URDF

URDF stands for ‘Unified Robot Description Format’. The Unified Robot Description Format (URDF) is an XML specification to describe a robot. We attempt to keep this specification as general as possible, but obviously the specification cannot describe all robots. The main limitation at this point is that only tree structures can be represented, ruling out all parallel robots. Also, the specification assumes the robot consists of rigid links connected by joints; flexible elements are not supported. The specification covers:

• Kinematic and dynamic description of the robot
• Visual representation of the robot
• Collision model of the robot

how URDF is written

• XML formatTags:
• Tags like Robot link, joint,transmission, … etc
• Kinematic tree structure
• Order in the file does not matter

---

Lets Explore URDF files and tags

The URDF see URDF file for below Robot

A typical xml file for the Robot ‘test.urdf’

test.urdf

<?xml version="1.0" encoding="UTF-8"?>
<robot name="test_robot">
     <link name="link1" />
      <link name="link2" />
     <link name="link3" />
     <link name="link4" />

     <joint name="joint1" type="continuous">
              <parent link="link1"/>
              <child link="link2"/>
             <origin xyz="5 3 0" rpy="0 0 0" />
     </joint>

     <joint name="joint2" type="continuous">
            <parent link="link1"/>
              <child link="link3"/>
                <origin xyz="-2 7 0" rpy="0 0 0" />
     </joint>

     <joint name="joint3" type="continuous">
     <parent link="link3"/>
     <child link="link4"/>
       <origin xyz="5 0 0" rpy="0 0 -1.57" />
     </joint>

   </robot>

you can run

 rosrun urdfdom_py display_urdf ~/ROS_Worksop/src/rosbasics/urdf/test.urdf

which outputs

gazebos: []
joints:
- {axis: None, calibration: None, child: link2, dynamics: None, limit: None, mimic: None,
  name: joint1, origin: None, parent: link1, safety_controller: None, type: continuous}
- {axis: None, calibration: None, child: link3, dynamics: None, limit: None, mimic: None,
  name: joint2, origin: None, parent: link1, safety_controller: None, type: continuous}
- {axis: None, calibration: None, child: link4, dynamics: None, limit: None, mimic: None,
  name: joint3, origin: None, parent: link3, safety_controller: None, type: continuous}
links:
- collisions: []
  inertial: None
  name: link1
  origin: None
  visuals: []
- collisions: []
  inertial: None
  name: link2
  origin: None
  visuals: []
- collisions: []
  inertial: None
  name: link3
  origin: None
  visuals: []
- collisions: []
  inertial: None
  name: link4
  origin: None
  visuals: []
materials: []
name: test_robot
transmissions: []
version: '1.0'

As said urdf is the stadard used to represent robot structue we need to have urdf xml praser in our system which can be enabled by

sudo apt install liburdfdom-tools

and afer install

urdf_to_graphiz ~/ROS_Worksop/src/rosbasics/urdf/test.urdf

which out puts

---

all dimetions are in SI units and base_link frame of Myfirst robot

NOTE

A Robot discription without dimensions is meaniningless lets add dimentions

---

Create A launch file

ROS has splitted a package as useage increased joint_state_publisher to ‘joint_state_publisher_gui’ to install it

sudo apt-get update
sudo apt-get install ros-melodic-joint_state_publisher_gui

‘urdf.launch’

<?xml version="1.0" encoding="UTF-8"?>
<launch>
  <arg name="model" default="box.urdf" />
  <arg name="file" default="$(find rosbasics)/urdf/$(arg model)"/>
   <arg name="gui" default="false" />
  <param name="robot_description" command="$(find xacro)/xacro --inorder $(arg file)" />
 <param name="use_gui" value="$(arg gui)"/>

 <node name="joint_state_publisher" pkg="joint_state_publisher_gui" type="joint_state_publisher_gui" />
 <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher" />

</launch>

---

Lets create shapes

BOX

box.urdf

<?xml version="1.0" encoding="UTF-8"?>
<robot name="box">
  <link name="base_link">
    <visual>
      <geometry>
        <box size= "1 1 1"/>
      </geometry>
    </visual>
  </link>
</robot>

CYLINDER

cylinder.urdf

<?xml version="1.0" encoding="UTF-8"?>
<robot name="cylinder">
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder radius="1.0" length="3"/>
      </geometry>
    </visual>
  </link>
</robot>

• open terminal

roslaunch roslaunch roslaunch rosbasics urdf.launch model:=box.urdf

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