Imported MJCF only moves in half the environments when loading multiple instances #2572

linusvwe · 2025-05-26T07:40:18Z

linusvwe
May 26, 2025

Hi,

I've imported a simple MJCF description of a knob with a revolute joint attached to a base, using the conversion scripts from the IsaacLab repo. I used the following flags: --import-sites --make-instanceable --fix-base
Additionally, I applied the workaround to move instanceable from the XForms to the geoms, as suggested in a another discussion (#2113)

Problem:
When I load multiple environments with this knob, only exactly half of the knobs move correctly. In the others, the knob appears frozen. The joint position stays fixed and the joint velocity remains zero. It’s as if the articulation is inactive or stuck.

I'm using a basic direct environment setup where a Shadow Hand is spawned above each knob to turn it.
The minimal mjcf I import:

<mujoco model="KnobArticulation">
    <asset>
        <mesh name="knob_mesh" file="assets/knob.stl"/>
    </asset>

    <worldbody>
        <body name="worldBody" pos="0 0 0" quat="1 0 0 0">
            <body name="knob" pos="0 0 0" quat="1 0 0 0">
                <joint name="knob_revolute" type="hinge" pos="0 0 0" axis="0 0 1"/>
                <geom name="knob_geom" type="mesh" mesh="knob_mesh"/>
            </body>
        </body>
    </worldbody>
</mujoco>

I also tried setting more default parameters like friction, armature, damping, etc. but it didn't change anything.

In my env_cfg:

knob_cfg: ArticulationCfg = ArticulationCfg(
        prim_path="/World/envs/env_.*/object",  
        spawn=sim_utils.UsdFileCfg(
            usd_path=f"{ISAACLAB_ASSETS_DATA_DIR}/Props/SRL/simple_knob/knob.usd",
            scale=(0.001, 0.001, 0.001),
            articulation_props=sim_utils.ArticulationRootPropertiesCfg(
                solver_position_iteration_count=8,
                solver_velocity_iteration_count=0,
                sleep_threshold=0.005,
                stabilization_threshold=0.0005,
            ),
        ),
        init_state=ArticulationCfg.InitialStateCfg(pos=(-0.028, 0.0, 0.0), rot=(1.0, 0.0, 0.0, 0.0)),
        actuators={
            "knob": ImplicitActuatorCfg(
                joint_names_expr=[
                    "knob_revolute",
                ],
                effort_limit_sim=None,
                velocity_limit_sim=200,
                stiffness=1.0,
                damping=0.5,
                friction=0.01,
            ),
        },    
    )

I have set replicate physics=True.

Has someone an idea how to fix this issue?
Thanks in advance!

Answered by linusvwe

May 28, 2025

I found that if I lift the object just slightly off the ground, so adjusting the initial z position slightly upwards, then it works correctly in all environments.

That suggests that in exactly half of the environments, the knob was slightly intersecting or embedded into the floor on spawn. This caused the articulation to become stuck or inactive. Slightly raising the object avoids this and resolves the issue.

View full answer

linusvwe · 2025-05-28T12:43:30Z

linusvwe
May 28, 2025
Author

I found that if I lift the object just slightly off the ground, so adjusting the initial z position slightly upwards, then it works correctly in all environments.

That suggests that in exactly half of the environments, the knob was slightly intersecting or embedded into the floor on spawn. This caused the articulation to become stuck or inactive. Slightly raising the object avoids this and resolves the issue.

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Imported MJCF only moves in half the environments when loading multiple instances #2572

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linusvwe May 28, 2025 Author

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May 26, 2025

linusvwe
May 28, 2025
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