How Does a Metal Detector Work

A metal detector is a specially designed device that can detect metals lying deep in the ground or water. Since its invention, the device has found a wide range of applications in the human society. Most commonly, they are used for security screening and for locating mines. In the industrial sector, metal detectors have found applications in the food, pharmaceutical, textile, chemicals, plastics and packaging industries. They play an important role in identifying the presence of metal shards in foods. Such metal shards can be the broken pieces of the processing machinery used in the food industry. However, occasionally you can find many people using electronic metal detectors for treasure hunting and coin shooting as well.

Metal Detecting Devices

They generally work on the basic principle that, electric current when passes through a coil produces a magnetic field around it. They usually consist of an oscillator, which produces alternating current. When alternating electric current passes through the transmit coil present in the metal detector, a magnetic field is produced around it. Now, if an electrically conductive metallic object is present near the coil, then eddy currents will be generated in the object, which will produce another magnetic field around it. Metal detectors contain another coil in its loop, called receiver coil, which can detect the changes in the magnetic field, due to the presence of the metal or the metallic object. The modern-day metal detectors usually use one of the three technologies, which are known as VLF or very low frequency, PI or pulse induction and BFO or beat-frequency oscillation. These three technologies are briefly explained below.

Very Low Frequency (VLF) Technology

VLF is the most commonly used technology in these devices. Metal detectors based on this technology contain two sets of coil, transmitter coil and receiver coil. Transmitter coil is the coil along which electricity is sent to create a magnetic field, that constantly pushes down into the ground and then pulls back. The magnetic filed generated in the metal detectors interact with any metallic or conductive object that comes in its way. If any such object is encountered, then eddy currents and subsequently a magnetic field is created around the conductive object.

The receiver coil on the other hand is shielded from any influence of the magnetic filed produced by the transmitter coil, so that it is affected only by the magnetic filed of the conductive or metallic object. Electric current runs through this receiver coil, whenever the metal detector passes over a conductive object that is producing a weak magnetic filed. The coil in turn amplifies and sends the frequency of the current (which is same with the frequency of the magnetic filed generated by the metal) to the control box for analysis. In this way, these devices working on a very low frequency technology can detect metals and determine the difference between different types of metals and the depth at which they are located.

Pulse Induction (PI) Technology

Pulse induction technology can use a single coil, which can be used both as the transmitter and receiver. Sometimes, it can also use two to three coils. The metal detectors based on this technology send short bursts or pulses of current through the coils, each of which generate a short magnetic field. At the end of each pulse, the magnetic filed generated reverses its polarity suddenly and then collapses. This creates electrical spikes, that can last for a very brief period. As the spikes and the pulse's magnetic field collapse, another current, known as reflected pulse runs through the coil. The reflected pulse lasts for an extremely short period.

But, when the metal detector encounters a metallic or conductive object, the reflective pulse lasts for a longer time period. This is because, the pulse sent by the metal detector produces an opposite magnetic filed in the object and this magnetic field causes the reflective pulse to last longer. The metal detectors contain a sampling circuit that closely monitors the spikes and the reflected pulses and sends these signals to the device, called integrator. The integrator reads, amplifies and then converts the signals to direct current. The audio circuit connected with the direct current's voltage produces a tone, which indicates the presence of a metal or metallic object.

Beat Frequency Oscillator (BFO) Technology

Like very low frequency technology, beat frequency oscillator uses two coils of wire. One coil is present in the control box of the device, while the other is located in the search head. The coil which is in the control box is usually smaller than the one present in the search head. Both the coils however, remain connected to the oscillators that sends out thousands of electric pulses in a single second. Radio waves are created when pulses pass through each coil of wire, which are then collected by a receiver located within the control box.

The receiver then creates audible tones on the basis of the frequency of the radio waves. But, when the metal detector pass over metals or metallic objects, the electric current running through the coil of the search head creates a magnetic filed, which in turn creates another magnetic filed around the metallic objects. The magnetic field created around the metallic objects interferes with the radio waves created by the coil in the search head. This brings about a change in the tones produced by the receiver and the change helps to detect the targeted object.

So, these are the technologies on which various types of metal detectors work that we see in our day-to-day life. The metal detectors not only detect the targeted metallic or electrically conductive objects, but can also distinguish the different objects and find out their location.