Arm Disappears when Gripper Added to MJCF xml File #3145

sayleuih · 2026-03-03T05:30:52Z

sayleuih
Mar 3, 2026

Intro

Hi!

I'm a student researcher working on simulating a cobot. Our cobot has a parallel gripper attached to its end effector.

My setup

Since the original MJCF model given by elephant robotics only includes the cobot, I am trying to model the gripper as well based off the URDF: https://github.com/automaticaddison/mycobot_ros2/blob/main/mycobot_description/urdf/mech/adaptive_gripper.urdf.xacro.

Here is my Arm.xml:
`

    <!-- gripper -->
    <mesh name="gripper_base" file="gripper_base.stl"/>
    <mesh name="gripper_left1" file="gripper_left1.stl"/>
    <mesh name="gripper_left2" file="gripper_left2.stl"/>
    <mesh name="gripper_left3" file="gripper_left3.stl"/>
    <mesh name="gripper_right1" file="gripper_right1.stl"/>
    <mesh name="gripper_right2" file="gripper_right2.stl"/>
    <mesh name="gripper_right3" file="gripper_right3.stl"/>
</asset>

<worldbody>
    <geom quat="0.000796327 0 0 1" type="mesh" contype="0" conaffinity="0" group="1" mesh="joint1_jet" />
    <geom quat="0.000796327 0 0 1" type="mesh" mesh="joint1_jet" />
    <body name="joint2" pos="0 0 0.15756">
        <inertial pos="-4.46252e-07 -0.0048982 -0.0203936" quat="0.988685 0.150004 1.75753e-05 -6.19332e-05" mass="0.153266" diaginertia="0.00010997 0.000104609 4.89092e-05" />
        <joint name="joint2_to_joint1" pos="0 0 0" axis="0 0 1" limited="true" range="-2.9321 2.9321" />
        <geom pos="0 0 -0.06096" quat="0.707105 0 0 -0.707108" type="mesh" contype="0" conaffinity="0" group="1" mesh="joint2" />
        <geom pos="0 0 -0.06096" quat="0.707105 0 0 -0.707108" type="mesh" mesh="joint2" />
        <body name="joint3" pos="0 0 -0.001" quat="0.499998 0.500002 0.5 -0.5">
            <inertial pos="-0.0548658 -3.79418e-06 0.0581352" quat="0.50338 0.49662 0.497294 0.502669" mass="0.4" diaginertia="0.000569683 0.000554829 8.82752e-05" />
            <joint name="joint3_to_joint2" pos="0 0 0" axis="0 0 1" limited="true" range="-2.0943 2.0943" />
            <geom pos="0 0 0.03256" quat="0.707105 0 -0.707108 0" type="mesh" contype="0" conaffinity="0" group="1" mesh="joint3" />
            <geom pos="0 0 0.03256" quat="0.707105 0 -0.707108 0" type="mesh" mesh="joint3" />
            <body name="joint4" pos="-0.1104 0 0">
                <inertial pos="-0.0454441 4.51021e-07 0.00478273" quat="0.508392 0.491471 0.491339 0.508508" mass="0.219069" diaginertia="0.000359328 0.000340078 6.35956e-05" />
                <joint name="joint4_to_joint3" pos="0 0 0" axis="0 0 1" limited="true" range="-2.6179 2.6179" />
                <geom pos="0 0 0.03056" quat="0.707105 0 -0.707108 0" type="mesh" contype="0" conaffinity="0" group="1" mesh="joint4" />
                <geom pos="0 0 0.03056" quat="0.707105 0 -0.707108 0" type="mesh" mesh="joint4" />
                <body name="joint5" pos="-0.096 0 0.06462" quat="0.707105 0 0 -0.707108">
                    <inertial pos="1.49997e-05 -0.00761485 -0.00688111" quat="0.903244 0.429125 -0.00118023 0.00112811" mass="0.0576212" diaginertia="1.88765e-05 1.81573e-05 1.0565e-05" />
                    <joint name="joint5_to_joint4" pos="0 0 0" axis="0 0 1" limited="true" range="-2.5307 2.5307" />
                    <geom pos="0 0 -0.03356" quat="0.707105 -0.707108 0 0" type="mesh" contype="0" conaffinity="0" group="1" mesh="joint5" />
                    <geom pos="0 0 -0.03356" quat="0.707105 -0.707108 0 0" type="mesh" mesh="joint5" />
                    <body name="joint6" pos="0 -0.07318 0" quat="0.499998 0.5 -0.5 0.500002">
                        <inertial pos="-3.34552e-08 0.00487808 -0.00751659" quat="0.395895 0.918296 6.92557e-06 2.44425e-06" mass="0.0904837" diaginertia="4.06947e-05 3.23387e-05 2.57661e-05" />
                        <joint name="joint6_to_joint5" pos="0 0 0" axis="0 0 1" limited="true" range="-2.8797 2.8797" />
                        <geom pos="0 0 -0.038" type="mesh" contype="0" conaffinity="0" group="1" mesh="joint6" />
                        <geom pos="0 0 -0.038" type="mesh" mesh="joint6" />
                        <body name="joint6_flange" pos="0 0.0456 0" quat="0.707105 -0.707108 0 0">
                            <inertial pos="-6.68861e-07 -1.54403e-07 -0.00514555" quat="0.276336 0.650875 -0.27633 0.650877" mass="0.0121397" diaginertia="1.94827e-06 1.13494e-06 1.13491e-06" />
                            <joint name="joint7_to_joint6" pos="0 0 0" axis="0 0 1" limited="true" range="-3.14 3.14159" />
                            <geom pos="0 0 -0.012" type="mesh" contype="0" conaffinity="0" group="1" mesh="joint7" />
                            <geom pos="0 0 -0.012" type="mesh" mesh="joint7" />
                            
                            <body name="gripper" pos="0 0 0">
                                <include file="gripper.xml"/>
                            </body>
                        </body>
                    </body>
                </body>
            </body>
        </body>
    </body>
</worldbody>

`

Here is my gripper.xml:
`

<geom type="mesh" mesh="gripper_base" pos="0 0 -0.012"/>

<!-- gripper left 3 (controller) -->
<body name="gripper_left3" pos="-0.012 0.005 0">
    <joint name="gripper_controller"
            type="hinge"
            axis="0 0 1"
            range="-0.7 0.15"/>

    <geom type="mesh" mesh="gripper_left3"/>

    <!-- gripper left 1 -->
    <body name="gripper_left1" pos="-0.027 0.016 0">
        <joint name="left3_to_left1"
                type="hinge"
                axis="0 0 1"
                range="-0.5 0.5"/>
        <geom type="mesh" mesh="gripper_left1"/>
    </body>
</body>

<!-- gripper left 2-->
<body name="gripper_left2" pos="-0.005 0.027 0">
    <joint name="base_to_left2"
            type="hinge"
            axis="0 0 1"
            range="-0.8 0.5"/>
    <geom type="mesh" mesh="gripper_left2"/>
</body>

<!-- gripper right 3 -->
<body name="gripper_right3" pos="0.012 0.005 0">
    <joint name="base_to_right3"
            type="hinge"
            axis="0 0 1"
            range="-0.15 0.7"/>
    <geom type="mesh" mesh="gripper_right3"/>

    <!-- gripper right 1 -->
    <body name="gripper_right1" pos="0.027 0.016 0">
        <joint name="right3_to_right1"
                type="hinge"
                axis="0 0 1"
                range="-0.5 0.5"/>
        <geom type="mesh" mesh="gripper_right1"/>
    </body>
</body>

<!-- gripper right 2 -->
<body name="gripper_right2" pos="0.005 0.027 0">
    <joint name="base_to_right2"
            type="hinge"
            axis="0 0 1"
            range="-0.5 0.8"/>
    <geom type="mesh" mesh="gripper_right2"/>
</body>

`

My question

Unfortunately, after I added my gripper in, the render in MuJoCo only shows my gripper. Weirdly when I control the arm joint sliders it rotates the gripper as intended.

How do I get MuJoCo to display both the gripper and the arm at the same time? The gripper needs to be connected to the end of the arm acting as the end effector and rotate with joint 7. Since it's a physics simulation picking up real objects, it would also be nice to have the gripper keep its mass.

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

Model:

minimal XML

<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
  </worldbody>
</mujoco>

Code:

import mujoco
import mediapy as media
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

Confirmations

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I searched previous Issues and Discussions, I am certain this has not been raised before.

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Arm Disappears when Gripper Added to MJCF xml File #3145

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Arm Disappears when Gripper Added to MJCF xml File #3145

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sayleuih Mar 3, 2026

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

sayleuih
Mar 3, 2026