Using multiple coils in close proximity, problem?

[monz]

Hi all,

I have a theoretical question that has been puzzling me for a while. Say you have a VLF metal detector with a coil diameter of say 40cm . Are you able to have multiple metal detector coil placed next to each other on a board (with enough offset) ? or would they interfere with each other and cause false readings?

My guess is yes you can place multiple metal detector on the same row with a big enough offset but not sure if thats right, and I wouldn't want to spend $600 to find out that it doesn't work .

What are your thoughts?

Thanks all

[Davor]

That would depend on a coil coupling type, and whether the other detectors are running, or are switched off.

Say other detectors are switched off...
With VLF detectors you'll most certainly have some coupling as the coil(s) are usually connected into (a) resonant tank(s), thus presenting very little losses and off resonance, which will result in a strong phase shifted response of some kind.
PI is effectively a coil in parallel with a damping resistor, so it presents itself as a target with very short tau well under 1us. So you can expect to have response similar to foil.

There are front ends that are different than this norm, and you'll have less response. However, all detectors will interfere if they are operating nearby. You can minimise this influence only by employing some anti-interference coil configuration.

[monz]

Hi Davor, thanks for your reply.

I need all the detectors to be ON at the same time to cover a span of 2 meters, so was thinking of attaching more 4 off the shelf coils in a row.

Trying to detect relatively big Iron plates (50cm diameter to 5cm), so sensitivity and discrimination can be ignored for this application.

By the anti-interference coil configuration, do you mean turning down discrimination and sensitivity? or did you mean adjusting something inside the control box itself?

[Davor]

Neither.

Your best bet for your application is using a single PI detector with a huge monocoil.

Anti-interference coils are designed in a way that they ignore far signals, by means of differential configuration. Such coils are usually avoided in daily use as they are more heavy, and they are not as sensitive as "normal" coils.
If you are familiar with electric guitars, humbucker pickup is an example of such configuration.

[Koala]

"(with enough offset)" This is no different to being on a rally with several people walking in a line.



But strange things happen when one coil is in the proximity of another. I left a spare one on the side of my bench well to the side of a coil I was testing. Every time the coil moved even a few .001" it would sound thought I had a problem till I moved the spare coil. Don't think you could mount them ridged enough not to sound unless there is very wide gaps which would then defeat the object of the exercise.

There was a TV program where a massive homemade PI coil of several meters square was towed behind a quad bike but they were looking for something made of iron several meters in diameter.

[monz]

I see your point.

I have got an idea though, what if we impelement multiple metal detector say (4 coils and their separate control boxes) in a row, and use a controller to enable/disable them?

so no two coils will be on at the same time, and we can do it fast enough so that it doesnt affect out application . what do you guys think ?

[Davor]

Too complicated.

[monz]

its our project at school haha, we have to do it
(just wanted to confirm that it would work in theory)

[Davor]

In theory... if they are PI detectors, and you magically synchronise their pulsing - yes. Otherwise - no.
Total energy obtained by such "distributed antenna" and sequential intermittent operation from targets in a ground would be equal to a single large coil operating constantly.
So instead of making your life miserable you can use a single rig to work with appropriate coil(s) to do it properly.

[Koala]

How do the multi zone walk through detectors work. The type you see at an airport that light up in line with where the metal has been found.

[Koala]

found this

[Davor]

So that's easy, just find a few commercial PI units, and convince them to synchronise their pulsing - easy /(sarc off)

[Carl-NC]

You can run multiple machines (either VLF or PI) concurrently by synchronizing them. Most security walk-throughs have a large TX coil and multiple induction-balanced RX coils, each with their own receive channel.

[monz]

I don't see why would I need to synchronize them. Each "unit" will have its own RX and TX. The only potential issue here is that there might be too much energy in the system

[6666]

I don't see why would I need to synchronize them. Each "unit" will have its own RX and TX.

Sometimes a group of 4-5 of us MD together , we all use VLF's, various makes and models
and we cannot get within 10 feet of each other because of the interference, try it yourself.