CHAPTER the simulation in the productive systems
The implementation of a model of discreet simulation on an electronic computer demands the employment of
a language of programming based on particular sintassi and specific acquaintances of logic.
The languages available can be classify to you in three categories:
To the first category which Pascal, C, Fortran, etc. belong those languages of programming to high level., than
being it specifically does not dedicate you to a simulation activity, demand from part of the programmatore one greater effort
for the implementation of all the mechanisms and the structures of necessary data in a simulator.
On the other hand, just because draft of languages generates them, and therefore a lot known, they do not create excessive
problems to the level of use.
The second category is constituted from those obtained languages adding to the languages generates them existing, typical
functions and costrutti of the simulation under the shape of bookcases of sottoprogrammi or richiamabili procedures from the program
of simulation written in a language general purpose.
Languages of simulation in a position to facilitating the task of the programmatore exist by now on the market various, than with the aforesaid
languages it is in a position to writing with little entire lines of program models or sottomodelli that otherwise they would demand
much longer times of drawing up with consequent increase of the error probability.
To these advantages, however, which can contrapporre some disadvantages the smaller efficiency (so that of usual
the time of elaboration increases) or to the insufficient spread of the language same due to the fact that only a narrow
number of persons is in a position to padroneggiare this type of language effectively.
With reference to the dynamics evolution of the system it is possible to distinguish between three various methodologies of simulation;
it is spoken therefore about:
The simulation oriented to the events, typical of the languages of programming defined previously as it generates them,
considers a series of events that can be taken place in the course of the evolution of the system; every event is described
from a subroutine that can modify or less the state than all the members of the system.
In this type of approach it comes defined a list, dictates calendar of the events, in which they come lists the various events
orders to you to you based on the temporal moment of accadimento. Once inizializzato the simulation model,
the evolution of the system comes described alternating itself of two distinguished ones is made said of scan and rescan:
In the approach oriented to the processes the simulator is constituted from with of procedures that describe all that that
succeeds to an entity that progressively it crosses the system; the model therefore is described from a diagram block by block,
everyone of which represents a process, it connects to you from each other from arches orients to you along which the entities flow.
In every language of simulation oriented to the processes it is present with of macro-instructions that automatically
translate a sure number of situations that are introduced more frequently during a simulation model.
The approach process-based particularly seems to be adapted for the description of the production processes, but it can
involve complications when the operations are involved various types of resources.
The approach oriented to the activities demands the definition of the necessary conditions for the beginning and the term of ciascuna
activity previewed from the model.
This presupposes the existence of a module of the program in order to define every activity that engages the entities; the same module
includes a test in order to determine if the activity can be begun and if the actions are eseguibili.
This type of approach is easiest for a not-specialist and is undoubtedly simplest from the point of view
of the programming.
The simulation languages base to you on last the two approaches concur to represent the system under investigation describing
to the sequence of processes (or activity) that ciascuna entity crosses at once in sequence from the moment of income in the fine
system of escape.
Of it remarkable simplification in the programming job achieves one. The system under investigation can that is be outlined
like a net of nodes or blocks constituted from blots some, tails and points of distribution, through which the covered entities
journey following differentiate to you to second of the specific attributes assign to you to ciascuna of they.
To every type of node it corresponds, from the point of view of the simulation program, one macroinstruction (as an example:
activity for one machine, queqe for one tail, etc).
To the programmatore the task only remains to choose the macroinstructions that describe the nodes of the system, to assign for
ognuna of they the parameters that they characterize the operation of the corresponding node and to establish the necessary connections
between the various macroinstructions (correspondents to the existing connections between the nodes of the net that describes the system).
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