MJX inertial import help (or maybe bug)

[SimpleGeometry]

Intro

Using MJX for RL experiments.

My setup

mujoco==3.1.2
brax==0.10.0

My question

Not sure if this belongs in brax or mujoco, but since the error doesn't seem to be anything due to brax's processing, and the XML is a standard mujoco XML, I thought it might be appropriate here. I could not find it in other discussions/issues when I looked for "inertial".

Took the sawyer.xml file from https://github.com/google-deepmind/mujoco_menagerie/tree/main/rethink_robotics_sawyer and imported it via brax's mjcf.load(), which ultimately calls mujoco's MjModel.from_xml_string(). Though there are a number of other small issues which I think are addressed by more recent versions of brax and mjx, I disabled those for now (implicitfast integrator, capsule and cylinder collision -> change cylinder to capsule, etc.).

The issue that I am having is when calling mjcf.load, MjModel.from_xml_string() gives the following:

ValueError: XML Error: Schema violation: unique sub-element 'inertial' found 2 time(s)
Element 'body', line 109

When I enable just a single inertial tag (the topmost one, under the first body tag), I instead get the following issue:

ValueError: XML Error: World body cannot have inertia
Element 'inertial', line 106

even though the inertial tag is nested under a body, not in worldbody.

Is this a bug? If not, how can I fix this (maybe I am misusing the import and we need to adjust the XML file between mujoco and MJX)? I'm guessing maybe that the inertial tags are somehow all being clustered under worldbody, which results in both the duplicate inertial tags for one element, and the error of worldbody having the inertial tag.

Minimal model and/or code that explain my question

Not quite minimal but it's copypastable. Uncommenting all inertial tags reproduces the first error, and uncommenting just the first inertial tags reproduces the second error.

Model:

XML

<mujoco model="sawyer">
  <compiler angle="radian" meshdir="assets" autolimits="true"/>

<!--   <option integrator="implicitfast"/> -->

  <default>
    <default class="sawyer">
      <material specular="0.5" shininess="0.25"/>
      <joint axis="0 0 1"/>
      <general dyntype="none" biastype="affine"/>
      <default class="visual">
        <geom group="2" type="mesh" contype="0" conaffinity="0"/>
      </default>
      <default class="collision">
        <geom group="3" type="capsule" rgba="0.5 0.5 0.5 1"/>
        <default class="eef_collision">
          <geom type="cylinder"/>
        </default>
      </default>
      <site size="0.001" rgba="0.5 0.5 0.5 0.3" group="4"/>

      <default class="large_joint">
        <joint armature="1" damping="0.1"/> <!-- <joint frictionloss="0.3" armature="1" damping="0.1"/> -->
        <general gainprm="800" biasprm="0 -800 -80" forcerange="-80 80"/>
      </default>
      <default class="medium_joint">
        <joint armature="1" damping="0.1"/> <!-- <joint frictionloss="0.3" armature="1" damping="0.1"/> -->
        <general gainprm="800" biasprm="0 -800 -80" forcerange="-40 40"/>
      </default>
      <default class="small_joint"> 
        <joint armature="1" damping="0.1"/> <!-- <joint frictionloss="0.1" armature="1" damping="0.1"/> -->
        <general gainprm="500" biasprm="0 -500 -50" forcerange="-9 9"/>
      </default>
    </default>
  </default>

  <asset>
    <material class="sawyer" name="red" rgba="0.5 0.1 0.1 1"/>
    <material class="sawyer" name="black" rgba="0.2 0.2 0.2 1"/>
    <material class="sawyer" name="white" rgba="1 1 1 1"/>
    <material class="sawyer" name="off_white" rgba="0.792157 0.819608 0.929412 1"/>
    <material class="sawyer" name="Mtl3" rgba="0.168627 0.152941 0.141176 1"/>
    <material class="sawyer" name="Mtl4" rgba="1 0.403922 0.121569 1"/>
    <material class="sawyer" name="Mtl7" rgba="0.250980 0.250980 0.250980 1"/>
    <material class="sawyer" name="dark_black" specular="0.0" shininess="0.25" rgba="0 0 0 1"/>
    <material class="sawyer" name="Mtl5.001" rgba="0.870588 0.309804 0.086275 1"/>
    <material class="sawyer" name="Mtl7.001" rgba="0.698039 0.698039 0.698039 1"/>
    <material class="sawyer" name="Mtl5.002" rgba="0.105882 0.105882 0.105882 1"/>
    <material class="sawyer" name="Mtl6.002" rgba="0.749020 0.749020 0.749020 1"/>
    <material class="sawyer" name="Mtl4_001" rgba="0.866667 0.905882 1 1"/>
    <material class="sawyer" name="Mtl5_001" rgba="0.501961 0.501961 0.501961 1"/>

    <mesh file="pedestal.obj"/>
    <mesh file="base_0.obj"/>
    <mesh file="base_1.obj"/>
    <mesh file="l0_0.obj"/>
    <mesh file="l0_1.obj"/>
    <mesh file="l0_2.obj"/>
    <mesh file="l0_3.obj"/>
    <mesh file="l0_4.obj"/>
    <mesh file="l0_5.obj"/>
    <mesh file="l0_6.obj"/>
    <mesh file="l1_0.obj"/>
    <mesh file="l1_1.obj"/>
    <mesh file="l1_2.obj"/>
    <mesh file="l2_0.obj"/>
    <mesh file="l2_1.obj"/>
    <mesh file="l2_2.obj"/>
    <mesh file="l2_3.obj"/>
    <mesh file="l2_4.obj"/>
    <mesh file="l3_0.obj"/>
    <mesh file="l3_1.obj"/>
    <mesh file="l3_2.obj"/>
    <mesh file="l3_3.obj"/>
    <mesh file="l4_0.obj"/>
    <mesh file="l4_1.obj"/>
    <mesh file="l4_2.obj"/>
    <mesh file="l4_3.obj"/>
    <mesh file="l4_4.obj"/>
    <mesh file="l4_5.obj"/>
    <mesh file="l4_6.obj"/>
    <mesh file="l4_7.obj"/>
    <mesh file="l5_0.obj"/>
    <mesh file="l5_1.obj"/>
    <mesh file="l5_2.obj"/>
    <mesh file="l5_3.obj"/>
    <mesh file="l5_4.obj"/>
    <mesh file="l6_0.obj"/>
    <mesh file="l6_1.obj"/>
    <mesh file="l6_2.obj"/>
    <mesh file="l6_3.obj"/>
    <mesh file="l6_4.obj"/>
    <mesh file="l6_5.obj"/>
    <mesh file="head_0.obj"/>
    <mesh file="head_2.obj"/>
    <mesh file="head_3.obj"/>
    <mesh file="head_5.obj"/>
    <mesh file="head_6.obj"/>
    <mesh file="head_7.obj"/>
    <mesh file="head_8.obj"/>
    <mesh file="head_9.obj"/>
  </asset>

    <worldbody>
        <light name="top" pos="0 0 2" mode="trackcom"/>
        <body name="base" childclass="sawyer">
          <inertial mass="2.0687" pos="-0.0006241 -2.8025E-05 0.065404"
             fullinertia="0.0067599 0.0067877 0.0074031 -4.2024E-05 -6.1904E-07 1.5888E-05"/>
          <geom mesh="base_0" material="black" class="visual"/>
          <geom mesh="base_1" material="red" class="visual"/>
          <body name="right_l0" pos="0 0 0.08">
<!--             <inertial mass="5.3213" pos="0.0243645 0.0109688 0.143631" quat="0.894821 0.0089945 -0.17028 0.412576"
              diaginertia="0.0651599 0.0510954 0.0186221"/> -->
            <joint name="right_j0" class="large_joint" range="-3.0503 3.0503"/>
            <geom mesh="l0_0" material="Mtl7" class="visual"/>
            <geom mesh="l0_1" material="off_white" class="visual"/>
            <geom mesh="l0_2" material="red" class="visual"/>
            <geom mesh="l0_3" material="Mtl4" class="visual"/>
            <geom mesh="l0_4" material="Mtl3" class="visual"/>
            <geom mesh="l0_5" material="white" class="visual"/>
            <geom mesh="l0_6" material="black" class="visual"/>
            <geom class="collision" fromto="0 0 -0.005 0 0 0.25" size="0.07"/>
            <body name="head" pos="0 0 0.2965">
<!--               <inertial mass="1.5795" pos="0.00532226 -2.65473e-05 0.1021"
                quat="0.999993 7.08384e-05 -0.00359812 -0.000626267" diaginertia="0.0118334 0.00827097 0.00496582"/> -->
              <!-- <joint name="head_pan" range="-5.0952 0.9064"/> -->
              <geom mesh="head_0" material="black" class="visual"/>
              <geom mesh="head_2" material="off_white" class="visual"/>
              <geom mesh="head_3" material="Mtl7" class="visual"/>
              <geom mesh="head_5" material="black" class="visual"/>
              <geom mesh="head_6" material="white" class="visual"/>
              <geom mesh="head_7" material="dark_black" class="visual"/>
              <geom mesh="head_8" material="red" class="visual"/>
              <geom mesh="head_9" material="Mtl5.001" class="visual"/>
              <geom class="collision" fromto="-0.005 0 0.02 -0.005 0 0.205" size="0.04"/>
              <geom class="collision" type="box" pos="0.025 0 0.1" size="0.01 0.13 0.08"/>
            </body>
            <body name="right_l1" pos="0.081 0.05 0.237" quat="1 -1 1 1">
<!--               <inertial mass="4.505" pos="-0.0030849 -0.026811 0.092521" quat="0.424888 0.891987 0.132364 -0.0794294"
                diaginertia="0.0224339 0.0221624 0.00970971"/> -->
              <joint name="right_j1" class="large_joint" range="-3.8095 2.2736"/>
              <geom mesh="l1_0" material="black" class="visual"/>
              <geom mesh="l1_1" material="red" class="visual"/>
              <geom mesh="l1_2" material="white" class="visual"/>
              <geom class="collision" fromto="0 0 -0.06 0 0 0.13" size="0.068"/>
              <body name="right_l2" pos="0 -0.14 0.1425" quat="1 1 0 0">
<!--                 <inertial mass="1.745" pos="-0.00016044 -0.014967 0.13582" quat="0.707831 -0.0524761 0.0516006 0.702537"
                  diaginertia="0.0257928 0.025506 0.00292516"/> -->
                <joint name="right_j2" class="medium_joint" range="-3.0426 3.0426"/>
                <geom mesh="l2_0" material="off_white" class="visual"/>
                <geom mesh="l2_1" material="Mtl5.002" class="visual"/>
                <geom mesh="l2_2" material="white" class="visual"/>
                <geom mesh="l2_3" material="black" class="visual"/>
                <geom mesh="l2_4" material="red" class="visual"/>
                <geom class="collision" fromto="0 0 -0.15 0 0 0.26" size="0.055"/>
                <body name="right_l3" pos="0 -0.042 0.26" quat="1 -1 0 0">
<!--                   <inertial mass="2.5097" pos="-0.0048135 -0.0281 -0.084154" quat="0.902999 0.385391 -0.0880901 0.168247"
                    diaginertia="0.0102404 0.0096997 0.00369622"/> -->
                  <joint name="right_j3" class="medium_joint" range="-3.0439 3.0439"/>
                  <geom mesh="l3_0" material="off_white" class="visual"/>
                  <geom mesh="l3_1" material="red" class="visual"/>
                  <geom mesh="l3_2" material="white" class="visual"/>
                  <geom mesh="l3_3" material="black" class="visual"/>
                  <geom class="collision" fromto="0 0 -0.115 0 0 0.03" size="0.055"/>
                  <body name="right_l4" pos="0 -0.125 -0.1265" quat="1 1 0 0">
<!--                     <inertial mass="1.1136" pos="-0.00188917 0.00689948 0.134095"
                      quat="0.803247 0.031244 -0.0298409 0.594077" diaginertia="0.0136555 0.0135498 0.00127385"/> -->
                    <joint name="right_j4" class="small_joint" range="-2.9761 2.9761"/>
                    <geom mesh="l4_0" material="Mtl7" class="visual"/>
                    <geom mesh="l4_1" material="black" class="visual"/>
                    <geom mesh="l4_2" material="Mtl6.002" class="visual"/>
                    <geom mesh="l4_3" material="white" class="visual"/>
                    <geom mesh="l4_4" material="off_white" class="visual"/>
                    <geom mesh="l4_5" material="dark_black" class="visual"/>
                    <geom mesh="l4_6" material="red" class="visual"/>
                    <geom mesh="l4_7" material="black" class="visual"/>
                    <geom class="collision" fromto="0 0 -0.13 0 0 0.27" size="0.045"/>
                    <body name="right_l5" pos="0 0.031 0.275" quat="1 -1 0 0">
<!--                       <inertial mass="1.5625" pos="0.0061133 -0.023697 0.076416" quat="0.404076 0.9135 0.0473125 0.00158335"
                        diaginertia="0.00474131 0.00422857 0.00190672"/> -->
                      <joint name="right_j5" class="small_joint" range="-2.9761 2.9761"/>
                      <geom mesh="l5_0" material="Mtl5_001" class="visual"/>
                      <geom mesh="l5_1" material="Mtl4_001" class="visual"/>
                      <geom mesh="l5_2" material="white" class="visual"/>
                      <geom mesh="l5_3" material="red" class="visual"/>
                      <geom mesh="l5_4" material="black" class="visual"/>
                      <geom fromto="0 0 -0.02 0 0 0.1" size="0.045" class="collision"/>
                      <geom fromto="0 0 0.105 0 -0.082 0.105" size="0.045" class="collision"/>
                      <body name="right_l6" pos="0 -0.11 0.1053" quat="0.0616248 0.06163 -0.704416 0.704416">
<!--                         <inertial mass="0.3292" pos="-8.0726e-06 0.0085838 -0.0049566"
                          quat="0.479044 0.515636 -0.513069 0.491321" diaginertia="0.000360268 0.000311078 0.000214984"/> -->
                        <joint name="right_j6" class="small_joint" range="-4.7124 4.7124"/>
                        <geom mesh="l6_0" material="Mtl4" class="visual"/>
                        <geom mesh="l6_1" material="white" class="visual"/>
                        <geom mesh="l6_2" material="red" class="visual"/>
                        <geom mesh="l6_3" material="black" class="visual"/>
                        <geom mesh="l6_4" material="black" class="visual"/>
                        <geom mesh="l6_5" material="black" class="visual"/>
                        <geom class="collision" size="0.045 0.02"/> <!-- <geom class="eef_collision" size="0.045 0.02"/> -->
                        <site name="attachment_site" pos="0 0 0.0245" quat="0 0 0 1"/>
                      </body>
                    </body>
                  </body>
                </body>
              </body>
            </body>
          </body>
        </body>
    </worldbody>

    <actuator>
        <general name="a0" joint="right_j0" class="large_joint" ctrlrange="-3.0503 3.0503"/>
        <general name="a1" joint="right_j1" class="large_joint" ctrlrange="-3.8095 2.2736"/>
        <general name="a2" joint="right_j2" class="medium_joint" ctrlrange="-3.0426 3.0426"/>
        <general name="a3" joint="right_j3" class="medium_joint" ctrlrange="-3.0439 3.0439"/>
        <general name="a4" joint="right_j4" class="small_joint" ctrlrange="-2.9761 2.9761"/>
        <general name="a5" joint="right_j5" class="small_joint" ctrlrange="-2.9761 2.9761"/>
        <general name="a6" joint="right_j6" class="small_joint" ctrlrange="-4.7124 4.7124"/>
    </actuator>
    
<!--     <keyframe>
        <key name="home" qpos="0 -1.18 0 2.18 0 0.57 3.3161" ctrl="0 -1.18 0 2.18 0 0.57 3.3161"/>
  </keyframe> -->
</mujoco>

Code:

from brax.io import mjcf
sys = mjcf.load("rethink_robotics_sawyer/sawyer.xml")

Confirmations

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

[kevinzakka]

I don't deeply know the inner workings of brax but upon quick investigation, the culprit seems to be this _fuse_bodies function which is probably fusing the base link (which is dofless) with right_l0. The merge is probably resulting in 2 inertial tags which explains the error.

[kevinzakka]

cc @erikfrey

[erikfrey]

@kevinzakka is correct - the culprit is _fuse_bodies. Brax doesn't expect a dofless body to have inertia. Honestly I'm not entirely sure what inertia even means for a static body, but hey if MJCF can express it then Brax should handle it.

@SimpleGeometry we can fix this in Brax, but in the meantime, I think you can get around this by changing the dofless body from a body to a frame:

  <worldbody>
    <frame name="base" childclass="sawyer"/>
      ...
    </frame>
  </worldbody>

[SimpleGeometry]

Oh, I see, that makes a lot of sense in hindsight. Am very new to this so thank you for the explanation and workaround!

[SimpleGeometry]

Sorry, reopening this thread -- I had a chance to try it but have another question (not sure what the convention is on reopening threads, I unmarked answer in case that matters for visibility but sorry if this is nonstandard). Changing the dofless body to a frame works if I additionally remove the inertial tag under it (this seems fine to me because it's fixed, as you mention).

I switched to an XML which has an end-effector (also swapped to the most recent versions, brax 0.10.5 and mjx 3.2.3) in Menagerie, xArm7: https://github.com/google-deepmind/mujoco_menagerie/tree/main/ufactory_xarm7.

I get a similar error if I leave the inertial tag of the gripper base link as-is, so I changed it to a frame and removed the inertial tag, leaving in the inertials of the children elements. However, I think in this case the inertial tag does matter because it's being moved by the rest of the arm. Even though I have the gripper base link as a frame, I get an error if I specify the inertial tag. Is there any workaround for this (or instead, am I misunderstanding something)?

Never mind, I can't reproduce the issue anymore, I may have messed something up that I didn't realize (inertial tag under frame works fine now). I guess then I just want to clarify, if I change the body to a frame but I add the inertial tag, does that mass information get used or ignored?

                      <frame name="xarm_gripper_base_link" quat="0 0 0 1">
                       <inertial pos="-0.00065489 -0.0018497 0.048028" quat="0.997403 -0.0717512 -0.0061836 0.000477479"
                        mass="0.54156" diaginertia="0.000471093 0.000332307 0.000254799"/>
                      <geom mesh="base_link"/>
                      <body name="left_outer_knuckle" pos="0 0.035 0.059098">
                        <inertial pos="0 0.021559 0.015181" quat="0.47789 0.87842 0 0" mass="0.033618"
                          diaginertia="1.9111e-05 1.79089e-05 1.90167e-06"/>
                        <joint name="left_driver_joint" axis="1 0 0" class="driver"/>
                        <geom material="black" mesh="left_outer_knuckle"/>
                        <body name="left_finger" pos="0 0.035465 0.042039">
                          <inertial pos="0 -0.016413 0.029258" quat="0.697634 0.115353 -0.115353 0.697634"
                            mass="0.048304" diaginertia="1.88037e-05 1.7493e-05 3.56792e-06"/>
                          <joint name="left_finger_joint" axis="-1 0 0" class="follower"/>
                          <geom material="black" mesh="left_finger"/>
                        </body>
                      </body>
                      <body name="left_inner_knuckle" pos="0 0.02 0.074098">
                         <inertial pos="1.86601e-06 0.0220468 0.0261335" quat="0.664139 -0.242732 0.242713 0.664146"
                          mass="0.0230126" diaginertia="8.34216e-06 6.0949e-06 2.75601e-06"/>
                        <joint name="left_inner_knuckle_joint" axis="1 0 0" class="spring_link"/>
                        <geom material="black" mesh="left_inner_knuckle"/>
                      </body>
                      <body name="right_outer_knuckle" pos="0 -0.035 0.059098">
                         <inertial pos="0 -0.021559 0.015181" quat="0.87842 0.47789 0 0" mass="0.033618"
                          diaginertia="1.9111e-05 1.79089e-05 1.90167e-06"/>
                        <joint name="right_driver_joint" axis="-1 0 0" class="driver"/>
                        <geom material="black" mesh="right_outer_knuckle"/>
                        <body name="right_finger" pos="0 -0.035465 0.042039">
                          <inertial pos="0 0.016413 0.029258" quat="0.697634 -0.115356 0.115356 0.697634"
                            mass="0.048304" diaginertia="1.88038e-05 1.7493e-05 3.56779e-06"/>
                          <joint name="right_finger_joint" axis="1 0 0" class="follower"/>
                          <geom material="black" mesh="right_finger"/>
                        </body>
                      </body>
                      <body name="right_inner_knuckle" pos="0 -0.02 0.074098">
                        <inertial pos="1.866e-06 -0.022047 0.026133" quat="0.66415 0.242702 -0.242721 0.664144"
                          mass="0.023013" diaginertia="8.34209e-06 6.0949e-06 2.75601e-06"/>
                        <joint name="right_inner_knuckle_joint" axis="-1 0 0" class="spring_link"/>
                        <geom material="black" mesh="right_inner_knuckle"/>
                      </body>
                      <site name="link_tcp" pos="0 0 .172"/>
                    </frame>