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All the methods described above are difficult to use for creating an accurate flexible model of a truck frame. In general, they are only capable of capturing the basic frame response: the first few bending and torsional modes and the gross frame stiffness. If each method is to work, a significant effort is required to tune its properties to match some reference, such as static deflection testing, modal testing, or finite element simulation results. Consequently, neither method is suitable for use in a concurrent design and analysis environment - it would simply take too long to make changes to the model, and it would not have adequate spatial resolution to capture subtle design changes to the frame.
COMPONENT MODE SYNTHESIS TECHNIQUE
Recent advances in the integration of FEA and MSS have overcome the difficulties in the methods described above .It is now possible to use a finite element model directly in a multi-body simulation using a modal superposition technique known as component mode synthesis (CMS).Using modal superposition, the deformation of a structure can be described by the contribution of each of its modes. Normally, a very large number of modes are required to accurately capture the deformations at points where constraints are applied to the structure. CMS was developed to alleviate this problem. It combines normal modes with constraint modes. These constraint modes, or static shapes, capture the deformation of key areas of the structure without having to maintain an excessive number of normal modes. As a result, they are computationally more efficient. The CMS procedure adopted in the ADAMS code is based on a modified version of the Craig-Bampton approach. In this method the structure is considered to have interface points where constraints and forces are applied, and each interface point can have up to six degrees-of-freedom: three translations and three rotations. Themotion of the structure is then described by a combination of two sets of modes: constraint modes for the interface points, and fixed interface normal modes. A constraint mode is calculated for each degree-of-freedom of an interface point, and it describes the static shape of the structure when that degree-of-freedom is given a unit deflection while keeping the degrees-of-freedom of all the other interface points fixed. This procedure is repeated to develop a family of constraint modes for all the interface points. Since the constraint modes are static shapes, their frequency information is unknown. The fixed interface normal modes represent the normal modes of the entire structure when all the degrees-of-freedom of all the interface points are held fixed.
In this form, the Craig-Bampton modes are not ideally suited for integration with the multi-body equations of motion. For example, the constraint modes add rigid body modes which conflict with the ADAMS non-linear rigid body motions. Also, the constraint modes may contain high frequencies that are difficult to solve. In the ADAMS implementation these problems are handled by orthogonalizing the Craig-Bampton modes. This identifies the rigid body modes making them easy to disable. It also adds frequency information to the constraint modes which is valuable for setting integration parameters during the multi-body simulation. After orthogonalization, a modified set of modes exist: normal modes for the unconstrained structure (free-free like modes similar to those calculated in a typical FEA eigenvalue run), and the interface degrees-of-freedom. See Ottarsson [3] for a complete description of this method.
All the modal calculations described above take place in the ANSYS environment and are performed on a finite element model offthe frame. To compute the modes, the user selects the nodes representing the interface points where forces and constraints enter the frame, and then runs a macro that executes the appropriate ANSYS commands. The number of normal modes to include in the calculations are passed as a parameter to the macro. The final set of modes are written to a modal neutral file MNF) that can be read by ADAMS. The advantage of this modal superposition method are many and include: