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sheng00125 / LIV_handhold

An open-source hardware-synchronized handheld device for FAST-LIVO.
GNU General Public License v3.0
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Is there a fixed deviation between the LiDAR frame and camera frame? #28

Open yunliuh opened 3 weeks ago

yunliuh commented 3 weeks ago

Hello, upon reviewing your code, I’ve noticed that the time synchronization between the LiDAR and the camera is achieved by sharing the timestamp from the LiDAR scan header. This aligns the camera frame timestamps with each LiDAR scan timestamp. However, theoretically, the trigger moments for the LiDAR and the camera are not the same. Therefore, should there be a fixed deviation between them (since the camera is triggered by a 10Hz PWM signal, whereas the LiDAR is not triggered by a PPS signal)?

xuankuzcr commented 2 weeks ago

There is no fixed delay; the time deviation mainly consists of the variable exposure time and ISP processing time. Let me explain the basic principle. All sensors are hard-synchronized using a 10 Hz trigger signal generated by synchronized timers on the STM32. Note that the Livox converter only supports a 1 Hz Pulse Per Second (PPS) trigger signal, and the Livox SDK automatically subdivides this into 10 Hz. Additionally, to segment the Livox LiDAR point cloud into scans at the rising edge of the square wave, we use the STM32 to generate a synthetic (false) GPRMC (containing an integer-second timestamp) and transmit it to the onboard PC via serial communication at 1 Hz. The Livox SDK then assigns the decoded timestamp to the LiDAR scan header, and the same timestamp is also applied to the corresponding camera image frame header, as the camera is triggered by the same rising edge. Since solid-state LiDAR lacks a scan synchronization signal, this approach ensures reliable hard synchronization, especially in indoor environments where GPS signals are unavailable.