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In your URDF file, your differential drive plugin is not between gazebo tags like your GPU LiDAR plugin is. Following code block should work,

<gazebo>
    <plugin filename="libignition-gazebo-diff-drive-system" name="ignition::gazebo::systems::DiffDrive">
        <left_joint>joint_left_wheel</left_joint>
        <right_joint>joint_right_wheel</right_joint>
        <wheel_separation>1.25</wheel_separation>
        <wheel_radius>0.3</wheel_radius>
        <topic>/cmd_vel</topic>
        <odom_topic>/odom</odom_topic>
    </plugin>
</gazebo>

Also before bridging over to ROS, it might be helpful to first use ign topic to check the working of any plugins/sensors that you may have