xacro文件是urdf文件的进阶版，可以通过宏定义，文件包含来精简模型文件。还可以通过定义常量、变量等来反复调用，相当有用哦～～

先贴代码：

<?xml version="1.0" ?>
<robot name="fourwheelrobot" xmlns:xacro="http://www.ros.org/wiki/xacro">
 <!--Defineing namespace-->

  <!--property list -->
  <xacro:property name="M_PI" value="3.141592"/>
  <xacro:property name="base_mass"   value="20" /> 
  <xacro:property name="base_length" value="0.80"/>
  <xacro:property name="base_width" value="0.550"/>
  <xacro:property name="base_height" value="0.30"/>

  <xacro:property name="wheel_mass"   value="2" />
  <xacro:property name="wheel_radius" value="0.122"/>
  <xacro:property name="wheel_length" value="0.062"/>
  <xacro:property name="wheel_joint_x" value="0.25"/>
  <xacro:property name="wheel_joint_y" value="0.306"/>
  <xacro:property name="wheel_joint_z" value="-0.1"/> 

  <!--Defineing the colors-->
  <material name="black">
    <color rgba="0 0 0 1"/>
  </material>
  <material name="yellow">
    <color rgba="1 0.4 0 1"/>
  </material>

  <!-- Macro for inertia matrix -->
   <xacro:macro name="cylinder_inertial_matrix" params="m r h">
     <inertial>
        <mass value="${m}" />
        <inertia ixx="${m*(3*r*r+h*h)/12}" ixy = "0" ixz = "0"
           iyy="${m*(3*r*r+h*h)/12}" iyz = "0"
           izz="${m*r*r/2}" /> 
        </inertial>
    </xacro:macro>

    <xacro:macro name="box_inertial_matrix" params="m a b c">
       <inertial>
          <mass value="${m}" />
           <inertia ixx="${m*(b*b+c*c)/12}" ixy = "0" ixz = "0"
              iyy="${m*(c*c+a*a)/12}" iyz = "0"
              izz="${m*(a*a+b*b)/12}" /> 
       </inertial>
    </xacro:macro>

  
  <!--Macro for the wheel -->
  <xacro:macro name="wheel" params="prefix1 prefix2 reflect1 reflect2">
     <joint name="${prefix1}_${prefix2}_wheel_joint" type="continuous">
       <origin xyz="${reflect1*wheel_joint_x} ${reflect2*wheel_joint_y} ${wheel_joint_z}"/>
       <parent link="base_link"/>
       <child link="${prefix1}_${prefix2}_wheel_link"/>
       <axis xyz="0 1 0"/>
     </joint>

     <link name="${prefix1}_${prefix2}_wheel_link">
       <visual>
         <origin xyz="0 0 0" rpy="${M_PI/2} 0 0" />
         <geometry>
            <cylinder radius="${wheel_radius}" length="${wheel_length}"/>
         </geometry>
         <material name="black"/>
      </visual>
      <collision>
         <origin xyz="0 0 0" rpy="${M_PI/2} 0 0" />
           <geometry>
               <cylinder radius="${wheel_radius}" length = "${wheel_length}"/>
           </geometry>
      </collision>
      <cylinder_inertial_matrix  m="${wheel_mass}" r="${wheel_radius}" h="${wheel_length}" />
     </link>
    
     <gazebo reference="${prefix1}_${prefix2}_wheel_link">
       <material>Gazebo/Black</material>
     </gazebo>

  <!-- Transmission is important to link the joints and the controller -->
     <transmission name="${prefix1}_${prefix2}_wheel_joint_trans">
         <type>transmission_interface/SimpleTransmission</type>
         <joint name="prefix1}_${prefix2}_wheel_joint" >
             <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
         </joint>
         <actuator name="${prefix1}_${prefix2}_wheel_joint_motor">
             <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
             <mechanicalReduction>1</mechanicalReduction>
         </actuator>
      </transmission>
  </xacro:macro>

  <!--start -->
   <xacro:macro name="fourwheelrobot_base_gazebo">
  <link name="base_footprint">
     <visual>
        <origin xyz="0 0 0" rqy="0 0 0"/>
         <geometry>
            <box size="0.001 0.001 0.001"/>
         </geometry>
     </visual>
   </link>

  <gazebo reference="base_footprint">
     <turnGravityOff>false</turnGravityOff>
  </gazebo>


  <joint name="base_footprint_joint" type="fixed">
            <origin xyz="0 0 ${base_height/2}" rpy="0 0 0" />        
            <parent link="base_footprint"/>
            <child link="base_link" />
   </joint> 

  <link name="base_link">
    <visual>
      <origin xyz="0 0 0" rqy="0 0 0"/>
      <geometry>
        <box size="${base_length} ${base_width} ${base_height}"/>
      </geometry>
      <material name="yellow"/>
    </visual>
   <collision>
      <origin xyz=" 0 0 0" rpy="0 0 0" />
      <geometry>
        <box size="${base_length} ${base_width} ${base_height}"/>
      </geometry>
   </collision>   
   <box_inertial_matrix  m="${base_mass}" a="${base_length}" b="${base_width}"  c="${base_height}" />
  </link>
 
  <gazebo reference="base_link">
     <material>Gazebo/Yellow</material>
  </gazebo>
    <wheel prefix1="left" prefix2="front" reflect1="1" reflect2="1"/>
    <wheel prefix1="left" prefix2="back" reflect1="-1" reflect2="1"/>
    <wheel prefix1="right" prefix2="front" reflect1="1" reflect2="-1"/>
    <wheel prefix1="right" prefix2="back" reflect1="-1" reflect2="-1"/>

<gazebo>
  <plugin name="skid_steer_drive_controller" filename="libgazebo_ros_skid_steer_drive.so">
      <rosDebugLevel>Debug</rosDebugLevel> 
       <publishWheelTF>true</publishWheelTF>
       <publishTf>1</publishTf>
       <publishWheelJointState>true</publishWheelJointState> 
       <alwaysOn>true</alwaysOn> 
    <updateRate>100.0</updateRate>
    <robotNamespace>/</robotNamespace>
    <leftFrontJoint>left_front_wheel_joint</leftFrontJoint>
    <rightFrontJoint>right_front_wheel_joint</rightFrontJoint>
    <leftRearJoint>left_back_wheel_joint</leftRearJoint>
    <rightRearJoint>right_back_wheel_joint</rightRearJoint>
    <wheelSeparation>${wheel_joint_y*2}</wheelSeparation>
    <wheelDiameter>${2*wheel_radius}</wheelDiameter>
        <broadcastTF>1</broadcastTF> 
    <robotBaseFrame>base_footprint</robotBaseFrame>
    <torque>20</torque>
    <topicName>cmd_vel</topicName>
    <odometryFrame>odom</odometryFrame> 
    <odometryTopic>odom</odometryTopic>
   
   </plugin>
</gazebo>

 

     
  </xacro:macro>
</robot>

 

 

 

 

 

 

 

 

 

1.常量定义：

 <xacro:property name="M_PI" value="3.141592"/>

  常量使用：

<origin xyz="0 0 0" rpy="${M_PI/2} 0 0" />

2.变量定义：

 变量使用：

（这次例子中没有哎～0-0)

3.宏定义：

<xacro:macro name="cylinder_inertial_matrix" params="m r h">
     <inertial>
        <mass value="${m}" />
        <inertia ixx="${m*(3*r*r+h*h)/12}" ixy = "0" ixz = "0"
           iyy="${m*(3*r*r+h*h)/12}" iyz = "0"
           izz="${m*r*r/2}" /> 
        </inertial>
    </xacro:macro>

宏定义调用：

  <box_inertial_matrix  m="${base_mass}" a="${base_length}" b="${base_width}"  c="${base_height}" />

这个例子中还添加了传动件（transmission），滑动驱动控制器插件（plugin),

<!-- Transmission is important to link the joints and the controller -->
     <transmission name="${prefix1}_${prefix2}_wheel_joint_trans">
         <type>transmission_interface/SimpleTransmission</type>
         <joint name="prefix1}_${prefix2}_wheel_joint" >
             <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
         </joint>
         <actuator name="${prefix1}_${prefix2}_wheel_joint_motor">
             <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
             <mechanicalReduction>1</mechanicalReduction>
         </actuator>
      </transmission>

<gazebo>
  <plugin name="skid_steer_drive_controller" filename="libgazebo_ros_skid_steer_drive.so">
      <rosDebugLevel>Debug</rosDebugLevel> 
       <publishWheelTF>true</publishWheelTF>
       <publishTf>1</publishTf>
       <publishWheelJointState>true</publishWheelJointState> 
       <alwaysOn>true</alwaysOn> 
    <updateRate>100.0</updateRate>
    <robotNamespace>/</robotNamespace>
    <leftFrontJoint>left_front_wheel_joint</leftFrontJoint>
    <rightFrontJoint>right_front_wheel_joint</rightFrontJoint>
    <leftRearJoint>left_back_wheel_joint</leftRearJoint>
    <rightRearJoint>right_back_wheel_joint</rightRearJoint>
    <wheelSeparation>${wheel_joint_y*2}</wheelSeparation>
    <wheelDiameter>${2*wheel_radius}</wheelDiameter>
        <broadcastTF>1</broadcastTF> 
    <robotBaseFrame>base_footprint</robotBaseFrame>
    <torque>20</torque>
    <topicName>cmd_vel</topicName>
    <odometryFrame>odom</odometryFrame> 
    <odometryTopic>odom</odometryTopic>
   
   </plugin>
</gazebo>

参考网站：

• 一个用xacro精简urdf文件的例子
• 官方wiki