This paper will describe one approach to the design and implementation of a multibody systems analysis code using an object-oriented architecture. The principal objective is to show the adequacy between object-oriented programming and the finite element method used for the treatment of three-dimensional multibody flexible mechanisms. It will show that object-oriented programming greatly simplifies the choice and the implementation of other formalisms concerning polyarticulated systems, thus conferring high flexibility and adaptability to the developed software.
The finite element method (FEM) has become the most popular numerical method for solving a wide variety of complex engineering problems. Over the years, FEM codes have emphasized the use of the Fortran programming language, with a so-called ‘procedural programming’. As a result, the codes contain numerous complex data structures, which can be called anywhere in the program. Because of this global access, the flexibility of the software is decreased: when a FEM program has many computational capabilities, it becomes very difficult to maintain the code and even more difficult to enlarge the program codes. The recoding of these finite element programs in a new language is not a solution to this inflexibility problem, thus a redesign is needed.
Object-oriented programming is currently seen as the most promising way of designing a new application. It leads to better structured codes and facilitates the development, the maintenance and the expansion of such codes.
The object-oriented philosophy was proposed as a general methodology for FEM implementation for the first time in Ref. [1]. Over the past decade, it has been successfully applied to various domains in finite element developments: constitutive law modeling [2] and [3], parallel finite element applications [4] and [5], rapid dynamics [6], multi-domain analysis for metal cutting, mould filling and composite material forming [7], [8] and [9], coupled problems [10], non-linear analysis [11], symbolic computation [12] and [13], variational approach [14], finite element analysis program architecture [15], [16], [17], [18], [19] and [20], neural networks [21], impact simulation [22], among others.
However, little effort has been made to implement object-oriented programming in multibody systems analysis: like many other engineering applications, multibody systems analysis codes (ADAMS [23], DADS [24], MBOSS [25]) are written in Fortran. Therefore, the main objective of this work is to describe one approach to the design and implementation of a multibody systems analysis code using an object-oriented architecture.
The first part of the paper deals with the formalism used for the treatment and the resolution of multibody systems. It will be shown that the structure of multibody systems presents similarities with object-oriented concepts and lends itself very well to object-oriented programming techniques [26] and [27]. The principal features of object-oriented programming are summarized in the second part of this paper. The architecture of the computational engine of the software is then presented, with the description of the most important classes. Emphasis is placed on the adequacy between object-oriented programming and the FEM within a given formalism and given hypotheses. It will be shown that object-oriented programming greatly simplifies the choice and the implementation of other formalisms concerning polyarticulated systems, conferring high flexibility and adaptability to the developed software. The last section is dedicated to numerical examples.
A finite element calculation software may advance in different directions. In particular, for software dealing with multibody systems, many formalisms and hypotheses can evolve and combine with each other, as for example, the choice of the finite elements, the choice of the referential frame, the definition of physical or material parameters, the choice of the parameters for the representation of the finite rotations, the choice of the solving algorithm, the formalism and the treatment of the joints, the flexibility or the rigidity of the bodies, among others.
In this paper, the architecture of a new finite element software for the simulation of flexible mechanisms has been presented. The program has been designed according to object-oriented principles. This approach allows us to simplify the architecture of the program and to take advantage of the inherent synergy between object-oriented design and multibody systems analysis. Additional benefits, such as improved maintainability, flexibility and ease of expansion, also accrue as a result. The flexibility of the software is made possible by the clear modularization that can be reached with object-oriented programming, with a marked separation of functionalities. Extensibility and reusability of object-oriented programming are clearly shown: the introduction of new formalisms or new solving strategies can be achieved easily, with small changes of some existing classes or the definition of new classes. In any case, it does not require the complete redefinition of the software architecture.
