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Now it's time to create moveable joints for the robot. First, we are going to change all the fixed joint types back to continuous. This will break your tf2 tree in RVIZ, but that’s okay because we are going to be adding Gazebo plugins to fix this.

Navigate to your tracer_wheels.xacro, tracer_castor_joints.xacro, and tracer_castor_wheels.xacro and change all the previously fixed joint types back to continuous.

Next, we are going to be creating a new XACRO called tracer_gazebo.xacro.

cd ~/humble_ws/src/tracer/tracer_description/models/xacro
touch tracer_gazebo.xacro

Open your tracer_gazebo.xacroe the following code:

<?xml version="1.0"?>

<robot name="tracer_gazebo" xmlns:xacro="<http://wiki.ros.org/xacro>">
    <xacro:macro name="diff_drive_control">
        <gazebo>
            <plugin name="gazebo_ros_diff_drive" filename="libgazebo_ros_diff_drive.so">
                <ros>
                    <namespace>${robot_namespace}</namespace>
                </ros>

                <!-- Update rate in Hz -->
                <update_rate>50</update_rate>

                <!-- wheels -->
                <left_joint>left_wheel_joint</left_joint>
                <right_joint>right_wheel_joint</right_joint>

                <!-- kinematics -->
                <wheel_separation>${wheelbase}</wheel_separation>
                <wheel_diameter>${2*wheel_radius}</wheel_diameter>

                <!-- limits -->
                <max_wheel_torque>20</max_wheel_torque>
                <max_wheel_acceleration>1.0</max_wheel_acceleration>

                <!-- input -->
                <command_topic>cmd_vel</command_topic>

                <!-- output -->
                <publish_odom>true</publish_odom>
                <publish_odom_tf>true</publish_odom_tf>
                <publish_wheel_tf>true</publish_wheel_tf>

                <odometry_topic>odom</odometry_topic>
                <odometry_frame>odom</odometry_frame>
                <robot_base_frame>base_link</robot_base_frame>

            </plugin>

            <plugin name="gazebo_ros_joint_state_publisher"
                filename="libgazebo_ros_joint_state_publisher.so">
                <ros>
                    <namespace>${robot_namespace}</namespace>
                </ros>
                <!-- Update rate in Hertz -->
                <update_rate>2</update_rate>
                <!-- Name of joints in the model whose states will be published. -->
                <joint_name>front_right_castor_joint</joint_name>
                <joint_name>front_right_wheel_joint</joint_name>

                <joint_name>front_left_castor_joint</joint_name>
                <joint_name>front_left_wheel_joint</joint_name>

                <joint_name>rear_right_castor_joint</joint_name>
                <joint_name>rear_right_wheel_joint</joint_name>

                <joint_name>rear_left_castor_joint</joint_name>
                <joint_name>rear_left_wheel_joint</joint_name>

            </plugin>
        </gazebo>
    </xacro:macro>
</robot>

Examine the code

The <gazebo> tag declares that the code following should be used in association with the Gazebo server. Within the <gazebo> tags, we are first going to call on the libgazebo_ros_diff_drive.so plugin.

<gazebo>
            <plugin name="gazebo_ros_diff_drive" filename="libgazebo_ros_diff_drive.so">

We then declare the robot namespace to match our launch file, and define an update rate for the controller. Next, we start passing configuration parameters into the plugin. We define the <left_joint> and <right_joint> such that the wheels will turn in the right frame. We also set the wheelbase, and wheel_diameter parameters such that odometry can be generated from the plugin.

								<left_joint>left_wheel_joint</left_joint>
                <right_joint>right_wheel_joint</right_joint>

                <!-- kinematics -->
                <wheel_separation>${wheelbase}</wheel_separation>
                <wheel_diameter>${2*wheel_radius}</wheel_diameter>

Next, we set some wheel limitations for torque and acceleration, and define the command topic for control. In this case, we are setting the topic to cmd_vel.

								<max_wheel_torque>20</max_wheel_torque>
                <max_wheel_acceleration>1.0</max_wheel_acceleration>

                <!-- input -->
                <command_topic>cmd_vel</command_topic>

Finally, we define the outputs we want the plugin to generate. We set the differential drive plugin to publish odometry, the transform between the base_link and odometry (will be used in NAVIGATION ODOMETRY TUTORIAL), and most importantly for our tf2 tree we set the plugin to publish_wheel_tf.

We also pass through the odometry topic, frame, and robot base link.