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How do Metal detector work

Writer£ºadmin    Postdate£º2014-4-3 14:56:11    Word£º¡¾big¡¿¡¾middle¡¿¡¾small¡¿

VLF Metal Detectors:

VLF, or Very Low Frequency, metal detectors are also known as induction balance detectors. They function by means of the disc-like plate which makes up the bottom of the device. The disc or "Broom" is actually made up of two separate sets of wire coil. The outer coil is called the transmitter coil, while the coil near the center of the broom is called the receiver coil. The transmitter coil projects an electromagnetic field down through the bottom of the broom, which passes down through anything the broom is swung over. If this field should contact anything conductive, namely metal, the metal will emit an electromagnetic field of its own that is opposite the transmitter coil's field. As the transmitter coil's field is aimed down, the metal's field comes back up to the broom. This is where the receiver coil comes into action. The receiver coil is shielded against the field the transmitter coil projects, but not the field the conductive material projects. It registers this field, sending a small electrical current through the coil. This current is registered by the control box of the metal detector and interpreted on the device's readout.

PI Metal Detectors

The Pulse Induction metal detector appears identical to the VLF detector, though it operates on different principles. Instead of the broom being made up of two separate coils, it possesses only one coil, which acts as both transmitter and receiver. Instead of transmitting a steady electromagnetic field, it sends one out in bursts or pulses. Each pulse takes a fraction of a second to complete; when the electromagnetic field collapses it generates a sharp electrical spike on the receiver, which takes a very specific time to die away before the next pulse can be sent. If the pulse should pass through a conductive material, it generates a sympathetic electromagnetic field for a very short time. This field is slightly out of phase with the one produced by the broom. This means that when the broom's field collapses, the collapsing of the field created by the conductive material will act like an echo, meaning that the generated electrical spike will take an abnormally long period to die down. The metal detector registers this time lag by means of an electronic sampling circuit, which reports its findings to the display on the device

BFO Metal Detectors

The Beat Frequency Oscillator metal detector is the simplest and least effective of the three metal detection devices. One large coil is in the broom, and a smaller coil is inside the control box near the handle. Each coil is connected to an oscillator that generates many thousands of pulses of electrical current per second. The frequency of these pulses is slightly different or offset between the two coils. This pulsing creates radio waves that pass through the broom. A receiver in the control box registers the radio waves and sends them to a headset in the form of tones or beats. The more beats that are heard, the greater the difference between the radio waves generated by each coil. The only way that the coils would generate significantly different radio waves would be if the broom passed over a conductive material. This would generate a weak magnetic field that would interfere with the radio waves generated by the broom.

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