OkiePC
Lifetime Supporting Member
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
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