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How to function a metal detector?
almost 2 years ago

The history of metal detectors is long and varied-starting from the detection of buried treasures, to the search for land mines and more recently, to include detection of metallic weapons. The basic technology for detecting metals is hinged on the principle of electromagnetic induction. In practice, metal detectors, are used for both detection and discrimination of metals from clutter.

It is worth examining the basic principles that metal detectors utilize.

The metal detectors function by transmitting a magnetic field, and analyzing a return signal from the target as well as the environment. The magnetic transmitter is in the form of a coil, with an alternating current (AC), which is generated by the transmitter circuitry, flowing through it. The receiver is in the form of a coil connected to a signal processing, electronic circuit. The transmit coil and the receive coil can sometimes be essentially the same. The coils are housed in a coil housing which is referred to as the coil. The accompanying circuitry is placed in what is known as the control box; and connected to the coils via an electric cable.


The alternating current in the transmit coil, induces a varying magnetic field in the same coil. This changing magnetic field is transmitted and induces an electromotive force in the metal targets. This EMF causes current to flow in the target metal. These currents are called eddyr currents. The eddy current in the target metal, in turn generates a weak electromagnetic field in the target metal. This field differs from the transmitted field in shape and strength. It is the kind of distortion produced by the metal target that enables it to be detected by the metal detectors. The regenerated magnetic field from the eddyr currents causes an alternating voltage signal at the receive coil. This is amplified by the receiver electronics because the returning signal is of several magnitudes weaker than the transmit signal.


The signal induced in the receive coil, by the magnetic field of the eddy current, can be thought of as made up of two simultaneous components, not just an altered component: One component is the same shape as the transmit signal. This is called the reactive signal (“X”) because it is of the same shape as the transmit field. This signal, by definition, is always in synchrony with the transmit signal. When this X component is subtracted from the eddyr current induced signal in the receive coil, the shape of the remaining signal depends only upon the history of the transmitted field, and not the instantaneous value. This signal is called the resistive or loss component (“R”).


Both the X and R signals of the target vary, depending on the distance of the target from the transmit coil; the further away it is, the weaker the transmitted magnetic field at the target, and the weaker the return signal from the eddy currents. The received signal is usually processed by the signal processing circuitry of the receiver to produce at least 2 signals: the strength of one signal is proportional to the R signal strength or magnitude, but is no longer an alternating signal.


Similarly, the other signal is also not an alternating signal, but rather, a signal simply related to X signal strength or magnitude only. Unfortunately, both the terms “X signal” and "R signal” may refer to both these two different meanings the one meaning referring to the alternating receive signal at the transmit frequency, and the other meaning to the strength of the received signals or magnitude (how big they are). So the term “X signal" may refer to the alternating in: signal waveform at the transmit frequency, or just the X signal strength or magnitude, which of course changes as the coil is moved about over different areas of ground. The same applies to the R signal.


These X and R signals (magnitudes) are further processed to give an output signal which may be reported to an operator in a number of different ways, the two most common being a ground balanced audio signal, whose loudness is usually proportional to the received signal strength from the eddy currents in metal targets; and a discriminated signal which only makes an audio “beep” when a target with selected properties is detected.


These properties may be varied by a metal detector Operator varying the controls of the metal detector. Most discriminating metal detectors also have a visual display which indicates properties of a detected metal target. The process of metal detection however is not without numerous challenges. We shall examine some of these, subsequently.


When metal detectors are used to check for the presence of landmines in certain geographical locations, a major problem arises; the need to distinguish between non-lethal, metal objects called clutter and landmines or metal weapons. These clutters result in false targets and false alarms. Stealth is a buried Indamine’s major defense against neutralization. Since the primary tool to find a landmine is the metal detector, landmine manufacturers have developed low metal content (LMC'), plastic-encased landmines to minimize the chance of detection.


These landmines have as little as 0.5g of metal content. Great effort has been expended by metal detector manufacturers to develop sensitive metal detectors to identify these small metal objects at depths of tens of centimeters in all soil types. Currently, the best metal detectors can find, with high confidence, these LMC landmines at a distance of about 20 cm. However, the increased metal detection sensitivity subjects the de-miner to increased false alarms owing to small metal clutter not previously detectable. It has been estimated that for every real landmine detected there are as many as 100 to 1000 metal clutter objects detected.


Obviously, it is desirable to be able to discriminate the metal clutter from the real landmine. Since 1991 a new technique using a high time resolution, wideband time domain metal detection sensor system called the Electromagnetic Target Discriminator {ETD}, has demonstrated, in laboratory and blind testing, the capabilities to detect and discriminate high- and medium-content metal landmines from metal clutter of similar metal content.

In addition, the system has shown the capability to detect, and in some cases, classify, some LMC plastic-encased landmines using time-delay and spatial features of the landmine signature. It differs from conventional metal detectors in several aspects.


First, the sensor's high-speed data collection system accurately measures the time-delay signature of the metal object. Second, its bandwidth is about 10 times that of other metal detectors, thus allowing the sensor to detect small, fast-decaying metal objects not normally detectable with a conventional metal detector. And third, the sensor uses a differential or gradiometer coil antenna design that has several advantages over most conventional metal detector coil antenna designs: automatic ground balance, mineralized soil effect rejection, void detection, far-field noise minimization, and cancellation of transmitter coil decay currents.


In conclusion, we have examined the underlying principle used by metal detectors; we have highlighted the challenging aspects of metal detection and how modern technology has helped minimized these problems. Metal detectors have proven to be of immense value; detectors are employed today by security agents for detecting hitherto concealed metallic weapons; by archaeologist, to find artifacts; and by hobbyist who hope to find some hidden treasure and make a fortune.

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