Home | Tutorials | Wiki | Issues
Ask Your Question
0

Including a Spherical (Ball) Joint in Robot Model

asked 2018-11-12 02:41:27 -0500

JeremySMorgan gravatar image

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...) 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 ...
(more)
edit retag flag offensive close merge delete

2 Answers

Sort by ยป oldest newest most voted
1

answered 2018-11-13 19:56:10 -0500

JeremySMorgan gravatar image

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
edit flag offensive delete link more
0

answered 2018-11-13 04:13:05 -0500

PeteBlackerThe3rd gravatar image

I can see a few things.

SDF and URDF are different formats that support different sets of joint types. Unfortunately ball joints are only supported by SDF and not URDF!

You have defined three continuous joints but also given them limits, this will not work. By definition a continuous joint doesn't have any limits, you want to use the revolute joint type instead. Hopefully this will get this showing up in Gazebo. You may still have some issues with your joint at singularities but it should be a good approximation of a ball joint in URDF.

NOTE: A universal joint is a specific type of joint which includes two revolute joints with axes at 90 degrees to each other. This composite joint type is only supported by SDF.

edit flag offensive delete link more

Comments

thanks! I didn't realize urdf and sdf had a different set of possible joint types. I would give you an upvote by I don't have the 3points required XD. I also realized that the issue was actually that I didn't have a <inertial> tag. Gazebo needs a <inertial> tag with <origin>, <axis> and <inertia> nested inside, instead of <origin>, <axis> and <inertia> inside the <link>

JeremySMorgan gravatar imageJeremySMorgan ( 2018-11-13 19:24:24 -0500 )edit

No problem, yes URDF and SDF are so similar and so different it can be confusing. I've just given you enough karma to up vote BTW ;-)

PeteBlackerThe3rd gravatar imagePeteBlackerThe3rd ( 2018-11-16 06:04:58 -0500 )edit
Login/Signup to Answer

Question Tools

Stats

Asked: 2018-11-12 02:41:27 -0500

Seen: 1,027 times

Last updated: Nov 13 '18