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Amp
03-01-2009, 05:05 PM
Hi All,

I am planning on building a lidar sensor system for my robot, similar to this sort of scanning (http://www.david-laserscanner.com/) . I have had a look at the lidar thread, and read peoples thoughts and results.

My design ideas so far & reasons:
- IR lasers - do not want scanning to interfere with visual web-cam currently on my robot or other people e.g red lasers flying about when your trying to watch TV :-p.

- Sony ps3 eye toy web-cam - 120fps at low res and 60fps normal, easy IR filter mod, linux driver written, & sensor interface doced by cmu cam (omnivision chip)

- Xilinx Sparta 3 dev board(tbd) - linux support, my qwerk control board uses spartan3.

- Stepper motor arrangement(tbd) - decide later when played with camera and lasers


My problem is IR lasers, I do not have much experience with this. I don't know if I need laser diodes or laser modules and how this effects generating lines or crosses. Also not sure what wavelength IR I need so that it is not visible to humans or what power is suitable.


Can anyone help me with suggesting what laser I should get?
Power, wavelength, module/diode, optics, or product/part number.

Here is a rough spec for what I need.
- Non human visable (IR not UV)
- project a line (or maybe cross)
- should easily project 4-8meters in indoor conditions and be vivid to the webcam
- cheep more preferable
- thiner line more preferable
- brigher (easier on webcam processing) preferable

BTW I am in the uk, if anyone knows of any good UK suppliers.

Thanks
Paul

Sienna
03-01-2009, 06:05 PM
The biggest concern with an IR laser is of course, that you can't see it! Which really becomes a safety issue. If you can't see it, then your eyes can't adjust to filter its intensity. Which in turn means, if you are not careful, an invisible laser can blind you without you realizing it.

That being said....

You most likely want a "laser module". A laser diode is often just that, a bare diode. A laser module has collimating optics, and occasionally a constant current driver. Modules can also be outfitted with diffraction gratings to produce your line or cross patterns (typically 50 to 60 degrees). As far as wavelength, you are looking between 800nm and 1000nm (808nm and 980nm are common I believe). Power output should still be 5mW or less.

For part numbers, etc, good luck. I was looking a few weeks ago for an IR laser, but there supply on ebay has dwindled :P

Adrenalynn
03-01-2009, 08:59 PM
I'm curious: You're going to build a backdrop, your robot will have to keep a scale in view at all times, it will process millions of point in a point cloud, and your robot will not ever be able to turn. What purpose will it have?

Why not just do ranging instead?

Amp
03-02-2009, 02:07 AM
Thanks for the responses.

Sienna, thanks for some of the specs. With saftey, can any laser blind you or is there is a power threshold. If so do you think I could meet my specs of about 4-8meters with a completely safe laser?

If not I think I would need to be very carfull about constantly moving, and having contigencey plans if the processor crashes or numerous other things. Maybe some little monostable timer circuit that puts the laser on for 5-10ms every time you toggle it. Thefore on a software lockup, the laser would switch of when it does not recive a new pulse.

From all of this talk of saftey I think I will deffineatly prototype with a visiable red laser first!!! Keep my eyeballs!!!

Adrenalynn, sorry for the confusion, I am planning on doing ranging. I just tried to use that website of an example of a camera and a line laser with an offset apart and using trig to calc the distance. I am basily trying to build a line laser range finder, using the fpga and quick camera rate to make the scan fast and high res (an entire line of points) tho probably not that accurate.

When you made your LIDAR, what resolution did you stepper need (angle per step)? (assuming you use a stepper motor). Also have you played about with a line laser in you lidar and sensing multiple points in one camera frame?

General,
I have seen videos on the internet about using wine stems as line makers, has anyone tried this and does it make a nice line? Also suggested are the line lasers is DIY levels, I dont really trust this as a good source (there pretty cheep), but if some one recomends it I may start here with red line lasers for prototyping/playing with.

Adrenalynn
03-02-2009, 03:30 AM
Ok - that makes sense.

I suggest you check out the line levels at Harbor Freight. Very cheap, VERY bright.

I didn't use a stepper. All I wanted to do was scan up and down with the camera fixed to the line level so the angle was always the same. A slow cheap motor worked fine for that.

The best way I found wasn't looking for "a line of points" but rather looking for the breaks in the line, and then looking at the parallax between the breaks and the lines. Higher = further away. Lower = closer.