Simulation Mode The Simulation Mode is used for experimentation. It has simulation setup to define duration of simulation, etc., plot windows, animation windows and variable browser.

Features of Modelica are tems. Modelica is an object-oriented language for modeling of large, complex and heterogeneous physical systems. It is suited for multi-domain modeling, for example for modeling of mechatronic systems within automotive, aerospace and robotics applications. Such systems composed of mechanical, electrical and hydraulic subsystems, as well as control sys- General equations are used for modeling of the physical phenomena. The language has been designed to allow tools to generate efficient code automatically. The modeling effort is thus reduced considerably since model components can be reused, and tedious and error-prone manual manipulations are not needed.

Background Modeling and simulation are becoming more important since engineers need to analyse in¬creasingly complex systems composed of components from different domains. Current tools are generally weak in treating multi-domain models because the general tools are block-oriented and thus demand a huge amount of manual rewriting to get the equations into explicit form. The domain-specific tools, such as circuit simulators or multibody pro¬grams, cannot handle components of other domains in a reasonable way. There is traditionally too large a gap between the user's problem and the model description that the simulation program understands. Modeling should be much closer to the way an en¬gineer builds a real system, first trying to find standard components like motors, pumps and valves from manufacturers' catalogues with appropriate specifications and interfaces.

Equations and reuse Equations are used in Modelica for modeling of the physical phenomena. No particular variable needs to be solved for manually because Dymola has enough information to decide that automatically. This is an important property of Dymola to enable handling of large models having more than hundred thousand equations. Modelica supports several formal¬isms: ordinary differential equations (ODE), differential-algebraic equations (DAE), bond graphs, finite state automata, Petri nets etc. The language has been designed to allow tools to generate very efficient code. Modelica models are used, for example, in hardware-in-the-loop simulation of automatic gearboxes, which have variable structure models. Such models have so far usually been treated by hand, modeling each mode of operation separately. In Modelica, component models are used for shafts, clutches, brakes, gear wheels etc. and Dymola can find the different modes of operation automatically. The modeling effort is considerably reduced since model com¬ponents can be reused and tedious and error-prone manual manipulations are not needed.

Modelica history Reuse is a key issue for handling complexity. There had been several attempts to define ob¬ject-oriented languages for physical modeling. However, the ability to reuse and exchange models relies on a standardized format. It was thus important to bring this expertise together to unify concepts and notations. A design group was formed in September 1996 and one year later, the first version of the Modelica language was available (http://www.Modelica.org). Modelica is intended to serve as a standard format so that models arising in different domains can be exchanged between tools and users. It has been designed by a group of more than 25 experts with previous know-how of modeling languages and differential-algebraic equation models. After more than 30 three-days' meetings during a five year period, version 2.0 of the language specifi¬cation was finished in January, 2002.

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