Solving static electricity problems by grounding the frame

[lukevanoort]

Originally Posted by 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.

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]

Originally Posted by lukevanoort

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]

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]

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]

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

[Dave Flowerday]

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]

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 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 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 two youtube videos.

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

[Al Skierkiewicz]

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]

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]

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

[Al Skierkiewicz]

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]

Originally Posted by ErichKeane

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.