The Dynasim Derivative

October 15, 1993

Dynasim one year

Dynasim started it's operation on August 1, 1992. It has been a very exciting year. There is now such a large interest in object-oriented modeling due to the need to model larger systems and to reuse modeling knowledge.

The development of Dymola during the last years has included: solving linear systems of equations symbolically, generating code for solution of non-linear systems of equations, index reduction by symbolic differentiation, support for the neutral Fortran subroutine format DSblock, symbolic simplifications, discrete features, etc. Simplifications are especially important when general model classes are developed, for example, for multibody systems. The single most important development is support for combined continuous/discrete event models .

When Dymola was originally defined, the computing power needed was not generally available. Since Dymola performs symbolic formula manipulation, large amount of memory is needed. Today, even PC's have large memory and can benefit from virtual memory management under Windows. Dymola has now been ported to several platforms: PC/Windows, Macintosh, VAX/VMS and Unix-machines.

Hilding Elmqvist
President, Dynasim AB

Dynasim newsletter

This newsletter will present information about new development of Dymola, libraries, activities etc. It will thus deal with changes, trends, i.e. derivatives, hence the name of the newsletter.

Sharing of modeling knowledge is very important. Dymola provides the mechanisms for reuse of models. It is encouraged that users of Dymola structure their models into classes for reuse. Dynasim would like to serve as a clearing-house for such libraries and thus redistribute the library files without charge to other Dymola users. One such library is provided by DLR (see below). If you want to share a library, please contact Dynasim.

Dymola extended with discrete event facilities

Models often contain discontinuities, hysteresis, computer control and other discrete effects. Information about these effects needs to be made available to the integration routines in the form of zero crossing functions enabling the integration routine to determine the time of state events.

In ACSL, this information is provided via schedule statements. Discrete blocks are used to describe the action to be performed at an event. One major drawback is that this description is not object-oriented.

Dymola uses high level discrete constructs and uses formula manipulation to get the information into suitable format for ACSL and DSblock. Discontinuities are simply described by if-expressions. The needed zero crossing functions are automatically generated. Instantaneous equations are used to model, for example, hysteresis and difference equations. Boolean variables are used to synchronise events. Discrete Boolean states can be used to, for example, model finite states machines.

Dymola at DLR

At DLR, we started experimenting with Dymola in June 1992. Personally, I became convinced that Dymola is a very useful and powerful tool for the realistic modeling of dynamic systems when Dymola was able to generate simulation code for a 6 degree of freedom robot and this code was as efficient as code from multibody programs I am aware of. Since Dymola can easily model also components from other domains, like controllers or electrical circuits, it became my favourite modeling environment.

Martin Otter, DLR Oberpfaffenhofen
Email: Martin.Otter@DLR.DE

Library for multibody mechanical models

A new Dymola library for the modeling of 3-dimensional mechanical systems like robots, satellites, or vehicles was developed at DLR. The library provides basic model classes for the modeling of rigid bodies, joints, force and measurement elements. A unique feature is the locking/unlocking feature of joints, which allows an easy modeling of brakes, clutches, stops or sticking friction. The multibody library is distributed together with Dymola.

Martin Otter, DLR Oberpfaffenhofen
Email: Martin.Otter@DLR.DE

New employee

Dag Brück is employed at Dynasim since August 1993. Dag was previously at the Department of Automatic Control, Lund Institute of Technology. He is an expert on C++ and the Swedish head of delegation in the ANSI and ISO standardisation groups for C++.