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.
How metal detectors work
Metal detectors work on the principles of electro physics. The device produces electromagnetic waves that go deep into the ground through metal search coils. These electro waves reach the object within its range that charges the other metal in its range. Electromagnetic waves are received by search coils. As soon as these waves reach the search coil, the detector beeps to inform the user that something important is in its way.
Science put into action
Some key concepts are applicable when we talk about the working of metal detectors. Few of them are explained below
· Frequency
There are two types of frequencies namely; low frequencies and high frequencies that work on different aspects of targets. A detector with low frequency is most suitable to detect metals that are deep into the piles of the earth’s crust. On the other hand, a high-frequency metal detector is to detect all types of metal that come into the direct range of the detector. This explains that a high-frequency detector is directly proportional to the high sensitivity of metals. However, there are metal detectors available in the market that have multiple frequencies and they work best in either case. Just choose the right frequency settings for finding deep large targets or small targets.
· Ground balance
This is an important mechanism when you not just have precious metals in the ground but also have various minerals in the ground. Sometimes, the minerals may hinder the signals from various underground metals that are covered with thousands of different minerals underground. Ground balance setting corrects this problem by ignoring unwanted detection and you can hear only the detections that you require. However, types of ground settings differ in different metal detectors. There are three types of ground balance settings:
- Manual Ground balance: people set the detector according to their requirements but sometimes they don’t get the desired results because the person is not an expert in this subject. He is using it as his hobby.
- Automatic Ground balance: All you have to do is specify your requirements and a metal detector will be set accordingly. This way of setting is much easier and efficient than manual.
- Tracking Ground balance: Continuously changing settings with a change in ground level. This is a more advanced setting for professional detection.
· Discrimination
Metal differs in properties such as how conducive they are to radiations, how well do they ionize or do they have ferrous properties and are prone to rust. The discrimination setting will help to efficiently distinguish between useful and wasteful targets even before digging. This setting saves a lot of time and effort of a person that can be used elsewhere for productive jobs.
Every detector differs in discrimination settings, hence, there are four different kinds of discrimination settings in different detectors. Four main types of discrimination settings include:
- Variable discrimination: the detector has a simple knob that controls the discrimination mechanism
- Iron mask/ Iron Reject: It avoids unwanted materials such as nails and detects valuable metals present in the soil.
- Notch discrimination: You customize both wanted and unwanted material that you want the detector to detect.
- Smartfind: as the name suggests, it is the most advanced type of discrimination in which you enter the target IDs in the detector. The system recognizes these IDs and sort material according to conductivity or ferrous property.
Different types of Metal Detectors
Metal detectors can be classified in different categories according to their properties. Differing magnetic fields due to differing electric discharge can enhance frequencies or make it worse. With altering currents, the detectors are named differently. Few types of metal detectors include:
- VLF (very low frequency) detectors are the ones that are simple and have fixed detecting frequency that ranges between 6 to 20 hertz. Generally, it is less than 30 hertz. This is considered as a simple treasure hunting hobby detector.
- PI (pulse induction) detector is another type of detector that senses pulses and requires high frequencies to detect the target. Due to these characteristics, they are especially for materials that lie deeper into the ground.
- FBS (Full band Spectrum) has such a design that it can detect multiple frequencies simultaneously. Simply, it can be said that multiple detectors are working on time.
- The most common type of metal detector is namely the walk-through metal detector that you may have seen outside office buildings, at airports, and many other areas. When a person passes through these detectors, any possession of harmful items underneath clothes or in bags enables the metal detector to beep according to its type.
- There are also handheld metal detectors, again used for security. These are swiped all over the person’s body and they are programmed to make a sound according to the type of metal found.
The Technology
- 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
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.