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Unread 29-03-2002, 15:40
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kmcclary kmcclary is offline
Founder 830/1015;Mentor 66/470/1502
FRC #0470 (Alpha Omega Robotics)
Team Role: Engineer
 
Join Date: Aug 2001
Rookie Year: 1994
Location: Ann Arbor, MI
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Re: Important info about light

Quote:
Originally posted by Jay Lundy
if anyone has any idea what is up with the rubber on the light, an explanation would be nice.
There are conductive rubbers, but your average hardware store rubber item and all rubber on the light are insulators.

First off, if your robot is entirely metal, your best bet is to mount the light onto a small square of plexy or wood, and mount THAT to something else at the corners. We did that with a scrap of 1/2" plywood. Piece of cake, and the whole problem vanishes.

Now assuming that is NOT desirable, and you STILL wish to mount directly to metal, you next need to verify that when you're torquing the screws down you haven't tightened it so hard it warped the bottom plate down and THAT is touching something.

If you are using a rubber SHEET, remember that the HOLE'S EDGE is in essence a ring of metal, and the screw will almost always lean against it if you don't place a nylon spacer or other insulator completely around the screw itself. It's better to use a grommet, which both lines the hole with rubber and places an insulating flange on both metal surfaces.

If you are already using a GROMMET, then either (1) your screw is too large for the grommet and is tearing the sides of it, (2) you could have a sharp protrusion from the screw such that it pierces the grommet under sufficient compression, or (3) the grommet is torn and migrates out of the screw's way when tightened. Grommets often tear along the inner edge if mistreated, but you have to lightly stretch or tug on them to see the tear.

Check your screw clearance in the grommet (it should fit loosely before compression), examine your grommet carefully for tears, and after looking at the screw for large burrs, check it carefully by touch to find any sharp points or edges you may not be able to see.

Note that using the wrong sized screwdriver often creates sharp points on screw head edges that'll piece insulators when tightened. If you find a burr, replace the screw, or smooth it off with a quick touch of a Dremel Tool, file, or sandpaper.

Quote:
Although it may be exciting to watch, I don't really want to see two robots, one with a negatively charged frame and one with a positively charged frame, get into a pushing match.
The frames are not "charged" in any way. No "static electricity" involved. You must have a complete circuit (wiring CIRCLE) to get a spark. Even if you DID have a Negative Ground robot contacting a Positive Ground robot, nothing would happen since there is no complete circuit.

To get a spark, there'd have to be TWO exposed conductors of different voltages on at least one of the robots before that can happen. (...And that is exactly why we have Robot Wiring Inspections, and disallow frame grounding on robots... )

BTW, this is also why birds and squirrels aren't electrocuted when they perch or walk on a high tension line. They're not completing a circuit. However, if the insulators are too small or the wires too close together, and they can STRADDLE the pole and a wire (or two wires) with two body parts, ZAAAP! Instant critter fritters! (Bug Zappers OTOH by design try to maximize that effect with a pair of opposite polarity concentric coarse screens placed VERY near each other... ) Wildlife is an additional reason insulators are so large and the wire spacing is so high between conductors even AT the pole. Wildlife likes to straddle and climb on everything!

Good luck, and please let us know what you find is causing your shorting problem!

- Keith McClary, Advisor, Huron High Team 830 "Rat Pack", Ann Arbor, MI
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Keith McClary - Organizer/Mentor/Sponsor - Ann Arbor MI area FIRST teams
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