MysteryLidar

Project MysteryLidar
Aliexpress Mystery Lidar
Status	In progress
Contact	bertrik
Last Update	2024-08-08

Introduction

I bought an inexpensive lidar from AliExpress, no guarantees, no datasheets. This page is about understanding it and reverse engineering the protocol

Hardware

On Aliexpress: https://nl.aliexpress.com/item/1005007119470172.html and https://nl.aliexpress.com/item/1005007204489515.html

Has stickers with the following codes:

• on the side of the plastic casing: data matrix HHCT3B6X9K00863
• on the PCB: data matrix FW0251022426001437
• next to the power/data connector: 2d bar code 20617158

Connecting a serial port:

• green: 5V
• yellow: TX
• black: ground
• red: RX

Protocol

Serial bit rate is 230400 bps.

Looks a bit similar (but not identical) to other LIDAR protocols:

• https://github.com/YDLIDAR/YDLidar-SDK/blob/master/doc/YDLidar-SDK-Communication-Protocol.md
• https://www.waveshare.com/wiki/D200_LiDAR_Kit
• https://gibbard.me/lidar/

From the Aliexpress page

Baud: 230400; Packet_length=60Bytes (in every ~4ms period)

• B0= 0x55
• B1=0xAA
• B2=0x23
• B3=0x10
• B4+B5=current resolution LSB+MSB
• B6+B7= Start Angle of this packet LSB+MSB
• 16x:
  • B8:B9= Distance LSB+MSB
  • B10= Quality (1 byte only) . . .
• B56+B57= Final Angle of this packet (LSB+MSB)
• B58+B59= CRC

The angle is encoded in steps of 1/64 degree, with a 640 offset.

Throughput

Some numbers:

• each packet is 60 bytes
• each packet contains 16 measurements
• each packet contains data for 10 degrees
• there are about 36 packets per rotation
• there are about 6 rotations per second
• => 6 rotations/s * 36 packets * 60 bytes = 12960 bytes/s
• => 6 rotations/s * 36 packets * 16 samples/packet = 3456 samples/s

Example packet:

 |<--     header    -->|  |<------------------------------------------------------------------data--------------------------------------------------------------------->|  angle check
 55 aa 23 10 ec 59 ac f7  7f 40 cd 7f 40 cc 7f 40 cf 7f 40 c6 7f 40 bf 7f 40 b4 80 40 b9 80 40 ba 80 40 bc 80 40 ba 80 40 b5 80 40 d0 81 40 ca 81 40 c2 82 40 c2 82 40 c8  0e a0 cd 2e

header

We see the following bytes:

• 0x55 0xAA fixed header
• 0x23 = fixed number = packet type?
• 0x10 = number of samples in this packet
• 0x59EC = 23020, angular resolution, in units of 1/64th step of a degree, nominally 360*64 = 23040
• 0xF7AC = start angle => 350 degrees

ranging data

Each sample is encoded in 3 bytes:

• first 2 are distance, e.g. 0x0407F, highest bits are not part of the range
• last is intensity, e.g. 0xCD

angle

Start and end angles are encoded in 1/64th of a degree. Encoded number = 640 + 64*angle.

• 0e a0 = 40974 => 0 degrees

check

I have not been able to figure out what the checksum is, a simple checksum, or a CRC, or even it is an actual checksum at all.

I noticed that bit 15 is always 0, this suggests some kind of 15-bit CRC, like the CANBUS CRC.

Software

See https://github.com/bertrik/MysteryLidar

This software allows you to see the raw data coming from the lidar. It also shows what the lidar sees as a 2d-point cloud in a graphical display.