MadSci Network: Engineering

Re: What/How does a metal detector read from the magnet it is attached to.

Date: Fri Jul 23 20:12:05 1999
Posted By: Abtin Spantman, , Electrical Engineering, L. S. Research, Inc.
Area of science: Engineering
ID: 930163390.Eg

I'm not sure if you mean a permanent magnet, or if you are assuming that 
the coil in the bottom is an electro-magnet.  I shall try to address both.

1) How metal detectors work: Most metal detectors, hand held, and the 
walk-through ones at the airport  utilize a field-disturbance detector.  
That is a fancy name for a simple concept.  When a conductor passes through 
a magnetic field, a portion of that magnetic field is disturbed, distorted 
and reduced.  That reduction in the magnetic field is related to a small 
amount of current that is induced or created in the conductor.  You can do 
the same thing by jumping rope with a piece of wire instead of the rope. As 
the wire cuts through the earth's magnetic field, a small amount of 
electricity is generated at the end of the wires.  Be careful not to 
electrocute yourself as you're trying this.

Most metal detectors create their own magnetic field by passing current 
trough a coil.  (I suspect that is what you thought it was an 
electro-magnet)  They do this at a rate of around 150kHz (150,000 cycles 
per second).  They monitor the current through the coil as they resonate it 
at 150kHz, and they set an initial reference level.  If an outside ferrous 
metal passes through the magnetic field setup by this coil, it will change 
the current going through the coil.  Detectors pick up this difference, 
amplify it, and give you a signal.  Some lower end or older detectors let 
you hear the sound change using head-phones, and some give you LED 
indicators when a change in detected current occurs.  To get a feel for 
this, using your voice as the resonating coil, try to hit a constant note 
at a constant amplitude (sound level).  Then bring a solid object like a 
book or a dish close to your mouth.  You should detect a difference in 
sound as you bring the objects closer and farther, and also, based on what 
objects you use.  The metal detectors also react the same.  The magnetic 
field disturbance is different if a copper-alloy coin passes through the 
field, than if a steal nail passed through it.  The sounds would be 

2) How expensive metal detectors work:  As you guessed, the concept is the 
same.  The more expensive units take some of the guess work out, by adding 
a microprocessor that has previous knowledge of the sound quality that each 
metal produces, and perhaps can take educated guesses about the size and 
distance from the electro-magnet.   Some of the more expensive units will 
have multiple coils, separated by very specific distances, to accentuate 
the field, direct it, or reduce the noise (errors) that the unit picks up 
before sending it to the microprocessor.  Some units even change the 
resonating frequency because they know different metals react differently 
to different frequencies, and again can take out more errors and guess-work 
by adding intelligence and post processing inside the unit.

3) What a magnet can possibly do:  I have never had the pleasure of tearing 
one apart that had a permanent magnet.  But - If you have a unit that has a 
"Permanent" magnet in the coil assembly, it can be for one of two reasons: 
Saving battery power, or directing the magnetic field.  It can save power 
by using the field from the permanent magnet, instead of having to use up 
some serious battery power to power up the coil to generate the equivalent 
magnetic field. In that way, it can enhance the performance and reduce 
battery consumption by a great amount, at a very expensive cost - your 
dollars.  Second, if magnets are placed in certain orientations, they can 
direct and assist the field of perhaps a smaller detector coil, and hence 
reduce the noise being introduced into the system.  My guess is that it 
would be a combination of both of the above.

Let me know if more questions arise.
Abtin Spantman

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