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There are several circuits, but the basic principle is the same for all of them. You use a "search coil" that you move over the area where the metal might be found.
As the moving coil passes over the metal object, the presence of the metal in the coil's field causes a change in the field.
Some detectors use a tuned circuit. When a piece of metal enters the field of the coil, the inductance of the coil changes. The inductance change is registered as a change in frequency for the tuned circuit, or a change in the amount of energy used by a resonant circuit. Either, or both, of the parameters can be used to give audible and/or visual indication.
Some detectors are made with two coils, like a big air core transformer. If a piece of metal enters the field of the transformer, the coupling between the coils is effected. That change in coupling again can be used to drive audible and/or visual indicators.
Wish to add some info - there are also radar alike metal detectors which are more capable than tuned detectors . These detectors output strong pulse signal or frequency (i do not remember exactly) and then go to receiver mode to detect waves generated back from metal.
Inductance variation is only observed with ferromagnetic metals (iron, nickel, a few alloys), having permeability considerable large than 1. It's the same effect as bringing a magnetic core to a coil.
Well, I've done some research and I found out (I'm not quite sure though) that permeability isn't the only cause of inductance change when an alternating current circulates in the coil. There is also the magnetic field from the original current and the secondary magnetic field from the eddy currents in the metal that change the inductance (Φ=Li).
The BEST answer depends on what type of machine you are using. The most common is Induction balance in which THREE coils ( TX, RX and BUCKING) are arranged in a manner that the output of the Rx is substantively ZERO (due to mutual coupling). When a metallic object (irrelevant whether it is ferrous or otherwise) is brought within the field of influence of the TX, is causes an imbalance. This imbalance is detected as a phase and amplitude shift, the direction and amplitude of which depends on the TYPE of metal. Ferrous will induce a NEGATIVE phase shift (WRT TX signal) in the RX coil whilst NON ferrous have the opposite effect.
NOW.. it's get a little more complicated :twisted:.
You CAN deliberately offset the null so that you introduce a permanent signal on the RX though generally the better or close to absolute zero you can get the better the detector will be as no DC offset will be present on the demodulator (lock in amps) outputs thus allowing higher levels of amplification to be used.
Basically modern IB detectors are have a GROUND channel or I(n phase) and a DISCRIMINATE or Q(uadrature) channel.
I could go on for ever, but the resource I pointed you to will have members who will give you (with all due respect) a FAR better answer than you will get here as, contrary to popular belief, there is a LOT more to metal detector design that most Engineers realise.
I've spent years simulating and refining various circuits only to find that when you apply them, the resultant machine just SUCKS for performance.
I agree (speaking from experience), however it's unlikely that the few mW required would trigger anything nasty.
I worked with a UK company on the laser system a number of years ago ;-)
BTW PI's CAN discriminate. Google "Pulse Devil" my friend Dave Emery in the US has developed one that can and has patents for it. I've played with this machine and let me tell you, in comparison to existing machines, IT'S DEEP!!!
I still don't fully understand why does the inductance of a coil change when we approach a metal. Today, I've done a small experiment and I found out that inductance change even when I approach a piece of aluminum.
My pass band filter was powered by alternating current. On the pictures, we see the filter and the aspect of the curves I obtained. The problem is that it detected both ferromagnetic metal and non ferromagnetic metals. How can this be explained ?
Thank you SO much and sorry for the english mistakes.
ALL metals whether ferrous or otherwise will cause an inductance change, some positive, some negative, and some like ferrite, usually neither ways, but just "focus" the magnetic field thereby increasing the "Q" factor of the coil.
I suggest you Google "inductance" and do some more research, as to explain here could take quite a while. You need to understand the fundamentals of electro-magnetism in order to understand what you are seeing with your device.
Permeability affects HOW much the particular material will change the inductance. That maths is quite involved and beyond the scope. I suggest you research the subject on Google, OR you could join the Geotech forum and have a look around there. Carl who runs the site is the SENIOR design Engineer at Whites (manufacturers of the worlds best metal detectors).