Ideas for table edge sensing?

The old (and new, I think) WAO robot shows a mechanical approach, which
is likened to a cat's whiskers. I dwscribe it in the book. Basically two
wires, with rounded bends to prevent snagging, probe downward. If the
robot reaches the edge of a table or stairs or whatever, the switch
triggers.

An advantage of the mechanical method is that it's "ahead" of the robot
by enough to provide time to stop, even with sluggish gear motors.

IR would work except in those instances where the surface absorbed all
the IR. You'd want a method of calibrating the IR sensor so that it
works in the most surfaces it's likely to encounter. You can put the
sensors on outriggers if your robot is larger and you don't want to have
it already 1/3 over the cliff before it realizes it's about to take a
plunge into a watery deep.

-- Gordon

Actually this isn't quite right. I think it's more correct to say
surface differences, regardless of whether they absorb or reflect. So, a
black table that absorbs all the light, and an "infinite" drop off the
table, where there'd be little or no return light, might look the same
to the robot. It's the difference in reflectivity you're looking for.

BTW, be sure to look at some of the Sharp sensors for close-up
detection. Many int he family are modulated so they aren't reactive to
ambient light.

-- Gordon

Would it not work to have it at perhaps a 45 degree angle, and then measure
the distance? If it detects a range longer than X amount of inches, it could
back up?

I used a sharp GP2D12 mounted at an angle looking down. Then I just
tested if there was ground ahead of the robot, If there wasn't any
thing then it would go away. I do not know if it would work on water.

-Kit

You'd have to try out the angle. Remembring the law of optics that says
the angle of reflectance equals the angle of incidence, any sum of
angles greater than the beamwidth may not provide a return path of the
light back into the sensor. Many of the Sharp distance sensors are made
for fairly perpendicular targets (paper, people standing at a toilet),
and have quite narrow bandwidths. However, anything is worth a shot!

-- Gordon

try an ir sensor facing down