I'm a student and I'm trying to use MuJoCo for my personal projects. (currently simulating a resisted-motion system that involves roller-chain transmissions).
At the moment I'm looking for some help with simulating a chain made of toroidal links, under gravity.
I modelled the chain in blender and made sure that none of the solids overlap. I have also set convexhull attribute of the compiler tag in MJCF to false. But still, the chain "explodes".
Here's a short video clip of the occurrence of the problem in simulation: https://github.com/google-deepmind/mujoco/assets/112756087/be0414ce-fcf9-4bcf-a18c-5afeceddd139
Here is a model which explains my question: Here's the link to the models's assets
<mujoco>
<compiler convexhull="false"/>
<asset>
<mesh name="link1" file="assets\Torus_001.obj" />
<mesh name="link2" file="assets\Torus_002.obj" />
<mesh name="link3" file="assets\Torus_003.obj" />
<mesh name="link4" file="assets\Torus_004.obj" />
<mesh name="link5" file="assets\Torus_005.obj" />
<mesh name="link6" file="assets\Torus_006.obj" />
<mesh name="link7" file="assets\Torus_007.obj" />
<mesh name="link8" file="assets\Torus_008.obj" />
<mesh name="link9" file="assets\Torus_009.obj" />
<mesh name="link10" file="assets\Torus_010.obj" />
<mesh name="link11" file="assets\Torus_011.obj" />
<mesh name="link12" file="assets\Torus_012.obj" />
<mesh name="link13" file="assets\Torus_013.obj" />
<mesh name="link14" file="assets\Torus_014.obj" />
<mesh name="link15" file="assets\Torus_015.obj" />
<mesh name="link16" file="assets\Torus_016.obj" />
<mesh name="link17" file="assets\Torus_017.obj" />
<mesh name="link18" file="assets\Torus_018.obj" />
<mesh name="link19" file="assets\Torus_019.obj" />
<mesh name="link20" file="assets\Torus_020.obj" />
<mesh name="link21" file="assets\Torus_021.obj" />
<mesh name="link22" file="assets\Torus_022.obj" />
<mesh name="link23" file="assets\Torus_023.obj" />
<mesh name="link24" file="assets\Torus_024.obj" />
</asset>
<worldbody>
<light diffuse="1 1 1" pos="6 -6 6" dir="-1 1 -1"/>
<geom type="plane" pos="0 0 0" size="2 2 0.01"/>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link1"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link2"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link3"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link4"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link5"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link6"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link7"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link8"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link9"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link10"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link11"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link12"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link13"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link14"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link15"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link16"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link17"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link18"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link19"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link20"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link21"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link22"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link23"/>
</body>
<body>
<joint type="free"/>
<geom type="mesh" mesh="link24"/>
</body>
</worldbody>
</mujoco>
As you can see, I turned off the generation of the convexhull collision profile. The expected behavior is that the chain should just fall on the worldbody surface (or it should hang in there when there is an obstacle on the inner-side of the chain, like a shaft)
Is it a problem with kinematic loops?
Any help regarding this problem will be greatly appreciated. Thanks in advance!