Planar 3DOF manipulator robot

This project implements a 3D planar robot in urdf. It also contain a launch with a robot_state_publisher and a joint_state_publisher_gui

This urdf create a planar robot with the next chain:

robot name is: planar_3dof
---------- Successfully Parsed XML ---------------
root Link: world has 1 child(ren)
    child(1):  base_link
        child(1):  link_1
            child(1):  link_2
                child(1):  gripper
                    child(1):  end

This is an adaptation of an old example of ros industrial tutorial(before docs ) about how to create and urdf files

where, $l_1=l_2=0.5$ y $l_3=0.18$

TODO: put the instruction about how to create the urdf The full name of URDF (United Robotics Description Format) is a language format used to describe robots in the XML grammar framework. URDF is very popular in ROS world. We can model the robot through URDF and put it in ROS for simulation and analysis.

The robot should be described through a tags, where the link represent the geometry of the robot and the joint represent the articulation or degree of fredoom of the robot. The tags are documented in wiki ros, where can be find other tags like sensor or transmission which let to describe this part of robot.

Steps to reproduce this repository:

• open a terminal inside the docker of ros2_start or create a workspace.

• create a package:

    cd src
    ros2 pkg create --build-type ament_python lesson_urdf

• Edit the files package.xml to add the next dependencies and your information about the package

      <depend>urdf</depend>
      <build_depend>launch_ros</build_depend>
      <exec_depend>launch_ros</exec_depend>
      <exec_depend>robot_state_publisher</exec_depend>
      <exec_depend>joint_state_publisher_gui</exec_depend>

• make a folder urdf inside package

    cd src/lesson_urdf
    mkdir urdf
    gedit planar_3dof.urdf

• Create the tag robot inside the file

    <?xml version="1.0"?>
    <robot name="planar_3dof" xmlns:xacro="http://ros.org/wiki/xacro">
    </robot>

• create a link without geometry which is the world (inside tag robot)

<link name="world" />

• create a link base_link which describe the base of the robot of the figure. Note that this link has a visual tag which describe the geometry an material of the piece. It also contain another geometry which is the collision geometry which could describe a simplified geometry to check collision or other tasks. In this case we represent the base of the robot with a cylinder of length 0.1 and width 0.2. The cilynde ris created

  <link name="base_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0" />
      <geometry>
        <cylinder length="0.1" radius="0.2" />
      </geometry>
      <material name="Cyan">
        <color rgba="0 1.0 1.0 1.0"/>
      </material>
    </visual>
  <collision>
    <origin rpy="0 0 0" xyz="0 0 0" />
    <geometry>
      <cylinder length="0.1" radius="0.2" />
    </geometry>
    <material name="grey"/>
  </collision>
    
  </link>

• create a joint fixed to the world

<joint name="fixed" type="fixed">
    <parent link="world" />
    <child link="base_link" />
  </joint>

• now create the link link_1, but now the geometry will be an stl file inside the folder meshes of our project (lesson_urdf). Note that we also can add inertial propierties of the robot like center of mass and inertia matrix of each link. Also note that collision and visual geometry could be diferrent. Here the cylinder made to simplify the link has a length 0.5 and radius 0.05, but this geometry is created over z axis since -0.25, so in origin we rotate the figure in y 90 degrees(1.5708) and translate in x 0.25 to start in zero.

    <link name="link_1">
    <visual>
      <geometry>
        <mesh filename="package://lesson_urdf/meshes/visual/arm_link.stl" scale="1 1 1" />
      </geometry>
      <material name="red"/>
    </visual>
    <collision>
      <origin rpy="0 1.5708 0" xyz="0.25 0 0" />
      <geometry>
        <cylinder length="0.5" radius="0.05" />
      </geometry>
      <material name="red"/>
    </collision>
    <inertial>
      <origin xyz="0 0 .25" rpy="0 0 0" />
      <mass value="1" />
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0" />
    </inertial>
  </link>

• now we create the joint with parent the base_link and child the link_1 of type revolute. Note that here the axis of rotation is over z and the link_1 is translated 0.05 over z from the origin of the base_link.

  <joint name="joint_1" type="revolute">
    <parent link="base_link" />
    <child link="link_1" />
    <origin xyz="0 0 0.05" rpy="0 0 0" />
    <axis xyz="0 0 1" />
    <limit lower="-1.57" upper="1.57" effort="0" velocity="0.5" />
  </joint>

• Repetae the process for link_2, gripper and a link without geomtery end ...

• save the file

• now copy the meshes,rviz and launch folder (TODO tutorial about launch)

• configure in setup.py the files to be copy:

  • Add the library at the beginning of the file

    from glob import glob
    

  • append these lines inside data_files

        ('share/' + package_name, glob('launch/*.py')),
        ('share/' + package_name+'/urdf/', glob('urdf/*')),
        ('share/' + package_name+'/rviz/', glob('rviz/*')),
        ('share/' + package_name+'/meshes/collision/', glob('meshes/collision/*')),
        ('share/' + package_name+'/meshes/visual/', glob('meshes/visual/*')),
    

• compile the workspace

    colcon build --symlink-install

• add to path the workspace

    . install/setup.bash

• run the launch file

    ros2 launch lesson_urdf view_robot_launch.py

Some error could be: To execute this package please add this to the .bashrc or to the specific console (Show basic URDF model in Rviz2 )

echo "export LC_NUMERIC="en_US.UTF-8"" >> ~/.bashrc