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→LIDAR Mechanical Info
[[:File:XV-11 LIDAR Mechanical Files.zip|XV-11 LIDAR Mechanical Files]] contains the following
*2D CAD Files
**XV-11_LDS CAD.zip
If anyone needs the file in some other format, just send me a request and I’ll try to help you out: rus.tech.studio ‘@’ gmail.com
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== LIDAR Sensor ==
The LIDAR sensor is what started this entire project. Find all the information you need below.
The angle of the data is not aligned with the natural axis of the device. It seems that the first sample of the first packet is in fact looking at a -10° angle, not 0°. Needs confirmation.
==== Data Format ====
===== Data format for firmware V2.4 (recent production units) =====
A full revolution will yield 90 packets, containing 4 consecutive readings each.
The length of a packet is 22 bytes.
This amounts to a total of 360 readings (1 per degree) on 1980 bytes.
Each packet is organized as follows:
start byte index speed Data 0 Data 1 Data 2 Data 3 checksum
where:
* start byte is always 0xFA
* index is the index byte in the 90 packets, going from 0xA0 (packet 0, readings 0 to 3) to 0xF9 (packet 89, readings 356 to 359).
* speed is a two-byte information, little-endian. It represents the speed, in 64th of RPM (aka value in RPM represented in fixed point, with 6 bits used for the decimal part).
* Data 0 to Data 3 are the 4 readings. Each one is 4 bytes long, and organized as follows :
byte 0 : Distance 7:0
byte 1 : “invalid data” flag : “strength warning” flag
byte 2 : Signal Strength 7:0
byte 3 : Signal Strength 15:8
As chenglung points out, the distance information is in mm, and coded on 14 bits. This puts the tests made by Sparkfun in a room of around 3.3m x 3.9m (11ft x 13 ft ?), which seems reasonable.
The minimum distance is around 15cm, and the maximum distance is around 6m.
When bit 7 of byte 1 is set, it indicates that the distance could not be calculated. When this bit is set, it seems that byte 0 contains an error code. Examples of error code are 0x02, 0x03, 0x21, 0x25, 0x35 or 0x50...
When it’s `21`, then the whole block is `21 80 XX XX`, but for all the other values it’s the data block is `YY 80 00 00`...
The bit 6 of byte 1 is a warning when the reported strength is greatly inferior to what is expected at this distance. This may happen when the material has a low reflectance (black material...), or when the dot does not have the expected size or shape (porous material, transparent fabric, grid, edge of an object...), or maybe when there are parasitic reflections (glass... ).
Byte 2 and 3 are the LSB and MSB of the strength indication. This value can get very high when facing a retroreflector.
* checksum is a two-byte checksum of the packet.
The algorithm is as follows, provided that `data` is the list of the 20 first bytes, in the same order they arrived in.
def checksum(data):
“”“Compute and return the checksum as an int.”“”
# group the data by word, little-endian
data_list = []
for t in range(10):
data_list.append( data2*t + (data2*t+1<<8) )
# compute the checksum on 32 bits
chk32 = 0
for d in data_list:
chk32 = (chk32 << 1) + d
# return a value wrapped around on 15bits, and truncated to still fit into 15 bits
checksum = (chk32 & 0x7FFF) + ( chk32 >> 15 ) # wrap around to fit into 15 bits
checksum = checksum & 0x7FFF # truncate to 15 bits
return int( checksum )
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==== Data format for firmware 2.1 (Sparkfun scans, pre-production units) ====
The periodicity of the data is 1446 bytes.
It is organized as follow :
5A A5 00 C0 XX XX data
where `XX XX` is an information about the current rotation speed of the module, in clock ticks (little endian). <nowiki>http://forums.trossenrobotics.com/showthread.php?t=4470&page=5</nowiki> posted interesting data about this.
`` is composed of 360 group of 4 bytes, organized like this :
byte 0 : Distance 7:0
byte 1 : “invalid data” flag : ”quality warning” flag : distance 13:8
byte 2 : Quality 7:0
byte 3 : Quality 15:8
As chenglung points out, the distance information is in mm, and coded on 13 or 14 bits. This would put the tests made by Sparkfun in a room of around 3.3m x 3.9m (11ft x 13 ft ?), which seems reasonable to me. 13 bits should be enough if the sensor is destined to work up to 6m. This needs some tests...
The bit 7 of byte 1 seems to indicate that the distance could not be calculated.
It’s interesting to see that when this bit is set, the second byte is always `80`, and the values of the first byte seem to be only `02`, `03`, `21`, `25`, `35` or `50`...
When it’s `21`, then the whole block is `21 80 XX XX`, but for all the other values it’s the data block is `YY 80 00 00`
maybe it’s a code to say what type of error ? (`35` is preponderant, `21` seems to be when the beam is interrupted by the supports of the cover) .
Another thing to have a look to is the temporal repartition of the data... the first sample after the sync seems to always be `21 80 XX XX`, and when this pattern appears again, it’s immediately after an other value, without the 0.2ms interval we can see most of the time between two blocks of 4...
The bit 6 of byte 1 is a warning when the reported strength is greatly inferior to what is expected at this distance. This may happen when the material has a low reflectance (black material...), or when the dot does not have the expected size or shape (porous material, transparent fabric, grid, edge of an object...), or maybe when there are parasitic reflections (glass... ).
Byte 2 and 3 are the LSB and MSB of the strength indication. This value can get very high when facing a retroreflector.
