Metal Detector Theory

[Unregistered]

I am looking for an introduction. Can anybody provide me with theory, schematics, equations, etc.

[UWLocator]

A Good Place to Start

The Metal Detector Handbook for Humanitarian Demining a pdf file published by the European Commission is a good place to start. Chapter 4 has a good basic intro to metal detection technologies. The book is geared toward finding mines but still gives a lot of good info on metal detection in general.

There are also several very good articles Here as well as several posts in the forums that should be helpful.

After looking this info over, if you have any specific questions please feel free to ask. There are several people on this forum that have a great deal of experience and a lot of technical know how on the subject.

Rip

[gef12]

Do a search on google ...
Reg Sniff and PI Metal Detector
Most informative

[gef12]

Do a search on google ...
Reg Sniff and PI Metal Detector
Most informative

[simonbaker]

There are two main types of hand-held metal detectors people seem to be interested in:

1 - IB (Induction Balance)
2 - PI (Pulse Induction)

The IB type are the ones that currently offer the best combination of depth and ability to discriminate (identify) the target under the ground. They are the metal detectors most often sold over the last couple of decades.

IB metal detectors have a search head with two coils, one called the Transmitter coil (abbreviated TX coil) and the other the Receiver coil (abbreviated the RX coil). The TX coil sends out a continous oscillating magnetic field. The RX coil is specially positioned and/or designed so it will not pick up the direct magnetic field from the TX coil (just a tiny bit). But it will pick up most of the magnetic field "induced" in the metalic target under the ground.

Because the electromagnetic signal from the target under the ground can be extremely small, special circuitry is needed to detect it and to ignore as much noise as possible that might hide the signal. The magical circuit to do that is called a "synchronous detector" and is kind of a variation of a circuit called a "lock-in amplifier". It is a very simple circuit to build but there is some mathematics behind how it does its job so well. I am attaching some introductory documents discussing how it works. I think it is the most important part of the metal detector circuit to understand well. (Note: only one document attached due to size limitations. You can search for others with google "lock-in amplifier").

The next most important thing to understand is how discrimination is done. That is related to the synchronous detector becauses the synchronous detector can give you phase information about detected signal (the TX signal is our basic reference for determining phase, the RX signal is same frequency as TX but different phase). Typically you will see at least two Synchronous Detectors working at the same time, each "tuned" to a different phase. You will also see a separate amplification "channel" for each detector. At the end of the amplification channels, the two signals can be compared to get an idea of what kind of metal the target is. A lot of fancy circuitry can be dedicated to trying to accurately identify the target metal, size, and depth. Computer processors are increasingly being used.

On this forum there is a thread about building one of the simplest IB type metal detectors -- the Tesoro Golden Saber (TGS). People (Ivconic) have simplified the circuit even more and call the design TGSL (lite version). I think looking at and building the TGSL circuit is the best way to understand the basics of an IB type metal detector. However, it is a very long thread and somewhat difficult to pinpoint what information you need get started.

Another type of metal detector is the Pulse Induction (PI) and it works on quite a different principle. It is known for searching deeper in the ground, but most designs are not capable of identifying the type of metal in the target to any degree. Basically it works with a single search coil through which a sudden short pulse of current is sent and quickly turned off. Then the same coil is used to detect the slight decaying magnetic field from the target. I don't know much about PI detectors myself but many people here do and there are some good simple circuits you can build.

Have fun!

-SB