6.5.1 xacro_快速体验

1.需求

使用xacro优化6.4.4 URDF练习中的小车底盘实现，需要使用变量封装车辆参数，并使用 xacro 宏封装轮子重复的代码并调用宏创建四个轮子(注意: 在此，演示 xacro 的基本使用，不必要生成合法的 URDF )。

2.实现

功能包cpp06_urdf的urdf/xacro目录下，新建xacro文件demo01_helloworld.urdf.xacro，并编辑文件，输入如下内容：

<robot name="mycar" xmlns:xacro="http://wiki.ros.org/xacro">
    <!-- 属性封装 -->
    <xacro:property name="wheel_radius" value="0.025" />
    <xacro:property name="wheel_length" value="0.02" />
    <xacro:property name="PI" value="3.1415927" />

    <!-- 宏 -->
    <xacro:macro name="wheel_func" params="wheel_name" >
        <link name="${wheel_name}_wheel">
            <visual>
                <geometry>
                    <cylinder radius="${wheel_radius}" length="${wheel_length}" />
                </geometry>

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

                <material name="wheel_color">
                    <color rgba="0 0 0 0.3" />
                </material>
            </visual>
        </link>
    </xacro:macro>
    <xacro:wheel_func wheel_name="left_front"/>
    <xacro:wheel_func wheel_name="left_back"/>
    <xacro:wheel_func wheel_name="right_front"/>
    <xacro:wheel_func wheel_name="right_back"/>
</robot>

终端下进入当前文件所述目录，输入如下指令：

xacro demo01_helloworld.urdf.xacro

终端将会输出如下内容：

<?xml version="1.0" ?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from demo01_helloworld.urdf.xacro   | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<robot name="mycar">
  <link name="left_front_wheel">
    <visual>
      <geometry>
        <cylinder length="0.02" radius="0.025"/>
      </geometry>
      <origin rpy="1.57079635 0 0" xyz="0 0 0"/>
      <material name="wheel_color">
        <color rgba="0 0 0 0.3"/>
      </material>
    </visual>
  </link>
  <link name="left_back_wheel">
    <visual>
      <geometry>
        <cylinder length="0.02" radius="0.025"/>
      </geometry>
      <origin rpy="1.57079635 0 0" xyz="0 0 0"/>
      <material name="wheel_color">
        <color rgba="0 0 0 0.3"/>
      </material>
    </visual>
  </link>
  <link name="right_front_wheel">
    <visual>
      <geometry>
        <cylinder length="0.02" radius="0.025"/>
      </geometry>
      <origin rpy="1.57079635 0 0" xyz="0 0 0"/>
      <material name="wheel_color">
        <color rgba="0 0 0 0.3"/>
      </material>
    </visual>
  </link>
  <link name="right_back_wheel">
    <visual>
      <geometry>
        <cylinder length="0.02" radius="0.025"/>
      </geometry>
      <origin rpy="1.57079635 0 0" xyz="0 0 0"/>
      <material name="wheel_color">
        <color rgba="0 0 0 0.3"/>
      </material>
    </visual>
  </link>
</robot>

显然的，通过xacro我们方便的实现了代码复用。