Spherical (Ball) Joint in Robot Model not Working in Gazebo

Question

I'm having trouble including a spherical/ball joint in my robot model. I have tried using the 'ball' joint type defined by sdf (http://sdformat.org/spec?ver=1.6&elem=joint#joint_type) but the urdf will no longer compile, throwing the error: 'Joint [joint_name] has no known type [ball]'.

I have also tried building a spherical joint using three orthogonal universal joints. The model does not throw any errors, and in rviz everything is displayed as expected and without issue. In gazebo however, neither the three universal joints composing the spherical joint nor the child link show up, or are listed as part of the model.

.xacro:

...
    <link name="${leg_name}_psi_link">
        <visual><geometry><sphere radius=".00001"/></geometry></visual>
        <collision><geometry><sphere radius=".00001"/></geometry></collision>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value=".00001"/>
        <inertia ixx="0.00001" ixy="0.0" ixz="0.0" iyy="0.00001" iyz="0.0" izz="0.00001" />
    </link>

    <link name="${leg_name}_theta_link">
        <visual><geometry><sphere radius=".00001"/></geometry></visual>
        <collision><geometry><sphere radius=".00001"/></geometry></collision>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value=".00001"/>
        <inertia ixx="0.00001" ixy="0.0" ixz="0.0" iyy="0.00001" iyz="0.0" izz="0.00001" />
    </link>

    <link name="${leg_name}_foot_link">
    <visual><geometry><mesh filename="${meshes_path}/foot_link.stl" /></geometry></visual>
        <collision><geometry><mesh filename="${meshes_path}/foot_link_collision.stl" /></geometry></collision>
    <xacro:default_inertial mass="${foot_mass}"/>
    </link>

    <joint name="${leg_name}_psi_joint" type="continuous" >
    <parent link="${leg_name}_tibia_link" />
        <child link="${leg_name}_psi_link" />
    <axis xyz="0 0 1" />
    <dynamics damping="0.005" />
    <origin xyz="0.075 0 0" rpy="0 0 0" />
    <limit lower="${-pi/2}" upper="${-pi/2}" velocity="${joint_velocity_limit}" effort="${joint_effort_limit}" />
    </joint>

    <joint name="${leg_name}_theta_joint" type="continuous" >
    <parent link="${leg_name}_psi_link"/>
        <child link="${leg_name}_theta_link"/>
    <axis xyz="0 1 0" />
    <dynamics damping="0.005" />
    <origin xyz="0 0 0" rpy="0 0 0" />
    <limit lower="${-pi/2}" upper="${-pi/2}" velocity="${joint_velocity_limit}" effort="${joint_effort_limit}" />
    </joint>

    <joint name="${leg_name}_phi_joint" type="continuous" >
    <parent link="${leg_name}_theta_link"/>
        <child link="${leg_name}_foot_link"/>
    <axis xyz="1 0 0" />
    <dynamics damping="0.005" />
    <origin xyz="0 0 0" rpy="0 0 0" />
    <limit lower="${-pi/2}" upper="${-pi/2}" velocity="${joint_velocity_limit}" effort="${joint_effort_limit}" />
    </joint>
...

launch file (relevent parts)

  <!-- Load robot description -->
  <param name="robot_description" command="xacro --inorder 'path/to/robot.xacro'" />

  <!-- load the controllers -->
  <node name="controller_spawner" pkg="controller_manager" type="spawner" respawn="false"
        output="screen" ns="/syropod" args="joint_state_controller
              AL_coxa_joint
              AL_femur_joint
              AL_tibia_joint
              AR_coxa_joint
              AR_femur_joint
              AR_tibia_joint
              BL_coxa_joint
              BL_femur_joint
              BL_tibia_joint
              BR_coxa_joint
              BR_femur_joint
              BR_tibia_joint"/>

  <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model"
        args="-urdf
        -model syropod
        -param robot_description
        -x 0.0
        -y 0.0
        -z 0.2
        -R 0.0
        -P 0.0
        -Y 0.0
        -J AL_coxa_joint 0.0
        -J AL_femur_joint 0.0
        -J AL_tibia_joint 0.0
        -J AL_psi_joint 0.0
        -J AL_theta_joint 0.0
        -J AL_phi_joint 0.0
        -J AR_coxa_joint 0.0
        -J AR_femur_joint 0.0
        -J AR_tibia_joint 0.0
        -J AR_psi_joint 0.0
        -J AR_theta_joint 0.0
        -J AR_phi_joint 0.0
        -J BL_coxa_joint 0.0
        -J BL_femur_joint 0.0
        -J BL_tibia_joint 0.0
        -J BL_psi_joint 0.0
        -J BL_theta_joint 0.0
        -J BL_phi_joint 0.0
        -J BR_coxa_joint 0.0
        -J BR_femur_joint 0.0
        -J BR_tibia_joint 0.0
        -J BR_psi_joint 0.0
        -J BR_theta_joint 0.0
        -J BR_phi_joint 0.0"
  respawn="false" output="screen"/>

***Why do none of the joints or links appear in the robot model?****What am I missing/doing wrong? Is there a better way to do this?*

Notes

the three orthogonal joints represent the roll pitch yaw of the spherical joint they compose. their mass and inertia matrices are close to zero intentionally. they are not zero because I understand that this can cause issues in gazebo.

${leg_name}_foot_link is the end effector link

Thanks

Answer 1

Answer:

The issue is that <origin>, <mass>, and <inertia> tags must be nested inside a <inertial> element for each of the links, instead of being nested in the <link> tag.

correct:

     <link name="${leg_name}_psi_link">
         <visual><geometry><sphere radius=".01"/></geometry></visual>
         <collision><geometry><sphere radius=".005"/></geometry></collision>
         <inertial>
             <origin xyz="0 0 0" rpy="0 0 0"/>
             <mass value="${gimbal_link_mass}"/>
             <inertia ixx="0.1" ixy="0.0" ixz="0.0" iyy="0.1"  iyz="0.0" izz="0.1"/>
         </inertial>
     </link>

incorrect:

     <link name="${leg_name}_psi_link">
     <visual><geometry><sphere radius=".01"/></geometry></visual>
     <collision><geometry><sphere radius=".005"/></geometry></collision>
     <origin xyz="0 0 0" rpy="0 0 0"/>
     <mass value="${gimbal_link_mass}"/>
         <inertia ixx="0.1" ixy="0.0" ixz="0.0" iyy="0.1" iyz="0.0" izz="0.1"/>
     </link>

additional notes:

• un-actuated joints should not have a <transmission> tag
• gazebo will not load links with zero mass