Support for multiple degrees of freedom joints

[ShravanTata]

Currently Phobos only supports single degrees of freedom joints. In my models it is essential to have multiple degrees of freedom. From a robotics points of view it is understandable to have single DOF since they represent motors. But I am working on more abstract simulation models which would require multi DOF joints.

Describe the solution you'd like
To support all the joints described in SDF format.
[http://sdformat.org/spec?ver=1.6&elem=joint]

[cjaverliat]

I encountered the same issue regarding ball joints.
As of now, there are three joints missing for the SDF format (Cf. joints section):

• ball: 3 rotational degrees of freedom
• screw: 1 coupled rotational/translational degree of freedom
• universal: 2 rotational degrees of freedom

As Phobos is mainly based on URDF it explains the lack of such joints.

What would be the ideal solution in that case ?

• On the one hand, adding all the SDF joints types would resolve the issue, but on the other hand they would be useless and not exported for the URDF format.
• It looks that you can build up a ball joint/spherical joint by composing fixed and continuous joints (Cf. this discussion). Maybe it would be possible to convert automatically the ball joint into the four equivalent joints in case of a URDF export.
• Do not change anything and let the user create its own ball joint the way I mentionned before, but adding it somewhere in the documentation for the kinematic skeleton.

[hwiedPro]

Adding a ball joint is no problem, exporting it to URDF will lead to an error message, so that the user has to take care of it. I'll update this when I found time to tackle this.

The other two need some more changes, especially the coupling of the screw joint. This will rather happen in a later iteration.