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Constraints: Calculated DOFs |
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Constraints: Calculated DOFs |
Calculated DOF constraints can be either Cartesian or curvilinear. The Cartesian type will be sufficient in the vast majority of modelling applications, but many advanced capabilities become available when using curvilinear constraints, such as the ability to model the motion of the out-frame along arbitrary curves and surfaces, mixed calculated and imposed motion, and dynamic release of the out-frame based upon user-specified criteria.
Determines which degrees of freedom (DOFs) are calculated. In each case, if free is checked then the DOF is calculated, otherwise it is fixed. The translational DOFs are x, y, z, and the rotational DOFs are Rx, Ry, Rz. These degrees of freedom are relative to the in-frame axes.
An initial value is required for each free DOF for the initial iteration of the static analysis. Often this can be left at the default of zero, but in some cases static convergence can only be achieved if this value is specified appropriately. The use calculated positions function can be used to capture this value following a successful statics analysis. If the constraint has a double-sided connection, then the initial DOF values are not available because they are superseded by the out-frame's initial position and orientation data.
| Note: | The degrees of freedom page is only relevant to Cartesian constraints. It is not available when the model type is curvilinear. |
The out-frame can be disconnected from the in-frame at the start of any stage of the simulation, to release it from the in-frame. It will then move completely independently of the in-frame, as if all six of its DOFs are free. Constraint stiffness and damping forces will be ignored, but any applied loads will still have an effect. Any child objects of the constraint will remain connected to the out-frame after it has been released. For no disconnection, set this item to '~'.
| Note: | If a constraint has no children connected to it, other than other constraint objects, then the dynamic equations associated with its free DOFs become singular because the constraint is effectively a massless object with no physical properties of its own. In this case, all free DOFs are ignored and the out-frame is assumed to remain rigidly attached to the in-frame in the position it was in when the last of its children (ignoring other constraints) was released. |
Multiple applied loads can be defined, with respect to either global axes or local out-frame axes. An applied load is defined by specifying its point of application, relative to the out-frame, and its components. Each component may be constant, vary with simulation time (via a tabular variable data source or a Python applied load) or be given by an external function. An applied load acts as if its point of application were rigidly attached to the out-frame.
For some models it may be desirable to explicitly set a characteristic length and characteristic force for the constraint. These characteristic scales directly affect the convergence criteria of the iterative solvers employed in the analysis. The data does not appear on the constraint data form but can be found on the all objects data form.
| Note: | These data appeared on the constraint data form prior to version 11.1; however, now that all objects with free degrees of freedom now have associated characteristic scales, the data have been moved to a single location on the all objects data form. |
Constraint stiffness and damping data is explained in a separate topic.