URDF 语法详解

简单实例

在 Rviz 中显示一个盒状机器人

URDF文件 demo01_helloworld.urdf

<robot name="mycar">
    <link name="base_link">
        <visual>
            <geometry>
                <box size="0.5 0.2 0.1" />
            </geometry>
        </visual>
    </link>
</robot>

launch文件 demo01_helloworld.launch

<launch>
    <!-- 1. 在参数服务器载入 urdf 文件; name值是固定的-->
    <param name = "robot_description" textfile = "$(find urdf01_rviz)/urdf/urdf/demo01_helloworld.urdf" />
    <!-- 2. 启动 rviz args用来保存rviz文件-->
    <node pkg = "rviz" type = "rviz" name = "rviz" args = "-d $(find urdf01_rviz)/config/show_mycar.rviz"/>
</launch>

source ./devel/setup.bash
roslaunch urdf01_rviz demo01_helloworld.launch

link

定义：urdf 中的 link 标签用于描述机器人某个部件(也即刚体部分)的外观和物理属性，比如: 机器人底座、轮子、激光雷达、摄像头…每一个部件都对应一个 link, 在 link 标签内，可以设计该部件的形状、尺寸、颜色、惯性矩阵、碰撞参数等一系列属性。

• Inertial：惯性矩阵
• Collision：碰撞参数
• Visual：可视化
  

URDF文件 demo02_link.urdf

<!-- 需求: 设置不同形状的机器人部件 -->
<robot name = "mycar">
    <link name = "base_link">
        <!-- 可视化标签 -->
        <visual>
            <!-- 1. 形状 -->
            <geometry>
                <!-- 1.1 立方体 x,y,z-->
                <!-- <box size = "0.3 0.2 0.1" /> -->
                <!-- 1.2 圆柱 -->
                <!-- <cylinder radius = "0.1" length = "2" /> -->
                <!-- 1.3 球体 -->
                <!-- <sphere radius = "1" /> -->
                <!-- 1.4 皮肤 -->
                <mesh filename = "package://urdf01_rviz/meshes/autolabor_mini.stl"/>
            </geometry>

            <!-- 2. 偏移量与倾斜弧度 -->
            <!-- xyz 设置机器人模型在 x, y, z轴上偏移量 rpy 设置倾斜弧度 x(翻滚) y(俯仰) z(偏航) -->
            <origin xyz = "0 0 0" rpy = "1.57 0 1.57" />

            <!-- 3. 颜色 -->
            <!-- rgba： r = red g = green b = blue a = 透明度 四者取值介于[0,1] -->
            <material name = "car_color">
                <color rgba = "0 1 0 1" />
            </material>
        </visual>
    </link>
</robot>

launch文件 demo02_link.launch

<launch>
    <!-- 1. 在参数服务器载入 urdf 文件; name值是固定的-->
    <param name = "robot_description" textfile = "$(find urdf01_rviz)/urdf/urdf/demo02_link.urdf" />
    <!-- 2. 启动 rviz args用来保存rviz文件-->
    <node pkg = "rviz" type = "rviz" name = "rviz" args = "-d $(find urdf01_rviz)/config/show_mycar.rviz"/>
</launch>

joint

urdf 中的 joint 标签用于描述机器人关节的运动学和动力学属性，还可以指定关节运动的安全极限，机器人的两个部件(分别称之为 parent link 与 child link)以"关节"的形式相连接，不同的关节有不同的运动形式: 旋转、滑动、固定、旋转速度、旋转角度限制…,比如:安装在底座上的轮子可以360度旋转，而摄像头则可能是完全固定在底座上。



origin 就是 上图的紫色部分，joint 关于 parent_link的位置关系

URDF文件 demo03_joint.urdf

urdf文件里使用中文注释可能会报错

<!-- 需求: 设置机器人底盘，并添加摄像头 -->
<robot name = "mycar">
    <!-- 1. 底盘link -->
    <link name = "base_link">
        <visual>
            <geometry>
                <box size = "0.3 0.2 0.1" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0" />
            <material name = "car_color">
                <color rgba = "0.8 0.5 0 0.5" />
            </material>
        </visual>
    </link>
    <!-- 2. 摄像头link -->
     <link name = "camera">
        <visual>
            <geometry>
                <box size = "0.02 0.05 0.05" />
            </geometry>
            <!-- 先使用默认(后期需要修改) -->
            <origin xyz = "0 0 0" rpy = "0 0 0" />
            <material name = "camera_color">
                <color rgba = "0 0 1 0.5" />
            </material>
        </visual>
    </link>
    <!-- 3. 关节-->
    <joint name = "camera2base" type = "continuous">
        <!-- 父节 link -->
        <parent link = "base_link" />
        <!-- 子节 link -->
        <child link = "camera" />
        <!-- 设置joint相对于父节的偏移量 -->
        <origin xyz = "0.12 0 0.05" rpy = "0 0 0" />
        <!-- 设置关节旋转参考的坐标轴,0表示不绕该轴旋转，1表示绕该轴旋转 -->
        <axis xyz = "0 0 1" />
    </joint>
</robot>

launch文件 demo03_joint.launch

<launch>
    <!-- 1. 在参数服务器载入 urdf 文件; name值是固定的-->
    <param name = "robot_description" textfile = "$(find urdf01_rviz)/urdf/urdf/demo03_joint.urdf" />
    <!-- 2. 启动 rviz args用来保存rviz文件-->
    <node pkg = "rviz" type = "rviz" name = "rviz" args = "-d $(find urdf01_rviz)/config/show_mycar.rviz"/>
    <!-- 只有上述两条语句: 表现: 设置头显示位置与颜色异常 提示: No transform from [camera] to [base_link] 缺少 camera 到 base_link 的坐标变换 原因: rviz 显示 URDF 时，必须发布不同部件之间的 坐标系 关系 解决: ROS中提供了关于机器人模型显示的坐标发布相关节点(两个) rosrun joint_state_publisher joint_state_publisher rosrun robot_state_publisher robot_state_publisher -->
    <!-- 关节状态发布节点 -->
    <node pkg = "joint_state_publisher" type = "joint_state_publisher" name = "joint_state_publisher" />
    <!-- 机器人状态发布节点 -->
    <node pkg = "robot_state_publisher" type = "robot_state_publisher" name = "robot_state_publisher" />

</launch>

优化

URDF文件 demo04_base_footprint.urdf

<!-- 需求: 设置机器人底盘，并添加摄像头 -->
<robot name = "mycar">
    <!-- 添加一个尺寸极小关节 link, 再去关联出时 link 与 base_link, 关节的高度刚好和 base_link 下沉的高度一致(半个底盘高度) -->
    <link name = "base_footprint">
        <visual>
            <geometry>
                <box size = "0.001 0.001 0.001" />
            </geometry>
        </visual>
    </link>

    <!-- 1. 底盘link -->
    <link name = "base_link">
        <visual>
            <geometry>
                <box size = "0.3 0.2 0.1" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0" />
            <material name = "car_color">
                <color rgba = "0.8 0.5 0 0.5" />
            </material>
        </visual>
    </link>

    <!-- 2. 摄像头link -->
     <link name = "camera">
        <visual>
            <geometry>
                <box size = "0.02 0.05 0.05" />
            </geometry>
            <!-- 先使用默认(后期需要修改) 默认情况下关节与连杆中心点重合-->
            <origin xyz = "0 0 0.025" rpy = "0 0 0" />
            <material name = "camera_color">
                <color rgba = "0 0 1 0.5" />
            </material>
        </visual>
    </link>

    <!-- 关联 base_footprint 与 base_link -->
    <joint name = "link2footprint" type = "fixed">
        <!-- 父节 link -->
        <parent link = "base_footprint" />
        <!-- 子节 link -->
        <child link = "base_link" />
        <!-- 设置子节相对于父节的偏移量 z为半个base_link的高度-->
        <origin xyz = "0 0 0.05" rpy = "0 0 0" />
    </joint>

    <!-- 3. 关节-->
    <joint name = "camera2base" type = "continuous">
        <!-- 父节 link -->
        <parent link = "base_link" />
        <!-- 子节 link -->
        <child link = "camera" />
        <!-- 设置子节相对于父节的偏移量 -->
        <origin xyz = "0.12 0 0.05" rpy = "0 0 0" />
        <!-- 设置关节旋转参考的坐标轴,0表示不绕该轴旋转，1表示绕该轴旋转 -->
        <axis xyz = "0 0 1" />
    </joint>
</robot>

launch文件 demo04_base_footprint.launch

<launch>
    <!-- 1. 在参数服务器载入 urdf 文件; name值是固定的-->
    <param name = "robot_description" textfile = "$(find urdf01_rviz)/urdf/urdf/demo04_base_footprint.urdf" />
    <!-- 2. 启动 rviz args用来保存rviz文件 -->
    <node pkg = "rviz" type = "rviz" name = "rviz" args = "-d $(find urdf01_rviz)/config/show_mycar.rviz"/>
    <!-- 只有上述两条语句: 表现: 设置头显示位置与颜色异常 提示: No transform from [camera] to [base_link] 缺少 camera 到 base_link 的坐标变换 原因: rviz 显示 URDF 时，必须发布不同部件之间的 坐标系 关系 解决: ROS中提供了关于机器人模型显示的坐标发布相关节点(两个) rosrun joint_state_publisher joint_state_publisher rosrun robot_state_publisher robot_state_publisher -->
    <!-- 关节状态发布节点 -->
    <node pkg = "joint_state_publisher" type = "joint_state_publisher" name = "joint_state_publisher" />
    <!-- 机器人状态发布节点 -->
    <node pkg = "robot_state_publisher" type = "robot_state_publisher" name = "robot_state_publisher" />
</launch>

练习

link偏移量是物体中心和大地坐标系原点的偏移
joint偏移量是两个link坐标系之间的偏移量

设计要求：创建一个四轮圆柱状机器人模型，机器人参数如下,底盘为圆柱状，半径 10cm，高 8cm，四轮由两个驱动轮和两个万向支撑轮组成，两个驱动轮半径为 3.25cm,轮胎宽度1.5cm，两个万向轮为球状，半径 0.75cm，底盘离地间距为 1.5cm(与万向轮直径一致)

launch文件

<launch>
    <!-- 1. 在参数服务器中载入URDF -->
    <param name = "robot_description" textfile = "$(find urdf01_rviz)/urdf/urdf/demo05_test.urdf" />

    <!-- 2. 启动rviz -->
    <node pkg = "rviz" type = "rviz" name = "rviz" args = "-d $(find urdf01_rviz)/config/show_mycar.rviz"/>

    <!-- 3. 添加关节状态发布节点 -->
    <node pkg = "joint_state_publisher" type = "joint_state_publisher" name = "joint_state_publisher" />

    <!-- 4. 添加机器人状态发布节点 -->
    <node pkg = "robot_state_publisher" type = "robot_state_publisher" name = "robot_state_publisher" />

    <!-- 5. 关节运动控制节点 -->
    <node pkg = "joint_state_publisher_gui" type = "joint_state_publisher_gui" name = "joint_state_publisher_gui" />

</launch>

添加极小球体

 <!-- 1. 添加 base_footprint 只需要是一个极小的球体或者立方体就行-->
    <link name = "base_footprint">
        <visual>
            <geometry>
                <sphere radius = "0.001" />
            </geometry>
            <material name = "car_color">
                <color rgba = "1 0 0 0.5" />
            </material>
        </visual>
    </link>

添加底盘

<!-- 2. 添加底盘 -->
    <!-- 形状: 底盘为圆柱状，半径 0.1m，高 0.08m 离地间距: 0.015m -->
    <!-- 2-1 link -->
    <link name = "base_link">
        <visual>
            <geometry>
                <cylinder radius = "0.1" length = "0.08" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0"/>
            <material name = "baselink_color">
                <color rgba = "1 0.5 0.2 0.5" />
            </material>
        </visual>
    </link>
    <!-- 2-2 joint 半个车体的高度0.08m/2+0.015m-->
    <joint name = "link2footprint" type = "fixed">
        <parent link = "base_footprint" />
        <child link = "base_link" />
        <!-- 关节z上的设置 = 车体高度/2 + 离地高度 -->
        <origin xyz = "0 0 0.055" rpy = "0 0 0" />
    </joint>

添加驱动轮

驱动轮在调试中抖动，是因为开了两个joint_state_publisher,一个是静态，一个是动态，造成tf冲突

 <!-- 4. 添加万向轮 -->
    <!-- 形状: 球 半径: 0.0075m -->
    <!-- 4-1 link -->
    <link name = "front_wheel">
        <visual>
            <geometry>
                <sphere radius = "0.0075" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0"/>
            <material name = "wheel_color">
                <color rbga = "0 0 0 0.3" />
            </material>
        </visual>
    </link>
    <link name = "back_wheel">
        <visual>
            <geometry>
                <sphere radius = "0.0075" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0"/>
            <material name = "wheel_color">
                <color rbga = "0 0 0 0.3" />
            </material>
        </visual>
    </link>
    <!-- 4-2 joint -->
    <joint name = "front2link" type = "continuous">
        <parent link = "base_link" />
        <child link = "front_wheel" />
        <!-- x: 0.08 小于半径 y: 0 z: 0.04 + 0.015 - 0.0075 = 0.0475 -->
        <origin xyz = "0.08 0 -0.0475" rpy = "0 0 0" />
        <!-- 转动时绕Y轴 -->
        <axis xyz = "0 1 0" />
    </joint>
       <joint name = "back2link" type = "continuous">
        <parent link = "base_link" />
        <child link = "back_wheel" />
        <!-- x: 0.08 小于半径 y: 0 z: 0.04 + 0.015 - 0.0075 = 0.0475 -->
        <origin xyz = "-0.08 0 -0.0475" rpy = "0 0 0" />
        <!-- 转动时绕Y轴 -->
        <axis xyz = "0 1 0" />
    </joint>

URDF工具

在终端中打开urdf文件夹
check_urdf demo05_test.urdf
urdf_to_graphiz demo05_test.urdf ， 出现以下

Created file mycar.gv
Created file mycar.pdf

evince mycar.pdf

urdf总程序 demo05_test.urdf

<robot name = "mycar">
    <!-- 1. 添加 base_footprint 只需要是一个极小的球体或者立方体就行-->
    <link name = "base_footprint">
        <visual>
            <geometry>
                <sphere radius = "0.001" />
            </geometry>
            <material name = "car_color">
                <color rgba = "1 0 0 0.5" />
            </material>
        </visual>
    </link>

    <!-- 2. 添加底盘 -->
    <!-- 形状: 底盘为圆柱状，半径 0.1m，高 0.08m 离地间距: 0.015m -->
    <!-- 2-1 link -->
    <link name = "base_link">
        <visual>
            <geometry>
                <cylinder radius = "0.1" length = "0.08" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0"/>
            <material name = "baselink_color">
                <color rgba = "1 0.5 0.2 0.5" />
            </material>
        </visual>
    </link>
    <!-- 2-2 joint 半个车体的高度0.08m/2+0.015m-->
    <joint name = "link2footprint" type = "fixed">
        <parent link = "base_footprint" />
        <child link = "base_link" />
        <!-- 关节z上的设置 = 车体高度/2 + 离地高度 -->
        <origin xyz = "0 0 0.055" rpy = "0 0 0" />
    </joint>

    <!-- 3. 添加驱动轮 -->
    <!-- 形状: 圆柱 半径: 0.0325m 长度: 0.015m -->
    <!-- 3-1 link -->
    <link name = "left_wheel">
        <visual>
            <geometry>
                <cylinder radius = "0.0325" length = "0.015" />
            </geometry>
            <!-- 正常情况下圆柱面朝上,这里驱动轮在左右,需要修改欧拉角,沿x轴翻滚90度,约为1.57弧度 -->
            <origin xyz = "0 0 0" rpy = "1.5708 0 0"/>
            <material name = "wheel_color">
                <color rgba = "0 0 0 0.3" />
            </material>
        </visual>
    </link>
    <link name = "right_wheel">
        <visual>
            <geometry>
                <cylinder radius = "0.0325" length = "0.015" />
            </geometry>
            <!-- 正常情况下圆柱面朝上,这里驱动轮在左右,需要修改欧拉角,沿x轴翻滚90度,约为1.57弧度 -->
            <origin xyz = "0 0 0" rpy = "1.5708 0 0"/>
            <material name = "wheel_color">
                <color rgba = "0 0 0 0.3" />
            </material>
        </visual>
    </link>
    <!-- 3-2 joint 左关节 -->
    <joint name = "lest2link" type = "continuous">
        <parent link = "base_link" />
        <child link = "left_wheel" />
        <!-- x: 0 无偏移 y: 0.1 车体半径 z: 0.04 + 0.015 - 0.0325 = 0.055 - 0.0325 = 0.0225,在下面为负数 车体高度 / 2 + 离地间距 - 车轮半径 -->
        <origin xyz = "0 0.1 -0.0225" rpy = "0 0 0" />
        <!-- 转动时绕Y轴 -->
        <axis xyz = "0 1 0" />
    </joint>
    <!-- 3-2 joint 左关节 -->
    <joint name = "right2link" type = "continuous">
        <parent link = "base_link" />
        <child link = "right_wheel" />
        <!-- x: 0 无偏移 y: 0.1 车体半径，这里要变成负值 z: 0.04 + 0.015 - 0.0325 = 0.055 - 0.0325 = 0.0225,在下面为负数 车体高度 / 2 + 离地间距 - 车轮半径 -->
        <origin xyz = "0 -0.1 -0.0225" rpy = "0 0 0" />
        <!-- 转动时绕Y轴 -->
        <axis xyz = "0 1 0" />
    </joint>


    <!-- 4. 添加万向轮 -->
    <!-- 形状: 球 半径: 0.0075m -->
    <!-- 4-1 link -->
    <link name = "front_wheel">
        <visual>
            <geometry>
                <sphere radius = "0.0075" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0"/>
            <material name = "wheel_color">
                <color rbga = "0 0 0 0.3" />
            </material>
        </visual>
    </link>
    <link name = "back_wheel">
        <visual>
            <geometry>
                <sphere radius = "0.0075" />
            </geometry>
            <origin xyz = "0 0 0" rpy = "0 0 0"/>
            <material name = "wheel_color">
                <color rbga = "0 0 0 0.3" />
            </material>
        </visual>
    </link>
    <!-- 4-2 joint -->
    <joint name = "front2link" type = "continuous">
        <parent link = "base_link" />
        <child link = "front_wheel" />
        <!-- x: 0.08 小于半径 y: 0 z: 0.04 + 0.015 - 0.0075 = 0.0475 -->
        <origin xyz = "0.08 0 -0.0475" rpy = "0 0 0" />
        <!-- 转动时绕Y轴 -->
        <axis xyz = "0 1 0" />
    </joint>
       <joint name = "back2link" type = "continuous">
        <parent link = "base_link" />
        <child link = "back_wheel" />
        <!-- x: 0.08 小于半径0.1 y: 0 z: 0.04 + 0.015 - 0.0075 = 0.0475 -->
        <origin xyz = "-0.08 0 -0.0475" rpy = "0 0 0" />
        <!-- 转动时绕Y轴 -->
        <axis xyz = "0 1 0" />
    </joint>
</robot>