Solving static electricity problems by grounding the frame

[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.

[ErichKeane]

Originally Posted by EricVanWyk

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.

I realize it is illegal, I was going to ask for a rules clarification in the Q&A, but in reading through here, I'm convinced that is what the rule means.

I just still haven't heard a single reasonable reason why grounding the frame is a bad idea. If car bodies are grounded because it is cheaper, why don't higher end cars electronically isolate the bodies if it is at all safer? The reason is that grounding the body has some huge advantages, 1st- It prevents static (which is especially unsafe in fuel-based vehicles), and 2nd- it isolates, identifies and removes any shorts immediately through smart fusing.

Another household device which has the body grounded is your computer case, for the same reasons. Most metal cases are grounded (as are many laptop cases, if not, at least the trays) for these reasons, and the computer manufacturers don't face huge risks from awry wiring! Again, it is to protect the user.

The ONLY somewhat reasonable reason i've seen so far is the delicacy of the cRio. In a grounded system, it is possible that a positive short to a grounded frame could cause problems with it. However, that begs the question: why isn't the cRio designed with removable diodes to prevent this problem?

We've already seen that the cRio is susceptible to ESD (a WHOLE 'nother rant!), which should be avoidable if ITS frame was grounded.

As for two vehicles exchanging electricity in a collision: the only way this happens is if the positive cables touch. Vehicles touching grounded bumpers happen all the time, and I've never seen, nor heard of a single one causing computer/fuse issues. Again, the only situation this causes a problem, is if one has a positive short that touches another ground.


So I guess this discussion has dropped into an academic one (since FIRST's rule, is FIRST's rule). Does anyone have a plausible true reason why this would cause an issue?

[Russ Beavis]

The cRIO isn't ESD sensitive - the Driver Station is. Have you ever caused the cRIO to "fail" with an ESD event?

The auto industry is ALWAYS doing safety vs. cost analyses. There are plenty of single-point failures that can result in dangerous situations but they've done everything "reasonable" to ensure that such events are unlikely. Imagine, for example, that your steering rack snaps or your brake pedal becomes disconnected. Automobiles are NOT fail-safe under every circumstance.

I don't agree with the rationale provided in Section 8 regarding the need for chassis isolation. Collisions would need to have multi-point/multi-differential voltage contact to get really exciting. For example, birds don't seem to mind sitting on high tension wires. Current needs to have a return path in order to flow.

I do, however, agree with the FIRST isolation rule for this very specific reason - if the chassis is grounded and ANY cable touches it, current will flow.

The chassis current will be limited by the breakers in most such failures but there are arcs that can be struck and maintained at < 40 amps. I've personally experienced this in non-FIRST applications. This is one of the primary reasons why UL often limits low-voltage devices to 8 amps.

The most dangerous instance, by far, is when the battery shakes loose and its positive rail touches the chassis. Now THAT'S exciting - a dead short on a 12V/18AHr lead acid battery!! The 120A main breaker won't help you then.

The second most exciting failure is when the output from the 120A main breaker is shorted to the chassis. That breaker takes a while (relatively speaking) to open since the short circuit current for the battery is "only" a few hundred amps. That's a lot of damage before the breaker opens.

The auto industry and PC industry have obviously weighed the pros and cons. I would NEVER consider shorting my car battery's positive output to the chassis to test for isolation and breaker operation but, if you care to try this on your own car, please post the video on Youtube for everyone to watch.

Russ

[Al Skierkiewicz]

OK,
Let's hit these one at a time.
Computer cases and most home appliances are at ground/earth not power supply common. No current flows in the case unless there has been a catastrophic failure and then the power breaker or fuse trips. This is an attempt to protect the user from death, not prevent failure in the components.
Cars do have static buildup but they are huge and produce a rather nice capacitively coupled connection to earth through that nice plate and conductive additives in the tires. Static sensitive components in your car are designed to operate in this environment through the use of shielding, isolated power supplies and internal protection. Ask any car electrical engineer and they will tell you that the electrical environment on a modern day vehicle is one of the worst environments they have to contend with.
Auto body common power supplies are not ground nor are they earth they are power supply common. No current flows between the car battery and the ground beneath the car. If the addition of one wire would add $.025 cents to the cost of a car, the manufacturer making 1 million cars would be spending an additional $250,000 a year per wire.
Metal bumpers are the exception rather than the norm these days. You will find them on some vehicles, mostly SUV or truck designs. Again you have to visualize the current flow. Where does it go. Does current flow if the positive lead of one battery is connected to the body of another car? No, there is no complete circuit. If the two bodies come together than the path is through the positive lead of the first battery, through the second car body, then through the common body contact and then through the first car body and back to the first battery. Where the current flows is anybody's guess and it is only limited by the resistance of the connections in between. There is a real reason why firefighters cut the battery leads when they first appear on an accident scene. High curent translates to sparks, heat, and explosive batteries.
Grounding the case of the Crio to the robot frame will not prevent ESD. Again the issue is with the discharge path, huge currents develop huge voltage potentials across resistance. Think about this...how much voltage is developed by 1000 amps flowing through a one ohm resistor? Ohm's Law says V=I*R. V=1000*1=1000 volts.
As I have discussed earlier, First wants it's teams to be able to play, as long as they want or can. Do you want to be dead during your finals match on Einstein because of an electrical mishap in a simple robot collision? Do you want your Crio to smoke when you have no money left to buy a new one? Do you want your students to watch in horror as smoke and flame rises from their robot or that of one of their alliance partners or both partners?

[MrForbes]

Originally Posted by ErichKeane

Another household device which has the body grounded is your computer case, for the same reasons.

Household device wiring is quite a different case...one side of AC line power is connected (literally) to the ground. The case of the computer is grounded to keep the exposed metal parts of the computer at ground potential.

Anyways, I don't see any advantage to connecting the wood frame of our robot to the negative battery terminal.

[ErichKeane]

Er, your computer case IS connected to the negative terminal of the DC power supply, through the brass stand-offs of the motherboard. The power supply itself is the only one routed directly to the 3rd pin AC Common.

And yes, grounding your wood frame would have minimal advantages...

[MrForbes]

Originally Posted by ErichKeane

The power supply itself is the only one routed directly to the 3rd pin AC Common.

The 3rd pin on the line cord (which most of us call the ground or earth terminal) connects directly to the metal power supply case, which is connected directly to the computer case by it's mechanical mounting. The negative output wires of the power supply are soldered to the ground path on the power supply circuit board, which is mechanically connected to the power supply case also.

[ErichKeane]

Ah, I forgot about the physical mount. I guess that makes more sense. In either case my point stands, the negative terminal is connected to the case body as well.

[Al Skierkiewicz]

Originally Posted by ErichKeane

Er, your computer case IS connected to the negative terminal of the DC power supply, through the brass stand-offs of the motherboard. The power supply itself is the only one routed directly to the 3rd pin AC Common.

And yes, grounding your wood frame would have minimal advantages...

In order to pass UL certification and NEC codes, all exposed metal on any line operated device must be tied to building electrical ground. That third pin in the plug connects to an isolated wire that connects to the same point in your power distribution panel that a ground stake, cold water pipe or other wire that leads directly to earth connects. In common 220 volt AC house wiring, the neutral connects to this point as well and only at this point. However, nuetral current does not flow in the isolated third wire in your home. In those cases where the manufacturer has decided to tie power supply common to the chassis ground connection, the power supply must be isolated from the power line as in a transformer. Line operated power supplies as in TVs (those without transformers) must not be connected to anything that the consumer can touch. Metal connectors on the outside of televisions, are not connected to ground, they are connected to a floating supply common.

[johnr]

The match is over. What do you do before you touch your bot to be safe? What about trailers? Will they develope a charge too?

[Alan Anderson]

Originally Posted by johnr

The match is over. What do you do before you touch your bot to be safe? What about trailers? Will they develope a charge too?

The best solution I know is to put a dragging ground wire on the robot to drain off charge as it accumulates. Make sure it stays within the BUMPER PERIMETER boundaries (and that it doesn't provide significant traction).

If you can't do that, contact the robot first through a large-valued resistor instead of directly with your fingers. You might even want to supply a special "touch this spot" finger target on your robot, electrically connected to the frame only through a 1 megohm resistor.

I think the trailers are largely plastic. They don't have a big metal frame to store charge in and conduct it quickly to someone touching it.

[MrForbes]

The trailer base and top plate are alumalite, so they could store a charge, maybe? There are also aluiminum brackets holding it togther.

[MCahoon]

Would the static charge accumulation on the chassis be discharged through a drag wire when the surface it is dragging against is the an insulating material?

The same problem exists with helicopters - in a past life, during training to help with rescue of mountain climbers - we are told DO NOT touch the helicopter until it touches the earth/snow or someone discharges it - it can seriously injure a person (kind of a mini-lightning strike). The rotors spinning through the air generate large amounts of static charge and with their large frames can store huge amounts of charge.

The wheels (insulator) spinning/rubbing against the plastic floor (insulator) generate the static charge. The contact point of the wire/chain on the floor will charge quickly to the same static voltage as the robot frame. Given some time and moisture in the air, etc, it will bleed off the surface. You can't "rub" it back off into the floor - and you can't force charge to flow into the insulator. Seems like if anything you have just added another rubbing contact to generate even more static?

Seems like we should be giving the team members who go onto the field to retrieve the robots a "wand" which is connected through something about 1 Mohm to a grounded plate somewhere which IS connected to earth (essentially a lightning rod) to touch the robot "touch here" spot. Should FIRST be looking into a conductive coating for the floor such as is used on factory floors to prevent static buildup?

My 2 cents