The map rotates in a ring

[CanCanNeedHT]

when i use vlp16 and gazebo-imu-sensor to build a map, the point cloud map will rotate, but this issus will not happen when i just use /velodyne_points topic. I have set loopClosureEnableFlag : true
In URDF, I install imu on the base_link and the origin is <origin xyz="0 0 0.3" rpy="0 0 0"/>. the vlp16`s location is

<xacro:VLP-16 parent="base_link" name="velodyne" topic="velodyne_points" hz="10" samples="1800" gpu="false">
    <origin xyz="0 0 0.175" rpy="0 0 0" />
  </xacro:VLP-16>