Component mode synthesis (CMS) is a form of substructure coupling analysis frequently used in structural dynamics.
CMS allows you to derive the behavior of the entire assembly from its constituent components. First, the dynamic behavior of each of the components is formulated. Then, by enforcing equilibrium and compatibility along component interfaces, the program forms the dynamic characteristics of the full system model.
The following CMS topics are available:
Although breaking up a single large problem into several reduced-order problems via substructuring saves time and processing resources, component mode synthesis (CMS) can be advantageous because it is more accurate than a Guyan reduction for modal, harmonic and transient analyses. CMS includes truncated sets of normal mode generalized coordinates defined for components of the structural model.
A typical use of CMS involves a modal analysis of a large, complicated structure (such as an aircraft or nuclear reactor) where various teams each design an individual component of the structure. With CMS, design changes to a single component affect only that component; therefore, additional computations are necessary only for the modified substructure.
Finally, CMS supports these substructuring features:
The following component mode synthesis methods are available:
For most analyses, the fixed-interface CMS method is preferable. The free-interface method and the residual-flexible free-interface method are useful when your analysis requires more accurate eigenvalues computed at the mid- to high-end of the spectrum. The following table describes the primary characteristics of each interface method:
| CMS Methods Supported | ||
|---|---|---|
| Fixed (CMSOPT,FIX) | Free (CMSOPT,FREE)[a] | Residual-Flexible Free (CMSOPT,RFFB)[a] |
| Interface nodes are constrained during the CMS superelement generation pass. | Interface nodes remain free during the CMS superelement generation pass. | Interface nodes remain free during the CMS superelement generation pass. |
| No requirement to specify rigid body modes. | You must specify the number of rigid body modes (CMSOPT). | If rigid body motion exists, you must specify pseudo-constraints (D). |
| Generally recommended when accuracy on only the lower modes of the assembled structure (use pass) is necessary. | Generally recommended when accuracy on both lower and higher modes of the assembled structure (use pass) is required. | Generally recommended when accuracy on both lower and higher modes of the assembled structure (use pass) is required. |
[a] For the free-interface and residual-flexible
free-interface CMS methods, you can specify
pseudo-constraints on some interface master nodes
(SUPPORT = ON on the
M command). In this case, there
is no requirement to specify rigid body modes. These
mixed-free interface and mixed-RFFB methods differ from
the fixed- and free-interface methods and the RFFB
method, which require all master DOFs to be either
constrained or free, regardless of whether or not they
belong to interfaces with other substructures. It is
particularly useful for obtaining better convergence
when master nodes are defined at locations other than
the interfaces, for example an observation node where
the displacement solution is wanted without the need of
an expansion pass.
For more information, see the discussion of component mode synthesis theory and methods in the Mechanical APDL Theory Reference.
