1、完整代码：

<?xml version="1.0"?>
<robot name="myrobot" xmlns:xacro="http://www.ros.org/wiki/xacro">
    <!-- include arm xacro file -->
    <xacro:include filename="$(find myrobot_description)/urdf/myarm.xacro" />
    <!-- Constants -->
    <xacro:property name="car_length" value="0.5" />
    <xacro:property name="car_width" value="0.4" />
    <xacro:property name="car_height" value="0.18" />
    <link name="base_footprint"/>
    <joint name="base_joint" type="fixed">
      <origin xyz="0 0 ${car_height/2+0.05}" rpy="0 0 0"/>
      <parent link="base_footprint"/>
      <child link="base_link"/>
    </joint>
    
    <link name="base_link">
      <visual>
        <origin rpy="0 0 0" xyz="0 0 0"/>
        <geometry>
          <box size="${car_length} ${car_width} ${car_height}"/>
        </geometry>
        <material name="orange" />
      </visual>
      <visual name="front_cast">
        <origin rpy="0 0 0" xyz="0.17 0 -${car_height/2+0.05/2}"/>
        <geometry>
          <sphere radius="0.0249"/>
        </geometry>
        <material name="light_grey" />
      </visual>
      <visual name="back_cast">
        <origin rpy="0 0 0" xyz="-0.17 0 -${car_height/2+0.05/2}"/>
        <geometry>
          <sphere radius="0.0249"/>
        </geometry>
        <material name="light_grey" />
      </visual>
      <collision>
        <origin rpy="0 0 0" xyz="0 0 0"/>
        <geometry>
          <box size="${car_length} ${car_width} ${car_height}"/>
        </geometry>
      </collision>
      <collision name="front_cast">
        <origin rpy="0 0 0" xyz="0.17 0 -${car_height/2+0.05/2}"/>
        <geometry>
          <sphere radius="0.0249"/>
        </geometry>
      </collision>
      <collision name="back_cast">
        <origin rpy="0 0 0" xyz="-0.17 0 -${car_height/2+0.05/2}"/>
        <geometry>
          <sphere radius="0.0249"/>
        </geometry>
      </collision>
      <box_inertial_matrix m="2.0" l="${car_length}" w="${car_width}" h="${car_height}"/>
    </link>

    <link name="left_wheel">
      <collision name="collision">
        <origin xyz="0 0 0" rpy="${M_PI_2} 0 0"/>
        <geometry>
          <cylinder radius="0.095" length="0.05"/>
        </geometry>
      </collision>
      <visual name="left_wheel_visual">
        <origin xyz="0 0 0" rpy="0 0 -${M_PI_2}"/>
        <geometry>
          <mesh filename="package://myrobot_description/meshes/wheel.dae" />
        </geometry>
      </visual>
      <cylinder_inertial_matrix m="1.0" r="0.095" l="0.05"/>
    </link>

    <joint name="left_wheel_joint" type="continuous">
      <axis xyz="0 1 0" rpy="0 0 0"/>
      <origin xyz="0 ${car_width/2+0.03} 0.095" rpy="0 0 0"/>
      <parent link="base_footprint"/>
      <child link="left_wheel"/>
      <limit effort="6" velocity="1.0"/>
      <joint_properties damping="0.0" friction="0.0"/>
    </joint>

    <link name="right_wheel">
      <collision name="collision">
        <origin xyz="0 0 0" rpy="-${M_PI_2}  0 0"/>
        <geometry>
          <cylinder radius="0.095" length="0.05"/>
        </geometry>
      </collision>
      <visual name="right_wheel_visual">
        <origin xyz="0 0 0" rpy="0 0 ${M_PI_2}"/>
        <geometry>
          <mesh filename="package://myrobot_description/meshes/wheel.dae" />
        </geometry>
      </visual>    
      <cylinder_inertial_matrix m="1.0" r="0.095" l="0.05"/>
    </link>
    <joint name="right_wheel_joint" type="continuous">
      <axis xyz="0 1 0" rpy="0 0 0"/>
      <origin xyz="0 -${car_width/2+0.03} 0.095" rpy="0 0 0"/>
      <parent link="base_footprint"/>
      <child link="right_wheel"/>
      <limit effort="6" velocity="1.0"/>
      <joint_properties damping="0.0" friction="0.0"/>
    </joint>
    <joint name="arm_base_joint" type="fixed">
      <parent link="base_link"/>
      <child link="arm_base_link"/>
      <origin rpy="0 0 0" xyz="0 0 ${car_height/2}"/>
    </joint>
    
</robot>

右轮设置：

  <link name="right_wheel">
      <collision name="collision">
        <origin xyz="0 0 0" rpy="-${M_PI_2}  0 0"/>
        <geometry>
          <cylinder radius="0.095" length="0.05"/>
        </geometry>
      </collision>
      <visual name="right_wheel_visual">
        <origin xyz="0 0 0" rpy="0 0 ${M_PI_2}"/>
        <geometry>
          <mesh filename="package://myrobot_description/meshes/wheel.dae" />
        </geometry>
      </visual>    
      <cylinder_inertial_matrix m="1.0" r="0.095" l="0.05"/>
    </link>

 <mesh filename="package://myrobot_description/meshes/wheel.dae" /> 车轮的模型

        下面是底盘base_link与arm_base_link的关节。

<joint name="arm_base_joint" type="fixed">
      <parent link="base_link"/>
      <child link="arm_base_link"/>
      <origin rpy="0 0 0" xyz="0 0 ${car_height/2}"/>
    </joint>

launch文件：

<launch>
    <arg name="model" />
    <!-- 加载机器人模型参数 -->
    <param name="robot_description" command="$(find xacro)/xacro $(find myrobot_description)/urdf/myrobot.xacro" />

    <!-- 运行joint_state_publisher_gui节点，发布机器人的关节状态  -->
    <node name="joint_state_publisher_gui" pkg="joint_state_publisher_gui" type="joint_state_publisher_gui" />

    <!-- 运行robot_state_publisher节点，发布tf  -->
    <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher" />

    <!-- 运行rviz可视化界面 -->
    <node name="rviz" pkg="rviz" type="rviz" args="-d $(find myrobot_description)/rviz/robot.rviz" required="true" />
</launch>