在NAV2中，编辑URDF文件后，进行运行，提示：

Unable to parse component [${-(wheel_radius+wheel_zoff)}] to a double (while parsing a vector value)

是由于URDF文件使用了宏，而在加载时没有指定使用宏的定义。

老代码（报错的代码）：

urdf = os.path.join(gps_wpf_dir, 'urdf', 'scout_mini_algo.urdf')
with open(urdf, 'r') as infp:
   robot_description = infp.read()
launch_ros.actions.Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        name='robot_state_publisher',
        output='both',
        parameters=[{'robot_description': robot_description}]
        ),

改为新代码：

urdf = os.path.join(gps_wpf_dir, 'urdf', 'scout_mini_algo.urdf')
launch.actions.DeclareLaunchArgument(name='robotmodel', default_value=urdf,