model set to be fixed to world keeps jittering

**Background**: I want to set one of the box object to be fixed to the world. Here is the [wall.xacro](https://github.com/Shawnnn/ros/blob/master/wall.xacro) // script I show has commented the below code Code below used to make box object fixed to world. **Problem**: However, the box turns out to be jittering all the time like dancing!!