MysteryLidar
Project MysteryLidar | |
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Aliexpress Mystery Lidar | |
Status | In progress |
Contact | bertrik |
Last Update | 2024-08-05 |
Introduction
I bought an inexpensive lidar from AliExpress, no guarantees, no datasheets. This page is about understanding it and reverse engineering the protocol
Similar protocol: https://www.waveshare.com/wiki/D200_LiDAR_Kit
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
Protocol
Serial bit rate is 230400 bps.
Looks a bit similar (but not identical) to other protocols:
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.
Calculation
Some numbers:
- each packet is 60 bytes
- each packet contains 16 measurements
- each packet contains data for 10 degrees
- there are about 35 packets per rotation
- there are about 6 rotations per second
=> 6 rotations/s * 36 packets * 60 bytes = 12960 bytes/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?
- 0x59EC = 23020 ??? seems to vary a bit, is about 1/10th of the serial bit rate
- 0xF7AC = start angle
angle
Start and end angles are encoded in 1/64th of a degree. Encoded number = 640 + 64*angle.
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.