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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Third hit on google for "metal detection in food"
]guide_to_metal_detection.pdf[/file]
But OK, if starting from zero, it may not be so obvious how to apply it.
The pdf describes a balanced coil configuration. That's because your receiver coil will detect external fields, so it needs to be balanced to reject them. It also needs to reject the trnasmitter coil, so the substrate needs to be very stable. Industrial metal detectors use a portal made from granite, but the amateur can use glass very sucessfully.
The thing to be detected is passed along the axis of the coils. The idea is that ferrous material increases the coupling on the side it's on. A conductive material weakens the coupling. In either case, as the sample passes along the axis of the portal, there is an in-phase then anti-phase output signal.
Unfortunately, the most common material used for sieves, presses etc in the manufacturing of phamaceuticals is stainless steel, which is both weakly conductive and weakly ferromagnetic. So both effects cancel each other to some extent and it gives a very weak signal. Unfortunately (2), any uncontaminated pill will give some sort of output, though it is hoped that it will be a) small and b) of a repeatable size and phase angle so that signals from additional metal can be discriminated. The effect is very much worse in food, where nitrites make wet stuff very conductive, and a ton of cole slaw may have excess mayo at the start of the pour, and excess cabbage at the end, so will be changing response throughout the whole run. Pills would be expected to be very repeatable, though water content could vary.
Generally, the smaller the portal, the higher frequency you will need. However, there is a lot to be said for the amatuer using a frequency under 20kHz, so that a standard PC soundcard can be pressed into service as the detector.
I haven't found any further information about the transmitter and receiver.I tried to build a coil but there was no different when I put some metal near the coil...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
"I haven't found any effect, what's going on?" is about the worst sort of question to ask. There's no information for exactly what you tried, and no information for exactly what it *did* do, and what it does is much more informative than what it doesn't.
You say you've tried to build a coil. Did you succeed? Let's assume you did actually build a coil. How did you use it? How was the transmitter and receiver connected to it, and what were they?
Review Diagram 1 of the reference. It shows three coils. The middle one is connected to the transmitter, this is a simple continuous sinewave source. The outer two receiver coils are connected in a balanced configuration to reject the influence of external fields, when there's nothing in the bore. The reciever simply amplitude detects the output of the two receiver coils connected in anti-series. Do you have this arrangement?
Do you understand what a balanced arrangement is, and why it's needed? If not then we can take another step or so back.
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metal detector for pharamaceutical tablet
