[EE] ESD dissipative or conductive casters
Sean Breheny
We have a robotics application which puts fairly high reversing loads
on caster wheels. We are looking for a source for heavy-duty casters
which are either conductive or dissipative (we can deal with roughly
anything from 0 ohms to 10^9 ohms from hub to floor) to counter some
ESD generation problems that we have. So far, all we have been able to
find are either very soft conductive casters which do not hold up very
long under load, or semi-dissipative ones whose resistance is almost
10^11 ohms from hub to floor, which is too high for our application.
Unfortunately, we seem to have found that most caster vendors, even
those who make ESD casters, are not too knowledgeable about ESD and
either don't understand why this is a problem for us or do not know
how to fix it. Has anyone here ever dealt with this or know of a
source for ESD casters?
Thanks,
Sean
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Russell McMahon
Can you add a 360 degree rotating trailing arm somewhere in the middle
- or a "static strap" :-)?
Given the needed resistance a strap could be reasonably rugged with a
relatively light loading of eg carbon black. An UHMWPE wear end should
last a long while.
Or, is spraying with a conductive coat on the castor face acceptable.
Nickel loaded sprays are available, or making something custom with
carbon black (again). (Maybe galvanising paint :-) ).
Could you bond a multidisc wheel together with summat tough in layers
with summat conductive?
R
Olin Lathrop
> We have a robotics application which puts fairly high reversing loads
> on caster wheels. We are looking for a source for heavy-duty casters
> which are either conductive or dissipative (we can deal with roughly
> anything from 0 ohms to 10^9 ohms from hub to floor) to counter some
> ESD generation problems that we have. So far, all we have been able to
> find are either very soft conductive casters which do not hold up very
> long under load, or semi-dissipative ones whose resistance is almost
> 10^11 ohms from hub to floor, which is too high for our application.
> Unfortunately, we seem to have found that most caster vendors, even
> those who make ESD casters, are not too knowledgeable about ESD and
> either don't understand why this is a problem for us or do not know
> how to fix it. Has anyone here ever dealt with this or know of a
> source for ESD casters?
Drag a wire from robot ground on the floor.
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M. Adam Davis
> We have a robotics application which puts fairly high reversing loads
> on caster wheels.
> We are looking for a source for heavy-duty casters
> which are either conductive or dissipative (we can deal with roughly
> anything from 0 ohms to 10^9 ohms from hub to floor) to counter some
> ESD generation problems that we have.
So the high reversing loads are causing the ESD (Van De Graaff between
floor and cart)?
Or are you merely trying to reduce ESD by using the wheels as the
discharge path?
If the first, then you might look into other wheel materials (and
possibly floor coverings/coatings) to reduce the generation of the
electrostatic energy.
If the latter, I'm sure such wheels exist, but my understanding is
that it's more common to have a dangling conductor (or sprung
conductor) in constant contact with the floor. You have much greater
control over the discharge resistance and path, and since the wheels
don't "wipe" they can become less effective over time due to dirt,
oil, etc coating them. A wiping ground contact can be made that will
self clean, won't mar or mark the floor, won't catch on minor ground
imperfections (cracks, concrete stress releif joints, etc), and will
wear well.
However, I think the search terms that you need are "condutive wheels"
and "anti-static wheels"
http://www.google.com/search?q=conductive+wheels
http://www.google.com/search?q=antistatic+wheels
And they appear to be widely used in, for instance, the medical industry.
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Sean Breheny
Unfortunately, for confidentiality reasons, I can't give all of the
details. However, I can say that we have already tried most of these
alternative suggestions. These robots need to operate in the
customer's environment with minimal modification of the floor. Some of
our customers have terrible floors - one customer has a floor which
has a half-inch wide expansion gap in the concrete every 10 feet.
We've tried dragging chains, wires, etc. and all of them have
eventually managed to get caught in such gaps and ripped off.
The ESD generation is not due to the reversing loads but rather, we
think, due to friction between the main wheels (not the casters) and
the floor. There are some maneuvers which the robot does which involve
rotating in place and this produces some slip between the main wheels
and the floor (due to the wheel not being an ideal thin disk).
One version of this robot uses neoprene wheels and doesn't have this
problem (ESD) when running on concrete. A larger, heavier version,
though, uses urethane wheels due to the necessary hardness, and these
have the problem. We have not been able to find a commonly-available
alternative material for the main wheels which can handle the loads,
although we are looking at getting conductive additives for the
urethane.
We are also looking at dragging a conductive neoprene strap, but some
of our testing has indicated that it will wear significantly due to
the very large distances traveled by these machines (they pretty much
move at 1 meter per second constantly, 16 hours per day).
It seems like a conductive or dissipative caster with a fairly hard
material would be a good option because it would not wear nearly as
much as something which is dragging. Because there are direction
reversals very often, I suspect that dust build-up on the caster would
not be a huge issue (we don't really see it on our existing casters).
Sean
Bob Blick
On Thu, 25 Mar 2010 11:39:37 -0400, "Sean Breheny" said:
> One version of this robot uses neoprene wheels and doesn't have this
> problem (ESD) when running on concrete. A larger, heavier version,
> though, uses urethane wheels due to the necessary hardness, and these
> have the problem. We have not been able to find a commonly-available
> alternative material for the main wheels which can handle the loads,
> although we are looking at getting conductive additives for the
> urethane.
Can you add an additional neoprene wheel or caster that is not
loadbearing but merely for ESD?
Cheerful regards,
Bob
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Alan B Pearce
> loadbearing but merely for ESD?
That was my immediate thought too. Mount it on a long stiff arm so it is
like a static strap, but has a wheel on the end instead of just dragging on
the floor.
But 1/2" expansion every 10 feet - what temperature range do they expect ???
Russell McMahon
Worms our specialty ! :-).
And, it makes sense that We'All track down the paths that you have followed.
Drilled castor radially with insert of conductive material?
I didn't see the spray on side material (eg Nickel) mentioned?
> We are also looking at dragging a conductive neoprene strap, but some
> of our testing has indicated that it will wear significantly due to
> the very large distances traveled by these machines (they pretty much
> move at 1 meter per second constantly, 16 hours per day).
UHMWPE is often used when this is an issue.
Making it conductive may be another matter :-).
Deep mind processes or random neuron firing suggests "phosphor bronze"
as a strap material.
Russell
M. Adam Davis
> On Thu, 25 Mar 2010 11:39:37 -0400, "Sean Breheny" said:
> Can you add an additional neoprene wheel or caster that is not
> loadbearing but merely for ESD?
I really like this idea. If you have room or a place for a strap, you
can probably mount a small dolly on a swing arm so it won't have
problems with cracks or high centering the machine.
Alternately, coat the machine with needles pointing outwards and let
the static dissipate through air discharge. ;-D
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Ruben Jönsson
> Alternately, coat the machine with needles pointing outwards and let
> the static dissipate through air discharge. ;-D
>
This was my thought also. Perhaps you can use carbon fibre antistatic brushes,
like this: <http://www.amstat.com/solutions/passive.html> at suitable places on
the robot. Note that the brush should not touch the ground.
Normally these brushes are connected to ground potential and located close to a
charged surface. Since the carbon fibres are so thin, the field strength around
them becomes high enough to ionize the air through corona discharges long
before the breakdown voltage in air is reached (which would cause a spark
instead).
/Ruben
==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ru...@pp.sbbs.se
==============================
Sean Breheny
needles and they made very little difference below 3kV. I had not
heard of this carbon fiber brush but I will look into it. I find it
hard to believe that it is going to be able to reduce the corona
threshold to below 1kV (which we would need), but I will look.
Thanks,
Sean
2010/3/25 Ruben Jönsson <ru...@pp.sbbs.se>:
John Ferrell
Christmas tinsel that can be frequently replaced to solve this sort of
problem.
John Ferrell W8CCW
"A nation of sheep will beget a government of wolves."
-Edward R. Murrow
Ruben Jönsson
guess), the higher the field strength on the tip of the carbon fibres becomes
and the lower the actual corona discharge voltage becomes.
Another option might be to use an active device. I have seen a couple of
chinese small, rather chaep, piezo based dischargers and, a bit depending on
how big the charge on the robot is and how fast it is generated, the resulting
voltage on the robot could be well below 100V. These devices are usually used
together with a fan in order to discharge a larger surface, like a workbench
when working with electronics assembly. They are also used in air nozzles to
reduce any charge buildup when blowing with compressed air.
A new thing I have not seen before is USB ionizers. A small USB powered ionizer
that looks like a thumbdrive. They only seem to produce negative ions though
(probably just rectified the output).
/Ruben
YES NOPE9
Perhaps a UV laser diode can be used to create a discharge path.
http://iopscience.iop.org/1367-2630/4/1/361/fulltext?ejredirect=migration
Gus
Jake Anderson
> Hi all,
>
> We have a robotics application which puts fairly high reversing loads
> on caster wheels. We are looking for a source for heavy-duty casters
> which are either conductive or dissipative (we can deal with roughly
> anything from 0 ohms to 10^9 ohms from hub to floor) to counter some
> ESD generation problems that we have. So far, all we have been able to
> find are either very soft conductive casters which do not hold up very
> long under load, or semi-dissipative ones whose resistance is almost
> 10^11 ohms from hub to floor, which is too high for our application.
> Unfortunately, we seem to have found that most caster vendors, even
> those who make ESD casters, are not too knowledgeable about ESD and
> either don't understand why this is a problem for us or do not know
> how to fix it. Has anyone here ever dealt with this or know of a
> source for ESD casters?
>
> Thanks,
>
> Sean
>
them retreaded with a carbon loaded PU material?
We make polyurithane wheels for robotic combat type things so it
shouldn't be too much of a stretch to dose it with carbon or some such
for you.
Drop me a line if your interested.
Dario Greggio
> A new thing I have not seen before is USB ionizers. A small USB powered ionizer
> that looks like a thumbdrive. They only seem to produce negative ions though
> (probably just rectified the output).
nice!
I used to have one when I was a bot, made of a large chain of diodes
rectifying 220V up to some -10000V. Nice, it seemed to generate "fresh
air" in summer... :)
They say it should clean air from dust etc. It did work indeed, though
dumping it all on the walls... !
Not sure if it could be done @5V with the same method... oh well, a
boost converter etc...
--
Ciao, Dario
--
Cyberdyne
Ruben Jönsson
> > A new thing I have not seen before is USB ionizers. A small USB powered
> > ionizer that looks like a thumbdrive. They only seem to produce negative ions
> > though (probably just rectified the output).
>
>
> nice!
> I used to have one when I was a bot, made of a large chain of diodes
> rectifying 220V up to some -10000V. Nice, it seemed to generate "fresh
> air" in summer... :)
> They say it should clean air from dust etc. It did work indeed, though
> dumping it all on the walls... !
>
>
Here is an article by the late Mr Niels Jonassen aka Mr Static that discusses
the myths about negative ions:
<http://www.ce-mag.com/archive/02/11/mrstatic.html>
And here is the index for all of his articles in CE Mag:
<http://www.ce-mag.com/mrstatic.html>
Well worth reading for anyone who deals with static electricity.
/Ruben
==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ru...@pp.sbbs.se
==============================
--