Brendan Moran wrote:
>
> A laser range finder is dead easy, but moderately expensive to build.

Not really. Suitable web cams are dirt cheap.
You can even get away with a single camera by using mirrors to
split the image to give you two (or more) viewpoints with one frame capture.

> It takes a laser pointer, two PSDs (position sensitive detectors), two
> lenses, some ADCs, and a micro with decent math handling. (you really
> want something capable of divide for this).
>
> It's a simple concept, really, you don't actually need to make the beam
> parallel.  You need to know the baseline between the centres of the two
> lenses, and that needs to be the same as the baseline between the
> centres of the two PSDs.
>
> For best results, I would mount the laser pointer between the two PSDs.
>
> The lenses focus the far-off laser spot into a point on the PSD, and
> since they should (ideally) be the brightest thing around, they should
> override any other light sources around.  You can improve the quality by
> putting a red filter behind the lenses.

The big problem with PSD's (areal photodiodes)
is that they are susceptible to errors from
inputs other than your laser reflection. IOW, other light sources.
You need to put a very narrow bandpass filter (optical of course)
in front of the detector to prevent it's responding to ANY other
light it may receive from the object/background. With a filter it
will still respond to spurious reflections of the laser
(e.g. glass diffusion) but the errors will be smaller. (Been there,
done that. You'd be amazed at how IR reflective supposedly 'black'
paint/cloth/objects really are).

I would suggest using line CCD detectors instead (or a webcam),
and modulate the beam at the frame rate so that you can see your
laser reflection  separately from the background illumination
(simple synchronous subtraction).
You compute the 'real' beam position from the diffraction blurring
by using a pixel intensity weighted response.

It really is 'dead easy' with a bit of trigonometry.
And be sure to use 'front surface' mirrors to minimize your diffraction
effects.

Robert

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics