Static buildup. 1

[Commodore64]

I work for a company that is into CMM, and we are having a static build up problem. We have a large measuring head that floats on a ceramic beam via air bearings. It is driven by a small rubber wheel which contacts the surface. Static builds up on the Ceramic beam through some mechanism unknown to me (the drive wheel / support wheel on the other side rubbing?). I have been looking at various static control products, but am having a tough time discerning which product is best suited to either dissipating the static, or possibly preventing it from building up. Any help is appreciated.

 

[itsmoked]

A moving stream of air will dissipate or carry away static. Consider trying a small fan or an air nozzle appropriately aimed. Also there are ion blowers specifically for static control primarily at soldering stations where static sensitive parts are handled. One of those could be pressed into service for you.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Commodore64]

Ive looked into the air systems, and the head is actually supported by air bearings at the moment. Is it possible that the air being supplied by the bearings is causing the build up by transferring ions to the ceramic beam, or does moving air solely dissipate static?
Also, is there any way to temporarily test this ion blowing method without purchasing a unit?
Also, would one of the static dissipating films be capable of removing static from the beam? Would I have to run it only in a small strip, or would it be necessary to cover the entire surface that is building the charge?

 

[itsmoked]

Be nice if you had a picture.....

If you have a small fan that can blow down the beam - try it! You know, like a computer fan or a small room fan. If that's enough you've got a solution. If it helps appreciatively but not quite enough then it's time to try a static augmented fan.

Very dry air can transport static just like the air being used to carry away your static. That said, there is probably no static source in your air to cause it to bring static to your beam. The beam is becoming charged via triboelectric generation from the insulated bar being rubbed by the insulated rubber wheel.

If you could blow moist air across your beam then the static would dissipate in a New York Minute!

Peel some scotch tape off a roll and then hold it next to a surface. You will see the tape attracted via the static charge generated by the unrolling tape.

Do it again, then before holding the tape next to a surface, blow on it, mouth open, like you want to fog your glasses before wiping them. Now hold the tape near a surface. NADA - the charge was shorted and conducted away on your breath and to your fingers.

This would work well on your beam too.

As for static films.. Could work, could not work! There are antistatic soaps that you can spray on things but your app doesn't sound like this would work too well, and would require frequent reapplication.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Compositepro]

Static charge can develop wherever molecules of different materials that are in contact are separated from each other. Compressed air flowing through an air bearing can easily generate a charge. The ceramic is non-conductive so the charge is stored a long time. There are anti-static liquids that you can wipe on but these will leave a thin deposit that absorbs humidity to make the surface slightly conductive. You may not want this on your slides. An ionizing blower can work but they are expensive and you have to select the right type. You need a negative ion blower if you have a positive charge build-up. Or an A.C. blower.
The first thing I would try is a static brush like Ion Cord attached to the moving part of the CMM close to the stationary slide that is showing the static. The brush does need to be grounded.
What is the problem that the static is causing?

 

[Commodore64]

Once the slide has traversed the bar a few times (varies in position and duration) a spark will jump from the bar to the position feedback sensor, which causes the safety to trip and immobilizes the machine for a few minutes until it can have the drivers reset. Anything that would interfere with the feedback sensor or air bearing surface is not acceptable. Its some sort of optical / non contact reader which picks up off of a metallic strip.
I tried using a pair of computer fans on either side of the rubber tensioning wheel, it being the only thing in contact with the surface.
I also checked all of the fittings in the air supply system for any slight charge, in case it *may* be causing the VERY dry air to transport charge, but did not find any. The air is piped in through well grounded pipes to a series of air dryers, which connect to the air bearings via nylon tubing. I checked against the machines chassis ground and the fittings as well as the piping and the fittings, and the piping / chassis ground.

Other than static buildup, is there any reason a spark would jump between the reader head and the beam? It seems like a short would got to one of the grounded screws holding the reader to the head rather than jump across 1/8in of air to an insulated ceramic beam. Is there a specific way I could determine precisely is this was a static buildup problem? Removing the position feedback reader is a last choice due to HIGH calibration costs associated with the unit.

Other cheap solutions I've seen are the copper tinsel that floats above the surface, or like composite pro suggested the static dissipating brushes. Also the inexpensive thin film that is conductive in some manner to dissipate / ground static. Ion knives/curtains/blowers are expensive, and they require expensive power supplies.

 

[IRstuff]

Is this item something that your company built? or purchased? If purchased, have you contacted the manufacturer?

Have you run a field strength meter on this structure? You need something like 20 kV to jump a 1/4-inch gap.

Have you looked to see if there's some sort of carbon-impregnated wheels that you could use as an alternate?

TTFN

FAQ731-376

 

[Commodore64]

The machine is a Zeiss CMM machine. We have contacted the tech, and his advice was to replace a piece of the wiring harness, in case it was faulty. It appears as if only one other person has had a problem "similar," and their problem was caused by this harness. I am really unsure of how similar their problem really was to ours. It is definitely jumping this gap, and I doubt the digital circuitry in the reader head is creating such a difference. I will have to look into this field strength meter idea. Its not a piece of equipment I own, but I could look around.

 

[Commodore64]

So I talked to my co-workers and the other companies problem was not causing sparking like ours was. We are calling a company whom we have ties with to borrow a field strength meter. What sort of readings should I look for on this machine?

 

[Commodore64]

Apparently this has been a problem on this machine for a long time. The company temporarily fixed it by rubbing some of the gold off of the scale strip to get to the copper underneath, and attached a copper strip to the scale with some rubber tape. In the past, the problem has reoccurred and the solution was to press the tape on harder. Something has changed recently to make this contact no longer sufficient.

What I believe is that this strip was previously removing the static charge from the beam, but is now insufficient. Is there some way to ensure a VERY good electrical contact between the two copper films, such as some sort of very conductive paste or some such that I can apply between / on top of them?

Right now I am liking the idea of a carbon fiber brush that rides above the surface of the beam. While sprays and cleaners would be easy, we cannot risk putting any sort of film or covering of cleaner on the air bearing surface or the position scale. Ionizing air units are prohibitively expensive.

 

[Compositepro]

Your problem really isn't all that complicated but static can be very mysterious if you aren't really familiar with the basic principles. You are having a charge build-up. It is almost certainly accumulating on the surface of your ceramic beam due to the compressed air flowing through your air bearing. The air molecules are adding or subtracting some electrons to the ceramic surface as they bounce off and the charged air is carrying off an opposite charge which soon is attracted to a grounded surface and neutralized. This accumulating charge creates an electric field (voltage). The charge is on the non-conuctive beam and when it get large enough it will breakdown the air and spark to the shortest path to ground. If you put Ion Cord near the charge the microscopic conductive fibers will concentrate the electric field at their tips (like a needle point concentrates force to penetrate a balloon). The air at the fiber tips ionizes and the ions are immediately attracted to your charge and neutralizes it. Only the air at the fiber tips is ionized (in a corona charge). So sparks do not occur. Check out

http://www.fibraplex.com/IonCord.htm

By the way, aluminum foil over your encoder and wiring will protect it from sparks but it, too, must be grounded as well as the entire conductive structure of your CMM.

 

[itsmoked]

If you know were the spark is jumping to can you mount a grounded needle there to intercept the spark or that will bleed the charge continuously to a safe ground preventing the jump to the encoder?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Commodore64]

It may be possible to mount a grounded needle immediately by the sensor that does not contact the beam or foil. Ill give that a try. It sounds like that ion cord stuff that compositepro is suggesting is essentially a more hardcore version of the grounded needle, what with the millions of tiny needle points of CF.

 

[IRstuff]

Similar, but not exactly. An array of sharp points causes the field to be concentrated enough to cause bleeding of charge directly to the air.

TTFN

FAQ731-376

 

[itsmoked]

I have nothing against the ion cord stuff I just can't picture it not being problematical in your beam's situation whereas a single needle/small wire may physically be more tenable. ???

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Your tech's recommendation may not be so dumb, after all.

A CMM always has a set of cables and tubes supplying electricity and air as well as bringing measurement signals back to the computer (you have Zeiss' own touch probes or some other make? Or optical edge sensing?). The cable harness usually has a copper strip bringing ground to the moving parts and also screening power and signal flat cables from each other. Being usually a thin copper or bronze band, this strip sometimes comes loose or breaks near the end or where flexure is at maximum.

It may be that something has happened to this grounding system.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Commodore64]

I am fairly certain the person working on it right now has checked that grounding strap, but I could be wrong. I just found out that there have been more discharge problems than just the positional feedback. The head has been shocking operators when they go to change probes unless they ground themselves to the control box fist!

 

[sreid]

You've probably found your problem. Check the resistance between the probe head and the machine frame. I'll bet you'll find it open circuit.

 

[Skogsgurra]

"unless they ground themselves to the control box first!"

That does not indicate an open circuit. It sounds more like the operator has accumulated some charge and that he discharges himself when touching the probe. Do they not get the same shock when they "ground themselves" to the control box? Are you sure that some kind of myth or mistake hasn't been added to actual observations?

Time for a thorough check of the whole installation. Check continuity (when moving through the whole operating volume). And also the environment, floor, operator's dress and shoes, air humidity. Are you on the southern hemisphere? (air is getting dry this time of the year)

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...