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  How a Metal Detector Works  An overview and in-depth article on how Metal Detectors work  . OVERVIEW (By Michael W. Davidson) The operation of metal detectors is based upon the principles of electromagnetic induction. Metal detectors contain one or more inductor coils that are used to interact with metallic elements on the ground. The single-coil detector illustrated below is a simplified version of one used in a real metal detector.    A pulsing current is applied to the coil, which then induces a magnetic field shown in blue. When the magnetic field of the coil moves across metal, such as the coin in this illustration, the field induces electric currents (called eddy currents) in the coin. The eddy currents induce their own magnetic field, shown in red, which generates an opposite current in the coil, which induces a signal indicating the presence of metal.    MORE ID-DEPTH INFORMATION ON HOW A METAL DETECTOR WORKS  (By Mark Rowan & William Lahr) Metal detectors are fascination machines. Many of the people who use them are  just as enthusiastic about extolling the virtues of their favorite metal detector as they are about setting off in search of buried treasure. Those of us who design and  build these instruments for a living listen carefully when one of our customers talks about his or her experience in the field, because this is the primary means by which we determine how well we are doing our jobs, and what sort of things we need to do better. Sometimes though, communication is difficult. Almost as though we and our customers speak different languages. Which in a sense, we do. The purpose of this page is to try to narrow that communication gap a little. And, to resolve some of that typical curiosity metal detector operators have regarding what is going on inside their instruments. Is it necessary to know how a metal detector works in order to use it effectively? Absolutely not. Will knowing how it works help someone to use it more effectively in the future? Quite possibly yes, but only with persistence and practice. The best metal detector available is still only as good as the person using it. VLF (Very Low Frequency) Transmitter & Receiver Transmitter  Inside the metal detector's loop (sometimes called a search head, coil, antenna, etc.) is a coil of wire called the transmit coil. Electronic current is driven through the coil to create an electromagnetic field. The direction of the current flow is reversed several thousand times every second; the transmit frequency operating frequency refers to the number of times per second that the current flow goes from clockwise to counterclockwise and back to clockwise again. When the current flows in a given direction, a magnetic field is produced whose  polarity (like the north and south poles of a magnet) points into the ground; when the current flow is reversed, the field's polarity points out of the ground. Any metallic (or other electrically conductive) object which happens to be nearby will have a flow of current induced inside of it by the influence of the changing magnetic field, in much the same way that an electric generator produces electricity by moving a coil of wire inside a fixed magnetic field. This current flow inside a metal object in turn produces its own magnetic field, with a polarity that tends to be pointed opposite to the transmit field.  Receiver  A second coil of wire inside the loop, the receive coil, is arranged (by a variety of methods) so that nearly all of the current that would ordinarily flow in it due to the influence of the transmitted field is cancelled out. Therefore, the field produced by the currents flowing in the nearby metal object will cause currents to flow in the receive coil which may be amplified and processed by the metal detector's electronics without being swamped by currents resulting from the much stronger transmitted field. The resulting received signal will usually appear delayed when compared to the transmitted signal. This delay is due to the tendency of conductors to impede the flow of current (resistance) and to impede changes in the flow of current (inductance). We call this apparent delay phase shift . The largest phase shift will occur for metal objects which are primarily inductive; large, thick objects made from excellent conductors like gold, silver, and copper. Smaller phase shifts are typical for objects which are primarily resistive; smaller, thinner objects, or those composed of less conductive materials. Some materials which conduct poorly or not at all can also cause a strong signal to  be picked up by the receiver. We call these materials ferromagnetic . Ferromagnetic substances tend to become magnetized when placed in a field like a  paper clip which becomes temporarily magnetized when picked up with a bar magnet. The received signal shows little if any phase shift. Most soils and sands contain small grains of iron-bearing minerals which causes them to appear largely ferromagnetic to the metal detector. Cast iron (square nails) and steel objects (bottle caps) exhibit both electrical and ferromagnetic properties. It should be pointed out that this discussion describes an Induction Balance metal detector, sometimes referred to as VLF Very Low Frequency (below 30kHz). This is the most popular technology at the present time, and includes the LF Low Frequency (30 to 300kHz) instruments made for prospecting. Discrimination  Since the signal received from any given metal object exhibits its own characteristic phase shift, it is possible to classify different types of objects and distinguish between them. For example, a silver dime causes a much larger phase shift than an aluminum pull-tab does, so a metal detector can be set to sound off on a dime yet remain quiet on the pull-tab, and/or show the identification of the target on a display or meter. This process of distinguishing between metal targets is called discrimination . The simplest form of discrimination allows a metal detector to respond with an audio output when passed over a target whose phase shift exceeds a certain (usually adjustable) amount. Unfortunately, with this type of discriminator the instrument will not respond to some coins and most jewelry if the  discrimination is adjusted high enough to reject common aluminum trash for example pull-tabs and screw-caps. A more useful scheme is what is called Notch Discrimination . With this type of system, a notch in the discriminate response allows the metal detector to respond to targets within a certain range (such as the nickel/ring range) while still rejecting targets above that range (pull-tabs, screw-caps) as well as below it (iron, foil). The more sophisticated notch metal detectors allow for each of several ranges to be set for either accept or reject responses. White's Spectrum XLT for example, provides 191 individually programmable notches. A metal detector may provide a numeric readout, meter indication, or other display mechanism which shows the target's likely identity. We refer to this feature as a Visual Discrimination Indicator, or V.D.I. Metal Detectors with this capability have the advantage of allowing the operator to make informed decisions about which targets they choose to dig rather than relying solely on the instruments audio discriminator to do all the work. Most, if not all, V.D.I. metal detectors are also equipped with audio discriminators. Metal detectors can distinguish metal objects from each other based on the ratio of their inductance to their resistivity. This ratio gives rise to a predictable delay in the receive signal at a given frequency. An electronic circuit called a phase demodulator can measure this delay. In order to separate two signals, such as the ground component and the target component of the receive signal, as well as to determine the likely identity of the target, we use two such phase demodulators whose peak response is separated from each other by one fourth of the transmitter  period, or ninety degrees. We call these two channels X and Y . A third demodulated signal, we call G , can be adjusted so that its response to any signal with a fixed phase relationship to the transmitter (such as the ground) can be reduced to zero regardless of the strength of the signal. Some metal detectors use a microprocessor to monitor these three channels, determine the targets's likely identity, and assigning it a number based on the ratio of the X and Y readings, whenever the G reading exceeds a predetermined value. We can find this ratio with a resolution of better than 500 to 1 over the full range from ferrite to pure silver. Iron targets are orientation sensitive; therefore as the loop is moved above them, the calculated numerical value may change dramatically. A graphic display showing this numerical value on the horizontal axis and the strength of the signal on the vertical axis is extremely useful in distinguishing trash from more valuable objects. We call this display the SignaGraph (TM).

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Jul 23, 2017

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Jul 23, 2017
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