3D laser slam: time synchronization of livox lidar hardware

Preface

In progress robot slam Often, a single sensor cannot achieve strong robustness , Many times, the fusion of multiple sensors is needed , For example, lidar + The camera +IMU+GPS+ Wheel speedometer + Millimeter wave radar and so on .

In the process of multi-sensor fusion, it involves the correspondence of data frames , If you need to use a timestamp to correspond , Then time synchronization is required . Due to different degrees of delay in the transmission and reception of data , The frequency of data generation is also different , If you only use soft synchronization , Then there must be some deviation , Resulting in inaccurate data alignment , Then it reflects the importance of hardware time synchronization .

This article mainly introduces in 3D laser SLAM in , How to Livox Lidar time hardware synchronization .

Livox Device support 3 Time synchronization ：

• PTP：IEEE 1588v2.0 PTP Network protocol synchronization ;
• GPS： Second pulse +GPRMC Time data , form GPS Time synchronization mode ;
• PPS： Second pulse synchronization , The upper application needs to pass other ways （ Such as ：uart） Get the time information of each pulse , And correct the point cloud time .

Synchronization principle

PTP The principle of time synchronization

The synchronization process uses IEEE 1588v2.0 PTP Of Delay request-response Mechanism （two steps）,Livox Equipment as slave End , and master Clock device ptp Time synchronization .

master and slave Clock pass Sync、Follow_Up、Delay_Req、Delay_Resp The interaction of these packets , obtain t1、t2、t3、t4 Time , The transmission path delay and the offset of the two clocks are calculated as follows ：
Transmission path delay ：

Time migration ：

GPS The principle of time synchronization

GPS Clock source PPS The port sends hardware pulses every second （PPS The signal ）, Then the data port sends a time information corresponding to the rising edge of the pulse （GPRMC Format ）.

Livox The device received PPS The rising edge of the signal , And by the GPRMC After the data is parsed into the correct time information , The point cloud time will be set to GPS Time , And keep this time benchmark continuously accumulating , To achieve and GPS Time synchronization of the device .

Be careful ：
Livox Hub Can receive directly RS485 Level GPRMC The signal ;
Livox LiDAR Cannot receive directly GPRMC The signal , Need to put GPRMC Data port access PC, And then through sdk The protocol is sent to radar

PPS Signals and GPRMC Signal timing requirements ：

PPS The principle of time synchronization

Livox LiDAR Every time I receive PPS After the rising edge of the signal , The point cloud time at the current time will be set to 0, Then start counting again until the next PPS The pulse is coming . We can use this feature , To achieve PPS Pulse pair LiDAR Synchronization of time .

GPS+PPS How to use time synchronization

In order to be compatible with other manufacturers LiDAR Hardware ,Livox The device also supports GPS Time synchronization .
because Livox There are different kinds of hardware （LiDAR/Hub）, In the use of GPS When the synchronization , Hardware interfaces can be divided into 3 class ：

• Use Livox Hub;
• Use Livox Converter 1.0 Connected LiDAR（ Such as ：Mid-40、Mid-100）;
• Use Livox Converter 2.0 Connected LiDAR（ Such as ：Tele-15、Horizon、Avia）;

The following will introduce how to use this 3 Interface GPS Time synchronization .

Livox Hub

If GPS The time signal of the module and PPS The signal is RS485 level , Connect the line directly Hub Of GPS Time synchronization port （GPS sync port） that will do .
If GPS The time signal of the module and PPS The signal is TTL level , The following level conversion is required , To connect the signal Hub Of GPS Time synchronization port （GPS sync port）.

Hub Use GPS When the synchronization , There is no need for SDK Software configuration .

Livox Converter 1.0

take GPS The time signal of the module （GPRMC） adopt TTL turn usb Module access PC,PPS( Must be RS485 level ) Signal access LiDAR Synchronous port of adapter box （Sync Port）.

View access PC Of usb Port name of the module ,
for example /dev/ttyUSB0,
Add to livox_lidar_config.json In file “timesync_config” Of “device_name”,
And then “enable_timesync” Configure to true,
Baud rate “baudrate_index” You can refer to Livox_ros_driver To configure specific values

"timesync_config": {
    
    "enable_timesync": true,
    "device_name": "/dev/ttyUSB0",
    "comm_device_type": 0,
    "baudrate_index": 2,
    "parity_index": 0
    }

then function launch file

Livox Converter 2.0

take GPS The time signal of the module （GPRMC） adopt TTL turn usb Module access PC,PPS（ Notice that this is TTL level ） Signal access LiDAR Synchronous port of adapter box （Sync Port）.

View access PC Of usb Port name of the module ,
for example /dev/ttyUSB0,
Add to livox_lidar_config.json In file “timesync_config” Of “device_name”,
And then “enable_timesync” Configure to true,
Baud rate “baudrate_index” You can refer to Livox_ros_driver To configure specific values

"timesync_config": {
    
    "enable_timesync": true,
    "device_name": "/dev/ttyUSB0",
    "comm_device_type": 0,
    "baudrate_index": 2,
    "parity_index": 0
    }

then function launch file

Status check

By viewing timestamp_type data ,
If timestamp_type by 3, It means that the equipment is in progress GPS Time synchronization ：


UTC Time format ：

PPS How to use time synchronization

Livox LiDAR Every time I receive PPS After the rising edge of the signal , The point cloud time at the current time will be set to 0, Then start counting again until the next PPS The pulse is coming . We can use this feature , To achieve PPS Pulse pair LiDAR Synchronization of time .
The following is the pseudocode to implement this process ：

// PPS Time Synchronization
static uint64_t lidar_time_last;
static uint64_t lidar_time_real;

// 1. Read the PPS rising edge time, Unit is nanosecond.
uint64_t pps_time_ns = get_pps_rising_nsecond();
// 2. Read LiDAR point time, Unit is nanosecond.
uint64_t lidar_time = get_lidar_pack_time();
// 3. Update real time.
if (lidar_time < lidar_time_last)
{
    
    //LiDAR time jump indicates the generation of PPS rising edge.
    lidar_time_real = pps_time_ns + lidar_time%(1000000000);
}
else
{
    
    lidar_time_real += lidar_time - lidar_time_last;
}
//Update history
lidar_time_last = lidar_time;

Obtain by other means PPS Time information of rising edge , Corresponds to... In the above code get_pps_rising_nsecond() Interface .