Add support for spherical joints

[Nicogene]

Right now here is the list of type of joints supported:

• Revolute
• Fixed
• Prismatic

It would be nice to add the support for the spherical joints (cc @FabioBergonti @valegagge @maggia80)

We need to:

• On creo2urdf side check how they are defined at the ElementTree level
• Add the support of this type of joints in idyntree (cc @traversaro)

[traversaro]

Hello @xela-95 @FabioBergonti @CarlottaSartore @GiulioRomualdi @Giulero,

the URDF does not support ball/spherical joints out of the box, see https://chatgpt.com/share/685d1830-bb10-8006-a9f5-ff840a8cf3ba for a related ChatGPT convo. So the question is, how do you want to export this info?

Possible options:

O1: iDynTree specific-extension

Export a joint named:

<joint name="my_joint_revolute" type="ball"> 
    <origin xyz="0 0 1" rpy="0 0 3.1416"/>
    <parent link="link1"/>
    <child link="link2"/>
</joint>

Pro:

• Simple

Cons:

• Ignored by any other non-iDynTree URDF parser

O2: Export as three consecutive revolute

Export:

<joint name="my_joint_revolute_x" type="revolute"> 
    <origin xyz=". . ." rpy=". . ."/>
    <parent link="link1"/>
    <child link="my_joint_revolute_dummy_body_1"/>
</joint>

<joint name="my_joint_revolute_y" type="revolute"> 
    <origin xyz=". . ." rpy=". . ."/>
    <parent link="my_joint_revolute_dummy_body_1"/>
    <child link="my_joint_revolute_dummy_body_2"/>
</joint>

<joint name="my_joint_revolute_y" type="revolute"> 
    <origin xyz=". . ." rpy=". . ."/>
    <parent link="my_joint_revolute_dummy_body_2"/>
    <child link="link2"/>
</joint>

Pro:

• Any URDF importer supports it

Con:

• The URDF importer needs to have some special logic to convert this sequence back to a spherical joint (o3 mentioned that the Pinocchio parser is able to automatically convert them back to spherical joints that is a nice feature, but we are not sure if that is an hallucination or not)

[traversaro]

Thinking a bit about it, I think I like more O2:

• In O1, the exported model is useless almost everywhere, and you need to control (and modify) how to parse models to make it usefl
• In O2, the exported model is ready to use everywhere, and if you control how to parse models, you can post-process your URDF (or model generated from URDF) to convert back the chain of revolute to a spherical joint

So I am leaning towards O2, but @xela-95 @FabioBergonti @CarlottaSartore @GiulioRomualdi @Giulero please provide any opinion, thanks!

[CarlottaSartore]

What we are currently doing for loading the model in Jaxsim is loading a URDF with a fixed joint and then imposing a connect constraint in Jaxsim (thus closing the kinematic chain and implementing a passive spherical joint).
We have also started to develop some utilities that convert 3 consecutive revolute joints into a fixed one and vice versa (see https://github.com/ami-iit/element_differentiable_simulators_for_codesign/issues/12#issuecomment-2988398971),

So I agree that O2 seems the best option

[FabioBergonti]

O2 looks good to me too. But, how do we handle inertial parameters for the dummy links? Using very small values might cause instabilities in simulation, right?

[CarlottaSartore]

It depends on how the simulation handles dummy link ( without inertial parameters ) it could be that they are able to identify them . For instance old gazebo is not able but jaxsim converts them to frames and I think also isaac sim have something to deal with massless link.
Anyhow in the transformation to spherical joint ( as I think you will also do for Isaac sim) the dummy links should be removed in favor of the spherical joint

[traversaro]

Yes, I agree with @CarlottaSartore . I think we need some post-processing logic in the URDF --> USD (IsaacSim), URDF --> SDF (JaxSim/gz sim) or eventually URDF --> MJCF (MuJoCo, this is something that could be include in https://github.com/ami-iit/mujoco-urdf-loader) to post-process loaded models and convert the revolute joints back to sphericals, removign the dummy link at all. For IsaacSim it would be cool to have a openusd-urdf-loader library to contain this kind of functionality, but for now it can be something hardcoded in any project that loads the URDF.

GitHub

GitHub - ami-iit/mujoco-urdf-loader

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