|MadSci Network: Engineering|
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 different. 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 SPANTMAN@EXECPC.COM
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