Skip to content

Links, joints and sensors

A physical link in the simulation contains inertia, collision and visual properties. A link must be a child of a robot and a robot can have multiple links.

# Import the element creator
from pcg_gazebo.parsers.urdf import create_urdf_element

# The link is empty by default
link = create_urdf_element('link')
print(link)

<link name="link"/>

# By using reset(), it is possible to see the optional elements of a link
link.reset(with_optional_elements=True)
print(link)

<link name="link">
  <inertial>
    <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <mass value="0"/>
  </inertial>
  <visual name="visual">
    <material name="">
      <color rgba="0 0 0 1"/>
      <gazebo/>
    </material>
    <geometry>
      <box size="0 0 0"/>
    </geometry>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <gazebo/>
  </visual>
  <collision name="collision">
    <geometry>
      <box size="0 0 0"/>
    </geometry>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <gazebo/>
  </collision>
  <gazebo>
    <mu1>0</mu1>
    <minDepth>0</minDepth>
    <kd>1</kd>
    <maxContacts>20</maxContacts>
    <selfCollide>0</selfCollide>
    <mu2>0</mu2>
    <maxVel>0.01</maxVel>
    <kp>1000000000000.0</kp>
  </gazebo>
</link>

# Let's create the elements dynamically at first
link = create_urdf_element('link')

# The link's name must be unique in a model
link.name = 'base_link'
print(link)

<link name="base_link"/>

# Mass of the link in kg
link.mass = 30
# The center of mass are the cartesian coordinates in link.inertial.pose
link.origin = create_urdf_element('origin')
link.origin.xyz = [0, 10, 0]
# The moments of inertia describe the elements of the 3x3 rotational inertial matrix
link.inertia.ixx = 0.5
link.inertia.iyy = 0.5
link.inertia.izz = 0.5
print(link)

<link name="base_link">
  <inertial>
    <inertia ixx="0.5" ixy="0" ixz="0" iyy="0.5" iyz="0" izz="0.5"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <mass value="0"/>
  </inertial>
</link>
Joints
# The joint is empty by default
joint = create_urdf_element('joint')
print(joint)

<joint name="joint" type="revolute">
  <parent link="link"/>
  <child link="link"/>
  <limit effort="0" lower="0" upper="0" velocity="0"/>
</joint>

# By using reset(), it is possible to see the optional elements of a joint
joint.reset(with_optional_elements=True)
print(joint)

<joint name="joint" type="revolute">
  <parent link="link"/>
  <child link="link"/>
  <dynamics damping="0" friction="0">
    <gazebo/>
  </dynamics>
  <axis xyz="1 0 0"/>
  <origin rpy="0 0 0" xyz="0 0 0"/>
  <safety_controller k_position="0" k_velocity="0" soft_lower_limit="0" soft_upper_limit="0"/>
  <limit effort="0" lower="0" upper="0" velocity="0">
    <gazebo/>
  </limit>
  <mimic multiplier="1" offset="0"/>
  <gazebo>
    <stopCfm>0.0</stopCfm>
    <stopErp>0.2</stopErp>
  </gazebo>
</joint>