Uncoupling of rigid-flexible multibody equations of motion using node annexation method

Abstract

This study presents the node annexation method for modeling kinematic joints between rigid and flexible bodies of rigid-flexible multibody systems. Each node of a flexible body is assumed to have lumped mass and three translational degrees of freedom, resulting in a diagonal mass matrix. Based on the node annexation method, both the nodal- and the modal-coordinate formulations for rigid-flexible multibody dynamics are developed. Conventionally rigid-to-flexible-body joints are treated as kinematic constraints using the Lagrange multiplier method. The formulations based on kinematic constraint method yield coupled equations of motion which have the difficulties associated with modal truncation. On the other hand, the node annexation method transfers the inertia and force effect of connected nodes of a flexible body to the connected rigid body. The mass matrix of the resultant equations of motion consists of two different kind of sub-matrices: one is rigid-body sub-system matrix containing the inertia of both rigid bodies and connected nodes of the flexible body and another is flexible-body sub-system matrix containing the inertia of free nodes of the flexible body. Since there is no off-diagonal terms coupling the sub-matrices, the node annexation method allows the division of the equations of motion into smaller sub-system equations. The node annexation method not only provides computational efficiency but also fundamentally eliminates any kinematic error at rigid-to-flexible-body joints. In addition, the node annexation method preserves the uncoupled nature of modal coordinates, allowing a mathematically justified modal truncation. Computer simulations are performed using a vehicle model with a flexible car body. The simulation results show computational advantage over the kinematic constraint method.