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Lidar


#1

Over the years I have done weekend projects to scan and sense stuff on my PC but only now am I playing with embedded stuff. I have a UAV which I’ve fitted with bits to capture my flights with tags so I can map them but I want to move my realtime projects across.
I have two LIDAR setups but porting them seems hard due to support of webcams as that is how I have done it. If I want to scan at 1Hz I can use an Arduino with a serial/uart connection but 1Hz when you are traveling 30mph isn’t exactly great. I used .net to program them so using a FEZ would be easier than most other options but I need webcam support. What boards support this? Being inside a plane size is also a constraint.


#2

Basically you are asking what boards support UART? All boards have multiple UARTS. Panda has 4 for example.


#3

Sorry I should have been clearer. UART interface cameras are fine for 1Hz which is completely useless. I require 20Hz+ which as far as I can find requires USB webcams. What boards support this?


#4

EMX (FEZ Cobra) or ChipworkX been used with IP cameras. Not sure about 20hz but it was surely more than 1Hz. Smaller

FEZes should do it but not tested … what do you do with images after you get them from the camera?


#5

The laser and webcam are parallel so the laser line visible in the image has a proportional relationship (resolution and lens are taken into account). Using a cocktail stirrer the laser dot is changed into a laser line making every row in the image a distance measurement.
Code finds bright red pixel distance from center of image then using a simple equation throws out the distance. Move down a row. Last row reached get new image. Start again.
On my desktop I have actually done away with the new image part as I can scan the entire image as fast as the video can stream 30fps. When travelling at 30mph and scanning at 30fps the platform will have traveled 45cm. That is a very large gap in vision so the more fps the better.


#6

Sounds very interesting


#7

Strange, I have been thinking about real LIDAR the past few days.

Theoretically a PIC with CMU(Charge Measurement Unit) can do time measurement down to 1ns, which gives you 15cm resolution when measuring time of flight of a laser pulse. That is a little low res for indoor robotics.

With some ASIC chips they can do down to 20ps, which gives you 3mm resolution. These chips can also do readings at 500000 samples per second, nice for scanning in X and Y. I’d like to play with one of these setups running in a FPGA. If I can get 100ps then I would be happy(1.5cm).

Then there are cameras with one of these laser ranging circuits behind every pixel. Some are up to 320x240 pixels, and can do 100 frames per second.

Sorry about the rambling. :slight_smile:


#8

Unfortunately TOF needs shiny things to work so scanning indoors is not possible. LIDAR police guns reflect off the license plate. Useless for my circumstance.

Visual ranging is done by trigonometry and the knowns of the system (distance between laser and camera, distance of dot from center of image). The way to increase accuracy is to get a good camera as the resolution used is half that provided. 320x240 becomes 160x240.
An annoying but necessary part is calibration. Measure pixels from center, distance of laser to camera and actual distance to the object. Throw it into excel and plot actual distance against pixels from center. Add a trendline with equation and you have calibrated equation. Best suited line differs dependent on camera.
If you have the camera and laser 5cm apart then the measurement will only be effective to about 40ft where as 20cm I have successfully used over 100ft.

FEZ mini is the smallest board and as size is important is it theoretically possible?


#9

Shiny isn’t required for TOF. If you can see the laser with a CCD then you can see if with a silicon sensor, used for TOF.

You only need shiny if you want to go long range, like 500 meters.

On the other hand, you get laser rangefinders for golf that can give you the range to the hole. The flag doesn’t need reflective tape, does it? Dunno…

You get 3D TOF LIDAR for indoor and outdoor robotics, but they are $10000 or more, not in my budget… I saw a robot in Servo Magazine that uses such a LIRAR to map and measure trees in a lumber forest.

On the mini, I don’t think it supports USB Host, so cammera interface is a problem.