# Constraints¶

Constraints are specific kinematic links and are solved similarly. While kinematic links are used to model realistic links between bodies, constraints are more abstract and can be used at a conceptual level.

They are based either on point, axis and plane geometric abstractions, which are relying, within FRyDoM, on nodes and reference frames. Since nodes belong to bodies, the constraints are applied in fine on the bodies. It is however easier to define these constraints using the geometric abstractions rather than the nodes and frames directly.

Seven different constraints are implemented in FRyDoM (see next table)

Name Description Constrained degrees of freedom
Distance between points 1 translation, 0 rotation
Distance to axis 2 translations, 0 rotation
Point on plane 1 translation, 0 rotation
Point on line 2 translations, 0 rotation
Plane on plane 1 translation, 2 rotations
Perpendicular 0 translation, 1 rotation
Parallel 0 translation, 2 rotations

Motions are constrained with respect to the $$x$$ , $$y$$ and $$z$$ of the first body. The constraints and joints are thus applied along the axes of the first body.