michael1309
commented
2 years ago The information you are looking for can be found in the following explanation:
- https://github.com/SICKAG/sick_scan_xd/blob/0c35a36cc4cf40d2a04325e2145112c27623acd1/doc/timing.md
- https://github.com/SICKAG/sick_scan_xd/blob/0c35a36cc4cf40d2a04325e2145112c27623acd1/doc/software_pll.md
In a nutshell: The lidars do not work with absolute time stamps but with "ticks". There are two types of tick timestamps:
- Tick timestamps for the generation
- Tick timestamps for sending the message To match the tick timestamps against the system time, a straight line equation is estimated via the so-called software PLL, which can be used to calculate from the system time to the ticks in the lidar (and vice versa). The assumption is that the tick timestamp for sending the message corresponds to the system time when receiving the message. In reality there will be a tiny delay here. This delay can be ignored.
With the help of this straight line equation, one can now calculate the system time at which the data is generated in the lidar.
Summary:
- lidar: stamp with ticks the generation (first shot in the scan) (TICK_GEN)
- lidar: stamp with ticks the sending of the scan message (TICK_TRANSMIT)
- PC: Stamp with system time the receiving of the message
- PC: Calculate back to system time of generation: System time of generation = System time of receiving - (TICK_TRANSMIT - TICK_GEN)/TICK_FREQUENCY
thanks for your apply for anwser for https://github.com/SICKAG/sick_scan_xd/issues/119#issue-1412769021, but i'm still confued for the code:
the code set the timestamp after accept the datagram, so why the time is not the timestamp of the last point ,since the timestamp of the last point is laster than the first point。（ROS2.0,Tim5XX ） By the way，what is the param use_generation_timestamp used for, does it effect the timestamp of /sensor_msgs/laser_scan? thanks sincerely.