AND (

1

ZIN

3

R

TrE

→

,

2

Z

1

Z

CV

TrE

→

,

1

M

2

Z

1

R

TrE

→

);

12

MM

Tr

→

= AND (

4

Z

2

M

2

R

TrE

→

,

1

Z

4

Z

CV

TrE

→

,

2

Z

1

Z

CV

TrE

→

,

1

M

2

Z

1

R

TrE

→

);

21

MM

Tr

→

= AND (

2

Z

1

M

1

R

TrE

→

,

3

Z

2

Z

CV

TrE

→

,

4

Z

3

Z

CV

TrE

→

,

2

M

4

Z

2

R

TrE

→

);

OUTM

1

Tr

→

= AND {

2

Z

1

M

1

R

TrE

→

,

3

Z

2

Z

CV

TrE

→

, OR [AND (

4

Z

3

Z

CV

TrE

→

,

1

Z

4

Z

CV

TrE

→

), AND

(

6

Z

3

Z

CV

TrE

→

,

1

Z

6

Z

CV

TrE

→

)],

OUT

1

Z

3

R

TrE

→

}.

Finally,

1

M

TrA = AND [OR (

1

MIN

Tr

→

,

12

MM

Tr

→

), OR

(

2

M

1

M

Tr

→

,

OUT

1

M

Tr

→

)].

4 CONCLUSIONS

In this paper our modeling method dedicated to

FMS

mode handling is extended. The FMS

functional model is obtained by a modular and

hierarchical decomposition leading to the

elementary machining and transfer operations. For

large scale systems, it is difficult to obtain all

possible redundancies of a plant. So we propose to

determine aggregate operations associated with

machining areas from the plant model represented

by the OAG. The aggregate operations are generic

concepts which depend only on the plant and not on

production goals. Such method enables to generate

automatically aggregate operations for an existing

system or a system being designed. The proposed

modeling steps are then illustrated through an

example of a manufacturing cell.

Further works aim at implementing the proposed

method within the information system

CASPAIM_soft (Ndiaye et al., 2002).

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