The PetriNets library allows to model discrete
components by a special kind of Petri nets with at most one
token on a place, as well as by state transition diagrams
(which are special kinds of Petri nets). Petri nets and state
transition diagrams are "higher level" constructs for the
description of switching elements, parallel activities or
syncronization. For several kinds of applications it is much easier
and clearer to use these components instead of modeling the
discrete behaviour directly with the basic language constructs of
Modelica ("if" or "when" statements). A typical Petri net is shown
in the following figure:
A Petri net is defined in the following way:
- It consists of a set of places and of a set of
transitions. The places are split into start places which
are "active" at the start of the simulation and of "normal" places
which are "non-active" at the beginning.
- Places are connected by transitions, whereby no places
and no transitions are directly connected (i.e., a place is
connected to a transition which in turn is connected to another
place). Any number of start places can be present.
- An "active" place is characterized by a "token"
placed on the place. In the ModelicaAdditions.PetriNets libray a
place is "active" when the public variable "state" of the
place is true.
- There are several transition elements in the library.
Whenever the states of all inputs to the transition
elements are active and when the condition of the
transition is true then the following actions are performed:
- all input states are marked as "inactive", i.e.,
the token is removed.
- all output states are marked as "active", i.e.,
they are marked by new tokens.
The conditionPort connector of a transition element is used
to signal via a Boolean signal whether the condition of a
transition is true or false. Alternatively, the condition can be
provided as an equation to set the public variable condition
of the corresponding transition element.
- There are several place components in this library (such
as Place01, Place10, Place11) which have different number of input
and output transition connectors. This is due to the current
limitations of the annotations of Modelica, which do not allow to
define the graphical location of the elements of a vector of
(transition) connectors with unknown length.
- If two or more transitions of a place would fire at the same
time instant, priorities are used in order that exactly one
of them fires. The highest priority has a transition connector of a
place with the lowest index (e.g. outTransition1 has a higher
priority as outTransition2).
The method used in this library to realize Petri nets in
Modelica is described in detail in:
- Mosterman P.J., Otter M. and Elmqvist H. (1998):
- Modeling Petri-Nets as Local Constraint Equations for Hybrid
Systems using Modelica. 1998 Summer Computer Simulation
Conference (SCSC'98), Reno, U.S.A., 19.-20. Juli (download from
here).
This package is not part of the Modelica standard library,
because it is planned to realize a package with only one
place and one transition component, once vector connectors
with unknown length have better support in Modelica.
- Main Author:
- Martin
Otter
Deutsches Zentrum für Luft und Raumfahrt e.V. (DLR)
Institut für Robotik und Mechatronik
Postfach 1116
D-82230 Wessling
Germany
email: Martin.Otter@dlr.de
Release Notes:
Copyright © 2000-2002, DLR.
The ModelicaAdditions.PetriNets package is free
software; it can be redistributed and/or modified under the terms
of the Modelica license, see the license conditions and the
accompanying disclaimer in the documentation of package
Modelica in file "Modelica/package.mo".
Contents
Name |
Description |
Interfaces |
|
Examples |
|
Place01 |
Place with one output transition |
Place10 |
Place with one input transition |
Place11 |
Place with one input and one output transition |
Place21 |
Place with two input and one output transition |
Place12 |
Place with one input and two output transitions |
Place22 |
Place with two input and two output transitions |
Transition |
Transition with one input and one output connection |
Parallel |
Transition with one input and two output connections |
Synchronize |
Transition with two input and one output connections |
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