Re: Solving static electricity problems by grounding the frame
 

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?

Re: Solving static electricity problems by grounding the frame
 

Two robots contacting only at the frames will not exhibit any issues. The potential for unwanted current flow happens if part of one robot accidentally touches a live circuit in another while the frames provide a current path. Isolating the frame removes the possibility of a single fault causing large electrical problems.

You can still get smoked wires if two things go wrong at the same time -- which the TechnoKats had happen (twice!) in the pits last year. One motor lead rubbed against an unnecessarily sharp corner and the conductor intermittently contacted the frame when the arm was being raised. A sensor wire was smashed between a drive motor and the pan at the bottom of the robot, with the ground wire intermittently contacting the frame when the robot was lifted from underneath. Drive the roller motor while the arm is moving up and the robot is in the pit on the cart -- and the AWG24 sensor ground wire gives its life trying to handle twenty amps.

Re: Solving static electricity problems by grounding the frame
 

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

Re: Solving static electricity problems by grounding the frame
 

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?

Re: Solving static electricity problems by grounding the frame
 

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.

Re: Solving static electricity problems by grounding the frame
 

Erich,

I once had a good friend who worked at Bell Labs back in the early seventies. One of the quotes of his I always loved was "The nice thing about standards are that there are so many to choose from."

"Double isolated" appliances and systems do exist. Many hand power tools boast full isolation from earth ground and are usually designated Class 2 (see this Wikipedia entry).

In addition, the "grounded frame" exists for low voltage car systems only. In electric vehicles, hybrid vehicles, and (one of my development jobs before I retired) fuel cell vehicles, the high voltage system is always double isolated.

Lastly, do not confuse legacy systems such as the automobile with "safe" systems. There are literally thousands of vehicle fires every year. I did some research about a decade ago and the number for non-crash related vehicle fires was about 4,000 per year in the US alone. I just did a quick Google and found this for crash related fires.

Regards,

Mike

Re: Solving static electricity problems by grounding the frame
 

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...

Re: Solving static electricity problems by grounding the frame
 

Vehicle fires are most often caused by heat hitting exposed fuel, not electricity. I don't seem to be able to find anything that says one way or the other, but of the ones I've dealt with and know of, they are all caused by fuel (other than in a direct collision, where positive is shorted to ground).

In my limited experience with the Prius, I have found that the body is used as a ground as well, though I'm not sure its the positive to the batteries as you say.

Re: Solving static electricity problems by grounding the frame
 

While you electricians are debating pros and cons of various methods, us programmers are getting zapped every time we reset the robot for autonomous! :) Who would have thought we're just building a portable Van de Graaff generator?

We have an electrician/parent/mentor and he isn't too sure about the ground chain attached to the robot frame, as it's just dragging on the plastic floor, but he thinks it may be enough as it's grounded by the carpet under it. I'm thinking it's better than nothing, but what do I know?

I do like the lighting rod idea. I'm sure the inspectors wouldn't mind several air terminals -- aka nails -- sticking up on top of the robot! ;) "It's only there for safety, sir!"

Re: Solving static electricity problems by grounding the frame
 

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.

Re: Solving static electricity problems by grounding the frame
 

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.

Re: Solving static electricity problems by grounding the frame
 

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.

Re: Solving static electricity problems by grounding the frame
 

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?

Re: Solving static electricity problems by grounding the frame
 

If your programmers were that worried about it, they'd write the program so you didn't have to reset the robot before autonomous ;-)

If you've loaded the latest DS firmware, you can reboot the robot from the DS. See http://forums.usfirst.org/showthread.php?t=11857

Re: Solving static electricity problems by grounding the frame
 

Heh -- this year we gave up. EVERYTHING is a programming problem!

But you don't want us to fix your hardware problems -- all we have is a hammer.

Even though everyone on the team blames the programmers for everything, it almost always turns out to be the hardware. I think the score ended up Programming +3, Hardware -1/4 before the hardware group gave up. (It's an odd scoring system, but yes, hardware lost a whole point then gained 3/4 point later.)

But it isn't the rebooting the robot, it's putting the robot back in the starting position to try, try, and try again. And get all the orbitballs back in. Though I did manage one time to get the robot to come back on it's own autonomously, attracting it with a second target panel. That was a cool trick!