Using mujoco.elasticity.solid and attaching two elastic objects to one body

[JedMuff]

Intro

Hi!
I am a phd researcher at Vrije Universiteit Amsterdam, I use MuJoCo for my research on Evolutionary Robotics and I am currently trying out mujocos elasticity plugins but I am missing some information.

My setup

Python 3.12.3, Mujoco 3.2.1, Ubuntu 22.04.4 LTS

My question

I am using the mujoco.elasticity.solid plugin. Firstly I notice how this elasticity plugin is not in the latest version of mujoco 3.2.4. My first question is why is that so? Should I be using mujoco.elasticity.shell?

My second question is I am trying to attach two solid elastic cuboids (see code) to a solid cube and I get the following error:
m = mujoco.MjModel.from_xml_string(XML, ASSETS) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ValueError: Error: engine error: Body 2 does not have plugin instance 0

I don't know what is causing this. Can someone help?

Minimal model and/or code that explain my question

    <flexcomp name="fr_flipper" type="grid" count="5 2 2" spacing="0.063 0.063 0.0603" radius="0" mass="5" rgba="255 5 0 1" pos="0.2147725 0 0" quat="1 0 0 0" dim="3">
      <contact condim="3" solref="0.01 1" solimp="0.95 0.98 0.0001" selfcollide="none"/>
      <plugin plugin="mujoco.elasticity.solid">
        <config key="poisson" value="0.9"/>
        <config key="young" value="5e-1"/>
      </plugin>
      <edge equality="true" damping="1"/>
      <pin id="1 2 3 4"/>
    </flexcomp>
    
    <flexcomp name="bl_flipper" type="grid" count="5 2 2" spacing="0.063 0.063 0.0603" radius="0" mass="5" rgba="255 5 0 1" pos="0.0 0.2147725 0" quat="0.70710678118654757 0 0 0.70710678118654746" dim="3">
      <contact condim="3" solref="0.01 1" solimp="0.95 0.99 0.0001" selfcollide="none"/>
      <plugin plugin="mujoco.elasticity.solid">
        <config key="poisson" value="0.9"/>
        <config key="young" value="5e-1"/>
      </plugin>
      <edge equality="true" damping="1"/>
      <pin id="1 2 3 4"/>
    </flexcomp>
  </body>
</body>

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[JedMuff]

...Well, almost 30mins later I found a solution to the bug 🤦. If you create a new body for each soft part it works. Does not answer my first question though. Also I wonder if you can connect two rigid bodies to one elastic solid. I tried but doesn't seem to work. I think this might be a limitation.

[quagla]

Hi the answer to the first question is that the solid plugin has now moved to the engine so you don't need a plugin at all :) see https://github.com/google-deepmind/mujoco/blob/main/model/flex/floppy.xml#L33

[JedMuff]

Oh thats really useful to know thank you!