Concept Design

URDF Design

Differential Drive Robots

ROS REP conventions
- The root link (main coordinate frmae) must be called base_link (Coordinate Frames for Mobile Platforms)
- The orientation of this link must be:
  - x- forward
  - y- left
  - z- up

Creating the URDF

Use ROS Snippets to insert Robot element with URDF

<?xml version="1.0"?>
<robot name="robot_name">
    
</robot>

Specify colors

    <material name="white">
        <color rgba="1 1 1 1" />
    </material>
    <material name="orange">
        <color rgba="1 0.3 0.1 1"/>
    </material>
 
    <material name="blue">
        <color rgba="0.2 0.2 1 1"/>
    </material>

Base link

The base_link refers to the coordinate frame that represents the main reference point of a robot. It is a standard name used in robot descriptions to define the robot’s body frame in the TF (Transform) tree, which is used to track the spatial relationships between various components of a robot.

Key Characteristics of `base_link`:

Reference Frame of the Robot:
- It serves as the root or origin of the robot’s physical body in the coordinate frame hierarchy.
- Other frames, such as sensor frames (camera_link, lidar_link) or actuator frames (wheel_link), are typically defined relative to the base_link.
Placement:
- The exact position of the base_link is often defined at the robot’s center, such as:
  - The geometric center of the robot.
  - The point of contact with the ground (e.g., the bottom of the chassis).
  - A convenient reference point, like the robot’s centroid or center of rotation.

<link name="base_link">
</link>

Role in Navigation and Localization:
- It is commonly used as the robot’s frame in navigation and localization tasks, making it the frame in which transformations to and from world coordinates (map, odom) are calculated.
Hierarchy in the TF Tree:
- base_link is often connected to higher-level frames like odom or map, representing the robot’s position and orientation in the world.

Example TF Tree with `base_link`:

map
 └── odom
      └── base_link
           ├── camera_link
           ├── lidar_link
           └── wheel_link

For a differential drive robot it is simplest to treat the center of the two drive wheels as the origin, since the rotation will be centred around this point.

Chassis

We can design the chassis as a box as follows:

dimensions: 300 x 300 x 150 mm (0.3 x 0.3 x 0.3 m)
We want the origin of the chassis to be just behind the center of the 2 drive wheels, so we can set the chassis_joint -0.1m in x behind the base_link
- But the box geometry will be centered around the chassis origin by default, so we can shift the box forward (in X) by half its length (0.15m), and up (in Z) by half its height (0.075m)

<joint name="chassis_joint" type="fixed">
    <parent link="base_link"/>
    <child link="chassis"/>
    <origin xyz="-0.1 0 0"/>
</joint>
 
<link name="chassis">
    <visual>
        <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        <geometry>
            <box size="0.3 0.3 0.15"/>
        </geometry>
        <material name="white"/>
    </visual>
</link>

Current robot.urdf.xacro

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="robot">
 
    <material name="white">
        <color rgba="1 1 1 1" />
    </material>
    <material name="black">
        <color rgba="0 0 0 1" />
    </material>
    <material name="orange">
        <color rgba="1 0.3 0.1 1" />
    </material>
    <material name="blue">
        <color rgba="0.2 0.2 1 1" />
    </material>
 
    <!-- Example link -->
    <link name="base_link"></link>
 
    <joint name="chassis_joint" type="fixed">
        <parent link="base_link" />
        <child link="chassis" />
        <origin xyz="-0.1 0 0" />
    </joint>
 
    <link name="chassis">
        <visual>
            <origin xyz="0.15 0 0.075" />
            <geometry>
                <box size="0.3 0.3 0.15" />
            </geometry>
            <material name="black" />
        </visual>
    </link>
 
</robot>

Create training_ws if needed

mkdir -p training_ws/src 
cd training_ws/src/

clone the repo

git clone git@github.com:Training-Dummy/differential_drive_robot.git
# git clone https://github.com/Training-Dummy/differential_drive_robot.git

Build the package from the root of the workspace (training_ws)

cd ../..
colcon build --symlink-install --packages-select differential_drive_robot

Source the setup file

source install/setup.bash

Launch the robot_state_publisher node

ros2 launch differential_drive_robot rsp.launch.py

Configure RViz settings
- Set fixed frame to world
- Select “Add”, then click RobotModel display
  - Set the topic to /robot_description
  - Set alpha to 0.8
- Select “Add”, then click TF display
  - Enable names
Save RViz configuration
- File → save as → navigate to config folder and save the file (e.g. config/diff-drive.rviz)

# cd into training_ws
rviz2 -d src/differential_drive_robot/config/diff-drive.rviz

Drive wheels

The wheels will be connected directly to the base_link (via continuous joints) since it is the center of rotation.

The wheel origin will be offset from the center by 175mm in y
The wheel links and have cylinders of 50mm radius and 40mm length
Since cylinders point upward by default, they need to be rotated 90 degrees about the x axis (roll)
For the axis of rotation, we want the wheel to spin positive around z axis (which is pointing outwards) when we have a positive motion on the wheels.

    <link name="left_wheel">
        <visual>
            <geometry>
                <cylinder radius="0.04" length="0.05" />
            </geometry>
            <material name="green" />
        </visual>
    </link>
 
 
    <joint name="right_wheel_joint" type="continuous">
        <parent link="base_link" />
        <child link="right_wheel" />
        <origin xyz="0.0 -0.175 0.0" rpy="${pi/2} 0.0 0.0" />
        <axis xyz="0.0 0.0 -1.0" />
 
    </joint>
 
    <link name="right_wheel">
        <visual>
            <geometry>
                <cylinder radius="0.04" length="0.05" />
            </geometry>
            <material name="green" />
        </visual>
    </link>

Note: For RViz to correctly display non-fixed joints (like the wheels), we need joint state publishers to update their positions dynamically
- We can use the joint_state_publisher_gui to publish fake joint states to the /joint_states topic

ros2 run joint_state_publisher_gui joint_state_publisher_gui

Castor Wheel

The caster wheel will be a frictionless sphere that is connected to the chassis with lowest point matching the base of the wheels

    <joint name="caster_wheel_joint" type="fixed">
        <origin xyz="0.24 0.0 0.0" rpy="0.0 0.0 0.0" />
        <parent link="chassis" />
        <child link="caster_wheel" />
    </joint>
 
    <link name="castor_wheel">
        <visual>
            <geometry>
                <sphere radius="0.05" />
            </geometry>
            <material name="white" />
        </visual>
    </link>

Collision

Add collision by copying and pasting geometry and origin from visual tags into collision tags
- Untick “Visual Enabled” and tick “Collision Enabled” to see the collision geometry in RViz

<link name="chassis">
    <visual>
        <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        <geometry>
            <box size="0.3 0.3 0.15"/>
        </geometry>
        <material name="white"/>
    </visual>
    <collision>
        <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        <geometry>
            <box size="0.3 0.3 0.15"/>
        </geometry>
    </collision>
</link>

Inertia

Create a intertial_macros.xacro file

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" >
 
    <!-- Specify some standard inertial calculations https://en.wikipedia.org/wiki/List_of_moments_of_inertia -->
    <!-- These make use of xacro's mathematical functionality -->
 
    <xacro:macro name="inertial_sphere" params="mass radius *origin">
        <inertial>
            <xacro:insert_block name="origin"/>
            <mass value="${mass}" />
            <inertia ixx="${(2/5) * mass * (radius*radius)}" ixy="0.0" ixz="0.0"
                    iyy="${(2/5) * mass * (radius*radius)}" iyz="0.0"
                    izz="${(2/5) * mass * (radius*radius)}" />
        </inertial>
    </xacro:macro>  
 
 
    <xacro:macro name="inertial_box" params="mass x y z *origin">
        <inertial>
            <xacro:insert_block name="origin"/>
            <mass value="${mass}" />
            <inertia ixx="${(1/12) * mass * (y*y+z*z)}" ixy="0.0" ixz="0.0"
                    iyy="${(1/12) * mass * (x*x+z*z)}" iyz="0.0"
                    izz="${(1/12) * mass * (x*x+y*y)}" />
        </inertial>
    </xacro:macro>
 
 
    <xacro:macro name="inertial_cylinder" params="mass length radius *origin">
        <inertial>
            <xacro:insert_block name="origin"/>
            <mass value="${mass}" />
            <inertia ixx="${(1/12) * mass * (3*radius*radius + length*length)}" ixy="0.0" ixz="0.0"
                    iyy="${(1/12) * mass * (3*radius*radius + length*length)}" iyz="0.0"
                    izz="${(1/2) * mass * (radius*radius)}" />
        </inertial>
    </xacro:macro>
 
 
</robot>

Add the following line near the top of robot.urdf.xacro

<xacro:include filename="inertial_macros.xacro" />

Add the inertial macros for each link
- You need to specify an tag (default 0’s) to match the link visual origin.
- example:

 
    <link name="chassis">
        <visual>
            <origin xyz="0.15 0 0.075" />
            <geometry>
                <box size="0.3 0.3 0.15" />
            </geometry>
            <material name="black" />
        </visual>
        <collision>
            <origin xyz="0.15 0 0.075" />
            <geometry>
                <box size="0.3 0.3 0.15" />
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0" />
        </xacro:inertial_box>
    </link>

Final `robot.urdf.xacro`

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="robot">
    <xacro:include filename="inertial_macros.xacro" />
 
    <material name="white">
        <color rgba="1 1 1 1" />
    </material>
    <material name="black">
        <color rgba="0 0 0 1" />
    </material>
    <material name="orange">
        <color rgba="1 0.3 0.1 1" />
    </material>
    <material name="blue">
        <color rgba="0.2 0.2 1 1" />
    </material>
    <material name="green">
        <color rgba="0 0.3 0 1" />
    </material>
 
 
    <!-- Example link -->
    <link name="base_link"></link>
 
    <joint name="chassis_joint" type="fixed">
        <parent link="base_link" />
        <child link="chassis" />
        <origin xyz="-0.1 0 0" />
    </joint>
 
    <link name="chassis">
        <visual>
            <origin xyz="0.15 0 0.075" />
            <geometry>
                <box size="0.3 0.3 0.15" />
            </geometry>
            <material name="black" />
        </visual>
        <collision>
            <origin xyz="0.15 0 0.075" />
            <geometry>
                <box size="0.3 0.3 0.15" />
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0" />
        </xacro:inertial_box>
    </link>
 
    <joint name="left_wheel_joint" type="continuous">
        <parent link="base_link" />
        <child link="left_wheel" />
        <origin xyz="0.0 0.175 0.0" rpy="-${pi/2} 0.0 0.0" />
        <axis xyz="0.0 0.0 1.0" />
    </joint>
 
    <link name="left_wheel">
        <visual>
            <geometry>
                <cylinder radius="0.04" length="0.05" />
            </geometry>
            <material name="green" />
        </visual>
        <collision>
            <geometry>
                <cylinder radius="0.04" length="0.05" />
            </geometry>
        </collision>
        <xacro:inertial_cylinder mass="0.1" radius="0.04" length="0.05">
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" />
        </xacro:inertial_cylinder>
    </link>
 
 
    <joint name="right_wheel_joint" type="continuous">
        <parent link="base_link" />
        <child link="right_wheel" />
        <origin xyz="0.0 -0.175 0.0" rpy="${pi/2} 0.0 0.0" />
        <axis xyz="0.0 0.0 -1.0" />
 
    </joint>
 
    <link name="right_wheel">
        <visual>
            <geometry>
                <cylinder radius="0.04" length="0.05" />
            </geometry>
            <material name="green" />
        </visual>
        <collision>
            <geometry>
                <cylinder radius="0.04" length="0.05" />
            </geometry>
        </collision>
        <xacro:inertial_cylinder mass="0.1" radius="0.04" length="0.05">
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" />
        </xacro:inertial_cylinder>
    </link>
 
    <joint name="caster_wheel_joint" type="fixed">
        <origin xyz="0.24 0.0 0.0" rpy="0.0 0.0 0.0" />
        <parent link="chassis" />
        <child link="caster_wheel" />
    </joint>
 
    <link name="caster_wheel">
        <visual>
            <geometry>
                <sphere radius="0.05" />
            </geometry>
            <material name="white" />
        </visual>
        <collision>
            <geometry>
                <sphere radius="0.05" />
            </geometry>
        </collision>
        <xacro:inertial_sphere mass="0.1" radius="0.04">
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" />
        </xacro:inertial_sphere>
    </link>
 
</robot>

Add to your own repo

Remove the current remote

git remote remove origin

Verify that the remote is removed (should see no output)

git remote -v

Create new repo on GitHub and add it as the origin remote • example:

git remote add origin git@github.com:Training-Dummy/diff_drive_robot.git

push to the new repo

git push -u origin main

Gazebo Simulation

Spawning robot in Gazebo

See gazebo-and-ros

Create launch file

Create a launch_sim.launch.py file in your launch/ directory

launch_sim.launch.py

import os
 
from ament_index_python.packages import get_package_share_directory
 
 
from launch import LaunchDescription
from launch.actions import IncludeLaunchDescription
from launch.launch_description_sources import PythonLaunchDescriptionSource
 
from launch_ros.actions import Node
 
 
 
def generate_launch_description():
 
 
    # Include the robot_state_publisher launch file, provided by our own package. Force sim time to be enabled
    # !!! MAKE SURE YOU SET THE PACKAGE NAME CORRECTLY !!!
 
    package_name='differential_drive_robot' #<--- CHANGE ME
 
    rsp = IncludeLaunchDescription(
                PythonLaunchDescriptionSource([os.path.join(
                    get_package_share_directory(package_name),'launch','rsp.launch.py'
                )]), launch_arguments={'use_sim_time': 'true'}.items()
    )
 
    # Include the Gazebo launch file, provided by the gazebo_ros package
    gazebo = IncludeLaunchDescription(
                PythonLaunchDescriptionSource([os.path.join(
                    get_package_share_directory('gazebo_ros'), 'launch', 'gazebo.launch.py')]),
             )
 
    # Run the spawner node from the gazebo_ros package. The entity name doesn't really matter if you only have a single robot.
    spawn_entity = Node(package='gazebo_ros', executable='spawn_entity.py',
                        arguments=['-topic', 'robot_description',
                                   '-entity', 'my_bot'],
                        output='screen')
 
 
 
    # Launch them all!
    return LaunchDescription([
        rsp,
        gazebo,
        spawn_entity,
    ])

Rebuild and run the launch file

ros2 launch differential_drive_robot launch_sim.launch.py

Note, this is equivalent to running these 3 commands separately:

ros2 launch my_bot rsp.launch.py use_sim_time:=true

ros2 launch gazebo_ros gazebo.launch.py

ros2 run gazebo_ros spawn_entity.py -topic robot_description -entity my_bot

Instructions for Gazebo Garden

Gazebo Garden

Create URDF file

 xacro robot.urdf.xacro > robot.urdf

Launch Gazebo using the empty.sdf world

ros2 launch ros_gz_sim gz_sim.launch.py gz_args:=empty.sdf

Create and spawn a robot model from a URDF file into the empty Gazebo world

gz service -s /world/empty/create --reqtype gz.msgs.EntityFactory --reptype gz.msgs.Boolean --timeout 1000 --req 'sdf_filename: "src/differential_drive_robot/description/robot.urdf", name: "urdf_model"'

sudo apt update && sudo apt install -y ros-humble-joint-state-publisher-gui

https://youtu.be/fH4gkIFZ6W8?feature=shared&t=264

Add Gazebo tags

Since Gazebo uses a different material/color system than RViz, add a Gazebo material for each link
- example:

    <link name="chassis">
        <visual>
            <origin xyz="0.15 0 0.075" />
            <geometry>
                <box size="0.3 0.3 0.15" />
            </geometry>
            <material name="black" />
        </visual>
        <collision>
            <origin xyz="0.15 0 0.075" />
            <geometry>
                <box size="0.3 0.3 0.15" />
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0" />
        </xacro:inertial_box>
    </link>
    <gazebo reference="chassis">
        <material>Gazebo/Black</material>
    </gazebo>

Adjust friction coefficients to reduce drag of the caster wheel

<gazebo reference="caster_wheel">
    <material>Gazebo/Black</material>
    <mu1 value="0.001"/>
    <mu2 value="0.001"/>
</gazebo>

Control with Gazebo

Whenever we want to use ROS to interact with Gazebo, we do it with plugins.
- The control system will be a plugin (ros2_control or, for now, gazebo_ros_diff_drive) and that will interact with the core Gazebo code which simulates the motors and physics.
The ros_control plugin broadcasts a transform from the odom frame to base_link
- odom frame is the robot start position, the world origin
- which gives us current position estimate for the robot
The gazebo_ros_diff_drive plugin doesn’t use /joint_states but publishes the left/right_wheel transforms directly

Add a gazebo_control.xacro file to configure the differential drive plugin
- Include it to robot.urdf.xacro (<xacro:include filename="gazebo_control.xacro" />)

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro">
 
    <gazebo>
        <plugin name='diff_drive' filename='libgazebo_ros_diff_drive.so'>
    
    
            <!-- Wheel Information -->
            <left_joint>left_wheel_joint</left_joint>
            <right_joint>right_wheel_joint</right_joint>
            <wheel_separation>0.35</wheel_separation>
            <wheel_diameter>0.1</wheel_diameter>
    
 
            <!-- Limits -->
            <max_wheel_torque>200</max_wheel_torque>
            <max_wheel_acceleration>10.0</max_wheel_acceleration>
    
 
            <!-- Output -->
            <odometry_frame>odom</odometry_frame>
            <robot_base_frame>base_link</robot_base_frame>
 
            <publish_odom>true</publish_odom>
            <publish_odom_tf>true</publish_odom_tf>
            <publish_wheel_tf>true</publish_wheel_tf>
    
    
        </plugin>
    </gazebo>
 
</robot>

Use the teleop_twist_keyboard node to send command velocities on the /cmd_vel topic

ros2 run teleop_twist_keyboard teleop_twist_keyboard

Visualize the result in RViz

Start RViz
- Set fixed frame to odom to tell us where the robot is with respect to Gazebo’s origin
- save config (e.g. diff-drive-odom.rviz)

rviz2 -d src/diff_drive_robot/config/diff-drive-odom.rviz

Create an obstacle course

In the Insert tab in Gazebo, click models to add
- Delete robot and save world (e.g. worlds/obstacles.world) to use later

Launch Gazebo with obstacle world

ros2 launch diff_drive_robot launch_sim.launch.py world:=src/diff_drive_robot/worlds/obstacles.world

View code https://github.com/Training-Dummy/diff_drive_robot

🪐Chris T.

Recent Notes

po-mobile-ros-robot

Perception

Microfloat SD Card Module

Explorer

Concept Design

URDF Design

Differential Drive Robots

Creating the URDF

Base link

Key Characteristics of `base_link`:

Example TF Tree with `base_link`:

Chassis

Drive wheels

Castor Wheel

Collision

Inertia

Final `robot.urdf.xacro`

Add to your own repo

Gazebo Simulation

Spawning robot in Gazebo

Create launch file

Instructions for Gazebo Garden

Add Gazebo tags

Control with Gazebo

Visualize the result in RViz

Create an obstacle course

Graph View

Table of Contents

Backlinks

🪐Chris T.

Recent Notes

po-mobile-ros-robot

Perception

Microfloat SD Card Module

Explorer

Concept Design

URDF Design

Differential Drive Robots

Creating the URDF

Base link

Key Characteristics of base_link:

Example TF Tree with base_link:

Chassis

Drive wheels

Castor Wheel

Collision

Inertia

Final robot.urdf.xacro

Add to your own repo

Gazebo Simulation

Spawning robot in Gazebo

Create launch file

Instructions for Gazebo Garden

Add Gazebo tags

Control with Gazebo

Visualize the result in RViz

Create an obstacle course

Graph View

Table of Contents

Backlinks

Key Characteristics of `base_link`:

Example TF Tree with `base_link`:

Final `robot.urdf.xacro`