Eric N

Metal Detector Theory

30 posts in this topic

Over the last decade or two, I've begun studying, building and using metal detectors.

The first thing I learned is everyone seems to have an opinion and most are questionable.

So in a fit of irritation, I started buying books.

Guess what? Most are just opinions as well. However, ...

 

Carl Moreland and George Overton, long time denizens of the Geotech website and MD designers in their own right wrote the book, "Inside the Metal Detector."

 

Not only does the book cover theory, but design principles and actual projects as well.

 

But again, the principal mechanisms presented have their own biases built into the explanations.

So here is my take.

 

ALL MDs use the same basic principles.

Emit an magnetic wave which induces eddy currents in the target.

Then compare the magnetic wave re-emitted by the target against the magnetic wave emitted by the detector.

 

That's it. Compare after target with before target.

 

MD coils are NOT, I say again, NOT antennas.

So for all practical purposes, forget about radio waves.

Think strictly magnetic.

MD coils are just one side of a transformer.

 

Now we have to consider the detection methods.

Unlike so many others, I content that there are three detection methodologies:

Frequency domain, time domain and phase domain.

 

In frequency domain, the target affects the frequency. Think of it as frequency shift.

I count two different types of detectors that use this method: BFO and Loaded Loops aka Off-resonance.

In the BFO, the target pulls the transmitted frequency and you hear the difference of the pull when it is compared to a stable oscillator.

In the Loaded Loop, the target pulls the transmitter either into a filter pass-band or away from the filter pass-band.

This pulling of frequency is due to conditionally stable oscillators using coils that have the coil characteristics (inductance) changed by metal getting within the coils area of influence. Altering the inductance changes the operating frequency.

 

In time domain, the re-emitted signal changes the decay characteristics of the soil. By shutting off the transmitter and coil real fast and then sampling, any noticeable change in the decay can be detected. This is the realm of Pulse Induction detectors. Stability of the oscillator is absolutely mandatory.

 

Then there is phase domain. In phase domain detection, we look at how does the target change the phase relationship designed into the detector.

Regular electricity is single phase 60 cycles per second. In industry, where there is a demand for far more power, we have three phase 60 cycle. There are three separate waveforms going down the wires with each 120 degrees out of phase with the others.

 

In phase detection MDs, the coil and electronics are designed to accommodate the difference in phase relationships between emitted and the re-emitted signal from ground. Any conductive target will alter that phase relationship. That phase difference is measured in degrees: plus or minus.

Among the phase domain detectors we have T-R and IB. Stability of the oscillator is once again mandatory.

In the T-R detector, we have a carefully phase balanced pair of transmitter and receiver. Each has a separate coil. Any conductive mineral will unbalance the system. These detectors are often called two-box and were used for surveys. Most are now airborne and much more complex.

 

This complexity is at it peak in the IB detector. It uses the same principles as the T-R, but adds discrimination. So how does it add this? Remember that phase relationship and how it can be plus or minus a few degrees? That is the key. You compare the emitted against the re-emitted in both short-term and slightly longer. The ground doesn't change much in the short-term while a target does. The ground changes slower and this is compared separately and alters the ground balance.

There is no change in frequency, ergo T-R and IB are NOT frequency domain detectors.

 

Now here it comes just because I'm old and cantankerous. You WON'T learn doodley squat from reading about detector theory on some website. It will only confuse you more. If MD theory really appeals to you, buy the book. Read it, do the experiments and build the project MDs covered in the book. You might wanna lurk for awhile on the Geotech forums. Do NOT go there if your sole interest is using detectors. It is the hang-out for designers, builders and flat-out unusual characters with odd ideas. Such as mine with frequency, time and phase domain design characteristics.

 

There is nothing mysterious about MDs. Once you learn the theory and gain experience building with electronics, you will notice that excellent MDs can be home-made.

 

Technically speaking, magnetometers and gradiometers are MDs. They detect a localized shift in the earth's magnetic field. Once again, it is ALL about magnetism. In using mags, there must be two mags in operation. One is the reference that logs the changes in the earth's magnetic field throughout the day while the other is the sampling mag that gets carried through the survey zone. In gradiometers, there are two mags that compare their readings. Both mags are separated by a few feet and the one closest to the iron target, remember this is about magnetism, detects the localized shift in the earth's magnetic field before the one further away. No separate reference magnetometer is required because this is only a comparative detector. Comparing here with back there. Or in cases where the gradiometer is used in vertical mode, here with just up there.

 

The key to understanding all this is forget everything you think you know about radio. This is about magnetic fields. Yes, there is an accompanying electrical field but it isn't relevant.

 

Hope this helps a bit. I'm still learning and have a long ways to go before I can claim any measure of proficiency. First, last and always, it is all about magnetism. Induced or natural. Oh and just a friendly aside. College doesn't teach metal detector theory or design. My electronics training background was military. Even it was awful weak. Fortunately the technical manuals covering each of the military MDs was quite well written and at an 8th grade level.

 

Nuf said. This isn't the best forum for MD theory and design. And I admit that I am pretty much a novice. Try Geotech. It is where I go for answers. Just be aware, they ain't gonna hold your hand. So you better get familiar with the website's search function. Hope you got the idea that I'm done with this thread. I presented it only for information purposes only.

 

eric

 

 

 

 

Clay and Tad R like this

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Thanks for that explanation Eric. Now it makes sense. :)

 

The all important distinction between magnetism and electrical fields was enlightening. The theories most offer about electrical field charges never made sense.

 

Now maybe you can explain why a simple coil glued into a plastic case can cost $300 or more? :lol:

Tad R likes this

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Holy coils batman! Alaska is truly expensive.

 

My wife handled several assembly contracts for a major defense contractor. Arizona and New Mexico as well as some work in Canada. Experienced electronic techs are going for around $30, $40 for a very experienced one, all have EEs. Good electronic assembly workers are making $12 - $14 on contract - usually piece work (no load). Maybe another 20% for internal overhead and management. All mil spec qualified.

 

It is getting harder finding good assembly workers. You can make more as a waitress at Denny's and many have chosen similar paths.

 

Maybe it's the "rare" litz wire you can buy just about anywhere since I was a kid? I've been told that by one of the coil makers. Looking at the winding standards I've seen in the industry high school interns could meet those quality standards.

 

It's a coil fer crikeys. It's not rocket science. There is more litz wire, electronics, moulding, assembly and design in an $80 induction cooker than in a metal detecting coil. The liability insurance, advertising and packaging are much more expensive too. I'm pretty sure that more than 80% of the retail cost of a coil has nothing to do with manufacturing expenses.

 

I'm all for coil makers and retailers making a good living for their efforts. As you can see the actual assembly workers don't. There is still a big gap between the $99 Bounty metal detector with a coil that doesn't false and the aftermarket $300 coil of the same size and design (no detector included) that sometimes does false.

 

I'm guessing the coil makers are caught between oodles of small shop competition splitting market share too fine and a dwindling market. I hope the coming shakeout at least produces some price competition. It seems to have already started. ;)

 

I'm not pointing fingers here. I asked an honest question. Why are these coils so expensive? I don't know the answer but I'm pretty sure it has no direct relationship to manufacturing expenses.

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This is interesting even tho I don't know much about such but I did own a White's detector back in the day. I'd say it was in the late 60's or early 70s, the group I was running with all had either White's or Garrett equipment. There was one in our bunch who knew a little about electronics and he purchased a DIY Heath Kit detector. He knew enough to add a feedback circuit into the kit where he was able to amplify the audio level of a target return signal which allowed him to easily pickup on a weak audio output. He had a unit that would out perform both the above mentioned detector's. I'm sure that the newer units are now far and above the modified DIY Kit he had but thought this story would make a good addition to this post saying how thing's used to be.

 

Hmmmm, I wonder what ever happened to Heath Kit?

 

dick

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I recently purchased the book inside the metal detector !1 This is not a good book on detectors the knowledge is 35 years old and not well presented , if you were working on a 1978 bounty hunter this would be the book for you !!! I am an Electronics Tech with a Degree in Industrial Tech and Electronics also I started my career by building industrial metal detectors and repairing them in the field and that was 30 years ago ,also only a small part of all the industrial tech work I performed , this book is a must miss on it's best day overpriced and under written !!!

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Ricky,

 

That is an excellent book review! :)

 

Books on technical subjects like metal detectors are written to a specific audience. Since most prospectors aren't scientific geniuses, they write in a form that is non-technical and use terms that are understood by prospectors.

 

Many prospectors become better prospectors as a result of reading information contained in books.

 

- Geowizard

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This is interesting even tho I don't know much about such but I did own a White's detector back in the day. I'd say it was in the late 60's or early 70s, the group I was running with all had either White's or Garrett equipment. There was one in our bunch who knew a little about electronics and he purchased a DIY Heath Kit detector. He knew enough to add a feedback circuit into the kit where he was able to amplify the audio level of a target return signal which allowed him to easily pickup on a weak audio output. He had a unit that would out perform both the above mentioned detector's. I'm sure that the newer units are now far and above the modified DIY Kit he had but thought this story would make a good addition to this post saying how thing's used to be.

 

Hmmmm, I wonder what ever happened to Heath Kit?

 

dick

Dick,

 

What happened to Heathkit is that other manufacturers improved their circuits so that they were as good as Heathkit and low cost Asian assembly work made it so that we could buy very good quality electronic products pre-assembled for less than Heathkits.  They hung on for awhile trying to sell wood furniture kits but I think they finally threw in the towel.

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Minelab and First Texas (maker of Fisher and Tecnetics detectors) both have have some good background info on metal detector operation on their websites.

 

See:

http://www.minelab.com/__files/f/11043/METAL%20DETECTOR%20BASICS%20AND%20THEORY.pdf

and

http://www.tekneticst2.com/tekfiles/Dave%27sGoldbook-reders.pdf

and

http://www.tekneticst2.com/tek-files.htm

 

Beyond just the operation of a device to detect metal is how the detector deals with ground mineralization. Iron bearing minerals in the ground react to the magnetic fields of a detector, and make detecting a target much more difficult.

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I recently purchased the book Inside The Metal Detector ! This is not a good book on detectors the knowledge is 35 years old and not well presented , if you were working on a 1978 bounty hunter this would be the book for you !!! I am an Electronics Tech with a Degree in Industrial Tech and Electronics also I started my career by building industrial metal detectors and repairing them in the field and that was 30 years ago ,also only a small part of all the industrial tech work I performed , this book is a must miss on it's best day overpriced and under written !!!

 

Underwritten for you no doubt Ricky. Many novices on the other hand would think most of it over their head, so just goes to show. I don't regret buying it. Either way, it is definitely overpriced. It is hard to print small runs of books at anything like a reasonable cost.

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They really should update the book with  more current concepts and circuits aimed at the layman or prospector in non technical terms if you don't have a Degree in Electronics a lot of the information can be very abstract , but is also very easy to pass off old information as new if you have no knowledge of Electronics !!!! They actually do teach you how to understand and repair metal detectors in Electronics studies in college , they start by teaching you theory D/C, A/C , ,inductive and capacitive reactance , analog and digital circuits,  integrated as well as  disc components , Soldering,  building and testing circuits using Oscopes and Digital and analog meters , LCR meters ,Meggers, not all detectors are built the same but they use the same components and basic circuits to perform the same job , this is just a small portion of what being an electronics technician is about !!

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If it has an LCD or Graphic display, it has a microcontroller! :)

 

Make your own metal detector? Show me the code. Oh, I forgot... "just download the code". Download the code for what? for a geotech roll your own detector with no pcb,.. maybe a pcb,.. well, not now but maybe from another source?  The placement was wrong... the diodes go the other way and the transistor is labeled wrong... yep... if you aren't a tech, you will be.

 

- Geowizard

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Over the last decade or two, I've begun studying, building and using metal detectors.

Rick, why dont you tell us some more about the detectors you have built, how they work for prospecting and some of the concepts they employ?

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Metal detectors are used for other applications besides prospecting. I think Ricky said the application was related to food service or security.

 

- Geowizard

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I think Ricky said the application was related to food service or security.

Geo - I think you are the one who should re-read the thread. Rick never mentions food service or security, he mentions he did repairs on commercial detectors, but that's not saying anything about they types of detectors he may have designed.

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There are many other applications for metal detectors. Commercial metal detectors are used for security of entry-ways in many government buildings.

 

- Geowizard

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All metal detectors operate on the same theory. Since Ricky probably works in the business and has signed non-disclosure agreements with his employer, he should not and probably will not disclose details pertaining to the detectors his company sells. Personally, I do not think it is ethical to push others to divulge insider knowledge. Discussion of relevant theory can help prospectors to understand why metal detectors have limitations and how to properly operate a metal detector.

 

- Geowizard

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Ricky probably

and

probably will not

Of these things you have no idea and are just speculating. I'd like to hear from Rick himself about Rick, rather that someone else who really does not know but like to speculate. If he cannot speak about something, fine, but allow him to speak for himself.

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Theory is after-all speculation. Metal detector theory is no exception. There are those that consider mathematics to be theoretical. The theory of metal detection is based on mathematic models (theory) of an electromagnetic response.

 

So, we are left to speculate on the probability of what the application is that Ricky is willing to provide information about. Any discussion on metal detector operation is theoretical. Probability of what the application is - doesn't matter.

 

- Geowizard

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