Solving static electricity problems by grounding the frame [Archive]

sbrumund

12-02-2009, 00:14

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

12-02-2009, 00:24

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

12-02-2009, 00:53

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

12-02-2009, 01:11

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

12-02-2009, 01:22

ErichKeane

12-02-2009, 02:19

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

12-02-2009, 03:20

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.

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.

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

12-02-2009, 07:37

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

12-02-2009, 09:32

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

12-02-2009, 10:20

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

12-02-2009, 10:30

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

12-02-2009, 10:32

ErichKeane

12-02-2009, 10:41

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?

Alan Anderson

12-02-2009, 11:04

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.

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.

Russ Beavis

12-02-2009, 11:10

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

12-02-2009, 11:12

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

12-02-2009, 11:14

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.

Mike Betts

12-02-2009, 11:15

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 (http://en.wikipedia.org/wiki/Double_isolation)).

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 (http://www.autosafety.org/1990-nhtsa-study-motor-vehicle-fires-and-fmvss-301) for crash related fires.

Regards,

Mike

ErichKeane

12-02-2009, 11:16

ErichKeane

12-02-2009, 11:19

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.

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 (http://en.wikipedia.org/wiki/Double_isolation)).

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 (http://www.autosafety.org/1990-nhtsa-study-motor-vehicle-fires-and-fmvss-301) for crash related fires.

Regards,

Mike

Roger

12-02-2009, 11:22

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!"

MrForbes

12-02-2009, 11:25

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

12-02-2009, 11:27

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

12-02-2009, 11:30

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

12-02-2009, 12:44

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?

Joe Ross

12-02-2009, 12:47

While you electricians are debating pros and cons of various methods, us programmers are getting zapped every time we reset the robot for autonomous!

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

Roger

12-02-2009, 13:44

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!

lukevanoort

12-02-2009, 13:48

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.

I don't think computers are a valid comparison, computer wiring operates in an enclosed environment that is not subject to many of the hazards a robot's is. I imagine it is very rare for computers to have their wires caught in roller chain, for example. In fact, as far as I am aware, the only moving parts in a computer that are not fully enclosed are the fans, and they generally have shields on them and are usually made of non-conductive materials.

ErichKeane

12-02-2009, 13:52

I don't think computers are a valid comparison, computer wiring operates in an enclosed environment that is not subject to many of the hazards a robot's is. I imagine it is very rare for computers to have their wires caught in roller chain, for example. In fact, as far as I am aware, the only moving parts in a computer that are not fully enclosed are the fans, and they generally have shields on them and are usually made of non-conductive materials.

Yep, the computer isn't a perfect comparison, but the automobiles/boats/anything with an alternator was I thought.

jgraber

12-02-2009, 14:28

Since the robot is sitting on insulated wheels, there is no way to ground it without using a drag chain of some kind. (as has been mentioned previously, but not lately in this thread) I suggest a 1Megohm resistor between the chain and the frame as well.

Thought experiment: Static discharge you can feel is > 10,000V.
Now illegally connect the conductive frame to the battery negative.
Now the battery positive terminal is at > 10,012V. So what?
Your finger is still at earth ground (or any other voltage if wearing insulated shoes).

Other (in addition to drag chains) potential (!) ways to reduce static electricity discharge into your fingertips when touching the robot after it has been driving around...
- Use a 1Mohm resistor to connect the frame to an otherwise insulated knob, and touch the knob first to more slowly (2 seconds) equalize the charge between your body and the frame. (like putting your hand on the ESD mat before picking up an IC)
- Hold your carkeys in your hand and touch the frame with a pointy key first. The spark point pain is distributed over the whole area of your hand that is holding the (conductive) car key, so you dont feel it much.
- Use moisturizing hand cream (really! it is required in ESD labs) to keep the surface of your hands equally resistive/conductive.
- Rap the robot frame with your knuckles first. Knuckles have fewer nerve endings, and if you rap it hard enough, the pain covers up the pain of static discharge.
- put on an ESD wrist strap, and touch the frame with the plug before touching the robot. (MegOhm resistor thing again)
- connect a CIM to a reciprocating linkage to a mechanical piezo-electric static discharge squeeze gun originally used to equalize the surface static charge on vinyl LP records.
- Do not use an illegal radioactive ionizer, of the type that is used inside automatic IC handling equipment.

FYI: Regarding Human Body Model ESD standard testing for ICs: Chips that fail 1KV test break too easily. 1kV and 2kV are common standards. It is hard to make chips that pass 4kV standard. Any static spark you can see or feel is typically 10kV-300kV, so if that was on the pin of a chip, its going to be a damaged chip.

MrForbes

12-02-2009, 14:31

Cars have the same problem to some extent....during certain weather conditions, when I get out of my wife's late model truck with it's nice cloth seats, I get a shock when I touch the metal door frame. The body is "grounded" (connected to the negative battery terminal), yet the static problem persists.

Did you try dragging a wire or chain or something from the robot frame, and see if it helps reduce the static problem?

Alan Anderson

12-02-2009, 14:36

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

12-02-2009, 14:50

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

MCahoon

12-02-2009, 16:13

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

Dave Flowerday

12-02-2009, 16:29

FWIW, if you're having issues getting zapped when touching the robot, a very simple solution is to grab something else metal (like a wrench or something) and touch that to the robot. The zap will still occur, but it will be between the tool and the robot frame. The pain associated with getting zapped is mostly from the burning caused by the arc as it enters your skin. If you move the arc to a metal extension instead of your finger you most likely won't even feel it. We've been doing this since 2003 (we had terrible static issues with our robot that year - that's what a dry Midwest build site does for you).

Roger

13-02-2009, 07:09

We added a wire strap under our prototype robot and still had some static buildup but I didn't really get a chance to really test it fully. It may have lessened it. Our electrician/parent/mentor also attached a wire from the floor to an electrical ground. This may or may not help as we have individual panels and each panel may have to be electrically connected in order to work. The official field has only three panels so -- if this is the solution -- there is not much additional work involved.

Glasteel (http://www.glasteel.com/ingles/DetalleSubMarcaProd_1_1261_1262_1266_1282_1283.htm l) doesn't have any specs as to electrical properties, but of course that isn't it's main selling point. It is great stuff for restaurants -- my company uses it all the time.

Here (http://www.chiefdelphi.com/forums/showthread.php?p=816918#post816918) is the robot in question. The top and bottom frames have been connected electrically with a wire. I'm guessing the orbit balls are generating the charge as they are being pushed up the ramp. They are also held in place, so there is additional charge buildup by the inner brushes as they spin.

Dave Flowerday: I'm not too worried about the zap -- I use the back of my hand to ground. But it does surprise some students. "Now maybe you'll listen to me when I say watch out!"

MCahoon: when you first mentioned helicopters I thought of these (http://www.youtube.com/watch?v=PlhhBhwtivY) two (http://www.youtube.com/watch?v=6_NEAEGeFIw) youtube videos.

Too bad we can't harness all that static electricitiy!

Al Skierkiewicz

13-02-2009, 07:52

Roger,
Nice videos. Not to confuse our audience, the arc that appears in these two videos is not caused by the helicopter static but by the high potential of the lines that are being worked on. It is neccessary to bring the helicopter and the people to the same potential so that current does not flow through either. The suits are not protection from electricity as much as a convenience for the worker. The high potential causes the hair on your body to stand up and it distracts the workers. By wearing the conductive suit, the workers are not distracted and therefore more safe. Power lines that are above 100,000 volts are common in the US for cross country transmission.

Ivan Helmrich

13-02-2009, 14:26

Al,
Sorry to go a bit off topic but I'm curious. Once the helecopter and line are at the same potential, why is there an arc again as the helecopter pulls away?

Alan Anderson

13-02-2009, 14:33

It's AC. Once physical contact is lost, there is very quickly a difference in voltage between the wire and the helicopter.

Al Skierkiewicz

13-02-2009, 14:38

As I understand this, these lines are AC voltage and the helicopter acts as a capacitor (plate) to ground, so some potential difference will exist whenever the helicopter and line are not connected. So current does flow albeit in small amounts. You will not find many birds sitting on these high voltage lines for the same reasons. At these voltages, a condition known skin effect may also come into play. The potential is so high that electrons actually start to push away from each other and so are looking for any other path. This is the part that causes you hair to stand up. In these lines, the majority of the electric current actually flows on the outside of the wire. (I am sure there are some power line workers lurking here that can jump in any time.) I have been told that on these lines, the wire is actually a steel core with a conductive jacket to take advantage of the strength of steel and the low loss of a conductive jacket. If you remember back a few years, you could pick up a little arcing from the faceplate of your color picture tube by running your hand over the face of the tube.

Rick TYler

13-02-2009, 21:54

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.

You're confusing 12v apples with 120v oranges. That computer case is grounded, but it's an earth ground to the house. Computer makers also don't run 120 volts through that case. Trust me. I experienced an ethernet error that carried 120 volts to the case of a PC and there was absolutely no doubt what happened. My arm still hurts when I think of it.

There is no way to provide a real earth ground on your moving robot, so that is simply not an issue or a possibility. There is no safety benefit to "grounding" your negative lead to the robot chassis mostly because there is no electrical shock hazard on your 12-volt battery. If you don't believe me, lick your thumb and forefinger and place them on the terminals of your battery. That battery is capable of amazing current flow, but humans have more than enough effective resistance to reduce that the nearly zero with only 12 volts. I still remember the day my friend Ed and I tried this on a car battery (we had both recently earned our Novice Amateur Radio licenses and knew intellectually that 12 volts wouldn't hurt us, but it took a certain amount of nerve to test this knowledge). It was exactly like placing our fingers on block of stone -- we couldn't feel a thing.