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The simulator
The work cell controller is a software application that runs on a workstation
interconnected with the physical devices through specialized networks. The
physical devices are reactive systems that receive stimuli from the work cell
controller (e.g. “start the next operation on a new piece”) and responds to
them by updating their state (“machine working”) and giving feedback
(“work done”). The dynamic of the physical devices is modelled as a
sequence of states, which they visit during a period of observation.
Event-driven systems are special kinds of reactive systems, where the
interaction with the external world (stimuli and feedback) takes place
through events. An event is characterized by two parameters: type (what
happened) and time (when did the event happen). State transitions are
triggered by occurrences of discrete events at given time instants. The
behaviour of the entire work cell system emerges from the exchange of
events between the physical devices and the work cell controller.
We want to develop an event-driven simulator that models and executes
the behaviour of the physical devices and of the work cell controller. The
simulation consists of the evolution of the work cell state from an initial
configuration that defines the levels of the input and output buffers. The
evolution is caused by the occurrence of discrete events and terminates after
a predefined number of state transitions or when there are no enabled
Problem analysis
The work cell simulator is an event-driven system that models and executes
the concurrent behaviour of a number of interconnected subsystems
(machine, AGV, inventory system, etc.).
The dynamic of each subsystem comprises short-lived actions such as
picking up a piece from the input buffer, or moving the AGV to the target
station. Groups of these actions, which are triggered by events, are present
inside a sequential process (e.g. piece machining, piece assembly and piece
Each process is a finite state automaton that offers the protected environ-
ment to short-lived actions and hides the local state from other processes.
The processes synchronize with each other through shared resources and
event communication. In the work cell, the input and output buffers play the
role of shared resources between the machines, the assembler and the AGV.
The production cycle
Three types of operation characterize the manufacturing process under
consideration: piece machining, bolt assembling and piece moving. Each
operation consists of a sequence of actions.
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