Static discharge beating up my 1769-WS

OkiePC

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I have recently installed a retrofit of the controls of an in motion checkweigher scale.

It uses four load cells under a 36" long conveyor to weigh cases of product (18 to 36 pounds) as they travel toward a sorting and palletizing area.

I used a Compactlogix 1769-L23E-QB1B with a Hardy Instruments 1769-WS scale module, a Hardy summing board with IT (Integrated technician), C2 certified load cell cable from the Compactlogix to the summing board, and I am having lots of problems with the module "internally resetting".

At first, I thought I had poor grounding, but I didn't really find anything too terribly wrong, so I called Hardy, and their very helpful techs explained how to detect what was happening (the ADC counter and weight data will freeze for ten seconds, but the backplane connection keeps on working), and that the two common causes are ground loop currents and static discharge.

I have tried all manner of changes to the grounding, and every time I so much as breath on the system, the symptoms change, but they always seem to come back. It's always worse when my shield is landed on the shield terminal of the scale module, (which is a direct path to the backplane ground) and a little better when I simply land that shield on the adjacent power supply ground. A floating shield is a worse still.

This thing will internally reset up to 4 times a minute, and will rarely go more than fifteen minutes without freezing. This has been going on over a week and a half and I have tried every combination of grounding, alternate power sources...everything has a short term effect, but the symptoms always come back.

Finally I decided that I should see if it might be static, and not a ground problem, so a co-worker recommended misting the conveyor belts with water which should kill static.

Low and behold, the machine went from resetting 3 times a minute to a solid hour of zero problems until the moisture evaporated. All I did was spray water (with sanitizer) on the belt leading into the weigh scale, the scale itself and the outfeeding belt.

There is one braided ground that is supposed to carry static around the load cells to the frame of the machine, and I have verified that the connection is good. I have tried rerouting the load cell cables to no avail. I have added (temporarily) more frame to frame load cell bypass wires and it didn't seem to help.

The infeeding belt is plastic (don't know exact material) intralox with plastic guides, and has a 90 degree curve, so there is a lot of friction. The outfeed belt is rubber and straight and short.

While the conveyors might be generating some static on their own, I really think the product is carrying it. I can run all the belts with no product and I have very little trouble, but it still might happen once an hour.

I do not have a static meter, but I did some experiments with my multimeter set on millivolts. I can put the black lead on the frame of the machine, and hold the red lead on top of the cardboard case and read up to -500 millivolts. Toward the bottom edge of the cases, the voltage is higher (-1.5v), and when the conveyor pauses to create gaps the voltage drops quickly.

I am pretty sure this is not a good way to get an indication of the magnitude of the static, but if there's 1.5vdc flowing through my meter for several seconds...from cardboard... then there must be a significant build-up in the cartons. The multimeter is the only tool I really have readily available besides a 30 year old o-scope that barely works...

I am not sure how this static is getting onto the load cell cable shielding and causing the 1769-WS module to hiccup. I am not sure if my WS module is just hypersensitive and a new one (due to arrive tomorrow) might cure the problem.

I've done some reading on static elimination bars and ionizers, but I have never had to deal with this problem affecting controls and would like advice from those of you who have.

Thanks,
Paul
 
Sounds like you have a static generator. We had a similar conveyor on a palletizer that would shoot arcs 4 inches to the closest ground. I used an anti-static spray to solve the problem, although metal "whiskers" can work too. The static just need a path to ground. The spray I used was called Techspray 1726-QT Anti Static Coating, available at Newark, Allied Electronics, Mouser and probably a lot of other places. It worked on the take-up reels on a labeler as well.
 
Excellent. I think that is something I can try out without wasting too much time and money. How long did ot work? I mean is this something that needs to be frequently re-applied? We don't wash down that palletizing area every night, I think it gets hosed down monthly...
 
While the conveyors might be generating some static on their own, I really think the product is carrying it. I can run all the belts with no product and I have very little trouble, but it still might happen once an hour.
That still suggests to me that the belt is generating the static. It just develops a bigger charge when loaded.

If I were you I'd get in touch with the belt supplier and ask if they have something available with inherent anti-static properties. That could be a coating or a conductive medium integrated into the construction of the belt. The most effective anti-static strategy is to prevent it from being generated.

If for some reason you can't prevent a static charge from building up , then the strategy becomes one of discharging it before it causes problems. One method you've already discovered is moisture. Static problems get worse as the humidity drops. If you can maintain a high relative humidity level in the air around the conveyor, that might be sufficient.

Hanging metal "tinsel" so that it contacts the surface carrying the static charge is another easy to implement remedy. It may not be the best method for your application because the cartons prevent you from getting it close to the surface of the conveyor across the full width.

The next method I've seen is to provide a source of ions of the opposite charge to the static carried on the conveyor. For your situation I'd look at creating a source of ionized air to blow onto the conveyor surface just upstream of the weigher. The link below is the first one on the list when I googled "Anti-Static ionized air. I've never used any of their products myself.

http://www.electrostatics.com/index.html
 
Regarding the Techspray product, I think I used it about once a month. A washdown will definitely remove it. The material is similar to a fabric softener, the MSDS says no hazard. The stuff is cheap and easy, tho.
 
That still suggests to me that the belt is generating the static. It just develops a bigger charge when loaded.

I believe its one of the other conveyors, either the infeed or the outfeed. If you read how a Van De Graaf generator develops it's charge you may see how this possible.
http://science.howstuffworks.com/transport/engines-equipment/vdg3.htm

By the way, it's the making and breaking of contact between the triboelectric materials, not rubbing that transfers the electrons to make the charge differential.

My guess is the material is transferring the charge from either the infeed or the outfeed conveyors.

Try positioning a metallic brush (or frayed metallic screen cloth) close but not touching the belt. Its the pointy ends that attract the charge.

static.jpg
 
Steve Bailey said:
That still suggests to me that the belt is generating the static. It just develops a bigger charge when loaded.

If I were you I'd get in touch with the belt supplier and ask if they have something available with inherent anti-static properties. That could be a coating or a conductive medium integrated into the construction of the belt. The most effective anti-static strategy is to prevent it from being generated.

The techs just installed the belting so it will be hard to convince mgmt to buy another new one so soon, but I will check into the manuf. for recommendations for the future.

Steve Bailey said:
One method you've already discovered is moisture. Static problems get worse as the humidity drops. If you can maintain a high relative humidity level in the air around the conveyor, that might be sufficient.

Can't do that. It's in a room that's kept at 45 degrees F, adjacent to a huge -5 degree storage freezer and the humidity is going be near zero...no chance fighting against that freezer.

Steve Bailey said:
Hanging metal "tinsel" so that it contacts the surface carrying the static charge is another easy to implement remedy. It may not be the best method for your application because the cartons prevent you from getting it close to the surface of the conveyor across the full width.

Tinsel on the bottom might help, but the boxes coming down the line are various heights so I can't contact the tops of the boxes, although I can contact the sides as they are aligned already. I am not sure this will help, since the cartons are already in contact with a grounded stainless steel plat for about two feet just prior to the check weigher, and it doenn't seem to do much good.

Steve Bailey said:
The next method I've seen is to provide a source of ions of the opposite charge to the static carried on the conveyor. For your situation I'd look at creating a source of ionized air to blow onto the conveyor surface just upstream of the weigher.

That is what I see as being the long term solution, but I don't want to guess at what I need. If we invest in some cheap little ionizing bars, that might be okay, but before I spent more than a few hundred dollars, I think I will bring in an expert to analyze the situation beforehand.

I also don't think I want the ionizer too close to the scale, as I might end up with a worse problem...too far away, and it might not help.

I am still hopeful that a new module cures the problem, or at least makes it better.

Thanks for the replies, and keep 'em coming.
 
Paul:

We had a similar problem with Glass Containers on a Plastic Single Line Conveyor. The Packers were getting nasty shocks off the Bottles. We solved it with a set of Cats Whiskers tied back to Ground that the Bottles brushed against as they went by. It was just some heavier Solid Copper Wire with the Insulation stripped back.

Of all the options suggested so far, that is probably the cheapest and easiest to implement. Try setting that up on the problem Conveyor, and see if it helps. You might also have to do the same to the Cartons as well.

Stu.....
 
Funny thing is I have never been shocked, not seen any sparks, and I have already tried plain copper wire brushing against the belt. I have not tried it against the cartons though.
 
Funny thing is I have never been shocked, not seen any sparks, and I have already tried plain copper wire brushing against the belt. I have not tried it against the cartons though.

I suppose it is possible the Cartons are transferring the charge. I am happy to say I have not had a similar problem with any load cells I have worked with. Even though the Humidity gets very low here in the winter. It sounds to me like you have done all the right things. It might be worth trying to discharge the Cartons as well.

Stu....
 
Left Field

2 things I have seen that may cause the issue you describe.

First 1 does have to do with grounding but not static. I have seen through a scope ground noise from a VFD get induced into my Weight controller. This will cause what you have seen

Second thing is my gross weight going below 0. In other words the Gross goes into the negative. The way around this is to calibrate then add weight so you are always positive on gross weight.

Like I said left feild, but I have seen both.
 
Well, the grounding is not entirely off the table, but I have done so many different things in that regard including temporarily replacing VFDs with contactors.

The only thing that has made any real difference is spraying the water/sanitizer mist, so I am pretty sure that it must be static. It might be easier to get rid of the static with changes to the grounding...

The scale doesn't seem to mind a negative gross weight. It has a dead weight of about 120 pounds so the mV signal is always positive. I once unplugged the individual load cells one at a time trying to isolate the problem and had gross weight readings well into the negative range during that time, but the overall symptoms didn't change.
 

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