Modal Solutions

Use the Modal solution type to determine vibrational natural frequencies of structures, along with the shapes of the various vibration modes. It is often desirable to avoid exciting structures at frequencies at or near their natural frequencies, due to resulting resonance. Where certain excitation frequencies are unavoidable, you can alter the design of the structure (mass, stiffness, support locations) to change the natural frequencies. In other cases, you might provide a means of damping to limit the severity of unavoidable resonant frequencies.

The results of a modal analysis include the vibrational natural frequencies and mass participation factors, which reflect the amount of a structure involved in a particular vibration mode. You can sum these factors to determine the cumulative mass participation from multiple vibration modes. Displacements are also calculated, but they are not scaled to any specific excitation (such as ground displacement or acceleration). Rather, the displacement magnitudes are arbitrary in nature, and the raw numeric values are meaningless. However, the relative displacement values (between one area of the structure and another area) provide a visual indication of the vibration mode shape. These deformed shapes typically must be scaled, as the arbitrary displacements can be orders of magnitude greater than the size of the structure or, in some cases, too small to be clearly seen without exaggeration.

Important:

Product licensing for Structural solutions differs from other design and solution types within the Ansys Electronics Desktop application. See the Mechanical Licensing Requirements topic for details.

This section contains information regarding the material assignment, setup options, and results that are available for a Mechanical design, Modal solution type. Specifically, the following topics are included:

Note:

For a valid modal solution, the following model characteristics are required:

  • The Mass Density, Young's Modulus, and Poisson's Ratio must be defined for the selected materials.

Regarding Boundaries and Contact:

  • The model does not have to be statically stable. That is, the model can be free to experience rigid-body motion in any or all six directions (X, Y, and Z translations and X, Y, and Z rotations). Rigid-body modes are supported by the solvers. When a body is free to move in any X, Y, and direction, the first six calculated modes will be the rigid-body modes, which do not involve distortion of the geometry but only rigid-body translation and rotation. For two unconstrained bodies, there would be twelve rigid-body vibration modes, eighteen rigid-body modes for three unconstrained objects, and so on. After all possible rigid-body modes are accounted for, the remaining modes will involve deformation of the geometry (bending, stretching/compression, and torsion).
  • Bonding occurs between parts with coplanar faces and also occurs by default between parts that overlap each other. Depending on the options selected in the Mechanical Design Settings, implicit subtraction of material may occur where solids overlap (eliminating the overlap from the solution). Alternatively, an error condition may be reported during model validation. Parts that do not touch each other are separate and do not interact.

Regarding Damping:

  • All modal solutions in the current release of Ansys Electronics Desktop are undamped. Damping is not currently supported.