﻿ Constraints: Common data

# Constraints: Common data

### Name

Used to refer to the constraint.

### In-frame connection

A constraint's in-frame can be free, fixed, anchored or connected to another object.

### In-frame initial position

Defines the position of the constraint in-frame in terms of the connection coordinates $x$, $y$ and $z$.

### In-frame initial orientation

The azimuth, declination and gamma angles, $\phi, \theta, \gamma$, that define the orientation, $\Ixyz$, of the constraint in-frame relative to the connection axes, $\Cxyz$. These angles specify the orientation of the in-frame as follows:

• Start with $\Ixyz$ aligned with $\Cxyz$.
• Then rotate $\Ixyz$ by an angle $\phi$ about $I\urm{z}$ ($= C\urm{z}$ at this point).
• Now rotate by an angle $\theta$ about the resulting $I\urm{y}$ direction.
• Finally rotate by an angle $\gamma$ about the resulting (and final) $I\urm{z}$ direction.

In all these rotations, a positive angle means rotation clockwise about the positive direction along the axis of rotation, and a negative angle means anti-clockwise.

Three-dimensional rotations are notoriously difficult to describe and visualise. When setting the azimuth, declination and gamma, it is best to check that the resulting $\Ixyz$ directions are correct by drawing the local axes on the 3D view.

### Constraint type

Either calculated DOFs or imposed motion.

### Solution method

Either direct or indirect.

### Double-sided connection

If the solution method is indirect, then the constraint's out-frame can be independently connected to another object. Such a constraint is said to have a double-sided connection.

 Note: If the constraint has a double-sided connection, then its connection data (detailed below) determines where the out-frame will be when the model is in reset state. This is independent of the position of the in-frame. OrcaFlex does not attempt to enforce the usual relationship between the in-frame and the out-frame (e.g. any fixed internal degrees of freedom for a Cartesian constraint) until the analysis begins. It is possible to specify out-frame connection data that is incompatible with the defined relationship between the in-frame and the out-frame; such models are insoluble and will fail to converge. It is important to be aware of such potential pitfalls when building models with constraints that have double-sided connections.

### Out-frame connection

If a constraint has a double-sided connection, then its out-frame can independently be free, fixed, anchored or connected to another object.

 Note: The out-frame connection applies in addition to the usual relationship between the in-frame and the out-frame. For instance, a free out-frame ostensibly has six free degrees of freedom; however, OrcaFlex may eliminate some or all of these (once the analysis is underway) in order to enforce the stated conditions between the in-frame and the out-frame (such as a particular internal degree of freedom, e.g. $x$, being fixed on the degrees of freedom page).

### Out-frame initial position

Defines the position of the constraint out-frame in terms of the connection coordinates $x$, $y$ and $z$. These data are only available if the constraint has a double-sided connection.

### Out-frame initial orientation

The azimuth, declination and gamma angles, $\phi, \theta, \gamma$, that define the orientation, $\Ixyz$, of the constraint out-frame relative to the connection axes, $\Cxyz$. These angles have the same meaning as their in-frame equivalents and are only available if the constraint has a double-sided connection.