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standable, we will adopt simplified system requirements. The experimental conditions
are inspired by [3]. Robots evolve on a football field. A video recorder system makes it
possible to know the position of each robot as well as of the ball. These positions are
periodically broadcasted to all robots. If the ball goes out of the limits of the field, a
robot of the nonfaulty team recovers the ball and plays (the order is given by the ref-
eree). If a robot has no more battery or is dysfunctioning, the match is stopped (the
order is given by referee for human safety reason) and the robot is withdrawn from the
field: all robots must be then motionless. At the beginning of a match the robots must
be located in their camp and the referee decides to give the gardian role to one robot of
each team. So, the game is open and the team, which marks the higher number of goals
in 90 minutes, wins.
3 Definition of needs
This preliminary stage begins by analysing the physical context of the system (identify-
ing workflow, main tasks, etc...). Then, we study the different actors and their participa-
tive user cases (using UML use case diagrams), the services requirements (using UML
sequence diagram) of these actors.
The second step consists in the study of the modes of steps and stops. This activity
is very significant because it enable to structure the global running of the system. It is
generally wishable that the system functions in autonomy. But working with physical
systems imposes to know all the other possible behaviors precisely when the system
starts, when it goes under maintenance etc.
This activity puts forward a restricted running of the system. It allows to specify the
first elements necessary for a minimal fault-tolerance. Moreover, it enable to identify
cooperative (or not) situations and to define recognition states in order to analyse, for
example, the self-organizational process of an application. This activity allows to take
into account the safety of the physical integrity of the users possibly plunged in the
physical system.
We have defined 15 differents modes that we regroup in 3 families. The stops modes
which are related to the different procedures for stopping and to define associate recog-
nition states. The steps modes which focuses on the definition of the recognition states
of normal functionning, test procedures etc. The failing operations modes which con-
centrates the procedure allowing to a human maintenance team to work in the system
or to specify rules for restricted running.
Application to our case study. We find the following actors. The referee (logical actor)
manages match parameters (choice of a goalkeeper and a camp for each team, verifica-
tion that robots respect the rules) and authorise the human to withdraw a robot when all
robots are motionless. The manager (physical actor) withdraws robot when a problem
occurs. The ball (physical actor) moves under the robot actions. The opposing team
(physical/logical actor) shares the field with us. The camera system broadcasts the co-
ordinate of each robot and of the ball.
There is two user cases. The configuration expresses that the referee chooses a field and