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Tom C.
Tom C.

Reputation: 127

Drake Parser error: Unable to read the 'value' attribute for the <drake:hunt_crossley_dissipation> tag

I am trying to write a URDF file for a simple pendulum that will act as a 'paddle' - i.e. upon collision with a bouncy ball, the ball will bounce off (like a volleyball player passing a volleyball).

Inspired by this bouncy ball example, I have written the URDF file given below, but loading it with parser.AddModelFromFile("filename.urdf") gives this error:

RuntimeError: filename.urdf:39: error: Unable to read the 'value' attribute for the <drake:hunt_crossley_dissipation> tag

I am surprised that this doesn't work, since it was adapted from the bouncing ball example (which does work). I would really appreciate any guidance, thanks!

URDF file:

<?xml version="1.0"?>

<robot name="pendulum-paddle">
  <material name="black">
    <color rgba="0 0 0 1" />
  </material>

  <link name="base">
    <inertial>
      <origin xyz="0 0 0" />
      <mass value="0.01" />
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0" />
    </inertial>
  </link>
  <joint name="base_weld" type="fixed">
    <parent link="world" />
    <child link="base" />
  </joint>

  <link name="arm">
    <inertial> 
      <origin xyz="0 0 -1" rpy="0 0 0" />
      <mass value="1" />
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/> 
    </inertial>
    <visual>
      <origin xyz="0 0 -.5" rpy="0 0 0" />
      <geometry>
         <box size="0.2 0.2 1"/>   
      </geometry>
      <material name="black" />
    </visual> 
    <collision name="collision">
      <origin xyz="0 0 -.5" rpy="0 0 0" />
      <geometry>
         <box size="0.2 0.2 1"/>   
      </geometry>
      <drake:proximity_properties>
            <drake:hunt_crossley_dissipation>
              0.1
            </drake:hunt_crossley_dissipation>
          <drake:point_contact_stiffness>
            1000000
          </drake:point_contact_stiffness>
        </drake:proximity_properties>
    </collision> 
  </link>
  <joint name="shoulder" type="continuous">
    <parent link="base"/>
    <child link="arm" />
    <axis xyz="0 1 0" />
  </joint>
  <transmission type="SimpleTransmission" name="shoulder_trans">
    <actuator name="shoulder" />
    <joint name="shoulder" />
    <mechanicalReduction>1</mechanicalReduction>
  </transmission>

</robot>

Upvotes: 0

Views: 52

Answers (1)

Russ Tedrake
Russ Tedrake

Reputation: 5523

Unfortunately, the syntax is URDF is different than in SDF, and we tried to make the Drake tags comply to the standard. https://drake.mit.edu/doxygen_cxx/group__multibody__parsing.html#tag_drake_hunt_crossley_dissipation

In urdf, the proximity properties look more like:

<drake:proximity_properties>
  <drake:mu_static value="0.8"/>
  <drake:mu_dynamic value="0.3"/>
</drake:proximity_properties>

so try

      <drake:proximity_properties>
         <drake:hunt_crossley_dissipation value="0.1"/>
         <drake:point_contact_stiffness value="1000000"/>
      </drake:proximity_properties>

Upvotes: 2

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