Voltage on Frame

[Greg]

Thanks for the ideas I'll need to make a list of things to check when I get to the regional. What's weird though is that this voltage was still there with all the fuses out. So the RC and the speed controllers would not have had power. It looks like it is going through a ground wire somewhere, possibly a motor. Any thoughts on this one?

[jacob_dilles]

how is your ground system wired? i.e. we have the battery going to the ground stud which then goes to the dist block and ground plane on the fuse pannel, etc... it may awaken an inkling

[Greg]

The battery ground goes to the ground stud. From there there is a ground wire to the ground plate on the small circuit breaker panel and several ground wires to speed controllers for the motors that require 40 amp fuses. Other speed controllers/relays use the ground connectors on the small fuse panel.

[jacob_dilles]

hmm if the ground stud doesnt touch anything...

with all the fuzes pulled, the victors shouldent out put ANYTHING

hmmmm there have been rare cases (one that i know of personaly) of spikes outputing with no signal, but there fuzed too. how bout the team color lights... they have a metal screw that comes pretty darn close to low voltage. but if the RC is off....

wow this is a toughie. if i think of anything ill be sure to post, but i realy am befuzled. compleatly befuzled.

[dez250]

Inspect all the wires that directly connect to the ground stud and make sure that there is not a scratch in any of them. And yes i mean a scratch because i have seen a wire arc out of a scratch due to it being deeper then it looked. Also disconnect your backup battery and only have your main battery connected, check for the charge then. then switch the batteries, plug in the backup and dissconnect the main battery, then check for the charge. If it is present in both cases you have more of a problem then you thought due to more then one thing chassis grounding. If it is only present while the backup battery is connected, then you have a chassis ground some wheres on your RC, most likely the radio modem port. If it is present though with the main battery you prob are looking at a chassis ground from something thats connected to your ground stud. If thats your case, i would suggest to change all connectors that are on the ground stud to be insulated connectors if they arent already. I hope that this helps.

[ErichKeane]

First, check to see that it is still happening at the competition.

BTW, according to my ap physics TB: when Resistance is raised, Current goes down, not voltage, therefore, there must be some kind of source for 3 volts going somewhere.

My guess would be something having to do with an external power source. Check for static electricity and things like that. see if the voltage is relatively constant accross the robot, otherwise, try and figure out the path of the current.

if it still occurs with the BATTERY disconnected, i would lay environmental issues to blame. The only other bet of mine is that the speed controller fans/other fans are spinning to point where a source is created.

That reminds me, when you turned everything off, did you kill the muffins?

[KenWittlief]

so where exactly did you measure this phantom 3V? from the ground stud to the frame?

[Mike Betts]

Originally Posted by Greg

When we shipped the robot, there was one small problem remaining that we could not fix. When we checked the frame for voltage there was about 3V going through it. Last year we had a similar problem, but there was 12V. It turned out to be a limit switch with pins touching the frame.

This year we were unable to find the source of this mysterious voltage. I pulled all fuses out. There was still voltage. Unplugged the RC and all limit switches. Still there. I wasted about and hour on this before putting the bot into the crate. I think we'll need to eliminate this problem to pass inspection (the frame is not supposed to be used for power distribution). Any ideas on what to look for when we get to our first regional?

Greg,

If your frame is indeed floating (as it should be) the voltage is UNDEFINED and not "zero" as it has no reference. It could just as easily be +100V or - 500V... Is is just not defined... In general, it will usually come to rest between 0V and 12V due to leakage resistance of the wires.

Take Ken's first suggestion to heart and use a 100K resistor to ground and measure. Then 100K to +12 and measure. If you get 0V in the first case and 12V in the second case, you have no problem.

We had an inspector at NE about 4 years ago who knew his stuff. He asked the students to make that exact measurement (frame to ground) and then asked them to explain where the voltage came from. It got them to thinking about Ohm's Law. I later used an extra 100K pot to prove to the students that there was no short.

[Tom Bottiglieri]

try disconecting the battery.

And by the way. E = I x R.

It is electrically impossible to drop voltage with resistance
The only way this might happen is if there was a cap on ampereage coming from the power source, but im sure if u can still touch the frame wihtout flying backwards then this is not happening.
also, an easy way to test if if the leak is coming from the in/out pins is to turn them all high in the program, test voltage, then do the same but low.

[bigqueue]

Originally Posted by Jay Lundy

In 2002 we spent a good part of one regional figuring out why our frame had a voltage on it.

It turns out the "rubber" that went around the bottom of the light conducted electricity and we had the light mounted on metal assuming the rubber was sufficient insulation. So we just mounted it on lexan and it solved that problem.

Not that this applies to this year, but I just thought it was interesting.

[Edit]By the way you should check the screws you use to mount your speed controllers. Those are often very close to the connectors for the speed controllers and if you have your victors mounted on metal (or some metal part of your robot is touching the screw) it could be a problem.

Hmmm....this is a good point. I didn't pay much attention to how the small flashing lights "beacon replacements" mounted this year, but I wonder if a screw through them might have shorted to an etch on their PCBs?

-Quentin

[KenWittlief]

If you want to find the source quick and easy, jumper the frame to the ground stud with a piece of #6 wire

then just replace the thing that explodes or spews smoke :^)

[Disclaimer: trained professional electrical engineer on a closed course. Do not try this at home. Void where prohibited. $.05 deposit in NY ME and PA. Your actual mileage may vary. Past performance does not guarentee future return. Batteries not included. If you read this far your warrenty is now null and void. FIRST is highly addictive. There is no known cure for addiction to FIRST. Shake before using.]

[mtaman02]

ken has lost it the disclaimer proves it lol He's addicted to FIRST when he makes up a disclaimer warning about the addicition to FIRST hehe j/k

with all this talk about staic electricty your not measuring the 3v while the robots on the rug are you. some carpets have bad habits forming static electricity. that is why computers do not sit directly on carpeted / rugged floors. :-) just a thought which reminds me gotta pc to work on !

[Jay Lundy]

Originally Posted by ErichKeane

BTW, according to my ap physics TB: when Resistance is raised, Current goes down, not voltage, therefore, there must be some kind of source for 3 volts going somewhere.

Right, but resistors also have a voltage drop across them equal to their resistance times the current running through them..

So if you have a simple circuit with a 9V battery and 2 resistors of equal resistance (say 100K) then the voltage drop across each resistor is 4.5V. You can see this by the equation V=IR. The total resistance is 200K, and with a 9V battery since I=V/R, I = 45 uA. Each resistor has a voltage drop V=IR = (45 uA) * (100KOhms) = 4.5V.

Therefore if your frame was somehow inbetween those two 100K resistors, your frame voltage would not be the same as the power supply voltage.

But I definately think the problem is static electricity. It's most likely from the carpet or the HDPE. Try the 100K resistor to the 12V and ground test. It should solve your problem.

[Andy Brockway]

In 2002 we ran into the same problem at the Championship. We finally traced it to the CIM motor. We can only quess that we had overheated during our Regional and caused a short in the motor somehow. This took four frantic hours of pulling the robot apart, we replaced the motor and the problem was gone!

[Al Skierkiewicz]

Guys,
You may be missing the obvious. We have a new battery this year, the 7.2 volt backup. This battery is still up when all the fuses are removed and if the outer case were to be pressed up against a sharp edge it is possible to short the frame to one of the cells internal to the battery. This would give you a multiple of cell voltage when reading ground to the frame. On a discharged Nicad battery, 1.2 volt/cell x 3 cells = 3.6 volts less the discharge voltage might read close to 3 volts. Try disconnecting the backup battery and test again.