It's one of those theory and practice
dilemmas... The only problem with this approach is that it only takes light
3ns to travel a meter. Now it is possible to resolve this kind of resolution
with fairly simple electronics. I could envision two square waves: one being
the square wave gating the laser on and off and the other being the
reflected signal. If you took the difference between these and ran it
through an integrator then 'theoretically' you'd have a dc signal
proportional to the phase difference. The phase difference is proportional
to the time-of-flight of the laser pulse.
Err... I think you may have misunderstood me. I was suggesting the use of a
rotating mirror to direct the attention of the photo diode across the plane
of the laser. For example, if the laser and mirror are mounted horizontally
from one another, the laser reflection will be somewhere on the horizontal
line scanned by the mirror. As the mirror turns, different points along that
line become visible to the photo diode. When the mirror reflects the point
that the laser "dot" is illuminating, the signal from the photo diode begins
oscillating at the frequency of the lasers modulation. This signal is
detected by a filter (analog or digital) and produces the pulse that is
detected by the processor. If the laser is on the left and the mirror on the
right, closer objects will cause the reflected "dot" to appear further to
the left from the viewpoint of the scanning mirror (pulse appears sooner)
and more distant objects will cause it to appear farther to the right (pulse
appears later) . The purpose of the modulation is only to quickly
distinguish the laser "dot" from other light sources as the mirror scans
across it.
The first picture at
http://www.cyberg8t.com/pendragn/actlite.htm
shows the mechanical setup except that I would replace the laser with the
photodiode so that different lasers could be used.
... NTSC input to PIC. ... already KNOW where
the spot is (which lines) and only need to scan them for which columns.
Using RLL encoding (how many pixels across _didn't_ have the spot, when it
appears, how many pixels does it occupy, then how many don't again. Three
1-byte entries per line. Very little RAM.
Andy, you are absolutely clever... the CCD NTSC to PIC pin and count the
lines then the level is brilliant. If the laser modulation was genlocked to
the NTSC frame then you could look for the dot to be there then not be there
etc... just one bit to hold the expected value. The final data after 2
frames is just the depth of the point the laser is hitting. My only
objection left is the cost of the CCD. And the advantage of the CCD is that
any of the scan lines can be searched which would allow a laser with a beam
to line lens to produce an entire (vertical) 3d profile using only as much
memory as you wanted to store the results. Shifting that side to side
(slowly) would get you an entire 3d map of the objects in front.
James Newton, webmaster http://get.to/techref
(hint: you can add your own private info to the techref)
mailto:jamesnewton@geocities.com
1-619-652-0593 phone