Problem Description
10.12
打开clearpath_playpen.world时小车无法正常运动，而打开默认资源路径（usr/share/gazebo-9）中的world小车可以正常运动。（都未实现转向）
For example: mud.world; willowgarage.world The car can move normally
10.14
It is found that the wheels are turning, but the position of the trolley remains the same, and the wheels have changed direction
Cause Analysis:
The friction force of the contact surface between the trolley and the environment is too small
solution:
Change mu1, mu2 in .xacro or .urdf or .sdf or .gazebo (depending on your model architecture)