8. FreeDyn: A Free, Flexible and State of the Art Multibody Simulation Package for Education, Research and Industrial Applications

FreeDyn (www.​freedyn.​at) is a free multibody simulation (MBS) package consisting of a graphical user interface (GUI) and a C++ solver in which a modified version of the HHT time integration method is implemented. In order to underline the solvers efficiency with respect to CPU time, comparative simulations of FreeDyn and a commercial software product are presented in this paper. The main modelling elements of FreeDyn are rigid and flexible bodies, constraints, forces and measures (angle, position, velocity …). The FreeDyn standard force elements can be specified by user defined expressions. However, if a more advanced force is required, the user can link a user written subroutine (DLL) to FreeDyn. FreeDyn can be used as standalone software, which is controlled via the mentioned GUI. Alternatively, FreeDyn can be linked via a C-Interface as a dynamic link library (DLL) to other software like MATLAB, Scilab or self-written codes. The C-interface provides control functions for time integration and access to model data, which are normally hidden (e.g. mass matrix, constraint forces, Jacobi matrices etc.). Beside industrial applications, FreeDyn can be used in education and research.

In Education: Beside the classical teaching elements like the creation and simulation of MBS models, it is furthermore possible, to look deeper inside the structure of the equations. This can be easily done by the use of the access routines of the C-Interface.

In Research: FreeDyn enables quick implementations of particular research ideas in the context of MBS. The integration of those ideas can be realized with a user written subroutine and/or alternatively, by the use of the C-Interface. A researcher can focus on the research topic, while FreeDyn handles the MBS model. Moreover, due to the availability of very specific information, the C-Interface of FreeDyn is suitable for extraordinary optimization problems, which is underlined by an example and several literature citations.

The paper contains a brief review of the theoretical concepts on which FreeDyn is based. The previously mentioned possibilities are briefly discussed, and underlined by illustrative and meaningful examples.