Metal detector technology is a huge part of our lives as it provides millions of people around the world an opportunity to discover hidden treasures; while also ensuring safety in airports, office buildings, schools, government agencies, and prisons by helping to ensure no one brings weapons onto such premises.
So, you’ve probably been wondering… How exactly does this stuff work?
Well, the internal working of metal detectors is based on the science of electromagnetism. It works by using technology to harness the unique relationship that exists between electricity and magnetism.
In this article, I would be explaining in details the basic working principle of a metal detector and the technologies that make different types work.
Bear in mind that it’s not necessary to know how a metal detector works in order to use it. However, it’s quite possible that knowing how it works can help you use it more effectively in the future.
The Basic Working Principle of Metal Detectors
The basic working principles of metal detectors are based on electromagnetic induction.
Metal detectors work by interacting with metallic elements on the ground through one or more inductor coils that are attached to them. They work by electromagnetic field transmission from the search coil attached into the ground. As a result of this electromagnetic transmission, any metal objects (targets) within the electromagnetic field range will then become energized and transmit an electromagnetic field of their own in return. The detector’s search coil then receives the re-transmitted field and alerts the user by producing a target response.
Most metal detectors are designed with a discrimination feature; which is the metal detector’s ability to identify underground targets based on its ferrous and/or conductive properties. This feature allows metal detectors discriminate between different types of targets and can be set to ignore unwanted targets.
Having had a basic understanding of how metal detectors work, let’s now talk about the technologies that power this principle.
- Very Low Frequency
- Pulse Induction
- Beat-Frequency Oscillation
Here’s a brief look at each of these technologies in detail to see how they work.
Very Low Frequency (VLF)
Very Low Frequency is the most popular metal detector technology in use today. And it employs the use of 2 distinct coils; namely the transmitter & receiver coil.
The transmitter coil is the outer coil loop. It contains a coil of wire through which electric current flows up and down (up to about 1000 times per second).
The electric current flowing up and down through this transmitter coil generates an electromagnetic field which interacts with any conductive substance it encounters and causes these substances to generate their own weak magnetic fields.
The receiver coil, on the other hand, is the inner coil that’s also made up of another wire coil. Again this wire coil works like an antenna to detect the frequencies of targets underground and amplify their signals.
To differentiate between different metals, the VFL relies on a phenomenon called “phase shifting”. Phase shift provides VLF-based metal detectors with a capability called discrimination; which is a metal detector’s ability to identify underground targets based on its ferrous and/or conductive properties.
And since the resistance and inductance in most metals vary, the amount of phase shift is examined by a VLF metal detector, using phase demodulates (which is a pair of electronic circuits), and compares it with the average for a particular type of metal. You then get notified with a visual indicator or audible tone indicating the range of metals the object is likely to be in.
PI Technology differs from Very Low Frequency (VLF), because it uses 1 coil as both a transmitter and receiver. Some, however, are known to use multiple coils (two or even three) working together at once.
Powerful pulses travel along the wire coil that creates small magnetic fields. When these run out, the polarity gets reversed by the magnetic fields then collapses to make an electric spike. In microseconds, the spike runs out after it generates another current to move through the search coil.
PI detectors are able to pick up metals buried far deeper than other metal detecting technologies.
Beat-Frequency Oscillation- BFO Technology
Beat-Frequency Oscillation (BFO) is the most basic form of metal detecting technology. In a BFO system, there are two coils of wire. The larger coil is put in the search head while the smaller one is placed inside the detector’s control box.
An oscillator connects each coil and generates thousands of pulses of current per second.
The coil generates radio waves as pulses travel through each coil. The radio wave is picked up within the control box by a tiny receiver and creates an audible series of tones depending on the difference between the frequencies.
When a metal object is passed over by the coil, the magnetic field caused by the current flowing through the coil generates a magnetic field around the object. The magnetic field of the object interferes with the radio wave frequency generated by the search-head coil.
As the frequency in the control box deviates from the frequency of the coil, the audible beats change in tone and duration.
The Beat-Frequency Oscillation, however, doesn’t offer the same degree of accuracy and control put forward by the other 2 technologies. And metal detectors that use the BFO technology are sold at a lower price because of its simple design.
Factors that can interfere with how a Metal Detector Works
The detection depth of a metal detector depends upon which of the above technology it employs and a number of environmental factors listed below…
- Ground Mineralization
- The Size of the Target
- The Shape of the Target
- The Orientation of the Target
- The Material of the Target
So, there you have it. You now have a basic understanding of how your metal detector works.