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Greg 29-02-2004 16:35

Voltage on Frame
 
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? :D

jacob_dilles 29-02-2004 17:00

Re: Voltage on Frame
 
well if you dont tell them they probably wont check it. but thats bad GP :D . so to get rid of frame voltage...
*check all MOTORS. all connectios where wires touch motors. unplug the motor and see if it is going thrugh the winding or watever. if it is, thats a BIG problem.
*electronics board. last yeah i build my own "ground stud". all of the electronics were on lexan, but we had a huge frame-grounding problem. it turns out that the screws that held the standofs that held the groundstud were touching a structal member.
*check wires. sometimes wire can get pinched and short with the frame. bad striping jobs, exsessive heat... etc
*sensor circutry. as well as custom circutry. make sure it FAR away from the frame. be libral with your electrical tape use!
*and finaly, the battery. one year our battery holder was made out of aluminum. must i go farther?

KenWittlief 29-02-2004 17:23

Re: Voltage on Frame
 
There is no place to get 3 volts from the robot RC or power system, unless you have something like the tether cable (RS-232 port) or maybe the link to the radio shorting to the chassis.

chances are you are getting a false reading - ifyou put a digital voltmeter on something that has very high resistance, you can pick up stray static and false readings on the meter. try putting something like a thousand ohm resistor across the voltmeter leads, and Ill bet your "3 volts" dissapears.

If by some chance it remains, then the only place I can think that it could come from is the radio cable from the RC. Does it run through the frame anywhere? is it possible it got pinched or sliced somewhere?

BTW - Im assuming you measured this with the black lead from the voltmeter on the negative (gnd) lead of the battery? another way to check for shorts to the frame is to unplug the batterys (both) and use the ohm meter part of the DVM - see if you have low resistance from the +12 or the gnd posts to the frame anywhere - it should be in the thousands or ten thousands of ohms -if you see <100 ohms to the frame from either one, then you have a shorted cable to the frame somewhere.

jacob_dilles 29-02-2004 17:25

Re: Voltage on Frame
 
Quote:

Originally Posted by KenWittlief
There is no place to get 3 volts from the robot RC or power system

or so one would think. resistance can drop voltage. it doesnt have to be a direct short. if a oxidized piece of aluminum 1/64 of a inch wide pierces a 5v wire partialy, it could have 3v on it easly.

just a thought.

KenWittlief 29-02-2004 17:29

Re: Voltage on Frame
 
Quote:

Originally Posted by jacob_dilles
or so one would think. resistance can drop voltage. it doesnt have to be a direct short. if a oxidized piece of aluminum 1/64 of a inch wide pierces a 5v wire partialy, it could have 3v on it easly.

just a thought.

only if the short was delivering power (currrent) to some device - but that would require two shorts, one to source the power and one to consume it.

jacob_dilles 29-02-2004 17:30

Re: Voltage on Frame
 
power --> short --> multimeter --> ground
Quote:

Originally Posted by KenWittlief
only if the short was delivering power (currrent) to some device - but that would require two shorts, one to source the power and one to consume it.


KenWittlief 29-02-2004 17:36

Re: Voltage on Frame
 
the impedance of a Digital Voltmeter (DVM) is over a million ohms - so the 'short' would have to have millions of ohms of resistance - that is a possiblility -if someone spilled something on the breaker or ground, and it was salty or only slightly condutive, then you would see voltage on the frame with a DVM - but if the resistance is that high its not a problem

you can put your fingers on the probes of a DVM and it will register several thousand ohms - much less than the input resistance of the meter iteself when its on the volts scale.

Yan Wang 29-02-2004 17:42

Re: Voltage on Frame
 
Each year, we build most of the robot on the frame before taking stuff off to have the frame welded (it's initially bolted together). We didn't take off electronics last year and the electromagnetic field caused by the welding completely fried all the electronics... that little messup cost us about $1000. Moral of story - large EM fields are bad around the electronics.

Tom Bottiglieri 29-02-2004 17:45

Re: Voltage on Frame
 
Just use a wood base.... duhHH!!!

Solace 29-02-2004 17:45

Re: Voltage on Frame
 
................. or you could just build your robot out of plywood, the way all the real teams do.............. :D

on a side note, does anyone remember the HDPE at the top of the ramp last year creating some serious static charge on the robots? has anyone had any problems like that this year? just wondering.

jacob_dilles 29-02-2004 19:02

Re: Voltage on Frame
 
i could only see it being a problem if you made your wheels out of something like wool, and had a wire running from them into your control board...

Solace 29-02-2004 19:05

Re: Voltage on Frame
 
never underestimate the creativity of tired people...

jacob_dilles 29-02-2004 19:07

Re: Voltage on Frame
 
lol its a shame EMP isnt alowed...

Elgin Clock 29-02-2004 19:29

Re: Voltage on Frame
 
I'm guessing you are in the middle of checking electrical continuity while you discovered this little problem. The problem with this that I see is that the robot contains a closed electrical syetem. Whereas the battery is the ground and the supply.

The only suggestion I can offer as to the extra 3V is that possibly your electrical sub system parts are not mounted on a non conductive part like wood or lexan.

Their is a ground built into the system, but of course like I stated before it is a closed sytem, so this is just a precautionary ground I believe.

Jay Lundy 29-02-2004 19:30

Re: Voltage on Frame
 
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.

Greg 29-02-2004 21:09

Re: Voltage on Frame
 
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 29-02-2004 21:11

Re: Voltage on Frame
 
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 29-02-2004 21:15

Re: Voltage on Frame
 
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 29-02-2004 21:18

Re: Voltage on Frame
 
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 29-02-2004 21:44

Re: Voltage on Frame
 
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 29-02-2004 21:49

Re: Voltage on Frame
 
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 29-02-2004 22:37

Re: Voltage on Frame
 
so where exactly did you measure this phantom 3V? from the ground stud to the frame?

Mike Betts 29-02-2004 22:40

Re: Voltage on Frame
 
Quote:

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? :D

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 29-02-2004 22:45

Re: Voltage on Frame
 
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 29-02-2004 22:59

Re: Voltage on Frame
 
Quote:

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 29-02-2004 23:15

Re: Voltage on Frame
 
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 01-03-2004 02:44

Re: Voltage on Frame
 
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 01-03-2004 04:16

Re: Voltage on Frame
 
Quote:

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 01-03-2004 08:00

Re: Voltage on Frame
 
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 01-03-2004 08:20

Re: Voltage on Frame
 
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.

Mark McLeod 01-03-2004 10:38

Re: Voltage on Frame
 
If you are using the current sensors then you also have capacitors storing some energy that's still around after you've pulled the fuses.

Mike Betts 02-03-2004 13:49

Re: Voltage on Frame
 
In an eariler post, I had said:


Quote:

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.
A former alumnus from that year's team responded via private message:


Quote:

...You explained it to me after the inspector drilled me for an answer... but all i can remember is induction.... maybe you can shed some light on this issue for them (and me!)...
To which I replied:


Quote:

Let me pose this in the form of a question. What is the voltage potential between a fragment of Eta Carinae and Haley's comet?

Don't know the answer? Neither do I. If the systems are truly isolated, the answer is undefined. In the real world, nothing is undefined but the coupling is so weak that the voltage could be trillions of volts. The probability that the voltage is zero is, for all purposes, zero.

In our robot, the DC "coupling" due to the leakage resistance of the system is quite weak (on the order of 10s to 100s of megohms so I would expect the DC voltage to be between 0V and 12V. Note also that the act of measuring with a voltmeter which is not ideal (less than infinite ohms) will change what you read (Heisenberg is alive and well).

Induction would likely have a more measurable effect on the AC coupling. If you measure AC volts, I can almost guarantee that the voltage will not be zero either.

Now to the 100K resistor: Its effect is much stronger than the weak coupling of the leakage resistance or of inducted fields. When I connect a 100K between ground and frame, I expect near zero volts unless there is indeed a short in the system. Likewise, a 100K between frame and +12 will raise the frame to a 12V potential unless there is indeed a short in the system.

The 100K resistor “defines” the undefined system and has a impedance low enough such that the non-ideal aspects of the meter’s input impedance is reduced dramatically.

Does this make more sense?
I hope this helps others understand the phenomenon better.


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