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Why do the wheels move away from joint when the robot falls over in gazebo?

asked 2021-07-16 12:29:45 -0500

vertical_beef576 gravatar image

updated 2021-07-19 08:42:41 -0500

The wheels seem to fall of the robot when it falls over in gazebo. When the model is first spawned the wheels seem to be in the correct position but when it falls over it looks like they detach. Any help for why this happens/ how to fix it would be much appreciated. Many thanks

The code/markup used is below:

    <?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="self balancing robot">

<link name="base">
    <visual>
        <origin xyz="0 0 0.2" />

        <geometry>
            <box size="0.1 0.2 0.3"/>
        </geometry> 
    </visual>
    <collision>
    <origin xyz="0 0 0.2" />
        <geometry>
            <box size="0.1 0.2 0.3"/>
        </geometry>
    </collision>
    <inertial>
        <origin xyz="0 0 0.2" />
        <mass value="2"/>
        <inertia ixx="0.4" ixy="0.0" ixz="0.0" iyy="0.4" iyz="0.0" izz="0.2"/>
    </inertial>
</link>
<link name="right_wheel">
    <visual>
        <origin xyz="0 0.12 0.05" rpy="${pi/2} 0 0" />

        <geometry>
            <cylinder length="0.04" radius="0.05"/>
        </geometry> 
    </visual>
    <collision>
        <origin xyz="0 0.12 0.05" rpy="${pi/2} 0 0" />
        <geometry>
            <cylinder length="0.04" radius="0.05"/>
        </geometry>
    </collision>
    <inertial>
        <origin xyz="0 0.12 0.05" rpy="${pi/2} 0 0" />
        <mass value="2"/>
        <inertia ixx="0.4" ixy="0.0" ixz="0.0" iyy="0.4" iyz="0.0" izz="0.2"/>
    </inertial>
</link>
<link name="left_wheel">
    <visual>
        <origin xyz="0 -0.12 0.05" rpy="${pi/2} 0 0" />

        <geometry>
            <cylinder length="0.04" radius="0.05"/>
        </geometry> 
    </visual>
    <collision>
        <origin xyz="0 -0.12 0.05" rpy="${pi/2} 0 0" />
        <geometry>
            <cylinder length="0.04" radius="0.05"/>
        </geometry>
    </collision>
    <inertial>
        <origin xyz="0 -0.12 0.05" rpy="${pi/2} 0 0" />
        <mass value="2"/>
        <inertia ixx="0.4" ixy="0.0" ixz="0.0" iyy="0.4" iyz="0.0" izz="0.2"/>
    </inertial>
</link>
<joint name="lwj" type="continuous">
    <parent link="base"/>
    <child link="left_wheel"/>
    <origin xyz="0 0 0" /> 
    <axis xyz="0 1 0"/>
</joint>

<joint name="rwj" type="continuous">
    <parent link="base"/>
    <child link="right_wheel"/>
    <origin xyz="0 0 0" />
    <axis xyz="0 1 0"/>
</joint>

</robot>
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answered 2021-07-19 10:43:11 -0500

Mike Scheutzow gravatar image

I see at least two problems with this urdf you show:

Collision specs are relative relative to the object they apply to. Your wheel collision specs should be centered on the wheel's origin.

You attach both left and right wheel cylinders at the origin of the base box. In the real world, two objects can't occupy the same physical volume. I would guess the physics simulator is shifting the wheels so that neither is "inside" another object.

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Asked: 2021-07-16 08:31:12 -0500

Seen: 24 times

Last updated: Jul 19