Important info about light

[Jay Lundy]

At the cleveland regional, our robot had a short and because of this we would always get a low battery after about 1 1/2 mins of operation. We thought we had solved it when we (a) put nylon spacers around the bolts from the light and (b) shortened a screw in the drill motor that possibly could have been touching a connection inside the drill motor.

Just to be safe we built a new panel completely out of plexiglass after the regional to mount the light on. We thought we had solved the problem, but during one of the practice rounds today our battery died early. It may have just been a low battery, but we decided to test it anyway and sure enough our robot was connected directly to the ground that runs throughout the entire robot (as in, we turned the robot on, stuck one side of the multimeter on the frame and the other on the negative terminal of the battery and there was a connection. We also could touch one side on the frame and the other side on the positive terminal and read 12 V).

We were again supicious of the light, and disconnected it from the relay, which opened the connection to the frame, so it was obviously the light. We decided the problem was a screw that has nothing to do with the light assembly was resting up against the rubber surrounding the light base. The rubber near the screw had been torn up a little (though we could not see an obvious connection between metal and metal) so we decided to try a new light with a fresh rubber base. Yet, when we hooked it up, we still had a problem. There was no way the screw was touching metal, only the 1/4" piece of rubber. Just out of curiousity, I tested the connection between the rubber around the light and the metal on the light and to our surprise there was a connection!

Anyway, we solved the problem by moving the screw, but we are still confused as to how rubber can conduct electricity. I also suggest everyone, especially those who are having problems with their batteries, to check this to make sure this is not happening to them. Although it may be exciting to watch, I don't really want to see two robots, one with a negatvely charged frame and one with a positively charged frame, get into a pushing match.

Also if anyone has any idea what is up with the rubber on the light, an explanation would be nice.

[kmcclary]

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.

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

[Jay Lundy]

Yeah we had already fixed this problem I just posted this to inform other teams.

[patrickrd]

Our light was giving us all sorts of trouble during Thursday at our first regional. While my teammates were instructed to flip our control system right-side-up by the inspectors, I tried to figure out what was wrong with our light.

Some of the interesting observations I noticed were:
- Light turns on when i pull the breaker
- Light turns on with no voltage provided to the light through the wires (checked with multimeter)
- Light changes from red to blue and blue to red spontaneously
- Light turns off randomly when the robot is on
- Light occasionally slows down to a stop

Eventually I decided that the connection to the frame must be causing the problem. Mounting a piece of Lexan in between was the solution.

- Patrick

p.s. I was joking about the light color changing (but it wouldn't have surprised me at that point on Thursday) but everything else was true.

[Al Skierkiewicz]

Lights?
By now if you have been reading this post, you know that the light needs to be insulated from the frame. This device is designed for mounting on a car which has it's frame tied to the negaive terminal of the battery and therefore supplies the much needed return to battery to work. The mounting hardware is part of the electrical circuit. Under robot rules, no electrical connection can be made to the robot frame so it is important to mount this on a non-conductive matierial and run electrical through two wires. We use Lexan but almost anything works.
As to the conductive rubber, my guess is your meter was telling untruths. Check again and see the resistance between the case and the rubber ring. If it reads anything at all it is probably reads in the megohm range. For this to be considered conductive it would need to be much lower, like in the 10-100 ohm range.
Earlier teams were having problems with the light not turning and a some other problems. We found that the hardware was loose in almost all cases of light failure. Check that the hardware is tight and the light is assembled correctly.
Good Luck All

[Jay Lundy]

I am 100% sure the rubber is conductive. I know we tested it to see how many ohms it was, and I don't remember exactly what it was but it was low.

Also we stuck one side of the multimeter on the frame and another on the positive terminal of the battery and read the voltage as we repetedly touched and removed the rubber from the frame. The reading would go 12V to 0V to 12V...

We had realized that the entire metal on the light was grounded, and we had mounted the light on lexan because of this. The only reason our frame was grounded was because a screw was touching the outside of the rubber (no other metal was touching the light).

[HotWheels2002]

At Johnson and Johnson, Firestorm 4 had a similar problem. We had to pull out that voltmeter for inspection, and when the leads touched the chassis and the screen showed 12.6 volts, our whole pit went quiet. We must have spent 25 crazy minutes inspecting the insulation on every wire before finding our problem. What we found was a spot no bigger than a pencil eraser on the negative battery lead that was not fully covered in silicone, which just barely contacted the aluminum frame.

[Beowulf]

Originally posted by patrickrd
- Light changes from red to blue and blue to red spontaneously

Ah, wha?!? I didn't know that was possible.

[Al Skierkiewicz]

"What we found was a spot no bigger than a pencil eraser on the negative battery lead that was not fully covered in silicone, which just barely contacted the aluminum frame."
This is a subject that is not given enough coverage but should be a warning for all new team members. The battery is capable of welding itself to metal and with that the possibility of self destruction. With the addition of the battery connector (I hope most teams are using some form of connector) accidents have been reduced. It was not uncommon for two anxious team members to reach in and each work on battery terminal hardware to change out the battery only to find that both their tools contacted the frame at the same time and poof!
Nice find on the problem.
Good Luck All

[Greg McCoy]

The rotating beacon is not the only source of a short. Bad wiring (exposed wires, loose crimps, etc) can get into contact with the frame and have the same effect. While I'm sure the light causing problems is very possible, you shouldn't overlook silly wiring mistakes as an equally probable cause.

I also doubt that 12V could pass through the rubber.

Remember: With enough voltage, all matter becomes conductive

[Andy Brockway]

We tested our beacon at UTC and the rubber is conductive. We had been experiencing batteries not lasting during practice and had suspected a low level drain.

We lost our light during the semi-finals, squashed flat during a pushing match, and our batteriy went low voltage at the end of the match.

I ordered two new lights from NA Signal and tested both for conductivity. Sure enough with the ohm meter set at 100 ohms the rubber passes approximately 25 ohms!

Just thought you might like to know.

Andy 716