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MPC184 Element Description
MPC184 comprises a general class of multipoint constraint elements that implement kinematic constraints. The elements are loosely classified here as “constraint elements” and “joint elements.” You can use these elements in situations that require some type of kinematic constraint to be imposed. The constraint may be as simple as that of identical displacements at a joint. Constraints can also be more complicated, such as those involving modeling of rigid parts, or kinematic constraints that transmit motion between flexible bodies in a particular way. The type of constraint used depends on the desired application. For example, a structure may consist of some rigid parts and some moving parts connected together by some rotational or sliding connections. The rigid part of the structure may be modeled using the MPC184 Link/Beam elements, while the moving parts may be connected with the MPC184 slider, spherical, revolute, slot, or universal joint element.
The kinematic constraints are imposed using one of the following two methods:
The direct elimination method, wherein the kinematic constraints are imposed by internally generated MPC (multipoint constraint) equations. The degrees of freedom of a dependent node in the MPC equations are eliminated in favor of an independent node.
The direct elimination method should be used whenever it is available since the degrees of freedom at the dependent nodes are eliminated, thereby reducing the problem size and solution time.
Since the dependent degrees of freedom are eliminated, the constraint forces and moments are not available from the element output table (ETABLE) for output purposes. However, the global constraint reaction forces are available at independent nodes in the results file, Jobname.rst (PRRSOL command, etc.).
The Lagrange multiplier method, wherein the kinematic constraints are imposed using Lagrange multipliers. In this case, all the participating degrees of freedom are retained.
The Lagrange multiplier method should be used when the direct elimination method is not available or not suitable for the analysis purposes.
In this method, the constraint forces and moments are available from the element output table (ETABLE).
The disadvantage of the Lagrange multiplier method is that the Lagrange multipliers are additional solution variables and, hence, the problem size and solution time become larger when compared with the direct elimination method.
Currently, the rigid beam/link MPC184 elements can use the direct elimination method or the Lagrange multiplier method. All other MPC184 element options use the Lagrange multiplier method only.
Constraint Elements
The following types of constraint elements are available:link/beam
slider
spherical
Link/Beam
Figure 184.1 MPC184 Geometry |
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发表于 2009-5-31 22:35
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