Solving static electricity problems by grounding the frame

[Kris Verdeyen]

Has anyone given any thought to the idea of purposely inserting a (>100kΩ) resistor between the frame and ground so as to allow any static charge built up on the robot to bleed off harmlessly? I'm a little paranoid about frying a cRIO.

I honestly can't see a downside here (other than possible ineffectiveness) but then we aren't any worse off than we are now.

Kris

[kevin.li.rit]

Shouldn't a large metal frame be enough to discharge static electricity without the resistor?

[Stuart]

I would be more worried about the frame gathering a static charge (what with the mandatory fabric and plastic rubbing devices covering 2/3 of your robot) than the cRio's Chassis.

[sbrumund]

Whenever you have two different materials sliding against each other there is a possibility of building up a static charge.

Last year at a scrimmage several teams had problems with static discharges that required reloading their control program (plastic wheels on dry carpet).

The simplest solution is to install a lightning rod. The main purpose of a lightning rod is to dissapate charge so there is no lightning strike. Usually the sharp edges of the robots frame will so this. If you are concerned several nails installed with the points up can be used as air terminals to dissapate any charge that builds up. If you know a physics teacher ask them for a demonstration with their VanDeGraff generator.

This may be a safety problem but if not located appropriatly

[lemon1324]

Alternatively (not sure if it will be legal at competitions) you can add a drag chain from the frame to the ground to prevent charge buildup.

[ErichKeane]

The easiest solution would be to ground the frame to the battery negative terminal, though that is questionable based on the rules. I'm terribly surprised that it isn't REQUIRED to ground the frame to the battery, as it is done with vehicles.

[Stuart]

no you cant ground your frame to your battery. there is a rule against this, and rightfully so. Remember ground is only ground when it really is ground (as in earth . . mostly) Ive personally measured a 3 volt potential between 2 "grounds" on the old 2006 batteries. If 2 robots were to come in to contact with each other even as little as 3 volts between their frames could be very bad.

I really hate to be a Debbie Downer on this subject.

[AdamHeard]

Originally Posted by Stuart

no you cant ground your frame to your battery. there is a rule against this, and rightfully so. Remember ground is only ground when it really is ground (as in earth . . mostly) Ive personally measured a 3 volt potential between 2 "grounds" on the old 2006 batteries. If 2 robots were to come in to contact with each other even as little as 3 volts between their frames could be very bad.

I really hate to be a Debbie Downer on this subject.

and don't forget that a grounded frame allows a single wire failure to short the battery (a single positively charged wire coming loose).

[ErichKeane]

The battery short issue is easily solved by using fuses and circuit breakers. For a competition like this, I've been very surprised that there isn't a rule requiring inline fuses on the positive source lines of all the batteries. Otherwise, the fuses will prevent a short from causing any issue.

As for the difference in potential between two robots, it would cause no problem whatsoever, since only grounds would touch, so there wouldn't be any power loop.

Think about this example: you have a car that needs a jump. What is the first step? You connect both grounds on the batteries to eachother. In a jump situation, the difference in potential is going to be much greater than 3 VDC, and yet there is never a problem.

I will connect negative terminals of batteries all day, there is no danger.

By disallowing the grounding of the frame, FIRST is causing an even more dangerous problem, the situation where a single positive wire contact would could give the entire frame a POSTIVE charge, which, through a human resistor, could cause extreme physical damage.

There is a very good reason all cars have their bodies/frames grounded, it is for the obvious safety benefits.

[RyanCahoon]

Originally Posted by ErichKeane

The battery short issue is easily solved by using fuses and circuit breakers. For a competition like this, I've been very surprised that there isn't a rule requiring inline fuses on the positive source lines of all the batteries. Otherwise, the fuses will prevent a short from causing any issue.

There is, that's what the breakers in the power distribution block are for, as well as the 120A main breaker. There'll always be some wire that has to connect directly to the battery, which gives you a risk of an unprotected short.

Originally Posted by ErichKeane

By disallowing the grounding of the frame, FIRST is causing an even more dangerous problem, the situation where a single positive wire contact would could give the entire frame a POSTIVE charge, which, through a human resistor, could cause extreme physical damage.

While I don't think there's any debate that a shorted chassis is bad, as you have pointed out yourself, in order for current to flow through a person and cause damage, they would have to be holding on to a negative connection while they were touching the chassis.

Conversely, with a grounded chassis, if a positive lead touches the frame, you've created a short to negative, meaning a massive current flow, causing the battery to heat due to internal resistance. Hot material expands, and the last thing first needs is a leaking lead-acid battery, much less the fire hazard.

Originally Posted by ErichKeane

There is a very good reason all cars have their bodies/frames grounded, it is for the obvious safety benefits.

Not really, it's more a convenience incentive, that way only one power wire (positive) has to be run to devices, leading to less cable runs.

--Ryan

[Al Skierkiewicz]

OK, time for a break here...
The static buildup most of you are experiencing is the plastic wheels moving inside a metal housing and the wheels slipping on the plastic floor. Plastic being an operative word here as it is non conducting. Therefore any of the normal methods one uses to dissipate charge have no effect since there is no path for the static to drain off other than through the humidity in the air.
The robot frame is insulated from the control and electrical system for one very important reason, we want you to be able to play. If you were allowed to add fuses or any other kind of protection device, when they tripped your robot would be dead in the water. The simple act of isolating the electrical system from the frame of the robot prevents abnormal robot to robot contact from taking you out of the match. A side benefit is that the control system remains unharmed and the chance of fire is reduced. Remember that the battery is capable of over 600 amps when fuly charged for about 7000 potential watts.
In addition to all of this protection, there is a 3 amp breaker built into the PD in the negative lead of the Crio power to protect the Crio from any stray shorts that may develop. (Thanks Russ and Erik) Please remember that Inspectors will be checking for electrical shorts to robot frame. Most often these come from poorly insulated sensors but this year will also come from the Crio and camera. Both of these devices must be insulated from the robot frame per R41...


<R41> All wiring and electrical devices, including all control system components, shall be electrically isolated from the ROBOT frame. The ROBOT frame must not be used to carry electrical current (e.g. this is necessary due to polarity reversals that occur under certain operating conditions such as during motor direction reversals).
The chassis for the cRIO Mobile Device Controller and the supplied camera have grounded enclosures. Under this rule (and for their protection), it is REQUIRED that they be electrically isolated from the ROBOT frame when installed on the ROBOT.

You are probably asking yourself 'has he really seen anything like this before?' and the answer would be YES. Smoke, flame, and robots out of commission. I know that inspectors may give you pause but our job is to insure that you play as long as you want or can. We want to see you play and will do everything in our power to insure that.

[EricVanWyk]

A drag chain is a good idea.

ErichKeane -

As several others have pointed out, connecting the battery's return to the chassis is a bad thing that is illegal for very good reasons. This has examined and agreed on by many EE and safety experts, including our friends at UL.

I've gathered a few quips from this thread:

AdamHeard - and don't forget that a grounded frame allows a single wire failure to short the battery (a single positively charged wire coming loose).

RyanCahoon - Not really, it's more a convenience incentive, that way only one power wire (positive) has to be run to devices, leading to less cable runs.

Al Skierkiewicz - The robot frame is insulated from the control and electrical system for one very important reason, we want you to be able to play.

And I'll add a slightly less elegant one:

The battery short issue is easily solved by using fuses and circuit breakers.

This is not true. In order to do this, all of the smaller paths need positive AND negative breakers. If a 5A return is sourced by a 40A source, the 5A return will fail. The converse is also true.

"One Breaker per Path" is only viable if your topology is a star. If the ground is already shorted, you are one mistake away from breaking that assumption.

[ErichKeane]

A fuse in the shorted circuit would prevent any discharge from causing significant damage if the frame was grounded.

Vehicles are grounded because it is safe. The simplicity of the electronics system is a byproduct of running the whole body as a ground, but in the end, it is still safe enough that every car made in the last half century does it!

Why would you need a ground-breaker? I've never seen a system where this is even done before. Again, in your car, each circuit has a fuse, but the master fusible link prevents the positive line from getting grounded and causing damage.

When it comes down to it, a grounded frame would be safer. I'd rather have a short cause a fuse to break, than have the chance at harming a person. A grounded frame makes a short immediately obvious! Otherwise, it causes massive electriction risk to anyone around the battery! Touching a ground wire on the bot is fairly easy, all it would take is someone installing/removing the battery while in contact with the frame.

I also still don't see how two colliding bots with grounded frames cause any issues. Attaching the ground wire from two batteries causes absolutely no problem.

[Al Skierkiewicz]

Eric,
Remember that in a car the frame is not "grounded" it is the common connection for the power supply. It was and is used to make manufacturing cheaper by eliminating the need to run two wires eveywhere in the vehicle. However, things go awry when body parts have high resistance, loose hardware and defective welds. And in a vehicle collision, electrical faults usually serve to stop the vehicle operation, a result we don't want to happen.
The battery is 12 volts and does not produce an electrocution hazard to First participants.
Electrical contact of auto body parts does produce electrical problems in modern day auto electronics. Although "jumping" a dead battery is normally though of as OK, two autos entwined in a collision can cause serious currents to flow in the bodies of the vehicles. If a car battery is capable of 5500 cold cranking amps and that passes through your car's computer instead of directly to the battery as in jumping, your are looking at replacing a very expensive piece of hardware. It depends on the path the current takes.

[EricVanWyk]

Originally Posted by ErichKeane

I also still don't see how two colliding bots with grounded frames cause any issues. Attaching the ground wire from two batteries causes absolutely no problem.

I don't either, I think this reason is simply wrong.

The other reasons I still agree with.

In any case, grounding the chassis is illegal in FRC.