How Metal Detectors Work Click here to print this article.
Mention the words metal detector and you'll get completely different reactions from different people. For instance, some people think of combing a beach in search of coins or buried treasure. Other people think of airport security, or the handheld scanners at a concert or sporting event. If you work in construction, you might think about looking for buried cables or pipes.
Photo courtesy Bounty Hunter Metal detectors can provide hours of enjoyment when you use them to search for buried treasure.
The fact is that all of these scenarios are valid. Metal-detector technology is a huge part of our lives, with a range of uses that spans from leisure to work to safety. The metal detectors in airports, office buildings, schools, government agencies and prisons help ensure that no one is bringing a weapon onto the premises. Consumer-oriented metal detectors provide millions of people around the world with an opportunity to discover hidden treasures (along with lots of junk).
Now, you'll learn about metal detectors and the various technologies they use. Our focus will be on consumer metal detectors, but most of the information also applies to mounted detection systems, like the ones used in airports, as well as handheld security scanners.
Shaft - connects the control box and the coil; often adjustable so you can set it at a comfortable level for your height
Search coil - the part that actually senses the metal; also known as the "search head," "loop" or "antenna"
Photo courtesy Garrett Electronics Garrett GTI 1500 metal detector
Most systems also have a jack for connecting headphones, and some have the control box below the shaft and a small display unit above.
Operating a metal detector is simple. Once you turn the unit on, you move slowly over the area you wish to search. In most cases, you sweep the coil (search head) back and forth over the ground in front of you. When you pass it over a target object, an audible signal occurs. More advanced metal detectors provide displays that pinpoint the type of metal it has detected and how deep in the ground the target object is located.
Metal detectors use one of three technologies:
Very low frequency (VLF)
Pulse induction (PI)
Beat-frequency oscillation (BFO)
In the following sections, we will look at each of these technologies in detail to see how they work.
Photo courtesy Bounty Hunter This LandRanger metal detector from Bounty Hunter uses VLF.
The current moving through the transmitter coil creates an electromagnetic field, which is like what happens in an electric motor. The polarity of the magnetic field is perpendicular to the coil of wire. Each time the current changes direction, the polarity of the magnetic field changes. This means that if the coil of wire is parallel to the ground, the magnetic field is constantly pushing down into the ground and then pulling back out of it.
As the magnetic field pulses back and forth into the ground, it interacts with any conductive objects it encounters, causing them to generate weak magnetic fields of their own. The polarity of the object's magnetic field is directly opposite the transmitter coil's magnetic field. If the transmitter coil's field is pulsing downward, the object's field is pulsing upward.
The animation above demonstrates VLF technology.
The receiver coil is completely shielded from the magnetic field generated by the transmitter coil. However, it is not shielded from magnetic fields coming from objects in the ground. Therefore, when the receiver coil passes over an object giving off a magnetic field, a small electric current travels through the coil. This current oscillates at the same frequency as the object's magnetic field. The coil amplifies the frequency and sends it to the control box of the metal detector, where sensors analyze the signal.
The metal detector can determine approximately how deep the object is buried based on the strength of the magnetic field it generates. The closer to the surface an object is, the stronger the magnetic field picked up by the receiver coil and the stronger the electric current generated. The farther below the surface, the weaker the field. Beyond a certain depth, the object's field is so weak at the surface that it is undetectable by the receiver coil.
How does a VLF metal detector distinguish between different metals? It relies on a phenomenon known as phase shifting. Phase shift is the difference in timing between the transmitter coil's frequency and the frequency of the target object. This discrepancy can result from a couple of things:
Inductance - An object that conducts electricity easily (is inductive) is slow to react to changes in the current. You can think of inductance as a deep river: Change the amount of water flowing into the river and it takes some time before you see a difference.
Resistance - An object that does not conduct electricity easily (is resistive) is quick to react to changes in the current. Using our water analogy, resistance would be a small, shallow stream: Change the amount of water flowing into the stream and you notice a drop in the water level very quickly.
Basically, this means that an object with high inductance is going to have a larger phase shift, because it takes longer to alter its magnetic field. An object with high resistance is going to have a smaller phase shift.
Phase shift provides VLF-based metal detectors with a capability called discrimination. Since most metals vary in both inductance and resistance, a VLF metal detector examines the amount of phase shift, using a pair of electronic circuits called phase demodulators, and compares it with the average for a particular type of metal. The detector then notifies you with an audible tone or visual indicator as to what range of metals the object is likely to be in.
Many metal detectors even allow you to filter out (discriminate) objects above a certain phase-shift level. Usually, you can set the level of phase shift that is filtered, generally by adjusting a knob that increases or decreases the threshold. Another discrimination feature of VLF detectors is called notching. Essentially, a notch is a discrimination filter for a particular segment of phase shift. The detector will not only alert you to objects above this segment, as normal discrimination would, but also to objects below it.
Advanced detectors even allow you to program multiple notches. For example, you could set the detector to disregard objects that have a phase shift comparable to a soda-can tab or a small nail. The disadvantage of discrimination and notching is that many valuable items might be filtered out because their phase shift is similar to that of "junk." But, if you know that you are looking for a specific type of object, these features can be extremely useful.
Photo courtesy Garrett Electronics This Garrett metal detector uses pulse induction.
If the metal detector is over a metal object, the pulse creates an opposite magnetic field in the object. When the pulse's magnetic field collapses, causing the reflected pulse, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echos: If you yell in a room with only a few hard surfaces, you probably hear only a very brief echo, or you may not hear one at all; but if you yell in a room with a lot of hard surfaces, the echo lasts longer. In a PI metal detector, the magnetic fields from target objects add their "echo" to the reflected pulse, making it last a fraction longer than it would without them.
A sampling circuit in the metal detector is set to monitor the length of the reflected pulse. By comparing it to the expected length, the circuit can determine if another magnetic field has caused the reflected pulse to take longer to decay. If the decay of the reflected pulse takes more than a few microseconds longer than normal, there is probably a metal object interfering with it.
The animation above demonstrates PI technology.
The sampling circuit sends the tiny, weak signals that it monitors to a device call an integrator. The integrator reads the signals from the sampling circuit, amplifying and converting them to direct current (DC).The direct current's voltage is connected to an audio circuit, where it is changed into a tone that the metal detector uses to indicate that a target object has been found.
PI-based detectors are not very good at discrimination because the reflected-pulse length of various metals are not easily separated. However, they are useful in many situations in which VLF-based metal detectors would have difficulty, such as in areas that have highly conductive material in the soil or general environment. A good example of such a situation is salt-water exploration. Also, PI-based systems can often detect metal much deeper in the ground than other systems.
As the pulses travel through each coil, the coil generates radio waves. A tiny receiver within the control box picks up the radio waves and creates an audible series of tones (beats) based on the difference between the frequencies.
If the coil in the search head passes over a metal object, the magnetic field caused by the current flowing through the coil creates a magnetic field around the object. The object's magnetic field interferes with the frequency of the radio waves generated by the search-head coil. As the frequency deviates from the frequency of the coil in the control box, the audible beats change in duration and tone.
The animation above demonstrates BFO technology.
The simplicity of BFO-based systems allows them to be manufactured and sold for a very low cost. You can even make one at home following the instructions on this page. But these detectors do not provide the level of control and accuracy provided by VLF or PI systems.
The size of the object - A dime is much harder to detect at deep levels than a quarter.
The makeup of the soil - Certain minerals are natural conductors and can seriously interfere with the metal detector.
The object's halo - When certain types of metal objects have been in the ground for a long time, they can actually increase the conductivity of the soil around them.
Interference from other objects - This can be items in the ground, such as pipes or cables, or items above ground, like power lines.
Photo courtesy Bounty Hunter The control unit for a Bounty Hunter Tracker IV metal detector
Hobbyist metal detecting is a fascinating world with several sub-groups. Here are some of the more popular activities:
Coin shooting - looking for coins after a major event, such as a ball game or concert, or just searching for old coins in general
Prospecting - searching for valuable metals, such as gold nuggets
Relic hunting - searching for items of historical value, such as weapons used in the U.S. Civil War
Treasure hunting - researching and trying to find caches of gold, silver or anything else rumored to have been hidden somewhere
Many metal-detector enthusiasts join local or national clubs that provide tips and tricks for hunting. Some of these clubs even sponsor organized treasure hunts or other outings for their members. Check out LostTreasure.com for more information on clubs.
Photo courtesy Garrett Electronics A Garrett walk-through metal detector
Some nonrecreational applications for metal detectors are:
Airport security - screen people before allowing access to the boarding area and the plane
Building security - screen people entering a particular building, such as a school, office or prison
Event security - screen people entering a sporting event, concert or other large gathering of people
Item recovery - help someone search for a lost item, such as a piece of jewelry
Construction - locate pipes and cables underground
Archaeological exploration - find metallic items of historical significance
Geological research - detect the metallic composition of soil or rock formations
Photo courtesy Garrett Electronics A Garrett Super Scanner handheld metal detector
Manufacturers of metal detectors are constantly tuning the process to make their products more accurate, more sensitive and more versatile. On the next page, you will find links to the manufacturers, as well as clubs and more information on metal detecting as a hobby.
For more information on metal detectors and related topics, check out the links on the next page.