Tutorial: Building a Differential Drive Robot

In this tutorial, you will configure a differential drive mobile robot in Blender and export it as an actuable URDF for ROS 2 or Gazebo.

What You Will Learn

How to create and configure Links.
How to connect links with Joints.
How to add a Lidar Sensor.
How to configure Control Interfaces.
How to Validate and Export your robot.

Kinematic Tree

Before we start building, here is the structure of the robot we are going to create:

        graph TD
    base_link[base_link] -->|continuous| left_wheel[left_wheel]
    base_link -->|continuous| right_wheel[right_wheel]
    base_link -->|fixed| lidar_link[lidar_link]

Step 1: Create the Base Link

Add a Mesh: In Blender, press Shift + A and select Mesh > Cube.
Scale the Base: Set the dimensions to roughly 0.4m x 0.3m x 0.1m.
Forge the Link:
- Open the Links panel in the LinkForge sidebar (N key).
- With the cube selected, click Create Link from Mesh.
- Name it base_link.
- Set Mass to 5.0 kg.
- Enable Auto-Calculate Inertia (LinkForge will automatically generate the inertia tensor for the box).
- Generate Collision: Click Generate Collision. LinkForge will create an optimized bounding box for the cube.

Creating the Base Link

Tip

Always keep LinkForge’s Auto-Calculate Inertia checkbox enabled rather than entering values manually. It ensures the physical consistency required by simulation engines like Gazebo.

Step 2: Create the Wheels

Add a Cylinder: Shift + A > Mesh > Cylinder.
Dimensions: Set Radius to 0.1m and Depth to 0.05m.
Rotate: Rotate it 90 degrees on the X-axis so it looks like a wheel.
Duplicate: Press Shift + D and move the new cylinder to the other side. You now have two generic cylinder meshes.

Forge the Left Wheel

Select the first cylinder.
Click Create Link from Mesh.
Name it left_wheel.
Set Mass to 0.5 kg.
Generate Collision: Click Generate Collision.

Forge the Right Wheel

Select the second cylinder.
Click Create Link from Mesh.
Name it right_wheel.
Set Mass to 0.5 kg.
Generate Collision: Click Generate Collision.

Step 3: Connect with Joints

Connect Left Wheel:
- Select left_wheel.
- In the LinkForge sidebar, go to the Joints panel and click Create Joint.
- Type: Select continuous (wheels don’t have limits).
- Parent: Select base_link.
- Axis: Set to (0, 1, 0) if your wheel rotates around the Y-axis.

Connecting the Wheels

Connect Right Wheel:
- Repeat the process for right_wheel, connecting it to base_link.

Step 4: Add a Lidar Sensor

Create Lidar Mesh: Add a small cylinder on top of the base.
Create Link from Mesh: Call it lidar_link.
Create Fixed Joint: Connect lidar_link to base_link using a fixed joint type.
Attach Sensor:
- Go to the Perceive panel in the LinkForge sidebar.
- With lidar_link selected, click Create Sensor.
- Select Type: LIDAR (LinkForge exports this as gpu_lidar for modern Gazebo).
- Set Update Rate to 30 Hz.

Step 5: Configure Control

To make our robot actuable in ROS 2 or Gazebo, we need to add standard interfaces (Velocity) to the wheels using the ros2_control system.

Open Control Dashboard:
- Go to the Control panel in the LinkForge sidebar.
- Check Use ROS2 Control.
- This enables the centralized dashboard.
Add Interfaces:
- Click the Add (+) button next to the “Joint Interfaces” list.
- Select left_wheel_joint.
- Click Add (+) again and select right_wheel_joint.
Configure Velocity Control:
- Select left_wheel_joint in the list.
- Check Velocity under Command Interfaces.
- Check Position and Velocity under State Interfaces (standard for feedback).
- Repeat this configuration for right_wheel_joint.

Configuring Control Dashboard

Step 6: Validate and Export

Validate: In the Validate & Export panel, click Validate Robot.
- LinkForge will check if all links are connected, physics data is valid, and control interfaces are set.

Warning

Exporting without validation may result in a URDF that causes simulators to crash or behave erratically. Fix all red markers before proceeding.

Export:
- Go to the Validate & Export panel.
- Select Format: URDF.
- Click Export URDF/XACRO and choose a location.

Success!

Final Robot

You now have a production-ready, actuable URDF file. You can now load this file into Gazebo or use it with ROS 2 and the diff_drive_controller to drive your robot!

Sample URDF Output

If you followed the steps correctly, your exported URDF should look similar to the following. Note the clean ros2_control block and absence of legacy <transmission> tags.

Click to view diff_drive_robot.urdf

<robot name="diff_drive_robot">
  <!-- Links -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.4 0.3 0.1" />
      </geometry>
    </visual>
    <collision>
      <geometry>
        <box size="0.4 0.3 0.1" />
      </geometry>
    </collision>
    <inertial>
      <mass value="5" />
      <inertia ixx="0.041667" ixy="0" ixz="0" iyy="0.070833" iyz="0" izz="0.104167" />
    </inertial>
  </link>
  <link name="left_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.1" length="0.05" />
      </geometry>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.1" length="0.05" />
      </geometry>
    </collision>
    <inertial>
      <mass value="0.5" />
      <inertia ixx="0.001354" ixy="0" ixz="0" iyy="0.001354" iyz="0" izz="0.0025" />
    </inertial>
  </link>
  <link name="right_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.1" length="0.05" />
      </geometry>
    </visual>
    <collision>
      <geometry>
        <cylinder radius="0.1" length="0.05" />
      </geometry>
    </collision>
    <inertial>
      <mass value="0.5" />
      <inertia ixx="0.001354" ixy="0" ixz="0" iyy="0.001354" iyz="0" izz="0.0025" />
    </inertial>
  </link>
  <link name="lidar_link">
    <visual>
      <geometry>
        <cylinder radius="0.03174" length="0.037866" />
      </geometry>
    </visual>
    <inertial>
      <mass value="1" />
      <inertia ixx="0.000371" ixy="0" ixz="0" iyy="0.000371" iyz="0" izz="0.000504" />
    </inertial>
  </link>
  <!-- Joints -->
  <joint name="left_wheel_joint" type="continuous">
    <origin xyz="0 0.175 0" rpy="1.570796 0 0" />
    <parent link="base_link" />
    <child link="left_wheel" />
    <axis xyz="0 1 0" />
    <limit effort="10" velocity="1" />
  </joint>
  <joint name="right_wheel_joint" type="continuous">
    <origin xyz="0 -0.175 0" rpy="1.570796 0 0" />
    <parent link="base_link" />
    <child link="right_wheel" />
    <axis xyz="0 1 0" />
    <limit effort="10" velocity="1" />
  </joint>
  <joint name="lidar_link_joint" type="fixed">
    <origin xyz="0 0 0.064282" rpy="0 0 0" />
    <parent link="base_link" />
    <child link="lidar_link" />
  </joint>
  <!-- ROS2 Control -->
  <ros2_control name="GazeboSimSystem" type="system">
    <hardware>
      <plugin>gz_ros2_control/GazeboSimSystem</plugin>
    </hardware>
    <joint name="right_wheel_joint">
      <command_interface name="velocity" />
      <state_interface name="position" />
      <state_interface name="velocity" />
    </joint>
    <joint name="left_wheel_joint">
      <command_interface name="velocity" />
      <state_interface name="position" />
      <state_interface name="velocity" />
    </joint>
  </ros2_control>
  <!-- Gazebo -->
  <gazebo>
    <plugin name="gazebo_ros2_control" filename="libgz_ros2_control-system.so">
      <parameters>$(find robot_description)/config/controllers.yaml</parameters>
    </plugin>
  </gazebo>
  <!-- Sensors -->
  <gazebo reference="lidar_link">
    <sensor name="lidar_link_sensor" type="gpu_lidar">
      <always_on>true</always_on>
      <update_rate>30</update_rate>
      <visualize>false</visualize>
      <topic>/lidar_link_sensor</topic>
      <ray>
        <scan>
          <horizontal>
            <samples>640</samples>
            <resolution>1</resolution>
            <min_angle>-1.570796</min_angle>
            <max_angle>1.570796</max_angle>
          </horizontal>
        </scan>
        <range>
          <min>0.1</min>
          <max>10</max>
          <resolution>0.01</resolution>
        </range>
      </ray>
    </sensor>
  </gazebo>
</robot>