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Delta Robot simulation using Gazebo

asked 2019-03-28 13:56:10 -0500

Cedric gravatar image

I am defining my delta robot using XACRO file. After implementing most of the parts, I have to place the "suction cup" object connected to three arms. This object should be fixed to all three arms. However, I am not able to define multiple joints (as parents) for this object. Is there a way to connect an object fixed to three other objects at the same time? I am really stuck in that project and hope to get help so I can advance with my project. Thank you in advance!

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answered 2019-03-29 13:36:44 -0500

As you observed, XACRO/URDF does not support closed kinematic loops.

However, SDF does support closed kinematic loops.

You either want to define your delta robot directly in SDF
or
include SDF elements within your XACRO/URDF using the <gazebo> tag.


Example Snippet (adding a gas-spring to a robot arm URDF model):

<?xml version="1.0" ?>
<robot name="HEBI_6_DOF_Arm_1">

  ...

  <link name="a_2040_01r_12Z">
    <visual>
      <origin rpy="0 0 0" xyz="0.00144337567 0.0366111248 -0.0110127649"/>
      <geometry>
        <mesh filename="package://hebi_6_dof_arm_generated_urdf/meshes/a_2040_01r_12z_prt.stl" scale="0.001 0.001 0.001"/>
      </geometry>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0.00144337567 0.0366111248 -0.0110127649"/>
      <geometry>
        <mesh filename="package://hebi_6_dof_arm_generated_urdf/meshes/a_2040_01r_12z_prt.stl" scale="0.001 0.001 0.001"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="-0.0048895444636 0.025165657448 0.0321043733508"/>
      <mass value="0.20946134593484614"/>
      <inertia ixx="0.000172350274153" ixy="6.95173997536e-06" ixz="-1.73640622883e-06" iyy="0.000214786314285" iyz="-3.45081093975e-05" izz="0.000121134255458"/>
    </inertial>
  </link>
  <gazebo reference="a_2040_01r_12Z">
    <selfCollide>True</selfCollide>
    <material>Gazebo/Black</material>
    <mu1>0.1</mu1>
    <mu2>0.1</mu2>
    <kp>10000000</kp>
    <kd>1</kd>
    <minDepth>0.0005</minDepth>
    <maxVel>100</maxVel>
  </gazebo>

  <joint name="a_2040_01r_12Z_to_hebi_x5_9_16Z" type="fixed">
    <parent link="a_2040_01r_12Z"/>
    <child link="hebi_x5_9_16Z"/>
    <origin rpy="1.57079632679 -0.0 -3.14159265359" xyz="-5.38184507892e-17 0.03750000003 0.055"/>
  </joint>

  <gazebo>
    <!-- Add a_2040_01r_12Z to cylinder ball joint -->
    <joint name="a_2040_01r_12Z_to_cylinder" type="ball">
      <parent>a_2040_01r_12Z</parent>
      <child>cylinder</child>
      <pose>0 0 0 0 0 0</pose>
      <axis>
        <limit>
          <lower>-0.785</lower>
          <upper>0.785</upper>
        </limit>
        <xyz>0.0 -1.0 0.0</xyz>
        <dynamics>
          <damping>2.0</damping>
          <friction>0.5</friction>
        </dynamics>
        <use_parent_model_frame>1</use_parent_model_frame>
      </axis>
      <physics>
        <ode>
          <implicit_spring_damper>1</implicit_spring_damper>
          <cfm_damping>1</cfm_damping>
          <limit>
            <cfm>0</cfm>
            <erp>0.2</erp>
          </limit>
        </ode>
      </physics>
    </joint>

    <!-- Add cylinder link -->
    <link name="cylinder">
      <pose frame=''>0.045 -0.05 0.05 0 1.3 0</pose>
      <inertial>
        <pose frame=''>0 0 0.06 0 0 0</pose>
        <mass>0.05</mass>
        <inertia>
          <ixx>6.1e-05</ixx>
          <ixy>0.0</ixy>
          <ixz>0.0</ixz>
          <iyy>6.1e-05</iyy>
          <iyz>0.0</iyz>
          <izz>1e-06</izz>
        </inertia>
      </inertial>
      <visual name="cylinder_visual">
        <pose frame=''>0 0 0.06 0 0 0</pose>
        <geometry>
          <cylinder>
            <length>0.12</length>
            <radius>0.0075</radius>
          </cylinder>
        </geometry>
        <material>
          <script>
            <uri>file://media/materials/scripts/gazebo.material</uri>
            <name>Gazebo/Green</name></script>
        </material>
      </visual>
      <collision name="cylinder_collision">
        <pose frame=''>0 0 0.06 0 0 0</pose>
        <geometry>
          <cylinder>
            <length>0.12</length>
            <radius>0.0075</radius>
          </cylinder>
        </geometry>
      </collision>
      <velocity_decay/>
      <gravity>1</gravity>
      <velocity_decay/>
      <self_collide>0</self_collide>
    </link>

    <!-- Add cylinder to piston prismatic joint -->
    <joint name="cylinder_to_piston" type="prismatic">
      <parent>cylinder</parent>
      <child>piston</child>
      <pose>0 0 0 0 0 0</pose>
      <axis>
        <limit>
          <lower>-0.1</lower>
          <upper>0.1</upper>
        </limit>
        <xyz>0.0 0.0 1.0</xyz>
        <dynamics>
          <spring_stiffness>2500</spring_stiffness>
          <!--  <spring_stiffness>2000</spring_stiffness> -->
          <!--  <spring_stiffness>111.1111</spring_stiffness> -->
          <spring_reference>0.06</spring_reference>
          <damping>20.0</damping>
          <friction>0 ...
(more)
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Comments

And in the case you defined the joint in the gazebo tag, can you still control the joints from ROS?

GuillaumeB gravatar imageGuillaumeB ( 2019-04-04 17:14:10 -0500 )edit

Yes, but you probably have to write your own ROS-Gazebo Plugin to interact with the joint and subscribe/publish to ROS Topics. Most Gazebo-ROS packages probably parse the URDF for joint names (and ignore anything in gazebo tags.

josephcoombe gravatar imagejosephcoombe ( 2019-04-09 14:47:28 -0500 )edit
0

answered 2019-05-06 22:25:01 -0500

updated 2019-05-09 22:37:59 -0500

The closed_loop_plugin can serve as a workaround for the issue of URDF not supporting closed kinematic loops. Please refer to https://github.com/wojiaojiao/pegasus_gazebo_plugins or http://wiki.ros.org/Angel_jj/closed_loop_plugin

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Comments

Elevating this from a comment to an answer.

josephcoombe gravatar imagejosephcoombe ( 2019-05-09 22:38:20 -0500 )edit
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Asked: 2019-03-28 13:56:10 -0500

Seen: 105 times

Last updated: May 09