Questions

Introduction

In this workshop, you will add elements to the robot by the various means introduced in the tutorials.

Task: Attach a Flag to the Robot Model

The following is meant to be an exercise of the various ways links can be joined in ROS. It is of course better to have static components of a robot linked statically, either as joints described in urdf, or in a simplified manner by creating a static transform. A dynamic transform, though not the usual way of attaching static links, is also explored in this exercise.

Exercise: Use Static Transforms

Create the static transforms in your launch file turtlebot3_tf.launch with the following characteristics:

→ Relationship between the parent frame base_link and the child frame pole_link:

  • Translation, t = (x, y, z)´ = (-0.22, 0.06, 0.038)´

  • Rotation, r = (roll, pitch, yaw)´ = (0, 0, 0)´

→ Relationship between the parent frame pole_link and the child frame flag_link:

  • Translation, t = (x, y, z)´ = (0, 0, 0.02)´

  • Rotation, r = (roll, pitch, yaw)´ = (0, 0, 0)´

Launch turtlebot3_tf.launch again and visualize the newly added frames in RViz.

Check the tf tree at this point

$ rosrun rqt_tf_tree rqt_tf_tree

  • Have a look at the structure and the names of the broadcasters of the links for comparision at various stages of this Exercise.

Exercise: Use Dynamic Transforms

A dynamic broadcaster, though not generally used for this purpose can also be used to accomplish the same task. Since, essentially, a dynamic broadcaster periodically keeps updating the tf tree with the new data, passing constant values to it is a way to exploit it to publish a static transform.

However, keep in mind that this is not the best way to add static tf frames to your robot. Instead, for adding static simplified frames, the former method of using *static_transform_publisher* is practiced.

In this exercise, we just create an equivalent tf tree by replacing the static transforms defined in the previous exercise.

Create a script in your package tb3_description that defines dynamic transforms between

  1. base_link and pole_link

  2. pole_link and flag_link

with the same characteristics(rotational and translational) as those defined in the previous exercise.

Comment out the static transforms for the same links added earlier, relaunch the launch file and then run your script.

  • Have a look at the tf tree again and watch for and differences.

  • Drive the robot around and observe the behaviour of the tf frames.

Exercise: Add Flag model to urdf

As explained in the tutorials, instead of using the simplified tf frames, you may create an elaborate robot description using something like URDF files.

Create a flag.urdf in the urdf folder of tb3_description.

Add the pole_link and the flag_link to the flag model:

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

  <link name="pole_link">
    <visual>
       <origin xyz="0 0 0" rpy="0 0 0" />
       <geometry>
         <cylinder radius="0.001" length="0.075"/>
       </geometry>
    </visual>

    <collision>
       <origin xyz="0 0 0" rpy="0 0 0" />
       <geometry>
         <cylinder radius="0.001" length="0.075"/>
       </geometry>
    </collision>

    <inertial>
      <mass value="0.001"/>
      <inertia ixx="0.001" ixy="0" ixz="0" iyy="0.001" iyz="0" izz="0.001"/>
    </inertial>
  </link>

  <link name="flag_link">
    <visual>
      <origin xyz="-0.022 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.045 0.001 0.03"/>
      </geometry>
    </visual>
    <inertial>
      <mass value="0.001"/>
      <inertia ixx="0.001" ixy="0" ixz="0" iyy="0.001" iyz="0" izz="0.001"/>
    </inertial>
  </link>

  <joint name="flag_pole_joint" type="continuous">
    <axis xyz="0 0 1"/>
    <origin xyz="0 0 0.02" rpy="0 0 0" />
    <parent link="pole_link"/>
    <child link="flag_link" />
  </joint>

</robot>

The flag model is now ready to be imported into the turtlebot xacro file. Include it by adding the required statement to the turtlebot3_waffle_base.urdf.xacro file.

Thus far we have just added the links of the flag to the robot description, without attaching the flag to the robot. Hence, define in the flag.urdf, this joint between base_link and pole_link using the following:

  • Name: flag_to_base_link

  • Parent Link: base_link

  • Child Link: pole_link

  • Joint Type: fixed

  • Origin:

    • Translation, t = (x, y, z)´ = (-0.22, 0.06, 0.038)´

    • Rotation, r = (roll, pitch, yaw)´ = (0, 0, 0)´

Make sure to add the joint within the <robot> tags.

Review the addition of the links by running turtlebot3_model.launch to see the updated model and check again the tf tree.