Read line by line to get a deeper understanding of these three files.
First, the launch file:
<launch>
<include file="$(find gazebo_ros)/launch/empty_world.launch">
<arg name="paused" value="false"/>
<arg name="use_sim_time" value="true"/>
<arg name="gui" value="true"/>
<arg name="headless" value="false"/>
<arg name="debug" value="false"/>
</include>
<!-- Load the robot description -->
<param name="robot_description"
command="$(find xacro)/xacro '$(find robot_model_pkg)/urdf/robot.xacro'"/>
<!-- Robot state publisher node -->
<node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher" />
<!-- Spawn the model -->
<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"
args="-urdf -model robot_model -param robot_description" />
</launch>
Second, the gazebo file
<?xml version="1.0"?>
<!-- ###################################### -->
<!-- GAZEBO ADDITIONAL DESCRIPTION OF THE 4-WHEELED ROBOT -->
<!-- October 2023 -->
<!-- ###################################### -->
<robot>
<!-- Everything is described here -->
<!-- http://classic.gazebosim.org/tutorials?tut=ros_urdf&cat=connect_ros -->
<!-- mu1 and mu2 are friction coefficients -->
<gazebo reference="body_link">
<mu1>0.2</mu1>
<mu2>0.2</mu2>
<material>Gazebo/Red</material>
</gazebo>
<gazebo reference="wheel1_link">
<mu1>0.2</mu1>
<mu2>0.2</mu2>
<material>Gazebo/Yellow</material>
</gazebo>
<gazebo reference="wheel2_link">
<mu1>0.2</mu1>
<mu2>0.2</mu2>
<material>Gazebo/Yellow</material>
</gazebo>
<gazebo reference="wheel3_link">
<mu1>0.2</mu1>
<mu2>0.2</mu2>
<material>Gazebo/Yellow</material>
</gazebo>
<gazebo reference="wheel4_link">
<mu1>0.2</mu1>
<mu2>0.2</mu2>
<material>Gazebo/Yellow</material>
</gazebo>
<!-- Controller for the 4-wheeled robot -->
<gazebo>
<plugin name="skid_steer_drive_controller" filename="libgazebo_ros_skid_steer_drive.so">
<!-- Control update rate in Hz -->
<updateRate>100.0</updateRate>
<!-- Leave this empty otherwise, there will be problems with sending control commands -->
<robotNamespace> </robotNamespace>
<!-- Robot kinematics -->
<leftFrontJoint>wheel4_joint</leftFrontJoint>
<rightFrontJoint>wheel3_joint</rightFrontJoint>
<leftRearJoint>wheel2_joint</leftRearJoint>
<rightRearJoint>wheel1_joint</rightRearJoint>
<wheelSeparation>${body_link_y_dim+wheel_link_length}</wheelSeparation>
<wheelDiameter>${wheel_link_radius}</wheelDiameter>
<!-- Maximum torque which the wheels can produce, in Nm, defaults to 5 Nm -->
<torque>1000</torque>
<!-- Topic to receive geometry_msgs/Twist message commands, defaults to `cmd_vel` -->
<commandTopic>cmd_vel</commandTopic>
<!-- Topic to publish nav_msgs/Odometry messages, defaults to `odom` -->
<odometryTopic>odom</odometryTopic>
<!-- Odometry frame, defaults to `odom` -->
<odometryFrame>odom</odometryFrame>
<!-- Robot frame to calculate odometry from, defaults to `base_footprint` -->
<robotBaseFrame>dummy</robotBaseFrame>
<!-- Set to true to publish transforms for the wheel links, defaults to false -->
<publishWheelTF>true</publishWheelTF>
<!-- Set to true to publish transforms for the odometry, defaults to true -->
<publishOdom>true</publishOdom>
<!-- Set to true to publish sensor_msgs/JointState on /joint_states for the wheel joints, defaults to false -->
<publishWheelJointState>true</publishWheelJointState>
<!-- This part is required by Gazebo -->
<covariance_x>0.0001</covariance_x>
<covariance_y>0.0001</covariance_y>
<covariance_yaw>0.01</covariance_yaw>
</plugin>
</gazebo>
</robot>Third, the xacro file for a simple four-wheel car bot
<?xml version="1.0"?>
<!-- ###################################### -->
<!-- DESCRIPTION OF THE 4-WHEELED ROBOT -->
<!-- Made by Aleksandar Haber -->
<!-- October 2023 -->
<!-- ###################################### -->
<robot name="differential_drive_robot" xmlns:xacro="http://www.ros.org/wiki/xacro">
<!-- Body dimensions -->
<xacro:property name="body_link_x_dim" value="1"/>
<xacro:property name="body_link_y_dim" value="0.6"/>
<xacro:property name="body_link_z_dim" value="0.3"/>
<!-- Wheel dimensions -->
<xacro:property name="wheel_link_radius" value="0.15"/>
<xacro:property name="wheel_link_length" value="0.1"/>
<xacro:property name="wheel_link_z_location" value="-0.1"/>
<!-- Material density -->
<xacro:property name="body_density" value="2710.0"/>
<xacro:property name="wheel_density" value="2710.0"/>
<!-- Pi constant -->
<xacro:property name="pi_const" value="3.14159265"/>
<!-- Robot body and wheel mass -->
<xacro:property name="body_mass" value="${body_density*body_link_x_dim*body_link_y_dim*body_link_z_dim}"/>
<xacro:property name="wheel_mass" value="${wheel_density*pi_const*wheel_link_radius*wheel_link_radius*wheel_link_length}"/>
<!-- Moments of inertia of the wheel -->
<xacro:property name="Iz_wheel" value="${0.5*wheel_mass*wheel_link_radius*wheel_link_radius}" />
<xacro:property name="I_wheel"
value="${(1.0/12.0)*wheel_mass*(3.0*wheel_link_radius*wheel_link_radius+wheel_link_length*wheel_link_length)}" />
<!-- This macro defines the complete inertial section of the wheel -->
<!-- It is used later in the cod e -->
<xacro:macro name="inertia_wheel">
<inertial>
<origin rpy="0 0 0" xyz="0 0 0"/>
<mass value="${wheel_mass}"/>
<inertia ixx="${I_wheel}" ixy="0.0" ixz="0.0" iyy="${I_wheel}" iyz="0" izz="${Iz_wheel}" />
</inertial>
</xacro:macro>
<!-- Over here we include the file that defines extra Gazebo options and motion control driver -->
<xacro:include filename="$(find robot_model_pkg)/urdf/robot.gazebo" />
<!-- ########################################################### -->
<!-- FROM HERE WE DEFINE LINKS, JOINTS -->
<!-- ########################################################### -->
<!-- We need to have this dummy link otherwise Gazebo will complain -->
<link name="dummy">
</link>
<joint name="dummy_joint" type="fixed">
<parent link="dummy"/>
<child link="body_link"/>
</joint>
<!-- ########################################################### -->
<!-- START: Body link of the robot -->
<!-- ########################################################### -->
<link name="body_link">
<visual>
<geometry>
<box size="${body_link_x_dim} ${body_link_y_dim} ${body_link_z_dim}" />
</geometry>
<origin rpy="0 0 0" xyz="0 0 0"/>
</visual>
<collision>
<geometry>
<box size="${body_link_x_dim} ${body_link_y_dim} ${body_link_z_dim}" />
</geometry>
<origin rpy="0 0 0" xyz="0 0 0"/>
</collision>
<inertial>
<origin rpy="0 0 0" xyz="0 0 0"/>
<mass value="${body_mass}"/>
<inertia
ixx="${(1/12)*body_mass*(body_link_y_dim*body_link_y_dim+body_link_z_dim*body_link_z_dim)}" ixy="0" ixz="0" iyy="${(1/12)*body_mass*(body_link_x_dim*body_link_x_dim+body_link_z_dim*body_link_z_dim)}" iyz="0"
izz="${(1/12)*body_mass*(body_link_y_dim*body_link_y_dim+body_link_x_dim*body_link_x_dim)}" />
</inertial>
</link>
<!-- ########################################################### -->
<!-- END: Body link of the robot -->
<!-- ########################################################### -->
<!-- ########################################################### -->
<!-- START: Back right wheel of the robot and the joint -->
<!-- ########################################################### -->
<joint name="wheel1_joint" type="continuous" >
<parent link="body_link"/>
<child link="wheel1_link" />
<origin xyz="${-body_link_x_dim/2+1.2*wheel_link_radius} ${-body_link_y_dim/2-wheel_link_length/2} ${wheel_link_z_location}" rpy="0 0 0" />
<axis xyz="0 1 0"/>
<limit effort="1000" velocity="1000"/>
<dynamics damping="1.0" friction="1.0"/>
</joint>
<link name="wheel1_link">
<visual>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</visual>
<collision>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</collision>
<xacro:inertia_wheel />
</link>
<!-- ########################################################### -->
<!-- END: Back right wheel of the robot and the joint -->
<!-- ########################################################### -->
<!-- ########################################################### -->
<!-- START: Back left wheel of the robot and the joint -->
<!-- ########################################################### -->
<joint name="wheel2_joint" type="continuous" >
<parent link="body_link"/>
<child link="wheel2_link" />
<origin xyz="${-body_link_x_dim/2+1.2*wheel_link_radius} ${body_link_y_dim/2+wheel_link_length/2} ${wheel_link_z_location}" rpy="0 0 0" />
<axis xyz="0 1 0"/>
<limit effort="1000" velocity="1000"/>
<dynamics damping="1.0" friction="1.0"/>
</joint>
<link name="wheel2_link">
<visual>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</visual>
<collision>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</collision>
<xacro:inertia_wheel />
</link>
<!-- ########################################################### -->
<!-- END: Back left wheel of the robot and the joint -->
<!-- ########################################################### -->
<!-- ########################################################### -->
<!-- START: Front right wheel of the robot and the joint -->
<!-- ########################################################### -->
<joint name="wheel3_joint" type="continuous" >
<parent link="body_link"/>
<child link="wheel3_link" />
<origin xyz="${body_link_x_dim/2-1.2*wheel_link_radius} ${-body_link_y_dim/2-wheel_link_length/2} ${wheel_link_z_location}" rpy="0 0 0" />
<axis xyz="0 1 0"/>
<limit effort="1000" velocity="1000"/>
<dynamics damping="1.0" friction="1.0"/>
</joint>
<link name="wheel3_link">
<visual>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</visual>
<collision>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</collision>
<xacro:inertia_wheel />
</link>
<!-- ########################################################### -->
<!-- END: Front right wheel of the robot and the joint -->
<!-- ########################################################### -->
<!-- ########################################################### -->
<!-- START: Front left wheel of the robot and the joint -->
<!-- ########################################################### -->
<joint name="wheel4_joint" type="continuous" >
<parent link="body_link"/>
<child link="wheel4_link" />
<origin xyz="${body_link_x_dim/2-1.2*wheel_link_radius} ${body_link_y_dim/2+wheel_link_length/2} ${wheel_link_z_location}" rpy="0 0 0" />
<axis xyz="0 1 0"/>
<limit effort="1000" velocity="1000"/>
<dynamics damping="1.0" friction="1.0"/>
</joint>
<link name="wheel4_link">
<visual>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</visual>
<collision>
<origin rpy="1.570795 0 0" xyz="0 0 0"/>
<geometry>
<cylinder length="${wheel_link_length}" radius="${wheel_link_radius}"/>
</geometry>
</collision>
<xacro:inertia_wheel />
</link>
<!-- ########################################################### -->
<!-- END: Front right wheel of the robot and the joint -->
<!-- ########################################################### -->
</robot>
Naixian Zhang 