MysteryLidar

Project MysteryLidar
Aliexpress Mystery Lidar
Status	In progress
Contact	bertrik
Last Update	2024-08-10

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.

Example packet:

 |<--     header    -->|  |<------------------------------------------------------------------data--------------------------------------------------------------------->|  |<footer->|
 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

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

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

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

• 0e a0 = 40974 => 0 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

footer

We see the following bytes:

• 0e a0 => 0xA00E = final angle
• cd => checksum
• 2e => unknown, is probably not a checksum, I've seen packets that are identical except for this byte

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.