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When a ticket has been delivered, it visualizes the ticket outside the
dispenser. The ticket dispenser simulates the printing operation with a
delay of 700 milliseconds.
It visualizes the stopping bar while it is raising and lowering step by step
every 100 milliseconds. The bar is down when it is in the horizontal
position (0 degrees). The bar is up when it is in the vertical position
(90 degrees). Every step corresponds to 9 degrees.
Problem analysis
The access control system is a typical reactive system: its dynamic is
governed by stimuli received from the external world. In this scenario, the
external world is represented by the car that proceeds on the access lane
and interacts with the physical devices. This case study shows several
similarities with those discussed in Chapter 9 and Chapter 10.
The work cell (Chapter 9) is an example of a discrete event dynamic
system: the dynamic behaviour is described with a set of finite state
machines that synchronize their activities by exchanging events. The work
cell system is characterized by a number of different macro events that notify
the start and completion time of the components' activities. When state tran-
sitions occur, physical devices generate events, which in turn trigger state
transitions in other devices. In terms of simulation, we were interested in
testing the correct interaction between the work cell components and thus
we disregarded the animation of the physical devices.
The mobile robot (Chapter 10) is an example of process-based dynamic
system: the dynamic behaviour emerges from the exchange of service
requests between autonomous components and the use of shared resources.
The system is characterized by frequent fine-grained interactions between
the environment and the robot's devices. We simulated the behaviour of the
laser and the mobile platform with independent threads that animate the
robot during the exploration. The robot controller was modelled as an inde-
pendent thread that interacts with the robot's components according to the
communication and synchronization patterns of client
server software
The specification of the access control system indicates four fundamental
At most, two cars can be present within the system simultaneously: one
car is proceeding along the access lane; another car is waiting for the
semaphore to go green.
The simulator should animate the behaviour of the cars and of the
physical devices.
The control logic should be encapsulated in a control module, which must
be easily reprogrammable without affecting the rest of the system.
The cars, the controller and the devices synchronize themselves by
exchanging events.
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