
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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