Ideas for table edge sensing?

As the topic describes, I'm working on my robotics project again after a 8 year hiatus. I remember reading in a book somewhere--not sure if it was
gordon's or david cook's--about coming up with some sensor ideas for sensing the edge of, say, a table. Seeing as my testing grounds include a large pool, I think it would be a good idea to mount a ground facing sensor to detect edges/cliffs/etc. Any suggestions on a suitable sensor for this application? IR? Sonar? Would those be able to detect water? Any input is appreciated, thanks!
-Chris Alas
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Chris Alas wrote:

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
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Gordon McComb wrote:

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
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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?

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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
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Chris Alas wrote:

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
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try an ir sensor facing down
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