Stationary metal detector

[Chupa]

I’ve been working on a project using FGM-3s to create a walk through metal detector much like you see at the air port. I'm using 8 sensors total, 4 on each side, and a atmel µC to read the pwm from the sensors and send it to a PC with a program that will display all the data on screen. I’ve been working on this for about 4 months now, and I’m at a dead end. The main problem is noise I think so I’ve come here looking for ideas and to confirm some things.

Being in FL and it being the rainy season most every night we have a storm or what looks like could be a storm so lightning is everywhere. Is it possible that the FGM-3s are fluctuating so much because of the surrounding lightning? I noticed one night when I was testing it readings were all over the place (very noisy when they should have been near idle) and it was stormy that day. But another day there were clear skies and the readings were less noisy.

With the sensors I really want them reading in only one direction, straight out toward the middle of the door way. Is there a way to shroud or insulate them so their only reading in a narrow cone in the direction in which they are pointing? I imagine if this is possible it would help reduce noise as well.

I saw this thread https://www.geotech1.com/thuntings/s...ad.php?t=12728 and was wonder what the thoughts are on using the Honeywell sensors instead of Speake’s. Do you think there is any improvement to be made if I were to switch?

As is the metal detector can pick up large objects and do a fairly decent job triangulating them on a person’s body when they go through the door. Things such as a small gun or a kitchen knife aren’t a problem. But it’s for smaller things such as a razor blade that I’m having issues. I’ve read things about the feedback coil on the FGM-3s but I don’t understand it enough to know if it will benefit me to use them or not. Is there any improvement to sensitivity that can be made by using the feedback coil? From what I understand it’s only used to keep readings linear which I don’t think I care about (do I need to?).

My mind is just all over the place right now as I don’t know what else to do to try and make this thing work. Any feedback or comments would be appreciated. Thanks!

[Wreckdetector]

Stationary metal detector

Hi Chupa,

I'd say it's very likely that the FGM3s will respond to the storm, i've not used them myself as I make my own fluxgates but it would not surprise me in the least. (You can detect lightning with a moving coil meter and a wire antenna so an FGM3 should have no trouble detecting it) -Note don't try the moving coil meter trick, it's highly dangerous!!!!

Screening them to read in a cone facing in one direction isn't a practical proposition.

If you are trying to detect things as small as a razor blade then you are looking for a very small signal which will be similar in size to the natural background. I think you will struggle with that. I'd expect the detector to detect objects passing outside the frame and mistake them for objects inside it at that level.

Of course, your detector will only detect ferrous metals, I assume you don't care about others?

Making a metal detector using FGM3s is a passive approach that will always be susceptible to background noise. I think you would be better off using a more conventional active method. (Pulse induction etc,etc) IMHO.

[Wreckdetector]

Stationary metal detector

Having said all that above, here's a hopefully helpful thought.

If the FGM3 are responding to the natural background, storms, diurnal variations and all that, then all the sensors should respond in about the same way and at the same time. You could take an average reading from all the sensors and then subtract that average from each individual sensor reading. Any of the sensors that are seeing a metal object would then give an output value. You would lose some sensitivity but gain some immunity from interference.

You might need to callibrate the individual sensor sensitivities.....

All the best......

[Chupa]

Thanks for the reply!

If the FGM3 are responding to the natural background, storms, diurnal variations and all that, then all the sensors should respond in about the same way and at the same time. You could take an average reading from all the sensors and then subtract that average from each individual sensor reading. Any of the sensors that are seeing a metal object would then give an output value. You would lose some sensitivity but gain some immunity from interference.

That’s kid of what I’m currently doing to deal with noise. Each sensors last 40 readings are averaged into a circular buffer. If a reading comes in that x amount (where x is the sensitivity) over the average then the program assumes it was a metal object.
But I hadn’t thought that I might be able to take all 8 sensors noise windows and attempt to use that to try and null it out. That’s something ill defiantly try.

I'd say it's very likely that the FGM3s will respond to the storm, i've not used them myself as I make my own fluxgates but it would not surprise me in the least. (You can detect lightning with a moving coil meter and a wire antenna so an FGM3 should have no trouble detecting it) -Note don't try the moving coil meter trick, it's highly dangerous!!!!

Screening them to read in a cone facing in one direction isn't a practical proposition.

If you are trying to detect things as small as a razor blade then you are looking for a very small signal which will be similar in size to the natural background. I think you will struggle with that. I'd expect the detector to detect objects passing outside the frame and mistake them for objects inside it at that level.

Of course, your detector will only detect ferrous metals, I assume you don't care about others?

Making a metal detector using FGM3s is a passive approach that will always be susceptible to background noise. I think you would be better off using a more conventional active method. (Pulse induction etc,etc) IMHO.?

Humm I’m wondering what if anything I can do to keep things somewhat working when there’s a thunderstorm out.

I have success with razorblades if I magnetize them, but it’s obvious that will make almost anything ferrous work with such sensors. I’m wondering if I could maybe use magnets before the doorway to somehow magnetize objects before they get to the door. Although that might seem kind of dangerous.

Is it possible to detect non ferrous objects somehow?

If I could use an active approach I defiantly would. Unfortunately my design constraints were it has to be passive. I think its not doing too bad of a job, but like I mentioned before, there’s problems that makes its obvious why they use active ones in the airport.

[Chupa]

Oh I forgot to ask. I know you said you make your own sensors, but do you think using the Honeywell sensors will have any advantage? It looks like they have an analog output so that would require me to completely redesign the components of the project. So at this point It seems like it would only be worth it if it were a somewhat guaranteed improvement.

[Wreckdetector]

Stationary metal detector

Good morning!

To magnetise objects before they go through the door would take a fairly hefty field, which your detector would probably be swamped by. Also anyone walking through it wearing an expensive mechanical watch would probably sue you.

Honeywell sensors - not very sensitive. Their resolution is something like 5nT if I recall.

Another problem for you....I assume you are trying to stop weapons getting through your door, dive knives are made of stainless steel so that they don't rust in sea water. Stainless is non magnetic so wouldn't be picked up.

I'd recommend going back to whoever gave you the project brief and telling them it won't work as a passive device.

[Chupa]

Thanks for all the info. Fortunately the limitations were known before starting the project so there's not too much of a problem with not being able to pick everything up. The main concern was hand guns, and I think It can pick most of those up with out any problem.

Again thanks for your input!