the criteria of the earliest stopping point. Whatever
the case, in order to manage more easily the
complexity of the analysis of the behaviour of his
process, the designer can first identify only the
independent physical processes or those interlinked
by causal precedence and temporal precedence
relationships, and only then can he proceed in a
more local way to a breakdown into coupled
physical processes. In the first instance the
relationships of causal and temporal precedence are
defined, and only after that the coupling
relationships. The textual formalism that we are
using allows us to key in and carry out automatically
the usual syntactic checks on the specification of the
behaviour of a process. It is then possible to convert
this specification automatically into a Petri net using
the conversion rules defined in Table 1 and in (Yan
and Caglayan, 1983). The existing tools surrounding
Petri nets thus allow us to assure that the described
behaviour respects certain properties of good
functioning: mutual exclusion, deadlock, liveness
and termination (Kara and all, 2009). Moreover, as
the places in our graphical model are associated with
physical states of the process, we can deduce
automatically all the accessible global states of the
process by constructing a graph of markings. These
global states must describe coherent situations.
4 LOGICAL STRUCTURE
CONSTRUCTION RULES
To describe the logical structure of a control system
we use the concept of communicating modules.
These are modular, multi-task entities which do not
communicate by variables, but communicate with
one another (for coordination purposes) via internal
ports and with the process (which they control) via
external ports. The inter-modular links defined
between the ports can be of different types:
1 towards 1, 1 towards n, n towards 1 or n towards
m. They allow us to describe the logical structure of
the system as a logical network of autonomous
communicating modules, in which checking is
decentralised, and where the following circulate:
- events reporting on the evolution of the process,
- controls, requests and reports,
- or again the data or the results of the data-
processing.
In this description, the modules are represented
graphically by rectangles, the input ports by the
symbol and the output ports by the symbol
The logical structure of a control system in terms
of communicating modules is largely directly
deduced by the graphical representation of the
process behaviour described in Section 2:
(1) Each transition is replaced by a communicating
module which abstracts the corresponding physical
process p
i
.
(2) The arcs which interlink the transitions thus
become inter-modular links via which control
transfer messages will circulate.
(3) Analysis of the control algorithms of each
physical process allows us to determine, for the
corresponding modules, the other ports where will
circulate external messages exchanged with the
process, as well as potential data shared between
these modules.
5 ILLUSTRATION OF THE
APPROACH
To illustrate our approach, we use as our example a
mixer, a test example, which has the feature of
bringing into play various physical processes.
This is a process which manufactures a product x by
mixing a given quantity of two liquid products a and
b, and a given quantity of soluble product y that we
call rolls. The liquid products are contained in two
vats A and B which feed vat C via controllable
valves Va and Vb. Vat C is equipped with a level
sensor, which allows it to measure the required
quantity of the two products, and a controllable
valve Vc which allows it to empty its contents into
the mixer. Moreover, the rolls are transported into
the mixer via a controllable, motorised conveyor
belt. There is also a device which detects the
passage of each roll. Finally, the mixer has a
controllable motor which operates both the mixing
process of its contents and also the emptying
process. For this last operation there is a sensor
which can detect the high and low positions of the
mixer.
Table 2 identifies the entities which constitute
the process and whose attributes are likely to evolve
with time. We have also defined in Table 2 the level
of observation of this evolution, by specifying for
each entity the attributes which describe it, and for
each attribute its domain of definition.
Four physical processes lead from one physical
state to another. These are: the emptying of vat C,
the filling of vat C, the transport of the rolls, and the
mixing-emptying of the mixer. The temporal
dependences which interlink these physical
processes define the behaviour of the process.
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