Frequently asked questions¶
The laser data in the 3D bags is much higher than the maximum reported 20 Hz rotation speed that the VLP-16 can do. Why?¶
The VLP-16 in the example bags is configured to rotate at 20 Hz. However, the frequency of UDP packets the VLP-16 sends is much higher and independent of the rotation frequency. The example bags contain a sensor_msgs/PointCloud2 per UDP packet, not one per revolution.
In the corresponding Cartographer configuration file you see TRAJECTORY_BUILDER_3D.scans_per_accumulation = 160 which means we accumulate 160 per-UDP-packet point clouds into one larger point cloud, which incorporates motion estimation by combining constant velocity and IMU measurements, for matching. Since there are two VLP-16s, 160 UDP packets is enough for roughly 2 revolutions, one per VLP-16.
Why is IMU data required for 3D SLAM, but not for 2D?¶
In 2D, Cartographer supports running the correlative scan matcher, which is normally used for finding loop closure constraints, for local SLAM. It is computationally expensive but can often render the incorporation of odometry or IMU data unnecessary. 2D also has the benefit of assuming a flat world, i.e. up is implicitly defined.
In 3D, an IMU is required mainly for measuring gravity. Gravity is an attractive quantity to measure since it does not drift and is a very strong signal and typically comprises most of any measured accelerations. Gravity is needed for two reasons:
1. There are no assumptions about the world in 3D. To properly world align the resulting trajectory and map, gravity is used to define the z-direction.
2. Roll and pitch can be derived quite well from IMU readings once the direction of gravity has been established. This saves work for the scan matcher by reducing the search window in these dimensions.