ros运行launch文件报错requires the 'model' arg to be set是什么原因

执行 roslaunch smart_car myrobot.launch

出现：

RLException: [/home/milk/DEMO05_WS/src/smart_car/launch/myrobot.launch]
The traceback for the exception was written to the log file

model是什么，为什么网上搜不到这个错误

.launch:

<?xml version="1.0"?>
<launch>
    <arg name="model"/>
    <arg name="gui" default="False"/>
    <param name="robot_description" textfile="$(arg model)"/>
    <para name="use_gui" value="$(arg gui)"/>
    <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher"/>
    <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"/>
</launch>

我改成 roslaunch smart_car myrobot.launch model:="../urdf/myrobot.urdf"
现在又显示出IOError: [Errno 2] No such file or directory: u'../urdf/myrobot.urdf'
明明有那个文件，怎么找不到呢

你贴一下你的myrobot.launch文件的内容