Open-Source LiDAR Time Synchronization System by Mimicking GNSS-clock¹

The LiDAR-to-IMU time synchronized system mimicing a GNSS-supplied clock interface by a microcontroller-powered platform that provides 1 microsecond synchronization precision.

To clone the repository with proper submodules utilize --recurse-submodules argument:

git clone --recurse-submodules https://github.com/MobileRoboticsSkoltech/lidar-sync-mimics-gps.git

System requirements

The project was tested using:

• Ubuntu 18.06
• ROS Melodic Morenia
• STM32CubeIDE for Linux

Hardware

The STM32 MCU-platform is chosen as it meets all the requirements described in the paper. The IMU is fed by external MCU reference clock for data rate stability.

Firmware

The MCU firmware aimed on IMU-data gathering and LiDAR-to-IMU time synchronization. It is designed using STM32CubeIDE for Linux. The main source code is placed in main.c while the other setups are stored in firmware.ioc and setup through the IDE GUI. It means there is no need to edit other source files except two mentioned.

IMU data format

The MCU outputs IMU data as are plain ASCII strings that can be used independently on ROS and may be parsed by another data recording/processing API. These strings contain timestamp (minutes, seconds, subseconds (1/25600000 of second) and IMU data (3D-angular velocity, temperature, 3D-acceleration) with format:

"i0%02x %02x %04x %04x %04x %04x %04x %04x %04x %04x %04x %04x\n"
 \/\__/ \__/ \__/ \_______/ \____________/ \__/ \____________/
 | hour mins secs  subsecs     3D gyro     temp     3D acc
IMU0 identifier for compatibility with other projects

Software

The software consists of ROS drivers for handling Lidar and IMU data and precise timestamping. Lidar ROS driver is based on common ROS package with our patch for hardware timestamping by mimicking GNSS-clock.
IMU ROS driver is developed from scratch and produces sensor_msgs/Imu messages.

Wiring

TIM5 (RTC) and TIM1(PPS signal generator, NMEA message transmission trigger) now share three tasks explained in the article. For optimization, only single timer can be utilized for all this tasks. However, to keep compatibility with current uasge of the firmware in ongoing projects, we do not plan to update it.

Wiring result schematically should look as shown in the figure:

Note about MCU to Lidar data signal inverter

Page 43 of section 7.4.3 Timing and Polarity Requirements VLP-16 User Manual states the following:

The serial connection for the NMEA message follows the RS232 standard. The interface is capable of handling voltages between ±15 VDC.

• Low voltages are marks and represent a logical 1.
• High voltages are spaces and represent a logical 0.

The serial line idle state (MARK) is a low voltage indicating a logical 1. When the start bit is asserted, the positive voltage will be asserted representing a logical 0.

This means that the polarity of the input UART signal must be inverse: "1" is "Low", "0" is "High". For that case some UART transmitters can provide such a signal polarity inversion. However, UART transmitter of STM32F4 MCU cannot. To solve the issue, separate signal invertor can be utilized that simply converts UART signal to the inverse one:

HIGH---    ┌─────────┐    ┌───────┐                   ───┐         ┌────┐       ┌──────
           │         │    │       │         ---->        │         │    │       │
LOW---- ───┘         └────┘       └──────                └─────────┘    └───────┘

Single Schmitt-trigger inverter SN74LVC1G14 is a good choice for that purpose, we used it in a board.
Googling of schmitt inverter, 74HC14 can help.

In case of questions

Any question — raise an issue, please.

¹ :

@misc{faizullin2021opensource,
  title={Open-Source LiDAR Time Synchronization System by Mimicking GNSS-clock}, 
  author={Marsel Faizullin and Anastasiia Kornilova and Gonzalo Ferrer},
  year={2021},
  eprint={2107.02625},
  archivePrefix={arXiv},
  primaryClass={cs.RO}
}