The Dynamic analyses all use the method of Modal Superposition and required the modal analysis to be run first or as part of the Dynamic design study. When dynamic loads are applied the response is calculated based on the dynamic characteristics of the structure determined by the modal analysis. Also, dynamic stresses are calculated based on the element P-level derived during the modal analysis convergence process. No stress convergence is performed during Dynamic design studies.
Modal Analysis can be run as Constrained, Unconstrained or Partially Constrained. In the Constrained case Creo Simulate expects that all degrees-of-freedom (DOF) are tied to ground in some way. If any free body modes are found you will get an error message about the model being un-sufficiently constrained. In the Unconstrained case no constraint sets are selected and Creo Simulate will search for rigid body modes. In this case the model may still be constrained by springs to ground. For the Partially Constrained case a constraint set is selected and the ‘with rigid mode search’ option is selected. In this case Creo Simulate will use the constraints defined but will also look for rigid body modes.
For rigid body modes Creo Simulate will calculate near zero frequencies for each. These calculations can be avoided by setting the minimum frequency to some number greater than zero (but less than the first real mode), however, the Dynamic design studies will not work if the minimum frequency is greater than zero.
You may choose to set the maximum frequency range or the maximum number of modes. If the maximum frequency range is set, pick a value that is about 1.5 times higher than the maximum frequency of interest. This will help on error estimation (for SPA) and convergence (for MPA) for the higher frequency modes. If you select the maximum number of modes you should calculate a least one mode higher in frequency than you require. This will serve as a check and may also help convergence.
Setting up convergence properly for the modal analysis is important since stresses calculated during the Dynamic studies are based on the element P-level derived during the modal analysis convergence. If SPA or MPA with a stress measure is selected for convergence then Creo Simulate will use the stresses for the normalized mode shapes as the basis for error estimation. This should result in the best P-levels possible for a good Dynamic stress calculation. If only displacement, acceleration or velocity results are desired then you will only need to converge on frequency.
