1 概述

机器人都是由link和joint进行描述.

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2 流程

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# 建立工程

mkdir -p urdf_ws/src
cd urdf_ws/src

ros2 pkg create --build-type ament_cmake --dependencies urdf robot_state_publisher --license Apache-2.0 my_robot_description

cd my_robot_description
# 名称固定为 urdf
mkdir -p urdf

写入urdf代码.

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# robot.urdf
<?xml version="1.0"?>
<robot name="my_robot">

  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.2 0.2 0.05"/>
      </geometry>
      <origin xyz="0 0 0.025"/>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
  </link>

  <joint name="joint1" type="fixed">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0 0 0.05" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit lower="-1.57" upper="1.57" effort="1.0" velocity="1.0"/>
  </joint>

  <link name="link1">
    <visual>
      <geometry>
        <cylinder length="0.4" radius="0.03"/>
      </geometry>
      <origin xyz="0 0 0.2" rpy="0 0 0"/>
      <material name="green">
        <color rgba="0 1 0 1"/>
      </material>
    </visual>
  </link>

  <joint name="joint2" type="fixed">
    <parent link="link1"/>
    <child link="link2"/>
    <origin xyz="0 0 0.4" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="1.0" velocity="1.0"/>
  </joint>

  <link name="link2">
    <visual>
      <geometry>
        <cylinder length="0.3" radius="0.02"/>
      </geometry>
      <origin xyz="0 0 0.15" rpy="0 0 0"/>
      <material name="red">
        <color rgba="1 0 0 1"/>
      </material>
    </visual>
  </link>

</robot>

编译并构建.

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colcon build
source install/setup.bash

创建 launch 目录, 在其中编写 launch.

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# display.launch.py
from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration, Command
from launch_ros.actions import Node
from launch_ros.parameter_descriptions import ParameterValue
from ament_index_python.packages import get_package_share_directory
import os

def generate_launch_description():
    pkg_path = get_package_share_directory('my_robot_description')
    default_model_path = os.path.join(pkg_path, 'urdf', 'robot.urdf')
    rviz_config_path = os.path.join(pkg_path, 'config', 'display_config.rviz')

    declare_model_arg_path = DeclareLaunchArgument(
        name='model',
        default_value=str(default_model_path),
        description='Absolute path to robot urdf or xacro file'
    )

    robot_description_result = Command(['xacro ', LaunchConfiguration('model')])
    robot_description_val = ParameterValue(robot_description_result, value_type=str)

    return LaunchDescription([
        declare_model_arg_path,

        Node(
            package='robot_state_publisher',
            executable='robot_state_publisher',
            parameters=[{'robot_description': robot_description_val}],
        ),

        Node(
            package='joint_state_publisher_gui',
            executable='joint_state_publisher_gui',
        ),

        Node(
            package='rviz2',
            executable='rviz2',
            arguments=['-d', rviz_config_path],
        )
    ])

添加安装指令.

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install(DIRECTORY urdf
  DESTINATION share/${PROJECT_NAME}
)

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

install(DIRECTORY config
  DESTINATION share/${PROJECT_NAME}
)

目录结构如下

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build
install
log
src/
└── my_robot_description
    ├── CMakeLists.txt
    ├── include
    │   └── my_robot_description
    ├── launch
    │   └── display.launch.py
    │── config
    │   └── display_config.rviz
    ├── LICENSE
    ├── package.xml
    ├── src
    └── urdf
        └── robot.urdf

3 描述机器人关系的组件

3.1 robot_state_publisher

  1. 输入

    • robot_description: 机器人模型(URDF / XACRO 的 XML)

    • /joint_states: 每个关节的角度

  2. 输出: /tf 和 /tf_static：每个 link 的位置变换

3.2 joint_state_publisher

/joint_states: 消息类型为 sensor_msgs/JointState

组件 功能说明
joint_state_publisher 自动发布所有关节角度为 0 的状态
joint_state_publisher_gui 提供滑块 GUI 让你拖动角度

joint_state_publisher_gui 的效果, 如果 joint 的关系是 revolute, 则可以滑动.

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4 xacro

xacro 是对 urdf 的语法增强.

我们将上面的 urdf 换成 xacro 版本.

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

  <!-- 定义颜色 -->
  <xacro:property name="blue" value="0 0 1 1"/>
  <xacro:property name="green" value="0 1 0 1"/>
  <xacro:property name="red" value="1 0 0 1"/>

  <!-- 定义基本尺寸参数 -->
  <xacro:property name="base_size" value="0.2 0.2 0.05"/>
  <xacro:property name="link1_length" value="0.4"/>
  <xacro:property name="link2_length" value="0.3"/>

  <link name="base_link">
    <visual>
      <geometry>
        <box size="${base_size}"/>
      </geometry>
      <origin xyz="0 0 0.025"/>
      <material name="blue">
        <color rgba="${blue}"/>
      </material>
    </visual>
  </link>

  <joint name="joint1" type="fixed">
    <parent link="base_link"/>
    <child link="link1"/>
    <origin xyz="0 0 0.05" rpy="0 0 0"/>
  </joint>

  <link name="link1">
    <visual>
      <geometry>
        <cylinder length="${link1_length}" radius="0.03"/>
      </geometry>
      <origin xyz="0 0 ${link1_length/2}" rpy="0 0 0"/>
      <material name="green">
        <color rgba="${green}"/>
      </material>
    </visual>
  </link>

  <joint name="joint2" type="revolute">
    <parent link="link1"/>
    <child link="link2"/>
    <origin xyz="0 0 ${link1_length}" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="1.0" velocity="1.0"/>
  </joint>

  <link name="link2">
    <visual>
      <geometry>
        <cylinder length="${link2_length}" radius="0.02"/>
      </geometry>
      <origin xyz="0 0 ${link2_length/2}" rpy="0 0 0"/>
      <material name="red">
        <color rgba="${red}"/>
      </material>
    </visual>
  </link>

</robot>

4.1 xacro 的多文件结构

alt text

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urdf/
├── base.xacro
├── macros.xacro
├── robot.xacro
├── robot.urdf
├── sensors.xacro
└── wheels.xacro

我们传入 robot.xacro 即可, xacro 会自动解析所有 include 文件.

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

  <!-- Include modules -->
  <xacro:include filename="macros.xacro"/>
  <xacro:include filename="base.xacro"/>
  <xacro:include filename="wheels.xacro"/>
  <xacro:include filename="sensors.xacro"/>

  <!-- Define properties (used as macro arguments) -->
  <xacro:property name="base_size" value="0.4 0.4 0.1"/>

  <!-- Call base macro with required params -->
  <xacro:base_link base_size="${base_size}"/>

  <!-- Wheels -->
  <xacro:wheel name="left_wheel" parent="base_link" xyz="0.1 0.2 0"/>
  <xacro:wheel name="right_wheel" parent="base_link" xyz="0.1 -0.2 0"/>

  <!-- Sensor -->
  <xacro:lidar_sensor parent="base_link" xyz="0 0 0.1"/>

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

  <!-- 颜色 -->
  <xacro:property name="color_red" value="1 0 0 1"/>
  <xacro:property name="color_green" value="0 1 0 1"/>
  <xacro:property name="color_blue" value="0 0 1 1"/>

  <!-- 底盘尺寸 -->
  <xacro:property name="base_size" value="0.4 0.3 0.1"/>
  <xacro:property name="wheel_radius" value="0.05"/>
  <xacro:property name="wheel_width" value="0.02"/>
</robot>

对于 base.xacro 我们可以建立一个虚拟组件对地面进行贴合.

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# base.xacro
<?xml version="1.0"?>
<robot xmlns:xacro="http://ros.org/wiki/xacro">
  <xacro:macro name="base_link" params="base_size">

    <xacro:property name="base_size_z" value="${float(base_size.split()[2])}"/>
    <link name="base_footprint" />

    <link name="base_link">
      <visual>
        <geometry>
          <box size="${base_size}"/>
        </geometry>
        <origin xyz="0 0 ${base_size_z / 2}"/>
        <material name="blue">
          <color rgba="${color_blue}"/>
        </material>
      </visual>
      <collision>
        <geometry>
          <box size="${base_size}"/>
        </geometry>
        <origin xyz="0 0 ${base_size_z / 2}"/>
        <material name="blue">
          <color rgba="${color_blue}"/>
        </material>
      </collision>
    </link>

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

  </xacro:macro>
</robot>
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# wheels.xacro
<?xml version="1.0"?>
<robot xmlns:xacro="http://ros.org/wiki/xacro">

  <xacro:macro name="mul_wheel" params="name parent xyz">

    <xacro:property name="wheel_mass" value="0.1"/>

    <link name="${name}_link">
      <visual>
        <geometry>
          <sphere radius="${wheel_radius}"/>
        </geometry>
        <origin xyz="0 0 ${wheel_radius}" rpy="0 0 0"/>
        <material name="gray">
          <color rgba="0.4 0.4 0.4 1"/>
        </material>
      </visual>
      <collision>
        <geometry>
          <sphere radius="${wheel_radius}"/>
        </geometry>
        <origin xyz="0 0 0" rpy="0 0 0"/>
      </collision>
      <inertial>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value="${wheel_mass}"/>
        <inertia ixx="1e-5" ixy="0.0" ixz="0.0"
                iyy="1e-5" iyz="0.0"
                izz="1e-5"/>
      </inertial>
    </link>

    <joint name="${name}_joint" type="fixed">
      <parent link="${parent}"/>
      <child link="${name}_link"/>
      <origin xyz="${xyz}" rpy="0 0 0"/>
    </joint>
  </xacro:macro>

  <xacro:macro name="wheel" params="name parent xyz">
    <xacro:property name="wheel_mass" value="0.3"/>
    <xacro:property name="inertia_ixx" value="${0.5 * wheel_mass * wheel_radius * wheel_radius}"/>
    <xacro:property name="inertia_iyy" value="${0.5 * wheel_mass * wheel_radius * wheel_radius}"/>
    <xacro:property name="inertia_izz" value="${0.5 * wheel_mass * wheel_radius * wheel_radius}"/>

    <link name="${name}_link">
      <visual>
        <geometry>
          <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
        <origin xyz="0 0 0" rpy="1.5708 0 0"/>
        <material name="black">
          <color rgba="0 0 0 1"/>
        </material>
      </visual>
      <collision>
        <geometry>
          <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
        <origin xyz="0 0 0" rpy="1.5708 0 0"/>
        <material name="black">
          <color rgba="0 0 0 1"/>
        </material>
      </collision>
      <inertial>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value="${wheel_mass}"/>
        <inertia
          ixx="${inertia_ixx}" ixy="0.0" ixz="0.0"
          iyy="${inertia_iyy}" iyz="0.0"
          izz="${inertia_izz}"/>
      </inertial>
    </link>

    <joint name="${name}_joint" type="continuous">
      <parent link="${parent}"/>
      <child link="${name}_link"/>
      <origin xyz="${xyz}" rpy="0 0 0"/>
      <axis xyz="0 1 0"/>
    </joint>
  </xacro:macro>

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

  <xacro:macro name="lidar_sensor" params="parent xyz">

    <link name="lidar_link">
      <visual>
        <geometry>
          <cylinder radius="0.03" length="0.02"/>
        </geometry>
        <origin xyz="0 0 0.01" rpy="0 0 0"/>
        <material name="gray">
          <color rgba="0.6 0.6 0.6 1"/>
        </material>
      </visual>
    </link>

    <joint name="lidar_joint" type="fixed">
      <parent link="${parent}"/>
      <child link="lidar_link"/>
      <origin xyz="${xyz}" rpy="0 0 0"/>
    </joint>

  </xacro:macro>

</robot>

5 物理属性

5.1 碰撞

collision 标签可以给组件添加碰撞体积, 一般写法与 visual 一致, 对于不规则的物体我们也可以用 collision 将碰撞体积近似.

这里详见 base.xacro 和 wheels.xacro 的写法.

通过开关我们可以看到碰撞体积, 比如这里只写了 base.xacro 的碰撞体积.

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5.2 惯性

这里可以查看 base.xacro 为例.

  • inertial 表示惯性: 需要用一个 3x3 的矩阵表示.

    • mass 的单位是千克.

    • origin 表示重心位置.

    • inertia 表示转动惯量.

alt text

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<link name="base_link">
  <visual>
    <geometry>
      <cylinder length="0.6" radius="0.2"/>
    </geometry>
    <material name="blue">
      <color rgba="0 0 .8 1"/>
    </material>
  </visual>
  <collision>
    <geometry>
      <cylinder length="0.6" radius="0.2"/>
    </geometry>
  </collision>
  <inertial>
    <mass value="10"/>
    <inertia ixx="0.4" ixy="0.0" ixz="0.0" iyy="0.4" iyz="0.0" izz="0.2"/>
  </inertial>
</link>

这里可以看一下 base.xacro 和 wheels.xacro.

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6 References