Modelica is an object-oriented, declarative, multi-domain modeling language for component-oriented modeling of complex systems, e.g., systems containing mechanical, electrical, electronic, hydraulic, thermal, control, electric power or process-oriented subcomponents. The free Modelica language is developed by the non-profit Modelica Association. The Modelica Association also develops the free Modelica Standard Library that contains about 1400 generic model components and 1200 functions in various domains, as of version 4.0.0.
Characteristics
While Modelica resembles object-oriented programming languages, such as C++ or Java, it differs in two important respects. First, Modelica is a modeling language rather than a conventional programming language. Modelica classes are not compiled in the usual sense, but they are translated into objects which are then exercised by a simulation engine. The simulation engine is not specified by the language, although certain required capabilities are outlined.
Second, although classes may contain algorithmic components similar to statements or blocks in programming languages, their primary content is a set of equations. In contrast to a typical assignment statement, such as x := 2 + y; where the left-hand side of the statement is assigned a value calculated from the expression on the right-hand side, an equation may have expressions on both its right- and left-hand sides, for example, x + y = 3 * z; Equations do not describe assignment but equality. In Modelica terms, equations have no pre-defined causality. The simulation engine may (and usually must) manipulate the equations symbolically to determine their order of execution and which components in the equation are inputs and which are outputs.
History
The Modelica design effort was initiated in September 1996 by Hilding Elmqvist. The goal was to develop an object-oriented language for modeling of technical systems in order to reuse and exchange dynamic system models in a standardized format. Modelica 1.0 is based on the PhD thesis of Hilding Elmqvist and on the experience with the modeling languages Allan, Dymola, NMF ObjectMath, Omola, SIDOPS+, and Smile. Hilding Elmqvist is the key architect of Modelica, but many other people have contributed as well (see appendix E in the Modelica specification). In September 1997, version 1.0 of the Modelica specification was released which was the basis for a prototype implementation within the commercial Dymola software system. In year 2000, the non-profit Modelica Association was formed to manage the continually evolving Modelica language and the development of the free Modelica Standard Library. In the same year, the usage of Modelica in industrial applications started.
This table presents the timeline of the Modelica specification history:
Implementations
Commercial front-ends for Modelica include AMESim from the French company Imagine SA (now part of Siemens Digital Industries Software), Dymola from the Swedish company Dynasim AB (now part of Dassault Systemes), Wolfram SystemModeler (formerly MathModelica) from the Swedish company Wolfram MathCore AB (now part of Wolfram Research), SimulationX from the German company ESI ITI GmbH, MapleSim from the Canadian company Maplesoft, JModelica.org (open source, discontinued) and Modelon Impact, from the Swedish company Modelon AB, and CATIA Systems from Dassault Systemes (CATIA is one of the major CAD systems).
Openmodelica is an open-source Modelica-based modeling and simulation environment intended for industrial and academic usage. Its long-term development is supported by a non-profit organization – the Open Source Modelica Consortium (OSMC). The goal with the OpenModelica effort is to create a comprehensive Open Source Modelica modeling, compilation and simulation environment based on free software distributed in binary and source code form for research, teaching, and industrial usage.
The free simulation environment Scicos uses a subset of Modelica for component modeling. Support for a larger part of the Modelica language is currently under development. Nevertheless, there is still some incompatibility and diverging interpretation between all the different tools concerning the Modelica language.
Examples
The following code fragment shows a very simple example of a first order system
():
model FirstOrder
parameter Real c=1 "Time constant"; Real x (start=10) "An unknown"; equation der(x) = -c*x "A first order differential equation"; end FirstOrder;
The following code fragment shows an example to calculate the second derivative of a trigonometric function, using OMShell, as a means to develop the program written below.