I have a metal detector which is beeping for no reason when people pass it by with the talkie-walkies. The talkie-walkies use UHF radio waves, and so I know that I have to somehow block the radio waves. I have tried chicken wire grids as small as 1cm by 1cm meshes, making a faraday Cage around the machine, but the small opening in the cage is enough for the Radio waves to get through now and then.
I also read that broken fluorescent lights can attenuate the RF waves, but I have no idea how and why that should work...
I was hoping for suggestions about metal detectors and RF interference, May be a way to fully shield the metal detector from the waves without having to completely seal it in a grid.
it's actually a LOMA IQ2 metal detector for food industry. The walkie-talkies use 450 Mhz signal. The metal detecotor itself works pretty much like an AM radio. It has 3 coils along the convoyer belt line. The central coil transmits a signal similar to that of AM broadcasting ( 530-1,710 kHz). Spaced equally from the centre transmitter coil, are the two recieving coils which act as aerials to pick up the radio signal. The metal detector measures the difference in voltage between 2 recieving coils. If a metalic object passes by, this difference voltage (obviously becomes more than a certain amount) and that triggers the alarm. But where does the RF interfere?
well, from what I've found out so far: this metal detector ( it is actually in a factory in a food line) works with 3 coils placed perpendicular to the conveyor belt that runs beneath. The centre coil is the transmitter of a signal similar to AM boradcasting readios, probably around is 530-1,710 kHz. ( or 153 kHz - 26100Khz). The other two coils equally spaced from the middle one are aerials that recieve the signals and produce a voltage. The difference between the voltae of the two reciever coils will be the metal detector signal. Upon entering of a metalic object in to the filed the voltage between the two reciever coils will differ significantly. My Guess is that the RF somehow penetrate the case of the metal detector and produce new voltage in the coils. ( and yes offcourse the walkie-talkies are on)Is there any other electronics that you think are penetrable by the UHF.
Strange things happen to metal detectors when water is present in the lining. If the equipment is exposed to harsh wash-down or is subject to rapid changes in temperatures (both common in food processing environments), moisture will collect under the lining. The detector may need drying. (I noted on your company's web site that there are openings for night cleaners. If there are new sanitation crew members, you might what to ensure they're well familiar with cleaning Loma metal detectors).
Loma's IQ series' phase discriminator can be adjusted to be more sensitive to ferrous or non-ferrous contaminants. It's been a while, but the phase adjustments can be made using the keypad/display. Fine tuning the phase discriminator might mitigate the interference problem as well as more favourably detect the dominant type of expected metal contaminants (SS, carbon steel, brass, aluminium).
I personally have experience with Gorring Kerr metal detectors, the principle is similar to the Loma that you describe. Although I assume you are using O frame detectors, as opposed to single sided units.
This application is in paper processing, so the product is not as wet as food, but it is on the borderline of what a metal detector considers wet. I have 3 units in similar installations, on 3 separate lines. Two behave flawlessly, the other is possessed. My problem is not equipment, but location, because I can physically move the entire installation, and it will work.
This problem unit is a single sided detector. I use very fine copper screen like is used in aircraft skins. There is a 4'x8' piece above the detector to shield and a similar size 12" below the detector face which acts as ballast, which allows us to raise the sensitivity, while still ignoring external interferance. A transmitting two-way radio still causes problems, but only within a few feet.
If your problem unit is an O frame, your problem may have been identified by a previous poster. The mutliple coils can shift their alignment over time, which requires a "Balance" adjustment which is typically >I have a metal detector which is beeping for no reason when people pass
I, too, have had experience with Goring-Kerr's single sider model. We used them in food processing and they were constructed in stainless steel frames with a food-grade epoxy on the sensing face. Goring-Kerr did what they could to water-proof them, but we only got about 4 or 5 years of service out of them.
In our application, we needed an unobstructed field of view of the product (meat) as it passed over the detector. We were looking for large metal objects (meat hooks, knives, bolts) lodged within piles of meat as they were conveyed to a grinder. The requirement for a clear line of sight of the product and the rather relaxed contaminant specs eliminated the need for the more conventional closed aperture style.
In your case, you describe a biasing screen added above the single sided detector that, I imagine, blocks the view of product. Why not use a closed aperture detector for that application? They are considerably more sensitive to contaminants and there is less fringing of the electric field (and hence, require a smaller metal-free zone).
Also, you mention the anomalous behaviour of one of your detectors seems to be "location sensitive". We experienced something similar to this. It turned out one of our detector conveyors, when positioned, had two legs resting on a pair of the many floor drain grates. As these legs were on opposite sides of the conveyor's eddy current break, the drains (cast iron) bridged the eddy current break, resulting in weird operation. We ended up insulated all our metal detector conveyors' feet from the floor. Probably not the same conditions you have, but maybe floor surface variations between one location and another cause vibrations in your conveyor that's affecting the detector.
At this point in the process there is no conveyor, the paper is self-supporting. We sample 1/2 of the product and assume an equal distribution. At 120" an O frame would not sense the 1.5 mm non-ferrous detection required by our customers
I went through the entire setup and unless an anchor bolt is contacting re-bar, then the ground is on one side only.