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CHAPTER the simulation in the productive systems

1.4 The languages of simulation

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:

  1. languages generate them (or general purpose);
  2. extension of languages generates orients them you to the simulation;
  3. simulation languages.
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:
  1. simulation oriented to the events (event-based);
  2. simulation oriented to the processes (process-based);
  3. simulation oriented to the activities (activity-based).
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 before (the phase of scan) the program it characterizes the first element in the calendar of the events and transfers the control to the associated ruotine to such event, which realizes the necessary transformations of the state of the system, while the simulation clock comes made to be left over at once until correspondent to the accadimento of the considered event;
  • to the term of the phase of scan the system it comes newly analyzed (phase of rescan), verifying if the conditions exist in order to begin more or of the relative activities that the system is in a position to carrying out and modernizing the variable ones of state. With of the under way activities during this phase it concurs to define the list of the future events, correspondents to the moments in which the under way activities finish. During this phase the time of simulation remains firm.
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).

1.5 the choice of the simulation software

In the choice of the products simulation software it is opportune to hold account of the following factors:
  • modality of development of the program: of type batch and/or interactive type. Some types of software concur the interactive development of the program by means of appropriate sequences of menù;
  • sintassi: if easy comprehensible and it deprives of ambiguity, it concurs an express and corrected development of the model;
  • modularity of the software, such to concur one modular development of the model in its various members (layout, characteristics of means of production, structure of the material flows, etc). In this way the single members of the model can be modified in independent way;
  • flexibility of the software, understanding as adaptability to the modellizzazione of systems variously shapes to you and/or characterizes from different operation modality to you. Normally the approach oriented to the events offers greater degrees of flexibility, however the development of the models turns out in such case particularly along and boring. The approach oriented to the processes turns out more express to price than one smaller flexibility, due to the necessity to use exclusively the instructions standards previewed from the prechosen software;
  • modellizzazione concision: how much more powerful they are the instructions standards previewed in the simulation language the much more model will turn out concise;
  • ability to describe systems of material handling is of continuous type is of intermittent type in adequate way (as an example industrial, convogliatori undercarriages that must easy be inseribili in the context under investigation, with opportune the logical ones of operation);
  • generation of relative relationships you to the analysis statistics of the obtained data of output in the various launch of the simulation program;
  • graphical facilitations as the animation on video of the simulated operation of the system, with various scales of blowup, and ability to dialogue with other languages;
  • debugging interactive of the model;
  • compatibility hardware: the model can initially be developed on personal computer and subsequently, to growing of the complexity, transferred on minicomputer or mainframe;
  • documentation (manual and operating guide exausted to you), training (examples of resolved problems or course finalize to you), attendance, continuous modernization of the product, possibility of exchange of information and experiences between utilizzatori.



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