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2 2012

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Joint reactions in rigid or flexible body mechanisms with redundant constraints

Wojtyra M., Frączek J.

Bulletin of the Polish Academy of Sciences. Technical Sciences

2012

Vol. 60, nr 3

617-626

The problem of joint reactions indeterminacy, in engineering simulations of rigid body mechanisms is most often caused by redundant constraints which are defined as constraints that can be removed without changing the kinematics of the system. In order to find a unique set of all joint reactions in an overconstrained system, it is necessary to reject the assumption that all bodies are rigid. Flexible bodies introduce additional degrees of freedom to the mechanism, which usually makes the constraint equations independent. Quite often only selected bodies are modelled as the flexible ones, whereas the other remain rigid. In this contribution it is shown that taking into account flexibility of selected mechanism bodies does not guarantee that unique joint reactions can be found. Problems typical for redundant constraints existence are encountered in partially flexible models, which are not overconstrained. A case study of a redundantly constrained spatial mechanism is presented. Different approaches to the mechanism modelling, ranging from a purely rigid body model to a fully flexible one, are investigated and the obtained results are compared and discussed.

Integration of the Equations of Motion of Multibody Systems Using Absolute Nodal Coordinate Formulation

Orzechowski G., Frączek J.

Acta Mechanica et Automatica

2012

Vol. 6, no. 2

75-83

Recently, a finite element formulation, called the absolute nodal coordinate formulation (ANCF), was proposed for the large rotation and deformation analysis of flexible bodies. In this formulation, absolute position and slope coordinates are used to define the finite element configuration. Infinitesimal or finite rotations are not used as nodal coordinates. The ANCF finite elements have many unique features that distinguish them from other existing finite element methods used in the dynamic analysis of the flexible multibody systems. In such systems, there appears the necessity of solving systems of differential-algebraic equations (DAEs) of index 3. Accurate solving of the DAEs is a non-trivial problem. However, in the literature about the ANCF one can hardly find any detailed information about the procedures that are used to solve the DAEs. Therefore, the current paper is devoted to the analysis of selected DAE solvers, which are applied to simulations of simple mechanisms.