1、简述

统一机器人描述格式（URDF）是描述机器人模型的 XML 文件，支持Xacro（XML宏），使用Xacro来引用已经存在的XML块，创建更短且可读的XML文件。

2、初始URDF描述文件

URDF描述文件为XML格式，如下示例，定义机器人的常量。

<?xml version="1.0"?>
<robot name="sam_bot" xmlns:xacro="http://ros.org/wiki/xacro">
  <!-- Define robot constants -->
  <xacro:property name="base_width" value="0.31"/>
  <xacro:property name="base_length" value="0.42"/>
  <xacro:property name="base_height" value="0.18"/>

  <xacro:property name="wheel_radius" value="0.10"/>
  <xacro:property name="wheel_width" value="0.04"/>
  <xacro:property name="wheel_ygap" value="0.025"/>
  <xacro:property name="wheel_zoff" value="0.05"/>
  <xacro:property name="wheel_xoff" value="0.12"/>

  <xacro:property name="caster_xoff" value="0.14"/>
</robot>

3、示例

3.1 最终效果

使用URDF创建并在RViz中显示一个三轮小车，如下图所示：

3.2 安装依赖

sudo apt install ros-humble-joint-state-publisher-gui
sudo apt install ros-humble-xacro

3.3 创建功能包

mkdir -p nav2/src
cd nav2/src
ros2 pkg create --build-type ament_cmake sam_bot_description

3.4 编辑源码

3.4.1 编辑URDF描述文件

cd sam_bot_description/src/
mkdir description
cd description/
vi sam_bot_description.urdf

<?xml version="1.0"?>
<robot name="sam_bot" xmlns:xacro="http://ros.org/wiki/xacro">

  <!-- 定义机器人主底盘的大小和两个后轮的形状 -->
  <xacro:property name="base_width" value="0.31"/> 	<!-- 底盘宽 -->
  <xacro:property name="base_length" value="0.42"/> <!-- 底盘长 -->
  <xacro:property name="base_height" value="0.18"/>	<!-- 底盘高 -->
  <xacro:property name="wheel_radius" value="0.10"/><!-- 后轮半径 -->
  <xacro:property name="wheel_width" value="0.04"/> <!-- 后轮厚度 -->
  <xacro:property name="wheel_ygap" value="0.025"/>	<!-- 两轮间隙 -->
  <xacro:property name="wheel_zoff" value="0.05"/> 	<!-- 后轮在Z轴的定位 -->
  <xacro:property name="wheel_xoff" value="0.12"/> 	<!-- 后轮在X轴的定位 -->
  <xacro:property name="caster_xoff" value="0.14"/> <!-- 前轮在X轴的定位 -->

  <!-- Robot Base  -->
  <link name="base_link"><!-- link 表示一个完整组件，通过link来链接 -->
    <visual>
      <geometry><!-- 小车形状 -->
        <box size="${base_length} ${base_width} ${base_height}"/> 
      </geometry>
      <material name="Cyan"><!-- 小车颜色 -->
        <color rgba="0 1.0 1.0 1.0"/>
      </material>
    </visual>
  </link>

  <!-- Robot Footprint -->
  <link name="base_footprint"/> <!-- 添加一个关节base_footprint将其连接到 base_link-->
  <joint name="base_joint" type="fixed">
    <parent link="base_link"/>
    <child link="base_footprint"/>
    <origin xyz="0.0 0.0 ${-(wheel_radius+wheel_zoff)}" rpy="0 0 0"/>
  </joint>

  <!-- Wheels 添加两个大驱动轮 -->
  <xacro:macro name="wheel" params="prefix x_reflect y_reflect">
    <link name="${prefix}_link">
      <visual>
        <origin xyz="0 0 0" rpy="${pi/2} 0 0"/>
        <geometry>
            <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
        <material name="Gray">
          <color rgba="0.5 0.5 0.5 1.0"/>
        </material>
      </visual>
    </link>

    <joint name="${prefix}_joint" type="continuous">
      <parent link="base_link"/>
      <child link="${prefix}_link"/>
      <origin xyz="${x_reflect*wheel_xoff} ${y_reflect*(base_width/2+wheel_ygap)} ${-wheel_zoff}" rpy="0 0 0"/>
      <axis xyz="0 1 0"/>
    </joint>
  </xacro:macro>
  <xacro:wheel prefix="drivewhl_l" x_reflect="-1" y_reflect="1" />
  <xacro:wheel prefix="drivewhl_r" x_reflect="-1" y_reflect="-1" />

  <!-- Caster Wheel 添加前脚轮，将前脚轮建模成一个球体-->
  <link name="front_caster">
    <visual>
      <geometry>
        <sphere radius="${(wheel_radius+wheel_zoff-(base_height/2))}"/>
      </geometry>
      <material name="Cyan">
        <color rgba="0 1.0 1.0 1.0"/>
      </material>
    </visual>
  </link>
  
  <joint name="caster_joint" type="fixed">
    <parent link="base_link"/>
    <child link="front_caster"/>
    <origin xyz="${caster_xoff} 0.0 ${-(base_height/2)}" rpy="0 0 0"/>
  </joint>
  
</robot>

3.4.2 编辑launch启动文件

cd sam_bot_description/
mkdir launch
cd launch
vi display.launch.py

import launch
from launch.substitutions import Command, LaunchConfiguration
import launch_ros
import os

def generate_launch_description():
    pkg_share = launch_ros.substitutions.FindPackageShare(package='sam_bot_description').find('sam_bot_description')
    default_model_path = os.path.join(pkg_share, 'src/description/sam_bot_description.urdf')
    default_rviz_config_path = os.path.join(pkg_share, 'rviz/urdf_config.rviz')

    robot_state_publisher_node = launch_ros.actions.Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        parameters=[{'robot_description': Command(['xacro ', LaunchConfiguration('model')])}]
    )
    joint_state_publisher_node = launch_ros.actions.Node(
        package='joint_state_publisher',
        executable='joint_state_publisher',
        name='joint_state_publisher',
        condition=launch.conditions.UnlessCondition(LaunchConfiguration('gui'))
    )
    joint_state_publisher_gui_node = launch_ros.actions.Node(
        package='joint_state_publisher_gui',
        executable='joint_state_publisher_gui',
        name='joint_state_publisher_gui',
        condition=launch.conditions.IfCondition(LaunchConfiguration('gui'))
    )
    rviz_node = launch_ros.actions.Node(
        package='rviz2',
        executable='rviz2',
        name='rviz2',
        output='screen',
        arguments=['-d', LaunchConfiguration('rvizconfig')],
    )

    return launch.LaunchDescription([
        launch.actions.DeclareLaunchArgument(name='gui', default_value='True',
                                            description='Flag to enable joint_state_publisher_gui'),
        launch.actions.DeclareLaunchArgument(name='model', default_value=default_model_path,
                                            description='Absolute path to robot urdf file'),
        launch.actions.DeclareLaunchArgument(name='rvizconfig', default_value=default_rviz_config_path,
                                            description='Absolute path to rviz config file'),
        joint_state_publisher_node,
        joint_state_publisher_gui_node,
        robot_state_publisher_node,
        rviz_node
    ])

3.4.3 编辑RViz配置文件

cd sam_bot_description/
mkdir rviz
cd rviz
vi urdf_config.rviz

Panels:
  - Class: rviz_common/Displays
    Help Height: 78
    Name: Displays
    Property Tree Widget:
      Expanded:
        - /Global Options1
        - /Status1
        - /RobotModel1/Links1
        - /TF1
      Splitter Ratio: 0.5
    Tree Height: 557
Visualization Manager:
  Class: ""
  Displays:
    - Alpha: 0.5
      Cell Size: 1
      Class: rviz_default_plugins/Grid
      Color: 160; 160; 164
      Enabled: true
      Name: Grid
    - Alpha: 0.6
      Class: rviz_default_plugins/RobotModel
      Description Topic:
        Depth: 5
        Durability Policy: Volatile
        History Policy: Keep Last
        Reliability Policy: Reliable
        Value: /robot_description
      Enabled: true
      Name: RobotModel
      Visual Enabled: true
    - Class: rviz_default_plugins/TF
      Enabled: true
      Name: TF
      Marker Scale: 0.3
      Show Arrows: true
      Show Axes: true
      Show Names: true
  Enabled: true
  Global Options:
    Background Color: 48; 48; 48
    Fixed Frame: base_link
    Frame Rate: 30
  Name: root
  Tools:
    - Class: rviz_default_plugins/Interact
      Hide Inactive Objects: true
    - Class: rviz_default_plugins/MoveCamera
    - Class: rviz_default_plugins/Select
    - Class: rviz_default_plugins/FocusCamera
    - Class: rviz_default_plugins/Measure
      Line color: 128; 128; 0
  Transformation:
    Current:
      Class: rviz_default_plugins/TF
  Value: true
  Views:
    Current:
      Class: rviz_default_plugins/Orbit
      Name: Current View
      Target Frame: <Fixed Frame>
      Value: Orbit (rviz)
    Saved: ~

3.5 添加编译依赖

1）修改package.xml

<exec_depend>joint_state_publisher</exec_depend>
<exec_depend>joint_state_publisher_gui</exec_depend>
<exec_depend>robot_state_publisher</exec_depend>
<exec_depend>rviz</exec_depend>
<exec_depend>xacro</exec_depend>

2）修改CMakeLists.txt

install(
  DIRECTORY src launch rviz
  DESTINATION share/${PROJECT_NAME}
)

3.6 编译

cd 功能包根目录
colcon build
. install/setup.bash

3.7 启动

ros2 launch sam_bot_description display.launch.py

3.8 效果

滑动Publisher窗口中的滑块，可以控制轮子旋转

4、URDF的官方示例

http://wiki.ros.org/urdf/Examples