Possible Static on Conveyor Belt causing Metal Detector Issues..... Possible ?

Good Afternoon ,

I just came out of a meeting , and the subject came up , about static buildup on a conveyor belt causing a metal detector to go "Haywire" . Have any of you had this problem before ? And if so , what did you guys do to solve it ? I was just thinking , if the lagging is removed from the head pulley , and a motor brush assembly was mounted on the shaft , hooked to a ground wire , would that work you think ?

Thanks in advance ,

Vibration can cause issues, at least the ones I've had to deal with in the past.

I have not heard of a metal detector reacting to static buildup. It might be a problem though.

I would recommend installing a set of anti-static air knives, probably on the return length just before the final turn to usage. At least that would eliminate that possibility if the problem continues.

McMaster has a few sizes and power supplies.

Something else to verify ,any "metal" rollers,sprockets or pulleys in close enough proximity to metal detector will create induced voltage that can be high enough to cause erroneous readings on any metal detector and even inductive proxes .

We had that. We found the part of the metal detectors guards was was touching the next conveyor down line. It might even state this in the manual, but it worked.

When ever we get called to the metal detector for issues that is the first thing we check.

we had to order an anti static belt.
the end rollers had to be 2 ft from each end minimum.

Big static discharges can screw up all *sorts* of electronic equipment, and metal detectors are just sensitive inductive sensors.

Storytime: The local paper mill had a packaged product distribution conveyor that was chewing up bearings because it was discharging static through them.

The solution was to switch to an insulated bearing. No more discharge through the bearings, right ? Problem solved !

A new problem appeared; the VFD that ran that conveyor would stop and recover a couple of minutes later. The only evidence ? A diagnostic code on the DeviceNet scanner. This became known as a "DeviceNet problem" and then a "DeviceNet bug" and then eventually led to a Company-wide e-mail blast that there would be no DeviceNet used anywhere because it was junk and unreliable and nothing Rockwell was able to do would help and Ken Roach was an idiot because he said the problem isn't the network.

Bearings are replaced by millwrights. Different union than the electricians who replace VFDs.

But both of them came to see when management hauled me up there to answer for the "DeviceNet bug". I brought my oscilloscope, and my protocol analyzer, and started asking questions.

At one point I had my hand on the conveyor frame and on a handrail when the steel laces came around to the head pulley and discharged static.

"OW ! Mother****er ! What was that !"

The millwright laughed. "Oh, yeah, it'll bite ya. I used to have to replace the bearings on the head pulley all the time. I put in a plastic hub bearing."

The electrician took notice. "You did ? Where's the grounding wire for this section ?" We found it hanging, probably left that way by a welder who repaired a walkway handrail.

With no route to ground through the bearings, and no route to ground through the frame, the static built up to voltages high enough to jump around the pulley bearing race and went to ground through the only path... right through the VFD's chassis ground wire.

The reason the VFD showed a DeviceNet error code is because it was rebooting after having its memory corrupted by the voltage spike.

Years later, I hired a new engineer who had been a junior guy at the paper mill around that time. I was showing him how to configure POINT I/O on DeviceNet, and asked if he had any experience with it.

"Oh, we had a little of it at the mill. But it was too unreliable. There was this one conveyor that would fault all the time because of the network and Rockwell never could figure it out."

Static certainly can mess with a metal detector. Also check the framework surrounding the metal detector. All conductive mechanical connections should be securely bonded or securely isolated. Ground loops in the surrounding framework that make and break with vibration are another common problem.

We had some tumblers that would build up static and discharge through a weigh belt and scramble controller parameters. We clamped a strip of brass shim stock against the drum to redirect the charge.

I am retired now but in my time I have seen many issues with metal detectors. The vast majority of problems were down to one of two causes:-
i) Earthing/Grounding. Make sure that all earth conductors including bonding are secure and in place.
ii) Contaminated belts. Belts should be meticulously clean and unmarked. Do not, under any circumstances, allow anybody to make pencil or ball point pen marks when fitting a new belt. The cobalt in the ink of many pens will cause mayhem.

I have seen metal detectors affected by static as well as load cells on weighing conveyors.

One machine gave me fits until I finally found the culprit...2 bolt flange bearings on a rubber conveyor where the flanges were composite (insulator). I finally caught the thing making sparks every few minutes that would jump over to the next belt where the load cells were getting spiked which in turn faulted the weigh module in the PLC.

I did a quick fix by putting a fat wire to act as a brush against the roller shaft. Later we swapped out the bearings for some with steel housing so the static in the end roller would have a place to go.

Another room in that same plant had hell with metal detectors which we fixed by scooting one platform away from the frame of the long conveyor with the metal detector. We didn't find the root cause, but as long as the two frames weren't touching the metal detector didn't fault.

FWIW: I once had an issue with a metal detector randomly tripping, and we eventually discovered that it occurred only when a nearby variable speed belt operating on a PID controller made a sudden change in speed. We had to move the metal detector to get it farther away from the variable speed conveyor's motor.

We had metal detector issues for a couple of years that gradually got worse overtime and the problem ended up being about 30 ACTech drives in a panel about 50 feet away from the metal detectors.

As the plant started to become more efficient and production needed to run faster, maintenance started turning all the speeds up on the drives where they were all at or above 90hz and the metal detectors wouldn't stay calibrated for more than a couple of hours at a time, if they would calibrate at all.

After a lot of head scratching, one of the maintenance guys noticed that they never had metal detector issues when doing rework and while doing rework only about 5 of the 30 drives were used. Then it started to make sense that the drives were probably causing the issues but they hadn't added any new drives over the years, so they started digging into it and the only thing that had changed was that the drives were all running at or above 90hz.


They ended up changing all the gearboxes on the motors so they were running below 60hz and they put line filters on the drives and the metal detector issues drastically went away.

They make metal brushes that you can mount against the belt that rub and can you ground that will help cut down on the static.

Another thing to take into consideration is induced eddy currents through the frame or rollers that could create a magnetic field.