How Do Metal Detectors Work?

Minelab Explorer SEMetal detectors play an important role in modern day security. They can be found at almost every airport around the world and are often used at large scale or high profile events. They are also the pride possession of enthusiasts constantly on the lookout for buried treasures, be they of monetary, historic or sentimental value. But how exactly do these curious machines work? Well, the technology is actually pretty simple and harnesses one of the most fundamental principles of science and indeed the universe itself: electromagnetism.

The Basics…

All metal detectors have a search coil that transmits an electromagnetic field. Any metal objects (known as targets) within that field become energised and so then retransmit a field of their own. When the detector’s search coil picks up the target’s field they alert the user by providing a target response – often in the form of short beeps. Many detectors can emit a variation of beeps allowing the user to distinguish between different types of targets, including ones to ignore.

Consumer metal detectors like the ones often spotted being used to comb beaches and fields comprise of the following basic parts: 

  • Search Coil – This is what actually senses the metal and is also known as a ‘loop’, ‘antenna’ or ‘search head’.
  • Control Box – This is the communication hub that contains all the circuitry and controls as well as the batteries, speaker and microprocessor.
  • Shaft – This connects the coil to the control box and is often adjustable in length.
  • Stabiliser – This part is optional though many users prefer to have it as it enables a steadier hold with more control.
  • Display Unit – More common on modern devices, this gives the user a visual guide of all the data being received by the coil, such as the type of metal detected and the depth of its location.

How To Use…

Consumer detectors are designed with ease in mind and so are relatively simple to use. You just turn the unit on and move it slowly over the area you wish to search, often by sweeping side-to-side whilst walking forwards. When you pass over a target object, the detector will emit an audible beep. If there is a display unit then you will see what type of metal the target is as well as its exact location. The key to success is really in the method of your search.

Metal Detecting Coils

The Technology…

Metal detectors use one of three types of technology:

  • Very Low Frequency (VLF)
  • Pulse Induction (PI)
  • Beat-Frequency Oscillation (BFO)

Very Low Frequency

Also known as induction balance, VLF is the most popular type of detector technology in use today. VLF detectors have two separate coils: one that transmits a field and another that receives them.

The transmitter coil is the outer loop and works by having a fast electrical current being sent around the wire, first one way, then the other. This happens thousands of times a second with the number of switches per second identifying the unit’s operating frequency. The current produces an electromagnetic field that polarises each time it changes direction. It is this action of the field pushing and pulling in and out of the ground that causes metal objects to react by producing their own weaker field that is in polarity to that of the coil.

The receiver coil is the inner loop, positioned carefully so as to shield it from the field generated by the transmitter coil. Its wire acts as an antenna to pick up and amplify the frequencies of any potential targets in the ground. This works by sending a small electric current that oscillates at the same frequency as the object’s field. The coil then amplifies the frequency and sends it to the control box where it is analysed and alerts the user.

VLF detectors can also distinguish between different types of metal by picking up on their phase shift: the difference in timing between the coil’s frequency and the target object’s frequency. An object with a high inductance has a larger phase shift than one with low inductance because it takes longer for its magnetic field to switch poles.

Part of VLF technology includes a pair of electronic circuits called phase demodulators that examine the phase shift and match it to the average of a particular type of metal. There are many advanced settings based on these analytics that enable the user to refine their search.

Pulse Induction

PI systems use a single coil that acts as both a transmitter and a receiver and sometimes have multiple coils working together. Once switched on, short powerful pulses of electricity are sent around the coil creating a brief magnetic field. As soon as the pulse ends, the polarity of the field reverses then collapses suddenly. The result is a sharp electrical spike that causes another incredibly short current to run through the coil, known as a reflected pulse. This process is repeated many times per second with the exact number differing greatly from one device to the next.

Similar to VLF, metal objects within range will react by generating their own magnetic field, opposite to that of the device. The target object’s field interferes with the reflected pulse, making it take longer to disappear. A sampling circuit monitors the length of reflected pulses, sending the tiny weak signals to an integrator that converts them into a direct current (DC). The voltage of this current is connected to an audio circuit that produces a sound synonymous with detection.

Beat-Frequency Oscillator

BFO is the most basic system used in metal detection. There are two coils of wire – one in the search head and the other inside the control box. Each coil is connected to an oscillator that generates thousands of pulses per second with the frequency being slightly offset between the two. These pulses cause the coils to generate radio waves that are picked up by a tiny receiver in the control box, creating audible tones or beats based on the difference between frequencies.

When the coil in the search head passes over a metal object, the magnetic field caused by the current creates a magnetic field around the object. This then interferes with the frequency of the radio waves, which in turn alters the tone and duration of the audible beats.

Other Facts…

Even the most advanced detectors have a limit on how deeply buried an object is before it is out of range. The normal maximum depth is around 8 – 12 inches, though this is subject to a number of things like the type of detector, the size and halo of the object, the makeup of the soil and the possibility of interference from other objects such as cables and pipes.

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